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UBC Theses and Dissertations

The chemical limnology of two meromictic lakes with emphasis on pyrite formation Perry, Karen Anne 1990

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THE C H E M I C A L L I M N O L O G Y O F T W O M E R O M I C T I C WITH EMPHASIS O N PYRITE F O R M A T I O N  LAKES  by K A R E N A N N E PERRY B.Sc. University of V i c t o r i a 1979 B.A.Sc. University of British C o l u m b i a 1983  A THESIS SUBMITTED IN PARTIAL FULFILLMENT THE REQUIREMENTS F O R THE D E G R E E O F D O C T O R O F PHILOSOPHY in THE FACULTY O F G R A D U A T E STUDIES Oceanography  W e a c c e p t this thesis a s c o n f o r m i n g to the required standard  THE UNIVERSITY O F BRITISH C O L U M B I A N o v e m b e r 1990 © K a r e n A n n e Perry 1990  OF  In  presenting this  degree at the  thesis  in  partial  fulfilment  University  of  British Columbia,  of  the  requirements  this thesis for scholarly  department  or  by  his  or  her  an  purposes may be  representatives.  It  is  permission for extensive  granted by the understood  head of my  that  publication of this thesis for financial gain shall not be allowed without permission.  Department of The University of British Columbia Vancouver, Canada Date  DE-6 (2/88)  Q  n  /  ^  O  advanced  I agree that the Library shall make it  freely available for reference and study. I further agree that copying of  for  copying  or  my written  ii ABSTRACT  P o w e l l a n d S a k i n a w Lakes a r e s t a b l y stratified ex-fjords, w h i c h b e c a m e i s o l a t e d f r o m t h e Strait of G e o r g i a a p p r o x i m a t e l y 11000 years a g o b y e m e r g e d sills d u e t o postg l a c i a l isostatic r e b o u n d . A l t h o u g h b o t h lakes c o n t a i n highly s u l p h i d i c relict s e a w a t e r ( P o w e l l 3.0 m M ; S a k i n a w 5.5 m M ) , t h e y h a v e distinct c h e m i c a l d i f f e r e n c e s , w h i c h m a y b e d u e t o S a k i n a w r e c e i v i n g o c c a s i o n a l inputs of s e a w a t e r o v e r t h e  barely-emerged  sill w h e n strong o n s h o r e w i n d s a r e c o i n c i d e n t w i t h spring tides. P o w e l l L a k e , n o w 50 m a b o v e s e a l e v e l , has not r e c e i v e d a d d i t i o n a l s e a w a t e r s i n c e t h e sill originally e m e r g e d . S a k i n a w h a s a v e r y s h a r p c h e m o c l i n e l o c a t e d just b e l o w t h e o x i c / a n o x i c i n t e r f a c e , w h e r e a s in P o w e l l , t h e i n t e r f a c e is s p r e a d out o v e r 200 m of t h e w a t e r c o l u m n . A l t h o u g h b o t h lakes h a v e f r e s h e n e d , t h e ratios of m a j o r ion c o n c e n t r a t i o n s relative t o c h l o r i d e in t h e b o t t o m saline vyaters a r e similar t o t h o s e of p r e s e n t - d a y s e a w a t e r . There a r e s o m e d i f f e r e n c e s , h o w e v e r , a n d t h e s e c a n b e e x p l a i n e d , in p a r t , b y t h e d i f f e r e n c e in m o l e c u l a r diffusivities for e a c h of t h e ions. The b o t t o m w a t e r s of P o w e l l a n d S a k i n a w Lakes a r e c h e m i c a l l y similar t o a n o x i c s e d i m e n t p o r e w a t e r s . c o n t a i n i n g h i g h c o n c e n t r a t i o n s of nutrients, D O C a n d alkalinity. Unlike S a k i n a w , h o w e v e r , P o w e l l L a k e h a s v e r y l o w c o n c e n t r a t i o n s of p h o s p h a t e in its b o t t o m w a t e r s , in spite of b o t h lakes h a v i n g similar p a r t i c u l a t e o r g a n i c N:P ratios in their u p p e r o x i c w a t e r s . This m a y b e a t t r i b u t a b l e t o m o r e r e c e n t a d d i t i o n of s u l p h a t e t o S a k i n a w , a l l o w i n g g r e a t e r m i n e r a l i z a t i o n of p h o s p h o r u s c o m p a r e d t o t h e  relatively  oxidant-starved Powell Lake. High c o n c e n t r a t i o n s of r e d u c e d iron, h y d r o g e n sulphide, a n d  polysutphides  result in f o r m a t i o n of iron m o n o s u l p h i d e s a n d pyrite in t h e a n o x i c w a t e r c o l u m n s of b o t h lakes. The p r e s e n c e  of t h e s e t w o m i n e r a l s c o r r e l a t e s w e l l w i t h t h e i r  calculated  s a t u r a t i o n states. Pyrite p r e c i p i t a t e s d i r e c t l y w i t h n o m o n o s u l p h i d e p r e c u r s o r a t d e p t h s w h e r e s u l p h i d e c o n c e n t r a t i o n s a r e l o w ; thus m o n o s u l p h i d e p h a s e s a r e u n d e r s a t u r a t e d . As s u l p h i d e levels i n c r e a s e w i t h d e p t h , iron m o n o s u l p h i d e s b e c o m e s a t u r a t e d a n d a r e d e t e c t e d in t h e w a t e r c o l u m n . Pyrite c a n t h e n f o r m v i a t h e slower r e a c t i o n of e l e m e n t a l sulphur w i t h m o n o s u l p h i d e . The l a r g e s e p a r a t i o n of t h e o x i c / a n o x i c i n t e r f a c e a n d t h e c h e m o c l i n e in S a k i n a w (-10 m) a n d e s p e c i a l l y in P o w e l l L a k e (-100 m) relative t o t h a t of s e d i m e n t p o r e w a t e r s a l l o w s e x c e l l e n t resolution of t h e s e p r o c e s s e s .  TABLE OF CONTENTS ABSTRACT LIST O F TABLES LIST O F FIGURES ACKNOWLEDGEMENTS  ii vi vii ix  ' 1. I N T R O D U C T I O N  1  2. PHYSIOGRAPHY, PHYSICAL A N D C H E M I C A L L I M N O L O G Y 2.1 I n t r o d u c t i o n Powell Lake Sakinaw Lake 2.2 M a t e r i a l s a n d M e t h o d s Water Collection Analytical Methods P h y s i c a l Parame.ters M a j o r Ions Sodium, m a g n e s i u m , potassium Strontium Calcium Borate 2.3 Results Physical D a t a M a j o r Ions 2.4 Discussion  5 5 5 8 10 10  3. R E D O X CHEMISTRY 3.1 I n t r o d u c t i o n Carbon Nitrogen Ammonification (Deamination) N i t r o g e n Fixation Nitrate R e d u c t i o n a n d Denitrification Nitrification 3.2 M a t e r i a l s a n d M e t h o d s O r g a n i c C a r b o n a n d Nitrogen Nutrients Nitrogen Dissolved Silicon" Alkalinity pH ; 3.3 Results O r g a n i c C a r b o n a n d Nitrogen Powell Lake Sakinaw Lake Nutrients Powell Lake Sakinaw Lake Alkalinity a n d p H Powell Lake Sakinaw Lake  26 26 29 32 33 33 35 36 38 38 39 39 39 40 40 41 41 41 41 41 41 42 42 42 42  10 10 10 11 11 12 13 13 13 21  '.  '.  3.4 Discussion Organic Carbon.. DOC POC DCOPCC POOPON Inorganic Nitrogen D i s s o l v e d Si l i c o n Alkalinity a n d p H  :  ; -.  48 48 48 51 53 53 55 58 61  4. P H O S P H O R U S CHEMISTRY 4.1 I n t r o d u c t i o n The P h o s p h o r u s C y c l e Forms of P h o s p h o r u s Dissolved Phosphorus Particulate Phosphorus B i o l o g i c a l I m p o r t a n c e of P h o s p h o r u s M i n e r a l i z a t i o n of P h o s p h o r u s 4.2 M a t e r i a l s a n d M e t h o d s Soluble R e a c t i v e Phosphorus Total P h o s p h o r u s Particulate Phosphorus 4.3 Results Powell Lake Sakinaw Lake 4.4 Discussion Sakinaw Lake Powell Lake Phosphorus R e m o v a l Due to Adsorption P h o s p h o r u s R e m o v a l D u e t o Direct P r e c i p i t a t i o n Biological Removal W h y P o w e l l a n d S a k i n a w Lakes A r e So Different  65 65 65 67 67 68 68 70 73 73 73 75 76 76 77 85 85 88 89 91 94 98  5. SULPHUR CHEMISTRY 5.1 I n t r o d u c t i o n Sulphur C o m p o u n d s I n o r g a n i c Sulphur C o m p o u n d s O r g a n i c Sulphur C o m p o u n d s The Sulphur C y c l e Sulphate-Reducing Bacteria Sulphide-Oxidizing Bacteria D i a g e n e s i s of Sulphur Iron S u l p h i d e s S e d i m e n t a r y Pyrite F o r m a t i o n L a b o r a t o r y Studies of Pyrite F o r m a t i o n 5.2 M a t e r i a l s a n d M e t h o d s Water Collection ' Sulphate Sulphite, Thiosulphate a n d Polythionates Z e r o v a l e n t Sulphur Dissolved Sulphide P a r t i c u l a t e S u l p h i d e Analysis Iron a n d M a n g a n e s e S p e c i a t i o n a n d Solubility C a l c u l a t i o n s  101 101 102 102 105 105 107 107 108 110 110 112 115 115 115 116 118 119 119 122 122  5.3 Results Powell Lake Sakinaw Lake 5.4 Discussion Sulphur O x y a n i o n s Sulphate T h i o s u l p h a t e , Sulphite a n d P o l y t h i o n a t e s R e d u c e d Sulphur S p e c i e s Dissolved Sulphide Z e r o v a l e n t Sulphur Iron a n d M a n g a n e s e Metal Complexes Particulate Sulphides Effect of p H o n Pyrite F o r m a t i o n O r g a n i c Sulphur M a n g a n e s e Sulphides  • •  i^i 123 124 133 133 133 135 136 136 137 144 147 148 152 153 154  6. S U M M A R Y A N D C O N C L U S I O N S  155  APPENDIX 1 SOLUBILITY A N D SPECIATION C A L C U L A T I O N S A P P E N D I X 2 D A T A TABLES REFERENCES  158 164 172  vi  LIST OF TABLES T a b l e 2-1  D e v i a t i o n s of o b s e r v e d b o t t o m w a t e r m a j o r i o n c o n c e n t r a t i o n s f r o m t h o s e c a l c u l a t e d f r o m chlorinity a s s u m i n g c o n s t a n t relative c o m p o s i t i o n of s e a w a t e r T a b l e 3-1 O x i d a t i o n r e a c t i o n s o f o r g a n i c m a t t e r (from Froelich et a l . 1979)...., T a b l e 3-2 M a j o r classes of r e a c t i o n s i n v o l v e d in alkalinity g e n e r a t i o n a n d c o n s u m p t i o n (from Schiff a n d A n d e r s o n 1987) T a b l e 5-1 I n o r g a n i c sulphur c o m p o u n d s c o m m o n l y f o u n d in t h e a q u e o u s environment T a b l e 5-2 R e a c t i o n s at t h e m e r c u r y e l e c t r o d e (from Luther et a l . 1986a) Table A-l Stability c o n s t a n t s u s e d in MINEQL c a l c u l a t i o n s T a b l e A-2 Solubility p r o d u c t s u s e d in MINEQL c a l c u l a t i o n s T a b l e A-3 P h y s i c a l a n d m a j o r i o n d a t a for P o w e l l L a k e T a b l e A-4 Nutrient, c a r b o n , p H , a n d alkalinity d a t a for P o w e l l L a k e T a b l e A-5 P h o s p h o r u s d a t a for stations o t h e r t h a n t h e S o u t h b a s i n in P o w e l l L a k e T a b l e A-6 Sulphur a n d t r a c e m e t a l d a t a for P o w e l l L a k e T a b l e A-7. P h y s i c a l a n d m a j o r i o n d a t a for S a k i n a w L a k e T a b l e A-8 Nutrient, c a r b o n , p H a n d alkalinity d a t a for S a k i n a w L a k e T a b l e A-9 Sulphur a n d t r a c e m e t a l d a t a for S a k i n a w L a k e  25 28 63 ICG 117 160 163 165 166 167 168 169 170 171  vii LIST OF FIGURES Fig. Fig. Fig. Fig.  2-1 2-2.. 2-3 2-4  Fig. 2-5 Fig. 2-6 Fig. 2-7 Fig. 2-8 Fig. 2-9 Fig. 2-10 Fig. 2-11 Fig. 2-12 Fig. 2-13 Fig. 2-14 Fig. 2-15 Fig. 2-16 Fig. 2-17 Fig. 3-1 Fig. 3-2 Fig. 3-3 Fig. 3-4 Fig. 3-5 Fig. 3-6 Fig. 3-7 Fig. 3-8 Fig. 3-9 Fig. 3-10 Fig. 3-11 Fig. 4-1 Fig. 4-2 Fig. 4-3 Fig. 4-4 Fig. 4-5 Fig. 4-6 Fig. 4-7 Fig. 4-8 Fig. 4-9 Fig. 4-10 Fig. 4-11 Fig. 4-12  L o c a t i o n m a p for P o w e l l a n d S a k i n a w Lakes M a p o f P o w e l l L a k e (after M a t h e w s 1962) M a p o f S a k i n a w L a k e (after N o r t h c o t e a n d J o h n s o n 1964) D i s s o l v e d o x y g e n a n d s u l p h i d e c o n c e n t r a t i o n s in P o w e l l L a k e . The horizontal line a t 150 m represents t h e o x i c / a n o x i c i n t e r f a c e T e m p e r a t u r e , c h l o r i d e a n d density profiles in P o w e l l L a k e for April 1984 D i s s o l v e d o x y g e n a n d s u l p h i d e c o n c e n t r a t i o n s in S a k i n a w L a k e . The horizontal line a t 30 m represents t h e o x i c / a n o x i c i n t e r f a c e T e m p e r a t u r e , chlorinity, a n d density profiles in S a k i n a w L a k e for July 1985 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of s o d i u m in P o w e l l L a k e O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s o f p o t a s s i u m in P o w e l l L a k e O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of m a g n e s i u m in P o w e l l Lake O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s o f c a l c i u m in P o w e l l L a k e O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s o f b o r a t e in P o w e l l L a k e O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s o f strontium in P o w e l l L a k e O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of s o d i u m in S a k i n a w L a k e O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of p o t a s s i u m in S a k i n a w L a k e O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of m a g n e s i u m in S a k i n a w L a k e O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of c a l c i u m in S a k i n a w L a k e The r e d o x c y c l e of n i t r o g e n (from Brock 1979) D i s s o l v e d a n d p a r t i c u l a t e o r g a n i c c a r b o n in P o w e l l L a k e D i s s o l v e d a n d p a r t i c u l a t e o r g a n i c c a r b o n in S a k i n a w L a k e P a r t i c u l a t e o r g a n i c c a r b o n a n d n i t r o g e n in Powell Lake P a r t i c u l a t e o r g a n i c c a r b o n a n d n i t r o g e n in S a k i n a w L a k e Nitrate a n d a m m o n i a in P o w e l l L a k e (nitrite u n d e t e c t a b l e ) Nitrate, nitrite a n d a m m o n i a in S a k i n a w Lake D i s s o l v e d silicon in P o w e l l L a k e D i s s o l v e d silicon in S a k i n a w Lake Alkalinity a n d p H in P o w e l l Lake Alkalinity a n d p H in S a k i n a w Lake A simplified r e p r e s e n t a t i o n of t h e p h o s p h o r u s c y c l e in a q u a t i c e n v i r o n m e n t s (from F e n c h e l a n d B l a c k b u r n 1979) M a p of P o w e l l L a k e s h o w i n g six stations s a m p l e d for p h o s p h o r u s analysis S o l u b l e r e a c t i v e p h o s p h o r u s in P o w e l l L a k e . SRP is u n d e t e c t a b l e a b o v e 175 m S o l u b l e r e a c t i v e p h o s p h o r u s in S a k i n a w Lake. U p p e r s c a l e is for t h e o x i c w a t e r c o l u m n ( t o p 30 m). B o t t o m s c a l e is for t h e a n o x i c w a t e r c o l u m n ( b e l o w 30 m) S o l u b l e r e a c t i v e , p a r t i c u l a t e a n d t o t a l p h o s p h o r u s in P o w e l l L a k e S o l u b l e r e a c t i v e , p a r t i c u l a t e a n d t o t a l p h o s p h o r u s in S a k i n a w L a k e P e r c e n t a g e of t o t a l p h o s p h o r u s consisting of s o l u b l e r e a c t i v e a n d p a r t i c u l a t e P in P o w e l l Lake. Total P w a s u n d e t e c t a b l e a b o v e 140 m P e r c e n t a g e o f t o t a l p h o s p h o r u s consisting of soluble r e a c t i v e a n d p a r t i c u l a t e P in S a k i n a w Lake.... M o l a r ratios of p a r t i c u l a t e o r g a n i c c a r b o n a n d n i t r o g e n t o p h o s p h o r u s in P o w e l l L a k e M o l a r ratios of p a r t i c u l a t e o r g a n i c c a r b o n a n d n i t r o g e n t o p h o s p h o r u s in S a k i n a w L a k e M o l a r n i t r o g e n t o p h o s p h o r u s ratios in P o w e l l Lake. Soluble r e a c t i v e p h o s p h o r u s is u n d e t e c t a b l e in t h e u p p e r 175 m , resulting in infinite d i s s o l v e d N:P ratios M o l a r n i t r o g e n t o p h o s p h o r u s ratios in S a k i n a w L a k e  6 7 9 14 14 15 15 16 16 17 17 18 18 19 19 20 20 34 43 43 44 44 45 45 46 46 47 47 66 74 78 78 79 79 80 80 81 81 82 82  viii  Fig. 4-13 Total p h o s p h o r u s in various basins a n d a t t h e h e a d of P o w e l l L a k e ( s e e Fig. 4-2 for l o c a t i o n s ) . The East b a s i n c o n t a i n s relict s e a w a t e r Fig. 4-14 P a r t i c u l a t e p h o s p h o r u s in various basins a n d a t t h e h e a d o f P o w e l l L a k e (see Fig. 4-2 for l o c a t i o n s ) . The East b a s i n c o n t a i n s relict s e a w a t e r Fig. 4-15 S a t u r a t i o n s t a t e of p h o s p h o r u s minerals a n d c a l c i t e in P o w e l l L a k e . . L o g (Ksp*IAP) > 0 = s u p e r s a t u r a t e d . < 0 = u n d e r s a t u r a t e d 'Fig. 4-16 S a t u r a t i o n s t a t e of p h o s p h o r u s minerals a n d c a l c i t e in S a k i n a w L a k e . L o g (Ksp*IAP) > 0 = s u p e r s a t u r a t e d , < 0 = u n d e r s a t u r a t e d Fig. 5-1 The sulphur c y c l e in a l a k e , with e m p h a s i s o n t h e m i c r o b i o l o g i c a l p r o c e s s e s M S = m e t a l s u l p h i d e s , (from W e t z e l 1983) Fig. 5-2 G e n e r a l Eh-pH e n v i r o n m e n t a l limits of: 1) c h e m o s y n t h e t i c (colourless) sulphur-oxidizing b a c t e r i a ; 2) p h o t o s y n t h e t i c p u r p l e b a c t e r i a ; 3) s u l p h a t e - r e d u c i n g b a c t e r i a ; a n d 4) g r e e n sulphur b a c t e r i a (from W e t z e l 1983) Fig. 5-3  Fig. Fig. Fig. Fig.  5-4 5-5 5-6 5-7  Fig. 5-8 Fig. 5-9 Fig. 5-10 Fig. 5-11 Fig. 5-12 Fig. 5-13 Fig. 5-14 Fig. 5-15 Fig. 5-16 Fig. 5-17 Fig. 5-18 Fig. 5-19 Fig. 5-20  Eh-pS " d i a g r a m for iron minerals a t p H = 7.37, l o g P o2= -2.40, T = 25°C. P = 1 a t m . M e a s u r e m e n t s of n a t u r a l s u l p h i d i c m a r i n e s e d i m e n t s fall n e a r t h e d a s h e d line.(from Berner 1971) Possible r e a c t i o n p a t h w a y s of pyrite f o r m a t i o n (from Raiswell 1982) D i s s o l v e d sulphur s p e c i e s in P o w e l l L a k e D i s s o l v e d sulphur s p e c i e s in S a k i n a w L a k e R a t i o o f t o t a l d i s s o l v e d s u l p h i d e (S(-2)) t o d i s s o l v e d z e r o v a l e n t p o l y s u l p h i d e (S(0)) in P o w e l l L a k e Ratio o f t o t a l d i s s o l v e d s u l p h i d e (S(-2)) t o d i s s o l v e d z e r o v a l e n t p o l y s u l p h i d e (S(0)) in S a k i n a w L a k e Dissolved iron a n d m a n g a n e s e in P o w e l l L a k e Dissolved iron a n d m a n g a n e s e in S a k i n a w L a k e P e r c e n t a g e s o f f r e e a n d c o m p l e x e d d i s s o l v e d iron in P o w e l l L a k e . All o t h e r c o m p l e x e s w e r e < 1% of t h e t o t a l d i s s o l v e d iron P e r c e n t a g e s of f r e e a n d c o m p l e x e d d i s s o l v e d iron in S a k i n a w L a k e . All o t h e r c o m p l e x e s w e r e < 1% of t h e t o t a l d i s s o l v e d iron P e r c e n t a g e s o f f r e e a n d c o m p l e x e d d i s s o l v e d m a n g a n e s e in P o w e l l L a k e . All o t h e r c o m p l e x e s w e r e < 1% o f t h e t o t a l d i s s o l v e d m a n g a n e s e P e r c e n t a g e s of f r e e a n d c o m p l e x e d d i s s o l v e d m a n g a n e s e in S a k i n a w L a k e . All o t h e r c o m p l e x e s w e r e < 1% of t h e t o t a l d i s s o l v e d m a n g a n e s e P a r t i c u l a t e sulphur in P o w e l l L a k e P a r t i c u l a t e sulphur in S a k i n a w L a k e S a t u r a t i o n state of iron sulphides in P o w e l l L a k e L o g (Ksp*IAP) > 0 = s u p e r s a t u r a t e d , < 0 = u n d e r s a t u r a t e d S a t u r a t i o n s t a t e of iron sulphides in S a k i n a w L a k e . L o g (Ksp*IAP) > 0 = s u p e r s a t u r a t e d , < 0 = u n d e r s a t u r a t e d S a t u r a t i o n s t a t e o f m a n g a n e s e sulphides in P o w e l l L a k e . L o g (Ksp*IAP) > 0 = s u p e r s a t u r a t e d , < 0 = u n d e r s a t u r a t e d S a t u r a t i o n s t a t e of m a n g a n e s e sulphides in S a k i n a w L a k e . L o g flVHAP) > 0 = s u p e r s a t u r a t e d , < 0 = u n d e r s a t u r a t e d 2  83 83 84 84 106  109  C  Ill 112 125 125 126 126 127 127 128 128 129 129 130 130 131 131 132 132  ACKNOWLEDGEMENTS This p r o j e c t w a s primarily f u n d e d b y a n N S E R C o p e r a t i n g g r a n t t o m y supervisor T o m P e d e r s e n . A d d i t i o n a l f u n d i n g w a s a l s o kindly p r o v i d e d b y M a c M i l l a n - B l o e d e l . P o w e l l River Division a n d b y t h e Bank o f Perry ( p a r e n t a l division). I w o u l d like t o t h a n k Tom P e d e r s e n f o r his input a n d a d v i c e d u r i n g t h e w r i t e u p o f this thesis, n o m a t t e r h o w little I w a n t e d t o h e a r it. I a l s o t h a n k L a w r e n c e L o w e , Bill Barnes a n d G e o r g e S p i e g e l m a n for ' sitting o n m y c o m m i t t e e t h r o u g h o u t t h e e x t e n d e d l e n g t h of this r e s e a r c h , a n d not falling a s l e e p o n c e . I e s p e c i a l l y t h a n k S t e v e C a l v e r t n o t just for sitting o n m y c o m m i t t e e , but also for b e i n g t h e k e e p e r of all k n o w l e d g e , a v e r i t a b l e walking-talking e n c y c l o p e d i a of s c i e n c e . ( A s t a t i n e ? A lovely element...) This thesis i n v o l v e d a g r e a t d e a l of f i e l d w o r k , a n d f e w t e c h n i c i a n s m a n a g e d t o e s c a p e m y t e n u r e a t U B C without h a v i n g t o d o t i m e a t o n e or b o t h lakes. M u r r a y Storm a c c o m p a n i e d m e o n m y first trips t o P o w e l l L a k e , R a m a n d H u g h M a c L e a n h e l p e d m e o u t o n s e v e r a l f i e l d s e a s o n s a n d H e i n z H e c k l s h a r e d his "wall of f l a m e " c o o k i n g t e c h n i q u e o n v a r i o u s o c c a s i o n s d u r i n g t h e s e v e r a l w e e k s h e s p e n t a t b o t h lakes. Their ingenuity a n d brute strength ( o o o o h h h those nitrogen cylinders) w a s m u c h a p p r e c i a t e d . B a c k at t h e l a b ( a n d briefly in t h e field) M a u r e e n S o o n w a s a g r e a t h e l p t o m e w i t h r e s p e c t t o t h e pilfering o f e q u i p m e n t , sharing of t e c h n i q u e s a n d g e n e r a l gossip.  N u m e r o u s c o l l e a g u e s h a v e h e l p e d m e stay s a n e d u r i n g t h e torture. The other late-nighters - Terri S u t h e r l a n d , D o n W e b b a n d A n n a M e t a x a s - h a v e h e l p e d t o k e e p m e a w a k e , a n d m o r e i m p o r t a n t l y , d i s t r a c t e d f r o m m y work. Lastly, I t h a n k m y off-world friends, p a r t i c u l a r l y Lynn G i r a u d , for p r o v i d i n g s u p p o r t , m o n e y a n d disbelief for t h e post millenium.  CHAPTER 1 INTRODUCTION  Elemental  cycling  under  suboxic  to anoxic  conditions  has  received  c o n s i d e r a b l e a t t e n t i o n in t h e p a s t f e w years. E x a m p l e s o f a n o x i c e n v i r o n m e n t s i n c l u d e h y d r o t h e r m a l systems, s o m e e s t u a r i n e , m a r i n e a n d l a c u s t r i n e s e d i m e n t s , a n d w a t e r c o l u m n s w i t h r e s t r i c t e d c i r c u l a t i o n . T h e possibility t h a t w i d e s p r e a d a n o x i a m a y h a v e o c c u r r e d in o c e a n s o f t h e g e o l o g i c p a s t n e c e s s i t a t e s a b e t t e r u n d e r s t a n d i n g o f g e o c h e m i c a l processes under such conditions. In a q u e o u s e n v i r o n m e n t s , t h e r e is a n e t c o n s u m p t i o n o f o x y g e n a t all d e p t h s b e l o w t h e c o m p e n s a t i o n d e p t h (the d e p t h a t w h i c h t h e c a r b o n f i x e d b y photosynthesis e q u a l s t h a t respired) d u e t o t h e f i x a t i o n o f c a r b o n ( R i c h a r d s 1965). A n o x i c c o n d i t i o n s arise w h e n t h e c o n s u m p t i o n o f o x y g e n b y m i c r o b i a l b r e a k d o w n o f o r g a n i c  matter  e x c e e d s t h e q u a n t i t y o f o x y g e n s u p p l i e d b y diffusion a n d w a t e r c i r c u l a t i o n , resulting in a redox discontinuity b e t w e e n discontinuity depending  occurs  a variable  o x y g e n a t e d a n d a n o x i c e n v i r o n m e n t s . This r e d o x distance  on the oxygen d e m a n d  below  the sediment-water  of t h e d e p o s i t e d o r g a n i c  matter  interface, a n d the  s e d i m e n t a t i o n rate. H o w e v e r , t h e r e d o x c l i n e m a y also b e f o u n d a t t h e s e d i m e n t - w a t e r i n t e r f a c e o r w i t h i n t h e w a t e r c o l u m n ( e u x i n i c ) , e i t h e r w i t h i n o r b e l o w t h e z o n e of p h o t o s y n t h e t i c activity. M a r i n e basins b e c o m e a n o x i c d u e t o r e s t r i c t e d c i r c u l a t i o n w h e r e s h a l l o w sills a n d s t e e p p y c n o c l i n e s a c t a s barriers t o restrain h o r i z o n t a l a n d v e r t i c a l c i r c u l a t i o n , r e s p e c t i v e l y ( R i c h a r d s 1965). This stratification m a y b e p e r m a n e n t or intermittent, t h e latter s t a t e b e i n g d e p e n d e n t u p o n s e a s o n a l , or a p e r i o d i c flushing e v e n t s .  Various  m a r i n e a n o x i c basins h a v e b e e n s t u d i e d e x t e n s i v e l y , i n c l u d i n g t e m p o r a l l y stratified basins s u c h a s S a a n i c h Inlet, a fjord o n t h e c o a s t o f British C o l u m b i a  ( E m e r s o n et a l .  1979), o r p e r m a n e n t l y stratified basins s u c h a s t h e B l a c k S e a (Luther e t a l . 1990c) a n d F r a m v a r e n Fjord in N o r w a y ( J a c o b s e t a l . 1985). O t h e r p e r m a n e n t l y a n o x i c m a r i n e basins t h a t h a v e b e e n e x a m i n e d i n c l u d e Nitinat L a k e , a British C o l u m b i a n fjord (Richards e t a l . 1965), t h e C a r i a c o T r e n c h , a n o c e a n i c basin north o f V e n e z u e l a ( J a c o b s et a l . 1987), a n d t h e brine-filled B a n n o c k a n d Tyro basins in t h e Eastern M e d i t e r r a n e a n S e a (Luther et a l . 1990b).  A n u m b e r of lakes also e x p e r i e n c e limited c i r c u l a t i o n a n d a r e classified as m e r o m i c t i c . E x a m p l e s o f this t y p e o f a n o x i c b a s i n i n c l u d e t h e S o l a r L a k e , Sinai ( J o r g e n s e n a n d C o h e n 1977), M a h o n e y L a k e (interior British C o l u m b i a ) ( N o r t h c o t e a n d Hall 1983), L a k e V a n d a , A n t a r c t i c a ( G r e e n et a l . 1986), a n d Big S o d a L a k e , N e v a d a ( O r e m l a n d et a l . 1988). The t w o lakes e x a m i n e d in this study, P o w e l l a n d S a k i n a w , a r e f o r m e r fjords w h i c h w e r e s e p a r a t e d f r o m t h e o c e a n a p p r o x i m a t e l y 11000 years a g o , d u e t o e m e r g e n t sills c a u s e d b y p o s t - g l a c i a l isostatic uplift, w h i c h t r a p p e d now-relict s e a w a t e r . T h e y a r e e x a m p l e s of e c t o g e n i c m e r o m i x i s , w h e r e e x t e r n a l e v e n t s inject e i t h e r fresh w a t e r into a saline l a k e , as in this c a s e , or salt w a t e r into a f r e s h w a t e r l a k e ( W e t z e l 1983). A n u m b e r of c h e m i c a l processes a n d products are d e p e n d e n t u p o n the redox p o t e n t i a l . P o w e l l a n d S a k i n a w L a k e s r e p r e s e n t c o n v e n i e n t n a t u r a l l a b o r a t o r i e s for studies o f d i a g e n e s i s b e c a u s e t h e w a t e r c o l u m n structure of b o t h lakes is similar t o t h a t o b s e r v e d in most n e a r s h o r e m a r i n e s e d i m e n t s w h e r e a n o x i d i z e d s u r f a c e l a y e r g r a d e s into u n d e r l y i n g r e d u c i n g d e p o s i t s . T h e r e f o r e , a c h e m i c a l a n a l o g y c a n b e  drawn  b e t w e e n t h e r e d o x c o n d i t i o n s in t h e w a t e r c o l u m n s o f t h e l a k e s a n d in c o a s t a l s e d i m e n t s . A l s o , c h e m i c a l g r a d i e n t s in t h e w a t e r c o l u m n c a n b e r e l a t i v e l y e a s i l y s t u d i e d a n d d e f i n e d b y d e t a i l e d w a t e r s a m p l i n g , a n d t h e w a t e r is p e r m a n e n t l y stratified a n d thus s t a b l e . The m o s t i m p o r t a n t p r o c e s s t h a t o c c u r s in a n o x i c b a s i n s is t h a t of s u l p h a t e r e d u c t i o n w i t h t h e resultant p r o d u c t i o n of h y d r o g e n s u l p h i d e . Sulphur is a n i m p o r t a n t , o f t e n m a j o r c o n s t i t u e n t in o r g a n i c - r i c h s e d i m e n t s w h e r e it o c c u r s in b o t h o r g a n i c a n d i n o r g a n i c p h a s e s , e s p e c i a l l y a s s u l p h i d e minerals. Pyrite (FeS2) is a c o m m o n a u t h i g e n i c m i n e r a l o f r e c e n t m a r i n e a n d l a c u s t r i n e s e d i m e n t s , a n d o f s e d i m e n t a r y r o c k s . The f o r m a t i o n a n d o c c u r r e n c e o f pyrite is o f interest t o t h e c o a l industry (Altschuler et a l . 1983), t o g e o c h e m i s t s s t u d y i n g t r a c e m e t a l i n c o r p o r a t i o n into iron s u l p h i d e minerals (Raiswell a n d  Plant  1980), t o e n v i r o n m e n t a l  scientists e x a m i n i n g h e a v y  e n r i c h m e n t s in s e d i m e n t s ( F e r g u s o n e t a l . 1983) a n d a s o n e o f s e v e r a l  metal  potential  i n d i c a t o r s of a n c i e n t e u x i n i c e n v i r o n m e n t s ( L e v e n t h a l 1983). There a r e t w o p r o p o s e d p a t h w a y s o f pyrite f o r m a t i o n : 1) t h e r e a c t i o n of iron m o n o s u l p h i d e (FeS) with e l e m e n t a l sulphur (Berner 1967a); a n d  2) t h e d i r e c t r e a c t i o n of ferrous iron w i t h e l e m e n t a l sulphur or p o l y s u l p h i d e ions in t h e p r e s e n c e o f d i s s o l v e d s u l p h i d e s p e c i e s ( H o w a r t h 1979). Studies o f e a r l y d i a g e n e s i s in m a r i n e a n d lacustrine s e d i m e n t s a r e o f t e n not c o n c l u s i v e a s t o w h i c h o f t h e t w o p a t h w a y s is r e s p o n s i b l e f o r t h e o c c u r r e n c e o f pyrite in a n y s p e c i f i c e n v i r o n m e n t . This is d u e t o t h e d i f f i c u l t y i n c u r r e d in s t u d y i n g c o m p l e x , i n h o m o g e n e o u s s e d i m e n t s , w h e r e r e d u c i n g m i c r o e n v i r o n m e n t s f r e q u e n t l y o c c u r in o x i d i z e d horizons. In a d d i t i o n , e l e m e n t a l sulphur, iron m o n o s u l p h i d e s , pyrite, d i s s o l v e d p o l y s u l p h i d e s , ferrous iron a n d h y d r o g e n s u l p h i d e s p e c i e s c a n c o e x i s t in t h e  same  s e d i m e n t - p o r e w a t e r systems, t h e r e b y o b s c u r i n g t h e m e c h a n i s m ( s ) b y w h i c h pyrite is p r o d u c e d ( L o r d a n d C h u r c h 1983). The p r i m a r y o b j e c t i v e o f this thesis is t o e x a m i n e pyrite f o r m a t i o n in a less c o m p l i c a t e d s y s t e m , s u c h a s t h e a n o x i c b o t t o m w a t e r s of P o w e l l a n d S a k i n a w L a k e s . In a n o x i c w a t e r c o l u m n s , d i a g e n e t i c p r o c e s s e s w h i c h n o r m a l l y t a k e p l a c e o v e r a f e w c e n t i m e t r e s in s e d i m e n t s o c c u r o v e r tens t o h u n d r e d s of metres. Thus, P o w e l l a n d S a k i n a w Lakes a r e i d e a l p l a c e s in w h i c h t o study b o t h r e d o x chemistry in g e n e r a l a n d sulphur c h e m i s t r y in particular. This thesis consists of essentially four s e p a r a t e , b u t l i n k e d parts, w h i c h d e t a i l t h e c h e m i s t r y o f t h e s e t w o lakes. The next c h a p t e r d e s c r i b e s t h e l a k e s in g e n e r a l w i t h a short r e v i e w o f t h e i r p h y s i c a l s e t t i n g a n d their s t r a t i f i c a t i o n . The p h y s i c a l s t r u c t u r e (density, salinity, t e m p e r a t u r e )  is d e s c r i b e d a n d t h e  major ion distributions are  e x a m i n e d t o s e e h o w m u c h this relict s e a w a t e r r e s e m b l e s t h a t of t h e p r e s e n t - d a y . C h a p t e r 3 e x a m i n e s w h a t h a s h a p p e n e d t o t h e w a t e r s i n c e it has b e e n c u t off f r o m t h e s e a , i.e. b i o c h e m i c a l r e a c t i v i t y w h i c h h a s a l t e r e d t h e c h e m i s t r y of t h e w a t e r c o l u m n . Alkalinity, p H , nutrients (nitrate, nitrite, a m m o n i u m , d i s s o l v e d silicon), a n d d i s s o l v e d a n d p a r t i c u l a t e o r g a n i c c a r b o n a n d n i t r o g e n a r e all d i s c u s s e d . This c h a p t e r outlines a n interesting f i n d i n g w h i c h is t h e f o c u s of C h a p t e r 4. The e x t r e m e l y l o w levels of p h o s p h a t e o b s e r v e d in t h e b o t t o m w a t e r s of P o w e l l a r e i n c o m p a t i b l e w i t h t h e v e r y h i g h a m o u n t s of a m m o n i u m present. V a r i o u s p h o s p h o r u s r e m o v a l p r o c e s s e s a r e r e v i e w e d a n d r e l a t e d t o w h a t m a y b e p r e v e n t i n g p h o s p h o r u s b u i l d u p in P o w e l l L a k e b o t t o m w a t e r s . The final s e c t i o n ( C h a p t e r 5) d e s c r i b e s t h e sulphur chemistry of b o t h l a k e s , w i t h e m p h a s i s o n iron s u l p h i d e f o r m a t i o n , t h e f o c u s o f this thesis. D i s s o l v e d s u l p h u r s p e c i e s o f v a r i o u s o x i d a t i o n states (sulphate, sulphite, thiosulphate, p o l y t h i o n a t e , e l e m e n t a l sulphur, p o l y s u l p h i d e , h y d r o g e n s u l p h i d e ) a n d d i s s o l v e d iron a n d m a n g a n e s e a r e e x a m i n e d .  4 The o c c u r r e n c e o f  iron m o n o s u l p h i d e a n d pyrite in t h e w a t e r c o l u m n s of P o w e l l a n d  S a k i n a w a r e t h e n d i s c u s s e d in t e r m s of t h e t w o p r o p o s e d m e c h a n i s m s of pyrite f o r m a t i o n a n d c o m p a r e d t o m o d e l - d e r i v e d solubility c a l c u l a t i o n s . All a n a l y t i c a l d a t a d i s c u s s e d in this thesis a r e listed in A p p e n d i x 2.  CHAPTER 2 PHYSIOGRAPHY, PHYSICAL AND CHEMICAL LIMNOLOGY  2.1 Introduction  The t w o l a k e s c h o s e n f o r this s t u d y a r e similar in t h e i r g e o l o g i c history a n d p h y s i c a l structure. B o t h c o n t a i n a t least o n e b a s i n w i t h a n o x i c relict s e a w a t e r a t d e p t h . In this c h a p t e r , t h e p h y s i c a l a n d c h e m i c a l l i m n o l o g y o f b o t h lakes a r e c o m p a r e d a n d c o n t r a s t e d in s o m e d e t a i l . Powell Lake Powell  is a d e e p  fjord-lake s i t u a t e d a p p r o x i m a t e l y  110 k m n o r t h w e s t of  V a n c o u v e r o n t h e southwest c o a s t of British C o l u m b i a (Fig. 2-1). It is, o n a v e r a g e , a b o u t t w o k m w i d e a n d 47 k m l o n g a n d is d i v i d e d into six d e e p , f l a t - b o t t o m e d basins b y a series o f sills o f differing d e p t h s (Fig. 2-2) ( M a t h e w s 1962). The lake w a s originally a m a r i n e fjord; w i t h t h e m e l t i n g o f t h e C o r d i l l e r a n I c e S h e e t a t t h e e n d o f t h e last i c e a g e a n d c o n s e q u e n t r e b o u n d o f t h e c o n t i n e n t , t h e s h a l l o w b e d r o c k sill a t t h e m o u t h of t h e fjord rose 4 6 m a b o v e s e a l e v e l , isolating t h e b a s i n f r o m t h e Strait o f G e o r g i a . L o c a l p o s t g l a c i a l isostatic history suggests t h a t s e a w a t e r w a s t r a p p e d in t h e l a k e b e t w e e n 12,500 a n d 10,500 y e a r s a g o ( M a t h e w s e t a l . 1970). A d a m w a s built o n t h e sill in 1924, raising t h e h i g h w a t e r m a r k t o 56 m a b o v e m e a n s e a level. Freshwater flows into P o w e l l L a k e , largely v i a P o w e l l River a t t h e h e a d , a t a rate of a b o u t 3 x 10 m » y r ' \ This is t h e o u t f l o w a t t h e d a m , a v e r a g e d o v e r t h e past 4 0 y e a r s 9  3  ( S a n d e r s o n e t a l . 1986). A f e w small streams a l s o f l o w into t h e l a k e , b u t their input o f f r e s h w a t e r is small. T h e h e a d w a t e r s o f t h e l a k e a r e u n d e r l a i n b y l a r g e l y u n f r a c t u r e d h o r n b l e n d e g r a n o d i o r i t e ( M a t h e w s 1962). B e c a u s e t h e s u r f a c e of t h e l a k e is m o r e t h a n 50 m a b o v e s e a l e v e l a n d s e p a r a t e d f r o m t h e Strait o f G e o r g i a b y b e d r o c k , t h e r e c a n n o t b e a n y s e e p a g e o f s e a w a t e r into t h e l a k e . Turbidity c u r r e n t s , w h i c h c o u l d b e c a u s e d b y m u d d y streams o r b y mudslides f r o m d e l t a s g r o w i n g a t t h e h e a d o f t h e l a k e , a p p e a r t o b e t r a p p e d in t h e u p p e r b a s i n , so t h a t in t h e lower basins t h e r e is unlikely t o b e invasion o f t u r b i d w a t e r a t d e p t h . O f t h e six basins in t h e l a k e , only t h e south a n d east (the t w o closest t o t h e Strait of G e o r g i a ) still c o n t a i n relict s e a w a t e r a n d a r e thus m e r o m i c t i c , with p e r m a n e n t l y a n o x i c m o n i m o l i m n i o n s . The s o u t h b a s i n is t h e d e e p e s t , w i t h a m a x i m u m d e p t h o f 358 m. It is  Fig. 2-2 M a p of Powell Lake (after Mathews 1962)  a p p r o x i m a t e l y 15 k m l o n g a n d 2 k m w i d e , a n d is f l o o r e d b y s e d i m e n t s c o n s i s t i n g of o r g a n i c - a n d pyrite-rich g e l a t i n o u s o o z e (Barnes a n d Barnes 1981). The small a m o u n t of c l a s t i c detritus p r e s e n t reflects t h e v e r y l o w d i r e c t runoff into this b a s i n . The s o u t h b a s i n h a s f r e s h e n e d less t h a n t h e e a s t a n d w a s t h e r e f o r e c h o s e n f o r d e t a i l e d study. O n e station (Fig. 2-2) w a s s e l e c t e d in t h e c e n t r e t o b e r e p r e s e n t a t i v e of t h e b a s i n as a w h o l e . Sakinaw Lake S a k i n a w L a k e a l s o a p p e a r s t o h a v e o n c e b e e n a fjord, a n d is n o w s e p a r a t e d f r o m t h e Strait o f G e o r g i a b y a n e m e r g e d sill d u e t o p o s t - g l a c i a l isostatic r e b o u n d ( N o r t h c o t e a n d J o h n s o n 1964). It is l o c a t e d n e a r t h e n o r t h e r n e n d o f t h e S e c h e l t P e n i n s u l a , a p p r o x i m a t e l y 9 0 k m northwest o f V a n c o u v e r (Fig. 2-1), a n d it is a b o u t 9 k m l o n g a n d 0.7 k m w i d e (Fig. 2-3). It lies in a b a s i n c o m p o s e d primarily o f q u a r t z dioritic a n d g r a n o d i o r i t i c rocks, w h i c h a r e b r o k e n a b o u t h a l f w a y a l o n g its l e n g t h b y a n a r r o w b a n d of a l t e r e d v o l c a n i c r o c k s ( B a c o n 1957, c i t e d in N o r t h c o t e a n d J o h n s o n 1964). The l a k e consists o f t w o basins: o n e relatively s h a l l o w (49 m ) a n d w h o l l y f r e s h , a n d t h e o t h e r a d e e p (140 m ) a n d m e r o m i c t i c b a s i n t h a t still c o n t a i n s relict s e a w a t e r . A n u m b e r o f small streams f l o w into S a k i n a w , a l t h o u g h t h e v o l u m e o f t h e f r e s h w a t e r i n p u t , i n c l u d i n g t h a t w h i c h drains n e a r b y R u b y L a k e , is small. S a k i n a w L a k e lies a p p r o x i m a t e l y 2 m a b o v e m e a n s e a l e v e l . The s m a l l river exiting t h e l a k e h a s b e e n partially o r c o m p l e t e l y b l o c k e d s i n c e t h e e a r l y 1900's a n d a p e r m a n e n t d a m a n d f i s h w a y w e r e installed in 1952. O u t f l o w h a s b e e n r e g u l a t e d s i n c e t h a t t i m e . Prior t o t h e c o n s t r u c t i o n of t h e weir, it is possible t h a t incursion of s e a w a t e r f r o m t h e Strait o f G e o r g i a into t h e l a k e m a y h a v e o c c u r r e d u n d e r c o n d i t i o n s o f s t r o n g onshore w i n d s a n d high tides. Visual i n s p e c t i o n of t h e outlet suggests that  such  incursions m a y a l s o h a v e o c c u r r e d s i n c e t h e d a m w a s built. Therefore, t h e s e a w a t e r in S a k i n a w m a y b e o f m u c h m o r e r e c e n t v i n t a g e t h a n t h a t in P o w e l l , w h i c h c a n n o t h a v e r e c e i v e d a n y s e a w a t e r s i n c e t h e sill e m e r g e d . A g a i n , o n e s t a t i o n in t h e c e n t r e o f t h e a n o x i c b a s i n w a s s e l e c t e d for study (Fig. 2-3).  Creek  |  k m  Fig. 2-3 M a p of S a k i n a w L a k e (after N o r t h c o t e a n d J o h n s o n 1964)  2.2 Materials a n d Methods  Water Collection All w a t e r s a m p l e s w e r e c o l l e c t e d f r o m a s m a l l , 5.5 m l a u n c h e q u i p p e d with a g a s - p o w e r e d w i n c h using e i t h e r 1.8 or 9 L Niskin bottles m o u n t e d o n s t a n d a r d iron or stainless s t e e l h y d r o w i r e . S a m p l e s w e r e c o l l e c t e d in p o l y e t h y l e n e o r p o l y p r o p y l e n e c o n t a i n e r s w h i c h h a d b e e n w a s h e d in 1 0 % HCI a n d rinsed in distilled, d e i o n i z e d w a t e r (DDW). They w e r e t h e n s t o r e d a t 6 ° C until analysis.  Analytical Methods Physical Parameters D i s s o l v e d o x y g e n w a s d e t e r m i n e d b y Winkler titration w i t h a d e t e c t i o n limit of 2 p.M a n d a p r e c i s i o n o f 0 . 5 % (1 a , rsd). T e m p e r a t u r e w a s m e a s u r e d u s i n g r e v e r s i n g t h e r m o m e t e r s (± 0.01°C). Chlorinity w a s d e t e r m i n e d b y s t a n d a r d K n u d s e n titration with a G i l m o n t m i c r o b u r e t t e , after e x p o s i n g t h e s a m p l e t o air a n d a l l o w i n g a n y H2S present t o either e x s o l v e or oxidize. The d e t e c t i o n limit w a s 0.18 m M w i t h a p r e c i s i o n o f 0 . 5 % (1 a , rsd). Density w a s c a l c u l a t e d f r o m t h e t e m p e r a t u r e a n d chlorinity d a t a . S u l p h i d e w a s d e t e r m i n e d a s d e s c r i b e d in C h a p t e r 5. Major Ions Sodium, Magnesium a n d Potassium Na  +  , K , +  and  Mg  2 +  were  determined  by  flame  atomic  absorption  s p e c t r o p h o t o m e t r y (AAS). S a m p l e s w e r e a c i d i f i e d with H N 0 t o a p H < 3 t o prevent 3  p r e c i p i t a t i o n o f C a C 0 3 , f i l t e r e d t h r o u g h 0.4  Nuclepore membranes a n d then  a s p i r a t e d into t h e f l a m e after a p p r o p r i a t e dilution w i t h D D W t o ensure a n a b s o r b a n c e < 0.4. S u l p h i d i c s a m p l e s w e r e b u b b l e d w i t h N  2  prior t o filtration t o r e m o v e t h e H S. 2  S t a n d a r d s w e r e p r e p a r e d b y diluting IAPSO S t a n d a r d S e a w a t e r w i t h D D W . S o d i u m a n d p o t a s s i u m a r e partially i o n i z e d in t h e a i r - a c e t y l e n e f l a m e , h o w e v e r , t h e p r e s e n c e of o t h e r a l k a l i salts lessens this e f f e c t . A l s o , i o n i z a t i o n s h o u l d h a v e b e e n similar in s a m p l e s a n d s t a n d a r d s . B e c a u s e Si a n d A l d e p r e s s M g a b s o r p t i o n in t h e a i r - a c e t y l e n e f l a m e , a nitrous o x i d e - a c e t y l e n e f l a m e w a s u s e d f o r M g  2 +  analysis. A l l s a m p l e s w e r e a b o v e t h e  d e t e c t i o n limit for t h e s e t h r e e ions a n d t h e p r e c i s i o n w a s 0 . 5 % for K a n d 5 % (1 a , rsd) for +  Na and Mg . +  2 +  Strontium B e c a u s e A A S w a s not sufficiently sensitive, Sr  2+  was measured via  atomic  e m i s s i o n s p e c t r o p h o t o m e t r y (AES). This differs w i t h r e s p e c t t o A A S in t h a t rather t h a n m e a s u r i n g t h e r a d i a t i o n a b s o r b e d d u r i n g a n e l e c t r o n i c transition o f t h e e l e m e n t of interest f r o m t h e g r o u n d s t a t e t o a n e x c i t e d s t a t e , t h e emission of e n e r g y is d e t e c t e d . B e c a u s e t h e r e w a s a salt e f f e c t , s t a n d a r d s w e r e p r e p a r e d in 0 . 5 % N a C I . A f t e r sulphide r e m o v a l ( w h e r e n e c e s s a r y ) , s a m p l e s w e r e filtered t h r o u g h 0.4 u.m N u c l e p o r e filters a n d a n a p p r o p r i a t e a m o u n t o f N a C I a d d e d t o bring its t o t a l c o n c e n t r a t i o n t o 0 . 5 % . S e v e r a l s a m p l e s fell b e l o w t h e d e t e c t i o n limit o f 0.5 u.M. The p r e c i s i o n of t h e a n a l y s e s w a s 5% (1 a , rsd). Calcium W a t e r w a s a c i d i f i e d u p o n c o l l e c t i o n t o a p H < 3 t o p r e v e n t p r e c i p i t a t i o n of CaCC>3, a n y H S p r e s e n t w a s d r i v e n off b y b u b b l i n g w i t h N a n d t h e n t h e s a m p l e w a s 2  2  f i l t e r e d t h r o u g h 0.4 u,m N u c l e p o r e m e m b r a n e s prior t o analysis. L o w p r e c i s i o n w a s e n c o u n t e r e d in A A S analysis a n d so t h e c o n c e n t r a t i o n of C a  2 +  was remeasured by  c o m p l e x o m e t r i c titration w i t h 1,2-di(2-aminoethoxy)ethane-A/,/V,N,/S/'-tetra-acetic  acid  (EGTA) (Tsunogai e t a l . 1968). This t e c h n i q u e is b a s e d o n t h e c h e l a t i o n of p o l y v a l e n t cations with a m i n o polycarboxylic acids (complexones) w h i c h h a v e the characteristic g r o u p i n g - N ( C H C O O H ) . A s p e c i a l c o m p l e x o n e , E G T A , allows a l m o s t s e l e c t i v e titration 2  2  of c a l c i u m in t h e p r e s e n c e of h i g h a m o u n t s o f m a g n e s i u m . The e n d p o i n t is d e t e c t e d b y t h e f o r m a t i o n of a  r e d inner c a l c i u m c o m p l e x with t h e  Schiff b a s e  di-(2-  h y d r o x y p h e n y l - i m i n o ) - e t h a n e ( G H A ) , w h i c h is q u i t e different in c o l o u r f r o m f r e e G H A a n d less s t a b l e t h a n t h e m e t a l - c o m p l e x o n e c o m p l e x . T h e r e f o r e , w h e n t h e titration p r o c e e d s , c a l c i u m ions a r e c o m p l e x e d t o m u c h stronger E G T A c o m p l e x e s a n d a t t h e s t o i c h i o m e t r i c e n d p o i n t t h e r e d c o l o u r of t h e C a - G H A c o m p l e x is r e p l a c e d b y t h e colourless free  indicator. A n  o r g a n i c solvent  (n-butanol)  is a d d e d  to  extract  q u a n t i t a t i v e l y ( a n d h e n c e c o n c e n t r a t e ) t h e real C a - G H A c o m p l e x . While stirring, 5 m M E G T A w a s a d d e d t o 1 mL of s a m p l e plus 0.5 m L of 0.04% G H A a n d 0.5 m L b o r a t e buffer. A f t e r stirring for 3 minutes, 1 mL n-butanol w a s a d d e d a n d t h e n t h e s a m p l e w a s stirred vigorously a n d t i t r a t e d w i t h 5 m M EGTA. S i n c e t h e c a l c i u m - G H A c o m p l e x is fairly u n s t a b l e . It w a s i m p o r t a n t t o c o m p l e t e t h e titration within 15 minutes. I a l s o f o u n d t h a t t h e a m o u n t of E G T A a d d e d t o t h e s a m p l e prior t o t h e 3 minutes of stirring  w a s c r i t i c a l . A l t h o u g h Grasshof (1976) r e c o m m e n d s a d d i n g 9 5 % of t h e E G T A r e q u i r e d t o r e a c h t h e e n d p o i n t , w i t h m y l o w salinity s a m p l e s , I f o u n d t h a t a d d i n g 9 0 % resulted in h i g h e r a c c u r a c y . A l l c o n c e n t r a t i o n s w e r e w e l l a b o v e t h e d e t e c t i o n limit a n d p r e c i s i o n w a s 0 . 5 % (1 a , rsd). S t a n d a r d s w e r e p r e p a r e d b y d i l u t i n g I A P S O  the  Standard  S e a w a t e r with DDW. Borate Borate w a s d e t e r m i n e d colourimetrically after c o m p l e x a t i o n with c u r c u m i n (a s u b s t i t u t e d B-diketone  R C O C H C O R , w h e r e R is t h e C H 3 ( O H ) ( C H 0 ) C H = G H 2  6  radical)  3  ( U p p s t r o m 1968). B o r o n forms t w o different c o m p l e x e s w i t h c u r c u m i n : r u b r o c u r c u m i n a n d r o s o c y a n i n . In this m e t h o d , r o s o c y a n i n is f o r m e d in t h e p r e s e n c e of strong sulphuric a c i d by the reaction b e t w e e n  b o r i c a c i d a n d c u r c u m i n . The e l i m i n a t i o n of w a t e r  ( n e c e s s a r y for c o m p l e x f o r m a t i o n ) is e f f e c t e d b y p r o p i o n i c a c i d a n h y d r i d e w i t h o x a l y l c h l o r i d e . The e x c e s s of p r o t o n a t e d  catalysed  c u r c u m i n is d e s t r o y e d  with  an  a m m o n i u m a c e t a t e buffer. To 0.5 m L s a m p l e , 1 m L g l a c i a l a c e t i c a c i d a n d 3 mL p r o p i o n i c a c i d a n h y d r i d e w e r e a d d e d a n d t h e mixture swirled. In a f u m e h o o d , 0.25 mL o x a l y l c h l o r i d e w a s a d d e d d r o p w i s e , d u e t o t h e v i o l e n t n a t u r e of t h e r e a c t i o n . A f t e r a l l o w i n g t h e s a m p l e t o sit for 15 t o 30 m i n u t e s t o c o o l t o r o o m t e m p e r a t u r e , 3 mL sulphuric-acetic a c i d reagent  a n d 3 mL c u r c u m i n w e r e a d d e d a n d t h e solution  t h o r o u g h l y m i x e d . The r e a c t i o n w a s a l l o w e d t o p r o c e e d for a m i n i m u m of 30 minutes a n d t h e n 2 0 m L of a m m o n i u m a c e t a t e b u f f e r w a s a d d e d . A f t e r c o o l i n g t o r o o m temperature  (about  15  minutes),  the  absorbance  was  measured  on  a  s p e c t r o p h o t o m e t e r a t 545 n m in 1 c m cells. As t h e final solution w a s v e r y sticky, it w a s i m p o r t a n t t o c l e a n t h e cells w e l l w i t h a c e t o n e b e t w e e n s a m p l e s . The d e t e c t i o n limit w a s 1 | i M a n d t h e p r e c i s i o n w a s 3 % (1 a , rsd). As t h e r e w a s n o salt e f f e c t ( U p p s t r o m 1968), s t a n d a r d s w e r e m a d e in D D W with H B 0 . 3  3  2.3 Results  Physical Data The o x i c / a n o x i c i n t e r f a c e in P o w e l l L a k e o c c u r s a t a p p r o x i m a t e l y 150 m d e p t h . S u r f a c e w a t e r s a r e w e l l o x y g e n a t e d t o 2 5 m , b e l o w w h i c h t h e c o n c e n t r a t i o n of 0  2  d e c r e a s e s fairly r a p i d l y until it b e c o m e s u n d e t e c t a b l e a t 150 m (Fig. 2-4). S u l p h i d e is first d e t e c t e d a t 150 m , b u t o c c u r s a t l o w c o n c e n t r a t i o n s until a t 275 m its c o n c e n t r a t i o n rises t o a m a x i m u m o f 3.1 m M in t h e b o t t o m w a t e r . C h l o r i d e is d e t e c t a b l e in t h e s u r f a c e w a t e r s o f P o w e l l L a k e (-1 m M ) , a n d g r a d u a l l y i n c r e a s e s t o 260 m M in t h e b o t t o m w a t e r (Fig. 2-5), a p p r o x i m a t e l y half t h e c o n c e n t r a t i o n o f o p e n o c e a n w a t e r . T h e d e n s i t y profile m i m i c s t h a t o f t h e c h l o r i d e . T e m p e r a t u r e initially d e c r e a s e s w i t h d e p t h , b u t t h e n b e g i n s t o rise a t t h e i n t e r f a c e s u c h t h a t t h e b o t t o m w a t e r a t 9.4°C is w a r m e r t h a n t h e s u r f a c e w a s a t t h e t i m e o f s a m p l i n g (April 1984). In S a k i n a w L a k e , o x y g e n r a p i d l y i n c r e a s e s b e t w e e n t h e s u r f a c e (78 u.M) a n d 10 m d e p t h (97 u.M), b e f o r e d e c l i n i n g sharply t o z e r o v a l u e s a t t h e o x i c / a n o x i c i n t e r f a c e a t 30 m , w h e r e s u l p h i d e is first d e t e c t e d (Fig. 2-6). H S i n c r e a s e s r a p i d l y w i t h d e p t h t o a 2  m a x i m u m o f 5.5 m M in t h e b o t t o m w a t e r . The m a x i m u m c h l o r i d e c o n c e n t r a t i o n , 170 m M (Fig. 2-7), is s o m e w h a t l o w e r t h a n t h a t in P o w e l l , a p p r o x i m a t e l y one-third t h a t o f o p e n o c e a n w a t e r . A g a i n , t h e density profile fairly m i m i c s t h a t o f t h e c h l o r i d e , e x c e p t in t h e u p p e r 10 m w h e r e t h e density w a s significantly l o w e r a t t h e t i m e o f s a m p l i n g ( J u n e , 1985). A t t h a t t i m e , d e n s i t y v a r i a t i o n r e f l e c t e d t h e r a p i d d r o p in t e m p e r a t u r e in t h e t o p 10 m f r o m 19°C a t t h e s u r f a c e t o a p p r o x i m a t e l y 5 ° C . The t e m p e r a t u r e t h e n i n c r e a s e s w i t h d e p t h t o a m a x i m u m o f 9.5°C a t t h e b o t t o m o f t h e lake. Major Ions Figs. 2-8 t o 2-13 s h o w t h e c o n c e n t r a t i o n profiles of t h e six m a j o r i n o r g a n i c ions in P o w e l l L a k e . The D v a l u e s s h o w n a r e t h e m o l e c u l a r diffusivities o f t h e ions a s d e t e r m i n e d e x p e r i m e n t a l l y b y Li a n d G r e g o r y (1974) for salt diffusing into distilled w a t e r , a n d h a v e b e e n a d j u s t e d t o a t e m p e r a t u r e o f 8 ° C . The o b s e r v e d (Obs.) profiles a r e t h o s e a c t u a l l y m e a s u r e d . T h e c a l c u l a t e d ( c a l c . ) c u r v e s a r e d e t e r m i n e d using m e a s u r e d CI* v a l u e s a n d assuming c o n s t a n t relative c o m p o s i t i o n of seawater.  This a s s u m e s t h a t t h e  e l e m e n t in q u e s t i o n is c h e m i c a l l y c o n s e r v a t i v e a n d lost f r o m t h e l a k e v i a u p w a r d diffusion a n d e x p o r t in t h e o u t f l o w , in t h e s a m e w a y a s CI" ( S a n d e r s o n e t a l . 1986). The  D i s s o l v e d o x y g e n (piM)  Dissolved s u l p h i d e ( m M )  Fig. 2-4 Dissolved o x y g e n a n d s u l p h i d e c o n c e n t r a t i o n s in Powell L a k e . The horizontal line at 150 m represents t h e o x i c / a n o x i c interface.  Fig. 2-5 T e m p e r a t u r e , c h l o r i d e a n d density profiles in Powell Lake for April 1984  Sigma t Temperature  Fig. 2-6 Dissolved o x y g e n a n d s u l p h i d e c o n c e n t r a t i o n s in S a k i n a w L a k e . The horizontal line a t 30 m represents t h e o x i c / a n o x i c interface.  (°C)  Fig. 2-7 T e m p e r a t u r e , chlorinity, a n d density profiles in S a k i n a w L a k e for July 1985  K  (mM)  +  1 2 3 4 5 QMj i i i i I i i i i I i i i i I i i i i I i i i i  50  D D  K  a  = .045 m »yr" 2  1  2 -1 = .035 m »Y  Powell  100  oxic anoxic  i i i i i i• i i i  Fig. 2-8 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of s o d i u m in Powell L a k e  Fig. 2-9 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of p o t a s s i u m in P o w e l l L a k e  Gci  Fig. 2-10 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of m a g n e s i u m in P o w e l l L a k e  (mM)  Fig. 2-11 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of c a l c i u m in P o w e l l Lake  B o r a t e (|j.M)  Fig. 2-12 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of b o r a t e in P o w e l l Lake  Sr  QxM)  Fig. 2-13 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of strontium in P o w e l l L a k e  Na  +  (mM)  Fig. 2-14 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of s o d i u m in S a k i n a w L a k e  K  +  (mM)  Fig. 2-16 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of m a g n e s i u m in S a k i n a w L a k e  Fig. 2-17 O b s e r v e d a n d c a l c u l a t e d c o n c e n t r a t i o n s of c a l c i u m in S a k i n a w L a k e  c a l c u l a t e d a n d o b s e r v e d c o n c e n t r a t i o n s of s o d i u m a n d b o r a t e (Figs. 2-8,2-12) a r e very c l o s e . R e l a t i v e t o c h l o r i d e , m a g n e s i u m , c a l c i u m , a n d strontium c o n c e n t r a t i o n s (Figs. 210, 2-11, 2-13) a r e h i g h e r t h a n e x p e c t e d , w h i l e p o t a s s i u m (Fig. 2-9) is lower. S a k i n a w shows similar results (Figs. 2-14 t o 2-17), a l t h o u g h t h e d i f f e r e n c e b e t w e e n t h e o b s e r v e d a n d c a l c u l a t e d p o t a s s i u m c u r v e s is n o t a s p r o n o u n c e d (Fig. 2-15).  2.4 Discussion  Both lakes c o n t a i n well-oxygenated surface waters, a l t h o u g h only Sakinaw d i s p l a y s a s u b s u r f a c e o x y g e n m a x i m u m a t 10 m (Fig. 2-6); this f e a t u r e most likely represents O2 p r o d u c t i o n b y b l o o m i n g p h y t o p l a n k t o n a t t h e t i m e of s a m p l i n g (July 1985). Light p e n e t r a t i o n in t h e u p p e r m o s t m e t r e m a y h a v e b e e n g r e a t e n o u g h t o c a u s e inhibition of photosynthesis. A similar s u b s u r f a c e m a x i m u m is n o t s e e n in t h e P o w e l l L a k e o x y g e n profile p r o b a b l y b e c a u s e o x y g e n w a s m e a s u r e d in e a r l y A p r i l , d u r i n g c o l d , rainy w e a t h e r , w h e r e a s S a k i n a w w a s s a m p l e d in h o t . sunny July. In P o w e l l , t h e o x y c l i n e is only a p p r o x i m a t e l y a s s o c i a t e d with t h e t o p of t h e saline d e e p water; significant c h l o r i d e c o n c e n t r a t i o n s o c c u r a t s o m e w h a t s h a l l o w e r d e p t h s (Figs. 2-4, 2-5). The H S 2  a n d CI* profiles In S a k i n a w m i m i c o n e a n o t h e r . A l t h o u g h t h e CI" c o n c e n t r a t i o n a p p e a r s t o i n c r e a s e m o r e g r a d u a l l y t h a n H S ( c o m p a r e Fig. 2-6 a n d Fig. 2-7), this is most likely a n 2  a r t e f a c t o f differing s a m p l i n g intervals, as n o CI" m e a s u r e m e n t w a s m a d e a t 60 m. In b o t h l a k e s t h e r e is a s u b s u r f a c e t e m p e r a t u r e m i n i m u m with t h e b o t t o m w a t e r i n c r e a s i n g t o a p p r o x i m a t e l y 9.5°C d u e t o g e o t h e r m a l h e a t i n g . This v a l u e in P o w e l l L a k e w a s s h o w n b y S a n d e r s o n e t a l . (1986) t o b e in a p p a r e n t equilibrium with t h e g e o t h e r m a l h e a t flux m e a s u r e d in t h e s o u t h b a s i n b y H y n d m a n (1976). The density profile of P o w e l l c l o s e l y m i m i c s t h a t of c h l o r i d e , i n d i c a t i n g t h a t t e m p e r a t u r e p l a y s a relatively small role in c o n t r o l l i n g w a t e r stability. This is a l s o true f o r S a k i n a w , a l t h o u g h t h e d e n s i t y profile d e v i a t e s s i g n i f i c a n t l y f r o m t h e c h l o r i d e in t h e u p p e r 5 m. This is d u e t o t h e v e r y l a r g e i n c r e a s e in t e m p e r a t u r e (from 8 t o 19°C) o b s e r v e d d u r i n g t h e m i d - s u m m e r s a m p l i n g , w h i c h c a u s e s t h e s i g m a t t o fall b e l o w zero. The most distinctive d i f f e r e n c e b e t w e e n t h e t w o lakes is in t h e g e n e r a l s h a p e of all their c o n c e n t r a t i o n versus d e p t h profiles. P o w e l l d i s p l a y s v e r y g r a d u a l c o n c a v e u p w a r d profiles, w h e r e r a p i d i n c r e a s e s t o m a x i m u m c o n c e n t r a t i o n s of most s p e c i e s  o c c u r 125 - 150 m b e l o w t h e o x i c / a n o x i c i n t e r f a c e ( e g . Fig. 2-5). S a k i n a w has a v e r y s h a r p i n t e r f a c e ; t h e d e n s i t y profile is a l m o s t s t e p - s h a p e d w i t h m a x i m u m c o n c e n t r a t i o n s of m o s t s p e c i e s o c c u r r i n g w i t h i n 4 5 m o f t h e i n t e r f a c e ( e g . Fig. 2-7). To e x p l a i n this d i f f e r e n c e , a brief discussion of t h e f r e s h e n i n g p r o c e s s is r e q u i r e d . The relict s e a w a t e r in P o w e l l L a k e h a s f r e s h e n e d c o n s i d e r a b l y s i n c e it w a s t r a p p e d a b o u t 12,000 years a g o . A s s u m i n g t h a t t h e original salinity w a s a p p r o x i m a t e l y t h a t o f p r e s e n t c o a s t a l w a t e r , or p e r h a p s slightly fresher d u e t o d i l u t i o n b y g l a c i a l m e t t w a t e r , t h e b o t t o m w a t e r salinity has d e c r e a s e d b y a f a c t o r of a b o u t 1.5. Streamflow f r o m P o w e l l River is s u b s t a n t i a l a n d w a s p r o b a b l y e v e n h i g h e r i m m e d i a t e l y after sill e m e r g e n c e d u e t o c o n t r i b u t i o n s f r o m g l a c i a l melt. W h e n t h e l a k e w a s initially f o r m e d , turbulent mixing w o u l d h a v e f l u s h e d out t h e s u r f a c e s a l t w a t e r a t t h e s a m e rate a t w h i c h f r e s h w a t e r e n t e r e d t h e system. Eventually, a s u r f a c e layer of fresh w a t e r overlying w a t e r of i n c r e a s i n g salinity w o u l d b e e s t a b l i s h e d . Freshening w o u l d t h e n c o n t i n u e m o r e slowly b y loss o f salt f r o m t h e l o w e r l a y e r b y v e r t i c a l diffusion. O n c e a s u r f a c e l a y e r of essentially z e r o salinity e x i s t e d , f r e s h w a t e r e n t e r i n g P o w e l l L a k e v i a P o w e l l River w o u l d t e n d t o f l o w o v e r t o p of t h e m o r e d e n s e w a t e r b e n e a t h . There w o u l d t h e n b e f e w if a n y p r o c e s s e s w i t h w h i c h t o mix t h e t w o layers of w a t e r . B e l o w t h e i n t e r f a c e , a n d particularly a t d e p t h , t h e v e r t i c a l diffusion is so small as t o b e c o m p a r a b l e t o m o l e c u l a r diffusivity ( S a n d e r s o n et a l . 1986). The south b a s i n has r e a c h e d this s t a g e at t h e present t i m e , with essentially z e r o salinity t o a b o u t 180 m a n d a g r a d u a l l y i n c r e a s i n g salt c o n t e n t until a s h a r p h a l o c l i n e is r e a c h e d a t 275 m (Fig. 2-5). A s salt diffuses into t h e o v e r l y i n g fresh w a t e r s , it is f l u s h e d f r o m t h e l a k e . The f o u r n o r t h e r n m o s t b a s i n s h a v e  freshened  c o m p l e t e l y , d u e t o their p r o x i m i t y t o t h e m a j o r f r e s h w a t e r s o u r c e . Turbidity currents a s s o c i a t e d w i t h s e d i m e n t a r y d i s c h a r g e m a y a l s o h a v e c o n t r i b u t e d t o t h e flushing p r o c e s s in t h e basins n e a r e r t h e h e a d of t h e l a k e (Williams et a l . 1961). The f r e s h e n i n g of S a k i n a w L a k e w o u l d h a v e o c c u r r e d v i a similar p r o c e s s e s a n d t h e l a k e is currently a t t h e s a m e s t a g e a s P o w e l l . A l t h o u g h f r e s h w a t e r input t o S a k i n a w is m u c h l o w e r , t h e l a k e h a s f r e s h e n e d m o r e d u e t o its c o n s i d e r a b l y s m a l l e r size. T h e r e f o r e , w h y is t h e r e s u c h a l a r g e d i f f e r e n c e in t h e s h a p e of its chlorinity profile ( a n d in f a c t , all its m a j o r i o n profiles)? To e x p l a i n this, t h e p r o c e s s e s i n v o l v e d in v e r t i c a l diffusion must b e d i s c u s s e d further.  V e r t i c a l diffusion i n c l u d e s b o t h e d d y a n d m o l e c u l a r diffusion. Unlike t h e latter, e d d y diffusion requires s o m e input o f kinetic e n e r g y t o t h e system. M o s t of t h e e n e r g y input t o P o w e l l L a k e w o u l d o c c u r a t t h e s u r f a c e . As t h e e n e r g y p r o p a g a t e s d o w n w a r d it is p a r t i a l l y d i s s i p a t e d , l e a d i n g t o l o w e r e n e r g y levels a n d less d i s s i p a t i o n a n d  eddy  diffusion a t g r e a t e r d e p t h s . In t h e unstratified f r e s h w a t e r l a y e r , v e r t i c a l o v e r t u r n i n g a s s o c i a t e d with c o o l i n g at the surface during winter p r o b a b l y contributes to vertical e d d y diffusivity. D u r i n g t h e D e c e m b e r t o F e b r u a r y p e r i o d , t h e m o n t h l y m e a n  air  t e m p e r a t u r e ( a v e r a g e d o v e r 1951 - 1980) a t P o w e l l River airport (4 k m s o u t h e a s t of t h e l a k e ) w a s b e l o w 4 ° C , t h e t e m p e r a t u r e o f m a x i m u m d e n s i t y of f r e s h w a t e r . This d e n s e r w a t e r will sink, l e a d i n g d i r e c t l y t o l a r g e v e r t i c a l e d d y diffusivities in t h e unstratified s u r f a c e layer. Stratification a t d e p t h p r e v e n t s this v e r t i c a l m i x i n g f r o m p e n e t r a t i n g far into t h e h a l o c l i n e . H o w e v e r , t h e o v e r t u r n i n g c o u l d g e n e r a t e internal w a v e s as sinking p a r c e l s of w a t e r b o u n c e a g a i n s t t h e h a l o c l i n e ( S a n d e r s o n et al. 1986), a n d t h e s e internal w a v e s c o u l d in turn c a u s e mixing in t h e stratified r e g i o n . N e a r t h e s u r f a c e , mixing d u e t o s u r f a c e g r a v i t y w a v e s must o c c u r , but this e n e r g y w o u l d d e c a y with d e p t h l o n g b e f o r e t h e i n t e r f a c e w o u l d b e r e a c h e d . A r o u g h c a l c u l a t i o n g i v e n a 15 k m f e t c h , a 2 h o u r d u r a t i o n a n d 10 m^s" w i n d s p e e d i n d i c a t e s t h a t t h e w a v e a m p l i t u d e w o u l d d e c a y t o < 1 1  c m b y a p p r o x i m a t e l y 15 m d e p t h . Therefore, in t h e stratified layer a n y mixing t h a t c o u l d o c c u r will b e r e l a t e d t o internal w a v e s r a t h e r t h a n s u r f a c e g r a v i t y w a v e s . There is a n a d d i t i o n a l s o u r c e of turbulent e n e r g y a s s o c i a t e d with t h e b o t t o m b o u n d a r y , s u c h t h a t a l o c a l i n c r e a s e in e d d y diffusion o c c u r s . This thin l a y e r a t t h e b o t t o m has little e f f e c t o n t h e n e t r a t e o f salt loss f r o m t h e s a l i n e l a y e r , w h i c h is c o n t r o l l e d b y t h e diffusivity m i n i m u m w h i c h lies w e l l a b o v e t h e b o t t o m diffusion layer ( S a n d e r s o n et a l . 1986). A t d e p t h , t h e r e f o r e , e d d y d i f f u s i o n is virtually n o n e x i s t e n t , a n d t h e v e r t i c a l diffusivity is so s m a l l t h a t it is c o m p a r a b l e t o t h e m o l e c u l a r diffusivity. S i n c e different c h e m i c a l s p e c i e s h a v e c o n t r a s t i n g m o l e c u l a r diffusivities, t h e y s h o u l d d i f f u s e a t different rates a n d thus b e lost f r o m t h e b o t t o m w a t e r s t o different d e g r e e s . In f a c t , t h e ratios o f t h e m a j o r i o n i c c o n s t i t u e n t s in t h e b o t t o m w a t e r o f b o t h l a k e s d o differ c o n s i d e r a b l y f r o m t h o s e f o u n d in m o d e r n s e a w a t e r (Figs. 2-6 t o 2-17). B e f o r e t h e r e a s o n s for t h e s e distributions a r e d i s c u s s e d , t h e d i f f e r e n c e b e t w e e n t h e g e n e r a l s h a p e of t h e P o w e l l a n d S a k i n a w ion profiles must b e e x p l a i n e d .  The p r o c e s s e s w h i c h c o n t r i b u t e t o v e r t i c a l diffusion in S a k i n a w will b e t h e s a m e a s t h o s e in P o w e l l , a s b o t h l a k e s , b e i n g c l o s e t o g e t h e r , a r e s u b j e c t t o essentially t h e s a m e w i n d r e g i m e . M o l e c u l a r diffusion is t h e d o m i n a n t p r o c e s s a s is c h a r a c t e r i s e d b y a s t r o n g c h e m o c l i n e , w h i c h is s i t u a t e d fairly n e a r t h e o x i c / a n o x i c i n t e r f a c e in S a k i n a w . The l a c k o f c o n c e n t r a t i o n g r a d i e n t t h r o u g h o u t most of t h e l o w e r a n o x i c w a t e r c o l u m n i n d i c a t e s t h a t e d d y diffusion p r o c e s s e s a r e i m p o r t a n t in this r e g i o n . A n y e n e r g y input f r o m t h e s u r f a c e w o u l d t e n d t o mix t h e c h e m o c l i n e , so t h e d i s t u r b a n c e must b e g e n e r a t e d a t d e p t h . A likely c a u s e o f t h e mixing is f r o m input of d e n s e , saline w a t e r at d e p t h . This w o u l d h a p p e n if s e a w a t e r f r o m t h e Strait of G e o r g i a w a s t o e n t e r S a k i n a w o v e r t h e b a r e l y e m e r g e d sill. The d e n s e r w a t e r w o u l d f l o w into t h e b a s i n , e n t r a i n s o m e fresh w a t e r f r o m t h e s u r f a c e layer, sink a n d mix with t h e b o t t o m w a t e r in t h e d e e p a n o x i c b a s i n . This p r o c e s s w o u l d e f f e c t i v e l y b u n t t h e c h e m o c l i n e ( w h i c h h a d b e e n e s t a b l i s h e d t h r o u g h m o l e c u l a r diffusion p r o c e s s e s ) u p w a r d s in t h e w a t e r c o l u m n (B. S a n d e r s o n , P.H. L e B l o n d pers. c o m m . ) , resulting in u n i f o r m , m i x e d w a t e r b e l o w t h e t h e r m o c l i n e . The ratios o f t h e major i o n i c constituents in t h e b o t t o m w a t e r of b o t h lakes differ c o n s i d e r a b l y f r o m t h o s e f o u n d in m o d e r n s e a w a t e r . Those ions w i t h l a r g e m o l e c u l a r diffusivities a r e lost m o r e r a p i d l y f r o m t h e d e e p w a t e r t h a n t h o s e w i t h smaller diffusion c o e f f i c i e n t s . The m o l e c u l a r diffusivities (D) of six m a j o r ions in P o w e l l L a k e a r e s h o w n o n t h e c o r r e s p o n d i n g c o n c e n t r a t i o n profiles in Figs. 2-8 t o 2-13. As w o u l d b e p r e d i c t e d b y their diffusivities, c o n c e n t r a t i o n s of M g  2 +  , Ca  2 +  a n d Sr a r e e n r i c h e d relative t o CI", w h e n +  c o m p a r e d t o m o d e r n s e a w a t e r (Figs. 2-8. 2-10. 2-11). The diffusivities of t h e s e ions a r e tower t h a n that of CI" a n d t h e r e f o r e t h e y diffuse a t a slower rate. K . c o n v e r s e l y , diffuses, +  faster t h a n C f a n d is relatively d e p l e t e d in t h e b o t t o m w a t e r (Fig. 2-9). N a  +  has a diffusion  c o e f f i c i e n t e q u a l t o t h a t of CI", a n d t h e o b s e r v e d a n d c a l c u l a t e d profiles a r e virtually i d e n t i c a l . A diffusion c o e f f i c i e n t c o u l d n o t b e f o u n d f o r b o r a t e ; f r o m Fig. 2-12 a n d a s s u m i n g c o n s e r v a t i v e b e h a v i o u r , it w o u l d b e p r e d i c t e d t o b e similar t o t h a t of CI*. S a k i n a w L a k e (Figs. 2-14 t o 2-17) shows similar b e h a v i o u r . M o l e c u l a r diffusion m a y n o t b e t h e only p r o c e s s o p e r a t i n g , h o w e v e r . N o t e t h a t Mg  2 +  and C a  2 +  h a v e i d e n t i c a l diffusivities a n d y e t C a  2 +  is m u c h m o r e e n r i c h e d in t h e  b o t t o m w a t e r t h a n M g . Similar results w e r e o b t a i n e d for S a k i n a w (Fig. 2-16). A p p a r e n t l y , 2 +  s o m e c h e m i c a l p r o c e s s t h a t c a u s e s extra e n r i c h m e n t of C a  2 +  (or p e r h a p s under-  e n r i c h m e n t o f M g ) is o p e r a t i n g in b o t h lakes. A p a r t i c u l a t e c a l c i u m m a x i m u m o c c u r s 2 +  a t t h e r e d o x i n t e r f a c e in F r a m v a r e n . a n d is a t t r i b u t e d t o b a c t e r i a l u p t a k e ( A n d e r s o n et a l . 1987). B a c t e r i a c a n b i n d c a t i o n s , i n c l u d i n g c a l c i u m i o n , v i a a d s o r p t i o n o n t o t h e s u r f a c e , b i n d i n g t o proteins, e t c . (Lalou 1957; M o r i t a 1980), a n d d e a d cells c a n t a k e u p as m u c h c a l c i u m a s living c e l l s ( G r e e n f i e l d 1963). In F r a m v a r e n , c a l c i u m is "released a t d e p t h d u e t o d e g r a d a t i o n o f t h e b a c t e r i a l c e l l s . This c a u s e s e l e v a t e d  calcium  c o n c e n t r a t i o n s in t h e b o t t o m w a t e r , a n d p r o m o t e s c a l c i t e p r e c i p i t a t i o n . B a c t e r i a a r e p r o b a b l y c o n c e n t r a t e d a t t h e i n t e r f a c e in P o w e l l a n d S a k i n a w a s w e l l , s o t h a t similar p r o c e s s e s m a y c a u s e t h e e n r i c h m e n t o f c a l c i u m in t h e b o t t o m w a t e r o f t h e s e lakes. C o m p a r i s o n o f t h e m a x i m u m i o n c o n c e n t r a t i o n s (those in t h e b o t t o m w a t e r ) , a n d t h e percent differences b e t w e e n t h e observed a n d the c a l c u l a t e d values for C a Mg  2 +  2 +  ,  a n d K ( T a b l e 2-1), s h o w t h a t m u c h l a r g e r d i f f e r e n c e s exist in P o w e l l r e l a t i v e t o +  S a k i n a w L a k e . This is further e v i d e n c e t h a t t h e r e h a s b e e n s e a w a t e r input t o S a k i n a w s i n c e sill e m e r g e n c e , a s t h e mixing of m o d e r n s e a w a t e r w i t h relict s e a w a t e r w o u l d t e n d t o p u s h t h e m a j o r i o n c o m p o s i t i o n o f t h e d e e p w a t e r c l o s e r t o t h a t of m o d e r n s e a w a t e r .  Table 2-1  Deviations of observed bottom water major ion concentrations from those  calculated from chlorinity assuming constant relative composition of seawater.  Ion Ca  2 +  Mg K  2 +  +  Powell Lake  Sakinaw Lake  +46%  +25%  +9.3%  +7.6%  -2.2%  -1.9%  In s u m m a r y , b o t h lakes c o n t a i n relict s e a w a t e r a t d e p t h w h i c h h a s f r e s h e n e d c o n s i d e r a b l y , primarily b y m o l e c u l a r diffusion. The d i f f e r e n c e in t h e g e n e r a l s h a p e of t h e m a j o r i o n profiles b e t w e e n P o w e l l a n d S a k i n a w Lakes is most likely d u e t o incursion of s e a w a t e r t o t h e d e e p w a t e r s of S a k i n a w s i n c e it w a s s e p a r a t e d f r o m t h e Strait of Georgia.  CHAPTER 3 REDOX CHEMISTRY  3.1 Introduction  In p u r e l y i n o r g a n i c c h e m i c a l systems, o x i d a t i o n - r e d u c t i o n ( r e d o x )  reactions  p r o c e e d w i t h a f l o w o f e l e c t r o n s b e t w e e n t h e o x i d i z e d a n d r e d u c e d states until e q u i l i b r i u m is a t t a i n e d . A l t h o u g h t h e r e is a t e n d e n c y f o r t h e r e d u c e d p h a s e t o lose e l e c t r o n s a n d b e t r a n s f o r m e d t o a n o x i d i z e d s t a t e , f r e e e l e c t r o n s usually inhibit this p r o c e s s , s u c h t h a t l a r g e q u a n t i t i e s o f f r e e i o n c a n exist t o g e t h e r in b o t h r e d u c e d a n d o x i d i z e d states. True r e d o x e q u i l i b r i u m d o e s n o t a c t u a l l y exist in a n y n a t u r a l a q u a t i c system  because  m o s t r e d o x r e a c t i o n s a r e e x t r e m e l y s l o w in t h e a b s e n c e  of  a p p r o p r i a t e b i o c h e m i c a l catalysis. O n l y a f e w e l e m e n t s - C , N , O , S, F e , M n , H - a r e i m p o r t a n t p a r t i c i p a n t s in a q u a t i c r e d o x p r o c e s s e s b e c a u s e t h e y e x h i b i t m u l t i p l e v a l e n c e states, a n d h e n c e c a n u n d e r g o o x i d a t i o n a n d r e d u c t i o n . A s n o t e d in C h a p t e r 2, t h e l i m n o l o g y of P o w e l l a n d S a k i n a w Lakes is d o m i n a t e d b y p e r m a n e n t stratification w h i c h is a s s o c i a t e d w i t h a p r o n o u n c e d d e c r e a s e in t h e Eh w i t h d e p t h . In this c h a p t e r , s u c h r e d o x v a r i a t i o n s will b e r e v i e w e d , a n d their i m p o r t a n c e in l a k e c h e m i s t r y will b e discussed. Photosynthesis, b y t r a p p i n g light e n e r g y a n d c o n v e r t i n g it t o c h e m i c a l e n e r g y , p r o d u c e s r e d u c e d states of h i g h e r f r e e e n e r g y (high-energy c h e m i c a l b o n d s ) a n d thus n o n e q u i l i b r i u m c o n c e n t r a t i o n s o f C . N , a n d S c o m p o u n d s ( S t u m m a n d M o r g a n 1981). In c o n t r a s t t h e r e s p i r a t o r y , f e r m e n t a t i v e , a n d o t h e r n o n p h o t o s y n t h e t i c p r o c e s s e s of o r g a n i s m s t e n d t o restore e q u i l i b r i u m b y c a t a l y t i c a l l y d e c o m p o s i n g t h e u n s t a b l e products of photosynthesis through energy-yielding redox reactions, t h e r e b y obtaining a s o u r c e o f e n e r g y f o r their m e t a b o l i c n e e d s . The o r g a n i s m s u s e this e n e r g y b o t h t o synthesize n e w cells a n d t o m a i n t a i n t h e o l d cells a l r e a d y f o r m e d . These o r g a n i s m s a r e p r i m a r i l y built u p f r o m " r e d o x e l e m e n t s " , a n d t h e i r rel ati vel y c o n s t a n t s t o i c h i o m e t r i c c o m p o s i t i o n (CiooFbttOnoNuP) ( R e d f i e l d e t al.1963) a n d t h e c y c l i c e x c h a n g e  between  c h e m i c a l e l e m e n t s of t h e w a t e r a n d t h e resident organisms affects t h e relative c o n c e n t r a t i o n s o f t h e e l e m e n t s in t h e e n v i r o n m e n t . Thus, t h e b i o l o g i c a l l y a c t i v e e l e m e n t s c i r c u l a t e in a p a t t e r n different t o that o f t h e w a t e r itself or t o t h a t o f t h e i n a c t i v e , i.e., c o n s e r v a t i v e , solutes. O r g a n i s m s a c t a s r e d o x catalysts b y m e d i a t i n g t h e r e a c t i o n s  a n d transfer o f e l e c t r o n s ; t h e o r g a n i s m s t h e m s e l v e s d o n o t oxidize substrates or r e d u c e c o m p o u n d s ( W e t z e l 1983). At t h e t e m p e r a t u r e . Eh, a n d p H of natural waters, a n d at t h e r m o d y n a m i c e q u i l i b r i u m , o n l y a small n u m b e r of c o m m o n d i s s o l v e d s p e c i e s m a k e u p almost all of t h e t o t a l c a r b o n , n i t r o g e n , sulphur, h y d r o g e n , a n d o x y g e n in a q u e o u s solution. They a r e  Haaq). Oaog), ChUaq), C O f , H C C i  H2CO3. N H , Nh& 3  N2.NO3", H£ . lOQ)  HS', a n d SO4  1970). A l l o t h e r s p e c i e s , s u c h a s S2O3* a n d NO2. a r e p r e s e n t  at m u c h  2  c o n c e n t r a t i o n s . In a e r o b i c s u r f a c e w a t e r s ( P  0 2  (Thorstenson lower  > 10" a t m ) a n d most b o t t o m w a t e r s , t h e 4  p r i n c i p a l s p e c i e s in e q u i l i b r i u m w i t h t h e a t m o s p h e r e a t neutral p H s h o u l d b e H C O 3 , N 0 a n d SO4  3  The f e w e x c e p t i o n s i n c l u d e b o t t o m w a t e r s o f s o m e m e r o m i c t i c lakes, a n d of  r e s t r i c t e d o c e a n b a s i n s , s u c h a s t h e B l a c k S e a o r F r a m v a r e n fjord. The p r e d o m i n a n t d i s s o l v e d s p e c i e s a c t u a l l y f o u n d in a e r o b i c w a t e r s a r e HCO3, S O 4 , a n d N . D i s s o l v e d 2  NO3 is r e l a t i v e l y m i n o r , e v e n t h o u g h , a c c o r d i n g t o t h e r m o d y n a m i c c o n s i d e r a t i o n s , it should b e m u c h m o r e a b u n d a n t t h a n N . B i o g e o c h e m i c a l reactions involving nitrogen 2  w o u l d s e e m t o result in a steady-state c o n c e n t r a t i o n o f d i s s o l v e d N in a e r o b i c w a t e r 2  w h i c h is far h i g h e r t h a n t h a t p r e d i c t e d for t h e r m o d y n a m i c equilibrium (Wetzel 1983). A n a e r o b i c w a t e r s (P02 < 10" a t m ) a r e usually o n l y f o u n d in t h e interstitial s p a c e s 4  of s e d i m e n t s ; a e r o b i c w a t e r s a r e o f t e n u n d e r l a i n b y a n a e r o b i c s e d i m e n t s . The l a c k of dissolved 0  2  in s u c h s e d i m e n t s is d u e t o t h e o x i d a t i o n o f o r g a n i c m a t t e r b y a e r o b i c  microorganisms w h i c h h a v e a high m e t a b o l i c rate a n d h e n c e use u p o x y g e n very q u i c k l y . T h e a e r o b i c b a c t e r i a live o n o r n e a r t h e s e d i m e n t s u r f a c e , a n d b y utilizing o x y g e n faster t h a n it c a n diffuse f r o m a b o v e , p r e v e n t it f r o m diffusing into t h e s e d i m e n t . This c a n only o c c u r if d e g r a d a b l e o r g a n i c m a t t e r is s u p p l i e d t o t h e s e d i m e n t faster t h a n it c a n b e d e s t r o y e d b y a e r o b i c o x i d a t i o n , a n d t y p i c a l l y o c c u r s in regions o f e x t r e m e l y h i g h p r o d u c t i v i t y o r in l o w e n e r g y e n v i r o n m e n t s w h e r e o r g a n i c m a t t e r is n o t c o n t i n u a l l y r e s u s p e n d e d a n d r e m o v e d b y currents. T h e b o t t o m w a t e r s o f m e r o m i c t i c l a k e s a r e a n a l o g o u s t o t h e latter: t h e r e is little w a t e r m o v e m e n t a n d s o n o a d v e c t i v e  oxygen  r e p l e n i s h m e n t . In s u c h s y s t e m s , d e c o m p o s i t i o n o f o r g a n i c m a t t e r b y a e r o b e s is restricted t o t h e o x i c p o r t i o n of t h e w a t e r c o l u m n , t h r o u g h w h i c h o r g a n i c m a t t e r settles. Once  d i s s o l v e d O2 is r e m o v e d , a n y further o x i d a t i o n o f o r g a n i c m a t t e r b y  b a c t e r i a must o c c u r b y t h e utilization o f o t h e r o x i d a n t s . These i n c l u d e NO3". SO4 a n d  28 o x i d i z e d o r g a n i c s p e c i e s . A s s o c i a t e d r e d o x r e a c t i o n s t e n d t o o c c u r in o r d e r o f their t h e r m o d y n a m i c f a v o u r a b i l i t y a n d a r e listed in T a b l e 3-1.  Table 3-1. Oxidation reactions of organic matter (from Froelich et al. 1979)  1. A e r o b i c D e c o m p o s i t i o n (CH2O),06(NH3)i (H3PO4) +138C-2 -> IO6CO2+ I6HNO3 + H3PO4 +122H20 6  A 3 . = -3190kJ»mor 2. M a n g a n e s e  1  reduction  (CH O),06(NH3)i6(H3PO4) + 2 3 6 M n a + 472H -> 2 3 6 M n + IO6CO2 + 8N2 + HPO +  2  ASo = -3090 Id • m o r  2+  A  + 366H 0 2  (birnessite)  1  4Go = -3050 kJ • mol"  1  &o = -2920 kJ • mol"  1  (nsutite) (pyrolusite)  3. Denitrification a n d nitrate r e d u c t i o n (CH O)i06(NH3) 6(H FO4) + 9 4 . 4 H N 0 -> 106CO2 + 55.2N2 + H3FO4 + 177.21^0 2  1  3  3  =-3030 k > mol"  1  (CH2O)i06(NH3))6(H PO4) + 84.8HNO3 -> 106CO2 + 42.4N2+I6NH3 + H3PO4 + I48.4H2O 3  4. Iron r e d u c t i o n (CH2O)i06(NH3)i6(H PO4) + 212Fe203 + 848H -» 424Fe + IO6CO2 + I6NH3 + H3PO4 + 530H O +  3  A S = -1410 kJ • mol" A 3 j = -1330 kj-mol"  1  1  2+  2  (hematite) (limonitic goethite)  5. S u l p h a t e r e d u c t i o n (CH2O),06(NH3),6(H PO4) + 53S04 -> 106CO2 + I6NH3 + 53S " + H3PO4 +106H2O 2  3  /5<Si = -380kJ»mor  1  6. F e r m e n t a t i o n (CHZOIMOMH^^^POA)  ->  530O2 + 53CH4 + I6NH3 + H3PO4  ^ 3 = -350kJ«mor  1  R e a c t i o n 1 o c c u r s in a e r o b i c e n v i r o n m e n t s . 2 - 4 in s u b o x i c z o n e s , a n d r e a c t i o n s 5 a n d 6 u n d e r a n o x i c c o n d i t i o n s . Thus, f o l l o w i n g O2 d e p l e t i o n , t h e s u c c e s s i o n of b a c t e r i a l p r o c e s s e s in n a t u r a l w a t e r s ( p H -7) is nitrate a n d m a n g a n e s e r e d u c t i o n , f o l l o w e d b y iron a n d s u l p h a t e r e d u c t i o n plus a m m o n i u m f o r m a t i o n , a n d f i n a l l y , methane  f o r m a t i o n . This results in t h e f o l l o w i n g fairly p r e d i c t a b l e s e q u e n c e  of  coexisting dissolved species: D H C C i + S C M + NO", 2) H C Q 3 + S C M  +N2  3) HCO", +HS" +NI-C 4) CH4 + HS" + N H I . O b s e r v a t i o n s o f m a r i n e s e d i m e n t s s h o w t h a t t h e c o m p o s i t i o n s o f p o r e w a t e r s a r e in g e n e r a l a g r e e m e n t with these predictions a l t h o u g h specific e x c e p t i o n s o c c u r (e.g. Froelich et a l . 1979). The o c c u r r e n c e o f t h e s e r e a c t i o n s in n a t u r a l s e d i m e n t s is n o t l i m i t e d t o welld e f i n e d d e p t h intervals within t h e s e d i m e n t c o l u m n . O v e r l a p is c o m m o n . The r e d u c t i o n of NO3 a n d M n 0 2 , for e x a m p l e , h a p p e n s a t a b o u t t h e s a m e pE levels a n d t h e r e f o r e c o n c u r r e n t l y in t h e s e d i m e n t c o l u m n (Stumm a n d M o r g a n 1981). In a d d i t i o n , b i o t u r b a t i o n ( G o l d h a b e r e t a l . 1977, B e r n e r a n d W e s t r i c h  1985) a n d d i f f u s i o n t e n d t o blur t h e  b o u n d a r i e s of t h e s e r e d o x p r o c e s s e s within t h e s e d i m e n t c o l u m n . In m e r o m i c t i c lakes, this series of r e d o x r e a c t i o n s is s p r e a d t h r o u g h o u t t h e w a t e r c o l u m n a n d h e n c e c a n b e m o r e easily o b s e r v e d t h a n in s e d i m e n t s , w h e r e t h e r e a c t i o n series s h o w n in T a b l e 3-1 is t y p i c a l l y c o m p r e s s e d . In sulphide-rich w a t e r s t h e c h i e f s p e c i e s a n t i c i p a t e d (at p H ~7) a r e H C O 3 , N H 4 , HS", H2S, C H , a n d N . The c h e m i s t r y of c a r b o n a n d n i t r o g e n is c o m p l e x , 4  2  a n d will b e d e s c r i b e d briefly in t h e f o l l o w i n g t w o s e c t i o n s . S u l p h u r s p e c i e s will b e d i s c u s s e d in a s u b s e q u e n t c h a p t e r .  Carbon The s o u r c e s a n d c o m p o s i t i o n of o r g a n i c m a t t e r in n a t u r a l w a t e r s a n d s e d i m e n t s a r e d i v e r s e a n d p o o r l y u n d e r s t o o d . N e a r l y all of t h e o r g a n i c c a r b o n o f n a t u r a l w a t e r s consists of d i s s o l v e d o r g a n i c c a r b o n ( D O C ) a n d d e a d p a r t i c u l a t e o r g a n i c c a r b o n ( P O C ) ; living b i o t a m a k e u p a v e r y s m a l l f r a c t i o n o f t h e t o t a l P O C , a l t h o u g h their m e t a b o l i s m results in reversible fluxes b e t w e e n t h e d i s s o l v e d a n d p a r t i c u l a t e p h a s e s of  d e t r i t a l c a r b o n . P r o d u c t i o n of d i s s o l v e d a n d p a r t i c u l a t e o r g a n i c c a r b o n is a result of a u t o t r o p h i c a n d h e t e r o t r o p h i c m e t a b o l i s m . I n s t a n t a n e o u s m e a s u r e m e n t s of D O C a n d P O C a r e h i g h l y b i a s e d t o w a r d r e f r a c t o r y c o m p o u n d s t h a t a r e relatively c h e m i c a l l y s t a b l e , of l o w solubility, a n d resistant t o r a p i d ( d a y s t o w e e k s ) m i c r o b i a l d e g r a d a t i o n . T h e s e r e f r a c t o r y c o m p o n e n t s of detrital o r g a n i c c a r b o n persist for l o n g e r p e r i o d s of t i m e t h a n t h e m o r e l a b i l e o r g a n i c c o m p o u n d s . H o w e v e r , t h e r e a d i l y utilizable l a b i l e c o m p o n e n t s c y c l e rapidly at low equilibrium c o n c e n t r a t i o n s , a n d represent  major  c a r b o n p a t h w a y s a n d e n e r g y fluxes. L a k e w a t e r s a r e relatively s t a t i c c o m p a r e d t o m o s t o c e a n i c e n v i r o n m e n t s , a n d thus s e d i m e n t a t i o n transfers a g r e a t e r p o r t i o n of c a r b o n a n d its a s s o c i a t e d m e t a b o l i s m t o t h e s e d i m e n t s ( V a l l e n t y n e 1962, W e t z e l et a l . 1972). A l l o c h t h o n o u s s o u r c e s of o r g a n i c m a t t e r t o a q u a t i c systems a r e primarily of terrestrial p l a n t origin a n d t h e D O M of s u r f a c e runoff is c o m p o s e d of relatively refractory o r g a n i c c o m p o u n d s resistant t o r a p i d m i c r o b i a l d e g r a d a t i o n . M o s t c a r b o n of n a t u r a l w a t e r s o c c u r s a s equilibrium p r o d u c t s of c a r b o n i c a c i d (H2CO3). O x i d i z e d c a r b o n , i.e. t o t a l C 0  (IC0 =C 0  2  2  2  + H C 0 + CO3"). is a d d e d t o n a t u r a l 3  w a t e r s b y a t m o s p h e r i c e x c h a n g e , l e a c h i n g of c a r b o n a t e f r o m soils or r o c k s , a n d b y t h e b i o l o g i c a l d e c o m p o s i t i o n of o r g a n i c m a t t e r . It c a n b e r e m o v e d v i a photosynthesis a n d mineral precipitation. Atmospheric C 0  2  dissolves in w a t e r a n d u n d e r g o e s hydrolysis t o f o r m c a r b o n i c  a c i d (at p H < 8.5):  CO2 (air) <-> C 0 (dissolved) + H 0 <-» H C 0 . 2  H C0 2  3  2  2  3  is a w e a k a c i d (pK= 16.30) ( M o r e l 1983). a n d r a p i d l y p a r t i a l l y d i s s o c i a t e s t o  b i c a r b o n a t e a n d c a r b o n a t e ions: hfeCQs <-» H + H C 0 +  3  «-> H + C O ! . +  A t e q u i l i b r i u m , h y d r o x y l ions (OH") a r e f o r m e d b y t h e hydrolysis of b i c a r b o n a t e  and  c a r b o n a t e ions:  HCO3 + H2O <-> HzCQj + OH" CO?+ H2O <-> HCO3+OH" hbCQj <-> HzO + COz. C a r b o n a t e s a r e s o l u b i l i z e d a s c a r b o n i c a c i d p e r c o l a t e s t h r o u g h soils a n d releasing d i s s o l v e d C a  2 +  rocks,  a n d H C O 3 . Slightly g r e a t e r c o n c e n t r a t i o n s of O H " i o n t h a n H  +  ion  result f r o m t h e d i s s o c i a t i o n o f H C O 3 . C O 3 . a n d H2CO3 a n d h e n c e m a n y fresh w a t e r s a r e w e a k l y a l k a l i n e ( W e t z e l 1983). The o x i d a t i o n o f o r g a n i c c a r b o n b y t h e respiration o f b o t h b a c t e r i a a n d higher organisms a d d s C 0  2  t o a e r o b i c waters:  e.g.GHsCfc + 6 a - * 6 C C b +6H2O  AGo = -3190kJ»mol" . 1  B e c a u s e CO2 forms a w e a k a c i d , H C 0 , t h e e f f e c t o f a e r o b i c d e c o m p o s i t i o n is t o 2  cause  3  a l o w e r i n g o f p H . In a n a e r o b i c  e n v i r o n m e n t s in w h i c h n i t r a t e , iron a n d  m a n g a n e s e oxides a n d sulphate a r e d e p l e t e d , o r g a n i c c a r b o n c a n n o t b e oxidized with O2 a n d is i n s t e a d d e c o m p o s e d b y t h e o x y g e n c o n t a i n e d within t h e o r g a n i c m a t t e r itself b y m i c r o b i o l o g i c a l f e r m e n t a t i o n :  *Gi = -35Q\dTno\'\  e.g. C6HcO6^3C02+3CH4  The g r e a t e r e n e r g y y i e l d for a e r o b i c o x i d a t i o n results in m o r e efficient m e t a b o l i s m , a n d thus faster o r g a n i c m a t t e r d e c o m p o s i t i o n relative t o t h a t of f e r m e n t a t i o n . The a m b i e n t p H a s s o c i a t e d with a n a e r o b i c f e r m e n t a t i o n d e p e n d s u p o n t h e a m o u n t s o f CO2 ( a n d o r g a n i c a c i d s ) f o r m e d relative t o t h e a m o u n t s o f n i t r o g e n o u s b a s e s r e l e a s e d . T h e t o p f e w c e n t i m e t r e s of a n a e r o b i c s e d i m e n t s g e n e r a l l y h a v e a l o w e r p H t h a n o v e r l y i n g a e r o b i c w a t e r s , i n d i c a t i n g t h a t f e r m e n t a t i o n r e a c t i o n s (plus s u l p h a t e r e d u c t i o n ) result in n e t a c i d p r o d u c t i o n (Berner 1971). V e r y l o w v a l u e s of p H (< 5) h a v e b e e n f o u n d in s w a m p s a n d b o g s d u e t o f e r m e n t a t i o n o f o r g a n i c c o m p o u n d s t h a t a r e l o w in n i t r o g e n ( e . g . , lignin a n d c a r b o h y d r a t e s ) . In t h e s e e n v i r o n m e n t s , n o CaC0  o r o t h e r b a s i c minerals a r e p r e s e n t t o b u f f e r t h e p H a n d n e u t r a l i z e e x c e s s  3  c a r b o n i c a n d other acids. Besides f e r m e n t a t i o n , m e t h a n e m a y also form b y t h e r e d u c t i o n of c a r b o n d i o x i d e u s i n g e i t h e r H2 o r a w i d e v a r i e t y o f o r g a n i c h y d r o g e n d o n o r s a s r e d u c i n g agents: e.g.  The s o u r c e o f C 0  2  C O 2 + 4 H - > Cl-U+2H2O. 2  is usually a n earlier f e r m e n t a t i o n r e a c t i o n (Brock 1979).  B a c t e r i a l s u l p h a t e r e d u c t i o n results in t h e net f o r m a t i o n o f b i c a r b o n a t e ion rather t h a n CO2, b e c a u s e a d i v a l e n t i o n , SOt. is c o n v e r t e d t o t h e m o n o v a l e n t i o n HS" or a n e u t r a l d i s s o l v e d s p e c i e s , H S, a n d t h e c h a r g e b a l a n c e must b e m a i n t a i n e d . The 2  b a c t e r i a ( e . g . Desulfovibrio  spp,) utilize o r g a n i c c a r b o n a s a r e d u c i n g a g e n t a n d t h e  resulting CO2 supplies t h e required n e g a t i v e c h a r g e in t h e form of HCO3 ion:  2CH2O + S 0 " -> 2HCa + H2S 2  or 2CH2O + SO4 -» HCO3+ HS" + CO2 + H2O. W h e r e a s in w a t e r s b e l o w t h e p h o t i c z o n e t h e r e is a n e t p r o d u c t i o n o f C 0 , 2  resulting in a l o w e r p H , in s u r f a c e w a t e r s CO2 c a n b e c o n s u m e d v i a photosynthesis b y phytoplankton:  6CO2 +6H2O <-> C6H a +6O2. 12  This p r o c e s s c a n o f t e n l e a d t o d r a m a t i c increases in p H (Wetzel 1983). Both s u l p h a t e r e d u c t i o n a n d photosynthesis c a n foster CaCG-3 p r e c i p i t a t i o n . If t h e solution of c a l c i u m b i c a r b o n a t e in equilibrium with C O 2 . H2CO3. a n d C O f loses a p o r t i o n of the C O 2 required t o maintain the equilibrium, s u c h as b y photosynthetic exceeding replacement  uptake  o f C 0 , o r b y H C O 3 f o r m a t i o n in s u l p h a t e r e d u c t i o n , t h e 2  relatively insoluble c a l c i u m c a r b o n a t e will p r e c i p i t a t e : CcKHCC^? «-* C a C O s + H ^ + C C ^ , until t h e e q u i l i b r i u m is r e e s t a b l i s h e d b y t h e f o r m a t i o n of sufficient C 0 (Kelts a n d Hsu 2  1978). A s CaCC>3 f o r m s a n d p r e c i p i t a t e s , i n o r g a n i c ( e . g . , P C M ) ions a n d o r g a n i c c o m p o u n d s c a n a d s o r b t o or c o p r e c i p i t a t e with t h e C a C 0 trophogenic  zone  of lakes, w h i c h  3  a n d a r e c a r r i e d o u t of t h e  m a y result in r e d u c e d  metabolic  activity.  M a i n t e n a n c e of s u p e r s a t u r a t i o n without p r e c i p i t a t i o n , h o w e v e r , m a y o c c u r d u e t o t h e inhibiting e f f e c t u p o n crystallization of d i s s o l v e d o r g a n i c m a t t e r (Suess 1970). Losses o f C 0  2  b y p h o t o s y n t h e t i c utilization or a d d i t i o n s o f C 0  2  from biotic  r e s p i r a t i o n t e n d t o c h a n g e t h e p H of t h e w a t e r . The b u f f e r i n g a c t i o n of t h e w a t e r , h o w e v e r , t e n d s t o resist c h a n g e s in p H as l o n g a s t h e e q u i l i b r i a of t h e C 0 system a r e 2  o p e r a t i o n a l . A d d i t i o n of H ions is n e u t r a l i z e d b y O H " ions f o r m e d b y t h e hydrolysis of +  H C 0 3 a n d C 0 . The p H r e m a i n s essentially t h e s a m e a s b e f o r e , unless t h e s u p p l y of 3  H C 0 3 a n d C 0 3 is e x h a u s t e d . Similarly, a d d e d O H " ions r e a c t w i t h H C O ' ions t o f o r m 3  carbonate:  HCO3 +OH" ^ C O t +H20. Nitrogen The e l e m e n t n i t r o g e n , a k e y c o n s t i t u e n t of p r o t o p l a s m , exists in a n u m b e r of o x i d a t i o n states, r a n g i n g f r o m -3 ( o r g a n i c N (R-NH2), N H a n d N H l ) t o 0 ( N ^ , +1 (N 0),+3 3  2  (NO2), a n d + 5 ( N 0 ) . In t h e p o r e w a t e r s o f fully a n o x i c m a r i n e s e d i m e n t s , t h e p r i n c i p a l 3  s p e c i e s a r e N a n d N H l (Thorstenson 1970; R i t t e n b e r g e t a l . 1955). The l a c k o f nitrate a n d 2  nitrite is d u e t o r a p i d b a c t e r i a l r e d u c t i o n o f t h e s e s p e c i e s t o N a n d N H 4 . Fig. 3-1 s h o w s 2  t h e redox c y c l e for nitrogen. Most of t h e key redox reactions of nitrogen a r e carried out in n a t u r e a l m o s t e x c l u s i v e l y b y m i c r o o r g a n i s m s . N i t r o g e n m a y e n t e r a l a k e a s d i s s o l v e d N . nitric a c i d , N H 4 . N O 3 . a s NH4 a d s o r b e d t o i n o r g a n i c p a r t i c u l a t e m a t t e r , a n d as o r g a n i c 2  c o m p o u n d s , w h i c h c a n o c c u r in e i t h e r d i s s o l v e d o r p a r t i c u l a t e p h a s e s ( W e t z e l 1983). Thermodynamically,  N  2  is t h e m o s t s t a b l e f o r m o f n i t r o g e n , a n d it is t o this f o r m t h a t  n i t r o g e n will revert u n d e r e q u i l i b r i u m c o n d i t i o n s . H e n c e t h e m a j o r reservoir for n i t r o g e n o n e a r t h is t h e a t m o s p h e r e . The d i s s o l v e d o r g a n i c n i t r o g e n ( D O N ) of fresh w a t e r s o f t e n constitutes o v e r 5 0 % of t h e t o t a l s o l u b l e n i t r o g e n ( W e t z e l 1983). G e o g r a p h i c v a r i a t i o n is g r e a t , h o w e v e r , in relation t o inputs o f i n o r g a n i c n i t r o g e n f r o m n a t u r a l a n d artificial s o u r c e s . O v e r one-half of t h e D O N is in t h e f o r m o f a m i n o n i t r o g e n c o m p o u n d s , of w h i c h a b o u t two-thirds is in t h e f o r m o f p o l y p e p t i d e s a n d c o m p l e x o r g a n i c c o m p o u n d s , a n d less t h a n  one-third  o c c u r s a s f r e e a m i n o n i t r o g e n (Wetzel 1983). Ammonification (Deamination) A m m o n i u m is g e n e r a t e d primarily b y h e t e r o t r o p h i c b a c t e r i a a s a p r i m a r y e n d p r o d u c t o f d e c o m p o s i t i o n o f o r g a n i c m a t t e r , e i t h e r d i r e c t l y f r o m proteins or f r o m o t h e r n i t r o g e n o u s o r g a n i c c o m p o u n d s . I n t e r m e d i a t e n i t r o g e n c o m p o u n d s a r e f o r m e d in t h e progressive  degradation  of o r g a n i c  m a t e r i a l , b u t rarely  accumulate,  because  d e a m i n a t i o n b y b a c t e r i a p r o c e e d s r a p i d l y . A l t h o u g h a m m o n i u m is a m a j o r e x c r e t o r y p r o d u c t o f a q u a t i c a n i m a l s , this n i t r o g e n s o u r c e is q u a n t i t a t i v e l y m i n o r in c o m p a r i s o n t o t h a t g e n e r a t e d b y b a c t e r i a l d e c o m p o s i t i o n ( W e t z e l 1983). S i n c e NO3" must b e r e d u c e d t o NI-£ b e f o r e it c a n b e a s s i m i l a t e d b y p l a n t s , a m m o n i u m is a n e n e r g y - e f f i c i e n t s o u r c e of n i t r o g e n f o r plants. T h e r e f o r e , v e r y l o w c o n c e n t r a t i o n s o f N H l a r e g e n e r a l l y f o u n d in s u r f a c e o x y g e n a t e d w a t e r s , a s t h e i o n is p r e f e r e n t i a l l y t a k e n u p b y p h y t o p l a n k t o n . W h e n a p p r e c i a b l e a m o u n t s of s e d i m e n t i n g o r g a n i c matter r e a c h t h e hypolimnion of stratified lakes, NH4 c a n a c c u m u l a t e . U n d e r a n a e r o b i c c o n d i t i o n s , b a c t e r i a l nitrification of NH4 t o N02" a n d N O 3 c e a s e s . Nitrogen Fixation A s m a l l n u m b e r o f o r g a n i s m s (primarily t h e p h o t o s y n t h e t i c c y a n o b a c t e r i a ) a r e a b l e t o f i x ' n i t r o g e n a n d r e d u c e it t o a m m o n i u m . H o w e v e r , a m m o n i u m f o r m e d in this  Fig. 3-1 The r e d o x c y c l e o f n i t r o g e n (from Brock 1979)  m a n n e r w o u l d b e f a r less a b u n d a n t t h a n t h a t f o r m e d b y t h e d e c o m p o s i t i o n of originallyd e p o s i t e d o r g a n i c n i t r o g e n c o m p o u n d s . O r g a n i c n i t r o g e n in a n a e r o b i c s e d i m e n t s is g e n e r a l l y o v e r a t h o u s a n d times m o r e a b u n d a n t t h a n d i s s o l v e d N ( a n d N0 ~) (Berner 2  3  1971). Nitrate Reduction a n d Denrtrification A s n i t r a t e is a s s i m i l a t e d b y a l g a e a n d l a r g e r h y d r o p h y t e s , it is r e d u c e d t o a m m o n i u m . A s m u c h a s 6 0 % o f t h e p h o t o a s s i m i l a t e d NO3" c a n b e e x c r e t e d a s d i s s o l v e d o r g a n i c n i t r o g e n c o m p o u n d s , s o m e of w h i c h a r e s i m p l e a m i n o a c i d s readily utilized b y b a c t e r i a ( C h a n a n d C a m p b e l l 1978). In lakes a n d o c e a n s , nitrate assimilation c a n g r e a t l y e x c e e d s o u r c e s of i n c o m e a n d r e g e n e r a t i o n , in s o m e c a s e s t o t h e point of r e d u c i n g NO3 t o b e l o w d e t e c t a b l e c o n c e n t r a t i o n s . In s e d i m e n t s , t w o m a j o r p a t h w a y s o f NO3" dissimilatory r e d u c t i o n h a v e  been  i d e n t i f i e d : d e n i t r i f i c a t i o n a n d "nitrate a m m o n i f i c a t i o n " (NO3" r e d u c t i o n t o NH4). W h i l e s o m e of t h e controlling factors h a v e b e e n identified for t h e t w o processes, a d e t a i l e d u n d e r s t a n d i n g o f t h e b a c t e r i a l g r o u p s i n v o l v e d - their m u t u a l i n t e r a c t i o n s , a n d their e c o l o g i c a l s i g n i f i c a n c e in n a t u r a l s e d i m e n t s is still l a c k i n g ( S 0 r e n s e n 1987). "Nitrate a m m o n i f i c a t i o n " , w h i c h has b e e n r e c o g n i z e d rather recently, seems t o b e most i m p o r t a n t in o r g a n i c - r i c h a n d relatively r e d u c e d s e d i m e n t s . The first e v i d e n c e f o r s i g n i f i c a n t NO3" dissimilation t o N H l w a s o b t a i n e d in a n o r g a n i c - r i c h a n d relatively r e d u c e d s e d i m e n t (Koike a n d Hattori 1978; S 0 r e n s e n 1978). The "nitrate a m m o n i f i c a t i o n " p a t h w a y : NO3 -* NO2 -> NH4 w a s originally o b s e r v e d in f e r m e n t i n g b a c t e r i a ( H a s a n a n d Hall 1975). These c o u l d use NO3 or NO2 a s " e l e c t r o n sinks" t o reoxidize N A D H , b u t w i t h o u t a d i r e c t c o u p l i n g t o ATP p r o d u c t i o n ( a s in t r u e ' d e n i t r i f i c a t i o n ) ( C o l e a n d B r o w n 1980). The r e d u c t i o n s t e p f r o m NC^ t o NH4 w a s later f o u n d t o o c c u r in a l a r g e n u m b e r o f b a c t e r i a w h i c h c a n respire with N C b , b u t d o n o t denitrify (Herbert e t a l . 1980; M a c F a r l a n e a n d H e r b e r t 1982). S e v e r a l strains o f Desulfovibrio  a p p e a r t o s w i t c h t o nitrate r e d u c t i o n w h e n s u l p h a t e is d e p l e t e d .  The dissimilatory r e d u c t i o n o f nitrate t o a m m o n i u m is n o t a l w a y s a respiratory p r o c e s s , s i n c e t h e n i t r a t e m a y a l s o f u n c t i o n a s a n e l e c t r o n sink in b a c t e r i a l f e r m e n t a t i o n s ( J 0 r g e n s e n a n d S 0 r e n s e n 1985).  Denitrificcrtion b y b a c t e r i a is t h e b i o c h e m i c a l r e d u c t i o n o f t h e o x i d i z e d n i t r o g e n a n i o n s , N O 3 a n d NO2, w i t h c o n c o m i t a n t o x i d a t i o n o f o r g a n i c m a t t e r . T h e g e n e r a l s e q u e n c e o f this p r o c e s s is: NO3' - * NO2  - * N2O -* Nb,  w h i c h results in a significant r e d u c t i o n o f c o m b i n e d n i t r o g e n t h a t c a n , in p a r t , b e lost f r o m t h e s y s t e m if it is n o t r e f i x e d . M a n y  Pseudomonas  facultative  anaerobic  bacteria, e.g.  s p p . , c a n utilize NO3* a s a n e x o g e n o u s t e r m i n a l H a c c e p t o r in t h e  o x i d a t i o n of o r g a n i c substrates ( A l e x a n d e r  1965). A n e x e m p l a r y r e a c t i o n of t h e  o x i d a t i o n o f g l u c o s e a n d c o n c o m i t a n t r e d u c t i o n of nitrate is: C i H u A + I2NO3" <-> I2NO2  + 6CQ2 +6H2O  A & = -1926kj.mol"',  a n d for t h e r e d u c t i o n o f nitrite t o m o l e c u l a r n i t r o g e n : C s H i i a +8NO2" «-* 4N2+CO2 + 4 C O t +6H2O  AG)=-3014kJ.mor . 1  A p p r o x i m a t e l y a s m u c h f r e e e n e r g y results a s in t h e a e r o b i c o x i d a t i o n o f g l u c o s e b y dissolved 0  2  ( A G o =-3190 k J ^ m o l " ) . T h e d e n i t r i f i c a t i o n r e a c t i o n s o c c u r i n t e n s e l y in 1  a n a e r o b i c e n v i r o n m e n t s , s u c h a s in t h e h y p o l i m n i a o f e u t r o p h i c lakes a n d in a n o x i c s e d i m e n t s , w h e r e o x i d i z a b l e o r g a n i c substrates a r e relatively a b u n d a n t . A b i o l o g i c a l N 0 r e d u c t i o n m a y a l s o o c c u r w h e r e possible r e d u c t a n t s c o u l d b e 3  r e d u c e d iron c o m p o u n d s w h i c h a c c u m u l a t e a t t h e l o w e r e d g e o f t h e N G v c o n t a i n i n g s u r f a c e l a y e r ( t h e " r e d o x c l i n e " ) . Nitrite s e e m s m o r e r e a c t i v e t h a n N 0 " t o w a r d s F e , b u t 3  2+  g e n e r a l l y v e r y little is k n o w n a b o u t r e a c t i o n kinetics (rates a n d p r o d u c t s ) a n d c a t a l y t i c e f f e c t s o f s u r f a c e s , e t c . (Sorensen 1987). Nitrification Nitrification m a y b e b r o a d l y d e f i n e d as t h e b i o l o g i c a l c o n v e r s i o n o f o r g a n i c a n d i n o r g a n i c nitrogenous c o m p o u n d s from a r e d u c e d state t o a m o r e oxidized state ( A l e x a n d e r 1965). The o v e r a l l r e a c t i o n c a n b e r e p r e s e n t e d as: N H l + I . 5 O 2 <-» 2 H + NG2"+H2O +  AG)=-276kJ.mof\  w h i c h p r o c e e d s b y a series o f o x i d a t i o n s t a g e s t h r o u g h h y d r o x y l a m i n e a n d p y r u v i c o x i m e t o nitrous a c i d : Nhtf -» N h b O H - * H2N2O2 - » HNO2. The nitrifying b a c t e r i a c a p a b l e o f t h e o x i d a t i o n o f NH4 -> NO2" a r e l a r g e l y c o n f i n e d t o  Nifrosomonas  s p p . O x i d a t i o n o f NO2" p r o c e e d s further t o N O 3 : N O " +O.5Q2 <-> N O i  AG =-75kJ»mol'\ J  Nitrobacter  is t h e p r i m a r y b a c t e r i a l g e n u s i n v o l v e d in this o x i d a t i o n . The o v e r a l l  nitrification r e a c t i o n , NHj  +2O2  <-»NOs" +HzO + 2 H  +  requires t w o m o l e s of 0 for t h e o x i d a t i o n of e a c h m o l e of N H l . A l t h o u g h c o n d i t i o n s must 2  b e a e r o b i c in o r d e r f o r n i t r i f i c a t i o n t o o c c u r , t h e s e p r o c e s s e s will c o n t i n u e  until  c o n c e n t r a t i o n s o f d i s s o l v e d O 2 d e c l i n e t o a b o u t 10 \iM ( D e v o l 1975, c i t e d in Murray et a l . 1978). This r e a c t i o n is e x c e e d i n g l y s l o w a t r o o m t e m p e r a t u r e a n d thus, nitrifying b a c t e r i a a c t a s c a t a l y s t s f o r t h e r e a c t i o n . B e c a u s e a m m o n i u m is r a p i d l y o x i d i z e d in n a t u r a l a e r o b i c w a t e r s , it d o e s n o t a c c u m u l a t e c o n c e n t r a t i o n s t o alter t h e p H .  in s u c h e n v i r o n m e n t s in sufficiently h i g h  3.2 Materials and Methods  Organic Carbon and Nitrogen W a t e r for particulate a n d dissolved o r g a n i c c a r b o n ( P O C  and DOC)  and  p a r t i c u l a t e o r g a n i c n i t r o g e n ( P O N ) a n a l y s e s w a s run directly f r o m 1.8 L Niskin bottles into well-rinsed  glass  containers  that  had  previously  been  used  only  for  storing  c o n c e n t r a t e d HCI a n d h e n c e s h o u l d h a v e b e e n c a r b o n - f r e e . The s a m p l e s w e r e k e p t o n i c e until t h e y c o u l d b e filtered later in t h e l a b . P O C a n d P O N s a m p l e s w e r e c o l l e c t e d o n p r e - c o m b u s t e d (4 hours a t 450°C) glass fibre ( G F / C ) filters with a n o m i n a l p o r e size of 1 u.m. The filters w e r e t h e n p l a c e d in p r e - c o m b u s t e d a l u m i n i u m foil p o u c h e s a n d f r o z e n . The filtrate w a s c o l l e c t e d for D O C analysis in 125 m L a c i d - c l e a n e d p y r e x bottles. All s a m p l e s w e r e f r o z e n i m m e d i a t e l y after filtering. P O C a n d P O N w e r e d e t e r m i n e d v i a c o m b u s t i o n in a Perkin-Elmer C H N A n a l y z e r . Filters w e r e t h a w e d a n d d r i e d for 24 - 48 hours in a n o v e n at 60°C. The filters w e r e t h e n a c i d i f i e d b y e x p o s i n g t h e m t o HCI f u m e s for 24 hours t o r e m o v e a n y c a r b o n a t e - c a r b o n p r e s e n t . Filters w e r e c o m b u s t e d a t 750°C a n d filter b l a n k s w e r e s u b t r a c t e d f r o m t h e s a m p l e s . S t a n d a r d s c o n s i s t e d of c a r e f u l l y w e i g h e d a m o u n t s of a c e t a n a l i d e (71.09% C a n d 10.36% N). All s a m p l e s w e r e w e l l o v e r t h e d e t e c t i o n limit a n d t h e p r e c i s i o n w a s 5% (1 a , rsd). D O C w a s a n a l y s e d using a n O c e a n o g r a p h y International Total C a r b o n System. A f t e r t h a w i n g t h e s a m p l e , 5 mL w a s p l a c e d in a n a m p o u l e c o n t a i n i n g a p p r o x i m a t e l y 0.2 g p o t a s s i u m p e r s u l p h a t e . T h e n 200 u.L of 8% H3PO4 w a s a d d e d a n d t h e solution p u r g e d w i t h p u r i f i e d o x y g e n for 6 m i n u t e s t o r e m o v e a n y c a r b o n a t e ions. The a m p o u l e w a s s e a l e d a n d t h e o r g a n i c c a r b o n o x i d i z e d t o c a r b o n d i o x i d e b y t h e p e r s u l p h a t e in a n a u t o c l a v e (1 hr a n d 20 min. a t 20 - 30 psi). All g l a s s w a r e a n d tools u s e d in t h e a m p o u l i n g p r o c e s s w e r e c o m b u s t e d a t 500°C for 4 hours prior t o use. The a m p o u l e w a s t h e n t a p p e d , a n d t h e CO2  p r e s e n t c a r r i e d t h r o u g h a non-dispersive infra r e d (IR) a n a l y z e r .  O r g a n i c c a r b o n Is thus r e a d o u t d i r e c t l y a s a f u n c t i o n o f CO2  c o n c e n t r a t i o n . The  d e t e c t i o n limit w a s 0.2 mg^L" a n d t h e p r e c i s i o n w a s 5% (1 a , rsd) for s a m p l e s < 2 mg»L 1  _1  a n d 1% (1 a , rsd) f o r m o r e c o n c e n t r a t e d s a m p l e s . S t a n d a r d s w e r e p r e p a r e d f r o m d e x t r o s e solutions in freshly d e i o n i z e d "Milli Q " w a t e r a n d w e r e run w i t h t h e s a m p l e s  t h r o u g h t h e s a m e p r o c e d u r e . The Milli Q w a t e r w a s c h e c k e d f o r c a r b o n c o n t e n t a n d w a s f o u n d t o h a v e a z e r o b l a n k a t t h e sensitivity u s e d . Nutrients Water  samples  were  collected  in  10%-HCI-washed  polyethylene  p o l y p r o p y l e n e b o t t l e s a n d w a s f i l t e r e d t h r o u g h 0 . 4 u.m p o l y c a r b o n a t e  or  Nuclepore  m e m b r a n e s . A n y s u l p h i d e p r e s e n t w a s r e m o v e d b e f o r e analysis either b y p r e c i p i t a t i o n w i t h ZnAc2 o r p u r g i n g w i t h N . A l l nutrient s a m p l e s d i s c u s s e d in this c h a p t e r 2  were  a n a l y z e d i m m e d i a t e l y after c o l l e c t i o n . Nitrogen Nitrate a n d nitrite w e r e m e a s u r e d c o l o u r i m e t r i c a l l y a c c o r d i n g t o t h e s t a n d a r d p r o c e d u r e of S t r i c k l a n d a n d Parsons (1972).  Nitrate is r e d u c e d t o nitrite b y running it  t h r o u g h a c o l u m n c o n t a i n i n g c a d m i u m filings c o a t e d w i t h m e t a l l i c c o p p e r . Nitrite diazotizes with s u l p h a n i l a m i d e a n d c o u p l e s with n a p h t h y l e t h y l e n e d i a m i n e t o form a highly c o l o u r e d a z o d y e . The d e t e c t i o n limit f o r t h e t w o nutrients w a s 0.5 u.M a n d t h e p r e c i s i o n 1% (1 a , r s d ) . A m m o n i u m w a s d e t e r m i n e d a c c o r d i n g t o t h e c o l o u r i m e t r i c m e t h o d o f Koroleff (1976). In a slightly a l k a l i n e s o l u t i o n , a m m o n i u m r e a c t s w i t h h y p o c h l o r i t e f o r m i n g m o n o c h l o r a m i n e . w h i c h , in t h e p r e s e n c e o f p h e n o l , nitroprusside ions a n d a n e x c e s s of h y p o c h l o r i t e , gives i n d o p h e n o l b l u e . Interference with M g  2 +  andCa  2 +  ions in s e a w a t e r is  e l i m i n a t e d b y c o m p l e x i n g t h e m w i t h c i t r a t e . I f o u n d t h a t it w a s n e c e s s a r y t o dilute s a m p l e s w i t h D D W t o < 50 u.M b e f o r e r e a g e n t a d d i t i o n or full c o l o u r d e v e l o p m e n t d i d not o c c u r . T h e d e t e c t i o n limit w a s 1 u M a n d t h e p r e c i s i o n 1% (1 a , rsd). Dissolved Silicon Dissolved silicon w a s m e a s u r e d b y t h e m e t h o d o f Strickland a n d Parsons (1972). S a m p l e s r e a c t w i t h m o l y b d a t e u n d e r c o n d i t i o n s w h i c h result in t h e f o r m a t i o n o f t h e silicomolybdate, phosphomolybdate a n d arsenomolybdate complexes. A reducing solution, containing  m e t o l a n d o x a l i c a c i d , is t h e n  added  which  reduces the  silicomolybdate c o m p l e x t o give a blue colour a n d simultaneously d e c o m p o s e s a n y p h o s p h o m o l y b d a t e or a r s e n o m o l y b d a t e , so that interference from p h o s p h a t e a n d a r s e n a t e is e l i m i n a t e d . I f o u n d t h a t s a m p l e s must b e d i l u t e d t o < 100 u.M with D D W b e f o r e r e a g e n t a d d i t i o n t o a v o i d u n d e r e s t i m a t i n g c o n c e n t r a t i o n s . The d e t e c t i o n limit w a s 0.5 u M a n d t h e p r e c i s i o n w a s 1% (1 a , rsd).  Alkalinity Alkalinity w a s d e t e r m i n e d b y t h e p o t e n t i o m e t r i c titration m e t h o d of G i e s k e s a n d Rogers (1973), w h i c h m e a s u r e s t h e n u m b e r of e q u i v a l e n t s of a c i d n e c e s s a r y t o titrate 1 L o f s e a w a t e r a t 2 5 ° C t o t h e b i c a r b o n a t e e n d p o i n t . S a m p l e s w e r e c o l l e c t e d in prew e i g h e d c o n t a i n e r s s o t h a t titrations c o u l d b e c a r r i e d o u t in t h e o r i g i n a l c o n t a i n e r s t o ensure t h a t a n y CaCG*3 w h i c h m i g h t h a v e p r e c i p i t a t e d d u r i n g s t o r a g e w a s i n c l u d e d in t h e analysis. S a m p l e s w e r e r a p i d l y t i t r a t e d t o a p H o f a p p r o x i m a t e l y 3.5 w i t h a c i d of k n o w n c o n c e n t r a t i o n (0.10042 N f o r l o w alkalinity s a m p l e s a n d 0.5454 N f o r m o r e c o n c e n t r a t e d s a m p l e s ) , w h i l e stirring v i g o r o u s l y . The s a m p l e w a s t h e n a l l o w e d t o stabilize b e f o r e c o n t i n u i n g t h e titration in small i n c r e m e n t s , r e c o r d i n g t h e v o l u m e a d d e d a n d its c o r r e s p o n d i n g EMF b e t w e e n p H 3.5 a n d 2.5. A t least 12 points w e r e r e c o r d e d for t h e G r a n plot (Dyrssen 1965) w h i c h is u s e d t o d e t e r m i n e t h e b i c a r b o n a t e e n d p o i n t of t h e titration b y e x t r a p o l a t i n g t h e linear p o r t i o n of a G r a n f u n c t i o n . The p r e c i s i o n w a s 0 . 3 % (1 a , rsd). PH B e c a u s e t h e r e w a s n o p o w e r s u p p l y o n t h e b o a t , p H w a s m e a s u r e d using colorpHasf® p H p a p e r ( M e r c k ) b y running w a t e r directly o u t of t h e Niskin b o t t l e o n t o t h e p a p e r . N o d i f f e r e n c e w a s f o u n d w h e n t h e p H w a s m e a s u r e d in a g l o v e b a g w h e r e t h e s a m p l e w a s n o t e x p o s e d t o air. Precision w a s a p p r o x i m a t e l y 0.1 p H unit.  3.3 Results  Organic Carbon a n d Nitrogen Powell Lake From a s u r f a c e c o n c e n t r a t i o n of 5.6 mg»L"\ d i s s o l v e d o r g a n i c c a r b o n ( D O C ) d e c r e a s e s t o 1.6 mg«L"' in t h e u p p e r l o w salinity layer of P o w e l l L a k e . In t h e a n o x i c layer, D O C increases rapidly t o nearly 50 mg«L'' a t 290 m (Fig. 3-2). Thereafter D O C d e c r e a s e s in t h e b o t t o m 15 m t o 23 mg«L"\ P a r t i c u l a t e o r g a n i c c a r b o n ( P O C ) s h o w s a similar distribution t o t h a t of D O C , i n c r e a s i n g a t d e p t h t o a m a x i m u m of 280 u.g»L'' a t 330 m a n d s u b s e q u e n t l y d e c r e a s i n g t o 211 u.g«L  0  at the bottom. D O C : P O C  ratios v a r y f r o m  a p p r o x i m a t e l y 3 0 in t h e o x i c p a r t of t h e w a t e r c o l u m n t o a h i g h of 186 in t h e b o t t o m water. The p a r t i c u l a t e o r g a n i c n i t r o g e n ( P O N ) profile is similar, a l t h o u g h n o t i d e n t i c a l , t o t h a t o f P O C (Fig. 3-4). C o n c e n t r a t i o n s r a n g e f r o m 4 t o 18 ng»L"'. The C : N w e i g h t ratio (all C:N  ratios in this c h a p t e r  are based  o n weight)  o f t h e p a r t i c u l a t e f r a c t i o n is  a p p r o x i m a t e l y 10 in t h e u p p e r , o x i c w a t e r s a n d g r a d u a l l y i n c r e a s e s t o 20 a t t h e b o t t o m . Sakinaw Lake Dissolved o r g a n i c c a r b o n ( D O C ) r e m a i n s c o n s t a n t t h r o u g h o u t t h e o x i c portion of t h e w a t e r c o l u m n a t a p p r o x i m a t e l y 3 mg«L"' (Fig. 3-3). It t h e n starts t o i n c r e a s e a b o u t 10 m b e l o w t h e o x i c / a n o x i c i n t e r f a c e t o a m a x i m u m of 16 mg«L*\ P a r t i c u l a t e  organic  c a r b o n ( P O C ) s h o w s t w o l a r g e m a x i m a , a t 10 m a n d a t t h e i n t e r f a c e , a n d t h e n is fairly c o n s t a n t in t h e a n o x i c w a t e r s , a t a b o u t 240 u.g»L"\ The D O C : P O C ratio r a n g e s from 7 a t t h e i n t e r f a c e t o 70 in t h e b o t t o m w a t e r . The p a r t i c u l a t e o r g a n i c n i t r o g e n ( P O N ) profile m i m i c s t h a t of P O C (Fig. 3-5), r a n g i n g f r o m a b o u t 22 t o 80 u.g»L"'. Thus t h e C : N profile is r e a s o n a b l y c o n s t a n t with d e p t h a t 5.5 t o 6.5. Nutrients Powell Lake Nitrate c o n c e n t r a t i o n s i n c r e a s e f r o m a s u r f a c e v a l u e of 2.9 u M t o a l m o s t 9 u M a t 50 m (Fig. 3-6). It t h e n d e c r e a s e s g r a d u a l l y t o z e r o c o n c e n t r a t i o n a t t h e i n t e r f a c e . N o nitrite is d e t e c t a b l e a n y w h e r e in t h e w a t e r c o l u m n . A m m o n i u m is n o t d e t e c t a b l e until just a b o v e t h e i n t e r f a c e a t 150 m ; t h e r e a f t e r it i n c r e a s e s t o almost 4 m M in t h e b o t t o m  w a t e r . C o n c e n t r a t i o n s o f d i s s o l v e d silicon a r e a r o u n d 43 n M in t h e u p p e r o x i c layer, a n d g r a d u a l l y i n c r e a s e t o 300 nM a t t h e b o t t o m (Fig. 3-8). Sakinaw Lake Nitrite a n d nitrate a r e b o t h l o w in s u r f a c e w a t e r s , i n c r e a s i n g r a p i d l y just a b o v e t h e i n t e r f a c e a n d d e c r e a s i n g just a s q u i c k l y b e l o w it (Fig. 3-7). The m a x i m u m c o n c e n t r a t i o n of nitrite o b s e r v e d is 0.24 \iM. while t h a t o f nitrate is 7 jxM. B e l o w t h e i n t e r f a c e , a m m o n i u m r a p i d l y i n c r e a s e s t o nearly 8 m M in t h e b o t t o m w a t e r (Fig. 3-7). There is a slight d e c r e a s e in t h e b o t t o m w a t e r . D i s s o l v e d s i l i c o n o c c u r s a t h i g h c o n c e n t r a t i o n s t h r o u g h o u t t h e w a t e r c o l u m n , rising q u i c k l y t o a b o u t 1000 n M in t h e b o t t o m w a t e r (Fig. 3-9). In the s u r f a c e w a t e r s it is a p p r o x i m a t e l y 4 0 | i M , similar t o t h e c o n c e n t r a t i o n in t h e u p p e r 150 m o f t h e Powell Lake w a t e r c o l u m n . Alkalinity and pH Powell Lake Alkalinity is v e r y l o w in s u r f a c e w a t e r s , i n c r e a s i n g g r a d u a l l y b e l o w t h e o x y c l i n e t o a m a x i m u m o f 31 m e q u i v - L " (Fig. 3-10). The p H w a s s t e a d y a t 5.7 d o w n t o 175 m (Fig. 31  10). It t h e n increases g r a d u a l l y t o a m a x i m u m o f 6.7 in t h e b o t t o m 90 m. Sakinaw Lake Alkalinity is v e r y l o w in s u r f a c e w a t e r s a n d g r a d u a l l y i n c r e a s e s w i t h d e p t h t o a m a x i m u m o f 27 mequiv«L'' (Fig. 3-11). The p H is c o n s t a n t in t h e u p p e r 37.5 m a t 5.7 a n d i n c r e a s e s s h a r p l y a t 4 0 m t o 6.5, t h e n c e r e m a i n i n g c o n s t a n t t o t h e b o t t o m of t h e l a k e (Fig. 3-11).  0  103  200  300  400  500  POCCug.L" ) 1  DOC/POC Fig. 3-2 Dissolved a n d p a r t i c u l a t e o r g a n i c c a r b o n in P o w e l l L a k e  Fig. 3-3 Dissolved a n d p a r t i c u l a t e o r g a n i c c a r b o n in S a k i n a w L a k e  0  5  10  15  20  P O N (jig.L" ) P O C / P O N (wt. ratio) 1  Fig. 3-4 Particulate o r g a n i c c a r b o n a n d nitrogen in P o w e l l L a k e  25  0  10  2 0 3 0 4 0 5 0 6 0 7 0 8 0 PON^g.L* ) 1  P O C / P O N (wt. ratio) Fig. 3-5 P a r t i c u l a t e o r g a n i c c a r b o n a n d n i t r o g e n in S a k i n a w L a k e  Fig. 3-6 Nitrate a n d a m m o n i u m in Powell Lake (nitrite u n d e t e c t a b l e )  Fig. 3-7 Nitrate, nitrite a n d a m m o n i u m in S a k i n a w L a k e  Fig. 3-8 Dissolved silicon in Powell Lake  Fig. 3-10 Alkalinity a n d p H in Powell Lake  Fig. 3-11 Alkalinity a n d p H in S a k i n a w L a k e  3.4 Discussion  Organic Carbon B e c a u s e most l a k e s a r e s m a l l , w i t h a h i g h p r o p o r t i o n of their s u r f a c e a r e a a s littoral z o n e , m u c h of t h e detrital c a r b o n is of a l l o c h t h o n o u s terrestrial origin. H o w e v e r , in l a r g e , s t e e p w a l l e d fjord-lakes s u c h a s P o w e l l a n d S a k i n a w , t h e littoral z o n e is small c o m p a r e d t o t h e l a r g e s u r f a c e a r e a , a n d t h e bulk of t h e c a r b o n is d e r i v e d f r o m a u t o c h t h o n o u s s o u r c e s . This is e s p e c i a l l y true w h e r e t h e r e is little f r e s h w a t e r input t o t h e l a k e a s in S a k i n a w , o r w h e r e t h e l a k e b a s i n is f a r r e m o v e d f r o m t h e f r e s h w a t e r s o u r c e , a s in P o w e l l . A u t o c h t h o n o u s s o u r c e s o f c a r b o n in lakes i n c l u d e : 1) littoral ( s h o r e l i n e )  where P O M a n d D O M are p r o d u c e d  by active  secretion  (extracellular r e l e a s e ) a n d autolysis of t h e m a c r o p h y t e s a n d a t t a c h e d m i c r o f l o r a ; 2) p r i m a r y  producers  chemosynthetic  including algal  phytoplankton, a n d photosynthetic  and  bacteria.  B e c a u s e o f t h e l a r g e a n o x i c z o n e in b o t h l a k e s , c h e m o s y n t h e t i c  bacteria may  c o n t r i b u t e c o n s i d e r a b l y t o t h e c a r b o n p o o l ; h o w e v e r , t h e m a j o r c a r b o n s o u r c e is p r o b a b l y t h a t g e n e r a t e d p h o t o s y n t h e t i c a l l y b y a l g a e in t h e e u p h o t i c z o n e . transformations b e t w e e n  P O C a n d D O C by heterotrophic microbes  Rapid  progressively  d e g r a d e o r g a n i c m a t t e r t o CO2 a n d h e a t . In s h a l l o w , small v o l u m e l a k e s , t h e bulk of h e t e r o t r o p h i c d e c o m p o s i t i o n o c c u r s in t h e s e d i m e n t s . H o w e v e r , a s t h e d e p t h a n d volume  i n c r e a s e , t h e o p e n w a t e r b e c o m e s t h e d o m i n a n t site o f h e t e r o t r o p h i c  m e t a b o l i s m , w h i c h is a l m o s t c o m p l e t e l y m i c r o b i a l . T h e a m o u n t of o r g a n i c  carbon  utilized a n d t r a n s f o r m e d b y a n i m a l s is q u a n t i t a t i v e l y a small p o r t i o n of t h a t of t h e w h o l e system. DOC The D O C p o o l consists primarily o f c a r b o n c o m p o u n d s w h i c h a r e relatively resistant t o b a c t e r i a l d e c o m p o s i t i o n . The major s o u r c e s t o t h e D O C p o o l a r e : 1) p h o t o s y n t h e t i c inputs o f t h e littoral a n d p e l a g i c f l o r a a d d e d t o t h e p o o l t h r o u g h s e c r e t i o n s a n d autolysis o f c e l l u l a r c o n t e n t s ; 2) a l l o c h t h o n o u s D O C , c o m p o s e d l a r g e l y of terrestrial h u m i c s u b s t a n c e s r e f r a c t o r y t o rapid bacterial degradation; 3) e x c r e t i o n s o f z o o p l a n k t o n a n d h i g h e r a n i m a l s ; a n d  4 ) b a c t e r i a l c h e m o s y n t h e s i s of o r g a n i c m a t t e r w i t h s u b s e q u e n t r e l e a s e of D O C . Phytoplanktonic productivity a n d allochthonous sources from t h e d r a i n a g e basin are t h e p r i m a r y s o u r c e s of t h e D O C p o o l of o l i g o t r o p h i c w a t e r s s u c h as P o w e l l a n d Sakinaw. In l a k e s o f this size, p h y t o p l a n k t o n i c photosynthesis a l o n e c a n d o m i n a t e inputs t o t h e D O C p o o l ; h o w e v e r , b a c t e r i a l p r o d u c t i o n m a y a l s o b e v e r y i m p o r t a n t in P o w e l l a n d S a k i n a w , g i v e n t h e u n i q u e c h a r a c t e r i s t i c s of t h e s e lakes. The D O C profiles in b o t h P o w e l l a n d S a k i n a w Lakes a r e similar in s h a p e t o those of t h e m a j o r ions ( c o m p a r e Figs. 3-2 a n d 3-3 w i t h Figs. 2-7 t o 2-17). In b o t h l a k e s , D O C i n c r e a s e s in t h e a n o x i c p o r t i o n of t h e w a t e r c o l u m n , w i t h t h e i n c r e a s e b e i n g m u c h m o r e r a p i d in S a k i n a w . This is a l s o similar t o t h e distribution o f D O C in p o r e w a t e r s in most a n o x i c m a r i n e s e d i m e n t s , w h e r e a l a r g e i n c r e a s e in t h e D O C c o n c e n t r a t i o n across t h e s e d i m e n t / w a t e r i n t e r f a c e is t y p i c a l l y s e e n ( e . g . N i s s e n b a u m e t a l . 1972; K r o m a n d Sholkovitz 1977; O r e m et a l . 1986). This is most p r o b a b l y d u e t o i n c o m p l e t e m i n e r a l i z a t i o n of s e d i m e n t a r y o r g a n i c m a t t e r b y a n a e r o b i c b a c t e r i a in t h e s e d i m e n t s (Otsuki a n d H a n y a 1972a,b). The c o n c e n t r a t i o n s of D O C f o u n d in P o w e l l a n d S a k i n a w L a k e s a r e t y p i c a l of t h o s e f o u n d in a n o x i c p o r e w a t e r s in s e d i m e n t s . For e x a m p l e , E m e r s o n (1976) f o u n d 20 m g . L " D O C in G r e i f e n s e e s e d i m e n t p o r e w a t e r s . K r o m a n d Sholkovitz (1977) m e a s u r e d 1  D O C in L o c h D u i c h , a fjord-type estuary h a v i n g a solid-phase o r g a n i c c a r b o n c o n t e n t of a b o u t 5%. They f o u n d t h a t t h e D O C i n c r e a s e d regularly with d e p t h in a n o x i c p o r e w a t e r s f r o m a s u r f a c e v a l u e o f a b o u t 15 mg-L" t o a m a x i m u m of 70 mg»L'' in t h e m e t h a n o g e n i c 1  z o n e . In o x i c c o r e s , t h e p o r e w a t e r D O C a l s o i n c r e a s e d f r o m s u r f a c e v a l u e s o f a b o u t 6 m g . L ' t o 16 m g « L ' \ s h o w i n g t h a t t h e r e must h a v e b e e n a n a u t o c h t h o n o u s p r o d u c t i o n 1  a n d a c c u m u l a t i o n o f D O C e v e n w i t h i n o x i c c o r e s w h e r e t h e r e is n o m e a s u r e d a c c u m u l a t i o n o f alkalinity or p h o s p h a t e . In M a n g r o v e L a k e , a s h a l l o w s a l i n e l a k e in B e r m u d a , O r e m e t a l . (1986) f o u n d c o n c e n t r a t i o n s of D O C r a n g i n g f r o m less t h a n 4 mg«L  _1  in t h e o v e r l y i n g w a t e r t o g r e a t e r t h a n 40 mg«L"' in t h e p o r e w a t e r a t a d e p t h of a b o u t 200 c m . This l a k e is m u c h m o r e p r o d u c t i v e t h a n e i t h e r P o w e l l o r S a k i n a w L a k e s , w i t h s e d i m e n t s c o n t a i n i n g 20 t o 2 5 % o r g a n i c c a r b o n ( s e d i m e n t s in t h e s o u t h b a s i n of P o w e l l L a k e c o n t a i n a b o u t 1 5 % o r g a n i c c a r b o n (Barnes a n d Barnes 1981)). A l t h o u g h M a n g r o v e L a k e is m u c h s m a l l e r t h a n b o t h P o w e l l a n d S a k i n a w , t h e o r g a n i c m a t t e r is a l s o p r e d o m i n a n t l y a l g a l in origin ( H a t c h e r et a l . 1983) a n d h e n c e s h o u l d b e e q u a l l y l a b i l e .  Lyons a n d G a u d e t t e (1979) h a v e s h o w n t h a t t h e n a t u r e of t h e o r g a n i c m a t t e r in m a r i n e s e d i m e n t s h a s a d o m i n a n t e f f e c t o n t h e rates of s u l p h a t e r e d u c t i o n . O r g a n i c m a t t e r d e r i v e d f r o m a l g a l r e m a i n s is m o r e r e a d i l y d e g r a d e d b y s e d i m e n t a r y b a c t e r i a t h a n o r g a n i c m a t t e r d e r i v e d f r o m v a s c u l a r plants. The r a t e of d e g r a d a t i o n d e c r e a s e s as o r g a n i c m a t t e r is progressively m e t a b o l i z e d : b a s e d o n a c o m p i l a t i o n o f a w i d e r a n g e of d a t a . E m e r s o n a n d H e d g e s (1988) f o u n d a n e a r l y o n e - t o - o n e inverse relationship b e t w e e n d e g r a d a t i o n r a t e a n d t h e a g e of t h e o r g a n i c m a t t e r o v e r m a n y orders of magnitude. In a s t u d y of o v e r 500 Wisconsin lakes. W e t z e l (1983) f o u n d a r a n g e of dissolved o r g a n i c c a r b o n c o n t e n t s o f t h e w a t e r s o f 1 t o 30 mg-L" with a n a v e r a g e of 15 mg»L'\ The 1  c o n c e n t r a t i o n s of D O C f o u n d in S a k i n a w fall within this r a n g e . H o w e v e r , h i g h e r D O C levels o c c u r in t h e b o t t o m w a t e r s o f P o w e l l L a k e , in s p i t e o f t h e p r e s e n c e  of  lower  c o n c e n t r a t i o n s o f r e m i n e r a l i z e d nutrients c o m p a r e d w i t h S a k i n a w ( c o m p a r e Fig. 3-7 a n d 3-6). The h i g h e r nutrient v a l u e s i n d i c a t e t h a t S a k i n a w is a m o r e p r o d u c t i v e l a k e , s i n c e h i g h e r c o n c e n t r a t i o n s of nutrients in t h e d e e p w a t e r s r e f l e c t a l a r g e r s u p p l y of nutrients a n d a l a r g e c o n c o m i t a n t r e m i n e r a l i z a t i o n of o r g a n i c m a t t e r . G e n e r a l l y , h i g h e r c o n c e n t r a t i o n s of D O C w o u l d a l s o b e e x p e c t e d t o r e f l e c t h i g h e r p r o d u c t i v i t y , as t h e primary  origin of  this m a t e r i a l  is r e l e a s e  by  phytoplankton  and  incomplete  mineralization. T h e r e f o r e , g i v e n a higher settling flux of p h y t o p l a n k t o n s e s t o n , m o r e D O C m i g h t b e e x p e c t e d in t h e h y p o l i m n i o n . The p r i m a r y r e m o v a l m e c h a n i s m for D O C is h e t e r o t r o p h i c u p t a k e b y b a c t e r i a , either in t h e w a t e r c o l u m n or a t t h e s e d i m e n t s u r f a c e . S a k i n a w L a k e w o u l d a p p e a r t o h a v e h i g h e r levels of b a c t e r i a l a c t i v i t y in its b o t t o m w a t e r s , a s s u l p h i d e c o n c e n t r a t i o n s a r e t w i c e a s h i g h a s t h o s e in P o w e l l L a k e . H i g h e r s u l p h i d e levels in S a k i n a w L a k e c o u l d b e d u e t o l o w e r c o n c e n t r a t i o n s o f F e  2+  resulting in  less p r e c i p i t a t i o n o f F e S ( d i s c u s s e d in C h a p t e r 5) a n d thus w o u l d n o t r e f l e c t g r e a t e r 2  b a c t e r i a l activity. H o w e v e r , b e c a u s e a m m o n i u m c o n c e n t r a t i o n s a r e a l s o m u c h h i g h e r in S a k i n a w , it is m o r e likely t h a t t h e t h e a d d i t i o n a l s u l p h i d e reflects a g r e a t e r s u p p l y of s u l p h a t e d u e t o o c c a s i o n a l intrusions o f s e a w a t e r  into S a k i n a w L a k e resulting in  i n c r e a s e d b a c t e r i a l activity. M e t h a n e levels a r e p r o b a b l y a l s o m u c h h i g h e r in S a k i n a w as b o t t o m w a t e r s a m p l e s d e g a s s e d m u c h m o r e vigorously w h e n b r o u g h t t o the s u r f a c e . This b e h a v i o u r persists t h r o u g h o u t t h e w a t e r c o l u m n in S a k i n a w L a k e (from 4 0 m d o w n ) , w h e r e a s in P o w e l l it w a s o n l y o b s e r v e d in t h e d e e p e s t w a t e r s . T h e r e f o r e , e v e n  t h o u g h p h o t o s y n t h e t i c p r o d u c t i v i t y is p r o b a b l y h i g h e r in S a k i n a w L a k e . D O C m a y b e l o w e r d u e t o c o n s u m p t i o n b y g r e a t e r n u m b e r s of h e t e r o t r o p h i c b a c t e r i a . A l a r g e d e c r e a s e in D O C c o n c e n t r a t i o n is s e e n in P o w e l l L a k e just a b o v e t h e s e d i m e n t - w a t e r i n t e r f a c e (Fig. 3-2). B a c t e r i a a r e usually f o u n d in m u c h h i g h e r n u m b e r s in this r e g i o n , e v e n in lakes w i t h a n o x i c b o t t o m w a t e r s . It is p o s s i b l e t h a t h i g h e r b a c t e r i a l activities c o u l d result in t h e d e p l e t i o n of D O C , d y n a m i c a l l y m a i n t a i n i n g t h e l o w e r v a l u e s in b o t t o m w a t e r . H o w e v e r , h i g h e r n u m b e r s of b a c t e r i a s h o u l d b e r e f l e c t e d with a n i n c r e a s e in P O C . S i n c e P O C a l s o d e c r e a s e s m a r k e d l y just a b o v e t h e s e d i m e n t - w a t e r i n t e r f a c e (Fig. 3-2), this is a n unlikely e x p l a n a t i o n . It m a y b e t h a t this is just a n a r t e f a c t as it is b a s e d o n o n l y o n e m e a s u r e m e n t . POC A s n o t e d earlier, a u t o c h t h o n o u s primary p r o d u c t i o n b y t h e p h y t o p l a n k t o n i c a n d littoral f l o r a is t h e major c o n t r i b u t o r (> 9 0 % ) t o t h e P O C of n a t u r a l l a k e systems. In l a r g e lakes like P o w e l l a n d S a k i n a w , p h y t o p l a n k t o n a n d b a c t e r i a m a k e u p t h e bulk of P O C ( W e t z e l 1983). A l l o c h t h o n o u s P O C is v e r y s m a l l , e s p e c i a l l y w h e n t h e r e is v e r y little f r e s h w a t e r input. B o t h lakes h a v e t w o P O C m a x i m a (Figs. 3-2 a n d 3-3). The u p p e r m o s t m a x i m u m in e a c h l a k e reflects t h e p h y t o p l a n k t o n s t a n d i n g s t o c k a t t h e t i m e of s a m p l i n g . In S a k i n a w L a k e , this o c c u r s b e l o w t h e s u r f a c e p e r h a p s d u e t o p h o t o i n h i b i t i o n within t h e u p p e r m o s t m e t r e . P h y t o p l a n k t o n , a n d h e n c e P O C , t h e n d e c r e a s e s w i t h d e p t h as light b e c o m e s limiting f o r p h o t o s y n t h e s i s . P O C b e g i n s t o i n c r e a s e in b o t h l a k e s a t t h e o x i c / a n o x i c i n t e r f a c e . In P o w e l l L a k e , t h e s e c o n d P O C m a x i m u m o c c u r s at t h e p y c n o c l i n e (300 m) (Fig. 3-2), w h e r e t h e s h a r p i n c r e a s e in d e n s i t y c a n a c t a s a barrier t o t h e sinking of particles. The b u i l d u p of p a r t i c l e s , c o m b i n e d w i t h t h e l a r g e i n c r e a s e in salt, m a y i n d u c e f l o c c u l a t i o n , f o l l o w e d b y m o r e r a p i d p a r t i c l e sinking, s u c h a s is s e e n w h e n river w a t e r s a r e m i x e d with s e a w a t e r (Sholkovitz 1976). S e v e r a l authors ( C l o e r n et a l . 1983; H a m n e r et a l . 1982; C u l v e r a n d Brunskill 1969) h a v e f o u n d t h a t t h e c h e m o c l i n e a n d / o r p y c n o c l i n e in m e r o m i c t i c lakes m a y b e t h e site of l o c a l i z e d a c c u m u l a t i o n of s e s t o n , i n d i c a t i n g t h a t it retards t h e sinking of b i o g e n i c particles. In S a k i n a w L a k e , a l a r g e P O C m a x i m u m is s e e n a t t h e o x i c / a n o x i c i n t e r f a c e (Fig. 3-3). This is i n d i c a t i v e o f a l a r g e c o n c e n t r a t i o n of b a c t e r i a . In P o w e l l L a k e t h e i n t e r f a c e is s p r e a d o v e r 150 m a n d so t h e b a c t e r i a a r e not as spatially c o n c e n t r a t e d as in S a k i n a w ,  w h e r e t h e i n t e r f a c e is s h arp . The v e r y h i g h levels of P O C f o u n d a t t h e S a k i n a w i n t e r f a c e (> 400u.g»L" ), a r e most likely a t t r i b u t a b l e t o t h e p r e s e n c e of l a r g e n u m b e r s of sulphate1  r e d u c i n g a n d s u l p h i d e - o x i d i z i n g b a c t e r i a , w h i c h m e t a b o l i z e sulphur c o m p o u n d s t h a t diffuse a c r o s s t h e i n t e r f a c e , a s w e l l a s denitrifiers, w h i c h r e d u c e nitrate a n d nitrite a n d nitrifiers w h i c h o x i d i z e a m m o n i u m (Fig. 3-7). This d e p t h is b e l o w t h e e u p h o t i c z o n e , so n o p h o t o s y n t h e t i c sulphur b a c t e r i a w o u l d b e e x p e c t e d . By b e i n g a c t i v e sites o f nutrient r e g e n e r a t i o n a n d d e c o m p o s i t i o n for o r g a n i c m a t t e r p r o d u c e d in t h e t r o p h o g e n i c z o n e , c h e m o c l i n e s c a n a c t as e f f i c i e n t "filters" for sinking o r g a n i c m a t t e r ( C u l v e r a n d Brunskill 1969). If t h e c h e m o c l i n e is a n inefficient filter, t h e n t h e m o n i m o l i m n i o n c a n b e a sink for nutrients a n d o r g a n i c m a t t e r ; o n t h e o t h e r h a n d , if t h e c h e m o c l i n e is a n efficient filter, t h e n nutrient c y c l i n g in t h e m i x o l i m n i o n (the u p p e r m i x e d layer) is similar t o t h a t in h o l o m i c t i c lakes (lakes in w h i c h t h e entire w a t e r c o l u m n periodically circulates). C h e m o s y n t h e t i c a n d photosynthetic b a c t e r i a  can  a c c o u n t for 8 5 % of t h e t o t a l primary p r o d u c t i o n a t times a n d t h e s e o r g a n i s m s sink v e r y slowly, w h e r e a s p h y t o p l a n k t o n sink m u c h faster ( C l o e r n et a l . 1987). B e c a u s e nutrients a n d o r g a n i c m a t t e r r e a c h v e r y h i g h c o n c e n t r a t i o n s in t h e b o t t o m w a t e r s of S a k i n a w a n d P o w e l l , it w o u l d a p p e a r t h a t in b o t h lakes t h e c h e m o c l i n e o p e r a t e s as a n inefficient filter. A g e n e r a l l y similar distribution o c c u r s in Big S o d a L a k e , a n a l k a l i n e , s a l i n e l a k e in N e v a d a , w h e r e a l a r g e , l o c a l i z e d p o p u l a t i o n o f b a c t e r i a exists a t t h e c h e m o c l i n e ( O r e m l a n d et a l . 1988; Zehr et a l . 1987; C l o e r n et a l . 1987). This suggests t h a t t h e density discontinuity m i g h t b e a n i m p o r t a n t site of o r g a n i c m a t t e r mineralization. H o w e v e r , most P O C (-70%) r e a c h i n g t h e c h e m o c l i n e p a s s e s t h r o u g h it. s o o n l y a b o u t 3 - 5% of d a i l y p r o d u c t i v i t y a c c u m u l a t e s t h e r e . In a d d i t i o n , v e r t i c a l f l u x e s o f s e s t o n b e l o w  the  c h e m o c l i n e a r e a b o u t 9 0 % of t h o s e m e a s u r e d a b o v e it, thus t h e c h e m o c l i n e o f this l a k e a c t s as a n inefficient filter for sinking p a r t i c u l a t e matter. It a p p e a r s , t h e r e f o r e , t h a t a n e x t r e m e d e n s i t y discontinuity resulting f r o m a salinity c h a n g e of 4%o o v e r <1 m d o e s not g r e a t l y inhibit t h e sinking of bulk p a r t i c u l a t e o r g a n i c m a t t e r or p e l a g i c d i a t o m s . The c h e m o c l i n e in Big S o d a L a k e d o e s inhibit m o r e s t r o n g l y t h e sinking o f b a c t e r i a , h o w e v e r , a s e v i n c e d b y t h e a c c u m u l a t i o n o f b a c t e r i a l cells a t t h a t horizon. H o w e v e r , o b s e r v e d n o n - e l e v a t e d ATP a n d p r o t e i n c o n c e n t r a t i o n s ( O r e m l a n d et a l . 1988) a n d low t h y m i d i n e assimilation rates (Zehr et a l . 1987) i n d i c a t e t h a t t h e s e b a c t e r i a p l a y a minor role in l a k e m e t a b o l i s m a n d nutrient r e g e n e r a t i o n ( C l o e r n et a l . 1987). Zehr et a l . (1987)  speculate  that  fermentation  processes  are  the  predominant  mechanism  of  d e c o m p o s i t i o n of t h e a n a e r o b i c m i x o l i m n i o n in Big S o d a L a k e . Similar c o n s i d e r a t i o n s w o u l d a p p e a r t o b e a p p l i c a b l e to Sakinaw, a n d p r o b a b l y t o Powell Lake. DOC:POC P o w e l l a n d S a k i n a w Lakes, r a n g i n g f r o m 23 t o 190 in t h e f o r m e r a n d 7 t o 70 in t h e latter (Figs. 3-2 a n d 3-3); t h e s e d a t a i n d i c a t e t h a t b o t h lakes a r e l a r g e l y o l i g o t r o p h i a as t h e ratio o f D O C t o P O C is r a t h e r c o n s t a n t a t a b o u t 10:1 in m o s t u n p r o d u c t i v e t o m o d e r a t e l y p r o d u c t i v e l a k e s ( W e t z e l 1983). A s l a k e s b e c o m e m o r e e u t r o p h i c , t h e D O C : P O C ratio d e c r e a s e s a n d f l u c t u a t e s g r e a t l y w i t h s e a s o n a n d d e p t h ; t h e ratio  can  d e c r e a s e t o <1 ( W e t z e l 1983). The l o w e r D O C . P O C ratio a t t h e i n t e r f a c e in S a k i n a w reflects t h e l a r g e c o n c e n t r a t i o n of b a c t e r i a a n d h e n c e p a r t i c u l a t e c a r b o n . The h i g h e r ratio in P o w e l l relative t o S a k i n a w L a k e c a n b e e x p l a i n e d in t w o w a y s . First, d e e p w a t e r in P o w e l l L a k e h a s b e e n i s o l a t e d for a l o n g e r p e r i o d t h a n t h a t of S a k i n a w , a n d h e n c e has h a d m o r e t i m e for d i s s o l v e d o r g a n i c m a t t e r t o a c c u m u l a t e . S e c o n d , S a k i n a w a l m o s t c e r t a i n l y h a s h i g h e r c o n c e n t r a t i o n s of b a c t e r i a in t h e w a t e r c o l u m n t h a n P o w e l l , a n d thus m o r e c a r b o n w o u l d b e e x p e c t e d in t h e p a r t i c u l a t e f r a c t i o n . The c o n t r a s t b e t w e e n t h e t w o l a k e s m a y a l s o r e f l e c t h i g h e r p r o d u c t i v i t y in S a k i n a w , w i t h h i g h e r n u m b e r s of p h y t o p l a n k t o n sinking out a n d c o n t r i b u t i n g t o t h e P O C p o o l . The D O C : P O C ratios in S a k i n a w a r e m o r e t y p i c a l o f t h o s e f o u n d in m o d e r a t e l y p r o d u c t i v e l a k e s a n d s u g g e s t t h a t S a k i n a w is in f a c t m o r e p r o d u c t i v e t h a n P o w e l l . POCrPON P O N c o n c e n t r a t i o n s a r e m u c h h i g h e r in S a k i n a w t h a n in P o w e l l L a k e , consistent w i t h t h e h i g h e r P O C levels in t h e f o r m e r b a s i n (Figs. 3-4 a n d 3-5). There a r e  major  d i f f e r e n c e s in t h e C : N ratios b e t w e e n t h e t w o lakes, h o w e v e r . The p a r t i c u l a t e C : N w e i g h t ratio in P o w e l l v a r i e s f r o m a l o w o f 5 n e a r t h e r e d o x c l i n e t o a m a x i m u m of 21 in t h e b o t t o m w a t e r s . In S a k i n a w L a k e , t h e C : N ratio r e m a i n s a t 5.7 t h r o u g h o u t t h e w a t e r c o l u m n , similar t o t h a t f o u n d for p h y t o p l a n k t o n u n d e r c o n d i t i o n s o f n i t r o g e n s a t u r a t i o n ( S a k s h a u g et a l . 1983). H i g h C : N ratios s u c h as t h o s e s e e n in P o w e l l L a k e a r e o f t e n u s e d a s a n i n d i c a t i o n t h a t o r g a n i c m a t t e r is of a l l o c h t h o n o u s origin. O r g a n i c m a t t e r o f terrestrial a n d m a r s h a r e a s initially h a s g r e a t e r C : N ratios d u e t o t h e h i g h e r c e l l u l o s e a n d lignin c o n t e n t a n d a l s o u n d e r g o e s v a r y i n g d e g r e e s o f d e c o m p o s i t i o n prior t o a n d d u r i n g t r a n s p o r t t o a  l a k e , d u r i n g w h i c h m u c h o f t h e o r g a n i c n i t r o g e n m a y h a v e b e e n utilized. T h e r e f o r e , a l l o c h t h o n o u s o r g a n i c m a t t e r c o n t a i n s roughly 6 % p r o t e i n a n d o f t e n h a s a C : N w e i g h t ratio o f a p p r o x i m a t e l y 4 5 t o 50 (Hutchinson 1957). Forest h u m u s a n d terrestrial plants a n d w o o d debris c o n t a i n i n g lignin particularly c o n t r i b u t e t o s u c h h i g h C : N ratios ( P o c k l i n g t o n and  Leonard  1979). In c o n t r a s t , a u t o c h t h o n o u s  organic  matter  produced  by  d e c o m p o s i t i o n of p l a n k t o n within t h e l a k e c o n t a i n s a b o u t 2 4 % c r u d e p r o t e i n a n d has a C : N w e i g h t ratio o f a r o u n d 12 (Wetzel 1983). High C : N ratios c o u l d also b e d e r i v e d from b e n t h i c m a c r o p h y t e s , h o w e v e r , P o w e l l L a k e is t o o d e e p f o r m a c r o p h y t e  growth.  B e c a u s e t h e r e is n o d i r e c t f r e s h w a t e r input t o t h e s o u t h b a s i n o f P o w e l l , t h e h i g h C : N ratios f o u n d most likely d o not reflect a l l o c h t h o n o u s input of o r g a n i c m a t t e r . Instead, t h e h i g h C : N ratios i n d i c a t e t h a t mineralization is p r o c e e d i n g in t h e w a t e r c o l u m n a s p l a n k t o n settle. The c a r b o n c o n t e n t is g e n e r a l l y a t least a n o r d e r of m a g n i t u d e g r e a t e r t h a n t h a t of n i t r o g e n c o n t e n t o f o r g a n i c m a t t e r . A s t h e c o m p l e x o r g a n i c m a t t e r within t h e w a t e r c o l u m n is m i n e r a l i z e d t o i n o r g a n i c c a r b o n (primarily as CO2) a n d i n o r g a n i c n i t r o g e n , t h e proteolytic m e t a b o l i s m of fungi a n d b a c t e r i a removes proportionately m o r e nitrogen t h a n c a r b o n . Rates of d e c o m p o s i t i o n slow as t h e residual o r g a n i c matter  becomes  i n c r e a s i n g l y refractory, b u t t h e s e l e c t i v e r e m o v a l of n i t r o g e n b y m i c r o b e s results in a net i n c r e a s e in C : N ratios. In F r a m v a r e n , a n a n o x i c N o r w e g i a n fjord c h e m i c a l l y similar t o P o w e l l  and  S a k i n a w L a k e s , C : N w e i g h t ratios o f settling m a t e r i a l c a p t u r e d v i a s e d i m e n t t r a p s a r e a p p r o x i m a t e l y 8 t h r o u g h o u t t h e w a t e r c o l u m n ( N a e s et a l . 1988). C o m p a r i s o n with t h e u s u a l C : N v a l u e s o f living p h y t o p l a n k t o n (6 - 10) r e p o r t e d b y S a k s h a u g et a l . (1983) i n d i c a t e s t h a t t h e o r g a n i c m a t e r i a l is mostly settling p h y t o p l a n k t o n . N a e s et a l . (1988) c o n c l u d e d t h a t m i n e r a l i z a t i o n in t h e w a t e r c o l u m n of F r a m v a r e n is r e s t r i c t e d t o t h e u p p e r 20 m w h e r e o x y g e n is present. G r a d i e n t s in alkalinity a n d H S a n d a d e c r e a s e in 2  c a r b o n c o n t e n t in t h e s e d i m e n t s of this fjord c o m p a r e d w i t h t h e m a t e r i a l c o l l e c t e d in t h e 160 m t r a p i n d i c a t e t h a t t h e m a i n o r g a n i c mineralization in t h e fjord m a y t a k e p l a c e in t h e s e d i m e n t s . Extensive d e g r a d a t i o n of o r g a n i c m a t t e r o c c u r s in B l a c k S e a  sediments  ( C a l v e r t a n d Karlin 1990), h o w e v e r , Ross a n d D e g e n s (1974) f o u n d t h a t o r g a n i c m a t t e r is a l s o m i n e r a l i z e d in t h e w a t e r c o l u m n . The r e s i d e n c e t i m e of o r g a n i c m a t t e r in t h e w a t e r c o l u m n is m u c h g r e a t e r t h a n in F r a m v a r e n , h o w e v e r , a s p a r t i c l e s h a v e t o settle t h r o u g h  2000 m c o m p a r e d t o 180 m in t h e fjord; t h u s , d e g r a d a t i o n d u r i n g settling w o u l d b e e x p e c t e d t o b e m o r e p r o f o u n d in t h e B l a c k S e a . A s in F r a m v a r e n , in S a k i n a w L a k e , w h i c h is only 140 m d e e p , t h e r e a l s o a p p e a r s t o b e little d e c o m p o s i t i o n t h r o u g h o u t t h e w a t e r c o l u m n : t h e C : N ratio r e m a i n s n e a r l y c o n s t a n t w i t h d e p t h a n d t h e P O C a n d P O N profiles almost o v e r l a p (Fig. 3-5). It s h o u l d b e n o t e d t h a t t h e c o n s t a n c y o f t h e C : N ratio m a y a l s o i n d i c a t e t h a t C a n d N a r e b e i n g m i n e r a l i z e d a t t h e s a m e r a t e , h o w e v e r , p r e f e r e n t i a l m i n e r a l i z a t i o n o f n i t r o g e n usually o c c u r s . P o w e l l is a l m o s t 200 m d e e p e r t h a n S a k i n a w , a n d t h e u p p e r 150 m is o x i c w h e r e a e r o b i c d e c o m p o s i t i o n p r o c e s s e s a r e m u c h faster t h a n t h o s e u n d e r s u l p h a t e - d e p l e t e a n a e r o b i c c o n d i t i o n s . The d e c r e a s e in t h e C : N ratio a t t h e o x i c / a n o x i c i n t e r f a c e in Powell Lake m a y reflect t h e p r e s e n c e of microorganisms. Bacteria c o n t a i n a higher p r o p o r t i o n o f n i t r o g e n t h a n p h y t o p l a n k t o n ( C : N -4.5) ( F e n c h e l a n d B l a c k b u r n 1979); h e n c e , if t h e y f o r m a n a p p r e c i a b l e f r a c t i o n o f t h e p a r t i c u l a t e o r g a n i c inventory t h e y will c a u s e a l o w e r C : N ratio in t h e bulk P O M f r a c t i o n t h a n if t h e o r g a n i c m a t t e r w a s d e r i v e d entirely f r o m p h y t o p l a n k t o n . In S a k i n a w L a k e , t h e C : N ratio similarly is slightly l o w e r a r o u n d t h e i n t e r f a c e , w h i c h is c o n s i s t e n t w i t h t h e h y p o t h e s i s t h a t b a c t e r i a a r e c o n c e n t r a t e d t h e r e (Fig. 3-5). B e l o w t h e i n t e r f a c e in P o w e l l , t h e C : N ratio a g a i n i n c r e a s e s w i t h d e p t h w h e r e b a c t e r i a p o p u l a t i o n s a r e e x p e c t e d t o b e l o w e r , a n d w h e r e n i t r o g e n is p r e f e r e n t i a l l y r e m i n e r a l i z e d . T h e C : N w e i g h t ratio o f t h e s e d i m e n t s in P o w e l l L a k e is a p p r o x i m a t e l y 18 (Barnes a n d Barnes 1981) a n d r e m a i n s fairly c o n s t a n t w i t h d e p t h . This ratio is v e r y similar t o t h a t f o u n d in P O M in t h e o v e r l y i n g w a t e r c o l u m n , i n d i c a t i n g t h a t most o f t h e c a r b o n m i n e r a l i z a t i o n is o c c u r r i n g prior t o d e p o s i t i o n o n t h e l a k e floor.  Inorganic Nitrogen A m m o n i u m is p r e s e n t in v e r y h i g h c o n c e n t r a t i o n s in t h e b o t t o m w a t e r s o f b o t h l a k e s , r e a c h i n g c o n c e n t r a t i o n s o f a l m o s t 4 m M in P o w e l l a n d n e a r l y 8 m M in S a k i n a w (Figs. 3-6 a n d 3-7). A l t h o u g h a m m o n i u m c a n b e g e n e r a t e d v i a s e v e r a l m e c h a n i s m s , t h e p r i m a r y p r o c e s s o p e r a t i n g in b o t h l a k e s is p r o b a b l y t h e d e c o m p o s i t i o n o f o r g a n i c matter by heterotrophic bacteria. Other ammonium-generating processes include nitrogen fixation, w h i c h c a n b e carried out photosynthetically b y c y a n o b a c t e r i a a n d various other b a c t e r i a , a n d non-photosynthetically b y numerous a n a e r o b i c bacterial g e n e r a . N i t r o g e n fixation usually d o e s n o t o c c u r t o a n y g r e a t e x t e n t in lakes w h e r e t h e r e  is c o n s i d e r a b l e n i t r o g e n a v a i l a b l e in o t h e r , m o r e easily a s s i m i l a t e d forms, as is t h e c a s e In P o w e l l a n d S a k i n a w L a k e s a n d h e n c e w o u l d n o t c o n t r i b u t e m u c h , if a n y , t o t h e a m m o n i u m p o o l . Dissimilatory nitrate r e d u c t i o n b y v a r i o u s b a c t e r i a a l s o results in a m m o n i u m f o r m a t i o n . As nitrate is not a v a i l a b l e in t h e d e e p w a t e r , this p r o c e s s c a n n o t y i e l d t h e h i g h c o n c e n t r a t i o n s of a m m o n i u m o b s e r v e d t h e r e . T h e r e f o r e , t h e bulk of t h e a m m o n i u m f o u n d in b o t h lakes must b e g e n e r a t e d b y h e t e r o t r o p h i c remineralization of o r g a n i c matter. There a r e normally t w o m e c h a n i s m s for extracting t h e remineralized dissolved n i t r o g e n f r o m b o t t o m w a t e r s . O n e is b a c t e r i a l u p t a k e a t d e p t h . H o w e v e r , t h e l o w e r g r o w t h yields o f a n a e r o b i c (relative t o a e r o b i c ) m e t a b o l i s m m e a n t h a t a n a e r o b e s must assimilate l a r g e r a m o u n t s of o r g a n i c c a r b o n a n d h e n c e , its a s s o c i a t e d n i t r o g e n ( a n d p h o s p h o r u s ) , re s u l ti n g in m i n e r a l i z a t i o n o f g r e a t e r q u a n t i t i e s o f a m m o n i u m  (and  p h o s p h a t e ) . T h e r e f o r e , it is unlikely t h a t a n a e r o b i c b a c t e r i a will t a k e u p i n o r g a n i c n i t r o g e n unless their o r g a n i c c a r b o n s o u r c e is n i t r o g e n - d e p l e t e . The o t h e r m e c h a n i s m for r e m o v a l is t o m o v e t h e nutrient-enriched b o t t o m w a t e r u p in t h e w a t e r c o l u m n b y s o m e p h y s i c a l p r o c e s s s u c h a s o v e r t u r n a n d d i s p l a c e m e n t or u p w e l l i n g . H o w e v e r , b e c a u s e P o w e l l a n d S a k i n a w Lakes a r e so s t a b l y stratified, s u c h a d v e c t i v e p r o c e s s e s d o n o t o c c u r ( S a n d e r s o n e t a l . 1986); virtually n o m i x i n g c a n o c c u r o t h e r t h a n b y m o l e c u l a r diffusion, e x c e p t  in S a k i n a w ,  where occasional seawater  incursions  c o n t r i b u t e e d d y diffusion. Therefore t h e basins serve as nutrient traps. A s t h e a m m o n i u m diffuses u p into t h e o x i c p o r t i o n o f t h e w a t e r c o l u m n , h o w e v e r , it is a s s i m i l a t e d or b e c o m e s o x i d i z e d , either b y nitrifying b a c t e r i a (very fast) or b y c h e m i c a l (very slow) p r o c e s s e s . The nitrite a n d nitrate g e n e r a t e d m a y b e u s e d b y denitrifying b a c t e r i a a s a l t e r n a t e e l e c t r o n a c c e p t o r s in l o w - o x y g e n e n v i r o n m e n t s . The o x y g e n c o n c e n t r a t i o n s b e l o w w h i c h t h e f a c u l t a t i v e l y a n a e r o b i c denitrifiers s w i t c h t o a nitrate-respiratory m o d e is not k n o w n w i t h g r e a t a c c u r a c y . O z r e t i c h (1976, c i t e d in Morris et a l . 1985), in a series o f c h e m o s t a t s t u d i e s , f o u n d this c r i t i c a l o x y g e n l e v e l t o v a r y b e t w e e n 0.5 -19.7 u.M. Morris et a l . (1985) o b s e r v e d t h a t nitrate d i s a p p e a r s a n d c o u n t s of denitrifiers i n c r e a s e a t a n o x y g e n c o n c e n t r a t i o n o f 18.8 u.M in t h e C a r i a c o T r e n c h . N o t e t h a t u n c e r t a i n t i e s in t h e Winkler d e t e r m i n a t i o n o f o x y g e n a t r e d u c e d O2 t e n s i o n s ( B r o e n k a w 1969; C l i n e 1973; b o t h c i t e d in Morris et a l . 1985) s u g g e s t t h a t t h e a c t u a l  c o n c e n t r a t i o n of dissolved o x y g e n at t h e p o i n t w h e r e N O i - r e d u c t i o n c o m m e n c e s  may  b e 20 - 4 0 % lower t h a n 19 p M In P o w e l l L a k e , nitrate b e g i n s t o d e c r e a s e b e t w e e n 50 a n d 75 m d e p t h (Fig. 3-6), where dissolved oxygen concentrations  remain  h i g h (-625  jiM). Therefore,  the  d e c r e a s e in nitrate c o n c e n t r a t i o n b e l o w 50 m c a n n o t b e d u e t o r e m o v a l b y denitrifying b a c t e r i a , as denitrification Is unlikely t o o c c u r a b o v e 140 m. The nitrate m a x i m u m a t 50 m m o s t likely r e p r e s e n t s  m i n e r a l i z a t i o n of s e t t l i n g o r g a n i c  matter with  subsequent  nitrification of t h e a m m o n i u m g e n e r a t e d . This is s u p p o r t e d b y t h e d e c r e a s e in d i s s o l v e d 0  2  b e l o w 25 m (Fig. 2-3). N e a r t h e i n t e r f a c e nitrate is c o n s u m e d b y denitrifying b a c t e r i a .  The l o w e r c o n c e n t r a t i o n s of nitrate in t h e u p p e r 25 m is primarily d u e t o p h o t o s y n t h e t i c u p t a k e b y p h y t o p l a n k t o n . The P O C c o r r e l a t e s inversely w i t h t h e n i t r a t e , s h o w i n g a m a x i m u m a t t h e s u r f a c e (Fig. 3-4), w h e r e nitrate is a t a m i n i m u m (Fig. 3-6). D u e t o little a l l o c h t h o n o u s input, t h e P O C a t t h e s u r f a c e consists a l m o s t entirely of p h y t o p l a n k t o n . The f a c t t h a t t h e nitrate levels r e m a i n h i g h , e v e n in t h e p h o t i c z o n e , i n d i c a t e s t h a t it is not t h e limiting nutrient for p l a n k t o n g r o w t h . The t w o lakes differ s o m e w h a t in their nitrate distributions. In P o w e l l L a k e , nitrate r e a c h e s a m a x i m u m v a l u e a t 50 m d e p t h , a g o o d 100 m a b o v e t h e o x i c / a n o x i c i n t e r f a c e , a n d t h e n g r a d u a l l y d e c l i n e s t o z e r o at t h e i n t e r f a c e (Fig. 3-6). In S a k i n a w L a k e , h o w e v e r , nitrate i n c r e a s e s t o a m a x i m u m v a l u e of 8 | i M , 2 m a b o v e t h e i n t e r f a c e a n d t h e n r a p i d l y falls t o z e r o i m m e d i a t e l y b e l o w (Fig. 3-7). Nitrite shows a strong m a x i m u m 2 m a b o v e t h e i n t e r f a c e in S a k i n a w , b u t is u n d e t e c t a b l e in P o w e l l L a k e . Nitrate is not entirely d e p l e t e d in t h e s u r f a c e , o x i c w a t e r s in S a k i n a w L a k e , a l t h o u g h it d o e s d e c r e a s e q u i t e substantially t o fairly l o w levels (0.16 \iM) a t 10 m d e p t h . This is most likely t h e d e p t h w h e r e p h y t o p l a n k t o n a r e c o n c e n t r a t e d , as i n d i c a t e d b y t h e l a r g e p e a k in P O C at t h a t h o r i z o n (Fig. 3-5). A b o v e this d e p t h , t h e a v a i l a b l e light m a y b e s u f f i c i e n t t o inhibit p h o t o s y n t h e s i s . In t h e b o t t o m w a t e r s of S a k i n a w L a k e , a m m o n i u m r e a c h e s m a x i m u m c o n c e n t r a t i o n s of 7.8 m M . U p w a r d diffusion a n d o x i d a t i o n of a m m o n i u m a c r o s s t h e i n t e r f a c e must s u p p o r t t h e nitrite a n d h i g h nitrate c o n c e n t r a t i o n s o b s e r v e d i m m e d i a t e l y a b o v e t h e o x y c l i n e . The s h a r p n e s s of t h e nitrite a n d nitrate p e a k s is m a i n t a i n e d b y p r o d u c t i o n b y nitrifiers in t h e  presence  of o x y g e n , a n d  reduction by  denitrifiers  i m m e d i a t e l y b e l o w . The l a r g e i n c r e a s e in P O C a t t h e s a m e d e p t h in t h e w a t e r c o l u m n , w h i c h is a t t r i b u t e d t o b a c t e r i a (Fig. 3-5), s u p p o r t s this ( a l t h o u g h sulphur-oxidizing  b a c t e r i a w o u l d a l s o c o n t r i b u t e t o t h e h i g h e r P O C ) . Nitrite is t h e m o r e e n e r g e t i c a l l y f a v o u r a b l e e l e c t r o n a c c e p t o r a n d h e n c e is u s e d u p first, f o l l o w e d b y nitrate. N o t e t h a t S a k i n a w L a k e h a s a p p r o x i m a t e l y t w i c e t h e a m m o n i u m c o n c e n t r a t i o n of P o w e l l in its b o t t o m w a t e r , y e t t h e c o n c e n t r a t i o n of nitrate in b o t h lakes is t h e s a m e . This m a y a g a i n i n d i c a t e h i g h e r p r i m a r y p r o d u c t i o n in S a k i n a w ; a s nitrate is p r o d u c e d it is q u i c k l y c o n s u m e d . H i g h e r a m m o n i u m levels in t h e b o t t o m w a t e r a l s o i n d i c a t e h i g h e r primary p r o d u c t i o n ; t h a t is, g r e a t e r a v a i l a b i l i t y o f d e g r a d a b l e o r g a n i c m a t t e r in d e e p w a t e r supports a  h i g h e r inventory of dissolved NH4. H o w e v e r ,  the  higher  ammonium  c o n c e n t r a t i o n s in S a k i n a w L a k e m a y a l s o b e d u e t o a l a r g e r s u p p l y o f o x i d a n t for o r g a n i c m a t t e r d e c o m p o s i t i o n . P o w e l l L a k e h a s not r e c e i v e d a n y s e a w a t e r input s i n c e it w a s s e p a r a t e d f r o m G e o r g i a Strait a p p r o x i m a t e l y 11000 y e a r s a g o , a n d t h e r e f o r e , it h a s b e e n o x i d a n t - d e p l e t e for a m u c h l o n g e r t i m e t h a n S a k i n a w L a k e . This w o u l d result in less o r g a n i c m a t t e r d e c o m p o s i t i o n , a n d h e n c e less m i n e r a l i z a t i o n of a m m o n i u m . O n t h e o t h e r h a n d , S a k i n a w L a k e has likely h a d p e r i o d i c incursions of s e a w a t e r c o n t a i n i n g various oxidants, w h i c h w o u l d allow greater o r g a n i c matter b r e a k d o w n a n d  hence  more a m m o n i u m release.  Dissolved Silicon The silicon c y c l e is t h e simplest of t h e t h r e e major nutrient (P, N a n d Si) c y c l e s , as it h a s o n l y o n e i n o r g a n i c d i s s o l v e d f o r m , orthosilicic a c i d (H Si04), with a n o x i d a t i o n state 4  of Si of +4 a n d n o k n o w n o r g a n i c forms of b i o l o g i c a l i m p o r t a n c e (Wetzel 1983). The m a i n c y c l i n g p a t h w a y is f r o m d i s s o l v e d i n o r g a n i c t o p a r t i c u l a t e a n d b a c k t o i n o r g a n i c f o r m . In b o t h P o w e l l a n d S a k i n a w Lakes, l a r g e c o n c e n t r a t i o n s o f d i s s o l v e d silicon a r e f o u n d a t d e p t h ( u p t o 300 \iM in t h e f o r m e r a n d 1000 u.M in t h e latter) (Figs. 3-8 a n d 3-9). There a r e five p r i n c i p a l s i l i c o n - c o n t a i n i n g m i n e r a l g r o u p s t h a t c a n a c t as s o u r c e s of silicon in l a k e waters:  1) d e t r i t a l q u a r t z , 2) d e t r i t a l a l u m i n o s i l i c a t e s . p r i m a r i l y c l a y m i n e r a l s  and  f e l d s p a r s , 3) o p a l i n e silica in t h e f o r m of d i a t o m a n d s p o n g e skeletal debris, 4) v o l c a n i c glass in v a r i o u s s t a g e s of h y d r a t i o n a n d d e v i t r i f i c a t i o n , a n d 5) a u t h i g e n i c q u a r t z a n d a l u m i n o s i l i c a t e s s u c h a s f e l d s p a r s , m o n t m o r i l i o n i t e a n d philiipsite (Berner 1975). Q u a r t z d o e s not normally play a  r o l e in t h e e n v i r o n m e n t a l g e o c h e m i s t r y o f Si a t  low  t e m p e r a t u r e s ( D r e v e r 1982), a n d t h e small a m o u n t o f f r e s h w a t e r input a n d t h e l a r g e d i s t a n c e of b o t h t h e Powell a n d S a k i n a w a n o x i c basins f r o m freshwater  sources  i n d i c a t e s t h a t little silica i n p u t is likely t o c o m e f r o m w e a t h e r i n g of q u a r t z a n d  clay  minerals. A l s o , t h e r e is a l a c k of c l a y s in s o u r c e r o c k s of t h e d r a i n a g e b a s i n of P o w e l l a n d S a k i n a w L a k e s (B. B a r n e s pers. c o m m . ) . T h e r e f o r e , t h e most p r o b a b l e s o u r c e of d i s s o l v e d silicon in P o w e l l a n d S a k i n a w Lakes is f r o m t h e dissolution of d i a t o m frustules. A t a p H of - 7 . o p a l i n e silica r e a c t s m u c h m o r e rapidly w i t h w a t e r t h a n b o t h c l a y minerals a n d q u a r t z , a n d in most l a k e s , t h e s e d i m e n t a t i o n of d i a t o m s constitutes t h e major silicon sink ( W e t z e l 1983). In d e e p l a k e s , frustules t y p i c a l l y u n d e r g o p a r t i a l dissolution b e f o r e r e a c h i n g t h e s e d i m e n t s . It is s u g g e s t e d , t h e r e f o r e , t h a t b e c a u s e dissolution of d i a t o m s must b e t h e p r i m a r y c o n t r i b u t o r of dissolved silicon t o t h e d e e p w a t e r s of b o t h lakes, t h e h i g h e r d i s s o l v e d silicon c o n c e n t r a t i o n s f o u n d a t d e p t h in S a k i n a w s u p p o r t t h e previous i n d i c a t i o n s t h a t primary p r o d u c t i o n is higher in this lake t h a n in P o w e l l . In P o w e l l L a k e , t h e s h a p e of t h e d i s s o l v e d silicon profile differs f r o m t h a t of a m m o n i u m (Fig. 3-6). A m m o n i u m is g e n e r a t e d w h e n o r g a n i s m s a r e m i n e r a l i z e d , i.e. w h e n b a c t e r i a f e e d o n t h e s e cells a n d r e l e a s e t h e i n o r g a n i c c o n s t i t u e n t s . A l t h o u g h m i n e r a l i z a t i o n a p p e a r s t o o c c u r in t h e P o w e l l L a k e w a t e r c o l u m n , p h y t o p l a n k t o n cells sink fairly r a p i d l y ( C l o e m et a l . 1987) a n d b a c t e r i a l activity is g r e a t e s t n e a r t h e b o t t o m , d u e t o t h e p r e s e n c e of m o r e substrate a n d e l e v a t e d t e m p e r a t u r e . Thus, t h e bulk of t h e m i n e r a l i z a t i o n o c c u r s a t d e p t h a n d t h e r e l e a s e d c o m p o u n d s diffuse u p into t h e w a t e r c o l u m n into r e g i o n s of l o w e r c o n c e n t r a t i o n . H o w e v e r , in c o n t r a s t t o n i t r o g e n  and  p h o s p h o r u s r e m i n e r a l i z a t i o n , d i s s o l v e d s i l i c o n is r e l e a s e d b y t h e dissolution of silica frustules, a strictly i n o r g a n i c h y d r o l y t i c p r o c e s s w h i c h c a n o c c u r t h r o u g h o u t t h e w a t e r c o l u m n . Thus, t h e s h a p e of t h e d i s s o l v e d s i l i c o n p r o f i l e m a y b e a t l e a s t p a r t i a l l y g e n e r a t e d b y dissolution of d i a t o m frustules as t h e y settle t h r o u g h t h e w a t e r c o l u m n . M o s t of t h e b i o g e n i c o p a l f o r m e d in o p e n o c e a n s u r f a c e s e a w a t e r dissolves b e f o r e it is i n c o r p o r a t e d into t h e s e d i m e n t ( B r o e c k e r 1971). The rate of solution of a single test is a f u n c t i o n o f solution t e m p e r a t u r e , d e g r e e of saturation of solution, s p e e d of w a t e r f l o w i n g p a s t t h e t e s t , a n d t h e a v a i l a b l e s u r f a c e a r e a of t h e test (Hurd 1972). The d o m i n a n t d i a t o m in p l a n k t o n t o w s in m i d - s u m m e r P o w e l l L a k e is a t h i n - w a l l e d , f r a g i l e d i a t o m  Rhizosolenia  sp. (Styan 1976). It is v e r y a b u n d a n t a n d is virtually t h e only d i a t o m p r e s e n t ,  a l t h o u g h 19 o t h e r g e n e r a of d i a t o m s h a v e b e e n i d e n t i f i e d in t h e l a k e . S i n c e  Rhizosolenia  is n o t f o u n d in t h e s e d i m e n t s of P o w e l l L a k e (B. B a r n e s , pers. c o m m . ) , t h e s e frustules must dissolve a s t h e y settle t h r o u g h t h e w a t e r c o l u m n . The d i a t o m s f o u n d in P o w e l l L a k e  s e d i m e n t s consist o f m o r e robust s p e c i e s . The majority o f t h e frustules in c o m p a r a t i v e l y s h a l l o w S a k i n a w L a k e p r o b a b l y sink without a p p r e c i a b l e dissolution. S o m e r e s e a r c h e r s h a v e f o u n d a n o x i c l a k e w a t e r s t o b e p a r t i c u l a r l y corrosive t o d i a t o m frustules. M e r i l d i n e n (1969) f o u n d a p a u c i t y of shells in t h e d e e p w a t e r s e d i m e n t s of L a k e S k j e n n u n g e n (Finland), w h i c h h e a t t r i b u t e d t o p o o r p r e s e r v a t i o n of t h e frustules in t h e m o n i m o l i m n i o n , a f a c t w h i c h w a s a p p a r e n t f r o m t h e s t r o n g c o r r o s i o n of t h e tests. K j e n s m o (1988) h a s s u g g e s t e d t h a t t h e p o o r p r e s e r v a t i o n o f d i a t o m frustules in t h e m o n i m o l i m n e t i c s e d i m e n t s of L a k e Skjennungen  m a k e s t h e c h a n g i n g c o n t e n t of  a m o r p h o u s silica in t h e s e d i m e n t s a n i m p o r t a n t p a r a m e t e r for t h e i n t e r p r e t a t i o n of t h e onset-time o f meromixis in t h e lake. H o w e v e r , M e r i l d i n e n (1969) f o u n d t h a t in s o m e other m e r o m i c t i c lakes t h e r e a p p e a r s t o b e n o c o r r o s i o n of frustules a t all. Silicon-rich w a t e r s h o u l d b e less a g g r e s s i v e t o w a r d siliceous s k e l e t o n s t h a n w a t e r p o o r e r in s i l i c o n , all o t h e r f a c t o r s b e i n g e q u a l . Thus a n o v e r a l l d e c r e a s e in dissolution r a t e w i t h d e p t h is usually s e e n , d u e t o t h e i n c r e a s e in t h e silicon c o n c e n t r a t i o n a n d t h e d e c r e a s e in t e m p e r a t u r e w i t h d e p t h . In P o w e l l a n d S a k i n a w L a k e s , h o w e v e r , t e m p e r a t u r e a c t u a l l y i n c r e a s e s in t h e b o t t o m waters. G r a z i n g c a n h a v e b o t h a d v e r s e a n d b e n e f i c i a l e f f e c t s o n test p r e s e r v a t i o n . C r u s h i n g of tests a n d b r e a k d o w n o f c e l l p r o t o p l a s m d u r i n g d i g e s t i o n c r e a t e s g r e a t e r s p e c i f i c s u r f a c e a r e a s a n d partially r e m o v e s p r o t e c t i v e o r g a n i c c o a t i n g s f r o m t h e test (Hurd 1972); b o t h f a c t o r s t e n d t o i n c r e a s e t h e r a t e of s o l u t i o n of t h e test. H o w e v e r , i n c o r p o r a t i o n in f a e c a l pellets m a y r e m o v e t h e test f r o m a n a r e a of m o r e a c t i v e t o less a c t i v e solution a n d m a y a l s o s p e e d settling o f seston. A l s o , g r a z i n g c a n only o c c u r in t h e a e r o b i c w a t e r s of b o t h lakes. Live d i a t o m cells t h a t a r e Si-limited t a k e u p silicon while settling t h r o u g h t h e  metalimnion a n d  h y p o l i m n i o n , thus a c t i n g t o p r e v e n t  the  a c c u m u l a t i o n of d i s s o l v e d silicon in t h e h y p o l i m n i o n f o l l o w i n g its r e l e a s e (by dissolution) a t t h e s e d i m e n t - w a t e r i n t e r f a c e (Stauffer 1986). T e s s e n o w (1966, c i t e d in Stauffer 1986) n o t e d t h a t Si-limited d i a t o m cells a c t i v e l y a b s o r b e d t h e nutrient in t h e d a r k , e . g . b e l o w t h e t r o p h o g e n i c z o n e . This p r o c e s s c a n o c c u r a s t h e c e l l is sinking as w e l l a s a f t e r t h e c e l l s h a v e s e t t l e d o u t . H o w e v e r , d u e t o t h e h i g h c o n c e n t r a t i o n s of d i s s o l v e d silicon p r e s e n t t h r o u g h o u t t h e w a t e r c o l u m n ( n e v e r < 3 0 |iM) in P o w e l l a n d S a k i n a w L a k e s , d i a t o m s a r e unlikely t o b e Si-limited. A l s o , s u l p h i d e is t o x i c t o most o r g a n i s m s a n d w o u l d p r o b a b l y halt all m e t a b o l i s m in t h e a n o x i c w a t e r s .  A n o t h e r p o s s i b l e r e l e a s e m e c h a n i s m f o r d i s s o l v e d s i l i c o n is r e d o x r e l a t e d . M o r t i m e r (1941, 1942) f o u n d t h a t t h e d i s s o l v e d silicon c o n t e n t of t h e w a t e r overlying t h e b o t t o m s e d i m e n t s in L a k e W i n d e r m e r e  i n c r e a s e d t o only 620 j i M u n d e r  aerobic  c o n d i t i o n s b u t u n d e r a n a e r o b i c c o n d i t i o n s it i n c r e a s e d g r e a t l y t o a b o u t 980 jxM, c l o s e t o t h e solubility o f a m o r p h o u s silica (Siever 1962). K a t o (1969) a l s o f o u n d t h a t a s a n o x i a p r o g r e s s e d in L a k e Kizaki, a s e a s o n a l l y a n o x i c J a p a n e s e l a k e , t h a t d i s s o l v e d s i l i con c o n c e n t r a t i o n s i n c r e a s e d a t d e p t h . He s h o w e d e x p e r i m e n t a l l y t h a t h y d r a t e d o x i d e s of iron a n d m a n g a n e s e c o - p r e c i p i t a t e a c o n s i d e r a b l e a m o u n t o f d i s s o l v e d silicon. S u c h c o p r e c i p i t a t i o n of s o l u b l e p h o s p h a t e  a n d s i l i c o n b y i r o n o x y h y d r o x i d e s is w e l l  established, a n d has b e e n postulated as the m e c h a n i s m controlling the dissolved c o n c e n t r a t i o n o f p h o s p h a t e a n d s i l i c o n in m a n y s u b - t i d a l s e d i m e n t s ( S t u m m L e c k i e 1970). L o d e r et a l . (1978) f o u n d t h a t 8 n M F e  3 +  and  c o p r e c i p i t a t e d a p p r o x i m a t e l y 1 \JM  d i s s o l v e d s i l i c o n a n d 0.7 \iM of p h o s p h a t e in a n o x i c m a r i n e p o r e w a t e r s .  Because  p h o s p h a t e is virtually non-existent in t h e o x i c p o r t i o n o f t h e w a t e r c o l u m n in b o t h P o w e l l a n d S a k i n a w Lakes ( C h a p t e r 4), larger a m o u n t s of d i s s o l v e d silicon m a y b e s c a v e n g e d b y iron a n d m a n g a n e s e o x i d e s . H o w e v e r , u n d e r a n a e r o b i c c o n d i t i o n s or a t l o w p H v a l u e s , t h e h y d r a t e d o x i d e s a r e c o m p l e t e l y d i s s o l v e d a n d c o - p r e c i p i t a t e d s i l i con is s o l u b i l i z e d . T h e r e f o r e , a n y s i l i c o n c o p r e c i p i t a t e d w i t h iron or m a n g a n e s e o x i d e s in P o w e l l a n d S a k i n a w Lakes w o u l d b e r e l e a s e d a t t h e o x i c / a n o x i c i n t e r f a c e . In S a k i n a w L a k e , t h e c o n c e n t r a t i o n of d i s s o l v e d silicon m a y also b e c o n t r o l l e d t o s o m e e x t e n t b y its solubility. The solubility o f a m o r p h o u s silica a t n e u t r a l p H v a l u e s varies f r o m a b o u t 1000 ^ M « k g  _1  a t 5 ° C t o 2000 j i M - k g ' a t 27°C (Siever 1962). Dissolved silicon 1  c o n c e n t r a t i o n s in S a k i n a w L a k e a p p r o a c h t h e s e v a l u e s (-980 \iM m a x i m u m ) . Thus, t h e d e c r e a s e in c o n c e n t r a t i o n just a b o v e t h e b o t t o m m a y b e d u e t o p r e c i p i t a t i o n of a m o r p h o u s silica. Solubility c a l c u l a t i o n s using M I N E Q L ( A p p e n d i x 1) i n d i c a t e t h a t S i 0 is 2  very near saturation b e l o w 40 m d e p t h .  Alkalinity and pH C a r b o n a t e alkalinity in n a t u r a l w a t e r s is d e f i n e d as t h e t o t a l c o n c e n t r a t i o n of all dissolved b i c a r b o n a t e  species  ( f r e e ions a n d  ion pairs)  plus t w i c e  the  total  c o n c e n t r a t i o n o f all d i s s o l v e d c a r b o n a t e s p e c i e s , a n d is a sensitive i n d i c a t o r of d i a g e n e t i c p r o c e s s e s (Presley a n d K a p l a n  1968). The m a j o r c l a s s e s o f  reactions  p r o d u c i n g o r c o n s u m i n g alkalinity in s e d i m e n t s or w a t e r c o l u m n s a r e listed in T a b l e 3-2. C a r b o n a t e alkalinity is p r o d u c e d b y t h e r e a c t i o n of C 0 with b a s e s ( C a C 0 , N H , S ") a n d 2  c o n s u m e d b y t h e reaction of H C 0 (Ca  3  3  o r C C i with a c i d s ( C a  2 +  2  3  . cation-free "acidic" clay).  is h e r e c l a s s i f i e d as a n a c i d in t h a t it neutralizes alkalinity). In t h e a n o x i c w a t e r s of  2 +  b o t h Powell a n d S a k i n a w Lakes m e t h a n o g e n e s i s , sulphate r e d u c t i o n , a m m o n i u m p r o t o n a t i o n , a n d iron a n d m a n g a n e s e o x i d e r e d u c t i o n all o c c u r . The p r i m a r y p r o c e s s c a u s i n g t h e l a r g e i n c r e a s e in alkalinity in t h e a n o x i c w a t e r c o l u m n o f b o t h lakes is p r o b a b l y b a c t e r i a l s u l p h a t e r e d u c t i o n a c c o m p a n i e d b y a m m o n i u m f o r m a t i o n (Figs. 310 a n d 3-11). D u r i n g s u l p h a t e r e d u c t i o n , f o r e v e r y e q u i v a l e n t o f S O " r e d u c e d , t w o 2  e q u i v a l e n t s of alkalinity a r e a d d e d . S i n c e N H is t h e c o n j u g a t e b a s e of t h e w e a k a c i d 3  NH4 ( p K = 9.3), e a c h e q u i v a l e n t of N H l i b e r a t e d f r o m o r g a n i c n i t r o g e n c o m p o u n d s 3  s h o u l d i n c r e a s e t h e titration alkalinity b y o n e e q u i v a l e n t . A t p o r e w a t e r p H v a l u e s most of t h e N H is p r o t o n a t e d a c c o r d i n g t o : 3  NHs + CO2 + H T O <-> NHJ + H C C b . This p r o t o n t r a n s f e r r e a c t i o n d o e s n o t d e c r e a s e t h e t i t r a t i o n a l k a l i n i t y s i n c e  each  e q u i v a l e n t o f N H p r o t o n a t e d results in t h e p r o d u c t i o n of o n e e q u i v a l e n t o f b i c a r b o n a t e . 3  It is interesting t h a t a l t h o u g h P o w e l l L a k e h a s a p p r o x i m a t e l y half t h e s u l p h i d e a n d a m m o n i u m o f S a k i n a w L a k e , alkalinity c o n c e n t r a t i o n s a r e higher in P o w e l l b o t t o m w a t e r . B a s e d o n t h e a m m o n i u m a n d s u l p h i d e , a n d a s s u m i n g all o t h e r f a c t o r s t o b e e q u a l , S a k i n a w L a k e alkalinity s h o u l d b e a p p r o x i m a t e l y 6 meq»L"' g r e a t e r t h a n t h a t of P o w e l l . I n s t e a d , t h e alkalinity in P o w e l l L a k e in a b o u t 3.5 m e q ^ L " h i g h e r , i n d i c a t i n g t h a t either 1  t h e r e is s o m e a d d i t i o n a l s o u r c e of alkalinity t o t h e b o t t o m w a t e r s o f P o w e l l , or t h a t alkalinity is c o n s u m e d in S a k i n a w . A s s h o w n in T a b l e 3-2, p r o c e s s e s o t h e r t h a n s u l p h a t e r e d u c t i o n a n d a m m o n i u m p r o d u c t i o n c a n c o n t r i b u t e t o alkalinity. H o w e v e r , t h e s e c a n all b e r u l e d o u t a s likely m e c h a n i s m s for alkalinity g e n e r a t i o n a t d e p t h in P o w e l l L a k e . First o f a l l , m e t h a n o g e n e s i s a d d s a l k a l i n i t y o n l y v i a t h e p r o t o n a t i o n o f a m m o n i a . Denitrification r e a c t i o n s o c c u r o n l y a b o v e t h e i n t e r f a c e . M a n g a n e s e a n d iron r e d u c t i o n o c c u r n e a r t h e i n t e r f a c e in b o t h lakes (Figs. 5-9 a n d 5-10), h o w e v e r , t h e c o n c e n t r a t i o n s of F e  2 +  and Mn  PL; b o t h F e  2 +  2 +  a r e l o w relative t o t h e titration alkalinity ( F e  and Mn  2 +  2+  m a x . 170 u.M, M n  2 +  30 u M in  ~7 u.M in SL) a n d thus t h e s e s p e c i e s d o n o t c o n t r i b u t e a significant  a m o u n t of alkalinity. M i n e r a l w e a t h e r i n g will n o t g e n e r a t e alkalinity a t d e p t h in t h e l a k e ,  Table 3-2 Major classes of reactions involved in alkalinity generation a n d consumption (from Schiff a n d A n d e r s o n 1987)  1) A e r o b i c Respiration - including nitrification  AAlk/Aoxidant  OrVT + 13802 + I8HCO3 -* 124C02 + I6NO3 + HPO4" + I4OH2O  -18/138  2) Denitrification a ) With o x i d a t i o n o f o r g a n i c nitrogen +92.4/94.4  O M + 94.4NG-3 -> 9 2 . 4 H C 0 3 + 1 3 . 6 C a + 55.2N2 + HPO4" + 84.8H 0 2  b) Without o x i d a t i o n o f o r g a n i c nitrogen  O M + 84.8N03" -> 98.8HC03+7.2C02 + 42.4Ns + 16NhC + HPO4 + 49.6H2O +98.8/84.8  c) A m m o n i a production  +120/53  OM+53NO3 +14C02 + 67H2O -* 12OHCO3+ 69NH4 + HPO4 + 53HzO  3) Mn(IV) R e d u c t i o n O M + 212MnC>2 + 332C02 + 12CHO ^ 4 3 8 H C 0 + 2 1 2 M n + I6NH4 + HPO4" 2+  3  +438/424  4) Fe(lll) R e d u c t i o n O M + 4 2 4 F e O O H + 756C02 + I2OH2O -»862HC03 + 424Fe 5) SulDhate  2+  +  I6NH4  + HPO4" +862/848  Reduction +120/106  O M + 53SO-4 -^67HCCb+ 53HS" +396C02 + I6NH4 + HPG4" + 3 9 H 0 2  6) M e t h a n e  Fermentation +14/53  O M + 14H -» 53CH4 + 53C02 + 16NI-C + HPO ; +  2  7) C a t i o n E x c h a n g e X-M  n+  + n H -» X-n(H ) + M +  +  n +  W h e r e X=mineral or o r g a n i c substrate; M = C a , M g 2 +  2 +  e . g . C a r b o n a t e dissolution/precipitation C0OO3 + 2 H -» 2HCO3+ C a +  , K , NHl,etc.  + 1/1  +  2 +  e.g. A m m o n i u m adsorption  (cation-clay) + N H j -* (NHl-clay) + c a t i o n  8) Organic Acid P r o t o n a t i o n R-COO" + H -»R-COOH +  9) M i n e r a l  Weathering  e.g. C a A f e S t A i + H 0 +2H -> C a 2  +  2 +  + Al2Si 0 (OH)4 2  + 1/1  5  * O M refers t o o r g a n i c m a t t e r with t h e c o m p o s i t i o n (CH O)i06(NH3)i6(H PO4) 2  (Redfield e t al. 1963)  3  a n d c l a y - c a t i o n e x c h a n g e r e a c t i o n s a r e n o t i m p o r t a n t d u e t o t h e s m a l l a m o u n t s of c l a y p r e s e n t in t h e o r g a n i c o o z e s e d i m e n t . C a C O s dissolution g e n e r a t e s alkalinity, a n d c a l c i t e p r e c i p i t a t i o n in s u r f a c e w a t e r s d u e t o t h e rise in p H a c c o m p a n y i n g p h o t o s y n t h e t i c utilization of C 0  2  is w e l l k n o w n in marl  l a k e s (Otsuki a n d W e t z e l 1972). A s this m a t e r i a l settles t h e c a r b o n a t e m a y solubilize, i n c r e a s i n g alkalinity. Extremely h i g h p r o d u c t i v i t y w o u l d b e r e q u i r e d t o raise t h e p H of P o w e l l s u r f a c e w a t e r s (5.7) sufficiently for C a C 0  3  p r e c i p i t a t i o n t o o c c u r (~9); as P o w e l l  L a k e is o l i g o t r o p h i c this is unlikely t o o c c u r . The h i g h c a r b o n a t e c o n c e n t r a t i o n s in P o w e l l a n d S a k i n a w b o t t o m w a t e r s a r e m o r e likely t o c a u s e C a C 0  3  precipitation. Calcite  p r e c i p i t a t i o n c o u l d c o n s u m e a l k a l i n i t y in S a k i n a w L a k e b o t t o m w a t e r s . solubility c a l c u l a t i o n s ( C h a p t e r 4) s h o w t h a t C a C 0  3  However,  is u n d e r s a t u r a t e d a t all d e p t h s in  S a k i n a w L a k e a n d b a r e l y s a t u r a t e d in t h e b o t t o m w a t e r o f P o w e l l L a k e . It is unlikely that C a C Q j p r e c i p i t a t i o n o c c u r s in Powell L a k e , as C a C 0  3  n u c l e a t i o n is p r o b a b l y i n h i b i t e d b y  h i g h levels of D O M a n d l a c k of s e e d crystals (Suess 1970). The p r o t o n a t i o n of o r g a n i c a c i d s h a s b e e n f o u n d t o c o n t r i b u t e as m u c h as 2 0 % of t h e alkalinity in soft w a t e r O n t a r i o Lakes ( H e r c z e g a n d Hesslein 1984). H o w e v e r , t h e s e lakes differ f r o m P o w e l l as t h e c o n c e n t r a t i o n o f D O C is substantially h i g h e r t h a n t h a t of D i C . In P o w e l l L a k e , D O C r e a c h e s c o n c e n t r a t i o n s of 50 mg«L'\ A s s u m i n g 6 c a r b o n a t o m s p e r m o l e c u l e , this w o u l d only g e n e r a t e 0.7 m e q - L " alkalinity. 1  All of t h e m e c h a n i s m s for g e n e r a t i n g alkalinity listed in T a b l e 3-2 c a n b e r u l e d out for P o w e l l L a k e . There a l s o d o n o t a p p e a r t o b e a n y m e a n s of s i g n i f i c a n t alkalinity c o n s u m p t i o n in S a k i n a w . T h e r e f o r e , I c a n n o t e x p l a i n w h y P o w e l l L a k e b o t t o m w a t e r s h a v e s o m u c h m o r e alkalinity t h a n S a k i n a w . The l a r g e i n c r e a s e in p H in t h e b o t t o m w a t e r s o f b o t h l a k e s is primarily d u e t o p r o t o n c o n s u m p t i o n v i a s u l p h a t e r e d u c t i o n a n d a m m o n i u m f o r m a t i o n (Figs. 3-10 a n d 311). The p H in S a k i n a w L a k e is slightly l o w e r t h a n t h a t of P o w e l l , a s e x p e c t e d s i n c e alkalinity is also s o m e w h a t lower.  CHAPTER 4  PHOSPHORUS CHEMISTRY  4.1 Introduction  In t h e p r e v i o u s c h a p t e r t h e alkalinity-, o r g a n i c c a r b o n - , a m m o n i u m - , a n d siliconrich b o t t o m w a t e r s o f P o w e l l a n d S a k i n a w Lakes w e r e d i s c u s s e d in terms o f their r e d o x chemistry. In g e n e r a l , similarities exist a m o n g t h e distributions of a l l o f t h e s e p a r a m e t e r s in b o t h basins. H o w e v e r , t h e t w o lakes differ c o n s i d e r a b l y w i t h r e s p e c t t o p h o s p h o r u s c h e m i s t r y . E x t r e m e l y l o w p h o s p h a t e c o n c e n t r a t i o n s , r e l a t i v e t o a m m o n i u m , o c c u r in t h e b o t t o m w a t e r s o f P o w e l l L a k e ; this c o n t r a s t warrants s p e c i a l a t t e n t i o n . H e n c e , in this c h a p t e r t h e distribution o f p h o s p h o r u s in b o t h lakes will b e d i s c u s s e d in d e t a i l , in a n a t t e m p t t o e x p l a i n w h y P o w e l l differs s o d r a m a t i c a l l y f r o m S a k i n a w in this r e s p e c t . The Phosphorus C y c l e P h o s p h o r u s exists in n a t u r e a l m o s t e x c l u s i v e l y a s p h o s p h a t e in w h i c h t h e o x i d a t i o n s t a t e o f P is +5. O f t h e k e y e l e m e n t s o f a l g a l p r o t o p l a s m ( C , N , a n d P), o n l y p h o s p h o r u s d o e s n o t c h a n g e v a l e n c e s t a t e s in n a t u r a l , b i o l o g i c a l , o r a b i o l o g i c a l c h e m i c a l t r a n s f o r m a t i o n s ( F e n c h e l a n d B l a c k b u r n 1979); p h o s p h a t e c a n b e r e d u c e d t o p h o s p h i t e , h y p o p h o s p h i t e , a n d p h o s p h i n e , but t h e redox p o t e n t i a l of p h o s p h a t e r e d u c t i o n is s o l o w t h a t r e d u c t i o n rarely, if e v e r , o c c u r s in n a t u r a l e n v i r o n m e n t s (Brock 1979). P h o s p h o r u s is usually f o u n d in high-energy p h o s p h a t e b o n d s a n d o c c u r s in m u c h l o w e r c o n c e n t r a t i o n s in cells t h a n t h e o t h e r t w o nutrients ( C a n d N). The most significant f o r m o f i n o r g a n i c p h o s p h o r u s is o r t h o p h o s p h a t e . The p h o s p h o r u s c y c l e (Fig. 4-1) h a s a t t r a c t e d a g r e a t d e a l o f a t t e n t i o n b e c a u s e this e l e m e n t h a s universal s i g n i f i c a n c e in a l g a l p h y s i o l o g y , a n d is most o f t e n b i o l o g i c a l l y "limiting" in n a t u r a l lakes u n a f f e c t e d b y a n t h r o p o g e n i c e u t r o p h i c a t i o n (Schindler 1977). The p h o s p h o r u s c y c l e , w h i c h is s l o w , differs f r o m t h o s e o f c a r b o n , h y d r o g e n , o x y g e n a n d n i t r o g e n , in w h i c h t h e e l e m e n t s a r e c y c l e d m u c h m o r e q u i c k l y . B e c a u s e it t a k e s millions o f y e a r s t o c o m p l e t e t h e p h o s p h o r u s c y c l e , t h e distribution o f t h e e l e m e n t c a n b e v i e w e d a s a o n e - w a y trip f r o m r o c k w e a t h e r i n g t o t h e w a t e r a n d s e d i m e n t s (Holtan et a l . 1988). In a d d i t i o n t o m i c r o b i a l m i n e r a l i z a t i o n activities, t h e c y c l i n g o f p h o s p h o r u s is r e g u l a t e d t o a l a r g e d e g r e e b y p h y s i c a l - c h e m i c a l r e a c t i o n s in s e d i m e n t s (Forsberg  Fig. 4-1 A simplified r e p r e s e n t a t i o n of t h e p h o s p h o r u s c y c l e In a q u a t i c e n v i r o n m e n t s (from F e n c h e l a n d B l a c k b u r n 1979).  1989). In this c o n t e x t , t h e s e d i m e n t - w a t e r i n t e r f a c e m a y a c t either a s a p e r m a n e n t sink or as a transient s o u r c e f o r p h o s p h o r u s .  Forms of Phosphorus In m o s t lakes, a l a r g e majority of t h e t o t a l p h o s p h o r u s is in a n o r g a n i c p h a s e , o f w h i c h a b o u t 7 0 % o r m o r e is within t h e p a r t i c u l a t e (sestonic) o r g a n i c m a t e r i a l , a n d t h e r e m a i n d e r is p r e s e n t a s d i s s o l v e d o r c o l l o i d a l o r g a n i c p h o s p h o r u s ( W e t z e l 1983). A significant q u a n t i t y o f t h e s o l u b l e o r g a n i c p h o s p h o r u s is in a c o l l o i d a l state. Dissolved Phosphorus The i n o r g a n i c f o r m s o f d i s s o l v e d p h o s p h o r u s i n c l u d e o r t h o p h o s p h a t e a n d p y r o p h o s p h a t e , a s w e l l a s their c y c l i c p o l y m e r s ( m e t a p h o s p h a t e s ) a n d linear polymers ( p o l y p h o s p h a t e s ) . O r g a n i c p h o s p h o r u s c o m p o u n d s a r e t y p i c a l l y p h o s p h a t e esters, s u c h a s p o l y o l or s u g a r p h o s p h a t e s , p h o s p h o r y l a t e d h y d r o x y a m i n e s a n d a m i n o a c i d s , n u c l e o t i d e s a n d p h o s p h o l i p i d s ( C e m b e l l a e t a l . 1986). A n a d d i t i o n a l g r o u p o f o r g a n i c p h o s p h o r u s c o m p o u n d s a r e t h e p h o s p h o n a t e s , in w h i c h t h e c a r b o n o f t h e o r g a n i c m o i e t y is b o u n d d i r e c t l y t o p h o s p h o r u s rather t h a n t h r o u g h a n ester b o n d (Kittredge a n d Roberts 1969). Typically, t h e total p o o l of dissolved phosphorus of natural waters varies b e t w e e n 0.1 a n d 35 | i M ( F e n c h e l a n d B l a c k b u r n 1979). O n l y a portion o f this p o o l , primarily t h e o r t h o p h o s p h a t e f r a c t i o n , is r e a d i l y t a k e n u p b y o r g a n i s m s . I n o r g a n i c  soluble  p h o s p h o r u s is consistently v e r y l o w , constituting only a f e w p e r c e n t o f t o t a l p h o s p h o r u s , a n d is c y c l e d v e r y r a p i d l y in t h e z o n e s o f utilization. T h e r a t i o o f i n o r g a n i c s o l u b l e p h o s p h o r u s t o o t h e r forms o f p h o s p h o r u s h a s b e e n f o u n d t o b e a p p r o x i m a t e l y 1:20 in a l a r g e v a r i e t y of lakes within t h e t e m p e r a t e z o n e (Wetzel 1983). The p e r c e n t a g e of t o t a l p h o s p h o r u s o c c u r r i n g a s truly ionic P O f . h o w e v e r , is p r o b a b l y c o n s i d e r a b l y less t h a n 5% in m o s t n a t u r a l waters. Orthophosphate phosphate  is d e r i v e d f r o m t h e n a t u r a l w e a t h e r i n g  minerals, t h e solubilization of p r e c i p i t a t e d  a n d e r o s i o n of  metallic phosphates a n d  a d s o r b e d p h o s p h a t e , a n d t h e excretion of b a c t e r i a a n d other organisms (Fenchel a n d B l a c k b u r n 1979). In r e c e n t t i m e s , soil fertilizers a n d industrial a n d d o m e s t i c w a s t e w a t e r s h a v e a l s o b e c o m e i m p o r t a n t s o u r c e s o f o r t h o p h o s p h a t e . P o l y p h o s p h a t e s (i.e. linearly c o n d e n s e d o r t h o p h o s p h a t e ) o f v a r y i n g m o l e c u l a r w e i g h t a r e s y n t h e s i z e d b y living  o r g a n i s m s . T h e y a r e u n s t a b l e in w a t e r , w h e r e t h e y a r e s l o w l y h y d r o l y s e d t o t h e o r t h o p h o s p h a t e f o r m ( A l e x a n d e r 1978). P h o s p h a t e , p y r o p h o s p h a t e , t r i p h o s p h a t e , a n d h i g h e r p o l y p h o s p h a t e a n i o n s f o r m c o m p l e x e s , c h e l a t e s , a n d i n s o l u b l e salts w i t h a n u m b e r o f m e t a l ions, t h e e x t e n t o f w h i c h d e p e n d s u p o n t h e relative c o n c e n t r a t i o n s of t h e p h o s p h a t e s a n d t h e m e t a l ions, t h e p H , a n d t h e p r e s e n c e o f o t h e r l i g a n d s (sulphate, c a r b o n a t e , f l u o r i d e , a n d o r g a n i c s p e c i e s ) ( S t u m m a n d M o r g a n 1981). A b o u t 15 t o 6 0 % of d i s s o l v e d ( a n d in p a r t c o l l o i d a l ) p h o s p h o r u s consists o f t h e o r g a n i c p h o s p h a t e esters w h i c h a r e c h e m i c a l l y p o o r l y d e f i n e d . These c o m p o u n d s a r e d e r i v e d f r o m t h e e x c r e t a or l e a c h a t e s of living o r g a n i s m s a n d f r o m t h e autolysis o f d e a d o r g a n i s m s ( F e n c h e l a n d B l a c k b u r n 1979). Particulate Phosphorus Particulate  phosphorus c a n b e c o m p o s e d of m a n y  minerals,  amorphous  p r e c i p i t a t e s , a n d s o r b e d r e a c t i o n p r o d u c t s . A l a r g e p r o p o r t i o n o f t h e p h o s p h o r u s in fresh w a t e r ( o f t e n g r e a t e r t h a n 9 0 % ) , o c c u r s a s o r g a n i c p h o s p h a t e s a n d c e l l u l a r constituents in t h e b i o t a a d s o r b e d t o i n o r g a n i c a n d d e a d p a r t i c u l a t e o r g a n i c materials i n c l u d i n g ( W e t z e l 1983): 1) p h o s p h o r u s in o r g a n i s m s , as: a ) relatively s t a b l e n u c l e i c a c i d s , D N A , R N A , a n d p h o s p h o p r o t e i n s (not i n v o l v e d in r a p i d c y c l i n g of p h o s p h o r u s ) ; b) l o w - m o l e c u l a r - w e i g h t esters o f e n z y m e s , v i t a m i n s , e t c . ; a n d c) n u c l e o t i d e phosphates, such as adenosine d i p h o s p h a t e (ADP) a n d a d e n o s i n e 5-triphosphate (ATP) u s e d in b i o c h e m i c a l p a t h w a y s of respiration a n d CO2 assimilation; 2) m i n e r a l p h a s e s o f r o c k a n d soil, s u c h a s h y d r o x y a p a t i t e , in w h i c h p h o s p h o r u s is p r e s e n t a s d i s c r e t e m i n e r a l p a r t i c l e s o r is a d s o r b e d o n t o i n o r g a n i c p a r t i c l e s s u c h a s c l a y s , c a r b o n a t e s , a n d ferric h y d r o x i d e s ; a n d 3) p h o s p h o r u s a d s o r b e d o n t o d e a d p a r t i c u l a t e o r g a n i c m a t t e r o r in m a c r o o r g a n i c aggregations.  Biological Importance of Phosphorus P h o s p h o r u s p l a y s a m a j o r role in b i o l o g i c a l m e t a b o l i s m . P h o s p h a t e is essential for t h e transfer o f e n e r g y a n d p h o s p h o r y l a t i o n s , a n d for t h e synthesis o f n u c l e i c a c i d s in  all living cells. In t h e c e l l , o r t h o p h o s p h a t e is c o u p l e d t o A D P t o f o r m ATP. In c o m p a r i s o n t o o t h e r m a c r o n u t r i e n t s r e q u i r e d b y f r e s h w a t e r b i o t a , p h o s p h o r u s is least a b u n d a n t a n d c o m m o n l y is t h e first e l e m e n t t o limit b i o l o g i c a l p r o d u c t i v i t y . P h o s p h o r u s is biolimiting in t e m p e r a t e z o n e lakes (Schindler 1977), a n d its c o n c e n t r a t i o n c a n b e u s e d t o p r e d i c t t h e t o t a l b i o m a s s of p h y t o p l a n k t o n t h a t will d e v e l o p in s u c h w a t e r b o d i e s (Dillon a n d Rigler 1974). O r t h o p h o s p h a t e is t h e most easily utilized f o r m of s o l u b l e i n o r g a n i c p h o s p h o r u s b y a l g a e , h i g h e r plants a n d b a c t e r i a , a n d t h e h i g h reactivity of this ion results in its c o m m o n d e p l e t i o n . P h o s p h a t e i n t e r a c t s w i t h m a n y c a t i o n s ( e . g . Fe a n d C a ) in solution t o f o r m , e s p e c i a l l y u n d e r oxidizing c o n d i t i o n s , relatively insoluble p r e c i p i t a t e s . S o m e o r g a n i s m s c a n t a k e u p o r g a n i c p h o s p h a t e s d i r e c t l y , w h i l e others first hydrolyze t h e m  using  extracellular alkaline  phosphatases.  However,  organic  p h o s p h o r u s in t h e w a t e r c o l u m n c a n b e v e r y resistant t o hydrolysis a n d not r e a d i l y available  to microorganisms. Bacteria  (and  some  other organisms)  may  store  p h o s p h o r u s in v o l u t i n g r a n u l e s w h e n t h e y a r e l i m i t e d b y s o m e o t h e r nutrient ( e . g . s u l p h a t e ) . This s t o r e d p h o s p h o r u s p o o l m a y m a k e u p a l a r g e f r a c t i o n of t h e t o t a l p h o s p h o r u s c o n c e n t r a t i o n in t h e c e l l ( F e n c h e l a n d B l a c k b u r n 1979), a n d is c o m p r i s e d of l i n e a r p o l y p h o s p h a t e s o f v a r i o u s lengths. W h e n t h e c e l l s b e g i n t o g r o w a g a i n , t h e p h o s p h a t e s a r e r a p i d l y m o b i l i z e d for D N A synthesis ( B a r s d a t e et a l . 1974). In n a t u r a l w a t e r s , t h e a m o u n t o f p h o s p h o r u s c o n t a i n e d in living o r g a n i s m s is usually m u c h g r e a t e r t h a n t h e a m o u n t of o r t h o p h o s p h a t e d i s s o l v e d in t h e w a t e r a n d t h e e x c h a n g e r a t e b e t w e e n t h e s e t w o p o o l s is m u c h g r e a t e r t h a n t h e n e t d e p o s i t i o n of u n d i s s o l v e d p h o s p h o r u s . W h e n t h e r e is h i g h b i o l o g i c a l a c t i v i t y ( a l g a l b l o o m s , b a c t e r i a l growth  on  easily  decomposed  organic  material, etc.),  the  turnover  time  of  o r t h o p h o s p h a t e m a y b e v e r y short (i.e. less t h a n 2 m i n ) , w h e r e a s u n d e r c o n d i t i o n s of l o w b i o l o g i c a l a c t i v i t y or unusually h i g h o r t h o p h o s p h a t e c o n c e n t r a t i o n , t h e t u r n o v e r t i m e m a y e x c e e d 100 hours ( F e n c h e l a n d B l a c k b u r n 1979). P h o s p h o r u s - s t a r v e d a l g a e possess e n h a n c e d u p t a k e rates for POt  (Blum 1966),  a n d t h e e x t e n t t o w h i c h t h e m a x i m u m cellular u p t a k e rates is e n h a n c e d is i n f l u e n c e d b y t h e d e g r e e o f p h o s p h o r u s limitation. Highest u p t a k e rates a r e m o s t o f t e n a s s o c i a t e d w i t h e i t h e r t h e g r e a t e s t nutrient limitation ( e . g . Fuhs e t a l . 1972; R h e e 1973; E p p l e y a n d R e n g e r 1974) or w i t h a n i n t e r m e d i a t e d e g r e e o f limitation ( e . g . Terry 1983; Suttle a n d Harrison 1986). U p t a k e rates for P O ^ a r e t y p i c a l l y g r e a t l y e n h a n c e d in P-deficient cells  w i t h r e s p e c t t o t h o s e t h a t a r e P-sufficient ( M c C a r t h y 1981). B a c t e r i a m a y o u t c o m p e t e p h y t o p l a n k t o n in t h e u p t a k e o f o r t h o p h o s p h a t e , e s p e c i a l l y a t l o w o r t h o p h o s p h a t e c o n c e n t r a t i o n s (Levine a n d Schindler 1980; L e a n 1984; Currie a n d Kalff 1984). In situ studies s u g g e s t t h a t t h e b a c t e r i o p l a n k t o n d o m i n a t e p e l a g i c p h o s p h o r u s c y c l i n g (Rigler 1956; Burnison 1975; Currie a n d Kalff 1984); Currie e t a l . (1986) f o u n d t h a t b a c t e r i o p l a n k t o n a r e responsible for > 9 5 % of t h e o r t h o p h o s p h a t e  uptake, except  in l a k e s t h a t a r e  phosphorus-replete.  Mineralization of Phosphorus M i n e r a l i z a t i o n o f p h o s p h o r u s involves its transfer f r o m detritus t o living b i o m a s s a n d s o m e t i m e s t o f r e e i n o r g a n i c p h o s p h a t e . The n e t result d e p e n d s o n t h e t y p e of o r g a n i c m a t t e r d e g r a d e d a n d t h e t y p e of r e s p i r a t i o n u s e d in t h e d e c o m p o s i t i o n process. The activity  of m i c r o b e s  c a n also  cause  changes  in t h e  chemical  e n v i r o n m e n t t h a t a f f e c t m o b i l i z a t i o n o f p h o s p h o r u s . Extracellular p r o d u c t s o f m i c r o b e s such as enzymes a n d chelating agents  (e.g. organic acids)  m a y also mobilize  p h o s p h o r u s f r o m o r g a n i c p h o s p h a t e - e s t e r s a n d i n o r g a n i c salts. N e t m i n e r a l i z a t i o n results in p h o s p h o r u s f r o m t h e b i o d e t r i t a l p o o l  (microbe  b i o m a s s a n d d e a d o r g a n i c m a t t e r ) b e i n g t r a n s f o r m e d into m o b i l i z e d p h o s p h o r u s ( o r t h o p h o s p h a t e ) . T h e e x t e n t of transfer b e t w e e n t h e s e t h r e e p h o s p h o r u s p o o l s d u r i n g m i n e r a l i z a t i o n d e p e n d s u p o n ( F e n c h e l a n d B l a c k b u r n 1979): 1) t h e p h o s p h o r u s c o n t e n t of t h e o r g a n i c m a t t e r b e i n g d e g r a d e d ; a n d 2) t h e g r o w t h y i e l d of t h e mineralizing b a c t e r i a . (The g r o w t h y i e l d is e q u a l t o t h e a m o u n t of c a r b o n in o n e c e l l d i v i d e d b y t h e a m o u n t of c a r b o n a s s i m i l a t e d , i.e. t h e e f f i c i e n c y of o r g a n i c m a t t e r d e c o m p o s i t i o n ) . In t h e m i n e r a l i z a t i o n of p h o s p h o r u s - d e p l e t e o r g a n i c m a t t e r , t h e r e is a n e t i m m o b i l i z a t i o n of nutrients; a l l p h o s p h a t e is k e p t in t h e b a c t e r i a l c e l l resulting in n o PO4 r e l e a s e t o t h e w a t e r c o l u m n (at least d u r i n g t h e initial p h a s e s of d e g r a d a t i o n ) . If sufficient p h o s p h o r u s f o r g r o w t h is u n a v a i l a b l e f r o m t h e o r g a n i c m a t t e r b e i n g d e c o m p o s e d ,  bacteria  assimilate inorganic phosphorus from t h e environment. M a n y a q u a t i c b a c t e r i a c a n g r o w a t t h e e x p e n s e of o r g a n i c m a t t e r h a v i n g little or n o p h o s p h o r u s a n d / o r n i t r o g e n , w h i l e g e t t i n g their nutrients f r o m d i s s o l v e d i n o r g a n i c s o u r c e s (Ostroff a n d Henry 1939; M a c L e o d e t a l . 1954; M a c L e o d a n d O n o f r e y 1956; Bick 1958; S k e r m a n 1963). In a e r o b i c  b a c t e r i a , g r o w t h y i e l d s a r e t y p i c a l l y in t h e r a n g e o f 20 - 8 0 % , w h e r e a s  anaerobes  g e n e r a l l y h a v e g r o w t h yields o f 5 - 3 0 % (Bostr&m et a l . 1988). W h e n g r o w t h yields a r e l o w , a s d u r i n g a n a e r o b i c respiration a n d f e r m e n t a t i o n , a v a i l a b i l i t y ( a n d thus p o t e n t i a l m o b i l i t y ) o f m i n e r a l i z e d nutrients r e l a t i v e t o t h e o r g a n i c substrate is h i g h e r . The gross p h o s p h o r u s t r a n s p o r t in a n d o u t o f b a c t e r i a l c e l l s is o f t e n m u c h g r e a t e r t h a n t h e m e a s u r e d n e t transfers. A s a n e x a m p l e , B a r s d a t e et a l . (1974) f o u n d t h a t in o n e w e e k i n c u b a t i o n s o f Carex  litter, t h e n e t m i n e r a l i z a t i o n o f p h o s p h o r u s w a s 0.027 j i M » h ' \  H o w e v e r , t h e a c t u a l u p t a k e a n d r e l e a s e rates o f p h o s p h o r u s b y b a c t e r i a w e r e 4.84  The p r o p o r t i o n o f p h o s p h o r u s t h a t r e m a i n s r e f r a c t o r y a f t e r long-term b a c t e r i a l d e c o m p o s i t i o n of a l g a e a n d / o r m i x e d p l a n k t o n i c c o m m u n i t i e s ranges from 0 - 100% ( L e e e t a l . 1980). There a r e n o c l e a r d i f f e r e n c e s b e t w e e n a e r o b i c a n d  anaerobic  d e c o m p o s i t i o n . G o l t e r m a n (1975) f o u n d a p p r o x i m a t e l y 8 0 % of a l g a l p h o s p h o r u s w a s converted to PCM  w i t h i n t h r e e d a y s a f t e r i n d u c e d autolysis. A l a r g e a m o u n t  of  phosphorus r e l e a s e d b y cell d e a t h m a y r e c y c l e r e p e a t e d l y within the biomass a n d detrital p o o l s b e f o r e net r e g e n e r a t i o n o c c u r s w i t h , of c o u r s e , s o m e loss t o t h e refractory pool e a c h cycle. The n e t i m p a c t o f m i n e r a l i z a t i o n o n p h o s p h o r u s s o l u b i l i z a t i o n in s u r f a c e s e d i m e n t s d e p e n d s o n (Bostr&m et a l . 1988): 1) t h e d e g r e e of d e c o m p o s i t i o n of s e t t l e d o r g a n i c m a t t e r a n d t h e d o m i n a n t t y p e of m i n e r a l i z a t i o n (i.e. a e r o b i c or a n a e r o b i c ) ; 2) t h e initial p h o s p h o r u s c o n t e n t of settling o r g a n i c m a t t e r ; 3) t h e a v a i l a b i l i t y of p h o s p h o r u s in surrounding m e d i a ; 4) w h e t h e r s e t t l i n g o f o r g a n i c m a t t e r is m o r e or less c o n t i n u o u s or if it o c c u r s episodically; a n d 5) t h e t e m p e r a t u r e , r e d o x p o t e n t i a l , a n d o t h e r e n v i r o n m e n t a l f a c t o r s t h a t  affect  m i n e r a l i z a t i o n a n d c h e m i c a l equilibria of s i g n i f i c a n c e t o p h o s p h o r u s mobility. S e v e r a l t y p e s of m i c r o b e s c a n t a k e u p p h o s p h o r u s u n d e r a e r o b i c c o n d i t i o n s , a n d t h e n r e l e a s e it w h e n t h e e n v i r o n m e n t b e c o m e s a n a e r o b i c ( S h a p i r o 1967; O s b o r n a n d Nicholls 1978; M a r a i s et a l . 1983; Fleischer 1986). R a p i d u p t a k e o f p h o s p h a t e u n d e r aerobic  c o n d i t i o n s is t h o u g h t t o  be  c o u p l e d with  storage  of  phosphorus  in  p o l y p h o s p h a t e g r a n u l e s ( M a r a i s et a l . 1983; Florentz et a l . 1984). This c a p a c i t y for luxury  c o n s u m p t i o n o f p h o s p h o r u s m a y result in u p t a k e of POf" e v e n w h e n b a c t e r i a g r o w o n o r g a n i c m a t t e r w i t h l o w C : P ratios (Levin a n d S h a p i r o 1965). This r a p i d u p t a k e o f POA  is  t h o u g h t t o involve t h e f o l l o w i n g t w o steps (Wetzel et a l . 1986): 1) utilization of o r t h o p h o s p h a t e in p o l y p h o s p h a t e c h a i n s for ATP f o r m a t i o n ; a n d 2) utilization o f ATP for synthesis a n d s t o r a g e of poly-3-hydroxybutyrate (PHB). These r e a c t i o n s c a u s e a c c u m u l a t i o n of o r t h o p h o s p h a t e in c y t o p l a s m . T h e r e f o r e , t h e o s m o t i c pressure inside t h e c e l l i n c r e a s e s a n d net diffusion of p h o s p h a t e o u t of t h e c e l l o c c u r s . PHB is a n e n e r g y s o u r c e u s e d w h e n a e r o b i c c o n d i t i o n s recur. P o l y p h o s p h a t e s t o r a g e u n d e r a e r o b i c c o n d i t i o n s , f o l l o w e d b y p o l y p h o s p h a t e utilization a n d  PHB  s t o r a g e in a n a e r o b i c w a t e r s c o u l d b e a c o m p e t i t i v e a d v a n t a g e for m i c r o b e s t h a t live w h e r e t h e r e a r e c o n s t a n t shifts b e t w e e n a e r o b i c a n d a n a e r o b i c c o n d i t i o n s . S e v e r a l a l g a l , c y a n o b a c t e r i a l a n d b a c t e r i a l s p e c i e s in s u r f a c e l a k e s e d i m e n t h a v e b e e n f o u n d t o p r a c t i c e p o l y p h o s p h a t e s t o r a g e ( R o d h e 1948; J e n s e n 1968; C m i e c h 1981), a n d PHB g r a n u l e s h a v e b e e n f o u n d in c y a n o b a c t e r i a l cells in s u r f a c e s e d i m e n t s ( C m i e c h 1981). F a c u l t a t i v e a n a e r o b e s ( a n d s o m e e u c a r y o t e s s u c h a s yeasts) c a n r a p i d l y t a k e u p a n d r e l e a s e e x c e s s p h o s p h o r u s u n d e r b o t h a e r o b i c a n d a n a e r o b i c c o n d i t i o n s . In a series o f P t r a c e r e x p e r i m e n t s Fleischer (1983, 1985, 1986) f o u n d t h a t p u r e a n d m i x e d 33  cultures of f a c u l t a t i v e a n a e r o b e s c o l l e c t e d f r o m l a k e s e d i m e n t s a n d a c t i v a t e d s l u d g e c o u l d t a k e u p p h o s p h o r u s f r o m n e w l y p r e c i p i t a t e d Fe(lll) g e l u n d e r a e r o b i c c o n d i t i o n s . This m i c r o b i a l p h o s p h o r u s u p t a k e c o i n c i d e d w i t h solubilization of Fe(lll) g e l . P e p t i z a t i o n or f o r m a t i o n of c o m p l e x e s w e r e t h o u g h t t o b e t h e p r i n c i p a l p r o c e s s e s i n v o l v e d . U n d e r a n a e r o b i c c o n d i t i o n s , the b a c t e r i a r e l e a s e d phosphorus, most of w h i c h h a d  been  t a k e n u p b e f o r e t h e y c a m e in c o n t a c t w i t h t h e g e l . These e x p e r i m e n t s c a n  be  s u m m a r i z e d as: Aerobic conditions: Fe(lll)-Pi + M i c r o b i a l s -> Soluble Fe(lll) + M i c r o b i a l - ^ + P£ A n a e r o b i c conditions: Soluble Fe(lll) + Microbial-(P + Pj) -> Soluble Fe(ll) + S o l u b l e P + Microbial-Pi t  2  The p h o s p h o r u s r e l e a s e d is p r o b a b l y f r o m s t o r e d p o l y p h o s p h a t e s (Fleischer  1986).  T h e r e f o r e , p h o s p h o r u s m o b i l i z a t i o n f r o m living c e l l s m a y c o n t r i b u t e t o p h o s p h o r u s r e l e a s e f r o m a n a e r o b i c s e d i m e n t . The q u a n t i t a t i v e s i g n i f i c a n c e o f this p r o c e s s is, however, unknown.  4.2 Materials and Methods  B e c a u s e o f t h e u n e x p e c t e d l y l o w d i s s o l v e d p h o s p h o r u s c o n c e n t r a t i o n s initially e n c o u n t e r e d in P o w e l l L a k e , a m o r e e x t e n s i v e s t u d y of p h o s p h o r u s w a s d o n e t h a n f o r t h e o t h e r nutrients: s a m p l e s w e r e c o l l e c t e d f r o m five a d d i t i o n a l stations a s s h o w n in Fig. 4-2. D i s s o l v e d r e a c t i v e  phosphorus ( P O f ) , total (organic) phosphorus, a n d total  p a r t i c u l a t e p h o s p h o r u s w e r e d e t e r m i n e d . W a t e r w a s c o l l e c t e d in n e w , 10%-HCIw a s h e d polyethylene or p o l y p r o p y l e n e bottles that h a d never b e e n e x p o s e d t o d e t e r g e n t . S a m p l e s w e r e pressure filtered w i t h n i t r o g e n t h r o u g h 0.4 urn p o l y c a r b o n a t e N u c l e p o r e m e m b r a n e filters in t h e f i e l d using Niskin bottles w i t h a d d e d pressure fittings. B o t h w a t e r s a m p l e s a n d filters c o n t a i n i n g p a r t i c u l a t e s w e r e f r o z e n o n d r y i c e immediately after collection. Soluble Reactive Phosphorus S o l u b l e r e a c t i v e p h o s p h o r u s (SRP) w a s m e a s u r e d b y t h e c o l o u r i m e t r i c m e t h o d of M u r p h y a n d Riley (1962). A f t e r filtration a n d r e m o v a l of s u l p h i d e b y p u r g i n g w i t h n i t r o g e n , a c o m p o s i t e r e a g e n t of m o l y b d i c a c i d , a s c o r b i c a c i d a n d trivalent a n t i m o n y w a s a d d e d t o t h e s a m p l e . The resulting c o m p l e x h e t e r o p o l y a c i d is r e d u c e d in situ t o p r o d u c e a b l u e c o m p l e x . S a m p l e s w e r e d i l u t e d w i t h D D W t o < 2.5 j i M b e f o r e r e a g e n t a d d i t i o n . Using 10 c m cells, t h e d e t e c t i o n limit w a s 0.02 j i M w i t h a p r e c i s i o n of 0 . 5 % ( l a , rsd). This f r a c t i o n is l a r g e l y c o m p o s e d o f o r t h o p h o s p h a t e , a l t h o u g h it a l s o i n c l u d e s s o m e d i s s o l v e d o r g a n i c p h o s p h o r u s a s w e l l . The a b s o r b a n c e w a s m e a s u r e d 10 m i n . a f t e r r e a g e n t a d d i t i o n t o minimize t h e c o n t r i b u t i o n o f o r g a n i c p h o s p h o r u s . P h o s p h a t e , SRP, a n d DIP ( d i s s o l v e d i n o r g a n i c p h o s p h a t e ) will b e u s e d i n t e r c h a n g e a b l y in this chapter. Total Phosphorus Total phosphorus (£P) w a s d e t e r m i n e d after o x i d a t i o n of all P present b y m a g n e s i u m nitrate t o p h o s p h a t e ( C e m b e l l a e t a l . 1986). T w o m L of 2 0 % m a g n e s i u m nitrate (in e t h a n o l ) w e r e a d d e d t o 4 0 m L s a m p l e s in Pyrex e r l e n m e y e r flasks c o n t a i n i n g w a s h e d b o i l i n g c h i p s . S a m p l e s w e r e t h e n b o i l e d vigorously until a p p r o x i m a t e l y 8 0 % of t h e v o l u m e h a d e v a p o r a t e d . G l a s s slides w e r e p l a c e d o v e r t h e flasks t o a l l o w v e n t i n g a n d t h e h e a t w a s r e d u c e d until s p a t t e r i n g c e a s e d ( w h e n t h e s a m p l e s w e r e a l m o s t dry). H e a t w a s t h e n i n c r e a s e d t o m a x i m u m a n d m a i n t a i n e d until e v o l u t i o n of b r o w n f u m e s of  Fig. 4-2 M a p o f P o w e l l L a k e s h o w i n g six stations (•) s a m p l e d for p h o s p h o r u s analysis.  N0  2  w a s c o m p l e t e d . A f t e r c o o l i n g , t h e s a m p l e s w e r e b r o u g h t b a c k t o their o r i g i n a l  v o l u m e w i t h 0.1 N HCI, s e a l e d with p a r a f i l m , a n d t h e n a l l o w e d t o sit a p p r o x i m a t e l y 24 h t o a l l o w t h e salts t o r e d i s s o l v e . The s a m p l e s w e r e t h e n a n a l y s e d f o r p h o s p h a t e  as  d e s c r i b e d a b o v e . This d i g e s t i o n t e c h n i q u e g i v e s 93 - 1 0 0 % r e c o v e r y f o r i n o r g a n i c p h o s p h o r u s , p h o s p h a t e esters, a n d p h o s p h o n a t e s , a n d a l s o r e l e a s e s p h o s p h o r u s b o u n d t o iron a n d  a d s o r b e d t o CaCG-3. H o w e v e r , it d o e s not r e l e a s e all p h o s p h o r u s  i n c o r p o r a t e d into a p a t i t e s . The d e t e c t i o n limit is 0.05 [iM a n d t h e p r e c i s i o n is 5% (1 a , rsd). Particulate Phosphorus P a r t i c u l a t e p h o s p h o r u s (PP) w a s d e t e r m i n e d v i a t h i n film X-ray f l u o r e s c e n c e s p e c t r o m e t r y (XRF). Filters w e r e f r o z e n i m m e d i a t e l y a f t e r s a m p l e c o l l e c t i o n a n d t h e v o l u m e f i l t e r e d w a s r e c o r d e d . S a m p l e s t a k e n f r o m t h e s a l i n e p o r t i o n of t h e w a t e r c o l u m n w e r e r i n s e d w i t h D D W t o r e m o v e salt b e f o r e t h e y w e r e f r o z e n . Filters w e r e t h a w e d a n d d r i e d in a d e s i c c a n t c h a m b e r . Filters w e r e t h e n m o u n t e d in n y l o n filter h o l d e r s a n d c o u n t e d in a Philips P W 1 4 0 0 X-ray f l u o r e s c e n c e s p e c t r o m e t e r e q u i p p e d w i t h a Rh t a r g e t X-ray t u b e . Fresh s t a n d a r d s w e r e p r e p a r e d e a c h d a y b y c a r e f u l l y d r i p p i n g l O O j i L o f dilute K H P 0 4 ( a n d N a ^ C M f o r S analysis) solutions s o a s t o c o v e r t h e 2  N u c l e p o r e m e m b r a n e fairly e v e n l y w i t h d r o p s of t h e s t a n d a r d . All s a m p l e s c o n t a i n e d «  3 m g of p a r t i c u l a t e s ; u n d e r s u c h c o n d i t i o n s , XRF signal r e s p o n s e is a linear f u n c t i o n of  s a m p l e mass (Holmes 1981). All s a m p l e s w e r e w e l l a b o v e t h e d e t e c t i o n limit w h i c h w a s 0.97 n m o l . N o t e t h a t b e c a u s e t h e m e t h o d u s e d f o r p a r t i c u l a t e p h o s p h o r u s w a s so sensitive, o f t e n PP w a s d e t e c t a b l e w h e r e t o t a l p h o s p h o r u s w a s not. The P O C : P P a n d P O N : P P ratios r e p o r t e d in this c h a p t e r a r e n o t c o m p l e t e l y a c c u r a t e , a s t h e s a m p l e s w e r e c o l l e c t e d with different e q u i p m e n t . P O C a n d P O N w e r e c o l l e c t e d o n G F / C filters w i t h a n o m i n a l p o r e size b e t w e e n 1 a n d 2 n m . P a r t i c u l a t e p h o s p h o r u s s a m p l e s w e r e c o l l e c t e d o n p o l y c a r b o n a t e N u c l e p o r e filters w i t h a n o m i n a l p o r e size o f 0.4 n m . T h e r e f o r e , p h o s p h o r u s m a y b e slightly o v e r e s t i m a t e d r e l a t i v e t o c a r b o n a n d n i t r o g e n , i.e. t h e C : P a n d N:P ratios r e p o r t e d a r e p r o b a b l y s o m e w h a t l o w e r t h a n t h e y a r e in situ. A l s o , P O C a n d P O N m e a s u r e m e n t s w e r e not m a d e o n t h e s a m e s a m p l e s as t h e PP s a m p l e s as t h e latter w e r e c o l l e c t e d o n different d a y s ,  and  s o m e t i m e s , a t different d e p t h s . To c a l c u l a t e t h e C : P a n d N:P ratios, s o m e v a l u e s h a v e b e e n i n t e r p o l a t e d f r o m t h e P O C , P O N , a n d PP curves (Figs. 4-9 a n d 4-10).  4.3 Results  Powell Lake S o l u b l e r e a c t i v e p h o s p h o r u s is u n d e t e c t a b l e in t h e South b a s i n a b o v e 175 m a n d r e a c h e s a m a x i m u m of -0.7 u M in t h e b o t t o m w a t e r s (Fig. 4-3). Total p h o s p h o r u s is also u n d e t e c t a b l e in t h e o x i c p a r t o f t h e S o u t h b a s i n w a t e r c o l u m n , b u t in t h e u n d e r l y i n g a n o x i c w a t e r s , c o n c e n t r a t i o n s r e a c h 3.7 p M (Fig. 4-5). P a r t i c u l a t e p h o s p h o r u s r a n g e s f r o m 0.005 - 0.013 \LM in t h e u p p e r 225 m a n d increases t o 0.14 \iM a t 345 m (Fig. 4-5). The bulk o f t h e t o t a l p h o s p h o r u s in t h e b o t t o m w a t e r s consists o f SRP (-40 - 6 0 % ) (Fig. 4-7), w h e r e a s P P m a k e s u p a b o u t 2 0 % of I P n e a r t h e i n t e r f a c e , d e c r e a s i n g t o 5 - 1 0 % in t h e u p p e r w a t e r s . The r e m a i n i n g p h o s p h o r u s p o o l p r o b a b l y consists o f d i s s o l v e d o r g a n i c p h o s p h o r u s . B e c a u s e I P is u n d e t e c t a b l e a b o v e 140 m , t h e c o n t r i b u t i o n o f t h e various p h o s p h o r u s f r a c t i o n s t o t h e I P p o o l c a n n o t b e c a l c u l a t e d in t h e u p p e r o x i c w a t e r column. Both t h e p a r t i c u l a t e o r g a n i c C : P ( P O C : P P ) a n d N:P ( P O N : P P ) m o l a r ratios s h o w l a r g e m a x i m a a t t h e s u r f a c e (1700 a n d 140 respectively) (Fig. 4-9). The P O C : P P ratio t h e n r a p i d l y d e c r e a s e s d o w n t o 5 0 m , b e l o w w h i c h it is relatively c o n s t a n t (-300) t o t h e o x i c / a n o x i c i n t e r f a c e . B e l o w t h e i n t e r f a c e , t h e P O C : P P ratio i n c r e a s e s slightly, a n d r e m a i n s fairly c o n s t a n t until t h e b o t t o m 25 m , w h e r e it d e c r e a s e s sharply. The P O N : P P m o l a r ratio profile is similar in s h a p e t o t h a t of P O C : P P , a l t h o u g h t h e i n c r e a s e in P O N : P P b e l o w t h e i n t e r f a c e is m u c h m o r e p r o n o u n c e d (Fig. 4-9). A l s o , P O N : P P  decreases  m a r k e d l y b e l o w 200 m f r o m a b o u t 8 0 t o 8 a t 3 4 0 m d e p t h . D i s s o l v e d i n o r g a n i c N:P (DIN:DIP) m o l a r ratios a r e p l o t t e d with P O N : P P in Fig. 4-11. B e c a u s e t h e r e is n o d e t e c t a b l e p h o s p h a t e a b o v e 175 m , t h e DIN:DIP ratios a r e infinite in t h e u p p e r o x i c p o r t i o n of t h e w a t e r c o l u m n . This ratio is lowest a t 175 m (-1100) a n d t h e n i n c r e a s e s t o a p p r o x i m a t e l y 7300 in t h e b o t t o m waters. In t h e o t h e r five sites s a m p l e d in P o w e l l L a k e , SRP is u n d e t e c t a b l e a t all d e p t h s a t all stations. All stations a l s o s h o w v e r y l o w levels of I P , w i t h most c o n c e n t r a t i o n s b e i n g < 0.2 u M (Fig. 4-13). I P i n c r e a s e s in t h e b o t t o m a n o x i c w a t e r s o f t h e East b a s i n t o 0.7 u M . PP c o n c e n t r a t i o n s a r e a l s o v e r y l o w , w i t h most s a m p l e s c o n t a i n i n g < 0.02 u M PP (Fig. 4-14). The a n o x i c East b a s i n c o n t a i n s t h e m o s t P P , w i t h m a x i m u m c o n c e n t r a t i o n s o f a p p r o x i m a t e l y 0.063 \iM.  Sakinaw Lake S a k i n a w c o n t a i n s m u c h h i g h e r levels o f SRP t h a n P o w e l l , w i t h a m a x i m u m c o n c e n t r a t i o n o f n e a r l y 300 \iM in t h e b o t t o m w a t e r s (Fig. 4-4). P h o s p h a t e is d e t e c t a b l e a t all d e p t h s in t h e w a t e r c o l u m n , a l t h o u g h in t h e u p p e r o x i c w a t e r s it is p r e s e n t a t v e r y l o w c o n c e n t r a t i o n s o f a b o u t 0.07 n M . B e l o w t h e o x i c / a n o x i c i n t e r f a c e , SRP r a p i d l y i n c r e a s e s w i t h d e p t h . X P is also present in l o w quantities in t h e o x i c p o r t i o n of t h e w a t e r c o l u m n a n d t h e n i n c r e a s e s b e l o w t h e i n t e r f a c e t o > 400 \iM in t h e b o t t o m w a t e r s (Fig. 46). P P d i s p l a y s t w o m a x i m a , a t 20 m a n d a t t h e i n t e r f a c e , w h e r e n e a r l y 0.15 \M a r e p r e s e n t . B e l o w t h e i n t e r f a c e PP r a p i d l y d e c l i n e s t o c o n c e n t r a t i o n s o f a p p r o x i m a t e l y 0.005 n M . A g a i n t h e bulk o f t h e I P p o o l consists o f SRP; h o w e v e r , in S a k i n a w d i s s o l v e d p h o s p h a t e m a k e s u p a p p r o x i m a t e l y 8 0 % o f t h e I P in t h e b o t t o m w a t e r s (Fig. 4-8). The p e r c e n t a g e o f I P a t t r i b u t a b l e t o SRP is m u c h less in t h e o x i c w a t e r s , a l t h o u g h right b e l o w t h e s u r f a c e SRP m a k e s u p a b o u t 5 0 % o f t h e I P . PP c o n t r i b u t e s a b o u t 2 0 % of t h e I P p o o l in t h e u p p e r o x i c w a t e r s , b u t b e l o w t h e i n t e r f a c e , a m i n o r a m o u n t o f t h e p h o s p h o r u s is p r e s e n t in t h e p a r t i c u l a t e p h a s e . Both P O C : P P a n d P O N : P P v a l u e s a r e e x t r e m e l y h i g h a t t h e s u r f a c e o f S a k i n a w , a t n e a r l y 5000 a n d 600 r e s p e c t i v e l y (Fig. 4-10). These ratios d e c r e a s e r a p i d l y in t h e u p p e r 10 m t o a p p r o x i m a t e l y 400 a n d 50. B e l o w t h e i n t e r f a c e b o t h ratios i n c r e a s e with d e p t h t o m a x i m a of 6300 ( P O C : P P ) a n d 900 (PON:PP). The DIN:DIP ratios also d i s p l a y t w o m a x i m a , a l t h o u g h a t different a r e a s o f t h e w a t e r c o l u m n (Fig. 4-12). DIN:DIP is g r e a t e s t a t t h e s u r f a c e a n d just a b o v e t h e i n t e r f a c e . B e l o w t h e i n t e r f a c e this ratio is relatively c o n s t a n t at a b o u t 27.  Soluble reactive phosphorus (uM) u p p e r 30 m 0  0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8  0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  •  20  I  OXIC  anoxic  40  E 60 a Q 80  100  Sakinaw  120  140' 0  Fig. 4-3 Soluble r e a c t i v e p h o s p h o r u s in Powell L a k e . SRP is u n d e t e c t a b l e a b o v e 175 m.  1  •  1 1 1 1  •  1  i  i  t  i  i  i  i  i  i  • i  i .  i i  50 100 150 200 250 300 S o l u b l e r e a c t i v e p h o s p h o r u s (jaM) d e e p e r t h a n 30 m  Fig. 4-4 Soluble r e a c t i v e p h o s p h o r u s in S a k i n a w Lake. U p p e r s c a l e is for t h e o x i c w a t e r c o l u m n (top 30 m). Bottom s c a l e is for t h e a n o x i c w a t e r c o l u m n ( b e l o w 30 m).  S o l u b l e R e a c t i v e P h o s p h o r u s (\LM) Total P h o s p h o r u s (jiM)  Fig. 4-5 Soluble r e a c t i v e , p a r t i c u l a t e a n d t o t a l p h o s p h o r u s in Powell Lake  Fig. 4-6 Soluble r e a c t i v e , p a r t i c u l a t e a n d total p h o s p h o r u s in S a k i n a w L a k e  % Total Phosphorus  Fig. 4-7 P e r c e n t a g e of t o t a l p h o s p h o r u s c o n sisting of soluble r e a c t i v e a n d p a r t i c u l a t e P in P o w e l l Lake. Total P w a s u n d e t e c t a b l e a b o v e 140 m.  Fig. 4-8 P e r c e n t a g e o f t o t a l p h o s p h o r u s c o n sisting of soluble r e a c t i v e a n d p a r t i c u l a t e P in S a k i n a w Lake.  POC/PP 0  0  1000  200  2000  3000  400  4000  600  5000  800  6000  1000  PON/PP  Fig. 4-9 M o l a r ratios of p a r t i c u l a t e o r g a n i c c a r b o n a n d nitrogen to p a r t i c u l a t e p h o s p h o r u s in P o w e l l Lake  Fig. 4-10 M o l a r ratios of p a r t i c u l a t e o r g a n i c c a r b o n a n d nitrogen to particulate p h o s p h o r u s in S a k i n a w L a k e  PON/PP  PON/PP 0  50  0  20  43  60  80  100  120 140 j -i-  I i i i | i i i | i i i | i i i | i i i | i i j ,|.  -  partic.  0 203 400 600 0|—i—i—i—|—i—i—i—|—r—i—i—| i  800 i  i—|—i  1003 i  i—  Powell  103  £ Q. <D Q  150  OXIC  anoxic  200 -  250  303 -  350  2000  4000  6000  8030  1^ I 0  i  i  i  t  I i 20  i  i  i  I i 40  t  i  i  I i 60  i  i  I i 80  i  i  i  I i i 100  i  i  i  120  DIN/DIP  DIN/DIP  Fig. 4-11 M o l a r nitrogen t o p h o s p h o r u s ratios in P o w e l l Lake. Soluble r e a c t i v e p h o s p h o r u s is u n d e t e c t a b l e in t h e u p p e r 175 m , resulting in infinite dissolved i n o r g a n i c N:P (DIN/DIP) ratios.  Fig. 4-12 M o l a r nitrogen t o p h o s p h o r u s ratios in S a k i n a w L a k e  Total P h o s p h o r u s (nM) 0  0.1  0.2  0.3  0.4  0.5  P a r t i c u l a t e P h o s p h o r u s (^M) 0.6  0.7  Fig. 4-13 Total p h o s p h o r u s in various basins a n d at t h e h e a d of Powell L a k e (see Fig. 4-2 for locations). The East b a s i n c o n t a i n s relict s e a w a t e r .  0  0.01  0.02  0.03  0.04  0.05  0.06  0.07  Fig. 4-14 Particulate p h o s p h o r u s in various basins a n d at t h e h e a d of P o w e l l L a k e (see Fig. 4- 2 for locations). The East b a s i n c o n t a i n s relict seawater.  Log (K *IAP) sp  -B-CoCQj --•-CasOHCPOOs - • o- • C a H P 0  4  —e-Ca H(P04)j 4  —0~Ca Fe(PO«)* 2  — F e  3  ( P 0  4  )  -l--Mn (PO) 3  2  2  — a — MgHPO-4 MgNH P0 4  —X— 3D  Mg (P04) 3  4  2  -  Fig. 4-16 Saturation s t a t e of p h o s p h o r u s minerals  Fig. 4-15 Saturation state of p h o s p h o r u s minerals a n d c a l c i t e in P o w e l l Lake. L o g (K *IAP) > 0 = 5p  a n d c a l c i t e in S a k i n a w L a k e . L o g (K  supersaturated, < 0 = undersaturated.  supersaturated, < 0 = undersaturated.  *IAP) > 0 = sp  g  4.4 Discussion  The most striking d i f f e r e n c e b e t w e e n P o w e l l a n d S a k i n a w L a k e s is s e e n in t h e distribution o f p h o s p h o r u s . In S a k i n a w L a k e , p h o s p h a t e is r e l e a s e d a t d e p t h (Fig. 4-4), similar t o a m m o n i u m a n d d i s s o l v e d silicon ( C h a p t e r 3). Interstitial w a t e r s o f o r g a n i c - r i c h , a n o x i c s e d i m e n t s a r e o f t e n h i g h l y e n r i c h e d in p h o s p h a t e , a m m o n i u m a n d d i s s o l v e d silicon c o m p a r e d w i t h t h e o v e r l y i n g w a t e r (e.g. R i t t e n b e r g e t a l . 1955; N i s s e n b a u m e t a l . 1972; H a r t m a n n e t a l 1973; Sholkovitz 1973; H a r t m a n n e t a l 1976; Price 1976; M a r t e n s et a l . 1978; M o r s e a n d C o o k 1978), w h i c h s u p p o r t s a n a p p r e c i a b l e u p w a r d flux o f t h e s e nutrients f r o m t h e s e d i m e n t s u r f a c e ( e . g . S c h i p p e l e t a l . 1973; Suess 1976; Aller 1980; Elderfield e t a l . 1981). B e c a u s e d e e p w a t e r s in S a k i n a w L a k e a r e also e n r i c h e d in t h e s e t h r e e nutrients, a s well a s H2S, D O C , a n d alkalinity, t h e a n o x i c p o r t i o n of t h e w a t e r c o l u m n c a n b e c o n s i d e r e d t o b e a n a n a l o g u e f o r t h e interstitial w a t e r o f r e d u c i n g c o a s t a l s e d i m e n t s . Unlike S a k i n a w L a k e , P o w e l l L a k e has v e r y l o w c o n c e n t r a t i o n s of p h o s p h a t e a t d e p t h (Fig. 4-3). S o l u b l e r e a c t i v e p h o s p h o r u s (SRP) is v e r y l o w in t h e b o t t o m w a t e r , w i t h a m a x i m u m c o n c e n t r a t i o n of 0.7 |xM, c o m p a r e d t o 300 p.M in S a k i n a w (Fig. 4-4). In most o t h e r a s p e c t s t h e c o m p o s i t i o n o f t h e b o t t o m w a t e r s of P o w e l l a n d S a k i n a w Lakes is very similar: P o w e l l a l s o c o n t a i n s h i g h levels of H2S, NH4, Si(OH)4, D O C , a n d alkalinity a t d e p t h . It is t h e r e f o r e surprising t h a t t h e p h o s p h o r u s levels a r e s o different. It is e s p e c i a l l y u n u s u a l f o r p h o s p h o r u s t o b e s o l o w , w h e n a m m o n i u m c o n c e n t r a t i o n s a r e so h i g h (almost 4 m M ) .  Sakinaw Lake D i s s o l v e d p h o s p h o r u s i n c r e a s e s t o v e r y h i g h c o n c e n t r a t i o n s in S a k i n a w  Lake  b o t t o m waters primarily d u e t o mineralization a n d u p w a r d diffusion of i n o r g a n i c p h o s p h a t e (Fig. 4-4). D u e t o t h e d e c r e a s e d g r o w t h e f f i c i e n c y o f a n a e r o b i c (relative t o a e r o b i c ) m e t a b o l i s m , a n a e r o b e s must assimilate l a r g e r q u a n t i t i e s o f c a r b o n , t h e r e b y also  assimilating more  associated  p h o s p h o r u s , r e s u l t i n g in r e l e a s e  of  excess  p h o s p h o r u s n o t r e q u i r e d f o r nutrition ( F e n c h e l a n d B l a c k b u r n 1979). In t h e p r e v i o u s c h a p t e r I c o n c l u d e d , b a s e d o n t h e c o n s t a n c y o f t h e C : N ratios in S a k i n a w L a k e (Fig. 3-5), t h a t o r g a n i c m a t t e r w a s b e i n g d e g r a d e d in t h e s e d i m e n t s r a t h e r t h a n in t h e w a t e r c o l u m n a s it s e t t l e d . H o w e v e r , t h e p a r t i c u l a t e C : P a n d N:P ratios d o i n c r e a s e with d e p t h  in t h e a n o x i c p o r t i o n of t h e w a t e r c o l u m n ( C : P i n c r e a s e s - 2 5 x f r o m i n t e r f a c e t o n e a r b o t t o m , N:P - 2 3 x (Fig. 4-10)). T h e r e f o r e , s o m e d e g r a d a t i o n d o e s o c c u r in t h e w a t e r c o l u m n , w i t h p h o s p h o r u s b e i n g r e l e a s e d preferentially relative t o c a r b o n a n d n i t r o g e n a s t h e p a r t i c l e s sink. The r e l e a s e of P O f c a n n o t b e d e t e c t e d in t h e DIN:DIP ratios, h o w e v e r . B e l o w t h e o x y c l i n e , t h e DIN-.DIP ratio is r e a s o n a b l y c o n s t a n t w i t h d e p t h , i.e. b o t h a m m o n i u m a n d p h o s p h a t e a r e r e l e a s e d a t d e p t h , b u t in a p p r o x i m a t e l y t h e s a m e p r o p o r t i o n . A l t h o u g h t h e P O N : P P profiles i n d i c a t e s o m e d e c o m p o s i t i o n o c c u r s in t h e w a t e r c o l u m n , t h e b u l k of it o c c u r s in t h e s e d i m e n t s a n d thus t h e s m a l l a m o u n t of p h o s p h o r u s t h a t is r e m o v e d f r o m t h e PP f r a c t i o n in t h e w a t e r c o l u m n (PP  decreases  f r o m -0.01 | i M t o 0.003 J I M ) is s w a m p e d b y t h e h i g h c o n c e n t r a t i o n s of P O ? r e l e a s e d f r o m the sediments. In S a k i n a w L a k e t h e PP m a x i m u m a t t h e i n t e r f a c e (Fig. 4-6) c o i n c i d e s with t h e P O C a n d P O N m a x i m a (Fig. 3-5), w h i c h is consistent with t h e i n t e r p r e t a t i o n a d v a n c e d earlier t h a t t h e p e a k represents b a c t e r i a . There is a l s o a PP m a x i m u m at 20 m , most likely d u e t o p h y t o p l a n k t o n , t h a t d o e s not c o i n c i d e w i t h t h e P O C m a x i m u m a t 10 m. This m a y r e f l e c t m o v e m e n t of t h e p h y t o p l a n k t o n b l o o m , as s a m p l e s w e r e n o t c o l l e c t e d o n t h e s a m e d a y for b o t h P O C a n d PP. In t h e DIN:DIP profile (Fig. 4-12), t h e r e a r e a l s o t w o m a x i m a , n e a r t h e s u r f a c e a n d just a b o v e t h e i n t e r f a c e , w h i c h c o r r e l a t e w e l l with t h e PP m a x i m a . These DIN:DIP m a x i m a i n d i c a t e r e g i o n s of a c t i v e . g r o w t h ( a n d  consequent  p r o p o r t i o n a t e l y h i g h e r p h o s p h o r u s u p t a k e ) of p h y t o p l a n k t o n ( s u r f a c e ) a n d b a c t e r i a (interface). The v e r y l o w DIN:DIP v a l u e a t 35 m m a y , in p a r t , r e f l e c t r e m o v a l of i n o r g a n i c n i t r o g e n v i a r e d u c t i o n of NO3 t o N  2  b y denitrifying b a c t e r i a (Fig. 3-7). H o w e v e r , it m a y  also b e d u e t o d e s o r p t i o n of p h o s p h a t e f r o m iron oxides just b e l o w t h e i n t e r f a c e , as t h e l o w DIN:DIP v a l u e c o r r e l a t e s w i t h t h e d i s s o l v e d iron m a x i m u m (Fig. 5-10). iron oxides very effectively  scavenge  phosphate  by  phosphate from water a n d the  iron o x i d e s is b e l i e v e d t o limit t h e  p r e c i p i t a t i o n of  inorganic  availability of phosphorus  in  t r o p h o g e n i c w a t e r , t h e r e b y r e d u c i n g p r i m a r y p r o d u c t i v i t y ( M o r t i m e r 1941). W h e n t h e iron o x i d e s a r e r e d u c e d b e l o w t h e o x i c / a n o x i c i n t e r f a c e ( d i s c u s s e d in C h a p t e r 5), a n y a d s o r b e d p h o s p h a t e is r e l e a s e d . In estuaries, t h e o b s e r v e d c o n c e n t r a t i o n of d i s s o l v e d p h o s p h a t e m a y b e entirely c o n t r o l l e d b y p r o c e s s e s of a d s o r p t i o n a n d d e s o r p t i o n o n iron ( a n d o t h e r ) p a r t i c l e s ( P o m e r o y et a l . 1965; Butler a n d Tibbitts 1972; Stirling a n d  W o r m a l d 1977). A s w e l l , a d s o r p t i o n o f s o l u b l e p h o s p h a t e b y iron o x y h y d r o x i d e s h a s b e e n p o s t u l a t e d a s t h e m e c h a n i s m c o n t r o l l i n g t h e p h o s p h o r u s distribution in m a n y subt i d a l s e d i m e n t s ( S t u m m a n d L e c k i e 1970) a n d l a k e s e d i m e n t s (Williams et a l . 1970; Shukla et a l . 1971; Li et a l . 1972; Lijklema 1980). A t p H < 8.5, a m o r p h o u s h y d r a t e d ferric o x i d e c a n r e m o v e a s m u c h a s 5% o f its o w n w e i g h t o f p h o s p h o r u s a s o r t h o p h o s p h a t e f r o m solution ( S t a m m a n d Kohlschutter 1965). Extraordinarily h i g h C:N:P ratios a r e f o u n d in t h e s u r f a c e w a t e r p a r t i c u l a t e s of b o t h S a k i n a w a n d P o w e l l Lakes ( C : N ; P ( m o l a r ) -4800:580:1 a n d -1700:180:1 r e s p e c t i v e l y ) , a l t h o u g h t h e P O C : P P ratios d e c r e a s e t o - 4 0 0 t h r o u g h o u t t h e rest of t h e o x i c portion of b o t h w a t e r c o l u m n s (Figs. 4-9 a n d 4-10). These ratios a r e c o n s i d e r a b l y h i g h e r t h a n t h e a v e r a g e C:N:P of p h y t o p l a n k t o n (106:16:1) (Redfield et a l . 1963). H o w e v e r , t h e w e l l k n o w n R e d f i e l d ratio is a c o n s t a n t s t o i c h i o m e t r i c relation w h i c h d o e s not a l w a y s exist in n a t u r e (Takahashi et a l . 1985); different a l g a l g r o u p s a c c u m u l a t e f a t , c a r b o h y d r a t e s a n d o t h e r s t o r a g e p r o d u c t s in v a r y i n g p r o p o r t i o n s a n d h e n c e c o n t a i n v a r i o u s p r o p o r t i o n s of c a r b o n , n i t r o g e n , a n d p h o s p h o r u s . This v a r i a t i o n c a n e v e n b e f o u n d within s p e c i e s , d e p e n d i n g o n various e n v i r o n m e n t a l f a c t o r s (Lewin 1962), a n d t h e sestonic C:P ratio c a n v a r y d a i l y ( K a g a w a et a l . 1988). The C : N : P ratios of p h y t o p l a n k t o n c a n a l s o v a r y w i d e l y a c c o r d i n g t o t h e nutritional status o f t h e p h y t o p l a n k t o n , w i t h h i g h C : P a n d N:P ratios i n d i c a t i n g p h o s p h o r u s d e f i c i e n c y (e.g. E p p l e y a n d R e n g e r 1974; Perry 1976; R h e e 1978; G o l d m a n et a l . 1979; H e a l e y a n d H e n d z e l 1975, 1979, 1980; G a c h t e r a n d B l o e s c h 1985; T e z u k a 1985; Harris 1976; U e h l i n g e r 1981). G a c h t e r a n d B l o e s c h (1985) f o u n d t h a t t h e b i o m a s s C : P ratio of a g r e e n a l g a , Chlamydomonas,  increases during summer, w h e n  p h o s p h o r u s is growth-limiting; d u r i n g sufficient or e x c e s s P s u p p l y , t h e  Chlamydomonas  C : P m o l a r ratio w a s o b s e r v e d t o b e less t h a n 106, w h e r e a s a d e c r e a s e in SRP s u p p l y p r o m o t e d a n i n c r e a s e in t h e C : P ratio t o m a x i m u m v a l u e s in t h e r a n g e 300 - 500. The C:P m o l a r r a t i o o f h e a l t h y a l g a e in p h o s p h o r u s - l i m i t e d c o n t i n u o u s c u l t u r e s i n c r e a s e s t o m o r e t h a n 900 (Uehlinger 1981; G a c h t e r a n d B l o e s c h 1985), a n d r a p i d P l e a c h i n g of d e a d a l g a e furthers this i n c r e a s e in t h e C : P ratio o f detritus u p t o 1470 ( G o l t e r m a n 1964; U e h l i n g e r 1986). H e a l e y a n d H e n d z e l (1980) f o u n d C : P m o l a r ratios a s h i g h as 1840 in s e v e r a l C a n a d i a n lakes. C : P a n d N:P ratios c a n also d r o p t o v e r y l o w levels w h e n a l g a e a s s i m i l a t e m o r e nutrients t h a n t h e y n e e d , t h e s o - c a l l e d "luxury* u p t a k e . This h a s b e e n  s e e n in a study o f 15 l a k e s h a v i n g different t r o p h i c levels, w h e r e N:P ratios in s u s p e n d e d solids v a r i e d b e t w e e n 4-13 (Forsberg e t a l . 1978). Thus, nutrient s u p p l y a p p e a r s t o r e g u l a t e t h e s t o i c h i o m e t r i c c o m p o s i t i o n of biomass.  C : P ratios c l o s e  to the Redfield  value  evidently  occur  only  when  p h y t o p l a n k t o n g r o w t h is n o t limited b y p h o s p h o r u s , a situation n o t t y p i c a l of t e m p e r a t e f r e s h w a t e r l a k e s , s u c h a s P o w e l l a n d S a k i n a w d u r i n g s u m m e r . In p h o s p h o r u s - l i m i t e d e n v i r o n m e n t s , this nutrient is m u c h m o r e e f f i c i e n t l y u s e d b y a l g a e f o r biosynthesis o f o r g a n i c m a t t e r , a l l o w i n g t h e p h y t o p l a n k t o n t o assimilate m u c h m o r e t h a n 106 m o l of c a r b o n p e r mol of phosphorus. H i g h C : P ratios o f s e s t o n c a n a l s o r e f l e c t a n a l l o c h t h o n o u s origin. C o n s i d e r a b l e p r o p o r t i o n s o f s u s p e n d e d P O C m a y b e detrital a n d n o t a l g a l , d e p e n d i n g o n l o c a t i o n . H o w e v e r , detrital input s h o u l d b e m i n i m a l in b o t h P o w e l l a n d S a k i n a w L a k e s , particularly in t h e f o r m e r w h e r e t h e r e is n o d i r e c t f r e s h w a t e r input t o t h e s o u t h b a s i n . For t h e s a m e r e a s o n . PIP s h o u l d b e m i n i m a l a n d h e n c e t h e P P c o n c e n t r a t i o n s m e a s u r e d in b o t h P o w e l l a n d S a k i n a w s h o u l d represent primarily P O P .  Powell Lake P h o s p h a t e is first d e t e c t a b l e in P o w e l l L a k e a t 175 m (Fig. 4-3), w h i c h is w h e r e N H l starts t o i n c r e a s e (Fig. 3-6). T h e r e f o r e it w o u l d a p p e a r t h a t t h e P O * is b e i n g g e n e r a t e d f r o m m i n e r a l i z a t i o n of o r g a n i c m a t t e r a s in S a k i n a w L a k e . H o w e v e r , t h e N H l c o n c e n t r a t i o n c o n t i n u e s t o i n c r e a s e fairly r a p i d l y w h i l e t h e c o n c e n t r a t i o n of P O * stays fairly c o n s t a n t t o 250 - 275 m; b e l o w this l e v e l , t h e PO4 c o n t e n t a l s o b e g i n s t o i n c r e a s e rapidly. This is r e a d i l y s e e n b y e x a m i n i n g t h e DIN:DIP ratio, w h i c h is "infinite" d o w n t o 175 m (Fig. 4-11). P e r h a p s p h o s p h o r u s is r e l e a s e d a t this d e p t h v i a d e s o r p t i o n f r o m iron o x i d e s . P a r t i c u l a t e iron w a s n o t m e a s u r e d in this s t u d y , b u t d i s s o l v e d iron ( F e ) r a p i d l y 2+  i n c r e a s e s t o h i g h c o n c e n t r a t i o n s (-170 t i M ) b e t w e e n  150 m a n d 2 0 0 m (Fig. 5-9),  i n d i c a t i n g t h a t a s i g n i f i c a n t a m o u n t o f p a r t i c u l a t e iron must b e p r e s e n t a t s h a l l o w e r d e p t h s . B e l o w 175 m , m o r e NH4 t h a n PO4 a p p e a r s t o b e r e l e a s e d , o r PO4 is b e i n g p r e f e r e n t i a l l y c o n s u m e d . B e l o w 250 m , w h e r e t h e PO4 c o n c e n t r a t i o n i n c r e a s e s , t h e DIN:DIP ratio b e c o m e s fairly c o n s t a n t ( i g n o r i n g t h e 3 2 5 m d a t u m ) . T h e P O N : P P ratio a c t u a l l y d e c r e a s e s w i t h d e p t h , w h e r e a s P O O P O N i n c r e a s e s (Fig. 3-4), i n d i c a t i n g t h a t  n i t r o g e n is b e i n g r e l e a s e d a n d p h o s p h o r u s is b e i n g preferentially i n c o r p o r a t e d into t h e particulate phase. The p a r t i c u l a t e p h o s p h o r u s c o n c e n t r a t i o n r e m a i n s v e r y l o w in P o w e l l L a k e until n e a r t h e b o t t o m . There is n o d e c l i n e in PP in t h e b o t t o m w a t e r c o m m e n s u r a t e w i t h t h e P O C a n d P O N d e c r e a s e s d e s c r i b e d earlier (Fig. 3-4). Thus, t h e r e is a l a r g e d e c r e a s e in t h e C : P a n d N:P ratios b e l o w -300 m (Fig. 4-9). In C h a p t e r 3,1 s u g g e s t e d t h a t t h e P O C a n d P O N d e c r e a s e s w e r e d u e t o f l o c c u l a t i o n a n d s u b s e q u e n t r a p i d sinking o f l a r g e r p a r t i c l e s ; b u t a t first g l a n c e , t h e h i g h PP v a l u e s i n d i c a t e t h a t this m a y n o t b e t h e c a s e . H o w e v e r , t h e h i g h e r c o n c e n t r a t i o n s of PP m a y reflect t h e p r e s e n c e o f g r e a t e r n u m b e r s of b a c t e r i a n e a r t h e s e d i m e n t - w a t e r i n t e r f a c e w h e r e t h e r e is m o r e substrate a n d t h e PO4 c o n c e n t r a t i o n is relatively h i g h . In s u p p o r t of this, n o t e t h a t t h e d e c r e a s e in P O C is larger t h a n t h a t of P O N n e a r t h e b o t t o m , resulting in a d e c r e a s e in t h e p a r t i c u l a t e C : N ratio. A s n o t e d in C h a p t e r 3, b a c t e r i a a r e k n o w n t o h a v e b o t h l o w e r C : N a n d C : P ratios than phytoplankton. The l o w c o n c e n t r a t i o n of PO4 in t h e b o t t o m w a t e r of P o w e l l L a k e is u n u s u a l a s PO4  is usually m i n e r a l i z e d in a n o x i c e n v i r o n m e n t s a s is s e e n in S a k i n a w L a k e . Three  e x p l a n a t i o n s f o r t h e l o w p h o s p h o r u s c o n c e n t r a t i o n s in P o w e l l L a k e a r e as follows: 1) t h e a n a l y t i c a l t e c h n i q u e u s e d d i d n o t d e t e c t t h e p h o s p h o r u s present; 2) t h e p h o s p h o r u s is b e i n g r e m o v e d i n o r g a n i c a l l y f r o m t h e w a t e r c o l u m n v i a m i n e r a l p r e c i p i t a t i o n or b y a d s o r p t i o n o n t o particles b e f o r e t h e y fall rapidly t o t h e b o t t o m ; a n d 3) t h e c o n c e n t r a t i o n of p h o s p h o r u s in t h e l a k e is naturally v e r y l o w s o t h a t v e r y little c a n b e t a k e n u p b y organisms. The first m e c h a n i s m c a n b e r u l e d o u t d i r e c t l y , a s s t a n d a r d a d d i t i o n s of KH2PO4 w e r e m a d e t o samples with subsequent normal c o l o u r d e v e l o p m e n t . Also, t h e t e c h n i q u e w o r k e d w e l l in t h e c h e m i c a l l y - s i m i l a r S a k i n a w L a k e w a t e r . B e c a u s e various p h o s p h o r u s m i n e r a l s a r e k n o w n t o f o r m in a q u a t i c e n v i r o n m e n t s , t h e s e c o n d o p t i o n w a r r a n t s d e t a i l e d a t t e n t i o n a n d will b e d i s c u s s e d in t h e f o l l o w i n g s e c t i o n . Phosphorus Removal Due to Adsorption P h o s p h a t e is r e a d i l y a d s o r b e d o n t o c l a y s , iron o x y h y d r o x i d e s a n d C a C 0 . O n c e 3  s o r b e d , t h e i o n is n o t s o r e a d i l y d e s o r b e d , a n d t h e p r o c e s s m a y b e irreversible (Barrow 1983). A s t i m e a n d d i s s o l v e d p h o s p h o r u s c o n c e n t r a t i o n s i n c r e a s e , s o d o e s t h e q u q n t i t y of p h o s p h o r u s t a k e n u p . G e n e r a l l y , s o r p t i o n i n c r e a s e s a s t h e i o n i c s t r e n g t h o f t h e  b a c k g r o u n d s o l u t i o n i n c r e a s e s . The s o u t h b a s i n of P o w e l l L a k e is f a r f r o m  major  t e r r i g e n o u s s e d i m e n t a n d t h e r e is a l a c k of detrital c l a y s in s o u r c e rocks of t h e d r a i n a g e b a s i n (B. B a r n e s pers. c o m m . ) ; t h e r e f o r e t h e r e is little e x t e r n a l input of detrital c l a y . As m e n t i o n e d earlier, a n y PO4  a d s o r b e d t o iron o x y h y d r o x i d e s in t h e u p p e r w a t e r s will b e  r e l e a s e d in t h e a n o x i c w a t e r s w h e n t h e iron is r e d u c e d . C a l c i u m plays a n  i m p o r t a n t role in p h o s p h o r u s r e m o v a l  in m a n y  lakes.  P h o s p h a t e h a s b e e n s h o w n t o a d s o r b strongly o n c a l c i u m c a r b o n a t e ( C o l e et a l . 1953; d e K a n e l a n d M o r s e 1978; Otsuki a n d W e t z e l 1972; K i t a n o et a l . 1978) a n d this has b e e n u s e d t o e x p l a i n w h y c a l c i u m c a r b o n a t e - r i c h s e d i m e n t s c o n t a i n l o w c o n c e n t r a t i o n s of d i s s o l v e d p h o s p h a t e in their p o r e w a t e r s (Berner 1973; M o r s e a n d C o o k 1978). Freshly precipitating C a C 0 absorbs p h o s p h a t e very effectively, with t y p i c a l P : C a C 0 3  3  ratios of  1:100at high PO4 c o n c e n t r a t i o n s or 1:1000 at low P O f c o n c e n t r a t i o n s (Lijklema et a l . 1983). The p r e s e n c e of e v e n a s m a l l a m o u n t of p h o s p h a t e ions in a p a r e n t solution f a v o u r s c a l c i t e f o r m a t i o n , a l t h o u g h p h o s p h a t e ions a r e c o p r e c i p i t a t e d m o r e e a s i l y w i t h a r a g o n i t e t h a n with c a l c i t e (Krtano et a l . 1978). CaC0 both C a C 0  3  3  p r e c i p i t a t i o n is f a v o u r e d b y h i g h t e m p e r a t u r e ( d e c r e a s i n g solubility of  a n d CO2)  a n d h i g h p H (Otsuki a n d W e t z e l 1972). The c o p r e c i p i t a t i o n of  p h o s p h a t e w i t h c a l c i t e d u e t o t h e rise in phi a c c o m p a n y i n g p h o t o s y n t h e t i c utilization of CO2 is w e l l k n o w n in m a r l l a k e s (Otsuki a n d W e t z e l 1972). Ishaq a n d K a u l (1988) f o u n d p h o t o s y n t h e t i c a l l y - i n d u c e d c o p r e c i p i t a t i o n in a H i m a l a y a n l a k e w h e r e t h e c a l c i u m - r i c h s e d i m e n t a c t s a s a n e x c e l l e n t p h o s p h o r u s t r a p . The p h o s p h o r u s u p t a k e w a s sufficient t o c o u n t e r a 4 0 % i n c r e a s e in t h e p h o s p h o r u s input t o t h e lake during t h e p e r i o d 1971 -1981: n o significant i n c r e a s e in p h o s p h o r u s c o n c e n t r a t i o n o c c u r r e d in l a k e w a t e r s d u r i n g t h a t t i m e ( I s h a q .1985). A similar p h e n o m e n o n h a s a l s o b e e n o b s e r v e d in B l a c k  Lake  ( C a n a d a ) w h e r e s o l u b l e r e a c t i v e p h o s p h o r u s (SRP) c o n c e n t r a t i o n s e x c e e d i n g 2.5 | i M ( M u r p h y et a l . 1983) fall t o z e r o in t h e p h o t i c z o n e d u r i n g b l o o m s of c y a n o b a c t e r i a a n d associated induced C a C 0  3  p r e c i p i t a t i o n . In b l o o m s w h e r e n o C a C 0  3  precipitated, the  SRP d e c r e a s e w a s small. The saturation s t a t e of C a C 0 w a s c a l c u l a t e d using a c o m p u t e r m o d e l (MINEQL), 3  a s d e s c r i b e d in A p p e n d i x 1. The results s h o w t h a t C a C 0  3  is u n d e r s a t u r a t e d at all d e p t h s  in P o w e l l L a k e , e x c e p t a t 3 0 0 m a n d b e l o w , w h e r e t h e a l k a l i n i t y a n d c o n c e n t r a t i o n s a r e v e r y h i g h (Fig. 4-15). A s s u m i n g t h a t t h e  calcium  M I N E Q L results  are  r e p r e s e n t a t i v e f o r t h e s e w a t e r s it is c l e a r t h a t C a C 0 3 c a n n o t r e m o v e p h o s p h a t e a t d e p t h s a b o v e 300 m. In a d d i t i o n , it is unlikely t h a t c a l c i t e is p r e c i p i t a t i n g a t > 300 m either, g i v e n t h a t c a l c i t e is o n l y b a r e l y s a t u r a t e d a t this l e v e l . T h e p r e s e n c e o f M g i m p e d e s t h e g r o w t h rate of C a C 0  3  strongly  2 +  crystals a n d thus a h i g h s u p e r s a t u r a t i o n is usually  r e q u i r e d t o e f f e c t p r e c i p i t a t i o n ( S t u m m a n d M o r g a n 1981). C a C 0  3  formation also  requires s e e d crystals f o r n u c l e a t i o n f o r w h i c h t h e r e is n o detrital s o u r c e in P o w e l l L a k e . A l s o , l a c u s t r i n e p l a n k t o n w h i c h p r o d u c e c a l c a r e o u s shells a r e e x t r e m e l y r a r e ; n o s u c h o r g a n i s m s w e r e s e e n in mid-July p l a n k t o n t o w s c o l l e c t e d in P o w e l l L a k e (Styan 1976; B. B a r n e s pers. c o m m . ) . T h e r e f o r e , g i v e n t h e c o m p a r a t i v e l y l o w p H (5.7-6.8) (Fig. 3-10), relatively l o w y e a r r o u n d s u r f a c e t e m p e r a t u r e s (usually < 10°C), h i g h c o n c e n t r a t i o n s o f M g \ a n d l a c k of 2  s e e d crystals, it is unlikely t h a t i n o r g a n i c C a C 0 p r e c i p i t a t i o n is a v e r y i m p o r t a n t p r o c e s s 3  in P o w e l l L a k e a n d h e n c e p h o s p h o r u s c a n n o t b e r e m o v e d v i a this p r o c e s s . N o t e t h a t CaC0  3  is s a t u r a t e d in t h e d e e p w a t e r s o f in S a k i n a w L a k e (Fig. 4-16).  However,  p r e c i p i t a t i o n in S a k i n a w L a k e is unlikely for t h e s a m e reasons a s in P o w e l l .  Phosphorus Removal via Direct Precipitation A n u m b e r o f p h o s p h a t e minerals a r e k n o w n t o f o r m a u t h i g e n i c a l l y in n a t u r a l waters or sediments, including chlorapatite (Ca5(P04) CI), fluorapatite (Ca (PCM) F), 3  h y d r o x y l a p a t i t e (Ca (PC>4) OH), strengite ( F e P 0 « 2 H 0 ) , vivianite 5  3  4  2  5  3  (Fe (P0 ) »H 0), 3  4  2  2  r e d d i n g i t e ( M n ( P C > 4 ) ) . struvite ( M g N H P 0 4 « 6 H 0 ) , a n d n e w b e r y i t e ( M g H P 0 4 « 3 H 0 ) . 3  2  4  2  2  E n v i r o n m e n t s s u i t a b l e f o r a u t h i g e n i c p h o s p h a t e m i n e r a l f o r m a t i o n a r e mostly f o u n d in highly r e d u c e d sediments, w h e r e mineralization a n d r e d u c t i o n typically a d d high c o n c e n t r a t i o n s o f i r o n , p h o s p h a t e a n d o t h e r ions t o interstitial solution. H o w e v e r , in P o w e l l L a k e , p h o s p h a t e r e m a i n s in l o w c o n c e n t r a t i o n until v e r y n e a r t h e b o t t o m , n e v e r e x c e e d i n g 0.7 yM (Fig. 4-3). The s a t u r a t i o n s t a t e o f v a r i o u s p h o s p h o r u s m i n e r a l s w a s c a l c u l a t e d v i a M I N E Q L f o r b o t h P o w e l l a n d S a k i n a w L a k e s (Figs. 4-15 a n d 4-16). A l l m i n e r a l s w e r e f o u n d t o b e u n d e r s a t u r a t e d in P o w e l l L a k e ; m o s t o f t h e m e x t r e m e l y s o . H y d r o x y a p a t i t e is t h e o n l y p h o s p h o r u s m i n e r a l t h a t a p p r o a c h e s s a t u r a t i o n in P o w e l l L a k e , a n d t h e n o n l y a t 325 m d e p t h . T h e r e f o r e , p h o s p h o r u s r e m o v a l in P o w e l l L a k e v i a m i n e r a l f o r m a t i o n is unlikely t o o c c u r in P o w e l l L a k e .  Unlike P o w e l l L a k e , s e v e r a l p h o s p h o r u s minerals a r e s a t u r a t e d in S a k i n a w L a k e . H y d r o x y a p a t i t e is s u p e r s a t u r a t e d t h r o u g h o u t m o s t of t h e a n o x i c w a t e r c o l u m n in S a k i n a w (Fig. 4-16). A p a t i t e is t h e r m o d y n a m i c a l l y t h e least soluble p h o s p h a t e m i n e r a l in natural waters, with c a l c i u m hydroxyapatite (Ca (P04)30H) a n d c a l c i u m fluorapatite 5  ( C a ( P 0 4 ) 3 F ) b e i n g t h e least s o l u b l e forms (Emerson 1976). A p a t i t e s c a n b e d i r e c t l y 5  p r e c i p i t a t e d o n s u r f a c e s of b i o g e n i c silica a n d / o r i n o r g a n i c p h a s e s s u c h a s c a l c i t e (Burnett 1977; S t u m m a n d L e c k i e 1970) or it c a n r e p l a c e C a C 0 ( D A n g l e j a n 1968; 3  M a n h e i m et a l . 1975). W e l l crystallized a u t h i g e n i c a p a t i t e a n d c a r b o n a t e a p a t i t e h a v e b e e n i d e n t i f i e d in m a r i n e s e d i m e n t s ( G u l b r a n d s e n 1969; A l t s c h u l e r 1973). The p r e s e n c e of p o o r l y crystalline p a r t i c l e s in lacustrine s e d i m e n t s has a l s o b e e n s u g g e s t e d (Emerson a n d W i d m e r 1978), b u t t h e p r e c i p i t a t i o n kinetics of a p a t i t e in s u c h e n v i r o n m e n t s s e e m t o b e sluggish (Frevert 1979). A p a t i t e p r e c i p i t a t i o n o c c u r s u n d e r similar c o n d i t i o n s t o t h a t of CaCC>3. e x c e p t t h a t r e a s o n a b l y h i g h c o n c e n t r a t i o n s of P 0  4  a r e r e q u i r e d , a s t h e r e a c t i o n rate is strongly  r e d u c e d a t h i g h C O f / P O j ratios ( S t u m m a n d L e c k i e 1970). S u p e r s a t u r a t i o n h a s b e e n f o u n d for a p a t i t e in m a n y s e d i m e n t s (Burnett 1977; D ' A n g l e j a n 1968; G a u d e t t e a n d Lyons 1980; K r o m a n d Berner 1980; M a n h e i m 1974; C a r i g n a n 1984; E m e r s o n 1976; H o l d r e n a n d A r m s t r o n g 1986; L o f g r e n a n d R y d i n g 1985a,b; Norvell 1974) a n d results f r o m n u c l e a t i o n barriers: t h e n e c e s s a r y s e e d crystals m a y b e a b s e n t ( d e B o e r 1977), or M g  2 +  (Martens  a n d Harriss 1970; H a n d s c h u h a n d O r g e l 1973), or o r g a n i c c o m p o u n d s (Stumm 1973) m a y a c t a s s u r f a c e inhibitors for n u c l e a t i o n . W h e r e t h e M g Ca  2 +  2 +  c o n c e n t r a t i o n is h i g h relative t o  a s in S a k i n a w L a k e ( a n d P o w e l l ) , s u p e r s a t u r a t i o n m a y r e a c h s u c h a d e g r e e t h a t  m a g n e s i a n c a l c i t e or a r a g o n i t e m a y p r e c i p i t a t e m e t a s t a b l y (e.g. Berner 1975). O r g a n i c a d s o r b a t e s a l s o inhibit t h e c r y s t a l l i z a t i o n r e a c t i o n s , e s p e c i a l l y a t l o w e r p H v a l u e s . S t u m m (1973) f o u n d t h a t a t y p i c a l n o n i o n i c o r g a n i c s u r f a c t a n t , Triton X-100, a t  a  c o n c e n t r a t i o n of 5 x 10" M c a u s e d a d e c r e a s e of a b o u t 5 0 % in t h e crystallization rate at 5  p H 6.8; n o significant i n t e r f e r e n c e o c c u r r e d at p H v a l u e s a b o v e 7.4. He s u g g e s t e d t h a t this r a t e r e d u c t i o n w a s d u e t o a d s o r p t i o n of t h e s u r f a c e - a c t i v e m o l e c u l e a t t h e crystalg r o w t h sites. A t h i g h e r p H t h e g r o w t h of t h e crystalline l a t t i c e m a y b e r a p i d e n o u g h t o s i m p l y c o v e r o v e r t h e a d s o r b e d m o l e c u l e ( S t u m m a n d L e c k i e 1970). D i s s o l v e d  PO4  d o e s d e c r e a s e in t h e b o t t o m 20 m of S a k i n a w L a k e , h o w e v e r , a p a t i t e is unlikely t o  p r e c i p i t a t e in S a k i n a w L a k e d u e t o t h e l a c k o f s e e d crystals, h i g h c o n c e n t r a t i o n s of Mg *, andDOC. 2  S e v e r a l o t h e r minerals a r e s a t u r a t e d in S a k i n a w L a k e b o t t o m w a t e r s . Mg (P04)2 is 3  s a t u r a t e d a t 75 m a n d d e e p e r . H o w e v e r , a l t h o u g h this m i n e r a l is v e r y i n s o l u b l e , struvite (MgNH4P04»6H 0) a n d n e w b e r y i t e ( M g H P 0 » 3 H 0 ) a l w a y s p r e c i p i t a t e p r e f e r e n t i a l l y t o 2  Mg (P04) 3  2  4  2  ( A b b o n a e t a l . 1982). B o t h m a r i n e a n d fresh p o r e w a t e r s h a v e b e e n r e p o r t e d  t o b e s a t u r a t e d or s u p e r s a t u r a t e d with r e s p e c t t o struvite (Elderfield et a l . 1981; M a r t e n s e t a l . 1978; M c C a f f r e y et a l . 1980). H a n d s c h u h a n d O r g e l (1973) f o u n d t h a t struvite i n s t e a d of a p a t i t e p r e c i p i t a t e d w h e n o r t h o p h o s p h a t e w a s a d d e d t o s e a w a t e r c o n t a i n i n g . N H l c o n c e n t r a t i o n s similar t o t h o s e f o u n d in n e a r s h o r e m a r i n e interstitial w a t e r s .  Struvite  formation has also b e e n o b s e r v e d during d e c o m p o s i t i o n of nitrogen-rich o r g a n i c materials in l a b o r a t o r y e x p e r i m e n t s ( M a l o n e a n d T o w e 1970) a n d in cultures of sulphatereducing  b a c t e r i a , under conditions closely approximating the anoxic  marine  s e d i m e n t a r y e n v i r o n m e n t ( H a l l b e r g 1972). N e w b e r y i t e is a l w a y s a s s o c i a t e d w i t h struvite a n d it m a y f o r m b y d i r e c t p r e c i p i t a t i o n or b y d e c o m p o s i t i o n of struvite (Boistelle a n d A b b o n a 1981). H o w e v e r , n e w b e r y i t e p r e c i p i t a t e s o n l y in a d e f i n i t e d o m a i n of p H a n d c o n c e n t r a t i o n i.e. p H < 5.8 a n d PO4 c o n c e n t r a t i o n s > 0.03 M (Boistelle a n d A b b o n a 1981). O u t s i d e this d o m a i n , struvite d e p o s i t s , o r n o p r e c i p i t a t i o n o c c u r s a t a l l . Struvite p r e c i p i t a t e s w h e r e p H is < 6.8 a n d p h o s p h a t e c o n c e n t r a t i o n s a r e > 10 m M ( A b b o n a et al. 1982) a n d is m e t a s t a b l e a t l o w NH4 a c t i v i t y , transforming into n e w b e r y i t e w h e n (NH ) < 4  15 n M . Thus, t h e p r e c i p i t a t i o n o f n e w b e r y i t e a n d struvite requires h i g h m a g n e s i u m a n d p h o s p h a t e c o n c e n t r a t i o n s ( a n d a m m o n i u m for struvite) t o p r e c i p i t a t e , all of w h i c h a r e p r e s e n t in S a k i n a w L a k e d e e p w a t e r . PO4. NH4 a n d M g  2 +  concentrations  do decrease  n e a r t h e b o t t o m in S a k i n a w L a k e (Figs. 2-15,3-7,4-6), h o w e v e r , b o t h t h e s e minerals a r e u n d e r s a t u r a t e d in S a k i n a w a n d thus a r e unlikely t o f o r m . S i n c e struvite a n d n e w b e r y i t e a r e u n d e r s a t u r a t e d in S a k i n a w L a k e , it is a l s o unlikely t h a t M g ( P 0 ) 3  4  2  p r e c i p i t a t e s in  S a k i n a w L a k e , e v e n t h o u g h this m i n e r a l a p p e a r s t o b e s a t u r a t e d . A n a p a i t e w a s also f o u n d t o b e s a t u r a t e d in S a k i n a w L a k e . N r i a g u a n d Dell (1974) s u g g e s t e d that a n a p a i t e ( C a F e ( P 0 4 ) » 4 H 0 ) c o u l d o c c u r t o g e t h e r with vivianite a n d 2  2  2  r e d d i n g i t e a n d t h a t a t t h e p H v a l u e s , a n d c a l c i u m a n d p h o s p h o r u s c o n c e n t r a t i o n s likely to  be  encountered  in m a n y  freshwater  sediments,  anapaite,  rather  than  h y d r o x y a p a t i t e , w o u l d b e t h e s t a b l e , C a - b e a r i n g p h o s p h a t e m i n e r a l . H o w e v e r , this m i n e r a l h a s n e v e r b e e n i d e n t i f i e d in m a r i n e or lacustrine s e d i m e n t s t o m y k n o w l e d g e . V i v i a n i t e (Fe (P04)2) is t h e s t a b l e p h o s p h a t e m i n e r a l in r e d u c i n g e n v i r o n m e n t s 3  ( N r i a g u 1972) a n d y e t it is highly u n d e r s a t u r a t e d in b o t h lakes. It is easily r e c o g n i z a b l e in t h e f i e l d a s it o c c u r s c o n c e n t r a t e d a s c l e a r w h i t e spots in s e d i m e n t s w h i c h turn bright b l u e u p o n e x p o s u r e t o air d u e t o o x i d a t i o n o f part o f t h e Fe(ll) within t h e vivianite crystal l a t t i c e , f o r m i n g a m i x e d iron (11,111) p h a s e o f i n d e t e r m i n a t e c o m p o s i t i o n , t e r m e d k e r t s c h e n i t e ( F a y e e t a l . 1968). H i g h ferrous iron a c t i v i t i e s a r e r e q u i r e d t o stabilize v i v i a n i t e in m o s t c h e m i c a l systems ( N r i a g u 1972), a n d , t h e r e f o r e , it is t h e F e  2 +  activity,  rather t h a n t h e p h o s p h a t e c o n c e n t r a t i o n , that a c t u a l l y d e t e r m i n e s w h e t h e r or not s a t u r a t i o n f o r v i v i a n i t e is a t t a i n e d ( P o s t m a  1982). A s a n e x a m p l e , C o r n w e l l (1987)  o b s e r v e d a u t h i g e n i c v i v i a n i t e f o r m a t i o n in Toolik L a k e , A l a s k a , a n o n - c a l c a r e o u s , nons u l p h i d i c , p h o s p h o r u s - p o o r u l t r a o l i g o t r o p h i c l a k e . V i v i a n i t e in l a k e s e d i m e n t s is g e n e r a l l y f o u n d in e u t r o p h i c systems a n d its u n u s u a l o c c u r r e n c e in Toolik L a k e is a t t r i b u t e d t o t h e Fe reduction-diffusion-oxidation c y c l e concentrations of porewater Fe  2 +  which  results  in h i g h  that also leads t o t h e release of a d s o r b e d phosphorus  ( C o r n w e l l 1987). Thus, d e s p i t e l o w p h o s p h o r u s inputs, vivianite p r e c i p i t a t i o n o c c u r s . The a b s e n c e o f h i g h rates o f s u l p h a t e r e d u c t i o n , e i t h e r b e c a u s e o f l o w inputs o f o r g a n i c m a t t e r o r l o w c o n c e n t r a t i o n s o f s u l p h a t e , is n e c e s s a r y t o p r e v e n t s c a v e n g i n g o f F e  2 +  by  s u l p h i d e s . The m o s t f a v o u r a b l e c o n d i t i o n s f o r v i v i a n i t e f o r m a t i o n t h e r e f o r e , a r e a n o x i c f r e s h w a t e r e n v i r o n m e n t s a n d , in f a c t , vivianite h a s n e v e r b e e n c o n c l u s i v e l y i d e n t i f i e d in marine sediments (Postma  1982). To illustrate this, Ellis-Evans & L e m o n (1989) f o u n d  v i v i a n i t e p r e c i p i t a t i o n in S o m b r e L a k e , a f r e s h w a t e r , m a r i t i m e l a k e in t h e A n t a r c t i c w h i c h c o n t a i n s v e r y l o w levels o f s u l p h i d e (4 j i M ) in t h e a n o x i c b o t t o m w a t e r . A m o s L a k e , another Antarctic lake,  h a d m u c h h i g h e r levels o f s u l p h i d e (25 u M m a x . ) a n d t o t a l  p h o s p h o r u s (-250 ^ M ) , a n d n o p h o s p h a t e p r e c i p i t a t i o n o c c u r r e d .  Biological Removal A s t h e l a c k o f p h o s p h a t e in P o w e l l L a k e c a n n o t b e e x p l a i n e d b y either a n a l y t i c a l error, o r a s a result o f i n o r g a n i c r e m o v a l , t h e possibility t h a t it is b i o l o g i c a l l y c o n t r o l l e d must b e c o n s i d e r e d . T h e r e f o r e , s o m e d i s c u s s i o n o f p h o s p h o r u s u p t a k e b y b a c t e r i a a n d p h y t o p l a n k t o n is r e q u i r e d .  U n d e r c o n d i t i o n s o f h i g h p h o s p h o r u s limitation w h i c h is c h a r a c t e r i z e d b y t h e e x t r a o r d i n a r i l y h i g h C : N : P ratios o f s u r f a c e p l a n k t o n in P o w e l l a n d S a k i n a w  Lakes,  b a c t e r i a a c t u a l l y t a k e u p , rather t h a n r e l e a s e P O ^ w h e n d e g r a d i n g s u c h p h o s p h o r u s d e p l e t e o r g a n i c m a t t e r . This p h o s p h o r u s u p t a k e of p h o s p h o r u s b y d e c o m p o s e r s w o u l d a p p e a r t o o c c u r in P o w e l l L a k e . The DIN:DIP ratios h a v e t w o m a x i m a in P o w e l l L a k e (Fig. 4-11). The first is e s s e n t i a l l y t h e e n t i r e u p p e r 175 m , w h e r e t h e r e is n o  detectable  p h o s p h a t e ; this results artificially, in infinite DINDIP m o l a r ratios. A t 175 m , p h o s p h a t e is first d e t e c t a b l e a n d it is a t this d e p t h t h a t t h e lowest m e a s u r a b l e DIN:DIP ratio o c c u r s . B e l o w this d e p t h t h e DIN:DIP ratio g r a d u a l l y i n c r e a s e s w i t h d e p t h , i n d i c a t i n g t h a t p h o s p h o r u s is n o t b e i n g m i n e r a l i z e d t o t h e s a m e e x t e n t a s n i t r o g e n , i.e., it is b e i n g a s s i m i l a t e d b y b a c t e r i a . B e l o w 175 m , SRP is r e l e a s e d b u t in m u c h l o w e r q u a n t i t i e s t h a n a m m o n i u m . The P O N : P P ratio profile s u p p o r t s this (Fig. 4-11). B e l o w 200 m , P O N : P P d e c r e a s e s w i t h d e p t h a n d mirrors t h e DIN:DIP p r o f i l e , i n d i c a t i n g t h a t p h o s p h o r u s is m o v i n g f r o m t h e dissolved fraction t o the particulate fraction. B a c t e r i a h a v e a m u c h l o w e r C P ratio t h a n a l g a e . F e n c h e l a n d B l a c k b u r n (1979) report a n a v e r a g e C : P a t o m i c ratio o f 48 for b a c t e r i a in g e n e r a l , w h e r e a s B r a t b a k (1985) f o u n d a t o m i c C : P ratios v a r y i n g b e t w e e n  8 a n d 56 f o r m i x e d b a c t e r i a l c u l t u r e s ,  d e p e n d i n g o n w h e t h e r their g r o w t h w a s phosphorus- or c a r b o n - l i m i t e d , a n d G d c h t e r et a l . (1988) f o u n d a m o l a r C : P ratio of 39 in n a t u r a l l a k e a s s e m b l a g e s of b a c t e r i a . A v e r a g e growth efficiency m a y approximate 5 0 % under aerobic conditions (Fenchel  and  B l a c k b u r n 1979). T h e r e f o r e , a s s u m i n g a b a c t e r i a l C : P ratio of 4 8 , SRP s h o u l d b e r e l e a s e d o n l y If t h e C : P ratio o f t h e o r g a n i c substrate is < 96. S i n c e t h e seston s u p p l i e d f r o m t h e e p i l i m n i o n o f b o t h P o w e l l a n d S a k i n a w Lakes h a s a C : P ratio c o n s i d e r a b l y > 9 6 , it is e v i d e n t t h a t a t least in t h e u p p e r p a r t o f t h e m o n i m o l i m n i o n m i n e r a l i z a t i o n p r o c e s s e s c a n n o t l e a d t o a n a c c u m u l a t i o n of SRP. V a r i o u s r e s e a r c h e r s h a v e r e p o r t e d u p t a k e o f SRP b y a q u a t i c b a c t e r i a ( e . g . B a r s d a t e et a l . 1974; P l a n a s 1978; F e n c h e l a n d B l a c k b u r n 1979; Fleisher 1983). In t h e p h o t i c z o n e , b a c t e r i a c a n o u t c o m p e t e p h y t o p l a n k t o n in t h e u p t a k e of SRP, at least a t l o w SRP c o n c e n t r a t i o n s (Currie a n d Kalff 1984; L e a n a n d W h i t e 1983; a n d L e a n 1984). Currie et a l . (1986) f o u n d t h a t t h e b a c t e r i o p l a n k t o n a r e responsible for > 9 5 % of t h e o r t h o p h o s p h a t e u p t a k e in situ, e x c e p t in lakes t h a t a r e not p h o s p h o r u s - d e f i c i e n t , a n d o r t h o p h o s p h a t e uptake by algae  was  greatest  in t h e l e a s t p h o s p h o r u s - d e f i c i e n t l a k e s s t u d i e d .  H y p o l i m n e t i c d e c o m p o s e r s c a n r a p i d l y a n d a c t i v e l y t a k e u p PO4.  keeping  the  p h o s p h o r u s c o n c e n t r a t i o n s o l o w t h a t little p h o s p h o r u s is a v a i l a b l e for i n o r g a n i c c o m p l e x a t i o n a t overturn (Levine a n d S c h i n d l e r 1980; D o r e m u s a n d C l e s c e r i 1982). Sherr e t a l . (1982) s h o w e d t h a t m i c r o b i a l b r e a k d o w n o f Peridinium  cells w a s a c c e l e r a t e d b y  t h e a d d i t i o n of p h o s p h a t e , suggesting that d e c o m p o s e r s w e r e phosphorus-limited. K a m p - N i e l s e n (1974) f o u n d t h a t p o i s o n e d s e d i m e n t s s o r b e d less SRP t h a n u n t r e a t e d systems, i n d i c a t i n g that phosphorus a c c u m u l a t i o n b y organisms continues w h e n settling  particles  reach  the  sediment  surface.  c h e m o a u t o t r o p h i c b a c t e r i a s u c h a s Beggiatoa  Production  of  biomass  by.  sp. a n d m e t h a n o g e n s will further  c o n t r i b u t e t o SRP fixation in l a k e s e d i m e n t s u n d e r a p p r o p r i a t e r e d o x c o n d i t i o n s . In a s t u d y o f t h e a e r o b i c d e c o m p o s i t i o n o f t h e g r e e n a l g a  reinhardii  Chlamydomonas  b y a m i x e d p o p u l a t i o n o f l a k e b a c t e r i a in b a t c h a n d c h e m o s t a t c u l t u r e s ,  Uehlinger (1986) f o u n d t h a t t h e b a c t e r i a l mineralization of a l g a l c a r b o n is linked with n o or m i n o r p h o s p h o r u s r e l e a s e . P h o s p h o r u s u p t a k e b y b a c t e r i a g r o w n o n heat-killed a l g a e v a r i e d a c c o r d i n g t o t h e C : P ratio of t h e a l g a e . A t a n a l g a l C : P of 114, 5 5 % of t h e t o t a l s o l u b l e p h o s p h o r u s (TSP)  w a s t a k e n u p b y t h e b a c t e r i a at t h e b e g i n n i n g of  an  e x p e r i m e n t , w h i l e a t a C : P o f 2 2 7 , 6 0 % w a s a s s i m i l a t e d , a n d at a C : P of 8 5 1 , 8 4 % of t h e TSP w a s i n c o r p o r a t e d . W h e n t h e b a c t e r i a l p o p u l a t i o n b e g a n t o d e c r e a s e b e c a u s e of s h o r t a g e o f f o o d , s o m e p h o s p h o r u s w a s r e g e n e r a t e d . The c h e m o s t a t e x p e r i m e n t s s h o w e d t h a t a c o n t i n u o u s s u p p l y of d e a d a l g a e p r e v e n t e d t h e d e c a y p h a s e of t h e b a c t e r i a l p o p u l a t i o n , a n d thus a n y n e t r e g e n e r a t i o n . U e h l i n g e r (1986) s u g g e s t e d t h a t p h o s p h o r u s in t h e e p i l i m n i o n is finally r e l e a s e d b y t h e g r a z i n g a c t i v i t y of b a c t i v o r o u s z o o p l a n k t o n or b y autolysis of t h e b a c t e r i a as a result of starvation. G a c h t e r a n d M a r e s (1985) f o u n d in t h e Swiss lakes Z u g , L u c e r n e a n d G r e i f e n t h a t t h e C : P ratio of seston c o n s i d e r a b l y e x c e e d e d t h e R e d f i e l d ratio (> 258 in L u c e r n e a n d G r e i f e n a n d - 2 3 2 in Zug). H o w e v e r , this ratio d e c r e a s e d w i t h i n c r e a s i n g w a t e r d e p t h ( a n d h e n c e i n c r e a s i n g a g e o f t h e seston), a n d e v e n t u a l l y b e c a m e as l o w as 25.8 in t h e a n o x i c h y p o l i m n i o n o f Z u g . A similar d e c r e a s e of t h e C : P ratio w i t h i n c r e a s i n g s a m p l i n g d e p t h w a s o b s e r v e d b y H o l m - H a n s e n (1972) in L a k e T a h o e , i n d i c a t i n g t h a t t h e seston t a k e s u p SRP w h i l e settling. G a c h t e r a n d M a r e s (1985) s u g g e s t e d t h a t t h e u p t a k e of SRP b y settling p a r t i c l e s m i g h t h a v e c o n t r i b u t e d t o t h e o b s e r v e d i n c r e a s e o f p a r t i c u l a t e  p h o s p h o r u s flux with i n c r e a s i n g d e p t h . They also f o u n d t h a t a f t e r p a r t i c u l a t e m a t t e r w a s d e p o s i t e d a t t h e l a k e b o t t o m it c o n t i n u e d t o a c c u m u l a t e SRP. T e z u k a (1986) s h o w e d t h a t n o SRP is r e l e a s e d f r o m t h e s e s t o n o f L a k e B i w a , a severely  P-limited, m e s o t r o p h i c J a p a n e s e l a k e , d u r i n g a e r o b i c d e c o m p o s i t i o n ,  a l t h o u g h DIN a c c u m u l a t e s a b u n d a n t l y in t h e h y p o l i m n i o n d u r i n g t h e s t a g n a t i o n p e r i o d (Tezuka 1984, 1985). T e z u k a (1985) s u g g e s t e d t h a t this l a c k of SRP r e l e a s e is d u e t o t h e h i g h C : P (516) a n d N:P ratios (27 - 40) o f t h e p h y t o p l a n k t o n in L a k e Biwa. To illustrate this, p h y t o p l a n k t o n a s s e m b l a g e s ( c o l l e c t e d f r o m L a k e Biwa) w i t h different C : N : P ratios (high (814:122:1), m e d i u m (266:38:1) a n d l o w (147:12:1)) w e r e g r o w n in t h e l a b o r a t o r y a n d subsequently d e c o m p o s e d u n d e r a e r o b i c conditions (Tezuka  1989a). The  algal  a s s e m b l a g e w i t h t h e highest N:P ratio (122) d i d not r e l e a s e SRP, w h e r e a s t h a t with t h e lowest N:P ratio (12) r e l e a s e d SRP a b u n d a n t l y . N i t r o g e n b e h a v e d m u c h like p h o s p h o r u s a s DIN w a s a b u n d a n t l y r e l e a s e d o n l y f r o m t h e a l g a l a s s e m b l a g e w i t h t h e highest N:P ratio (122), w h i l e t h e r e w a s little or n o r e l e a s e of  DIN f r o m t h e a l g a l a s s e m b l a g e s w i t h  l o w e r N:P ratios (38 a n d 12). T e z u k a (1989a) further s t u d i e d t h e e f f e c t o f C : N : P ratios of o r g a n i c substrates o n t h e r e g e n e r a t i o n of DIN a n d SRP b y n a t u r a l a s s e m b l a g e s of l a k e b a c t e r i a , using simple o r g a n i c C , N a n d P c o m p o u n d s ( g l u c o s e , a s p a r a g i n e  and  s o d i u m g l y c e r o p h o s p h a t e ) . In t h e s e e x p e r i m e n t s , w h e n t h e C : N a n d N:P ratios w e r e b o t h l o w e r t h a n 11.7 a n d 22 r e s p e c t i v e l y , b o t h DIN a n d SRP w e r e r e g e n e r a t e d . H o w e v e r , w h e n t h e C : N a n d N:P ratios w e r e higher t h a n 17.6 a n d 55 respectively, neither DIN nor SRP w e r e r e g e n e r a t e d . O n t h e other h a n d , w h e n t h e C : N ratio w a s lower t h a n 11.7 a n d t h e N:P ratio w a s higher t h a n 22, only DIN w a s r e g e n e r a t e d . In c o n t r a s t , in L a k e K a s u m i g a u r a , a highly e u t r o p h i c J a p a n e s e l a k e , t h e seston h a s v e r y l o w a v e r a g e C : N a n d N:P ratios o f 7.1 - 8.1 a n d 14.4 - 22.5, r e s p e c t i v e l y (Aizaki a n d O t s u k i 1987). W h e n p h y t o p l a n k t o n c o l l e c t e d f r o m this l a k e w e r e d e c o m p o s e d a e r o b i c a l l y t h e y r e l e a s e d b o t h DIN a n d SRP (Aizaki a n d T a k a m u r a 1986). T e z u k a (1989b) s h o w e d t h a t b o t h Microcystis  (C:N:P= 191:29:1) a n d Anabaena  (C:N:P= 150:18:1), t w o b l u e  g r e e n a l g a l s p e c i e s w h i c h g r o w in e u t r o p h i c e n v i r o n m e n t s a n d h a v e l o w C : N : P ratios, a l s o r e l e a s e b o t h DIN a n d SRP a b u n d a n t l y d u r i n g a e r o b i c d e c o m p o s i t i o n . Note that the  C : N : P ratios of Microcystis  (119:29:1) a n d Anabaena  (150:18:1)  o b s e r v e d in t h e p r e v i o u s study a n d of t h e p h y t o p l a n k t o n of L a k e K a s u m i g a u r a (Aizaki a n d Otsuki 1987) fall a p p r o x i m a t e l y into t h e C:N:P r a n g e t h a t releases b o t h DIN a n d SRP,  w h e r e a s t h e C : N : P ratio (516:44:1) of t h e p h y t o p l a n k t o n in t h e north b a s i n o f L a k e B i w a in s u m m e r falls into t h e C : N : P r a n g e t h a t releases DIN a l o n e . Thus it is e v i d e n t t h a t t h e C:N:P ratio o f p h y t o p l a n k t o n is a n i m p o r t a n t p a r a m e t e r for d e t e r m i n i n g t h e relative a m o u n t s of DIN a n d SRP r e l e a s e d b y a e r o b i c d e c o m p o s i t i o n . In a l l t h e e x a m p l e s o f b a c t e r i a l p h o s p h o r u s u p t a k e p r e v i o u s l y d i s c u s s e d , decomposition took p l a c e under aerobic conditions. Under a n a e r o b i c conditions, b a c t e r i a l g r o w t h e f f i c i e n c i e s a r e o n l y 5 - 3 0 % ( F e n c h e l a n d B l a c k b u r n 1979). H e n c e , m u c h m o r e o r g a n i c m a t t e r must b e d e c o m p o s e d f o r t h e b a c t e r i a t o a s s i m i l a t e t h e s a m e a m o u n t o f c a r b o n a n d thus relatively m o r e p h o s p h o r u s is a s s i m i l a t e d . T h e r e f o r e , in a n a e r o b i c d e c o m p o s i t i o n p h o s p h o r u s r e l e a s e g e n e r a l l y o c c u r s , a s b a c t e r i a c a n a c q u i r e s u f f i c i e n t p h o s p h o r u s e v e n f r o m o r g a n i c m a t t e r w i t h h i g h e r C : P ratios. To illustrate this, B a c c i n i (1985), using a l g a l m a t t e r l a b e l l e d w i t h  M  C and  3 2  P b a c t e r i a in t h e  o x i c p o r t i o n o f a s e d i m e n t c o r e s t o r e d p h o s p h o r u s a n d d e c r e a s e d t h e C : P ratio in this horizon.  A  significant  release  of phosphorus  from  the sediment  s i m u l t a n e o u s l y , b u t this p h o s p h o r u s w a s u n l a b e l l e d a n d o r i g i n a t e d f r o m  occurred deeper,  r e d u c e d s e d i m e n t layers. The o r g a n i c m a t t e r in P o w e l l L a k e is e x t r e m e l y p h o s p h o r u s d e p l e t e d ( C : P - 3 0 0 - 400). P e r h a p s , e v e n a t l o w e r g r o w t h e f f i c i e n c i e s , b a c t e r i a simply c a n n o t fulfil their p h o s p h o r u s r e q u i r e m e n t s w h e n C : P ratios a r e s o h i g h . T h e r e f o r e , in c o n t r a s t t o c l a s s i c a l t h e o r y , d e c o m p o s e r s d o n o t r e l e a s e , b u t t a k e u p SRP.  Why Powell a n d Sakinaw Are So Different The d i f f e r e n c e s in p h o s p h o r u s distribution b e t w e e n P o w e l l a n d S a k i n a w a r e s u m m a r i z e d in Figs. 4-7 a n d 4-8. In t h e b o t t o m w a t e r s o f P o w e l l L a k e , t h e p r o p o r t i o n o f p h o s p h o r u s in t h e p a r t i c u l a t e f r a c t i o n (5 - 2 0 % ) is m u c h g r e a t e r t h a n in S a k i n a w . In t h e latter, a l t h o u g h P P c o n t r i b u t e s u p t o 2 6 % o f t h e t o t a l p h o s p h o r u s a t t h e i n t e r f a c e a n d a r o u n d 2 0 % a b o v e , b e l o w t h e i n t e r f a c e P P c o n t r i b u t e s virtually n o t h i n g t o t h e t o t a l p h o s p h o r u s p o o l . By t h e s a m e t o k e n , m o s t o f t h e p h o s p h o r u s in S a k i n a w is in t h e SRP f r a c t i o n (-80%), w h e r e a s in P o w e l l it is half t h a t m u c h . In light o f t h e v a r i o u s studies d e s c r i b e d in t h e previous s e c t i o n , t h e small a m o u n t of p h o s p h o r u s r e l e a s e d in P o w e l l L a k e b o t t o m w a t e r s is n o t t h a t u n u s u a l , a s o r g a n i c m a t t e r in this l a k e is e x t r e m e l y P-deplete. H o w e v e r , this d o e s n o t e x p l a i n w h y t h e t w o lakes a r e s o different, as S a k i n a w p h y t o p l a n k t o n also h a v e very high C : P ratios.  c o n s i d e r a b l y h i g h e r t h a n t h o s e o f P o w e l l . T h e r e f o r e , w h y h a s P o w e l l d e v e l o p e d this unusual b o t t o m water containing high a m m o n i u m a c c o m p a n i e d with low p h o s p h a t e c o n c e n t r a t i o n s , w h e n Sakinaw b o t t o m waters show a m u c h m o r e normal mineralization of all t h e nutrients a s is usually s e e n in a n o x i c p o r e w a t e r s ? The first c o n s i d e r a t i o n is t h a t p h o s p h o r u s i n p u t t o S a k i n a w is unlikely t o  be  a p p r e c i a b l y h i g h e r t h a n in P o w e l l L a k e . A l t h o u g h S a k i n a w s u r f a c e w a t e r s c o n t a i n d e t e c t a b l e q u a n t i t i e s of SRP, t h e y a r e still v e r y low. The s o u r c e for this SRP is most likely diffusion f r o m t h e P-replete w a t e r s b e l o w . A n t h r o p o g e n i c s u p p l y t o t h e m o r e d e n s e l y p o p u l a t e d S a k i n a w Lake m a y serve as a n a d d i t i o n a l phosphorus s o u r c e , h o w e v e r , c o n s i d e r i n g t h e l a r g e a m o u n t o f t i m e t h a t t h e s e lakes h a v e b e e n a n o x i c , t h e relatively recent  i n f l u e n c e of a n t h r o p o g e n i c input s h o u l d b e  negligible. Phosphorus  was  m e a s u r e d a t five o t h e r stations in P o w e l l (Fig. 4-13 a n d 4-14) t o d e t e r m i n e if t h e  low  c o n c e n t r a t i o n s in t h e s o u t h b a s i n w e r e d u e t o r e m o v a l in t h e basins c l o s e r t o t h e h e a d . H o w e v e r , SRP w a s u n d e t e c t a b l e a t a l l d e p t h s in a l l b a s i n s . B o t h I P  a n d PP w e r e  e x t r e m e l y l o w . E v e n in t h e a n o x i c w a t e r s of t h e East b a s i n , I P c o n c e n t r a t i o n s d i d not e x c e e d 0.7 p M . T h e r e f o r e , p h o s p h o r u s is e x t r e m e l y limiting t h r o u g h o u t P o w e l l L a k e , e v e n in t h e river w a t e r e n t e r i n g t h e lake. The d i f f e r e n c e b e t w e e n t h e t w o lakes in t h e p h o s p h o r u s c h e m i s t r y m a y r e f l e c t t h e v a r y i n g c h e m i c a l histories o f t h e t w o lakes. A t p r e s e n t , neither P o w e l l nor S a k i n a w h a v e a n y s u l p h a t e in their b o t t o m w a t e r s . Thus, t h e d e e p w a t e r s a r e essentially oxidants t a r v e d , with f e r m e n t a t i o n b e i n g t h e only m e a n s of o r g a n i c matter d e c o m p o s i t i o n . A d d i t i o n a l s e a w a t e r input t o S a k i n a w s i n c e sill e m e r g e n c e w o u l d p r o v i d e S a k i n a w w i t h e x t r a o x i d a n t s u p p l y , in t h e f o r m of s u l p h a t e . This w o u l d result in m o r e s u l p h a t e r e d u c t i o n in t h e b o t t o m w a t e r s , w i t h a s s o c i a t e d m i n e r a l i z a t i o n o f nutrients. B e c a u s e P o w e l l h a s p r o b a b l y n o t h a d a n y input o f s u l p h a t e s i n c e it w a s s e p a r a t e d f r o m t h e Strait of G e o r g i a ~11000 y e a r s a g o , it h a s b e e n o x i d a n t - s t a r v e d for a m u c h l o n g e r p e r i o d , a n d thus less m i n e r a l i z a t i o n h a s o c c u r r e d . H o w e v e r , h i g h levels of a m m o n i u m d o o c c u r in P o w e l l a n d this m u s t b e e x p l a i n e d . B o t h l a k e s a r e o l i g o t r o p h i c , w i t h e x t r e m e l y h i g h s e s t o n i c C P ratios. P r e s u m a b l y , t h e lakes w e r e e q u a l l y p h o s p h o r u s - l i m i t e d in their g e o l o g i c history, a n d thus C P ratios w e r e p r o b a b l y a l w a y s h i g h . O v e r t h e l o n g p e r i o d of t i m e t h a t P o w e l l h a s b e e n i s o l a t e d it e v e n t u a l l y b e c a m e  o x i d a n t - s t a r v e d , a n d all t h e p h o s p h o r u s  r e m i n e r a l i z e d v i a s u l p h a t e r e d u c t i o n p r o c e s s e s (prior t o s u l p h a t e e x h a u s t i o n ) w a s likely  e x h a u s t e d b y c h e m o a u t o t r o p h s s u c h as m e t h a n o g e n s a n d fermenters  degrading  p h o s p h o r u s - d e p l e t e d o r g a n i c m a t t e r . Fermenters w o u l d n o t h a v e t o t a k e u p i n o r g a n i c n i t r o g e n a s t h e o r g a n i c m a t t e r b e i n g d e c o m p o s e d w o u l d c o n t a i n sufficient n i t r o g e n t o satisfy their nutritional n e e d s . The h i g h levels of a m m o n i u m , t h e r e f o r e , r e f l e c t t h e m a n y years of remineralization that h a v e t a k e n p l a c e before the lake b e c a m e starved.  oxidant  CHAPTER 5 SULPHUR CHEMISTRY  5.1 Introduction  Sulphur c a n exist in a n u m b e r o f v a l e n c e states b e t w e e n +6 a n d -2. The most a b u n d a n t f o r m s o f t h e e l e m e n t in n a t u r e h a v e v a l e n c e s o f +6 ( s u l p h a t e s , s u l p h a t e esters), 0 ( e l e m e n t a l s u l p h u r ) , a n d -2 ( s u l p h i d e s , r e d u c e d o r g a n i c sulphur). S u l p h u r d i o x i d e a n d sulphite (+4) a n d p o l y s u l p h u r c o m p o u n d s w i t h m i x e d v a l e n c e states ( e g . t h i o s u l p h a t e , p o l y t h i o n a t e s ) o c c u r a s transient s p e c i e s . P o w e l l a n d S a k i n a w L a k e s a r e i d e a l sites f o r t h e s t u d y o f sulphur c h e m i s t r y b e c a u s e their w a t e r s e n c o m p a s s a w i d e r a n g e o f E h , i n c l u d i n g t h e l o w Eh e n v i r o n m e n t r e q u i r e d f o r pyrite f o r m a t i o n . V a r i o u s s u l p h u r c o m p o u n d s w e r e m e a s u r e d in P o w e l l a n d S a k i n a w L a k e s , a n d thus sulphur c h e m i s t r y will b e briefly r e v i e w e d , b e f o r e d i s c u s s i n g t h e s p e c i f i c s o f iron s u l p h i d e f o r m a t i o n in t h e t w o lakes. Sulphur m a k e s u p a b o u t 0.05% o f t h e earth's crust, of w h i c h a p p r o x i m a t e l y 90% o c c u r s in s e d i m e n t s a n d d e e p o c e a n i c a n d s e d i m e n t a r y r o c k s a s Se ( n a t i v e ) , FeS2 ( p y r i t e ) , P b S ( g a l e n a ) , H g S ( c i n n a b a r ) , ZnS ( s p h a l e r i t e ) , C u S ( c h a l c o c i t e ) , 2  ( c h a l c o p y r i t e ) , CaSC>4»2H 0 ( g y p s u m ) , BaSO,* (barite), a n d M g S C W H 0 2  2  CuFeS  2  (epsomite)  ( M o r t i m e r 1979)). The r e m a i n d e r o c c u r s l a r g e l y a s s u l p h a t e in s e a w a t e r . The e l e m e n t c y c l e s b e t w e e n reservoirs v i a a w e l l d e f i n e d o x i d a t i o n - r e d u c t i o n c y c l e  (Goldhaber  a n d K a p l a n 1974). Sulphur is a d d e d t o n a t u r a l w a t e r s f r o m t h e w e a t h e r i n g o f r o c k s , fertilizers, a n d a t m o s p h e r i c p r e c i p i t a t i o n a n d d r y d e p o s i t i o n ( W e t z e l 1983). S u l p h i d e s a n d / o r f r e e sulphur a r e o x i d i z e d in t h e p r e s e n c e o f w a t e r t o f o r m sulphuric a c i d , v i a : FeS2 +7/20 +H 0-> FeSCU + H2SO4 2  2  2S° + 3O2 + 2HzO -> 2H S0 . 2  4  L a r g e q u a n t i t i e s o f r e d u c e d sulphur (as H S) a r e r e l e a s e d i n t o t h e a t m o s p h e r e f r o m 2  v o l c a n o e s a n d b i o g e n i c a n d industrial s o u r c e s . The e m i t t e d H S is o x i d i z e d t o sulphur 2  d i o x i d e (SOiD. sulphur trioxide (SO3), a n d sulphuric a c i d (H S04). A p p r o x i m a t e l y 95% of 2  t h e sulphur e v o l v e d f r o m t h e b u r n i n g o f fossil fuels is p r o d u c e d a s S0 , w h i c h is r a p i d l y 2  o x i d i z e d t o sulphuric a c i d a s it dissolves in a t m o s p h e r i c w a t e r . Sulphur is a n essential c o m p o n e n t o f all living m a t t e r ; t h e S c o n t e n t o f organisms r a n g e s f r o m 0.05 - 5% ( d r y w e i g h t ) , a n d a v e r a g e s 0.2% ( W e t z e l 1983). The b i o l o g i c a l  sulphur p o o l is g e n e r a l l y s m a l l c o m p a r e d t o t h e i n o r g a n i c f o r m s of s u l p h u r in most n a t u r a l w a t e r s , t h e p r e d o m i n a n t s p e c i e s b e i n g s u l p h a t e . M o s t of t h e sulphur in seston o c c u r s as ester s u l p h a t e s a n d p r o t e i n sulphur (Wetzel 1983), w h i c h c o n t r i b u t e u p t o 8 0 % of t h e r e c e n t l y d e p o s i t e d s e d i m e n t sulphur in p r o d u c t i v e lakes. The r e m a i n d e r of t h e s e d i m e n t sulphur consists of pyrite, a c i d - v o l a t i l e s u l p h i d e s , s u l p h i d e s d i s s o l v e d in p o r e w a t e r s , e l e m e n t a l s u l p h u r , a n d d i s s o l v e d s u l p h a t e . O r g a n i c sulphur c o m p o u n d s  are  m o r e resistant t o d e c o m p o s i t i o n t h a n sulphur-free o r g a n i c m a t t e r ( W e t z e l 1983).  Sulphur Compounds T h e r e a r e a l a r g e n u m b e r of k n o w n s u l p h u r - c o n t a i n i n g c o m p o u n d s , i n c l u d i n g b o t h c o m p l e x o r g a n i c s u b s t a n c e s a n d s i m p l e i n o r g a n i c s p e c i e s . H o w e v e r , a t t h e Eh, p H , a n d t e m p e r a t u r e v a l u e s c o m m o n l y f o u n d in m a r i n e a n d lacustrine s e d i m e n t s , a n d a t t h e r m o d y n a m i c e q u i l i b r i u m , o n l y a s m a l l n u m b e r of c o m m o n d i s s o l v e d s p e c i e s (H S aq), HS", a n d SO4) 2  (  (Thorstenson  m a k e u p a l m o s t all of t h e t o t a l sulphur in a q u e o u s s o l u t i o n  1970). All o t h e r s p e c i e s , s u c h as S O f , a r e 2  present at m u c h  lower  c o n c e n t r a t i o n s . The p r i n c i p a l sulphur s p e c i e s in a e r o b i c s u r f a c e w a t e r s a t n e u t r a l p H s h o u l d b e S O 4 , a n d in a n a e r o b i c w a t e r s in t h e p H r a n g e of 5 - 9, t h e p r e d o m i n a n t sulphur s p e c i e s a r e H ^ a n d HS". Inorganic Sulphur Species Inorganic  sulphur species  c o n s i s t of t h e  reduced  compounds,  including  dissolved sulphide, polysulphide a n d particulate m e t a l sulphides, the oxidized species, i n c l u d i n g s u l p h a t e , sulphite, p o l y t h i o n a t e a n d thiosulphate, a n d e l e m e n t a l sulphur (Table 5-1). H ^ is m o d e r a t e l y s o l u b l e in w a t e r a n d is a w e a k , d i p r o t i c a c i d in a q u e o u s solution. It ionizes in t w o steps: H2S(oq) <-> H (aq) + HS' q)  \OQ K, = 7.02  HS" ) «-» H q) + S " ^  log 1(2=18.57.  +  (ocl  (a  +  (0  2  This s p e c i a t i o n is t h e r e f o r e p H d e p e n d e n t : a t t h e p H of most n a t u r a l w a t e r s ( 5 - 9 ) , t h e S " 2  s p e c i e s is n o t v e r y a b u n d a n t . The  two  primary  thermodynamically  forms  of  elemental  sulphur  are  s t a b l e f o r m ) a n d c o l l o i d a l S. E l e m e n t a l  orthorhombic  (the  s u l p h u r is e s s e n t i a l l y  i n s o l u b l e in w a t e r b u t dissolves in a n u m b e r of o r g a n i c s o l v e n t s , m o s t n o t a b l y C S . 2  Sulphur dissolves in solutions of s o l u b l e s u l p h i d e s a n d forms a mixture of p o l y s u l p h i d e  Table 5-1 Inorganic sulphur compounds commonly found in the aqueous environment  C h e m i c a l Species  Name  Oxidation State  SG-4  sulphate ion  S (+6)  SO?  sulphite i o n  S (+4)  HSO3  bisulphite i o n  so  sulphur d i o x i d e  S (+4)  SnC?  p o l y t h i o n a t e ion of l e n g t h n  SO3 (+5)  e g . S4O?  tetrathionate  c e n t r a l s (0)  S2O3 "  thiosulphate ion  t e r m i n a l s (-2)  2  2  c e n t r a l S (+6) e l e m e n t a l sulphur  S (0)  polysulphide ion of length n  t e r m i n a l s (-1)  eg. Sf  tetrasulphide  c e n t r a l S (0)  H2S  h y d r o g e n sulphide  S (-2)  HS"  bisulphide ion  S"  sulphide ion  FeS  iron m o n o s u l p h i d e  s  8  2  S (-2)  amorphous, mackinawite. pyrrhotite Fe S 3  4  greigite  S (-2) with 2 Fe (III) a n d 1 Fe(ll)  FeS  2  pyrite or m a r c a s i t e  S (-1)  a n i o n s , St, Sf" a n d S ," b e i n g t h e m o s t c o m m o n ( B o u l e g u e 1976, 1977; B o u l e g u e a n d 2  M i c h a r d 1978):  HS" + l/8Sa <-> S | + H  +  HS" + l/4Se <-> £ + H  +  HS' + 3/8S8 <-> £ + H  +  HS" + l/2Sa «-> S| + H  +  HS' + 5/8Sa <-> £ + H  +  The S^" s p e c i e s c a n r e a c t with H t o f o r m H S j o n s ( B o u l e g u e a n d M i c h a r d 1978): +  St + H <-» HS4 +  St + H <-» H S +  A l t h o u g h polysulphides c o n t a i n i n g over a h u n d r e d sulphur a t o m s h a v e  been  s y n t h e s i z e d , o n l y p o l y s u l p h i d e s w i t h five sulphur a t o m s o r less a r e s t a b l e in a q u e o u s solution a t m e a s u r a b l e c o n c e n t r a t i o n s for a n a p p r e c i a b l e l e n g t h o f t i m e (Pickering a n d T o b o l s k y 1972). A t r o o m t e m p e r a t u r e , p o l y s u l p h i d e s d e c o m p o s e in a c i d s o l u t i o n t o y i e l d m a i n l y H2S a n d f r e e S. Reversibility h a s a l s o b e e n s e e n b y V o g e (1939) w h o f o u n d r a p i d e x c h a n g e o f r a d i o a c t i v e ^S b e t w e e n d i s s o l v e d s u l p h i d e a n d sulphur. A t a g i v e n p H , t h e t o t a l r e d u c e d sulphur (ST) is g i v e n b y ( J a c o b s a n d E m e r s o n 1982):  S  T  = (HzS) + (HS") + (S ") + (£) + (St) + ( £ ) + (S?) + ( £ ) + (HSj + (HS5). 2  The m o s t s t a b l e f o r m o f sulphur in a e r o b i c e n v i r o n m e n t s is s u l p h a t e , w h i c h is o f t e n a l s o p r e s e n t in l o w c o n c e n t r a t i o n s in r e d u c e d waters. The t h i o s u l p h a t e i o n , S O f , 2  m a y b e r e g a r d e d as a s u l p h a t e ion in w h i c h o n e o x y g e n a t o m has b e e n r e p l a c e d b y a sulphur a t o m (the prefix thio is u s e d t o n a m e a n y s p e c i e s t h a t m a y b e c o n s i d e r e d t o b e d e r i v e d f r o m a n o t h e r c o m p o u n d b y r e p l a c i n g a n o x y g e n a t o m b y a sulphur a t o m ) . The t w o sulphur a t o m s of t h e thiosulphate i o n a r e not equivalent. W h e n a thiosulphate compound  is p r e p a r e d  from sulphite a n d r a d i o a c t i v e  sulphur ( S), a n d t h e n 35  d e c o m p o s e d b y a c i d i f i c a t i o n , all t h e a c t i v i t y o c c u r s in t h e e l e m e n t a l sulphur:  SO3 (oq) + S( ) -> ^SSCfcaq) S  ^SSOicoq) + 2H<oq) -» +  + SOxg) + H2O.  The c e n t r a l sulphur a t o m in t h i o s u l p h a t e is g e n e r a l l y a s s i g n e d a n o x i d a t i o n n u m b e r o f +6 (as in t h e s u l p h a t e i o n ) , a n d t h e c o o r d i n a t e d sulphur is usually a s s i g n e d a n o x i d a t i o n n u m b e r o f -2 ( c o r r e s p o n d i n g t o t h e o x i d a t i o n n u m b e r o f t h e o x y g e n it r e p l a c e s ) ; t h e a v e r a g e o x i d a t i o n s t a t e o f sulphur in t h e ion is +2. Thiosulphate i o n is r e a d i l y o x i d i z e d t o  t h e t e t r a t h i o n a t e i o n , SAOI. O t h e r t h i o n a t e ions a l s o exist, e . g . d i t h i o n a t e , S2O6. a n d trithionate, S3O?. Organic Sulphur Compounds O r g a n i c a l l y - c o m b i n e d sulphur is s y n t h e s i z e d v i a d i r e c t assimilation o f s u l p h a t e b y living p l a n t s a n d b a c t e r i a . The synthesis o f o r g a n i c sulphur b y m a r i n e o r g a n i s m s leads t o a variety of c o m p o u n d s comprising mainly sulphur-containing a m i n o a c i d s , s u l p h o n a t e s in w h i c h s u l p h u r is d i r e c t l y b o n d e d t o c a r b o n , a n d o r g a n i c esters o f sulphuric a c i d w h i c h c o n t a i n a C-O-SO3H l i n k a g e . O r g a n i c sulphur s p e c i e s c o m m o n l y f o u n d d i s s o l v e d in o c e a n i c w a t e r s i n c l u d e d i m e t h y l s u l p h i d e (DMS), d i m e t h y l d i s u l p h i d e (DMDS), c a r b o n  disulphide (CS ), methyl m e r c a p t a n 2  ( m e t h a n e t h i o l - CH3SH),  d i b e n z o t h i o p h e n e (DPT), n a p t h o b e n z o t h i o p h e n e , b e n z o t h i a z o l , t h i a m i n e , a n d biotin (Balzer 1981). A l t h o u g h o r g a n i c sulphur a c c o u n t s f o r less t h a n 1 0 % o f t h e t o t a l s e d i m e n t a r y s u l p h u r ( G o l d h a b e r a n d K a p l a n 1974). t h e r e is p r o b a b l y a l a r g e n u m b e r o f a s y e t u n d e t e c t e d o r g a n i c sulphur c o m p o u n d s , e s p e c i a l l y in c o m b i n e d o r c o n d e n s e d f o r m , w h i c h t e n d t o a c c u m u l a t e in t h e h u m i c f r a c t i o n o f t h e m a r i n e s e d i m e n t s ( N i s s e n b a u m a n d K a p l a n 1972). C u r r e n t l y , t h r e e f r a c t i o n s o f o r g a n i c sulphur a r e d i s t i n g u i s h e d using m e t h o d s b o r r o w e d f r o m soil s c i e n c e (Williams 1975): a f r a c t i o n t h a t is r e d u c e d b y h y d r o i o d i c a c i d t o H2S, consisting m a i n l y o f ester sulphates a n d s u l p h a m a t e s ; a f r a c t i o n c o n t a i n i n g t h e c a r b o n - b o n d e d sulphur; a n d a f r a c t i o n w h i c h is c h a r a c t e r i z e d a s Raneyn i c k e l - r e d u c i b l e sulphur. T h e latter is a l s o a s u b t r a c t i o n o f c a r b o n - b o n d e d s u l p h u r r e p r e s e n t i n g m a i n l y t h e cystine a n d m e t h i o n i n e in soil o r g a n i c matter.  The Sulphur C y c l e The sulphur c y c l e in lakes is s u m m a r i z e d in Fig. 5-1. T h e b i o g e o c h e m i c a l sulphur c y c l e c a n b e d i v i d e d into a s s i m i l a t o r y a n d dissimilatory p r o c e s s e s . In t h e f o r m e r , i n o r g a n i c s u l p h u r is t a k e n u p b y m i c r o b e s , p l a n t s a n d a n i m a l s ( v i a p l a n t s ) a n d t r a n s f o r m e d into a m i n o a c i d s , p r o t e i n s a n d c o e n z y m e s . D u e t o its p r e d o m i n a n c e , virtually all assimilation o f sulphur b y b i o t a is as s u l p h a t e . During t h e synthesis o f proteins b y plants a n d a n i m a l s , s u l p h a t e is r e d u c e d t o sulphydryl (-SH), w h i c h is further r e d u c e d t o H2S b y s u b s e q u e n t d e c o m p o s i t i o n o f this o r g a n i c m a t t e r b y v a r i o u s h e t e r o t r o p h i c b a c t e r i a ( W e t z e l 1983). B e c a u s e H S is o x i d i z e d fairly r a p i d l y , little is f o u n d in a e r a t e d 2  Surface runoff (SO/)  Precipitation ft dry fallout (S0 \ SOj)  Outflow S (SO/)  4  ORGANIC S] SQ, -*-R SH  Assimilation G'Oundwater  so;  ThiobiioHuS th.oo«Kl,ins done* (Chiomatium.ChlofObium. Other photosynthetic and chemosynthetic S bacteria)  ThtobacilHis denilridcins 1  ^ N 0  3  Deposition  ORGANIC? S  Fig. 5-1 The sulphur c y c l e In a l a k e , with e m p h a s i s o n t h e m i c r o b i o l o g i c a l p r o c e s s e s . MS = m e t a l sulphides, (from W e t z e l 1983)  TROPHOGENIC ZONE  TROPHOLYTIC ZONE  SEDIMENTS  w a t e r s . T h e t u r n o v e r r a t e s o f assimilatory s u l p h a t e r e d u c t i o n a r e slow r e l a t i v e t o t h e dissimilatory p r o c e s s e s , d u r i n g w h i c h s u l p h u r c o m p o u n d s o f v a r y i n g o x i d a t i o n states a r e u s e d a s e i t h e r e l e c t r o n d o n o r s o r a c c e p t o r s in m i c r o b i a l m e t a b o l i s m . The different m e t a b o l i c t y p e s i n c l u d e a n a e r o b i c dissimilatory s u l p h a t e (or sulphur, o r t h i o s u l p h a t e etc.) r e d u c t i o n , a n a e r o b i c p h o t o t r o p h i c sulphur o x i d a t i o n , a n d c h e m o l i t h o t r o p h i c sulphur o x i d a t i o n , w h i c h c a n b e b o t h a n a e r o b i c o r a e r o b i c . C o m m u n i t i e s of sulphurr e d u c i n g a n d sulphur-oxidizing b a c t e r i a f o r m s u l p h u r e t a , w h i c h a r e best d e s c r i b e d a s mini, s e l f - c o n t a i n e d sulphur c y c l e s . Sulphate-Reducing Bacteria Dissimilatory s u l p h a t e r e d u c t i o n is t h e p r i m a r y m e a n s o f o r g a n i c  carbon  m i n e r a l i z a t i o n in a n o x i c w a t e r s a n d s e d i m e n t s . T h e s u l p h a t e - r e d u c i n g b a c t e r i a ( e g .  Desulfovibrio.  Desulfotomaculum,  Desulfomonas)  a r e strict a n a e r o b e s , a n d u s e  s u l p h a t e a s t h e t e r m i n a l e l e c t r o n a c c e p t o r in t h e o x i d a t i o n o f either o r g a n i c m a t t e r o r molecular hydrogen:  SO4 + 2CH2O + 2H -> 2CO2 + 2H2O + H2S +  SO4+4H +2H ->4H 0+H S 2  +  2  A3o= -251.2 k j . m o f ' .  2  These reactions d o n o t use u p o x y g e n directly, b u t t h e H S g e n e r a t e d 2  is r e a d i l y  o x i d i z e d , w h i c h c o n s u m e s o x y g e n in a e r o b i c waters. The s u l p h a t e - r e d u c i n g b a c t e r i a c a n also r e d u c e sulphite m o r e rapidly ( a n d t h i o s u l p h a t e less rapidly) t h a n s u l p h a t e , a n d c a n r e d u c e c o l l o i d a l sulphur ( b u t n o t o r t h o r h o m b i c sulphur) v e r y slowly (Wetzel 1983). Sulphur-Oxidizing Bacteria The sulphur-oxidizing b a c t e r i a a r e m o r e diverse t h a n t h e r e d u c e r s a n d c a n b e d i f f e r e n t i a t e d into t w o g r o u p s , t h e first b e i n g t h e colourless sulphur b a c t e r i a , w h i c h a r e c h e m o s y n t h e t i c , a n d usually strict a e r o b e s . These b a c t e r i a oxidize primarily H S a n d 2  c a n b e d i v i d e d into t w o t y p e s , t h e first of w h i c h deposits sulphur inside t h e c e l l , v i a : H2S+1/202 -> S° + H20  AGo = -171.7 kJ»mof'.  This sulphur a c c u m u l a t e s until H2S b e c o m e s d e p l e t e d , w h e n t h e internally s t o r e d sulphur is t h e n o x i d i z e d t o s u l p h a t e : S° + 3/2O2 + hfeO -> SO4 +2H  +  T w o g e n e r a w h i c h oxidize b a c t e r i u m , a n d Thiothrix.  AGo= -494.0kJ.mol" .  a n d store S° internally a r e Beggiatoa,  1  a long, filamentous  The s e c o n d t y p e of c h e m o s y n t h e t i c , sulphur-oxidizing b a c t e r i a d e p o s i t s sulphur o u t s i d e t h e c e l l , t h e most c o m m o n g e n u s b e i n g Thiobacillus,  w h i c h oxidizes s u l p h i d e ,  S ° , a n d o t h e r r e d u c e d sulphur c o m p o u n d s s u c h as t h i o s u l p h a t e : 2 s o ; + a -»2s°+2S04\ 2  M a n y of t h e s p e c i e s of Thiobacillus  2  ( e g . T. thiooxidans)  ( p H = 1 t o 5), while others s u c h as T. thioparus  a r e o n l y f o u n d in a c i d i c w a t e r s  g r o w in neutral t o a l k a l i n e c o n d i t i o n s . The  sulphur-oxidizing b a c t e r i a a r e c o m m o n l y f o u n d a d h e r i n g t o e l e m e n t a l sulphur particles. The  second  photosynthetic  (Jhiorhodaceae)  group  of  sulphur-oxidizing  bacteria, which  are  composed  microbes of t h e  is  the  anaerobic  purple sulphur  bacteria  (Chlorobacteriaceae).These  a n d t h e g r e e n sulphur b a c t e r i a  organisms require light as a n e n e r g y s o u r c e , a n d use H S-sulphur as a n e l e c t r o n d o n o r in 2  t h e p h o t o s y n t h e t i c r e d u c t i o n of C 0  2  (Wetzel 1983):  CO2 + 2H2S  CH2O + H 0 + 2S° 2  2C02 + 2H2O + H2S The g r e e n sulphur b a c t e r i a ( e g . Chlorobium  20-feO + SC * + 2 H . 2  a n d Pelodictyon)  +  are generally unicellular,  n o n - m o t i l e , a n d d e p o s i t sulphur extracellularly. Unlike t h e p u r p l e sulphur b a c t e r i a , t h e g r e e n s c a n t o l e r a t e fairly h i g h c o n c e n t r a t i o n s of H S. 2  Chromatium  a n d Thiocystis)  The p u r p l e sulphur g r o u p ( e g .  c a n use o t h e r r e d u c e d sulphur c o m p o u n d s , e s p e c i a l l y  t h i o s u l p h a t e , in p l a c e of H S a s a n e l e c t r o n d o n o r . These b a c t e r i a a r e l a r g e , a c t i v e l y 2  motile a n d d e p o s i t f r e e S° intracellularly. The o c c u r r e n c e a n d distribution of t h e sulphur b a c t e r i a a r e restricted b y Eh a n d p H c o n d i t i o n s , a c c o r d i n g t o t h e a m o u n t of 0  2  p r e s e n t a n d t h e o x i d a t i o n s t a t e of t h e  sulphur c o m p o u n d s (Fig. 5-2). The s p e c i f i c r e q u i r e m e n t s f o r g r o w t h of sulphur b a c t e r i a o f t e n result in distinct, thin layers of c e r t a i n b a c t e r i a l p o p u l a t i o n s in t h e w a t e r c o l u m n s of stratified lakes. The p h o t o s y n t h e t i c p u r p l e b a c t e r i a a r e o f t e n f o u n d in a d e n s e  layer  i m m e d i a t e l y a t t h e o x i c / a n o x i c i n t e r f a c e in m e r o m i c t i c lakes, b e l o w w h i c h a thin b a n d of g r e e n sulphur b a c t e r i a d o m i n a t e s . Light levels a t b a c t e r i a - s u s t a i n i n g i n t e r f a c e s a r e usually a b o u t 1 0 % of t h o s e at t h e s u r f a c e (Pfennig a n d W i d d e l 1982).  Diagenesis of Sulphur The d i a g e n e s i s of sulphur d e p e n d s o n t h e p r e s e n c e or a b s e n c e of d i s s o l v e d o x y g e n . Sulphur c a n b e a d d e d t o s e d i m e n t s ( a n d a n o x i c b o t t o m w a t e r s ) as i n o r g a n i c  Fig. 5-2 G e n e r a l Eh-pH e n v i r o n m e n t a l limits of: 1) c h e m o s y n t h e t i c (colourless) sulphur-oxidizing b a c t e r i a ; 2) p h o t o s y n t h e t i c p u r p l e b a c t e r i a ; 3) s u l p h a t e - r e d u c i n g b a c t e r i a ; a n d 4) g r e e n sulphur b a c t e r i a (from W e t z e l 1983).  s u l p h a t e a n d a s various o r g a n i c sulphur c o m p o u n d s . In a e r o b i c e n v i r o n m e n t s , o r g a n i c s u l p h u r is o x i d i z e d t o s u l p h a t e , w h i c h a d d s t o t h e s u l p h a t e p o o l . U n d e r a n a e r o b i c c o n d i t i o n s , h o w e v e r , s u l p h a t e - r e d u c i n g b a c t e r i a use s u l p h a t e a s a t e r m i n a l e l e c t r o n a c c e p t o r , r e d u c i n g it t o s u l p h i d e . O t h e r b a c t e r i a a l s o p r o d u c e s u l p h i d e b y t h e d e g r a d a t i o n o f sulphur in o r g a n i c m a t t e r . The s u l p h i d e p r o d u c e d m a y i n t e r a c t w i t h various m e t a l s t o f o r m insoluble s u l p h i d e minerals, or, d u e t o m i g r a t i o n , it m a y c o m e into c o n t a c t w i t h o x y g e n a n d b e o x i d i z e d either a b i o t i c a l l y or b y sulphur-oxidizing b a c t e r i a t o sulphur c o m p o u n d s o f v a r i o u s o x i d a t i o n states. The p r i m a r y e n d p r o d u c t o f sulphur d i a g e n e s i s in m o s t systems is pyrite (FeS2). Iron Sulphides The c o m m o n iron s u l p h i d e minerals w h i c h f o r m u n d e r s e d i m e n t a r y c o n d i t i o n s include  pyrite  (cubic  FeS2), its o r t h o r h o m b i c  dimorph, marcasite,  a n d the  m o n o s u l p h i d e s : a m o r p h o u s FeS ( w h i c h is p r o b a b l y a mixture o f f i n e - g r a i n e d g r e i g i t e a n d m a c k i n a w i t e (Morse e t a l . 1987)), m a c k i n a w i t e (also k n o w n as t e t r a g o n a l s u l p h i d e , kansite, or hydrotroilite (FeSo.94) ( W a r d 1970)), g r e i g i t e (also k n o w n a s m e l n i k o v i t e Fe S4), 3  a n d pyrrhotite ( F e S u ) . Fig. 5-3 shows t h e Eh-pS " d i a g r a m f o r s u l p h i d i c s e d i m e n t s a t a 2  f i x e d p H o f 7.37. N o t i n c l u d e d in this d i a g r a m a r e m a c k i n a w i t e a n d g r e i g i t e b e c a u s e of their instability r e l a t i v e t o pyrrhotite a n d pyrite. A u t h i g e n i c pyrrhotite is o c c a s i o n a l l y f o u n d in s e d i m e n t s (Berner 1971) a n d it h a s b e e n s u g g e s t e d t h a t it is a precursor t o pyrite f o r m a t i o n ( R i c k a r d 1969; S w e e n e y a n d K a p l a n 1973); h o w e v e r , its o c c u r r e n c e is v e r y rare. T h e m o n o s u l p h i d e s a r e o f t e n r e f e r r e d t o a s " a c i d - v o l a t i l e sulphides" d u e t o their solubility in h o t HCI. They a r e m o r e c o r r e c t l y c a l l e d t h e m e t a s t a b l e iron s u l p h i d e s , s i n c e t h e y transform t o pyrite in t h e p r e s e n c e of HS" a t c o n c e n t r a t i o n s a b o v e pyrite saturation. W h e n e x c e s s s u l p h i d e is n o t p r e s e n t , t h e y c a n persist f o r l o n g p e r i o d s o f t i m e (Berner  1974, 1981). Pyrite is t h e s t a b l e m i n e r a l a t t h e l o w Eh a n d h i g h s u l p h i d e c o n c e n t r a t i o n s t y p i c a l o f o r g a n i c - r i c h s e d i m e n t s . Detrital pyrite is c h e m i c a l l y a n d p h y s i c a l l y u n s t a b l e , a n d is thus v e r y rare in s e d i m e n t s . Sedimentary Pyrite Formation S e d i m e n t a r y pyrite (FeS2) is a c o m m o n a u t h i g e n i c m i n e r a l in r e c e n t m a r i n e a n d l a c u s t r i n e e n v i r o n m e n t s , a s w e l l a s i n s e d i m e n t a r y r o c k s . It t y p i c a l l y o c c u r s a s m i c r o s c o p i c single crystal grains 1 t o 10 urn in size, a s f r a m b o i d a l spherules u p t o 250 urn in d i a m e t e r ( L o v e a n d A m s t u t z 1966) a n d a s g r o u p s o f f r a m b o i d a l s p h e r e s t e r m e d  Ill  0.00 -0.10  /  £ -0.30  Pyrite FeS  /  2  Pyrrhotite  -0.40  -0.60  4.0  H  6.0  1  8.0  1  10.0  e 0  F  2  3  Magnetite Fe 0 3  4  Siderite  F e S ^  -0.50 -  /  / Hematite  FeC0  3  2  1  12.0  l  i  14.0 16.0  i  18.0 20.0  Fig. 5-3 Eh-pS " d i a g r a m for iron minerals a t p H = 7.37. l o g Pccs= -2.40. T = 25°C. P = 1 a t m . 2  M e a s u r e m e n t s o f n a t u r a l s u l p h i d i c m a r i n e s e d i m e n t s fall n e a r t h e d a s h e d line, (from Berner 1971)  p o l y f r a m b o i d s ( L o v e 1971). T h e t e r m " f r a m b o i d a l " refers t o a u n i q u e , raspberry-like m i c r o t e x t u r e . T h e single crystals a n d f r a m b o i d s a r e p r e s e n t in t h e c l a y a n d silt-size s e d i m e n t f r a c t i o n , o r a s infillings of f o r a m , d i a t o m , a n d r a d i o l a r i a n tests ( G o l d h a b e r a n d K a p l a n 1974). Berner (1970) p r o p o s e d a t h r e e s t e p m e c h a n i s m for t h e f o r m a t i o n o f pyrite: 1) r e d u c t i o n o f s u l p h a t e t o s u l p h i d e b y b a c t e r i a ; 2) r e a c t i o n o f iron minerals w i t h this s u l p h i d e t o f o r m iron m o n o s u l p h i d e s ; a n d 3) r e a c t i o n o f e l e m e n t a l sulphur with t h e iron m o n o s u l p h i d e s t o f o r m pyrite. The n e t r e a c t i o n c a n b e written as: FeS + S° -> FeSz. Pyrite is t h e o n l y t h e r m o d y n a m i c a l l y s t a b l e p h a s e  o f t h e p o s s i b l e iron-sulphur  c o m p o u n d s in m a r i n e s e d i m e n t s (Berner 1967b). Laboratory Studies of Pyrite Formation A v a r i e t y o f p y r i t e synthesis s c h e m e s h a v e b e e n p r o p o s e d , i n c l u d i n g t h e r e a c t i o n o f m a c k i n a w i t e with e l e m e n t a l sulphur, t h e d i s s o c i a t i o n o f g r e i g i t e , a n d t h e d i r e c t r e a c t i o n of F e  2+  with p o l y s u l p h i d e (Fig. 5-4).  F e O O H + HS"  FeS Euhedral 2  FeS Framboidal 2  Fig. 5-4 Possible r e a c t i o n p a t h w a y s of pyrite f o r m a t i o n , (from Raiswell 1982)  A n u m b e r o f e x p e r i m e n t a l studies h a v e d e m o n s t r a t e d t h a t pyrite c a n b e s y n t h e s i z e d r a p i d l y in o n e t o a f e w d a y s in i n o r g a n i c solution u n d e r suitable c o n d i t i o n s (Berner 1964a; Roberts e t a l . 1969). A s m e n t i o n e d a b o v e , pyrite f o r m a t i o n is g e n e r a l l y t h o u g h t t o require a p r e c u r s o r iron m o n o s u l p h i d e p h a s e , s u c h a s m a c k i n a w i t e or g r e i g i t e , w h i c h t h e n  r e a c t s w i t h e x c e s s s u l p h i d e a n d e l e m e n t a l sulphur t o f o r m pyrite (Berner 1969, 1970; R i c k a r d 1969), In t h e earliest e x p e r i m e n t a l w o r k , pyrite w a s o b t a i n e d f r o m H2S only w h e n air o r F e  3 +  w a s p r e s e n t ( B e r n e r 1964a); o t h e r w i s e iron m o n o s u l p h i d e s w e r e f o r m e d .  Berner (1964a) f o u n d n o pyrite f o r m a t i o n a t p H > 7, e v e n o v e r 200 d a y s , w h e r e a s a t m o r e a c i d i c p H , pyrite f o r m a t i o n c o u l d b e a c h i e v e d within 24 h. B e c a u s e m a r i n e s e d i m e n t s t y p i c a l l y fall in t h e p H r a n g e o f 7-8, Berner (1970) h y p o t h e s i z e d t h a t pyrite f o r m a t i o n must t a k e c o n s i d e r a b l e t i m e . H o w e v e r , H a l l b e r g (1972) e x p e r i m e n t a l l y p r e c i p i t a t e d pyrite in short t i m e p e r i o d s a t p H v a l u e s b e t w e e n 6 a n d 8. R o b e r t s e t a l . (1969) f o r m e d pyrite r a p i d l y w h e n F e however, when Fe  2 +  3 +  w a s m i x e d with H S; 2  w a s u s e d n o pyrite w a s f o r m e d . These w o r k e r s s u g g e s t e d t h a t  pyrite f o r m a t i o n requires f o r m a t i o n o f p o l y s u l p h i d e first, w h i c h t h e n r e a c t s v e r y q u i c k l y with F e  2+  t o f o r m pyrite. G o l d h a b e r a n d K a p l a n (1974) s u g g e s t e d t h a t this m a y b e a major  p a t h w a y o f pyrite f o r m a t i o n . R o b e r t s e t a l . (1969) h y p o t h e s i z e d t h a t H S w a s t h e 2  2  p o l y s u l p h i d e s p e c i e s f o r m e d , b u t s e v e r a l studies h a v e s h o w n t h a t S * d o e s n o t exist in 2  solution at neutral p H ( S c h w a r z e n b a c h  a n d Fisher 1960; T e d e r 1969; P i c k e r i n g a n d  Tobolsky 1972) a n d t h e r e f o r e s o m e o t h e r p o l y s u l p h i d e must b e present. R i c k a r d (1975)  suggested that  monosulphides were  precursors t o pyrite  f o r m a t i o n , b u t t h a t b o t h FeS a n d e l e m e n t a l s u l p h u r m u s t g o t h r o u g h d i s s o l u t i o n r e a c t i o n s , w h e r e e l e m e n t a l s u l p h u r dissolves in s u l p h i d e s o l u t i o n s , w h i c h  produce  p o l y s u l p h i d e i o n s , a n d ferrous s u l p h i d e dissolves t o f o r m a q u e o u s ferrous ions a n d sulphur s p e c i e s . Pyrite t h e n p r e c i p i t a t e s d i r e c t l y t h r o u g h t h e r e a c t i o n b e t w e e n  ferrous  ions a n d p o l y s u l p h i d e ions: SnS " + & - » S^S " 2  2  SrveS " + SqS " -> S ^ " * SpS " 2  2  FeS + H -> F e +  2  2+  Fe * + sf + HS" -» FeS 2  2  2  + HS" + S i + HS".  H o w e v e r , Berner (1964a) f o u n d t h a t pyrite f o r m a t i o n only o c c u r r e d in t h e p r e s e n c e of e x c e s s e l e m e n t a l sulphur, a n d n o t w h e n p o l y s u l p h i d e s w e r e t h e only z e r o v a l e n t sulphur present. S w e e n e y a n d K a p l a n (1973) f o u n d t h a t w h e n g r e i g i t e r e a c t e d w i t h o x y g e n f r a m b o i d a l pyrite f o r m e d , w h i l e G o l d h a b e r a n d K a p l a n (1974) s u g g e s t e d t h a t e u h e d r a l pyrite r e s u l t e d f r o m t h e r e a c t i o n o f m a c k i n a w i t e w i t h e l e m e n t a l sulphur. Berner e t a l .  (1979) a r g u e d t h a t t h e c o n v e r s i o n of m a c k i n a w i t e a n d g r e i g i t e t o pyrite requires t h e p r e s e n c e of e x c e s s s u l p h i d e a n d if t h e c o n c e n t r a t i o n s a r e l o w t h e n t h e s e m e t a s t a b l e p h a s e s m a y persist for s o m e t i m e . Pyrite m a y a l s o b e f o r m e d directly w i t h o u t a m a c k i n a w i t e or g r e i g i t e p r e c u r s o r under conditions where monosulphides are undersaturated ( G o l d h a b e r a n d Kaplan 1974). H o w a r t h (1979) initially f o u n d t h a t pyrite is rapidly f o r m e d in this m a n n e r in salt marsh p e a t s . S i n c e t h e n , s e v e r a l authors ( H o w a r t h a n d T e a l 1979; Luther et a l . 1982; G i b l i n a n d H o w a r t h 1984) h a v e f o u n d t h a t in s u c h e n v i r o n m e n t s , w h i c h a r e c h a r a c t e r i z e d b y l o w p H (5 t o 6.5), l o w d i s s o l v e d s u l p h i d e c o n c e n t r a t i o n s , h i g h o r g a n i c c o n t e n t s a n d r a p i d rates of s u l p h a t e r e d u c t i o n , pyrite p r e c i p i t a t e s r a p i d l y a n d d i r e c t l y w i t h o u t f o r m a t i o n of p r e c u r s o r iron s u l p h i d e p h a s e s . H o w a r t h (1979) a n d Berner et a l . (1979) h a v e stressed t h e i m p o r t a n c e o f l o w p H in c a u s i n g u n d e r s a t u r a t i o n o f i r o n m o n o s u l p h i d e s w h i l e m a i n t a i n i n g s u p e r s a t u r a t e d c o n d i t i o n s for pyrite. The l o w s u l p h i d e c o n c e n t r a t i o n s a n d p H a r e c a u s e d b y p e r i o d i c o x y g e n a t i o n v i a m a r s h grass roots b e l o w t h e s e d i m e n t s u r f a c e . D u r i n g t h e s e e v e n t s , highly r e a c t i v e p o l y s u l p h i d e s a n d e l e m e n t a l sulphur a r e formed which  c a n r e a c t w i t h ferrous iron; thus pyrite c a n p r e c i p i t a t e r a p i d l y w i t h o u t  c o m p e t i t i o n f r o m iron m o n o s u l p h i d e s . A l t h o u g h it is c o m m o n in a n o x i c s e d i m e n t s , pyrite h a s rarely b e e n s t u d i e d in a n o x i c w a t e r c o l u m n s (Skei 1988). S a k i n a w a n d P o w e l l L a k e s a r e c o n v e n i e n t n a t u r a l b e a k e r s in w h i c h t o study pyrite f o r m a t i o n d u e t o t h e c o - o c c u r r e n c e o f d i s s o l v e d F e , 2+  d i s s o l v e d s u l p h i d e , a n d highly r e a c t i v e p o l y s u l p h i d e s , a n d a l s o d u e t o t h e e x p a n d e d d e p t h , r e l a t i v e t o s e d i m e n t p o r e w a t e r s , o v e r w h i c h t h e f o r m a t i o n o f p a r t i c u l a t e iron sulphides o c c u r .  5.2 Materials a n d Methods  Water Collection W a t e r s a m p l e s w e r e c o l l e c t e d in 1.8 L or 9 L Niskin bottles e q u i p p e d with pressure fittings. I m m e d i a t e l y after b e i n g b r o u g h t o n b o a r d , t h e full bottles w e r e c o u p l e d t h r o u g h s p e c i a l fittings t o a n N pressure line. W a t e r w a s run d i r e c t l y f r o m t h e Niskin bottles u n d e r 2  a slight positive pressure o f N into a nitrogen-filled g l o v e b a g a n d c o l l e c t e d in 10%-HCI2  w a s h e d p o l y e t h y l e n e o r p o l y p r o p y l e n e bottles t h a t h a d b e e n previously f l u s h e d w i t h n i t r o g e n . All s a m p l e s w e r e k e p t u n d e r n i t r o g e n d u r i n g transport t o t h e l a b . S a m p l e s t h a t r e q u i r e d filtration w e r e pressure-filtered in t h e g l o v e b a g d i r e c t l y f r o m t h e Niskin b o t t l e , t h r o u g h 0.4 jam p o l y c a r b o n a t e N u c l e p o r e m e m b r a n e filters. A l l n i t r o g e n u s e d in t h e g l o v e b a g s w a s s c r u b b e d f r e e o f o x y g e n b y p a s s i n g it t h r o u g h v a n a d o u s  chloride  solution prior t o e n t e r i n g t h e g l o v e b a g . Sulphate T w o a n a l y t i c a l t e c h n i q u e s w e r e u s e d . For b o t h t e c h n i q u e s , w a t e r s a m p l e s w e r e pressure-filtered in t h e g l o v e b a g a n d a n y H S p r e s e n t w a s p r e c i p i t a t e d a s ZnS, w h i c h 2  w a s later r e m o v e d b y filtration b e f o r e analysis. 1) Titration T e c h n i q u e S u l p h a t e w a s first a n a l y z e d b y t h e titration m e t h o d o f H o w a r t h (1978). In this t e c h n i q u e , 3 m L o f 0.0100 M EDTA a n d 4 m L o f 0.4 M HCI w e r e a d d e d t o 1 m L o f s a m p l e , b o i l e d gently for 2 min t o s p e e d t h e chelation of a n y metals present, a n d t h e n c o o l e d after a d d i t i o n o f 10 m L of 0.05 M HCI. Then 5 m L o f 1 0 % B a C I w a s a d d e d a n d t h e solution 2  w a s left for 20 m i n t o a l l o w t h e B a S 0 t o p r e c i p i t a t e . The B a S 0 w a s t h e n c o l l e c t e d o n a 4  4  0.45 n m M i l l i p o r e filter (with i n t e r f e r i n g c a t i o n s w a s h e d  a w a y in t h e filtrate) a n d  resolubilized w i t h 5.00 m L 0.0100 M EDTA a n d 4.0 m L N H O H . The mixture w a s h e a t e d for 15 4  m i n t o a i d d i s s o l u t i o n a n d a f t e r c o o l i n g t o r o o m t e m p e r a t u r e , 0.5 m L o f p H 10 N H C I / N H O H b u f f e r w a s a d d e d a n d t h e e x c e s s ( u n c o m p l e x e d ) EDTA t i t r a t e d w i t h 0.025 4  M MgCI  4  2  solution using a 2.5 m L G i l m o n t m i c r o b u r e t t e . The e n d p o i n t w a s d e t e r m i n e d  using e r i o c h r o m e b l a c k T a s a n i n d i c a t o r . C o p e n h a g e n S t a n d a r d S e a w a t e r w a s d i l u t e d w i t h D D W for s t a n d a r d s . All s a m p l e s w e r e b e l o w t h e t h e d e t e c t i o n limit o f 0.5 m M .  2) C h r o m a f o a r a p h i c Analysis o f S u l p h a t e S u l p h a t e w a s r e d e t e r m i n e d v i a i o n c h r o m a t o g r a p h y using a D i o n e x 2110i Ion C h r o m a t o g r a p h . In this t e c h n i q u e , S 0  4  is a d s o r b e d t o a n i o n e x c h a n g e resin a n d t h e n  e l u t e d w i t h a 2.5 m M s o d i u m c a r b o n a t e a n d 3.1 m M s o d i u m b i c a r b o n a t e buffer. A 0.25 m M H2SO4 s o l u t i o n w a s u s e d a s t h e a n i o n f i b e r s u p p r e s s o r r e g e n e r a n t .  Standards  (N02S04) w e r e m a d e u p in a p p r o p r i a t e c o n c e n t r a t i o n s of N a C I in D D W as a l a r g e CI p e a k interferes w i t h resolution o f t h e SO4 p e a k w h e n CI" is p r e s e n t a t c o n c e n t r a t i o n s > 2 m M . The d e t e c t i o n limit w a s < 1 j i M a t a salt c o n c e n t r a t i o n o f l%o a n d a p p r o x i m a t e l y 5 n M a t t h e m a x i m u m chlorinity o f 9%o ( a b o u t 5 0 % t h a t of s e a w a t e r ) . The p r e c i s i o n w a s 0 . 5 % (1 a , rsd). Sulphite, Thiosulphate a n d Polythionate These sulphur o x y a n i o n s w e r e d e t e r m i n e d p o l a r o g r a p h i c a l l y in b o t h P o w e l l a n d Sakinaw  L a k e s . In P o w e l l  Lake sulphite a n d thiosulphate w e r e also  measured  colourimetrically. S a m p l e s w e r e run within 1 h after c o l l e c t i o n , a n d usually within 30 min. 1) C o l o u r i m e t r i c Analysis o f Thiosulphate T h i o s u l p h a t e w a s d e t e r m i n e d b y t h e c o l o u r i m e t r i c m e t h o d o f U r b a n (1961), w h e r e t h i o s u l p h a t e is c y a n o l i z e d using c u p r i c c h l o r i d e a s a c a t a l y s t . The t h i o c y a n a t e f o r m e d is d e t e r m i n e d c o l o u r i m e t r i c a l l y a s its r e d ferric c o m p l e x . W a t e r w a s pressurefiltered into a b o t t l e c o n t a i n i n g 1 M Z n A c w h i c h h a d b e e n b u b b l e d with N 2  a n y d i s s o l v e d o x y g e n . T h e ZnS w a s f i l t e r e d w i t h 0.4 n m p o l y c a r b o n a t e  2  to remove Nuclepore  m e m b r a n e s while still in t h e g l o v e b a g , a n d t h e n 2.5 m L 1% N a C N , 1.5 mL 0.1 M C u C i a n d 2  2.5 m L ferric nitrate r e a g e n t  (0.74 M F e ( N 0 ) 3  3  in 2 2 % HNO3) w e r e a d d e d t o 25 mL of  s a m p l e (neutralized with 10% NH4OH t o p H 5). After mixing a n d dilution with D D W t o 50 mL, t h e s a m p l e w a s left for > 5 m i n in t h e d a r k ( d u e t o d e g r a d a t i o n o f t h e ferric c o m p l e x in light) for c o l o u r d e v e l o p m e n t . The a b s o r b a n c e w a s t h e n d e t e r m i n e d at 460 n m in 10 c m cells. S t a n d a r d s w e r e m a d e w i t h Na2S 0 «5H 0 in D D W . The d e t e c t i o n limit w a s 1 11M, 2  3  2  a n d p r e c i s i o n w a s 1% ( l a , rsd). 2) C o l o u r i m e t r i c Analysis o f Sulphite Sulphite w a s a n a l y z e d b y t h e c o l o u r i m e t r i c m e t h o d o f W e s t a n d G a e k e (1956) in w h i c h s u l p h i t e f o r m s a s t a b l e d i s u l p h i t o m e r c u r a t e c o m p l e x t h a t is resistant t o air o x i d a t i o n for 12 h. In a g l o v e b a g , 10 m L o f filtered s a m p l e w a s a d d e d t o 25 m L o f 0.1 M s o d i u m t e t r a c h l o r o m e r c u r a t e . A f t e r a l l o w i n g a t least 3 0 m i n f o r t h e sulphite c o m p l e x  f o r m a t i o n , t h e s a m p l e w a s r e m o v e d f r o m t h e g l o v e b a g , a n y HgS filtered o u t , a n d 1 mL of 1% p-rosaniline a n d 1 m L of 0 . 2 % f o r m a l d e h y d e w e r e a d d e d . C o l o u r d e v e l o p m e n t w a s e f f e c t e d for £ 30 min a n d t h e n a b s o r b a n c e w a s d e t e r m i n e d a t 560 n m in 10 c m cells. Standards w e r e m a d e with N a S 0 2  3  in D D W . The d e t e c t i o n limit w a s 0.25 \LM. a n d  p r e c i s i o n w a s 1% ( l a , rsd). 3) P o l a r o g r a p h i c Analysis of Sulphite. T h i o s u l p h a t e . a n d P o l v t h i o n a t e These sulphur oxyanions w e r e m e a s u r e d via differential pulse p o l a r o g r a p h y (DPP) using t h e t e c h n i q u e of Luther et a i . (1985). All m e a s u r e m e n t s w e r e m a d e with a P r i n c e t o n A p p l i e d R e s e a r c h m o d e l 374 P o l a r o g r a p h i c A n a l y z e r a n d a m o d e l 303 static d r o p m e r c u r y e l e c t r o d e (SDME). DPP w a s p e r f o r m e d with a 5 mV»s'' s c a n rate a n d a 0.47 s d r o p t i m e for t h e S D M E in t h e d r o p p i n g m e r c u r y e l e c t r o d e m o d e . T a b l e 5-2 s h o w t h e r e l e v a n t r e a c t i o n s of sulphur s p e c i e s a t t h e m e r c u r y e l e c t r o d e .  Table 5-2  Reactions at the mercury electrode (from Luther e t a l . 1986a)  1) 2S2O3 + Hg -» Hg(8*Oa£ + 2e"  Ei/2 =-0.12 V  2) 2SO? + H g  Ei/2 = -0.60 V  Hg(S0 )2 + 2e" 3  E  3 ) S 0 : + 2e- -> 2S2O3" 4  2  1/2  4)HS" + H g - > HgS + H + 2e"  E  5) f £ + H g -» HgS + (n-l)S + 2e"  E  6) 2RSH + H g -> (RS) Hg + 2 H + 2e"  E  7) RSSR + 2 e " -> 2RS*  E  +  2  +  =-0.32 V  1/2  = -0.68 V  1/2  = -0.68 V  1/2  1/2  = -0.68 V =-0.54 V  E1/2 for 1-3 are for 5 0 % p H 5 a c e t a t e buffer. E1/2 for 4-7 are for p H 10 buffer. E1/2 for 6 a n d 7 varies with R.  P r e p u r i f i e d "zero o x y g e n " n i t r o g e n ( m i n i m u m purity 99.998%) w a s u s e d f o r p u r g i n g t h e a n a l y t i c a l c e l l . W a t e r w a s pressure-filtered into bottles c o n t a i n i n g d e a e r a t e d Z n A c in a 2  g l o v e b a g a n d t h e p r e c i p i t a t e d ZnS w a s t h e n filtered out. Three m L of p H 5.0 a c e t a t e buffer w a s a d d e d t o t h e a n a l y t i c a l c e l l a n d p u r g e d with n i t r o g e n for 10 min. Three mL of  s a m p l e w a s t h e n c o l l e c t e d t h r o u g h a small o p e n i n g in t h e g l o v e b a g w i t h a Finn p i p e t t e w h i c h h a d b e e n f l u s h e d w i t h n i t r o g e n a n d rinsed w i t h s a m p l e t w i c e . The s a m p l e w a s t h e n q u i c k l y a d d e d t o t h e d e a e r a t e d a c e t a t e buffer in t h e e l e c t r o d e c e l l , w h i c h w a s in c l o s e proximity t o t h e g l o v e b a g so t h a t transfer i n v o l v e d m i n i m u m e x p o s u r e t o air. A f t e r stirring for 30 s (by p u r g i n g with N ) , t h e p o l a r o g r a p h s w e e p w a s run. The initial s c a n w a s 2  run b e t w e e n -0.45 t o -0.80 V t o d e t e c t sulphite w h i c h h a s a h a l f - w a v e p o t e n t i a l of a p p r o x i m a t e l y -0.6 V , b e c a u s e it is m o r e easily o x i d i z e d t h a n t h i o s u l p h a t e . Then a s c a n w a s run f r o m 0 t o -0.45 V t o d e t e c t t h i o s u l p h a t e (E1/2 —0.1 V) a n d p o l y t h i o n a t e ( E  V 2  —0.3  V ) . B e c a u s e t h e i o n i c s t r e n g t h v a r i e d w i t h d e p t h in t h e l a c u s t r i n e w a t e r c o l u m n s , s t a n d a r d a d d i t i o n s of N a S 0 » 5 H 0 a n d N a ^ G a 2  2  3  2  in D D W w e r e m a d e t o e a c h s a m p l e .  S o m e s a m p l e s w e r e a n a l y s e d w i t h o u t a d d i t i o n of Z n A c a n d s u l p h i d e w a s r e m o v e d b y 2  a c i d i f y i n g with d e a e r a t e d HCI if n e c e s s a r y (to bring t h e p H d o w n t o a b o u t 5) a n d p u r g i n g with nitrogen. N o difference w a s s e e n b e t w e e n these samples a n d those that  had  s u l p h i d e r e m o v e d as ZnS. The d e t e c t i o n limit w a s 2 \iM a n d 0.25 j i M for t h i o s u l p h a t e a n d sulphite r e s p e c t i v e l y , a n d t h e p r e c i s i o n w a s 1% ( l a , rsd) for b o t h ions. Z e r o v a l e n t Sulphur This f r a c t i o n i n c l u d e s b o t h e l e m e n t a l sulphur a n d z e r o v a l e n t sulphur c o n t a i n e d in p o l y s u l p h i d e s . This p r o c e d u r e follows t h a t o f Luther et a l . (1985) a n d involves r e a c t i n g sulphur with sulphite a n d h e a t t o form thiosulphate, w h i c h c a n t h e n b e  measured  p o l a r o g r a p h i c a l l y . The o v e r a l l r e a c t i o n is: H 0 + S? + (n-l)SOf -> HS" + (n-DSzOf + OH". 2  In a g l o v e b a g , 10 m L o f s a m p l e w a s a d d e d t o d e a e r a t e d a m p o u l e s c o n t a i n i n g d e a e r a t e d 1 M SOf  (25 \il P o w e l l , 55 i± S a k i n a w ) . A m p o u l e s w e r e c a p p e d w i t h s e p t a  a n d t h e n s e a l e d with a n o x y g e n - p r o p a n e t o r c h . Sealing h a d t o b e d o n e outside the g l o v e b a g ; h o w e v e r , t h e c a p s w e r e r e m o v e d o n l y w h e n t h e n e c k of t h e a m p o u l e w a s p l a c e d in t h e f l a m e a n d thus n o i n t r o d u c t i o n of air s h o u l d h a v e o c c u r r e d (the h e a t drives N out o f t h e a m p o u l e ) . The a m p o u l e s w e r e t h e n i m m e d i a t e l y i m m e r s e d in a 60°C 2  w a t e r b a t h f o r t w o hours t o c o m p l e t e t h e r e a c t i o n . S a m p l e s w e r e a n a l y z e d f o r t h i o s u l p h a t e p o l a r o g r a p h i c a l l y a s d e s c r i b e d a b o v e , w i t h t h e e x c e p t i o n t h a t dilutions w e r e m u c h h i g h e r f o r m a n y o f t h e s a m p l e s . P o l y s u l p h i d e sulphur w a s d e t e r m i n e d f r o m filtered w a t e r a n d  p a r t i c u l a t e e l e m e n t a l sulphur from unfiltered s a m p l e s  after  s u b t r a c t i n g t h e p o l y s u l p h i d e sulphur. T h e r e f o r e , t h e s e p a r a t i o n b e t w e e n t h e t w o p h a s e s  is o p e r a t i o n a l l y d e f i n e d , resulting in u n d e r e s t i m a t i o n o f e l e m e n t a l s u l p h u r , a s t h e p o l y s u l p h i d e f r a c t i o n u n d o u b t e d l y c o n t a i n s s o m e c o l l o i d a l e l e m e n t a l sulphur. A n y t h i o s u l p h a t e originally p r e s e n t in t h e w a t e r w a s s u b t r a c t e d f r o m t h e s e m e a s u r e m e n t s . Three r e p l i c a t e a m p o u l e s (all t a k e n f r o m t h e s a m e w a t e r s a m p l e ) w e r e run f o r e a c h s a m p l e . T h e c o n v e r s i o n e f f i c i e n c y w a s f o u n d t o fall b e t w e e n 98 - 9 9 % (± 5 % l a , rsd) a n d t h e d e t e c t i o n limit w a s t h a t o f t h i o s u l p h a t e (-1 \iM S). The s a m p l e s w e r e n o t run until s e v e r a l w e e k s a f t e r c o l l e c t i o n a n d thus u n t r e a t e d w a t e r w a s c o l l e c t e d a n d s t o r e d in a m p o u l e s i d e n t i c a l l y a n d c h e c k e d for o x i d a t i o n a t t h e s a m e t i m e t h e s a m p l e s w e r e run. N o sulphite o r t h i o s u l p h a t e w a s f o u n d in a n y o f t h e six a m p o u l e s t e s t e d ( i n c l u d i n g s o m e from t h e most sulphidic w a t e r ) , indicating that n o oxidation o c c u r r e d during handling a n d storage. Dissolved Sulphide D i s s o l v e d s u l p h i d e w a s d e t e r m i n e d a c c o r d i n g t o t h e m e t h o d o f C l i n e (1969). In this t e c h n i q u e , H S r e a c t s w i t h N.N d i m e t h y l - p - p h e n y l e r i e d i a m i n e s u l p h a t e in a c i d 2  m e d i u m , with  FeCI  2  as a catalyst, t o form methylene  b l u e w h i c h is m e a s u r e d  c o l o u r i m e t r i c a l l y . W a t e r w a s c o l l e c t e d in a g l o v e b a g , a n d s u l p h i d e w a s t r a p p e d with deaerated ZnAc  2  a s ZnS, t o b e a n a l y s e d s e v e r a l w e e k s later. Different c o n c e n t r a t i o n s  of r e a g e n t s a r e r e q u i r e d for different s u l p h i d e levels. B e c a u s e C l i n e (1969) only p r o v i d e d r e c i p e s f o r s u l p h i d e c o n c e n t r a t i o n s u p t o 1000 p.M, a n a d d i t i o n a l m o r e c o n c e n t r a t e d r e a g e n t c o n t a i n i n g 10.0 g N,N d i m e t h y l - p - p h e n y l e n e d i a m i n e s u l p h a t e plus 15.0 g F e C I » 6 H 0 in 100 m L 5 0 % (v/v) HCI w a s f o r m u l a t e d f o r t h e 3 - 5.5 m M s u l p h i d e 3  2  c o n c e n t r a t i o n s f o u n d in P o w e l l a n d S a k i n a w  Lakes. S t a n d a r d s w e r e m a d e  from  N a S » 9 H 0 in d e a e r a t e d D D W , a n d w e r e t r e a t e d in t h e s a m e w a y as t h e s a m p l e s (i.e. 2  2  p r e c i p i t a t e d w i t h Zn) a n d s t o r e d for a p e r i o d of t i m e b e f o r e analysis. The d e t e c t i o n limit w a s 0.3 \iM a n d p r e c i s i o n w a s a b o u t 2 % (1 a , rsd). Particulate Sulphide Analysis W a t e r s a m p l e s w e r e c o l l e c t e d in d e o x y g e n a t e d 4 L jugs in a g l o v e b a g . The jugs w e r e rinsed 3 times with s a m p l e w a t e r , a n d w e r e t h e n r e t u r n e d i m m e d i a t e l y t o t h e l a b . T h e p r o c e d u r e f o l l o w e d w a s t h a t o f H o w a r t h a n d J o r g e n s e n (1984), m o d i f i e d for w a t e r s a m p l e s . This m e t h o d i n v o l v e d essentially f o u r steps: 1) r e m o v a l o f S°; 2) c o l l e c t i o n o f p a r t i c u l a t e s b y filtration;  3) c o l l e c t i o n of m o n o s u l p h i d e s v i a a c i d distillation; a n d 4) c o l l e c t i o n of pyrite b y c h r o m i u m r e d u c t i o n . All steps w e r e c a r r i e d out in nitrogen-filled g l o v e b a g s , n S° R e m o v a l T w o L of s a m p l e w a s stirred w i t h 200 mL d e o x y g e n a t e d CS2 a t t h e highest s p e e d o n a m a g n e t i c stirrer for 1 hr. The o r g a n i c CS2 layer c o n t a i n i n g t h e S° w a s t h e n s e p a r a t e d f r o m t h e a q u e o u s l a y e r in a s e p a r a t o r y f u n n e l . The s e p a r a t i o n w a s not c o m p l e t e , w i t h s o m e frothing o c c u r r i n g at t h e i n t e r f a c e of t h e t w o layers. H e n c e , c a r e w a s t a k e n not t o c o l l e c t a n y of t h e a q u e o u s f r a c t i o n t h a t h a d l a r g e d r o p s of CS2 in it. The v o l u m e of t h e aqueous fraction collected was recorded.  7) Filtration The a q u e o u s f r a c t i o n f r o m 1) w a s filtered t h r o u g h 0.4 urn N u c l e p o r e m e m b r a n e s . B e c a u s e o f t h e l a r g e a m o u n t of o r g a n i c "fluff" p r e s e n t in t h e b o t t o m w a t e r s of b o t h lakes (as w a s n o t e d in all filtrations d o n e o n this w a t e r ) , t h e filters c l o g g e d easily a n d h a d t o b e c h a n g e d f r e q u e n t l y . For t h e a p p r o x i m a t e l y 8 L of w a t e r filtered for e a c h s a m p l e , a s m a n y a s 25 filters w e r e r e q u i r e d . B e f o r e r e m o v i n g filters f r o m v a c u u m , t h e y w e r e rinsed w e l l w i t h d e a e r a t e d D D W t o r e m o v e d i s s o l v e d s u l p h i d e . Filters w e r e f o l d e d a n d a d d e d t o t h e d e a e r a t e d distillation a p p a r a t u s flask in t h e g l o v e b a g . 3) M o n o s u l D h i d e The r e a c t i o n flask w a s h o o k e d u p t o t h e distillation a p p a r a t u s ( a c y a n i d e distillation a p p a r a t u s w h i c h h a d previously b e e n f l u s h e d w i t h N ) 2  f l u s h e d with N  2  Wheaton and  then  for 15 min. D e a e r a t e d c o n c e n t r a t e d HCI (30 mL) w a s t h e n a d d e d with a  syringe a n d r e l e a s e d  H S w a s t r a p p e d w i t h Z n A c . After 15 m i n , t h e r e a c t i o n flask w a s 2  2  b r o u g h t t o b o i l i n g t o r e l e a s e a n y greigite sulphur. The r e a c t i o n w a s a l l o w e d t o p r o c e e d for a n o t h e r 4 5 m i n , after w h i c h t h e h e a t w a s r e m o v e d a n d t h e t w o Z n A c traps (in series) 2  w e r e r e m o v e d . A t r a p c o n t a i n i n g 100 m L of 1 M p H 4 p h o s p h a t e buffer w a s installed b e t w e e n t h e c o l d f i n g e r a n d t h e Zn traps t o r e m o v e HCI v a p o u r . 4) Pyrite A f t e r r e p l a c i n g t h e t w o Z n A c t r a p s a n d letting t h e distillation a p p a r a t u s c o o l for 2  15 m i n , 2 0 m L of E t O H a n d 6 0 m L of 1M C r C b , b o t h d e a e r a t e d , w e r e a d d e d . Fifteen m i n after r e a g e n t a d d i t i o n , h e a t w a s a p p l i e d a n d t h e s a m p l e w a s b o i l e d g e n t l y for 45 min.  S a m p l e s c o l l e c t e d a s ZnS w e r e m e a s u r e d a s d e s c r i b e d a b o v e f o r s o l u b l e s u l p h i d e s . The d e t e c t i o n limit w a s d e p e n d e n t o n t h e v o l u m e of w a t e r f i l t e r e d ; for a n a v e r a g e s a m p l e v o l u m e of 7700 mL, it w a s 0.8 n M S, or 1.6 n M FeS2. B e c a u s e s a m p l e analysis t o o k so l o n g (from start t o finish - 6 - 7 h p e r s a m p l e ) , r e p l i c a t e s w e r e not d o n e . O n e pyrite m e a s u r e m e n t w a s r e p l i c a t e d d u e t o loss of t h e m o n o s u l p h i d e p o r t i o n of t h e s a m p l e a n d t h e t w o m e a s u r e m e n t s d i f f e r e d b y 3.7% (41.59 n M versus 40.09 n M ) . N o t e t h a t this t e c h n i q u e is s p e c i f i c t o i n o r g a n i c s u l p h i d e s , a s o r g a n i c sulphur is not r e d u c e d v i a this p r o c e d u r e ( Z h a b i n a a n d V o l k o v 1978; H o w a r t h a n d J o r g e n s e n 1984; C a n f i e l d e t a l . 1986). O n e serious p r o b l e m w i t h this a n a l y t i c a l f r a c t i o n a t i o n s c h e m e u s e d is t h e h o t a c i d distillation t h a t is r e q u i r e d t o solubiiize g r e i g i t e . B e c a u s e g r e i g i t e is t h e thiospinel of iron ( F e F e F e S 4 ) . w h e n hot HCI is a d d e d F e 2+  3+  3+  3+  is r e l e a s e d , w h i c h c a n t h e n  oxidize H2S t o S°. The S° g e n e r a t e d is n o t v o l a t i l i z e d v i a a c i d , h o w e v e r , a n d i n s t e a d it is r e d u c e d b y c h r o m i u m in t h e s e c o n d distillation. Thus, s o m e of t h e s u l p h i d e f r o m t h e a c i d - v o l a t i l e p h a s e s is m e a s u r e d in t h e c h r o m i u m - r e d u c e d "pyrite" f r a c t i o n ( B e r n e r 1964a, 1974), resulting in t h e pyrite f r a c t i o n b e i n g o v e r e s t i m a t e d , a n d t h e m o n o s u l p h i d e f r a c t i o n u n d e r e s t i m a t e d . To o b v i a t e this, S n C b , w h i c h r e d u c e s t h e F e , c a n b e a d d e d , 3+  t h e r e b y p r o t e c t i n g t h e H2S f r o m o x i d a t i o n ( C h a n t o n a n d M a r t e n s 1985). H o w e v e r , w h e n pyrite is b o i l e d in a c i d in t h e p r e s e n c e of S n C I . s o m e of t h e pyrite is r e l e a s e d t o t h e 2  m o n o s u l p h i d e f r a c t i o n . This results in m o n o s u l p h i d e b e i n g o v e r e s t i m a t e d a n d  pyrite  u n d e r e s t i m a t e d . W h e n t h e a c i d distillation is c a r r i e d out w i t h S n C b at r o o m t e m p e r a t u r e , h o w e v e r , n o pyrite is r e l e a s e d t o t h e m o n o s u l p h i d e p o o l . This c o n s i d e r a t i o n a l s o has a d r a w b a c k : g r e i g i t e c a n n o t b e r e l e a s e d w i t h o u t h e a t (R. Berner pers. c o m m . , c i t e d in H o w a r t h a n d J0rgensen 1984). T h e r e f o r e , m o n o s u l p h i d e s a r e a g a i n  underestimated  a n d pyrite is o v e r e s t i m a t e d . This c o n u n d r u m w a s s o l v e d b y H o w a r t h a n d J0rgensen (1984) b y p e r f o r m i n g a n a d d i t i o n a l CS2 e x t r a c t i o n after t h e a c i d distillation t o r e m o v e t h e e l e m e n t a l sulphur f o r m e d d u r i n g t h e a c i d i f i c a t i o n s t e p . B e c a u s e of t h e l o w p a r t i c u l a t e sulphur c o n c e n t r a t i o n s in t h e lakes a n d t h e difficulty in d e a l i n g with t h e l a r g e v o l u m e s of w a t e r r e q u i r e d t o c o l l e c t e n o u g h m a t e r i a l for analysis, this e x t r a s t e p w a s not c a r r i e d out in this study. T h e r e f o r e , it is possible t h a t t h e pyrite f r a c t i o n h a s b e e n o v e r e s t i m a t e d in this s t u d y a t t h e e x p e n s e of t h e m o n o s u l p h i d e f r a c t i o n . It s h o u l d a l s o b e n o t e d t h a t s o m e p y r i t e m a y b e e x t r a c t e d b y t h e h o t a c i d distillation b e c a u s e this s o l u t i o n is  u n d e r s a t u r a t e d w i t h r e s p e c t t o pyrite; t h u s , t h e e x t r a c t i o n is a k i n e t i c a l l y - d o m i n a t e d p r o c e s s ( M o r s e a n d C o r n w e l l 1987). Iron a n d Manganese Bottles for t h e c o l l e c t i o n of t r a c e  metals w e r e  n e w p o l y p r o p y l e n e or  p o l y e t h y l e n e bottles w h i c h h a d b e e n w a s h e d in h o t 2 0 % HNO3 a n d rinsed with N a n o p u r e w a t e r . S a m p l e w a t e r w a s pressure-filtered i n t o b o t t l e s c o n t a i n i n g a n a p p r o p r i a t e v o l u m e o f Ultrapure Seastar® HNO3 t o bring t h e p H t o 2. W a t e r s a m p l e s w e r e a n a l y z e d using g r a p h i t e f u r n a c e a t o m i c a b s o r p t i o n s p e c t r o m e t r y . S a k i n a w L a k e s a m p l e s w e r e a n a l y z e d v i a d i r e c t i n j e c t i o n into p y r o l i t i c a l l y - c o a t e d g r a p h i t e t u b e s using a Perkin Elmer 560 A A w i t h d e u t e r i u m b a c k g r o u n d c o r r e c t i o n a n d s t a n d a r d s p r e p a r e d in a c i d i f i e d , c h e l e x e d artificial s e a w a t e r a t t h e a p p r o p r i a t e salinities. The d e t e c t i o n limit for M n w a s 3.6 n M a n d p r e c i s i o n 2 % ( l a , rsd), w h e r e a s f o r Fe it w a s 9 n M a n d 2 % . P o w e l l L a k e s a m p l e s w e r e a n a l y z e d w i t h a V a r i a n S p e c t r A A 300 s p e c t r o p h o t o m e t e r w i t h Z e e m a n b a c k g r o u n d c o r r e c t i o n a n d a PSD 96 a u t o s a m p l e r using pyrolitic g r a p h i t e t u b e s w i t h L'vov p l a t f o r m s a n d a p a l l a d i u m m o d i f i e r (9.39 m M P d in 1% HCI a n d 1% HNO3) t o r e m o v e matrix e f f e c t s . S t a n d a r d s w e r e p r e p a r e d in 0 . 1 % HNO3 N a n o p u r e w a t e r . A l l d i s s o l v e d m e t a l c o n c e n t r a t i o n s w e r e a b o v e t h e d e t e c t i o n limit. Speciation a n d Solubility Calculations S p e c i a t i o n o f iron a n d m a n g a n e s e , a s w e l l a s t h e s a t u r a t i o n s t a t e o f iron a n d m a n g a n e s e solid p h a s e s w a s c a l c u l a t e d using M I N E Q L a s d e s c r i b e d in A p p e n d i x 1.  5.3 Results  Powell Lake T h i o s u l p h a t e a n d sulphite a r e u n d e t e c t a b l e t h r o u g h o u t t h e w a t e r c o l u m n o f P o w e l l L a k e . S u l p h a t e o c c u r s a t a c o n c e n t r a t i o n o f a b o u t 8 m-M in t h e u p p e r o x i c w a t e r s (Fig. 5-5), i n c r e a s e s sharply t o 11.3 |xM a t t h e o x i c / a n o x i c i n t e r f a c e , a n d t h e n d e c r e a s e s r a p i d l y w i t h d e p t h , b e c o m i n g u n d e t e c t a b l e a t 185 m. D i s s o l v e d s u l p h i d e (H S) is first 2  d e t e c t a b l e b e t w e e n 145 a n d 150 m. The c o n c e n t r a t i o n remains l o w (< 10 n.M) t o 180 m , b e l o w w h i c h it i n c r e a s e s t o a m a x i m u m of 3.1 m M in t h e b o t t o m w a t e r . P o l y s u l p h i d e (Si^) is first d e t e c t e d a t a d e p t h o f 160 m , a n d t h e n i n c r e a s e s m a r k e d l y t o m a x i m u m levels of 2.2 m M in t h e b o t t o m w a t e r (Fig. 5-5). T h e ratio o f t o t a l d i s s o l v e d s u l p h i d e t o t o t a l d i s s o l v e d z e r o v a l e n t sulphur d e c r e a s e s slightly f r o m just b e l o w t h e i n t e r f a c e (-2.0) t o t h e b o t t o m w a t e r s (-1.4) (Fig. 5-7). D i s s o l v e d iron c o n c e n t r a t i o n s a r e t y p i c a l l y 0.04 p.M in t h e o x i c w a t e r s w i t h a s u r f a c e m a x i m u m of 0.09 j i M (Fig. 5-9). C o n c e n t r a t i o n s i n c r e a s e r a p i d l y just a b o v e t h e i n t e r f a c e a t 145 m , r e a c h i n g a m a x i m u m o f 171 \iM a t 210 m. B e l o w this d e p t h , t h e iron c o n t e n t d e c r e a s e s sharply t o 0.06 | i M a t t h e b o t t o m . From m o d e l l i n g ( A p p e n d i x 1), iron o c c u r s p r i m a r i l y (98-99%) a s f r e e F e  2 +  in t h e o x i c w a t e r s (Fig. 5-11). T h e e l e m e n t is  i n c r e a s i n g l y c o m p l e x e d w i t h CI" w i t h d e p t h t o a m a x i m u m of 3 4 % a s F e C I in t h e b o t t o m +  water. D i s s o l v e d m a n g a n e s e is also l o w in t h e u p p e r o x i c w a t e r s w i t h c o n c e n t r a t i o n s o f a b o u t 0.03 n M (Fig. 5-9). Levels b e g i n t o i n c r e a s e a t 100 m , a n d a v e r y l a r g e i n c r e a s e in c o n c e n t r a t i o n is o b s e r v e d b e t w e e n 125 m d e p t h a n d t h e i n t e r f a c e . T h e c o n c e n t r a t i o n t h e n remains fairly invariant a t a b o u t 27 j i M t o a d e p t h o f 250 m , b e l o w w h i c h t h e e l e m e n t is d e p l e t e d t o a m i n i m u m o f 0.7 | i M a t t h e b o t t o m . M a n g a n e s e is p r e s e n t a s f r e e M n  2 +  in  t h e u p p e r o x i c w a t e r s ; in t h e b o t t o m s u l p h i d i c w a t e r s , M n H S i c o m p l e x e s p r e d o m i n a t e , c o m p r i s i n g 7 8 % o f t h e t o t a l M n (Fig. 5-13). O t h e r m a n g a n e s e s p e c i e s i n c l u d e M n H C 0 ( m a x . 8%), M n C I  +  3  +  ( m a x . 18%) a n d MnSf" ( m a x . 10%) ( s e e A p p e n d i x 1 f o r d e s c r i p t i o n o f  calculations). P a r t i c u l a t e iron m o n o s u l p h i d e s a r e u n d e t e c t a b l e a b o v e a d e p t h o f 200 m (4 n M ) (Fig. 5-15). T h e c o n c e n t r a t i o n t h e n i n c r e a s e s t o a m a x i m u m o f 2 4 0 n M a t 275 m , b e l o w w h i c h it r a p i d l y d e c l i n e s t o 84 n M a t t h e b o t t o m . Pyrite is first d e t e c t a b l e a t 150 m (41 n M ) ,  i n c r e a s e s t o a m a x i m u m of 4 3 5 n M a t 250 m , a n d t h e n d e c r e a s e s r a p i d l y t o 150 n M a t t h e bottom.  Sakinaw Lake Thiosulphate a n d sulphite a r e a l s o u n d e t e c t a b l e t h r o u g h o u t t h e w a t e r c o l u m n of S a k i n a w L a k e . S u l p h a t e is p r e s e n t in t h e u p p e r o x i c w a t e r s o f S a k i n a w  Lake at a  c o n c e n t r a t i o n o f a b o u t 5 0 n M (Fig. 5-6), a n d i n c r e a s e s r a t h e r s h a r p l y just a b o v e t h e i n t e r f a c e t o 64 \iM b e f o r e d e c l i n i n g r a p i d l y b e l o w t h e i n t e r f a c e . The i o n is u n d e t e c t a b l e b e l o w 50 m. B o t h s u l p h i d e a n d p o l y s u l p h i d e sulphur first a p p e a r a t 30 m d e p t h a n d their profiles a r e similar in s h a p e . C o n c e n t r a t i o n s i n c r e a s e r a p i d l y b e t w e e n 4 0 a n d 6 0 m , a n d r e m a i n a p p r o x i m a t e l y c o n s t a n t (H S - 5.3 m M , Sv, - 4.5 m M ) t o t h e b o t t o m . D i s s o l v e d 2  s u l p h i d e c o n c e n t r a t i o n s a r e a l m o s t t w o t i m e s t h o s e o f P o w e l l L a k e . The ratio o f t o t a l d i s s o l v e d s u l p h i d e t o p o l y s u l p h i d e sulphur is fairly c o n s t a n t w i t h d e p t h (-1.2) b e l o w 40 m (Fig. 5-8). D i s s o l v e d iron a n d m a n g a n e s e c o n c e n t r a t i o n s a r e l o w e r t h a n in P o w e l l L a k e . Dissolved iron is p r e s e n t in t h e o x i c w a t e r s , w i t h a m a x i m u m c o n c e n t r a t i o n of 0.3 j i M a t 5 m (Fig. 5-10). B e l o w this h o r i z o n , t h e c o n c e n t r a t i o n d e c r e a s e s t o 0.04 | i M until just b e l o w t h e i n t e r f a c e a t 32.5 m w h e r e it rapidly rises t o a m a x i m u m o f 6.7 \iM. B e l o w this d e p t h t h e c o n c e n t r a t i o n r a p i d l y falls t o a b o u t 0.09 jxM, a n d r e m a i n s fairly c o n s t a n t b e l o w 75 m. M a n g a n e s e is u n d e t e c t a b l e in t h e s u r f a c e w a t e r s , first a p p e a r i n g a t t h e i n t e r f a c e a t 30 m. It t h e n r a p i d l y i n c r e a s e s t o a m a x i m u m c o n c e n t r a t i o n of a p p r o x i m a t e l y 6 j i M a t 45 m a n d r e m a i n s fairly c o n s t a n t a t g r e a t e r d e p t h . The s p e c i a t i o n o f Fe a n d M n is similar t o t h a t c a l c u l a t e d f o r P o w e l l  Lake  ( A p p e n d i x 1). In t h e o x i c w a t e r s b o t h Fe a n d M n a r e p r e s e n t a l m o s t entirely a s t h e f r e e s p e c i e s (Fig. 5-12,5-14). In t h e a n o x i c w a t e r s , iron o c c u r s as F e C I (-28%) (Fig. 5-12), while +  M n is c o m p l e x e d primarily a s MnHSli in t h e s u l p h i d i c b o t t o m w a t e r s (-87%), with minor contributions f r o m M n C l \ M n H C O s , a n d MnSl" (all < 10%) (Fig. 5-14). Iron m o n o s u l p h i d e is first d e t e c t e d a t 4 0 m d e p t h (124 n M ) (Fig. 5-16), a n d g r a d u a l l y Increases in c o n c e n t r a t i o n t o 543 n M a t a d e p t h of 100 m. B e l o w this horizon t h e c o n c e n t r a t i o n d e c r e a s e s a n d a g a i n i n c r e a s e s t o 545 n M n e a r t h e b o t t o m . Pyrite is first d e t e c t e d right a t t h e i n t e r f a c e (30 m ) , increases t o a m a x i m u m o f 80 n M a t 60 m a n d t h e n d e c r e a s e s with d e p t h .  Sulphate  (piM)  Dissolved S u l p h i d e ( m M ) P o l y s u l p h i d e Sulphur ( m M ) Fig.5-5 Dissolved sulphur s p e c i e s in P o w e l l L a k e  Fig. 5-6 Dissolved sulphur s p e c i e s in S a k i n a w L a k e  Fig. 5-7 Ratio of t o t a l dissolved s u l p h i d e (S(-2)) t o d i s s o l v e d z e r o v a l e n t p o l y s u l p h i d e (S(0)) in Powell Lake  Fig. 5-8 Ratio of t o t a l dissolved s u l p h i d e (S(-2)) t o dissolved z e r o v a l e n t p o l y s u l p h i d e (S(0)) in Sakinaw Lake  Fe  Fig. 5-9 Dissolved iron a n d m a n g a n e s e in Powell L a k e  . M n ^ " QiM)  P e r c e n t Dissolved Iron  Fig. 5-11 P e r c e n t a g e s of free a n d c o m p l e x e d dissolved iron in P o w e l l L a k e . All other c o m plexes w e r e < 1% of the t o t a l dissolved iron.  P e r c e n t D i s s o l v e d Iron  Fig. 5-12 P e r c e n t a g e s of f r e e a n d c o m p l e x e d dissolved iron in S a k i n a w L a k e . All o t h e r c o m plexes w e r e < 1% of t h e t o t a l d i s s o l v e d iron.  P e r c e n t Dissolved M a n g a n e s e  Fig. 5-13 P e r c e n t a g e s of free a n d c o m p l e x e d d i s s o l v e d m a n g a n e s e in P o w e l l Lake. All o t h e r c o m p l e x e s w e r e < 1% of t h e t o t a l dissolved manganese.  Fig. 5-14 P e r c e n t a g e s of f r e e a n d c o m p l e x e d dissolved m a n g a n e s e in S a k i n a w Lake. All o t h e r c o m p l e x e s w e r e < 1% of t h e t o t a l d i s s o l v e d manganese.  Fig. 5-15 Particulate sulphur in Powell Lake  Fig. 5-16 P a r t i c u l a t e sulphur in S a k i n a w L a k e  Log (K *IAP) sp  -15 -i  -10  -5  ri i | i i i i |i i ii  0  5 11  11  .  "  r  -  11  10  11  0  11  15  11  11  •  - %  •  t  o  -  o  -  i  —  •  (  I  i  :  i 1 1 1 -  |  Sakinaw  1  (  •  4  •  o  FeS /  <  2  i •  .... i,... i... .  Fig. 5-17 Saturation state of iron sulphides in P o w e l l Lake. L o g (K *IAP) > 0 = supersaturated, < 0 = undersaturated.  6  • O  \  -  -  ; -  i y •  o  -  •'' • i • • • • i • ' ' •  Fig. 5-18 Saturation state of iron sulphides in S a k i n a w Lake. L o g (K *IAP) > 0 = super5p saturated, < 0 = undersaturated.  Log (K *IAP) sp  -15  -10  -5  i iii iiii iii  0 i i  ii  5  10  i i i i i  i  i i  15 11  MnCO, 20 OXIC  If  40  anoxic  Sakinaw  60  80  /i '  100  120  140  Fig. 5-19 Saturation state of m a n g a n e s e sulphides in P o w e l l Lake. L o g (K *IAP) > 0 = supersp saturated, < 0 = undersaturated.  Mn(OH).  /  MnS,  ' MnS  f ^  mil  i •' ' *' • ' 1  1  Fig. 5-20 Saturation s t a t e of m a n g a n e s e sulphides in S a k i n a w L a k e . L o g (K  *IAP) > 0 = supersp  saturated, < 0 = undersaturated.  5.4 Discussion  Sulphur Oxyanions Sulphate D e s p i t e b e i n g t h e t h e s e c o n d most a b u n d a n t a n i o n in s e a w a t e r (at S = 3 5 % , t h e 0  SO4 c o n c e n t r a t i o n is 28 m M ) , s u l p h a t e is d e p l e t e d a t d e p t h in b o t h lakes (Figs. 5-5 a n d 56). S u c h d e p l e t i o n s r e f l e c t t h e f a c t t h a t s u l p h a t e is t h e m a j o r e l e c t r o n a c c e p t o r for anaerobic  r e s p i r a t i o n in s t r a t i f i e d , o x y g e n - d e p l e t e d w a t e r  masses a n d a n o x i c  s e d i m e n t s ; m u c h o f t h e d e c o m p o s i t i o n in n e a r - s h o r e m a r i n e s e d i m e n t s a n d salt marshes is m e d i a t e d b y s u l p h a t e - r e d u c i n g b a c t e r i a (J0rgensen 1977,1982; H o w a r t h a n d T e a l 1979; H o w a r t h a n d G i b l i n 1983). There a r e t h r e e s o u r c e s for s u l p h a t e in t h e o x i c w a t e r s of P o w e l l a n d S a k i n a w Lakes: d i r e c t a t m o s p h e r i c i n p u t , river input a n d diffusion u p w a r d a n d o x i d a t i o n o f s u l p h i d e f r o m t h e a n o x i c b o t t o m w a t e r s . The h i g h e r s u l p h a t e c o n c e n t r a t i o n s a b o v e 30 m in S a k i n a w L a k e a r e most likely d u e t o t h e larger a m o u n t of s u l p h i d e p r e s e n t in t h e b o t t o m w a t e r , w h i c h s u p p o r t s a m u c h h i g h e r u p w a r d flux of r e d u c e d sulphur t o t h e o x i c s u r f a c e layer w h e r e o x i d a t i o n t o SOt c a n o c c u r . H o w e v e r , S a k i n a w L a k e is a l s o c l o s e r t o t h e Strait o f G e o r g i a w h e n c e s u l p h a t e c o u l d b e s u p p l i e d b y s e a s p r a y . In a d d i t i o n , S a k i n a w e x t e n d s less f a r t h e r i n l a n d t h a n P o w e l l L a k e , w h i c h must result in less dilution o f t h e salt a e r o s o l input. The s u l p h a t e m a x i m u m t h a t o c c u r s a t t h e i n t e r f a c e o f b o t h lakes is p r o b a b l y d u e t o c y c l i n g of sulphur s p e c i e s a t t h e i n t e r f a c e . A s s u l p h i d e diffuses u p w a r d s , it is o x i d i z e d b o t h c h e m i c a l l y b y c o n t a c t w i t h d i s s o l v e d o x y g e n a n d iron a n d m a n g a n e s e o x i d e s as w e l l as b i o l o g i c a l l y v i a c h e m o s y n t h e t i c b a c t e r i a , s u c h as Beggiatoa  s p p . The latter a r e  o b l i g a t e a e r o b e s a n d thus a r e o n l y f o u n d a b o v e t h e o x i c / a n o x i c i n t e r f a c e .  Once  o x i d i z e d t o s u l p h a t e , s u b - i n t e r f a c e s u l p h a t e - r e d u c i n g b a c t e r i a will r e d u c e SO ," w h i c h 2  diffuses d o w n w a r d f r o m t h e m a x i m u m . C o n t i n u e d u p w a r d diffusion of s u l p h i d e results in a c y c l i n g o f s u l p h u r a t t h e o x i c / a n o x i c b o u n d a r y . The i n t e r f a c e in b o t h l a k e s is t o o d e e p f o r p h o t o s y n t h e t i c b a c t e r i a t o survive, a n d , in f a c t , n o p u r p l e b a c t e r i a c o u l d b e visually o b s e r v e d . S u l p h a t e coexists w i t h s u l p h i d e in b o t h lakes, a l t h o u g h in S a k i n a w L a k e t h e z o n e of o v e r l a p is m u c h shallower (30 - 4 5 m ) t h a n in P o w e l l L a k e (150- 180 m)(Figs. 5-5 a n d 5-6). This reflects t h e s h a r p e r i n t e r f a c e in S a k i n a w L a k e . Diffusion p r o c e s s e s a l o n e a r e unlikely  t o a c c o u n t for t h e p r e s e n c e of s u l p h a t e u p t o 3 0 m b e l o w t h e o x i c / a n o x i c i n t e r f a c e , a n d thus sufficient o x i d a n t s must b e present t o g e n e r a t e s u l p h a t e a t t h e s e d e p t h s . Likely o x i d a n t s a r e iron a n d m a n g a n e s e o x i d e s w h i c h i n t e r a c t w i t h sulphur s p e c i e s a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n s (Aller a n d R u d e 1988): 1) 5 H + 2FeOOH + HS" -» 2 F e +  2+  + S° + 4H2O  2) 10H + 6FeOOH + S° -> 6Fe* + S 0 +  3) 16H + 8FeOOH + FeS -> 9 F e +  2+  + 8H2O  4  + S O * + I2H2O  4) 3 H + M n O z + HS" -> M n * + S° + 2HzO +  2  5) 4 H + 3MnG2 + S° -» 3 M n +  2 +  6) 8 H + 4 M n 0 2 + FeS - > 4 M n +  + SO4 + H2O 2 +  + SO? + F e  2+  + 4H2O.  A n e q u i v a l e n t set o f r e a c t i o n s c a n b e w r i t t e n f o r M n O O H or f o r o t h e r M n  3 +  or M n  4 +  c o m p o u n d s . R e a c t i o n 1) is a b i o g e n i c a n d well-known f r o m l a b o r a t o r y a n d f i e l d studies (Berner 1964a; L a n d i n g a n d Lewis 1990; Pyzik a n d S o m m e r 1981; R i c k a r d 1974). C o m p l e t e o x i d a t i o n s u c h as in 2) a n d 3) (with F e  3+  a s r e a c t a n t ) has b e e n d o c u m e n t e d a t l o w p H in  n o n m a r i n e e n v i r o n m e n t s a n d a t h i g h p H in pyrite o x i d a t i o n e x p e r i m e n t s , b u t n o t in n a t u r a l m a r i n e s e d i m e n t s (Berner 1970; B r o c k a n d G u s t a f s o n 1976; Pyzik a n d S o m m e r 1981; M o s e s e t a l . 1987). Aller a n d R u d e (1988) h a v e s h o w n t h a t c o m p l e t e a n o x i c o x i d a t i o n of s u l p h i d e t o s u l p h a t e c a n o c c u r in t h e a b s e n c e o f f r e e O2. w h e n Mn-oxides a r e p h y s i c a l l y m i x e d or in o t h e r w i s e c l o s e c o n t a c t w i t h s u l p h i d i c , a n o x i c m a r i n e s e d i m e n t . A l t h o u g h l i m i t e d c o m p a r a b l e o x i d a t i o n c o u l d a l s o t a k e p l a c e in t h e p r e s e n c e of Fe-oxides, Aller a n d R u d e (1988) f o u n d n o e v i d e n c e for this. Their e x p e r i m e n t s i m p l i e d t h a t M n - o x i d e s a r e 4+  more effective than M n  3 +  (as M n 0 4 / M n O O H ) in o x i d i z i n g s u l p h i d e . T h e s e o x i d a t i o n 3  r e a c t i o n s c a n b e e x p e c t e d t o o c c u r in a n o x i c basins like P o w e l l a n d S a k i n a w w h e r e o x i d e - b e a r i n g or d i s c r e t e o x i d e p a r t i c l e s a r e m i x e d w i t h a n o x i c , sulphur-rich m a t e r i a l . The r e a c t i o n o f M n - o x i d e w i t h HS" is r a p i d (< 1 m i n half-life HS" in p r e s e n c e of e x c e s s Mno x i d e ) a n d p r o d u c e s S ° as a p r i m a r y , a l t h o u g h not e x c l u s i v e , p r o d u c t ( B u r d i g e a n d N e a l s o n 1986). S u c h a n o x i c s u l p h i d e o x i d a t i o n is i n h i b i t e d b y t h e m e t a b o l i c inhibitors a z i d e a n d 2,4-dinitrophenol (DNP) b u t not b y c h l o r a t e (or p r o d u c e d chlorite), i n d i c a t i n g t h a t t h e r e a c t i o n is b i o l o g i c a l l y m e d i a t e d , possibly b y t h i o b a c i l l i or a similar g r o u p of chemolithotrophic bacteria.  Thus, t h e s u l p h a t e distributions o b s e r v e d in P o w e l l a n d S a k i n a w L a k e s c a n b e a t t r i b u t e d t o d i r e c t a e r o s o l a n d s u r f a c e runoff i n p u t s , a n d u p w a r d d i f f u s i o n a n d oxidation of sulphide at or near t h e oxic/anoxic b o u n d a r y , thereby establishing a steady-state sulphate m a x i m u m . Thiosulphate, Sulphite a n d Polythionates The p a r t i a l l y o x i d i z e d f o r m s o f s u l p h u r m a y b e p r o d u c e d b y a v a r i e t y o f r e a c t i o n s , i n c l u d i n g t h e o x i d a t i o n o f H2S ( C h e n a n d Morris 1972a,b; H o f f m a n n 1977), t h e o x i d a t i o n o f s u l p h i d e minerals ( G o l d h a b e r 1983), a n d m i c r o b i a l p r o c e s s e s ( G o l d h a b e r a n d K a p l a n 1974). D e s p i t e this variety o f p o t e n t i a l f o r m a t i o n p a t h w a y s for partly o x i d i z e d sulphur c o m p o u n d s , in this s t u d y , t h i o s u l p h a t e , s u l p h i t e a n d p o l y t h i o n a t e s w e r e n o t d e t e c t e d a t a n y d e p t h in e i t h e r l a k e . This is a n u n e x p e c t e d result, g i v e n t h a t t h e s e s p e c i e s a r e i n t e r m e d i a t e s in t h e o x i d a t i o n o f b o t h s u l p h i d e ( C h e n a n d Morris 1972a,b) a n d pyrite ( G o l d h a b e r  1983; M o s e s e t a l . 1987) t o s u l p h a t e . S u l p h i t e ( S O ? ) a n d  t e t r a t h i o n a t e (S^Of) a r e k n o w n t o o c c u r , a l b e i t rarely, in n a t u r a l e n v i r o n m e n t s . B o u l e g u e et a l . (1982) a n d Luther e t a l . ( 1 9 8 5 , 1 9 8 6 a ) , for e x a m p l e , f o u n d small a m o u n t s o f sulphite ( 7 - 4 0 \iM) in a f e w salt m a r s h p o r e w a t e r s a m p l e s , a n d Luther e t a l . (1986b) f o u n d u p t o 300 p.M t e t r a t h i o n a t e in t h e p o r e w a t e r s f r o m o n e salt m a r s h c o r e . In c o n t r a s t t o t h e s e i n t e r m e d i a t e s p e c i e s , t h i o s u l p h a t e is a significant c o m p o n e n t ( t y p i c a l l y 20 - 700 n M ) of d i s s o l v e d sulphur a t a l l d e p t h s in salt m a r s h p o r e w a t e r s ( B o u l e g u e e t a l 1982; H o w a r t h a n d Teal 1980; H o w a r t h e t a l 1983; Luther e t a l . 1985,1986a,b), s u b t i d a l p o r e w a t e r s (Luther et a l . 1985), l a k e p o r e w a t e r s ( N r i a g u e t a l . 1979) a n d s o m e a n o x i c w a t e r c o l u m n s ( J a c o b s 1984). T h i o s u l p h a t e is o n e o f t h e first p r o d u c t s o f t h e o x i d a t i o n o f pyrite (Goldhaber  1983; M o s e s e t a l . 1987; Luther 1987), a n d is a l s o p r o d u c e d f r o m t h e  d e c o m p o s i t i o n o f p o l y s u l p h i d e s ( C h e n a n d Morris 1972a). U n d e r w e a k l y  acidic  c o n d i t i o n s , s u c h a s t h o s e in b o t h S a k i n a w a n d P o w e l l L a k e s , t h i o s u l p h a t e is r e a d i l y o x i d i z e d t o t e t r a t h i o n a t e b y w e a k oxidizing a g e n t s (Lyons a n d Nickless 1968), viz: 2S2O : + H + I/2O2 -> S4CI + H2O. 2  +  P o l y t h i o n a t e s a r e relatively s t a b l e u n d e r a c i d i c c o n d i t i o n s (Lyons a n d Nickless 1968; D o w s o n a n d J o n e s 1974). The distributions o f sulphur o x y a n i o n s in P o w e l l a n d S a k i n a w Lakes a r e similar t o t h e B l a c k S e a , w h e r e n o S 0 T,S406 o r S O l a r e d e t e c t a b l e a t a n y d e p t h (Luther et a l . 2  2  1990c). This is in spite o f t h e f a c t t h a t Luther e t a l . (1990c) f o u n d a n a e r o b i c s u l p h i d e  o x i d a t i o n in s t o r e d s a m p l e s . This o b s e r v a t i o n i n d i c a t e s t h a t t h e i n t e r m e d i a t e oxidationstate sulphur o x y a n i o n s a r e either not f o r m e d , or, if t h e y a r e p r o d u c e d t h e y a r e o x i d i z e d r a p i d l y a n d essentially q u a n t i t a t i v e l y , or p e r h a p s u s e d a s a l t e r n a t e e l e c t r o n a c c e p t o r s (Truper 1982). H o w e v e r , Luther et a l . (1990c) o b s e r v e d t h a t t h i o s u l p h a t e w a s g e n e r a t e d w h e n a i r w a s p u r p o s e l y a l l o w e d t o c o n t a c t their s a m p l e s . T h e p a u c i t y o f sulphur o x y a n i o n s in b o t h t h e B l a c k S e a a n d P o w e l l a n d S a k i n a w L a k e s is c o n s i s t e n t w i t h a n a p p a r e n t l a c k o f a p p r o p r i a t e c h e m i c a l o x i d a n t s (O2, Fe(lll), Mn(IV)) in t h e a n o x i c w a t e r s . T h e inability t o d e t e c t a n y of t h e s e s p e c i e s i n d i c a t e s t h a t o x i d a t i o n of t h e s a m p l e s d i d n o t o c c u r in t h e f i e l d , o r in t h e various m a n i p u l a t i o n s o f t h e s a m p l e w a t e r such as ampouling.  Reduced Sulphur Species Dissolved Sulphide Dissolved s u l p h i d e c o n c e n t r a t i o n s a r e a l m o s t t w i c e a s h i g h in S a k i n a w L a k e as in P o w e l l (Figs. 5-5 a n d 5-6). B e c a u s e c h l o r i d e a n d o t h e r m a j o r i o n c o n c e n t r a t i o n s in t h e f o r m e r a r e a b o u t two-thirds t h o s e of P o w e l l , S a k i n a w L a k e must h a v e h a d a n a d d i t i o n a l s u l p h a t e s o u r c e s i n c e it w a s c u t off f r o m t h e o c e a n a p p r o x i m a t e l y 11000 y e a r s a g o . The most likely s o u r c e is f r o m o c c a s i o n a l intrusions of s e a w a t e r o v e r t h e b a r e l y e m e r g e d sill. H o w e v e r , s u l p h i d e levels m a y a l s o b e h i g h e r in S a k i n a w b e c a u s e of limitation o n irons u l p h i d e f o r m a t i o n d u e t o l a c k o f iron. The s u l p h i d e c o n c e n t r a t i o n m a x i m a in t h e t w o lakes differ b y 2500 | i M w h e r e a s t h e d i f f e r e n c e in m a x i m u m iron c o n t e n t s is only 160 n M . This o r d e r of m a g n i t u d e d i f f e r e n c e i n d i c a t e s t h a t t h e h i g h e r c o n c e n t r a t i o n s of s u l p h i d e in S a k i n a w a r e most likely a t t r i b u t a b l e t o a n a d d i t i o n a l s u l p h a t e s o u r c e . N o t e it h a s b e e n a s s u m e d t h a t t h e a n a l y t i c a l m e t h o d u s e d (Cline 1969) m e a s u r e s all d i s s o l v e d s u l p h i d e , i n c l u d i n g t h e t e r m i n a l S " o f p o l y s u l p h i d e s . H o w e v e r , 2  Jacobs  (1984) a n d Luther e t a l . (1985) f o u n d t h a t only a p p r o x i m a t e l y 20 - 2 5 % of S$ a d d e d t o H S2  b e a r i n g solutions w a s d e t e c t a b l e ( 7 4 % of Na S2). P r e s u m a b l y , t h e e l e m e n t a l sulphur 2  l i b e r a t e d w h e n t h e a c i d i c r e a g e n t is a d d e d t o t h e s a m p l e s inhibits t h e d e v e l o p m e n t of t h e m e t h y l e n e b l u e c o l o u r . C l i n e (1969) i n d i c a t e d t h a t t h i o s u l p h a t e , b u t n o t sulphite, has a similar e f f e c t o n c o l o u r d e v e l o p m e n t . Thus, this t e c h n i q u e u n d e r e s t i m a t e s t o t a l S(-2) w h e n p o l y s u l p h i d e s a r e p r e s e n t . To a v o i d this p r o b l e m I a t t e m p t e d t o m e a s u r e d i s s o l v e d s u l p h i d e p o l a r o g r a p h i c a l l y a s in Luther et a l . (1985), a t e c h n i q u e w h i c h d o e s  n o t suffer f r o m s u c h i n t e r f e r e n c e s a n d is a l s o m u c h m o r e sensitive t h a n t h e C l i n e t e c h n i q u e . H o w e v e r , d u r i n g t h e short t i m e b e t w e e n c o l l e c t i o n o f s a m p l e s a n d their d e l i v e r y t o t h e n e a r b y l a b , a l a r g e a m o u n t o f t h e H2S h a d b e e n lost ( w h e n c o m p a r e d t o t h e C l i n e m e t h o d ) . This is most likely d u e t o t h e s t r i p p i n g o f H S f r o m t h e s a m p l e s b y 2  v i g o r o u s o u t g a s s i n g o f m e t h a n e a s s a m p l e s a r e b r o u g h t t o t h e s u r f a c e f r o m d e p t h . The a m o u n t o f H2S lost w a s d e p e n d e n t o n p H : a t l o w e r d e p t h s , w h e r e t h e p H w a s less a c i d i c (6.8), o n l y a b o u t 2 5 % w a s lost d u e t o t h e p r e s u m e d o u t g a s s i n g . A t m i d - d e p t h s , w h e r e t h e p H r a n g e d f r o m 5.9 - 6.4, t h e r e w a s 4 0 - 6 4 % loss. A t t h e u p p e r m o r e a c i d i c d e p t h s ( p H 5.7), u p t o 7 5 % o f t h e H S m e a s u r e d v i a t h e C l i n e t e c h n i q u e w a s n o t d e t e c t e d 2  p o l a r o g r a p h i c a l l y . This is d u e t o t h e m u c h larger p r o p o r t i o n o f s u l p h i d e o c c u r r i n g a s t h e H S s p e c i e s ( 9 5 % a t p H 5.7 versus 4 5 % a t p H 6.8) a t t h e m o r e a c i d i c p H levels. A s n o 2  t h i o s u l p h a t e o r s u l p h i t e w e r e d e t e c t e d , s u l p h i d e loss d i d n o t a p p e a r t o b e d u e t o o x i d a t i o n . If all o f t h e p o l y s u l p h i d e p r e s e n t w a s S4 ( s e e next s e c t i o n ) a n d t h e r e f o r e , t h e d i s s o l v e d s u l p h i d e c o n c e n t r a t i o n s a r e p r o b a b l y a b o u t 2 5 % u n d e r e s t i m a t e d (Luther et al. 1985). Zerovalent Sulphur P o l y s u l p h i d e s a r e i m p o r t a n t i n t e r m e d i a t e s in p y r i t e f o r m a t i o n ( G i b l i n 1988; H o w a r t h 1979; Lord a n d C h u r c h 1983; Luther et a l . 1982; R i c k a r d 1975; S w e e n e y a n d K a p l a n 1974), a n d a r e a l s o p o t e n t i a l l y i m p o r t a n t in t h e c y c l i n g o f m e t a l s d u e t o their h i g h c o m p l e x i n g a b i l i t y ( B o u l e g u e e t a l . 1982: B o u l e g u e a n d Denis 1983). P o l y s u l p h i d e c o n c e n t r a t i o n s a r e e x t r e m e l y h i g h in b o t h P o w e l l a n d S a k i n a w L a k e s , m a k i n g u p a l a r g e f r a c t i o n o f t h e t o t a l i n o r g a n i c sulphur p o o l (Figs. 5-5 a n d 5-6). Free e l e m e n t a l sulphur w a s not d e t e c t e d in e i t h e r l a k e . T h e v e r y l a r g e q u a n t i t y o f polysulphide-sulphur m a y h a v e interfered w i t h t h e d e t e r m i n a t i o n of S ° , h o w e v e r , s i n c e in this study t h e e l e m e n t a l sulphur f r a c t i o n is o p e r a t i o n a l l y d e f i n e d a s t h e z e r o v a l e n t sulphur w h i c h is r e t a i n e d o n a 0.4 | i m N u c l e p o r e filter. T h e f r e e S ° is d e t e r m i n e d b y d i f f e r e n c e b e t w e e n t o t a l z e r o v a l e n t sulphur a n d dissolved z e r o v a l e n t sulphur, b u t t h e high polysulphide-sulphur c o n t e n t w o u l d p r o b a b l y interfere b y s w a m p i n g a n y small a m o u n t o f f r e e S ° present. A l s o , a n y colloidal S° present w o u l d b e m e a s u r e d as polysulphide. The p o l y s u l p h i d e c o n c e n t r a t i o n s f o u n d in b o t h S a k i n a w a n d P o w e l l Lakes a r e e x t r e m e l y h i g h (Figs. 5-5 a n d 5-6) w h e n c o m p a r e d t o t h o s e r e p o r t e d f o r o t h e r a n o x i c basins, w h i c h rarely h a v e p o l y s u l p h i d e present. For e x a m p l e , in t h e h y p e r s a l i n e a n o x i c  basins o f t h e Eastern M e d i t e r r a n e a n S e a , H2S r e a c h e s 3 a n d 2.2 m M r e s p e c t i v e l y in t h e B a n n o c k a n d Tyro b a s i n brines a n d y e t n o i n t e r m e d i a t e o x i d a t i o n s t a t e c o m p o u n d s of sulphur CSzOl", S O 3 , a n d Srf) a r e d e t e c t a b l e (Luther et a l . 1990b). The l a c k of t h e s e i n t e r m e d i a t e s is p r o b a b l y d u e t o t h e p a u c i t y of o x i d a n t s in t h e u p p e r f e w metres of t h e b r i n e s , i m m e d i a t e l y b e l o w t h e o x i c / a n o x i c i n t e r f a c e . In t h e B l a c k S e a , Luther et a l . (1990c) d i d d e t e c t l o w levels (< 60 n M ) of p a r t i c u l a t e S°, b u t w e r e u n a b l e t o d e t e c t a n y p o l y s u l p h i d e sulphur. J a c o b s et a l . (1985) also d i d not f i n d a n y z e r o v a l e n t sulphur in t h e a n o x i c w a t e r c o l u m n of F r a m v a r e n Fjord in southern N o r w a y . Polysulphides c a n b e g e n e r a t e d v i a t w o major p a t h w a y s : 1) t h e o x i d a t i o n of d i s s o l v e d s u l p h i d e a n d s u l p h i d e minerals; a n d 2) t h e r e a c t i o n of e l e m e n t a l sulphur with h y d r o g e n s u l p h i d e ( G i g g e n b a c h 1972). 1) E l e m e n t a l s u l p h u r dissolves in a q u e o u s s o d i u m s u l p h i d e s o l u t i o n a n d  combines  r e a d i l y w i t h s u l p h i d e t o f o r m p o l y s u l p h i d e ions, e . g . s i " , S3, S4 a n d Sf ( P e s c h a n s k i V a l e n s i 1949). P o l y m e r i z a t i o n d o e s n o t e x c e e d t h e Sf  and  s t a g e a n d e l e m e n t a l sulphur  r e m a i n s in e x c e s s , f o r m i n g a s a t u r a t e d p o l y s u l p h i d e solution m a d e u p primarily of St a n d sf", w h i l e S t a n d S? a r e e i t h e r t o o l o w in c o n c e n t r a t i o n or u n s t a b l e a n d a r e thus s u b j e c t e d t o r a p i d d i s p r o p o r t i o n a t i o n ( S c h w a r z e n b a c h a n d Fischer 1960), viz:  3S|" + 2H2O <-» 2HS" + 2 0 H " + S4 3S ; + hbO <-> HS" + 2 0 H " + 2S4. 2  The f o r m a t i o n of p o l y s u l p h i d e ions h a s a n a u t o c a t a l y t i c e f f e c t o n t h e r a t e of sulphur dissolution, s i n c e in t h e r e a c t i o n s e q u e n c e HS' + S <-> £  + H  +  Si" + S <r> S?  Sf  + S <-» s£  S  + S <-» s l  4  e a c h p r o d u c t is a l s o a r e a c t a n t for t h e next s t e p . Thus, a u t o c a t a l y s i s is t o b e e x p e c t e d if r e a c t i o n s after t h e first o n e a r e r a p i d ( C h e n a n d Morris 1972a). 2) A l o n g w i t h t h i o s u l p h a t e , p o l y s u l p h i d e a n d e l e m e n t a l sulphur a r e i m p o r t a n t p r o d u c t s of t h e i n c o m p l e t e o x i d a t i o n of H2S in n a t u r a l w a t e r s . They a r e f o r m e d f r o m t h e r e a c t i o n of s u l p h i d e w i t h z e r o v a l e n t sulphur f o r m e d f r o m s u l p h i d e o x i d a t i o n ( B o u l e g u e 1972; C h e n a n d Morris 1972a; G o u r m e l o n et a l . 1977; H o f f m a n n 1977), viz:  2HS" + O2 -> V4Se + 20H" HS" + (rvl)/8S8 ^ S " + H 2  +  St + 0 s + H2O -> S2O! + S^i +2H  +  2HS" + 2 O 2 - * S2O! + H2O. A s t h e c h a i n l e n g t h (n) i n c r e a s e s , p o l y s u l p h i d e s d e c o m p o s e t o e l e m e n t a l sulphur a n d sulphide ( G i g g e n b a c h  1972). These r e a c t i o n s p r o c e e d easily a n d r a p i d l y in n a t u r a l  w a t e r s , a n d , t h e r e f o r e , p o l y s u l p h i d e ions s h o u l d b e f o u n d in r e d u c i n g e n v i r o n m e n t s w h e r e t h e o x i d a t i o n o f H2S h a s b e e n i n c o m p l e t e . B o u l e g u e a n d Denis (1983) s h o w t h a t p o l y s u l p h i d e s c a n c o n s t i t u t e a l a r g e f r a c t i o n o f t h e t o t a l r e d u c e d sulphur s p e c i e s , a n d a r e s t a b l e e v e n a t v e r y l o w s u l p h i d e c o n c e n t r a t i o n s ( a b o u t lO^-lO^ M ) (Teder 1971; G i g g e n b a c h 1972; B o u l e g u e a n d M i c h a r d 1978). The c o n c e n t r a t i o n o f p o l y s u l p h i d e ions m a i n l y d e p e n d s o n t h e p h y s i c a l state o f e l e m e n t a r y sulphur (i.e. c o l l o i d a l vs. r h o m b i c ) p r o d u c e d d u r i n g t h e o x i d a t i o n p r o c e s s ( G o u r m e l o n e t a l . 1977). E l e m e n t a l sulphur is mostly f o u n d a s s t a b l e r h o m b i c sulphur (S ) 8a  a n d m e t a s t a b l e c o l l o i d a l sulphur (Ss ). The G i b b s free e n e r g y o f Sea is z e r o , while t h a t of c  Sac is larger t h a n 3.5 kJ»mof' ( B o u l e g u e 1976,1978). Thus, a c c o r d i n g t o t h e r e a c t i o n (n-l)/8S + HS' <-> Sh + H 8  +  t h e a m o u n t o f p o l y s u l p h i d e ions in t h e p r e s e n c e o f c o l l o i d a l e l e m e n t a l sulphur will b e a t least t w i c e t h e a m o u n t in t h e p r e s e n c e o f r h o m b i c sulphur, a n d this h a s b e e n f o u n d in t h e field ( B o u l e g u e 1977; B o u l e g u e e t a l . 1982). The o x i d a t i o n of H2S b y o x y g e n , b a c t e r i a , M n 0 2 a n d iron(lll) minerals is t h e m a i n s o u r c e of c o l l o i d a l e l e m e n t a l sulphur. The p e a k of p o l y s u l p h i d e c o n c e n t r a t i o n s fall within t h e p H r a n g e 6.6 - 7.4 a n d d e c r e a s e s drastically in b o t h a c i d i c a n d a l k a l i n e solutions ( C h e n a n d G u p t a 1973). U n d e r o n e a t m o s p h e r e pressure o f o x y g e n a n d s u l p h i d e c o n c e n t r a t i o n s o f 1 m M t o 10 m M , t h e c o r r e s p o n d i n g m a x i m u m y i e l d o f p o l y s u l p h i d e varies f r o m 14 t o 8% o f t h e t o t a l H2S. B e c a u s e t h e y a r e h i g h l y r e a c t i v e a n d t h e r m o d y n a m i c a l l y u n s t a b l e in n a t u r e , p o l y s u l p h i d e s a r e further o x i d i z e d t o f o r m m o r e s t a b l e p r o d u c t s like S 2 O ? . S O 4 a n d sulphur. H o w e v e r ,  Boulegue  a n d M i c h a r d (1978) f o u n d t h a t p o l y s u l p h i d e ions a r e s t a b l e p r o v i d e d n o o x i d a n t enters t h e system a n d f o u n d n o c h a n g e in s p e c i a t i o n in s t o r e d s a m p l e s o v e r t h e c o u r s e o f a y e a r . B o w e r s e t a l . (1966) s u g g e s t e d t h a t p o l y s u l p h i d e ions a r e m u c h m o r e s u s c e p t i b l e t o r e a c t i o n with o x y g e n t h a n H2S s p e c i e s .  High levels of polysulphides are  u s u a l l y f o u n d in o x i d i z i n g  environments.  B o u l e g u e et a l . (1979) m e a s u r e d sulphur s p e c i e s in t h e w a t e r s o f P u z z i c h e l l o ( F r a n c e ) , w h i c h a r e c h a r a c t e r i z e d b y a t m o s p h e r i c o x i d a t i o n of a n initially sulphide-rich w a t e r . In , t h e early s t a g e s of t h e o x i d a t i o n p r o c e s s , t h e y f o u n d m a i n l y p o l y s u l p h i d e  and  t h i o s u l p h a t e ; in t h e f i n a l s t a g e , s u l p h a t e w a s t h e m a i n p r o d u c t . B o u l e g u e (1977) a l s o f o u n d p o l y s u l p h i d e ions a n d t h i o s u l p h a t e in t h e g r o u n d w a t e r s o f a F r e n c h f r e s h w a t e r spring ( E n g h i e n ) , a n d B o u l e g u e a n d Denis (1983) n o t e d t h e o c c u r r e n c e of polysulphides a n d S ° in p o r e w a t e r s o f s e d i m e n t s f r o m t h e Walvis B a y , N a m i b i a . T h e y s u g g e s t t h e p r e s e n c e of p o l y s u l p h i d e in t h e s e p o r e w a t e r s m a y b e d u e t o a n i n p u t of S° resulting f r o m t h e a c t i v i t y o f s u l p h i d e - o x i d i z i n g b a c t e r i a in t h e s e d i m e n t s (Beggiatoa  were  present) a n d a g i n g of t h e p r o d u c e d c o l l o i d a l sulphur. Luther et a l . (1986b) f o u n d p o l y s u l p h i d e levels o f > 300 \M in t h e p o r e w a t e r s of G r e a t M a r s h , D e l a w a r e a n d Luther et a l . (1985) f o u n d t h a t m a r s h s a m p l e s consistently c o n t a i n h i g h e r levels o f S(0) t h a n s u b t i d a l p o r e w a t e r s . They s u g g e s t t h a t this d i f f e r e n c e results f r o m t h e t w o d i f f e r e n t m e c h a n i s m s o f p o l y s u l p h i d e f o r m a t i o n . In s u b t i d a l p o r e w a t e r s , p o l y s u l p h i d e s a r e f o r m e d f r o m t h e r e a c t i o n o f S° w i t h b i s u l p h i d e i o n ( G i g g e n b a c h 1972), w h e r e a s , in salt m a r s h p o r e w a t e r s , t h e y s h o u l d f o r m primarily f r o m t h e o x i d a t i o n o f h y d r o g e n s u l p h i d e ( C h e n a n d Morris 1 9 7 2 a , b ) . E x p o s u r e t o a t m o s p h e r e a n d infiltration b y t i d a l w a t e r a l l s e r v e t o d e l i v e r o x i d a n t s t o  the  marsh  s e d i m e n t . These o x i d a n t s c a n r e a c t w i t h s u l p h i d e s t o p r o d u c e v a r i o u s o x i d i z e d a n d partially o x i d i z e d sulphur c o m p o u n d s so t h a t p o l y s u l p h i d e s a r e r e a d i l y a v a i l a b l e in t h e u p p e r p o r t i o n s o f t h e s e d i m e n t s ( G i b l i n a n d H o w a r t h 1984). H o w e v e r , p o l y s u l p h i d e sulphur c o n c e n t r a t i o n s in G r e a t M a r s h w e r e p r e s e n t e v e n in t h e strongly r e d u c i n g z o n e (Luther e t a l . 1986b). In v i e w o f t h e s e n u m e r o u s p r e v i o u s s t u d i e s , it is difficult t o e x p l a i n w h y  the  p o l y s u l p h i d e l e v e l s in P o w e l l a n d S a k i n a w a r e s o h i g h . B o t h l a k e s r e p r e s e n t fairly s t a g n a n t b a s i n s , m u c h like t h e B l a c k S e a , t h e M e d i t e r r a n e a n b r i n e s , or F r a m v a r e n , w h e r e levels o f p o l y s u l p h i d e s a r e l o w d u e t o t h e l a c k of o x i d a n t s . Y e t , t h e e x t r e m e l y h i g h c o n c e n t r a t i o n s o f p o l y s u l p h i d e o b s e r v e d in t h e t w o lakes c o u l d only b e g e n e r a t e d b y o x i d a t i o n . There is n o r e a d i l y a p p a r e n t s o u r c e o f o x i d a n t in t h e b o t t o m w a t e r s of t h e s e l a k e s . To c o n f i r m t h e s e results, t h e r e f o r e , t h e first c o n s i d e r a t i o n must b e t o d e t e r m i n e w h e t h e r m e a s u r e d levels a r e a c c u r a t e , or r e f l e c t a r t e f a c t s i n t r o d u c e d  d u r i n g s a m p l i n g , o r d u r i n g t h e a n a l y t i c a l p r o c e d u r e , i.e.. d o t h e y m e r e l y reflect o x i d i z e d dissolved sulphide. There is c o n s i d e r a b l e e v i d e n c e t h a t t h e m e a s u r e d c o n c e n t r a t i o n s a r e c o r r e c t . 1) T h e r e w e r e n o p r o b l e m s w i t h o x i d a t i o n in a n y of t h e o t h e r a n a l y s e s  performed  t h r o u g h o u t this study. N o t h i o s u l p h a t e or sulphite w e r e e v e r d e t e c t e d in a n y s a m p l e s , b o t h b e i n g c o m m o n p r o d u c t s of s u l p h i d e o x i d a t i o n . Usually, o x i d a t i o n of s u l p h i d e generates  thiosulphate  together  with  polysulphide  and  n o t just  polysulphide.  U n f o r t u n a t e l y , b e c a u s e t h e i n d i r e c t m e t h o d for d e t e r m i n i n g p o l y s u l p h i d e  involves  c o n v e r s i o n t o t h i o s u l p h a t e , t h e p r e s e n c e of t h i o s u l p h a t e c o u l d not b e directly c h e c k e d on the polysulphide samples themselves. However,  a m p o u l e s w e r e flushed with  n i t r o g e n prior t o s a m p l e i n t r o d u c t i o n a n d t h o s e t h a t w e r e n o t u s e d for p o l y s u l p h i d e analysis s h o w e d n o e v i d e n c e of o x i d a t i o n . A l s o , t h e s a m p l e s f r o m t h e t w o lakes w e r e c o l l e c t e d in t w o d i f f e r e n t y e a r s . T h e r e f o r e , t o p r o d u c e t h e o b s e r v e d  polysulphide  c o n c e n t r a t i o n s , t h e s a m e d e g r e e of o x i d a t i o n w o u l d h a v e h a d t o o c c u r a t t w o different t i m e s , in t w o different p l a c e s . A l s o , a m u c h m o r e erratic p o l y s u l p h i d e profile w o u l d b e e x p e c t e d if o x i d a t i o n of s u l p h i d e w a s significant. I n s t e a d , t h e profiles a r e s m o o t h , with little s c a t t e r , a n d n o c l e a r l y a n o m a l o u s points. The p o l y s u l p h i d e profiles a l s o d o not c o m p l e t e l y m i m i c t h o s e of d i s s o l v e d s u l p h i d e . In t h e l o w e r m o s t d e p t h s of P o w e l l L a k e , p o l y s u l p h i d e c o n c e n t r a t i o n s d o n o t b e c o m e c o n s t a n t until t h e b o t t o m 10 m , w h e r e a s s u l p h i d e levels a r e c o n s t a n t for t h e b o t t o m 25 m. 2)  No  precipitate  was  observed  in t h e  ampoules.  Besides  polysulphide  t h i o s u l p h a t e . S ° is a c o m m o n p r o d u c t of s u l p h i d e o x i d a t i o n . In d u p l i c a t e  and  ampoules  w h i c h w e r e not o p e n e d , t h e r e w a s n o p r e c i p i t a t e u p t o t w o y e a r s after c o l l e c t i o n ; after f o u r y e a r s , s o m e h a d a p p e a r e d , i n d i c a t i n g t h a t s u l p h i d e w a s e v e n t u a l l y o x i d i z e d (or t h i o s u l p h a t e w a s d i s s o c i a t e d t o S° a n d S O f ) . 3) S u l p h i t e r e m a i n e d in t h e a m p o u l e s . A s m a l l e x c e s s of s u l p h i t e w a s a d d e d t o t h e a m p o u l e s a t t h e t i m e of s a m p l e c o l l e c t i o n t o r e a c t w i t h t h e z e r o v a l e n t sulphur. Sulphite is o x i d i z e d easily b y O2 ( o n t h e o r d e r of 100 m o l e ^ L ' ^ m i n " ) a n d h a s b e e n u s e d as a n 1  o x y g e n s c r u b b e r f o r this r e a s o n . O n a l l p o l a r o g r a p h runs, s u l p h i t e w a s  present,  i n d i c a t i n g t h a t it h a d n o t b e e n e x p o s e d t o o x i d a t i o n . H o w e v e r , o x i d a t i o n of sulphite is inhibited b y t h e p r e s e n c e of s u l p h i d e , t h e half-life a t 5 x 1 0 M S O f b e i n g i n c r e a s e d f r o m 5  a f e w m i n u t e s t o hours a t p H 8.4 in t h e p r e s e n c e o f 0.25 m M s u l p h i d e ( C h e n a n d Morris 1972a). Therefore a n y o x i d a n t present m i g h t h a v e b e e n e x h a u s t e d in oxidizing s u l p h i d e . 4) The b o t t o m w a t e r o f b o t h l a k e s h a s a distinct s t r a w c o l o u r , w h i c h i n d i c a t e s t h e p r e s e n c e of p o l y s u l p h i d e s (Morse e t a l . 1987). H o w e v e r , t h e c o l o u r c o u l d b e d u e t o t h e p r e s e n c e of h u m i c material, w h i c h also tends t o impart a yellow c o l o u r to w a t e r (Williams 1975). B o t h lakes h a v e v e r y h i g h levels of D O C ( C h a p t e r 3 ) , a n d so t h e latter c o n s i d e r a t i o n c a n n o t b e d i s c o u n t e d . The y e l l o w c o l o u r diminishes s o m e w h a t , a l t h o u g h n o t e n t i r e l y , u p o n o x i d a t i o n , i n d i c a t i n g t h a t a l a r g e p o r t i o n o f it is likely d u e  to  p o l y s u l p h i d e s , a l t h o u g h t h e r e m a i n i n g c o l o u r must b e d u e t o o r g a n i c m a t t e r . 5) The m o s t p e r s u a s i v e e v i d e n c e t h a t p o l y s u l p h i d e s o c c u r in h i g h c o n c e n t r a t i o n s is t h a t w h e n s a m p l e s w e r e run into bottles c o n t a i n i n g e i t h e r c o n c e n t r a t e d HCI, or H N 0 dissolved metals analyses), a white-coloured precipitate f o r m e d immediately  3  (for (the  p r e c i p i t a t e w a s slightly y e l l o w - c o l o u r e d in Powell). This w a s a t t r i b u t e d t o a c i d i f i c a t i o n of p o l y s u l p h i d e , w h i c h releases t h e s u l p h i d e a s H S, t h e r e b y f r e e i n g t h e z e r o v a l e n t sulphur 2  w h i c h t h e n precipitates. A n y dissolved sulphide present should also b e c o n v e r t e d to H S a n d q u i c k l y d e g a s , p a r t i c u l a r l y c o n s i d e r i n g t h a t , a t t h e p H of t h e w a t e r s s t u d i e d (< 2  6.8), m o s t o f t h e d i s s o l v e d s u l p h i d e must o c c u r a s H S. H o w e v e r , S h o l k o v i t z (1976) 2  s h o w e d t h a t a c i d i f i c a t i o n o f n a t u r a l o r g a n i c - r i c h w a t e r s ( l o w e r i n g t h e p H t o b e l o w 3), c a u s e s h u m i c m a t t e r t o p r e c i p i t a t e . To c h e c k this possible c o n t r i b u t i o n , t h e p r e c i p i t a t e w a s f i l t e r e d a n d a n a l y z e d o n a C N S a n a l y z e r a n d f o u n d t o b e 1 0 0 % sulphur in t h e S a k i n a w b o t t o m w a t e r a n d 9 2 % sulphur ( 8 % c a r b o n ) in P o w e l l L a k e . In s u m m a r y , it w o u l d a p p e a r t h a t t h e r e is a l a r g e store of z e r o v a l e n t sulphur p r e s e n t in b o t h lakes. It s h o u l d b e n o t e d t h a t s o m e o f this sulphur m a y b e c o l l o i d a l S° r a t h e r t h a n p o l y s u l p h i d e sulphur, a s t h e f r a c t i o n is o p e r a t i o n a l l y d e f i n e d as t h a t w h i c h p a s s e s t h r o u g h a 0.4 p.m N u c l e p o r e filter. Usually, e l e m e n t a l s u l p h u r co-exists w i t h p o l y s u l p h i d e s ( B o u l e g u e a n d M i c h a r d 1978). S p e c i a t i o n c a l c u l a t i o n s w i t h M I N E Q L ( A p p e n d i x 1) i n d i c a t e d t h a t a l m o s t all t h e p o l y s u l p h i d e in b o t h lakes s h o u l d b e present as  St. A s n o t e d earlier, t h e r e must b e , or h a v e b e e n , s o m e o x i d a n t a v a i l a b l e w h i c h  c o u l d h a v e c a u s e d t h e g e n e r a t i o n of t h e z e r o v a l e n t sulphur. O x i d a n t s m a y h a v e b e e n i n t r o d u c e d t o S a k i n a w L a k e b y o c c a s i o n a l intrusions o f s e a w a t e r o v e r t h e sill. S u c h inputs t o d e e p w a t e r s h a v e likely o c c u r r e d s e v e r a l times s i n c e S a k i n a w w a s s e p a r a t e d  f r o m t h e Strait of G e o r g i a . D u e t o t h e h i g h levels of s u l p h i d e , t h e o x i d a n t s m a y h a v e b e e n e x h a u s t e d in o x i d i z i n g t h e s u l p h i d e t o s u l p h u r a s w e l l a s t h i o s u l p h a t e ,  (and  possibly t o s u l p h a t e ) . B e c a u s e t h e r e is a s h o r t a g e of e l e c t r o n a c c e p t o r s in t h e b o t t o m w a t e r s , a n y t h i o s u l p h a t e s o g e n e r a t e d w o u l d likely h a v e b e e n r a p i d l y r e m o v e d s u l p h a t e - r e d u c i n g b a c t e r i a , m o s t of w h i c h a r e c a p a b l e o f r e d u c i n g sulphite  by and  t h i o s u l p h a t e (Truper 1982). In P o w e l l L a k e , h o w e v e r , it is m u c h m o r e difficult t o establish a n o x i d a n t s o u r c e . It is w e l l k n o w n t h a t b o t h iron a n d m a n g a n e s e o x i d e s o x i d i z e s u l p h i d e r a p i d l y . I n d e e d , L a n d i n g a n d Lewis (1990) s h o w e d e x p e r i m e n t a l l y t h a t m i c r o m o l a r s u l p h i d e levels almost i n s t a n t a n e o u s l y r e d u c e s o l u b l e Fe(lll). Thus t h e y b e l i e v e t h e r e is little s o l u b l e Fe(lll) p r e s e n t b e l o w 110 m (the s u l p h i d e i n t e r f a c e ) in t h e B l a c k S e a . B u r d i g e a n d N e a l s o n (1986) f o u n d c h e m i c a l r e d u c t i o n of Mn(IV) b y s u l p h i d e t o b e r a p i d a n d c o m p l e t e , w i t h all s u l p h i d e b e i n g o x i d i z e d p r e d o m i n a n t l y t o S° within 5-10 m i n . B e c a u s e t h e s e m e t a l o x i d e s a r e r e d u c e d so r a p i d l y in t h e p r e s e n c e of s u l p h i d e , h o w e v e r , t h e y s h o u l d all b e r e d u c e d just b e l o w t h e  oxic/anoxic interface  in P o w e l l  L a k e . This  observation  essentially rules o u t t h e p a r t i c i p a t i o n of Fe a n d M n o x i d e s a s o x i d a n t s a t d e p t h in t h e lake. In b o t h lakes, t h e ratio of s u l p h i d e t o sulphur d e c r e a s e s w i t h d e p t h (Figs. 5-7 a n d 5-8). This is u n e x p e c t e d , g i v e n t h a t t h e s o u r c e of most o x i d a n t s is a t t h e i n t e r f a c e . The lowest S(-2):S(0) v a l u e s w o u l d t h e r e f o r e b e e x p e c t e d t o o c c u r just b e l o w t h e i n t e r f a c e . A l s o , t h e l o w e r ratio in S a k i n a w L a k e i n d i c a t e s t h a t a g r e a t e r p r o p o r t i o n of t h e s u l p h i d e p o o l t h e r e consists of p o l y s u l p h i d e s . This is c o n s i s t e n t w i t h t h e h y p o t h e s i z e d s p o r a d i c injection of o x i c w a t e r into S a k i n a w d e e p waters. Luther et a l . (1990c) h a d similar difficulty in f i n d i n g a n o x i d a n t t o e x p l a i n a subi n t e r f a c e z e r o v a l e n t sulphur m a x i m u m in t h e B l a c k S e a . The l o w levels of S(0) a n d t h e inability t o d e t e c t sulphur o x y a n i o n s (typically < 200 n M ) a r e consistent w i t h t h e a p p a r e n t l a c k of t y p i c a l c h e m i c a l o x i d a n t s (O2.  Fe(lll), M n ( I V ) ) in t h e a n o x i c / n o n s u l p h i d i c  transition z o n e . D e s p i t e this, Luther et a l . (1990c) f o u n d t h a t s u l p h i d e is o x i d i z e d in situ a t d e p t h in t h e B l a c k S e a a n d s u g g e s t s t h a t t h e o x i d a n t m a y b e CO2 ( J a r g e n s e n et a l . 1990b, c i t e d in Luther et a l . 1990c; R e p e t a et a l . 1989) or o x i d i z e d o r g a n i c matter. M a n g a n e s e c o m p o u n d s m a y b e r e s p o n s i b l e for t h e a b i o t i c , a n o x i c s u l p h i d e o x i d a t i o n o b s e r v e d in t h e B l a c k S e a a n d Eastern M e d i t e r r a n e a n S e a brines b y Luther et  a l . ( 1 9 9 0 b , c ) . These a u t h o r s h y p o t h e s i z e t h a t s o l u b l e c o m p o u n d s or c o m p l e x e s s u c h as M n ( l l l ) - o r g a n i c c o m p l e x e s or M n ( l l ) - o r g a n i c c o m p l e x e s w i t h u n s a t u r a t e d  organic  m a t t e r ( e . g . c a r b o x y l i c a c i d s , olefins, e t c . ) , m a y a c t as t h e e l e c t r o n a c c e p t o r s for s u l p h i d e o x i d a t i o n . L i g a n d - c e n t r e d o x i d a t i o n o f M n ( l l ) t o M n ( l l l ) in  manganese  c o m p l e x e s c o n t a i n i n g c a r b o x y l a t e l i g a n d s h a s b e e n d e m o n s t r a t e d b y Richert et a l . (1988), a n d in this m e c h a n i s m , a n e l e c t r o n f r o m Mn(ll) is a c c e p t e d b y a c a r b o x y l i c a c i d l i g a n d . Luther et a l . (1990c) s u g g e s t t h a t Mn(lll) p r o d u c e d in this m a n n e r c a n r e a c t with a n d o x i d i z e s u l p h i d e . O x i d a t i o n o f Mn(ll) w o u l d n o t b e o b s e r v e d in situ  because  sulphide w o u l d r e d u c e t h e oxidized m a n g a n e s e readily. Overall t h e process should l e a d t o t h e r e d u c t i o n of u n s a t u r a t e d (oxidized) o r g a n i c m a t t e r (R) a n d t h e n e t o x i d a t i o n of s u l p h i d e w i t h Mn(ll) as t h e c a t a l y s t , viz: (R)-Mn"-S -»(R)"-Mn'"-S -+ (R)"-Mn"-S -> p r o d u c t s ( r e d u c e d o r g a n i c m a t t e r , SO ") + Mn(ll) +  2  M a n y different o r g a n i c e l e c t r o n a c c e p t o r s w o u l d b e a b l e t o c a u s e t h e net o x i d a t i o n of s u l p h i d e b e c a u s e o f t h e c a t a l y t i c e f f e c t o f Mn(ll). This p r o c e s s is a n a l o g o u s t o t h e t r a c e - m e t a l - c a t a l y z e d o x i d a t i o n of s u l p h i d e b y o x y g e n (Luther 1990a). There is a l a r g e q u a n t i t y of o r g a n i c m a t t e r in t h e b o t t o m w a t e r o f b o t h P o w e l l a n d S a k i n a w L a k e s , s o m e of w h i c h m a y a c t a s a s o u r c e o f o x i d a n t s . The p r o d u c t i o n of sulphur i n t e r m e d i a t e s in b o t t o m w a t e r s u n d e r t h e s e a n o x i c c o n d i t i o n s s h o u l d l e a d t o their r a p i d u p t a k e or d i s p r o p o r t i o n a t i o n b y o r g a n i s m s w h i c h require t h e m as a s o u r c e of e n e r g y ( J a r g e n s e n e t a l . 1990b). In s u m m a r y , t h e o n l y s o u r c e s o f o x i d a n t t h a t  could generate  the  high  p o l y s u l p h i d e c o n c e n t r a t i o n s o b s e r v e d a t d e p t h in P o w e l l L a k e a r e o r g a n i c c o m p l e x e s s u c h a s t h e Mn(ll)- a n d Mn(lll)-organic c o m p l e x e s p r o p o s e d b y Luther et a l . (1990c). B e c a u s e o f t h e h i g h c o n c e n t r a t i o n s of o r g a n i c m a t t e r a n d m a n g a n e s e in P o w e l l L a k e b o t t o m w a t e r s , a n d t h e l o n g p e r i o d of t i m e t h a t t h e w a t e r has b e e n a n o x i c , it is possible t h a t t h e s e c o m p o u n d s m i g h t h a v e g e n e r a t e d sufficient o x i d a n t t o c a u s e t h e h i g h l e v e l of p o l y s u l p h i d e f o u n d a t d e p t h in this l a k e . In S a k i n a w L a k e , o c c a s i o n a l incursions of s e a w a t e r o v e r t h e sill w o u l d a d d c o n s i d e r a b l y t o t h e o x i d a n t p o o l .  Iron and Manganese The s u b - i n t e r f a c e d i s s o l v e d iron m a x i m a in P o w e l l a n d S a k i n a w L a k e s (Figs. 5-9 a n d 5-10) arise f r o m t h e r e d u c t i v e dissolution of m e t a l o x i d e s settling f r o m t h e overlying  o x i c w a t e r , f o l l o w e d b y p r e c i p i t a t i o n or a d s o r p t i o n o n t o solid p h a s e s a t d e p t h ( J a c o b s et a l . 1987). These profiles a r e t y p i c a l of transition m e t a l s w h i c h exhibit r e d o x c y c l i n g of m e t a l o x i d e p a r t i c l e s a n d d i s s o l v e d m e t a l s p e c i e s a t t h e O2/H2S i n t e r f a c e , a n d m e t a l s u l p h i d e f o r m a t i o n ( J a c o b s et a l . 1985). In o x i c w a t e r s , iron a n d m a n g a n e s e a r e p r e s e n t primarily a s t h e e x t r e m e l y insoluble Fe(lll) a n d Mn(IV) a n d Mn(lll) o x y h y d r o x i d e s . These p a r t i c l e s a r e r e d u c e d w h e n t h e y e n c o u n t e r a n o x i c w a t e r s in m e r o m i c t i c lakes or s e a s , a n d s o p r o v i d e a s o u r c e of s o l u b l e m e t a l c a t i o n s w h i c h a r e t r a n s p o r t e d a t c o m p a r a b l e r a t e s b o t h u p w a r d t o b e o x i d i z e d a n d d o w n w a r d t o w a r d s d e e p e r w a t e r s . This solubilization at or n e a r the c h e m o c l i n e a n d the two-way  transport causes  the  c h a r a c t e r i s t i c m a x i m u m in t h e c o n c e n t r a t i o n - d e p t h profiles of t h e s o l u b l e s p e c i e s . D i s s o l v e d Fe(ll) a n d  Mn(ll)  in t h e s u b - o x i c z o n e d i f f u s e u p w a r d  along a  steep  c o n c e n t r a t i o n g r a d i e n t a n d a r e o x i d i z e d t o Mn(IV) a n d Fe(lll) at or a b o v e t h e i n t e r f a c e . Highly insoluble p h a s e s , s u c h a s 8 M n 0 2 a n d F e ( O H ) a r e f o r m e d w h i c h resettle t o w a r d 3  t h e a n o x i c z o n e . As iron is r e d u c e d , h o w e v e r , it is p r e c i p i t a t e d as iron s u l p h i d e p h a s e s in t h e p r e s e n c e o f H S a n d is thus r e m o v e d f r o m c y c l i n g a t t h e i n t e r f a c e . D u e t o t h e h i g h 2  solubility o f t h e l e a s t s o l u b l e Mn(ll) p r e c i p i t a t e (MnS2), M n  2 +  is rarely r e m o v e d v i a  precipitation. C o m p a r i s o n of t h e m a n g a n e s e a n d iron profiles reveals t h a t t h e l a r g e i n c r e a s e in iron c o n c e n t r a t i o n d e v e l o p s b e t w e e n 150 a n d 200 m in t h e w a t e r c o l u m n in P o w e l l L a k e , w h i l e t h a t of m a n g a n e s e b e g i n s earlier a t 125 m (Figs. 5-9 a n d 5-10). In S a k i n a w L a k e , t h e t w o s t e e p g r a d i e n t s o v e r l a p d u e t o t h e m u c h s h a r p e r i n t e r f a c e . The d i f f e r e n c e in t h e b e h a v i o u r o f t h e s e t w o m e t a l s reflects their different r e d o x chemistries, a n d is c o n s i s t e n t w i t h t h e f a c t t h a t m a n g a n e s e r e d u c t i o n o c c u r s a t s o m e w h a t h i g h e r ( m o r e oxidizing) p o t e n t i a l s t h a n d o e s t h e r e d u c t i o n o f Fe (Stumm a n d M o r g a n 1981). S e v e r a l studies h a v e i n d i c a t e d t h a t Fe(lll) a n d Mn(lll) r e d u c t i o n in n a t u r a l w a t e r s a n d s e d i m e n t s is b i o l o g i c a l l y - m e d i a t e d ( K a m u r a et a l . 1963; S e r e n s e n 1982; J o n e s et a l . 1983; B u r d i g e a n d  Nealson  1985; M y e r s a n d  Nealson  1988). G e n e r a l l y , t h e  net  a c c u m u l a t i o n o f Fe(ll) in w a t e r a n d s e d i m e n t is not o b s e r v e d until a f t e r t h e r e m o v a l of d i s s o l v e d o x y g e n a n d nitrate a n d t h e a c c u m u l a t i o n o f Mn(ll) ( P o n n a m p e r u m a 1972; Y o s h i d a 1975), a n d this h a s b e e n e x p l a i n e d a s a p r e f e r e n t i a l r e d u c t i o n o f e l e c t r o n a c c e p t o r s in o r d e r o f d e c r e a s i n g e n e r g y y i e l d . Thus, Mn(IV) r e d u c t i o n p r e c e d e s Fe(lll) r e d u c t i o n (see T a b l e 3-1). The latter p h e n o m e n o n h a s a l s o b e e n e x p l a i n e d a s resulting  f r o m c o m p e t i t i v e e x c l u s i o n o f Fe(lll)-reducing b a c t e r i a b y o t h e r o r g a n i s m s ( H a m m a n n a n d O t t o w 1974; P o n n a m p e r u m a 1972; Y o s h i d a 1975). Lovley a n d Phillips (1988) f o u n d that dissimilatory Fe(lll)-reducing o r g a n i s m s c a n r e d u c e Fe(lll) in t h e p r e s e n c e o f M n ( I V ) , a n d t h e y s u g g e s t t h a t t h e d o m i n a n t f a c t o r p r e v e n t i n g t h e a c c u m u l a t i o n of Fe(ll) in s e d i m e n t s t h a t c o n t a i n m i c r o b i a l l y r e d u c i b l e Mn(IV) is t h e n o n e n z y m a t i c o x i d a t i o n of Fe(ll) b y Mn(IV). A l t h o u g h m u c h o f t h e Fe(lll) a n d Mn(IV) r e d u c t i o n in n a t u r a l e n v i r o n m e n t s a p p e a r s t o b e microbially c a t a l y z e d , these elements c a n also b e r e d u c e d abiotically b y c o m p o u n d s s u c h a s s u l p h i d e (Ehrtich 1981; B u r d i g e a n d N e a l s o n 1986; L a n d i n g a n d Lewis 1990). V a r i o u s o r g a n i c c o m p o u n d s a r e a l s o c a p a b l e o f c h e m i c a l l y r e d u c i n g Fe(lll) a n d Mn(IV) ( M o r g a n a n d S t u m m 1964; S t o n e a n d M o r g a n 1984a, b). M i c r o b e s c a n c a t a l y z e iron a n d  manganese  reduction by excreting r e d u c e d metabolic  end  p r o d u c t s s u c h as sulphide a n d o r g a n i c c o m p o u n d s , w h i c h t h e n r e a c t abiotically with m e t a l o x i d e s ( B u r d i g e a n d N e a l s o n 1985). Iron a n d m a n g a n e s e o x i d a t i o n a r e a l s o t h o u g h t t o b e primarily b i o l o g i c a l l y m e d i a t e d - p r o c e s s e s . S e v e r a l studies suggest that  b i o l o g i c a l catalysis must  be  o c c u r r i n g ( E m e r s o n et a l . 1979, 1982; Wollast et al.1979; T e b o e t a l . 1984)) b e c a u s e t h e rates a r e t o o fast t o b e  e x p l a i n e d by purely inorganic mechanisms.  However,  a d s o r p t i o n a n d a u t o c a t a l y t i c o x i d a t i o n o f m a n g a n e s e o c c u r s ( M o r g a n 1967; Wilson 1980; Kessick a n d M o r g a n 1975; Sung a n d M o r g a n 1981). The d e p l e t i o n o f iron a t d e p t h in b o t h l a k e s (Figs. 5-9 a n d 5-10) is d u e t o p r e c i p i t a t i o n o f iron s u l p h i d e s . The Fe(ll) g e n e r a t e d in t h e a n o x i c w a t e r s r e a c t s w i t h s u l p h i d e , or m o r e likely w i t h t h e highly r e a c t i v e p o l y s u l p h i d e , t o f o r m either FeS or FeS . 2  This c a n b e s e e n b y c o m p a r i n g t h e iron a n d s u l p h i d e profiles in b o t h l a k e s : a s iron c o n c e n t r a t i o n s d e c r e a s e s u l p h i d e i n c r e a s e s (Figs. 5-5, 5-6 versus 5-9,5-10). The f a c t t h a t iron is a l m o s t entirely d e p l e t e d a t d e p t h i n d i c a t e s t h a t iron s u l p h i d e f o r m a t i o n is ironlimited in b o t h o f t h e s e lakes. The u l t i m a t e s o u r c e o f iron a n d m a n g a n e s e t o t h e lakes is s u r f a c e run off, a n d d i s s o l v e d iron a n d m a n g a n e s e levels a r e m u c h h i g h e r in t h e a n o x i c w a t e r s of P o w e l l t h a n in S a k i n a w L a k e . The h e a d w a t e r s of  b o t h lakes drain areas  d o m i n a n t l y u n d e r l a i n b y h o r n b l e n d e g r a n o d i o r i t e ( M a t h e w s 1962; N o r t h c o t e  and  J o h n s o n 1964); input o f iron a n d m a n g a n e s e s h o u l d b e similar f o r t h e t w o lakes. R u b y L a k e is u p s t r e a m o f S a k i n a w , a n d thus m a y t r a p iron b i o l o g i c a l l y b e f o r e it r e a c h e s  S a k i n a w L a k e . H o w e v e r , t h e situation f o r P o w e l l L a k e is similar as w a t e r f r o m P o w e l l River travels t h r o u g h s e v e r a l o t h e r basins b e f o r e r e a c h i n g t h e s o u t h e r n m o s t b a s i n . The higher c o n c e n t r a t i o n s of iron in P o w e l l L a k e most likely reflect t h e e x t r e m e l y diffuse i n t e r f a c e as c o m p a r e d t o t h e s h a r p i n t e r f a c e in S a k i n a w L a k e . In S a k i n a w , t h e z o n e s of iron a n d s u l p h a t e r e d u c t i o n v e r y n e a r l y o v e r l a p : t h e r e f o r e a s iron is r e d u c e d , it is i m m e d i a t e l y p r e c i p i t a t e d a s iron s u l p h i d e s a n d r e m o v e d f r o m t h e system. This results in a v e r y s h a r p p e a k in iron c o n c e n t r a t i o n just b e l o w t h e i n t e r f a c e (Fig. 5-9). In P o w e l l L a k e , t h e o x i c / a n o x i c i n t e r f a c e a n d t h e c h e m o c l i n e a r e s e p a r a t e d b y o v e r 100 m.  Hence,  s u l p h i d e c o n c e n t r a t i o n s a r e l o w in t h e z o n e of iron r e d u c t i o n a n d iron c a n  cycle  b e t w e e n o x i d i z e d a n d r e d u c e d forms, a n d thus r e a c h h i g h e r c o n c e n t r a t i o n s . Metal Complexes The c o m p u t e d s p e c i a t i o n of iron a n d m a n g a n e s e in t h e t w o lakes (Figs. 5-11 a n d 5-12) is similar t o t h a t c a l c u l a t e d for t h e B l a c k S e a b y L a n d i n g a n d Lewis (1990). S o l u b l e Fe(ll)-sulphide c o m p l e x e s w e r e o m i t t e d f r o m t h e c a l c u l a t i o n s s i n c e e v i d e n c e  from  F r a m v a r e n Fjord suggests t h a t t h e s e c o m p l e x e s , as w e l l as Fe(ll) c o m p l e x e s w i t h thiols, D O C , or p o l y s u l p h i d e s , a r e not i m p o r t a n t in s t a b l e , s u l p h i d i c m a r i n e basins ( L a n d i n g a n d Westerlund  1988). The d e c r e a s e in d i s s o l v e d iron c o n c e n t r a t i o n w i t h i n c r e a s i n g t o t a l  s u l p h i d e c o n c e n t r a t i o n is c h a r a c t e r i s t i c of a transition m e t a l t h a t d o e s n o t c o m p l e x significantly w i t h r e d u c e d s u l p h i d e ( E m e r s o n et a l . 1983). This results in a c o n c e n t r a t i o n m a x i m u m of t h e m e t a l a t l o w s u l p h i d e levels. The results for t h e B l a c k S e a h a v e b e e n corroborated  b y Luther et al. (1990c), w h o  found that  b o t h "free" a n d  metal  " c o m p l e x e d " forms of s u l p h i d e exist in t h e w a t e r c o l u m n . Thus, s u l p h i d e in t h e u p p e r o x i c w a t e r c o l u m n (0-100 m) of t h e B l a c k S e a is " c o m p l e x e d " b y m e t a l s , w h e r e a s t h e s u l p h i d e b e l o w 100 m is p r e d o m i n a n t l y "free" (H S, HS"). B a s e d o n e x p e r i m e n t a l d a t a 2  a n d t h e r m o d y n a m i c c o n s i d e r a t i o n s (Dyrssen 1988), Luther e t a l . (1990c) s u g g e s t t h a t Mn  2 +  s h o u l d b e t h e m e t a l w h i c h h a s t h e best o p p o r t u n i t y t o c o m p l e x s u l p h i d e , a n d  t h e r e f o r e , Mn(ll)-sulphide c o m p l e x e s w e r e i n c l u d e d in s p e c i a t i o n c a l c u l a t i o n s . The e q u i l i b r i u m t r a c e m e t a l m o d e l l i n g d i s c u s s e d a b o v e i n c l u d e d o n l y  the  e f f e c t s of i n o r g a n i c l i g a n d s , b u t m e t a l ions c a n a l s o b e c o m p l e x e d b y o r g a n i c l i g a n d s . s u c h a s h u m i c s u b s t a n c e s or thiols. L a n d i n g a n d Lewis (1990) f o u n d t h a t in a m o d e l s y s t e m , Mn(ll) a n d Fe(ll) a r e c o m p l e x e d w i t h nitrilotriacetic a c i d (NTA), a n a n a l o g u e for m a r i n e h u m i c s u b s t a n c e s . T h e y o b t a i n e d similar results using c y s t e i n e ( a thiol) as t h e  m o d e l l i g a n d ; b u t in this c a s e , o n l y Fe w o u l d b e likely t o c o m p l e x w i t h c y s t e i n e a t naturally o c c u r r i n g levels. A cursory a t t e m p t w a s m a d e t o m e a s u r e thiols in P o w e l l L a k e , a n d n o n e w e r e d e t e c t e d , a l t h o u g h t h e t e c h n i q u e u s e d w a s n o t v e r y sensitive ( m i n i m u m d e t e c t i o n limit -0.5 n M ) . Total t h i o l c o n c e n t r a t i o n s o f s e v e r a l h u n d r e d n M h a v e b e e n r e p o r t e d in t h e a n o x i c w a t e r s o f t h e B l a c k S e a ( M o p p e r a n d K i e b e r 1988; Luther e t a l . 1990c), s u g g e s t i n g t h a t only Fe(ll)-thiol c o m p l e x a t i o n m i g h t b e e x p e c t e d t o b e significant. L a n d i n g a n d Lewis (1990) d e t e c t e d s o m e a n i o n i c Fe(ll) s p e c i e s (10 - 3 0 % ) ; h o w e v e r , t h e y suggest that s u c h c o m p l e x e s a r e kinetically very labile. Therefore, c o m p l e x a t i o n b y o r g a n i c c o m p o u n d s w a s n o t i n c l u d e d in t h e c a l c u l a t i o n s d u e t o l a c k of t h e r m o d y n a m i c a n d f i e l d d a t a for t h o s e p o t e n t i a l ligands.  Particulate Sulphides B o t h pyrite a n d iron m o n o s u l p h i d e s a r e p r e s e n t in t h e w a t e r c o l u m n of P o w e l l a n d S a k i n a w Lakes (Figs. 5-15 a n d 5-16). X-ray d i f f r a c t o m e t r y of t h e s e d i m e n t s in P o w e l l s h o w e d t h a t pyrite is t h e d o m i n a n t c r y s t a l l i n e p h a s e in t h e l a r g e l y o r g a n i c o o z e , q u a n t i t a t i v e l y e x c e e d i n g e v e n d e t r i t a l silicates (T. P e d e r s e n pers. c o m m . ) . In b o t h P o w e l l a n d S a k i n a w L a k e s , pyrite w a s d e t e c t e d a t a h i g h e r l e v e l in t h e w a t e r c o l u m n t h a n t h e d e p t h of first d e t e c t i o n of iron m o n o s u l p h i d e . In P o w e l l , m o n o s u l p h i d e s a r e first d e t e c t a b l e a t 2 0 0 m d e p t h . 5 0 m b e l o w t h e h o r i z o n w h e r e pyrite is a n a l y t i c a l l y first o b s e r v e d (i.e. right a t t h e i n t e r f a c e ) . In S a k i n a w L a k e , h o w e v e r , m o n o s u l p h i d e s o c c u r a t t h e next s a m p l i n g d e p t h b e l o w t h e l e v e l o f first a p p e a r a n c e o f pyrite. The a p p e a r a n c e of  monosulphides  corresponds  well  with  the increase  in d i s s o l v e d  sulphide  c o n c e n t r a t i o n . It w o u l d a p p e a r t h a t in t h e u p p e r horizons of P o w e l l a n d S a k i n a w L a k e s , pyrite o u t c o m p e t e s m o n o s u l p h i d e s for iron w h e n H S c o n c e n t r a t i o n s a r e low. O n c e t h e 2  H2S levels i n c r e a s e (to ~100's o f n M in this c a s e ) , m o n o s u l p h i d e c a n t h e n p r e c i p i t a t e . In Sakinaw Lake, sulphide concentrations increase with d e p t h m u c h more quickly d u e t o t h e s h a r p e r i n t e r f a c e , a n d thus m o n o s u l p h i d e s a p p e a r a t s h a l l o w e r d e p t h s relative t o pyrite. P r e s u m a b l y t h e m o n o s u l p h i d e f o r m e d c a n t h e n r e a c t w i t h p o l y s u l p h i d e o r sulphur t o f o r m pyrite. This f o r m a t i o n o f pyrite w i t h o u t a p p a r e n t m o n o s u l p h i d e p r e c u r s o r s h a s a l s o b e e n o b s e r v e d in v a r i o u s s e d i m e n t s , e s p e c i a l l y in salt m a r s h e s ( B o u l e g u e e t a l . 1982; C u t t e r a n d Velinsky 1988; D a v i s o n e t a l . 1985; H o w a r t h 1979; H o w a r t h a n d G i b l i n 1983;  H o w a r t h a n d M a r i n o 1984; H o w a r t h a n d M e r k e l 1984; King 1988; Lord a n d C h u r c h 1983; L u t h e r e t a l . 1982; Skyring a n d L u p t o n 1984). H o w a r t h a n d T e a l (1979) f o u n d , v i a r a d i o t r a c e r s t u d i e s , t h a t pyrite f o r m s v e r y r a p i d l y (hours t o d a y s ) in m a r s h s e d i m e n t s , s u g g e s t i n g d i r e c t r e a c t i o n b e t w e e n iron a n d p o l y s u l p h i d e s rather t h a n f o r m a t i o n b y t h e s l o w m e c h a n i s m s o b s e r v e d in o t h e r situations. Salt m a r s h e s a r e c h a r a c t e r i z e d b y a l a r g e s u p p l y of o x i d a n t s r e q u i r e d for pyrite f o r m a t i o n d u e t o a ) t h e d i r e c t diffusion of o x y g e n into t h e a n o x i c s e d i m e n t s w h e n t h e d e p o s i t s a r e e x p o s e d d u r i n g l o w t i d e ( D a c e y a n d H o w e s 1984); b ) infiltration of o x y g e n a t e d t i d a l w a t e r ; a n d c ) 0  2  release from  t h e marsh-grass r h i z o s p h e r e , resulting in t h e f o r m a t i o n of p o l y s u l p h i d e s a n d  elemental  sulphur. Iron c a n t h e n directly r e a c t w i t h p o l y s u l p h i d e s t o f o r m pyrite w i t h o u t t h e n e e d for FeS i n t e r m e d i a t e s , viz: Fe  2+  + S* + HS" <-» FeS2 +  + H. +  M a r s h p o r e w a t e r s c a n c o n t a i n p o l y s u l p h i d e s in c o n c e n t r a t i o n s f a r a b o v e e x p e c t e d e q u i l i b r i u m v a l u e s , a s o b s e r v e d in P o w e l l a n d S a k i n a w L a k e s , i m p l y i n g p r o d u c t i o n v i a s u l p h i d e o x i d a t i o n as d i s c u s s e d earlier ( B o u l e g u e et a l 1982; Luther et a l . 1985). The o c c u r r e n c e of s u s p e n d e d pyrite in t h e a b s e n c e of m o n o s u l p h i d e s in t h e l a k e s c o n t r a s t s w i t h most m a r i n e s e d i m e n t s w h e r e s o l u b l e s u l p h i d e s (H S, HS") a n d iron 2  m o n o s u l p h i d e (FeS) a r e t h e m a j o r short-term e n d p r o d u c t s of s u l p h a t e r e d u c t i o n , a n d w h e r e pyrite forms slowly t h r o u g h t h e s l o w r e a c t i o n of FeS w i t h S° (Berner 1970). As a n e x a m p l e , f r o m 80 t o 1 0 0 % of t h e S O * recoverable  r e d u c e d during one-day incubations  a s m o n o s u l p h i d e s in b l a c k , h i g h l y r e d u c i n g m u d s f r o m W o o d s  was Hole  h a r b o u r ( H o w a r t h 1979) a n d f r o m t h e P e r u u p w e l l i n g system ( R o w e a n d H o w a r t h 1985). Similar results w e r e o b t a i n e d f o r L o n g Island S o u n d ( W e s t r i c h a n d B e r n e r 1988), t w o D a n i s h fjords (Kysing Fjord a n d Limfjorden) ( H o w a r t h a n d J 0 r g e n s e n 1984), shallow-water c a r b o n a t e s e d i m e n t s in A u s t r a l i a ( F e r g u s o n et a l . 1983) a n d s e d i m e n t s f r o m t h e N o r t h Sea-Baltic transition ( J a r g e n s e n et a l . 1990a). A l t h o u g h pyrite m a k e s u p t h e bulk of t h e s u l p h u r in t h e s e s e d i m e n t s , FeS a n d S° usually d e c r e a s e  with d e p t h , while  pyrite  i n c r e a s e s , s u g g e s t i n g t h a t F e S forms f r o m t h e s l o w c o n v e r s i o n of iron m o n o s u l p h i d e s 2  ( H o w a r t h a n d J a r g e n s e n 1984). G e n e r a l l y , o x y g e n c a n  enter subtidal a n d  lake  s e d i m e n t s o n l y b y diffusion a n d b y t h e a c t i o n s of b e n t h i c a n i m a l s (Aller 1978) a n d thus t h e o x i d a t i o n of s u l p h i d e s t o f o r m p o l y s u l p h i d e s a n d e l e m e n t a l sulphur m a y b e primarily  limited t o s l o w r e a c t i o n s w i t h detrital iron o x i d e minerals. The limited availability of S° a n d p o l y s u l p h i d e s results in s l o w f o r m a t i o n of pyrite. The inability t o d e t e c t m o n o s u l p h i d e a t t h e s h a l l o w e r d e p t h s in t h e P o w e l l a n d S a k i n a w w a t e r c o l u m n s w h e r e pyrite o c c u r s d o e s not necessarily m e a n t h a t t h e m i n e r a l is n o t f o r m i n g a t t h e s e h i g h e r levels; r a p i d c o n v e r s i o n t o pyrite c o u l d r e m o v e it as quickly as  it is p r o d u c e d , t h u s m a s k i n g its f o r m a t i o n . H o w e v e r ,  conversion  of  m o n o s u l p h i d e s t o pyrite is t h o u g h t t o b e a s l o w p r o c e s s (Berner 1970) a n d t h e r e f o r e , if m o n o s u l p h i d e p r e c i p i t a t i o n o c c u r s , s o m e q u a n t i t y of FeS s h o u l d b e d e t e c t e d . B e r n e r (1964b) s u g g e s t e d t h a t this o c c u r s in t h e S a n t a C a t a l i n a Basin w h e r e s i g n i f i c a n t c o n c e n t r a t i o n s of pyrite b u t not of iron m o n o s u l p h i d e s o c c u r in t h e s u r f a c e s e d i m e n t s ( K a p l a n e t a l . 1963). B e c a u s e t h e s e d i m e n t a c c u m u l a t i o n rate in this b a s i n is l o w , l o n g e r e x p o s u r e t o o x i d i z i n g c o n d i t i o n s n e a r t h e s e d i m e n t - w a t e r i n t e r f a c e m a y result in a n a b u n d a n t s u p p l y of z e r o v a l e n t sulphur. A sufficiently slow rate of d e p o s i t i o n w o u l d a l l o w t h e t r a n s f o r m a t i o n of FeS t o F e S t o g o t o c o m p l e t i o n a t t h e u p p e r s u r f a c e of t h e 2  s e d i m e n t , a n d thus n o b l a c k transition z o n e w o u l d result. H o w e v e r , a p p a r e n t  absence  o f m o n o s u l p h i d e s in r e g i o n s w i t h l o w s e d i m e n t a c c u m u l a t i o n r a t e s m a y a l s o  be  e x p l a i n e d b y t h e u n d e r s a t u r a t i o n of s u c h p h a s e s d u e t o t h e l o w e r d i s s o l v e d s u l p h i d e (or p e r h a p s iron) c o n c e n t r a t i o n s c h a r a c t e r i s t i c of s u c h c o n d i t i o n s . In t h e S a n t a C a t a l i n a B a s i n , s e d i m e n t s a r e t y p i c a l l y o x i c t o s u b o x i c a n d t h e interstitial w a t e r s c o n t a i n l o w levels o f s u l p h i d e (usually u n d e t e c t a b l e ) ;  h e n c e , m o n o s u l p h i d e s a r e likely t o  be  u n d e r s a t u r a t e d , resulting in d i r e c t p r e c i p i t a t i o n of pyrite. Solubility c a l c u l a t i o n s w e r e p e r f o r m e d using M I N E Q L t o s e e if t h e t h e o r e t i c a l d e g r e e of s a t u r a t i o n of iron m o n o s u l p h i d e a n d pyrite a s a f u n c t i o n of d e p t h in t h e lakes c o r r e s p o n d s t o their a p p e a r a n c e in b o t h w a t e r c o l u m n s . It h a s b e e n s u g g e s t e d t h a t , d u e t o t h e u n c e r t a i n t y in t h e s e c o n d d i s s o c i a t i o n c o n s t a n t of s u l p h i d e , solubility c a l c u l a t i o n s f o r m e t a l s u l p h i d e s s h o u l d b e d o n e using t h e c o n c e n t r a t i o n of b i s u l p h i d e ion rather t h a n t h a t of S " (Emerson et a l . 1983) a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n : 2  H  +  + M S «-> M  2 +  + HS"  IAP=(M)(HS")/(H XWHS/YH) +  w h e r e M is t h e m e t a l of interest ( F e  2+  in this c a s e ) , () is solute c o n c e n t r a t i o n a n d y is t h e  t o t a l a c t i v i t y c o e f f i c i e n t . S e v e r a l lAPs w e r e r e c a l c u l a t e d in this m a n n e r a n d  no  d i f f e r e n c e w a s f o u n d in t h e s a t u r a t i o n state of t h e iron sulphides. Thus, t h o s e c a l c u l a t e d with S " v i a M I N E Q L a r e g i v e n here. 2  A s c a n b e s e e n in Figs. 5-17 a n d 5-18, t h e saturation i n d e x o f b o t h m o n o s u l p h i d e s a n d p y r i t e c o r r e s p o n d s fairly w e l l w i t h t h e d e t e c t i o n o f t h e s e s p e c i e s in t h e  water  c o l u m n s . In S a k i n a w L a k e , pyrite is s u p e r s a t u r a t e d t h r o u g h o u t t h e a n o x i c p o r t i o n of t h e w a t e r c o l u m n , w h e r e a s m a c k i n a w i t e a n d g r e i g i t e d o n o t b e c o m e s a t u r a t e d until 40 m depth, where  monosulphides are  first d e t e c t e d .  In P o w e l l  Lake,  pyrite  is a l s o  s u p e r s a t u r a t e d t h r o u g h o u t t h e a n o x i c p o r t i o n of t h e w a t e r c o l u m n , w h i l e g r e i g i t e a n d m a c k i n a w i t e d o not r e a c h saturation until 210 m d e p t h . Pyrite w a s d e t e c t e d a t all d e p t h s in t h e a n o x i c z o n e , w h e r e a s m o n o s u l p h i d e first a p p e a r s in t h e w a t e r c o l u m n a t 200 m , w h e r e g r e i g i t e is v e r y n e a r l y s a t u r a t e d . V e r y l o w levels of m o n o s u l p h i d e w e r e f o u n d at this d e p t h (4 n M ) (Fig. 5-15), i n d i c a t i n g t h a t t h e s p e c i e s w e r e just a t s a t u r a t i o n . The d a t a m a t c h t h e p r e d i c t e d solubilities e x t r e m e l y w e l l , e s p e c i a l l y c o n s i d e r i n g t h e u n c e r t a i n t i e s in t h e c a l c u l a t i o n s a n d t h e small d i f f e r e n c e s of K,p v a l u e s a m o n g most o f t h e m i n e r a l p h a s e s . A l s o , t h e s e c a l c u l a t i o n s w e r e d o n e a s s u m i n g i n o r g a n i c c o m p l e x a t i o n only. O t h e r iron m i n e r a l solubilities w e r e a l s o c a l c u l a t e d ( F e S i 0 , F e O H , F e C 0 , F e ( P 0 ) . 3  2  3  3  4  2  C a F e ( P 0 4 ) 2 ) ; all o f t h e s e minerals w e r e u n d e r s a t u r a t e d , with t h e e x c e p t i o n o f a n a p a i t e 2  ( C a F e ( P 0 4 ) ) in t h e b o t t o m - m o s t w a t e r s of S a k i n a w L a k e . B e c a u s e iron sulphides f o r m 2  2  so r a p i d l y , it is unlikely t h a t this m i n e r a l w o u l d r e m o v e significant a m o u n t s of iron, a n d , t o m y k n o w l e d g e , a n a p a i t e has not b e e n r e p o r t e d in m a r i n e s e d i m e n t s . The s a t u r a t i o n c a l c u l a t i o n s i n d i c a t e t h a t w h e n t h e solubility p r o d u c t of t h e least s o l u b l e a c i d - v o l a t i l e s u l p h i d e p h a s e is n o t e x c e e d e d (pyrite in e q u i l i b r i u m w i t h e x c e s s elemental  s u l p h u r is m a n y  orders  of  magnitude  less s o l u b l e t h a n  greigite  or  m a c k i n a w i t e ) pyrite c a n f o r m d i r e c t l y , w i t h o u t c o m p e t i t i o n f r o m m o n o s u l p h i d e s for iron. T h e r e f o r e , t h e w a t e r c o l u m n s of P o w e l l a n d S a k i n a w L a k e s c a n b e d i v i d e d i n t o t w o z o n e s o f pyrite f o r m a t i o n : 1) t h e u p p e r z o n e w h e r e pyrite p r e c i p i t a t e s a n d m o n o s u l p h i d e is u n d e r s a t u r a t e d  (and  n o n - d e t e c t a b l e ) (150 - 200 m PL, 30 - 40 m SL); a n d 2) t h e d e e p e r w a t e r s w h e r e b o t h iron m o n o s u l p h i d e s a n d pyrite a r e s a t u r a t e d a n d thus b o t h c a n form. These  observations  correspond  to the  two  previously  proposed  pathways  of  p y r i t i z a t i o n , t h e first b e i n g d i r e c t r e a c t i o n b e t w e e n Fe(ll) a n d p o l y s u l p h i d e s y i e l d i n g  single pyrite crystals ( H o w a r t h 1979) a n d t h e s e c o n d b e i n g p r o d u c t i o n o f pyrite v i a t h e slower r e a c t i o n with a m o n o s u l p h i d e p r e c u r s o r (Berner 1970). C u t t e r a n d Velinsky (1988) o b t a i n e d similar results In s e d i m e n t s f r o m t h e G r e a t M a r s h , D e l a w a r e , w h e r e t h e d e p t h s o f t h e FeS a n d F e S 3  4  m a x i m a c o i n c i d e with p r e d i c t e d m a c k i n a w i t e a n d greigite  s a t u r a t i o n (Berner 1967b; B o u l e g u e e t a l 1982; Lord a n d C h u r c h 1983). C u t t e r a n d Velinsky (1988) f o u n d o v e r l a p p i n g greigite a n d pyrite p e a k s in t h e d e e p e r G r e a t M a r s h s e d i m e n t , a distribution w h i c h is c o n s i s t e n t w i t h t h e s l o w pyritization m e c h a n i s m p r o p o s e d b y S w e e n e y a n d K a p l a n (1973), in w h i c h greigite is t h o u g h t t o b e a n e c e s s a r y p r e c u r s o r t o t h e f r a m b o i d a l f o r m of pyrite. In this r e a c t i o n s e q u e n c e , m a c k i n a w i t e c o m b i n e s w i t h e l e m e n t a l sulphur (in t h e f o r m o f polysulphides) t o p r o d u c e greigite (Berner 1967a): 3FeS + S° <-> FeaS* G r e i g i t e in t u r n p r o d u c e s f r a m b o i d a l p y r i t e , e i t h e r t h r o u g h d i s p r o p o r t i o n a t i o n or b y further r e a c t i o n w i t h e l e m e n t a l sulphur (as polysulphides): Fe S4 <-> 2FeS + FeS2 3  FeaSa + 2S° <-> 3FeS2. M o r s e a n d C o r n w e l l (1987), h o w e v e r , h a v e s u g g e s t e d t h a t t h e c o n v e r s i o n o f greigite t o pyrite c a n n o t b e t h e major m e c h a n i s m for f o r m a t i o n o f f r a m b o i d a l pyrite, a s t h e y c o u l d not f i n d g r e i g i t e in a w i d e v a r i e t y o f d i a g e n e t i c a l l y - a c t i v e s e d i m e n t s w h e r e f r a m b o i d s occur. Effect of pH on Pyrite Formation Berner e t a l . (1979) h a s stressed t h a t a n a c i d i c e n v i r o n m e n t is r e q u i r e d for r a p i d pyrite f o r m a t i o n w i t h o u t m o n o s u l p h i d e i n t e r m e d i a t e s , n o t i n g t h a t pyrite w o u l d only f o r m in l a b e x p e r i m e n t s w h e n t h e p H w a s < 6. U n d e r m o r e a l k a l i n e c o n d i t i o n s , o n l y monosulphides f o r m e d . Other researchers h a v e also f o u n d that a c i d conditions favour t h e r a p i d f o r m a t i o n o f FeS2 (Roberts e t a l . 1969; G o l d h a b e r a n d K a p l a n 1974; R i c k a r d 1975). H o w e v e r , p H c a n h a v e n o d i r e c t e f f e c t o n t h e n a t u r e o f t h e iron s u l p h i d e p r o d u c e d ; it c a n o n l y a f f e c t t h e n a t u r e o f t h e iron or sulphur s p e c i e s in solution or t h e n a t u r e o f t h e initial m a c k i n a w i t e s u r f a c e . The p H h a s n o e f f e c t o n t h e iron s p e c i e s (at t h e p H r a n g e c o m m o n l y f o u n d in n a t u r a l w a t e r s a n d s e d i m e n t s ) , a n d its e f f e c t s o n t h e s u l p h i d e s p e c i e s a r e simply t o c h a n g e d o m i n a n t HS' (at p H > 7) t o d o m i n a n t H2S ( p H < 7) a n d d e c r e a s e t h e a m o u n t o f S " a s p H d e c r e a s e s . H o w a r t h (1979) p r o d u c e d pyrite 2  rather t h a n m o n o s u l p h i d e a t p H 7.5 w h e n t h e p a r t i a l pressure o f H2S w a s m a i n t a i n e d a t  0.4 a t m (vs. 1 a t m ) . T h e r e f o r e , a s H o w a r t h (1979) o b s e r v e d , t h e t e n d e n c y f o r pyrite t o f o r m a t l o w e r p H a n d iron m o n o s u l p h i d e s a t higher p H only reflects t h e e f f e c t of p H o n S ' 2  activity. D e c r e a s i n g t h e p H d e c r e a s e s t h e c o n c e n t r a t i o n ( a n d activity) o f S * relative t o 2  o t h e r s u l p h i d e s p e c i e s ; thus iron m o n o s u l p h i d e s a r e m o r e likely t o b e u n d e r s a t u r a t e d a t l o w e r p H . These o b s e r v a t i o n s c o l l e c t i v e l y e x p l a i n r a p i d f o r m a t i o n o f pyrite in salt m a r s h s e d i m e n t s , a s t h e p H in t h e s e deposits is usually b e t w e e n 5 a n d 6.5. In s u m m a r y , s e v e r a l f a c t o r s i n d i c a t e t h a t pyrite is f o r m i n g d i r e c t l y in t h e u p p e r a n o x i c w a t e r s of P o w e l l a n d S a k i n a w Lakes, rather t h a n v i a a m o n o s u l p h i d e precursor: 1) pyrite o c c u r s a t s h a l l o w e r d e p t h s t h a n m o n o s u l p h i d e ; 2) w a t e r s a t t h e s e d e p t h s a r e u n d e r s a t u r a t e d w i t h r e s p e c t t o m o n o s u l p h i d e ( a n d s a t u r a t e d with r e s p e c t t o pyrite); a n d 3) highly r e a c t i v e p o l y s u l p h i d e s a n d F e  2 +  a r e p r e s e n t , a n d c o n c e n t r a t i o n s of d i s s o l v e d  s u l p h i d e a r e l o w . In a d d i t i o n , t h e p H is l o w , d e c r e a s i n g t h e activity of S ". 2  In t h e d e e p e r w a t e r s w h e r e m o n o s u l p h i d e is s a t u r a t e d , pyrite c a n f o r m v i a t h e m o r e g r a d u a l c o n v e r s i o n of FeS. It is c l e a r t h a t t h e u n i q u e s e p a r a t i o n o f t h e o x i c / a n o x i c i n t e r f a c e a n d t h e c h e m o c l i n e in P o w e l l a n d S a k i n a w Lakes a l l o w s b e t t e r resolution of t h e t w o r e g i m e s in w h i c h t h e a l t e r n a t e p a t h w a y s of pyrite f o r m a t i o n d o m i n a t e . In c o n t r a s t , t h e r e l a t i v e c o m p r e s s i o n of t h e a n o x i c z o n e in p o r e w a t e r profiles o b v i a t e s s u c h a d e l i n e a t i o n of formation pathways. Organic Sulphur N o a t t e m p t w a s m a d e in this s t u d y t o d e t e r m i n e o r g a n i c sulphur. In b o t h P o w e l l a n d S a k i n a w Lakes t h e i n o r g a n i c sulphur p o o l is e x t r e m e l y l a r g e (~3 m M in t h e f o r m e r a n d 5.5 m M in t h e latter). H o w e v e r , t h e r e a r e also v e r y high levels of D O C a n d P O C in t h e b o t t o m w a t e r s of t h e s e lakes (Figs. 3-2 a n d 3-3), a n d thus, t h e r e m a y b e a l a r g e q u a n t i t y of a s s o c i a t e d o r g a n i c s u l p h u r p r e s e n t . H i g h c o n c e n t r a t i o n s o f thiols (2.4 m M ) h a v e b e e n f o u n d in salt m a r s h p o r e w a t e r s ( L u t h e r e t a l . 1986b). H o w e v e r ,  although  o r g a n o s u l p h u r c o m p o u n d s m a y m a k e u p a sizable contribution of t h e total sulphur p o o l , it is unlikely t h a t t h e y p a r t i c i p a t e d i r e c t l y in iron s u l p h i d e f o r m a t i o n , a s iron will p r e f e r e n t i a l l y r e a c t w i t h the- l a r g e q u a n t i t i e s o f h i g h l y r e a c t i v e p o l y s u l p h i d e s a n d d i s s o l v e d i n o r g a n i c s u l p h i d e t h a t a r e present. O x i d i z e d o r g a n o s u l p h u r c o m p o u n d s c a n i n d i r e c t l y p l a y a role in pyrite f o r m a t i o n , a c t i n g a s e l e c t r o n a c c e p t o r s f o r s u l p h a t e  r e d u c t i o n , t h e r e b y b e i n g c o n v e r t e d t o dissolved sulphide that c o u l d t h e n r e a c t with i r o n , a s h a s b e e n s u g g e s t e d f o r E v e r g l a d e s p e a t s ( A l t s c h u l e r e t a l . 1983). T h e r e f o r e , a l t h o u g h t h e o r g a n i c sulphur p o o l m a y b e l a r g e in P o w e l l a n d S a k i n a w L a k e s , this f r a c t i o n is unlikely t o p l a y a d i r e c t role in pyrite f o r m a t i o n . M a n g a n e s e Sulphides In t h e b o t t o m w a t e r s o f P o w e l l L a k e m a n g a n e s e a p p e a r s t o b e r e m o v e d a t d e p t h (Fig. 5-9). T h e t w o m a n g a n e s e s u l p h i d e minerals, a l a b a n d i t e ( M n S ) a n d h a u r i t e (MnS2), b o t h o f w h i c h a r e relatively s o l u b l e , a r e u n d e r s a t u r a t e d in b o t h l a k e s , a l t h o u g h h a u r i t e d o e s a p p r o a c h s a t u r a t i o n in P o w e l l L a k e b o t t o m w a t e r s (Figs. 5-19 a n d 5-20). MnC0  3  a p p r o a c h e s , b u t d o e s n o t a t t a i n s a t u r a t i o n in P o w e l l L a k e a n d M n O H is highly 2  u n d e r s a t u r a t e d in b o t h lakes. The lAPs for M n S i 0 a n d M n ( P 0 ) w e r e a l s o c a l c u l a t e d , 3  3  4  2  b u t t h e s e m i n e r a l s a r e e v e n less s a t u r a t e d . The o n l y k n o w n o c c u r r e n c e o f M n S is in r e c e n t s e d i m e n t s o f t h e Baltic S e a (Suess 1979); M n S is o t h e r w i s e o n l y k n o w n f r o m h y d r o t h e r m a l d e p o s i t s ( P a l a c h e e t a l 1952). H i g h d i s s o l v e d M n  2 +  concentrations are  t y p i c a l o f Baltic S e a basins ( B a k e r 1978; Djafari 1976, c i t e d in Suess 1979), a n d , t h e r e f o r e , M n S m a y p r e c i p i t a t e in this e n v i r o n m e n t rather t h a n iron sulphides a s is usually t h e c a s e under anoxic marine conditions. A n o t h e r m e c h a n i s m f o r m a n g a n e s e r e m o v a l is a d s o r p t i o n a n d c o p r e c i p i t a t i o n with a n o t h e r m i n e r a l p h a s e . M a n g a n e s e c a n c o p r e c i p i t a t e w i t h C a C 0  3  (Pingitore et a l .  1988; T h o m s o n et a l . 1986), h o w e v e r , as w a s d i s c u s s e d in C h a p t e r 4, C a C 0  3  f o r m a t i o n in  P o w e l l L a k e is unlikely. J a c o b s e t a l . (1985) s u g g e s t t h a t f r a m b o i d a l pyrite p a r t i c l e s p r e c i p i t a t i n g in F r a m v a r e n Fjord a c t a s a carrier p h a s e f o r M n  2 +  , as framboids c o l l e c t e d  f r o m t h e fjord w a t e r c o l u m n h a v e m a n g a n e s e c o n t e n t s o f 1.0%. S i n c e f r a m b o i d a l pyrite is p r e s e n t in t h e w a t e r c o l u m n o f P o w e l l L a k e (T. P e d e r s e n pers. c o m m . ) , p e r h a p s this is h o w t h e m a n g a n e s e is b e i n g r e m o v e d f r o m t h e d e e p w a t e r s . Pyrite c o n c e n t r a t i o n s in P o w e l l L a k e a r e m u c h h i g h e r t h a n t h o s e in S a k i n a w a n d this m a y e x p l a i n w h y M n d e p l e t e d a t d e p t h in P o w e l l L a k e only. C o m p a r i s o n of t h e M n  2 +  2 +  is  a n d FeS2 profiles in P o w e l l  L a k e (Figs. 5-9 a n d 5-15) shows t h a t b o t h a r e r e m o v e d f r o m t h e w a t e r c o l u m n b e l o w 250 m , s u g g e s t i n g t h a t similar p r o c e s s e s a r e r e m o v i n g b o t h s p e c i e s ; c o p r e c i p i t a t i o n o f Mn  2 +  with FeS  2  is t h e m o s t p l a u s i b l e e x p l a n a t i o n . T h e d e c r e a s e  in pyrite ( a n d  m o n o s u l p h i d e ) n e a r t h e c h e m o c l i n e is p r o b a b l y d u e t o f l o c c u l a t i o n a n d r a p i d settling w h e r e t h e r e is a n a b r u p t c h a n g e in salinity as d e s c r i b e d in C h a p t e r 3.  CHAPTER 6 SUMMARY AND CONCLUSIONS  P o w e l l a n d S a k i n a w Lakes a r e m e r o m i c t i c ex-fjords w h i c h w e r e s e p a r a t e d f r o m t h e Strait o f G e o r g i a a p p r o x i m a t e l y 11000 y e a r s a g o d u e t o isostatic uplift f o l l o w i n g t h e last d e g l a c i a t i o n . B o t h a r e s t a b l y stratified, w i t h a n o x i c , highly s u l p h i d i c b o t t o m w a t e r s underlying fresh, o x y g e n a t e d waters. Sakinaw Lake has a m u c h sharper oxic/anoxic i n t e r f a c e t h a n P o w e l l L a k e , w h i c h reflects p e r i o d i c incursions o f s e a w a t e r into S a k i n a w s i n c e sill e m e r g e n c e , w h e r e a s t h e relict s e a w a t e r in P o w e l l L a k e is t h a t originally t r a p p e d . This d i f f e r e n c e in t h e a g e s o f t h e b o t t o m w a t e r o f t h e t w o lakes results in s e v e r a l n o t a b l e d i f f e r e n c e s in c h e m i s t r y . The m o s t o b v i o u s c o n s e q u e n c e  is t h a t  S a k i n a w L a k e h a s a l m o s t t w i c e t h e c o n c e n t r a t i o n o f s u l p h i d e , e v e n t h o u g h it h a s freshened considerably more t h a n Powell, reflecting a n additional source of sulphate. G e o t h e r m a l h e a t i n g h a s p r o d u c e d w a r m b o t t o m w a t e r s in b o t h lakes, w h i c h c r e a t e a m i d - d e p t h t e m p e r a t u r e m i n i m u m in b o t h basins. H o w e v e r , d u e t o t h e h i g h e r salinity o f t h e b o t t o m w a t e r s , t h e w a t e r c o l u m n r e m a i n s s t a b l y stratified in spite o f t h e h i g h e r temperature at depth. A l t h o u g h b o t h lakes h a v e f r e s h e n e d , t h e ratios o f t h e major i o n c o n c e n t r a t i o n s in their m o n i m o l i m n i a , relative t o chlorinity, a r e similar t o t h o s e o f p r e s e n t - d a y s e a w a t e r . There a r e s o m e d i f f e r e n c e s , h o w e v e r , a n d t h e s e c a n b e p a r t l y e x p l a i n e d b y t h e d i f f e r e n c e s in m o l e c u l a r diffusivities f o r e a c h o f t h e s p e c i e s . M a g n e s i u m , c a l c i u m , b o r a t e , a n d strontium a r e all relatively e n r i c h e d with r e s p e c t t o c h l o r i d e w h e n compared seawater.  t o t h e c o n c e n t r a t i o n s e x p e c t e d f r o m t h e c o n s t a n t c o m p o s i t i o n of These distributions h a v e b e e n  e x p l a i n e d as resulting from t h e lower  diffusivities o f t h e s e ions, w h i c h results in their b e i n g lost m o r e slowly v i a diffusion t o t h e u p p e r , fresher w a t e r s . P o t a s s i u m , o n t h e o t h e r h a n d , is relatively d e p l e t e d , d u e t o its h i g h e r diffusivity a n d thus f a s t e r u p w a r d diffusion. In c o n t r a s t , t h e N a t o CI" r a t i o is +  c o n s t a n t , w h i c h is c o n s i s t e n t w i t h t h e i d e n t i c a l diffusivities o f c h l o r i d e a n d s o d i u m . A m a t h e m a t i c a l m o d e l o f t h e history o f t h e f r e s h e n i n g in P o w e l l L a k e only partially explains the differences b e t w e e n o b s e r v e d major ion concentrations a n d those p r e d i c t e d o n t h e basis o f their differential rates o f diffusion ( S a n d e r s o n e t a l . 1986), w h i c h in p a r t , m a y r e f l e c t n o n - c o n s e r v a t i v e b e h a v i o u r o f s o m e o f t h e ions.  The b o t t o m w a t e r s o f P o w e