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Phytoplankton dynamics and the distribution of fish larvae and their nutritional resources across an… Levasseur, Maurice 1990

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PHYTOPLANKTON DYNAMICS AND THE DISTRIBUTION OF FISH LARVAE AND THEIR NUTRITIONAL RESOURCES ACROSS AN ESTUARINE PLUME FRONT By MAURICE EDGAR LEVASSEUR B . S c , Universite Laval, 1979 M.Sc, Universite Laval, 1984 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Oceanography) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1990 © Maurice Edgar Levasseur, 1990 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British Columbia Vancouver, Canada Department Date \ H | 0 ^ 1 ^ T f Q DE-6 (2/88) ABSTRACT In the marine environment, export production leading to the t r a d i t i o n a l food chain i s a r e l a t i v e l y rare event taking place p r i m a r i l y i n hydrographic features such as f r o n t a l areas. When export production p e r s i s t s , massive reproduction of herbivores i s expected to occur. Since copepod eggs and n a u p l i i are the main prey of a majority of f i s h postlarvae, the spawning of dominant f i s h species i s expected to be associated with fronts. The aims of t h i s study were to determine the influence of an estuarine plume front upon the phytoplankton dynamics ( d i s t r i b u t i o n and phy s i o l o g i c a l status) and to assess the ro l e of the cross-f r o n t a l c i r c u l a t i o n upon the d i s t r i b u t i o n of f i s h larvae and t h e i r prey. The f r o n t a l area under study i s located i n the northwestern part of the Gulf of St. Lawrence, at the int e r f a c e between a coastal j e t (Gaspe Current) flowing along the Gaspe Peninsula and the A n t i c o s t i Gyre. In early June, maximum phytoplankton concentrations (up to 35 /jg c h l a L~^) were found i n the Gaspe Current. In the s a l i n i t y gradient, a s i g n i f i c a n t c o r r e l a t i o n was found between s a l i n i t y and phytoplankton concentrations (and seston i n general), i n d i c a t i n g that physical processes ( v e r t i c a l and horizontal mixing) were more important i n c o n t r o l l i n g the seston d i s t r i b u t i o n than b i o l o g i c a l processes. The dominance of physical processes i s probably due to the high current v e l o c i t i e s and shear stress i n the Gaspe Current i n early June. Later during the season, the c r o s s - f r o n t a l mixing was less vigorous due to the lower freshwater runoff, and the front acted as a retention zone for estuarine plankton. Maximum diatom concentrations (up to 50 pq c h l a L~*) were measured i n the front per se. Measurements of nitrogen and s i l i c a t e concentrations (ambient and i n t r a c e l l u l a r ) and uptake rates suggested that s i l i c a t e generally l i m i t e d diatom growth across the front. In June, estuarine larvae (capelin, Mallotus villosus and sand lance, Ammodytes hexapterus) were concentrated i n the diatom-rich Gaspe Current and front where immature copepod stages were abundant. The f i v e - f o l d increase i n immature copepod concentrations between the gyre and the current/front resulted probably from a food-induced increase i n copepod reproduction. Thus i t appears that the dispersion strategy of the estuarine species i n r e l a t i o n with l o c a l hydrography favours the e x p l o i t a t i o n of the resource-rich Gaspe current and front by the f i r s t - f e e d i n g postlarvae. The extrusion of r e d f i s h (Sebastes spp.) larvae appears to be synchronized with the copepod reproduction that followed the gyre April/May bloom. Later, r e d f i s h larvae were also found i n abundance i n the resource-rich front. i v TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES ix LIST OF FIGURES xiv ACKNOWLEDGEMENTS x x v i i GENERAL INTRODUCTION 1 Frontal area - D e f i n i t i o n 3 Frontal area - Primary production 3 Frontal area - Secondary and t e r t i a r y production 5 Objectives 8 Study area 11 CHAPTER 1. INFLUENCES OF THE CROSS-FRONTAL CIRCULATION UPON THE PHYSIOLOGY AND ECOLOGY OF PHYTOPLANKTON COMMUNITIES IN THE GASPE CURRENT ESTUARINE PLUME FRONT 13 Background 13 Materials and Methods 16 F i l t r a t i o n and chemical analyses 20 Use of N:C r a t i o as i n d i c a t o r of nutrient d e f i c i e n c y 20 Photosynthetic parameters 22 Results 24 Sampling year 1985 24 Cruise A 24 Horizontal d i s t r i b u t i o n 24 V e r t i c a l d i s t r i b u t i o n 34 V C r u i s e B 45 H o r i z o n t a l d i s t r i b u t i o n 45 V e r t i c a l d i s t r i b u t i o n 55 Samp l i n g y e a r 1986 57 C r u i s e C 57 V - F I N R t r a n s e c t s 57 H o r i z o n t a l d i s t r i b u t i o n 61 V e r t i c a l d i s t r i b u t i o n 68 C r u i s e D 72 H o r i z o n t a l d i s t r i b u t i o n 72 V e r t i c a l d i s t r i b u t i o n 7 9 D i s c u s s i o n 84 P h y s i c a l c h a r a c t e r i s t i c s o f t h e f r o n t a l a r e a 84 C h e m i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s of t h e f r o n t a l a r e a 91 L a t e s p r i n g c o n d i t i o n s 94 Summer c o n d i t i o n s 105 The n o n - u p w e l l i n g f r o n t 107 The u p w e l l i n g f r o n t 109 Summary I l l CHAPTER 2. SILICATE AND NITROGEN DEFICIENCY OF A PHYTOPLANKTON COMMUNITY IN THE GASPE CURRENT ESTUARINE PLUME FRONT 113 Background.... 113 M a t e r i a l s and Methods 116 Sa m p l i n g and l a b o r a t o r y p r o c e d u r e s 116 v i Uptake r a t e s measurements 118 S i n k i n g r a t e s 121 R e s u l t s 122 Temperature and s a l i n i t y d a t a 122 B i o l o g i c a l d a t a 122 N u t r i e n t s 125 Ambient n u t r i e n t c o n c e n t r a t i o n s 125 I n t e r n a l n u t r i e n t s 129 N u t r i e n t u p t a k e 133 N:C r a t i o s 135 S i n k i n g r a t e s 139 D i s c u s s i o n 144 N u t r i e n t S t a t u s o f t h e P h y t o p l a n k t o n Community 144 N i t r o g e n N u t r i t i o n 144 S i l i c a t e N u t r i t i o n 146 E f f e c t s o f N u t r i e n t S t r e s s on S i n k i n g Rates 149 E f f e c t s o f N u t r i e n t S t r e s s on N:C r a t i o . . 150 CHAPTER 3. DIATOM ABUNDANCE AND THE DISTRIBUTION OF FISH LARVAE AND THEIR RESOURCE IN AN ESTUARINE PLUME FRONT 156 Background 156 M a t e r i a l s and Methods 158 C a l c u l a t i o n o f l a r v a l f i s h f o o d r e s o u r c e . 159 R e s u l t s 161 Hydrography 161 v i i I c h t h y o p l a n k t o n c o m p o s i t i o n , l e n g t h f r e q u e n c y and d i e t 161 C r o s s - f r o n t a l d i s t r i b u t i o n o f p h y t o p l a n k t o n and copepod r e p r o d u c t i o n 165 C r o s s - f r o n t a l d i s t r i b u t i o n o f f i s h l a r v a e and t h e i r r e s o u r c e . . . . 175 D i s c u s s i o n 184 Copepod r e p r o d u c t i o n - t h e s h o r t f o o d c h a i n 184 F i s h l a r v a e and p r e y r e l a t i o n s h i p - t h e match/mismatch t h e o r y 187 The Gaspe C u r r e n t f r o n t as a r e t e n t i o n zone. . 191 C o n c l u s i o n s 195 GENERAL CONCLUSIONS 198 P h y s i c a l / b i o l o g i c a l i n t e r a c t i o n s - t h e i m p o r t a n c e o f s c a l e s 199 The i m p o r t a n c e of c i r c u l a t i o n 203 D i v e r g e n c e 204 Convergence 205 Eddy m o t i o n s 205 F r o n t a l zones as entrapment zones f o r f i s h l a r v a e i n t h e S t . Lawrence 207 S i l i c a t e l i m i t a t i o n i n t h e S t . Lawrence -Causes and consequences 211 Causes 211 v i i i Consequences 215 REFERENCES 218 i x LIST OF TABLES Ta b l e 1.1. C r u i s e i d e n t i f i c a t i o n , types of sampling ( h o r i z o n t a l and/or v e r t i c a l t r a n s e c t ) , and date and t i d a l amplitude a t which the c r u i s e s were conducted (s = s p r i n g t i d e , n = neap t i d e ) 18 T a b l e 1.2. C r u i s e A - M i c r o p l a n k t o n s p e c i e s and abundance (determined a t 3 m depth a l o n g the t r a n s e c t ) i n the Gaspe Current ( s t a t i o n 75), the f r o n t per se ( s t a t i o n 99) and i n the G u l f Gyre ( s t a t i o n 24).. 31-32 T a b l e 1.3. C r u i s e A - C o e f f i c i e n t s ( s l o p e s and Y - i n t e r c e p t ) of the l i n e a r r e g r e s s i o n a n a l y s e s conducted between the independent v a r i a b l e c h l o r o p h y l l a and the dependent v a r i a b l e s POC and PON i n each p a r t of the f r o n t a l area (n = number of data p o i n t s , r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = not s i g n i f i c a n t ) 33 T a b l e 1.4. C r u i s e A - V e r t i c a l d i s t r i b u t i o n of m i c r o p l a n k t o n abundance (10 3 c e l l s l - 1 ) a t s e l e c t e d depths i n the Gaspe Current (Stn A l ) 38-39 Tabl e 1.5. C r u i s e A - V e r t i c a l d i s t r i b u t i o n of m i c r o p l a n k t o n abundance ( 1 0 3 c e l l s l - 1 ) a t s e l e c t e d depths i n the f r o n t (Stn A2) 40-41 X T a b l e 1.6. C r u i s e A - V e r t i c a l d i s t r i b u t i o n of m i c r o p l a n k t o n abundance ( 1 0 3 c e l l s l - 1 ) a t s e l e c t e d depths i n the G u l f Gyre (Stn A3) 42-43 Ta b l e 1.7. C r u i s e B - M i c r o p l a n k t o n s p e c i e s and abundance ( 1 0 3 c e l l s l - 1 ) (determined a t 3 m a l o n g the t r a n s e c t ) i n the Gasp6 Cu r r e n t ( s t a t i o n 47), the non-upwelling p a r t of the f r o n t ( s t a t i o n 20), the u p w e l l i n g p a r t of the f r o n t ( s t a t i o n 36) and i n the G u l f Gyre ( s t a t i o n 1) 52-5 3 T a b l e 1.8. C r u i s e B - C o e f f i c i e n t s ( s l o p e and Y - i n t e r c e p t ) of the l i n e a r r e g r e s s i o n a n a l y s e s conducted between the independent v a r i a b l e c h l o r o p h y l l a and the dependent v a r i a b l e s POC and PON i n each p a r t of the f r o n t a l area (n = number of data p o i n t s , r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = not s i g n i f i c a n t ) 54 T a b l e 1.9. C r u i s e C - M i c r o p l a n k t o n s p e c i e s and abundance (determined a t 3 m a l o n g the t r a n s e c t ) i n the Gaspe C u r r e n t ( s t a t i o n 19), the f r o n t per se ( s t a t i o n 12) and i n the G u l f Gyre ( s t a t i o n 2) 65-66 Ta b l e 1.10. C r u i s e C- C o e f f i c i e n t s ( s l o p e and Y - i n t e r c e p t ) of the l i n e a r r e g r e s s i o n a n a l y s e s conducted between the independent variable chlorophyll a and the dependent variables POC and PON i n each part of the f r o n t a l area (n = number of data points, r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = not s i g n i f i c a n t ) Table 1.11. Cruise C - C o e f f i c i e n t s (slope and Y-intercept) of the l i n e a r regression analyses conducted between the independent variable chlorophyll a and the dependent variables POC and PON at each s t a t i o n across the f r o n t a l area during the transect of v e r t i c a l p r o f i l e s (n = number of data points, r = c o r r e l a t i o n c o e f f i c i e n t , p = probability,and n.s. = not s i g n i f i c a n t ) 73 Table 1.12. Cruise D - Microplankton species and abundance (determined at 3 m along the transect) i n the Gaspe Current (station 24), the front per se (station 20) and i n the Gulf Gyre (station 1) 77-78 Table 1.13. Cruise D - V e r t i c a l d i s t r i b u t i o n of microplankton abundance (10 3 c e l l s l - 1 ) at selected depths i n the front (Station D7) 82-83 Table 2.1. Mid-day (10:00-15:00 h) chlorophyll a s p e c i f i c uptake rates f o r n i t r a t e , ammonium, urea and t o t a l nitrogen (N0 3 + NH4 + urea) measured i n the Gyre and xi 67 x i i i n the f r o n t . (n.d. = non- d e t e c t a b l e ) 134 T a b l e 2.2. /Ambient s i l i c a t e c o n c e n t r a t i o n s , s i l i c a t e t r a n s p o r t r a t e s ( P S i 0 4 ) and c h l o r o p h y l l a s p e c i f i c s i l i c a t e uptake r a t e s ( V S i 0 4 ) measured a t 3 m a c r o s s the f r o n t a l area (n.d.=non-detectable).... 136 T a b l e 2.3. C o e f f i c i e n t s ( s l o p e and Y - i n t e r c e p t ) of the l i n e a r r e g r e s s i o n a n a l y s e s conducted between the independent v a r i a b l e c h l o r o p h y l l a and the dependent v a r i a b l e s POC and PON a t each s t a t i o n a c r o s s the f r o n t a l a r e a . S t a t i o n s 6 and 7 were po o l e d t o g e t h e r i n o r d e r t o i n c r e a s e the number of data p o i n t s (n = number of data p o i n t s , r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = not s i g n i f i c a n t ) 152 T a b l e 3.1. S p e c i f i c composition and abundance (%) of i c h t h y o p l a n k t o n i n the f r o n t a l area of the Gaspe C u r r e n t i n e a r l y June ( c r u i s e A) and a t the end of June ( c r u i s e B) 1985 162 T a b l e 3.2. Percent composition of l a r v a l c a p e l i n d i e t by l e n g t h c l a s s e s i n June 1985 ( c r u i s e B) 166 T a b l e 3.3. Percent composition of l a r v a l sand l a n c e d i e t by l e n g t h c l a s s e s i n e a r l y June 1985 ( c r u i s e A ) . . . 167 x i i i T a b l e 3.4 P e r c e n t c o m p o s i t i o n o f l a r v a l r e d f i s h d i e t by l e n g t h c l a s s e s i n e a r l y June 1985 ( c r u i s e A) 168 T a b l e 3.5. N i t r a t e and s i l i c a t e ambient c o n c e n t r a t i o n s , h o u r l y t r a n s p o r t r a t e s (mid-day) and e s t i m a t e d t i m e u n t i l n u t r i e n t e x h a u s t i o n (T) i n t h e u p w e l l i n g p a r t o f t h e f r o n t o f t h e Gasp6 C u r r e n t ( d a t a a r e from c r u i s e E, c h a p t e r 2) 197 x i v LIST OF FIGURES F i g u r e 1.1. Map o f t h e n o r t h w e s t e r n G u l f o f S t . Lawrence showing s u r f a c e c i r c u l a t i o n ( f r o m E l - S a b h 1976), t h e a p p r o x i m a t e l o c a t i o n o f t h e f r o n t and t h e s t u d y a r e a (boxed a r e a ) 17 F i g u r e 1.2. P o s i t i o n s o f t h e s t a t i o n s a l o n g t h e h o r i z o n t a l t r a n s e c t s (numbers, 1 t o 100) and where v e r t i c a l p r o f i l e s were c o n d u c t e d ( c r u i s e l e t t e r / n u m b e r s , A l t o A3) d u r i n g c r u i s e A. F i s h l a r v a e were sampled a t t h e same t i m e a t a h i g h e r f r e q u e n c y a l o n g t h e t r a n s e c t (see c h a p t e r 3 ) . Numbers i n b r a c k e t s c o r r e s p o n d t o l a r v a l f i s h s t a t i o n s 25 F i g u r e 1.3. C r u i s e A - H o r i z o n t a l d i s t r i b u t i o n a t 3 m o f : (A) s a l i n i t y , (B) t e m p e r a t u r e , (C) t h e s u r f a c e l a y e r d e p t h , (D) n i t r a t e , (E) s i l i c a t e , (F) c h l o r o p h y l l a, (G) POC, (H) PON, ( I ) N:C r a t i o of p h y t o p l a n k t o n (•) and s e s t o n (=)(dashed l i n e r e p r e s e n t s t h e R e d f i e l d r a t i o ) , ( J ) p h y t o p l a n k t o n POC/CHL a r a t i o , (K) p h y t o p l a n k t o n PON/CHL a r a t i o , (L) P Bm, (M) a B , and (N) I i * on a h o r i z o n t a l t r a n s e c t a c r o s s t h e f r o n t a l a r e a o f t h e Gasp§ C u r r e n t i n t h e G u l f of S t . Lawrence, (see F i g . 1.2 f o r p o s i t i o n o f s t a t i o n s 0 t o 100; FRT = f r o n t ) 26-27 XV F i g u r e 1.4. H o r i z o n t a l v a r i a t i o n s o f s a l i n i t y a t 3 m (A) and v e r t i c a l d i s t r i b u t i o n o f t e m p e r a t u r e (B) a l o n g t h e t r a n s e c t c o n d u c t e d d u r i n g c r u i s e A (see F i g . 1.3 f o r o t h e r v a r i a b l e s measured a t 3 m). Dots r e p r e s e n t XBT s t a t i o n s 29 F i g u r e 1.5. C r u i s e A - V e r t i c a l d i s t r i b u t i o n o f : (A) s a l i n i t y and t e m p e r a t u r e , (B) n i t r a t e , s i l i c a t e and phosphate and (C) c h l o r o p h y l l a a t s t a t i o n A l i n t h e Gaspe C u r r e n t (see F i g . 1.2 f o r p o s i t i o n o f s t a t i o n A l ) 35 F i g u r e 1.6. C r u i s e A - V e r t i c a l d i s t r i b u t i o n o f : , ( A ) s a l i n i t y and t e m p e r a t u r e , (B) n i t r a t e , s i l i c a t e and phosphate and (C) c h l o r o p h y l l a a t s t a t i o n A2 i n t h e f r o n t (see F i g . 1.2 f o r p o s i t i o n o f s t a t i o n A2 ) 36 F i g u r e 1.7. C r u i s e A - V e r t i c a l d i s t r i b u t i o n o f : (A) s a l i n i t y and t e m p e r a t u r e , (B) n i t r a t e , s i l i c a t e and phosphate and (C) c h l o r o p h y l l a a t s t a t i o n A3 i n t h e G u l f Gyre (see F i g . 1.2 f o r p o s i t i o n o f s t a t i o n A3 ) 37 F i g u r e 1.8. P o s i t i o n s o f t h e s t a t i o n s a l o n g t h e h o r i z o n t a l t r a n s e c t s (numbers 1 t o 49) and a t w h i c h v e r t i c a l p r o f i l e s were c o n d u c t e d ( c r u i s e l e t t e r / n u m b e r s , B l t o B17)) d u r i n g c r u i s e B. F i s h l a r v a e were sampled a t t h e same t i m e a t a h i g h e r f r e q u e n c y a l o n g t h e t r a n s e c t (see C h a p t e r 3 ) . Numbers i n b r a c k e t s c o r r e s p o n d t o l a r v a l f i s h s t a t i o n s F i g u r e 1.9. C r u i s e B - H o r i z o n t a l d i s t r i b u t i o n a t 3 m o f : (A) s a l i n i t y , (B) t e m p e r a t u r e , (C) t h e s u r f a c e l a y e r d e p t h , (D) n i t r a t e , (E) s i l i c a t e , (F) c h l o r o p h y l l a, (G) POC, (H) PON, ( I ) N:C r a t i o of p h y t o p l a n k t o n (dashed l i n e r e p r e s e n t s t h e R e d f i e l d r a t i o ) , ( J ) p h y t o p l a n k t o n POC/CHL a r a t i o , (K) p h y t o p l a n k t o n PON/CHL a r a t i o , (L) P sm, (M) a B , and (N) I k on a h o r i z o n t a l t r a n s e c t a c r o s s t h e Gyre (GY), f r o n t and Gaspe C u r r e n t (G. C u r r e n t ) i n t h e G u l f of S t . Lawrence (see F i g . 1.8 f o r p o s i t i o n of s t a t i o n s 0 t o 50) 47 F i g u r e 1.10. H o r i z o n t a l v a r i a t i o n s i n s a l i n i t y a t 3 m (A) arid v e r t i c a l d i s t r i b u t i o n o f t e m p e r a t u r e (B) a l o n g t h e t r a n s e c t c o n d u c t e d d u r i n g c r u i s e B (see F i g . 1.9 f o r o t h e r v a r i a b l e s measured a t 3 m; GY = g y r e and G.C. = Gasp6 C u r r e n t ) . Dots r e p r e s e n t XBT s t a t i o n s F i g u r e 1.11. C r u i s e B - V e r t i c a l d i s t r i b u t i o n o f : (A) s a l i n i t y , and (B) t e m p e r a t u r e on a t r a n s e c t a c r o s s t h e Gaspe C u r r e n t ( B l t o B 6 ) , t h e f r o n t (B7 t o B l l ) x v i i and t h e Gyre (B12 t o B17) i n t h e G u l f o f S t . Lawrence (see F i g . 1.8 f o r p o s i t i o n s o f t h e s t a t i o n s B l t o B17). 56 F i g u r e 1.12. P o s i t i o n s o f t h e s t a t i o n s a l o n g t h e h o r i z o n t a l t r a n s e c t s (numbers) and a t w h i c h v e r t i c a l p r o f i l e s were c o n d u c t e d ( c r u i s e l e t t e r / n u m b e r s ) d u r i n g c r u i s e C 58 F i g u r e 1.13. C r u i s e C - High r e s o l u t i o n c r o s s - s e c t i o n s o f s a l i n i t y and t e m p e r a t u r e i n t h e s t u d y a r e a . The t r a n s e c t s were c o n d u c t e d June 3 ( p a n e l s A and B) and June 7 ( p a n e l s C and D). Data were o b t a i n e d by t o w i n g a v e r t i c a l l y o s c i l l a t i n g CTD probe (V-FIN 1* s y s t e m ) . The d u r a t i o n o f t h e t r a n s e c t s was 6 h (June 3) and 2 h (June 7 ) . The f i r s t t r a n s e c t was c o n d u c t e d s i m u l t a n e o u s l y w i t h t h e r e g u l a r h o r i z o n t a l s a m p l i n g ( d a t a p r e s e n t e d i n F i g u r e 1.14 ) . . . . 59-60 F i g u r e 1.14. C r u i s e C - H o r i z o n t a l d i s t r i b u t i o n a t 3 m o f : ( A ) s a l i n i t y , (B) t e m p e r a t u r e , (C) t h e s u r f a c e l a y e r d e p t h , (D) n i t r a t e , (E) s i l i c a t e , (F) c h l o r o p h y l l a, (G) POC, (H) PON, ( I ) N:C r a t i o o f p h y t o p l a n k t o n (•) and s e s t o n (°) (dashed l i n e r e p r e s e n t s t h e R e d f i e l d r a t i o ) , ( J ) p h y t o p l a n k t o n POC/CHL a r a t i o , (K) p h y t o p l a n k t o n PON/CHL a r a t i o , (L) P Bm, (M) a B , and (N) I * on a h o r i z o n t a l t r a n s e c t a c r o s s t h e f r o n t a l a r e a o f t h e x v i i i Gasp<§ C u r r e n t i n t h e G u l f o f S t . Lawrence (see F i g . 1.12 f o r p o s i t i o n of s t a t i o n s 0 t o 24) 62-63 F i g u r e 1.15. C r u i s e C - V e r t i c a l d i s t r i b u t i o n o f : (A) s a l i n i t y , (B) t e m p e r a t u r e , (C) n i t r a t e , (D) s i l i c a t e , (E) c h l o r o p h y l l a, (F) s e s t o n N:C r a t i o and (G) p h y t o p l a n k t o n N:C r a t i o d u r i n g 24 h a t a f i x e d s t a t i o n l o c a t e d i n t h e f r o n t (see F i g . 1.12 f o r p o s i t i o n o f s t a t i o n CI) 69-70 F i g u r e 1.16. P o s i t i o n s of t h e s t a t i o n s a l o n g t h e h o r i z o n t a l t r a n s e c t s (numbers, 1 t o 24) and a t w h i c h v e r t i c a l p r o f i l e s were c o n d u c t e d ( c r u i s e l e t t e r / n u m b e r s , D l t o D l l ) d u r i n g c r u i s e D 74 F i g u r e 1.17. C r u i s e D - H o r i z o n t a l d i s t r i b u t i o n a t 3 m o f : (A) s a l i n i t y , (B) t e m p e r a t u r e , (C) n i t r a t e , (D) s i l i c a t e , (E) c h l o r o p h y l l a, (F) POC, (G) PON and (H) N:C r a t i o o f s e s t o n (dashed l i n e r e p r e s e n t s t h e R e d f i e l d r a t i o ) on a t r a n s e c t a c r o s s t h e G y r e , t h e f r o n t and t h e Gasp£ C u r r e n t (G.C.) i n t h e G u l f of S t . Lawrence (see F i g . 1.16 f o r p o s i t i o n o f s t a t i o n s 1 t o 24) 75 F i g u r e 1.18. C r u i s e D - V e r t i c a l d i s t r i b u t i o n o f : (A) s a l i n i t y , (B) t e m p e r a t u r e , (C) n i t r a t e , (D) s i l i c a t e , (E) c h l o r o p h y l l a and (F) N:C r a t i o o f s e s t o n on a x i x t r a n s e c t a c r o s s the Gaspe Cu r r e n t (DI to D7), the f r o n t (D8 to D9) and the Gyre (D10 t o D l l ) i n the G u l f o f St. Lawrence (see F i g . 1.16 f o r p o s i t i o n of s t a t i o n s DI t o D l l ) 80-81 F i g u r e 1.19. Temperature vs s a l i n i t y r e l a t i o n s h i p s a t 3 m d u r i n g the t r a n s e c t s conducted d u r i n g : (A) c r u i s e A, (B) c r u i s e B, (C) c r u i s e C and (D) c r u i s e D 86 F i g u r e 1.20. Schematic r e p r e s e n t a t i o n of the model of c r o s s - f r o n t a l c i r c u l a t i o n proposed by Tang (1980b) f o r the Gaspe Cu r r e n t f r o n t 88 F i g u r e 1.21. Schematic r e p r e s e n t a t i o n s of the v e r t i c a l d i s t r i b u t i o n of s a l i n i t y , n u t r i e n t s ( n i t r a t e o r s i l i c a t e ) and phytoplankton biomass a c r o s s the f r o n t a l area d u r i n g : (A) s p r i n g , (B) summer and (C) d u r i n g an u p w e l l i n g event (Hatched areas r e p r e s e n t zone of maximum c o n c e n t r a t i o n s ) 93 F i g u r e 1.22. C h l o r o p h y l l a c o n c e n t r a t i o n s vs s a l i n i t y measured a t 3 m d u r i n g the t r a n s e c t s conducted d u r i n g : (A) c r u i s e A and (B) c r u i s e C 95 F i g u r e 1.23. C h l o r o p h y l l a c o n c e n t r a t i o n s vs s a l i n i t y measured a t 3 m d u r i n g the two t r a n s e c t s a c r o s s the f r o n t conducted d u r i n g c r u i s e A 97 XX F i g u r e 1.24. (A) P h y t o p l a n k t o n N:C r a t i o s vs ambient s i l i c a t e and (B) p h y t o p l a n k t o n POC/CHL a vs ambient s i l i c a t e measured a t 3 m d u r i n g c r u i s e C 101 F i g u r e 1.25. P h y t o p l a n k t o n POC/CHL a r a t i o s vs (A) t h e mixed l a y e r d e p t h and (B) c h l o r o p h y l l a measured a t 3 m d u r i n g c r u i s e A 104 F i g u r e 1.26. P h o t o s y n t h e t i c parameter I k vs t h e mixed l a y e r d e p t h d u r i n g c r u i s e C 106 F i g u r e 2.1. Map o f t h e n o r t h w e s t e r n G u l f o f S t . Lawrence showing s u r f a c e c i r c u l a t i o n ( f r o m E l - S a b h 197 6) and l o c a t i o n o f t h e s a m p l i n g s t a t i o n s . Based on t h e s a l i n i t y d i s t r i b u t i o n , t h e t r a n s e c t was d i v i d e d i n t o t h r e e p a r t s : (1) t h e Gasp6 C u r r e n t ( s t a t i o n s 6, 7 and 8 ) ; (2) t h e f r d n t ( s t a t i o n s 2, 3, 4 and 5 ) ; and (3) t h e G u l f Gyre ( s t a t i o n s 1 and 1') 117 F i g u r e 2.2. V e r t i c a l d i s t r i b u t i o n o f (A) s a l i n i t y and (B) t e m p e r a t u r e on a t r a n s e c t a c r o s s t h e f r o n t a l a r e a o f t h e Gaspe C u r r e n t i n t h e G u l f o f S t . Lawrence.. 123 F i g u r e 2.3. V e r t i c a l d i s t r i b u t i o n s o f (A) c h l o r o p h y l l a, and o f c e l l abundance ( 1 0 s c e l l s . I - 1 ) of t h e dominant p h y t o p l a n k t o n s p e c i e s , (B) Chaetoceros debilis, x x i (C) Thalassiosira gravida, (D) Skeletonema costatum, and (E) Chaetoceros pelagicus on t h e t r a n s e c t a c r o s s t h e Gaspe C u r r e n t i n t h e G u l f of S t . Lawrence. The shaded a r e a s i n d i c a t e t h e h i g h e s t c o n c e n t r a t i o n s . . . 124 F i g u r e 2.4. V e r t i c a l d i s t r i b u t i o n s o f (A) u n i d e n t i f i e d f l a g e l l a t e s , (B) Leucocryptos marina and (C) Monosiga sp. on t h e t r a n s e c t a c r o s s t h e Gasp6 C u r r e n t i n t h e G u l f o f S t . Lawrence. The shaded a r e a s i n d i c a t e t h e h i g h e s t c o n c e n t r a t i o n s . . . 126 F i g u r e 2.5. V e r t i c a l d i s t r i b u t i o n o f (A) n i t r a t e , (B) ammonium, (C) u r e a and (D) s i l i c a t e a c r o s s t h e f r o n t . The shaded a r e a s i n d i c a t e t h e l o w e s t c o n c e n t r a t i o n s . . . . 127 F i g u r e 2.6. V e r t i c a l d i s t r i b u t i o n o f p l a n k t o n i n t e r n a l n u t r i e n t p o o l c o n c e n t r a t i o n s a c r o s s t h e f r o n t : (A) n i t r a t e ( I N - N 0 3 ) , (B) ammonium ( I N - N H 4 ) , (C) u r e a ( I N - u r e a ) , and (D) s i l i c a t e ( I N - S i 0 4 ) . The shaded a r e a s i n d i c a t e t h e l o w e s t c o n c e n t r a t i o n s 130 F i g u r e 2.7. (A) i n t e r n a l n i t r a t e p o o l c o n c e n t r a t i o n s vs ambient n i t r a t e c o n c e n t r a t i o n s f o r a l l s t a t i o n s (0-12 m), and (B) i n t e r n a l s i l i c a t e p o o l c o n c e n t r a t i o n s vs ambient s i l i c a t e c o n c e n t r a t i o n s f o r a l l s t a t i o n s (0-3 m) ( s t a t i o n 1 = o) 132 x x i i F i g u r e 2.8. V e r t i c a l d i s t r i b u t i o n o f t h e p a r t i c u l a t e n i t r o g e n : c a r b o n r a t i o (by atoms) o f s e s t o n a c r o s s t h e f r o n t 137 F i g u r e 2.9. N:C r a t i o s of s e s t o n vs A: t o t a l ambient n o r g a n i c n i t r o g e n ; B: t o t a l ambient i n o r g a n i c i t r o g e n + u r e a ; and C: ambient s i l i c a t e . O n l y a l u e s between 0 and 3 m a r e p r e s e n t e d 138 F i g u r e 2.10. S i n k i n g r a t e o f Chaetoceros debilis vs ambient (•) and i n t e r n a l (°) s i l i c a t e (A) and n i t r a t e (B) i n s u r f a c e (0-1 m) w a t e r s 140 F i g u r e 2.11. S i n k i n g r a t e o f Thalassioslra gravida vs ambient (•) and i n t e r n a l (°) s i l i c a t e (A) and n i t r a t e (B) i n s u r f a c e (0-1 m) w a t e r s 141 F i g u r e 2.12. S i n k i n g r a t e o f Skeletonema costatum vs ambient (•) and i n t e r n a l (°) s i l i c a t e (A) and n i t r a t e (B) i n s u r f a c e (0-1 m) w a t e r s 142 F i g u r e 2.13. S i n k i n g r a t e o f Chaetoceros pelagicus vs ambient (•) and i n t e r n a l (°) s i l i c a t e (A) and n i t r a t e (B) i n s u r f a c e (0-1 m) w a t e r s 143 F i g u r e 2.14. s i l i c a t e N:C r a t i o s of p h y t o p l a n k t o n vs ambient i n t h e t o p 3 m o f t h e w a t e r column x x i i i a c r o s s the f r o n t . Dotted l i n e s r e p r e s e n t the N:C r a t i o f o r Skeletonema costatum (S.k.), Chaetoceros debills (Cd.) and Thalassloslra gravida (T.g.) when n u t r i e n t - s u f f i c i e n t 154 F i g u r e 3.1. S i z e c l a s s frequency of the major f i s h l a r v a e c o l l e c t e d d u r i n g c r u i s e A (empty bar) and d u r i n g c r u i s e B ( f i l l e d bar) 164 F i g u r e 3.2. C r u i s e A - H o r i z o n t a l d i s t r i b u t i o n of s a l i n i t y ( 3 m ) , c h l o r o p h y l l a ( 3 m ) , and c o n c e n t r a t i o n s of copepod eggs (0-20 m), n a u p l i i (0-20 m), copepodites and s m a l l (< 4 mm) copepods (0-20 m) and 203-500 u.m p a r t i c l e s (0-20 m) on a h o r i z o n t a l t r a n s e c t a c r o s s the f r o n t a l area of the Gasp6 C u r r e n t i n the G u l f of S t . Lawrence (see F i g . 1.2 f o r p o s i t i o n of s t a t i o n s 0-360) 169 F i g u r e 3.3. Copepod eggs, n a u p l i i and copepodite abundances vs c o n c e n t r a t i o n of 203-500 um p a r t i c l e s measured between 0 and 20 m i n t r a n s e c t A (A) and B (B) 171 F i g u r e 3.4. Abundance of p a r t i c l e s 203-500 um (0-20 m) vs (A) f l u o r e s c e n c e (3 m) and (B) s a l i n i t y (3 m) d u r i n g the h o r i z o n t a l t r a n s e c t conducted d u r i n g c r u i s e A 172 F i g u r e 3.5. C r u i s e B - H o r i z o n t a l d i s t r i b u t i o n of s a l i n i t y ( 3 m ) , c h l o r o p h y l l a ( 3 m ) , and c o n c e n t r a t i o n s of copepod eggs (0-20 m), n a u p l i i (0-20 m), copepodites and s m a l l (< 4 mm) copepods (0-20 m) and 203-500 nm p a r t i c l e s (0-20 m) on a h o r i z o n t a l t r a n s e c t a c r o s s the f r o n t a l area of the Gasp6 Cu r r e n t i n the G u l f of S t . Lawrence (see F i g . 1.8 f o r p o s i t i o n of s t a t i o n s 1 t o 180) F i g u r e 3.6. P a r t i c l e s 203-500 um (0-20 m) vs (A) f l u o r e s c e n c e (3 m) and (B) s a l i n i t y (3 m) d u r i n g the h o r i z o n t a l t r a n s e c t conducted d u r i n g c r u i s e B F i g u r e 3.7. C r u i s e A - H o r i z o n t a l d i s t r i b u t i o n of s a l i n i t y ( 3 m ) , sand l a n c e (Ammodytes hexapterus) l a r v a e (0-20 m) and t h e i r r e s o u r c e , c a p e l i n (Mallotus villosus) l a r v a e (0-20 m) and t h e i r r e s o u r c e , r e d f i s h (Sebastes spp.) l a r v a e (0-20 m) and t h e i r r e s o u r c e , and A r c t i c shanny (Stichaeus punctatus) l a r v a e (0-20 m) on a h o r i z o n t a l t r a n s e c t a c r o s s the f r o n t a l area of the Gaspe C u r r e n t i n the G u l f of S t . Lawrence (see F i g . 1.2 f o r p o s i t i o n of s t a t i o n s 0-360) F i g u r e 3.8. R e l a t i o n s h i p between s a l i n i t y a t 3 m and the abundance of sand l a n c e (Ammodytes hexapterus), c a p e l i n (Mallotus villosus) and r e d f i s h (Sebastes spp.) l a r v a e c o l l e c t e d between 0 and 20 m on a h o r i z o n t a l t r a n s e c t a c r o s s t h e f r o n t a l a r e a o f t h e Gaspe C u r r e n t i n t h e G u l f o f S t . Lawrence F i g u r e 3.9. R e l a t i o n s h i p between s a l i n i t y and d i f f e r e n t s i z e c l a s s e s o f sand l a n c e l a r v a e d u r i n g c r u i s e A.... F i g u r e 3.10. C r u i s e B - H o r i z o n t a l d i s t r i b u t i o n o f s a l i n i t y (3 in) , sand l a n c e (Ammodytes hexapterus) l a r v a e (0-20 m) and t h e i r r e s o u r c e , c a p e l i n (Mallotus villosus) l a r v a e (0-20 m) and t h e i r r e s o u r c e , r e d f i s h (Sebastes spp.) l a r v a e (0-20 m) and t h e i r r e s o u r c e , and A t l a n t i c h e r r i n g (Clupea harengus) l a r v a e (0-20 m) on a h o r i z o n t a l t r a n s e c t a c r o s s t h e f r o n t a l a r e a o f t h e Gaspe C u r r e n t i n t h e G u l f o f S t . Lawrence (see F i g . 1.8 f o r p o s i t i o n o f s t a t i o n s 0-180) F i g u r e 3.11. R e l a t i o n s h i p between s a l i n i t y and d i f f e r e n t s i z e c l a s s e s o f c a p e l i n l a r v a e d u r i n g c r u i s e B F i g u r e 4.1. L o c a t i o n s of f i s h l a r v a e r e t e n t i o n zones a s s o c i a t e d w i t h f r o n t a l systems i n t h e S t . Lawrence e s t u a r y and G u l f o f S t . Lawrence. Zone 1: r e t e n t i o n o f r a i n b o w s m e l t and tomcod l a r v a e a s s o c i a t e d w i t h a t u r b i d i t y f r o n t ; zone 2: r e t e n t i o n of A t l a n t i c h e r r i n g l a r v a e a s s o c i a t e d w i t h a t i d a l f r o n t ; zone 3: r e t e n t i o n o f sand l a n c e and c a p e l i n l a r v a e a s s o c i a t e d w i t h a c o a s t a l j e t f r o n t g u r e 4 . 2 . C r o s s - s e c t i o n showing t h e a p p r o x i m a t e l o c a t i o n o f t h e t h r e e main l a y e r s f o r m i n g t h e w a t e r column i n summer ( A ) , and v e r t i c a l d i s t r i b u t i o n o f n i t r a t e (B) and s i l i c a t e (C) a l o n g a t r a n s e c t e x t e n d i n g from t h e head o f t h e L a u r e n t i a n Channel t o Cabot S t r a i t i n t h e S t . Lawrence (adapted from Coote and Y e a t s 19 79) x x v i i ACKNOWLEDGEMENTS I s i n c e r e l y acknowledge my supervisor, Professor P.J. Harrison, for his support during t h i s study. Working under his supervision and within his group was a stimulating and unforgetable experience. I wish also to thank the other members of my supervisory committee, Professors T.R. Parsons, F.J.R Taylor and S. Pond for t h e i r guidance during t h i s work. I thank Drs. L. F o r t i e r and J.-C. T h e r r i a u l t who were involved i n t h i s project from the very beginning and p a r t i c i p a t e d i n a l l phases of i t s r e a l i z a t i o n . I am s p e c i a l l y indebted to Dr. L. F o r t i e r who shared with me his expertise on f i s h l a r v a l ecology and provided the indispensable technical support I needed for the ichthyoplankton part of t h i s project. I am also g r a t e f u l to Professor B. Heimdal for her taxonomic expertise and contribution to part of t h i s study and to Dr. Q. Dortch for improvements to e a r l i e r d r a f t s of Chapter 2. I also s i n c e r e l y appreciated the discussions I had with Drs. Y. de Lafontaine and J. Runge. Discussions with my U.B.C. colleagues P.A. Thompson, W.P. Cochlan, G.J. Doucette, K. Yin, A. Waite, M. St. John and P. C l i f f o r d contributed s i g n i f i c a n t l y to my thesis and other research. I thank Drs. C.L. Tang, P.A. Yeats and M. El-Sabh for allowing me to use some of t h e i r data i n my t h e s i s . Many people a s s i s t e d me i n c o l l e c t i n g and processing the f i e l d data. I s i n c e r e l y thank the following persons from 1' I n s t i t u t Maurice Lamontagne (Fisheries and Oceans Canada) for t h e i r excellent technical support: E. Bonneau, S. Cantin, J.-Y. Couture, A. Gagn6, R. Gagnon, P. J o l i , R. Pigeon, G. Lobb, J . Pauz§ and A. Ducharme. I also appreciated the help of the following students: S. Gosselin, R. Drolet, M. Provencher, M.-J. Martineau, M. Bolduc, S. Toutant, M. Gosselin and K. Hebert. I have a s p e c i a l thought for L. Caron. I wish to thank the captains and crew members of the vessels L.M. Lauzier and Pet r e l V for t h e i r patience and supportive a t t i t u d e . Moreover, I wish to thank my wife Line P e l l e t i e r f or her support a l l through t h i s project. I also thank my sons Francis and David who supported me, i n t h e i r own way. I am also g r a t e f u l to my mother Al i n e B. Levasseur and parents-in-law, Therese and Bernard P e l l e t i e r , f or t h e i r f i n a n c i a l support. I acknowledge Fisheries and Oceans Canada for allowing me to complete t h i s degree and for t h e i r f i n a n c i a l assistance. In that respect, I s i n c e r e l y thank Dr. J.-C. T h e r r i a u l t , Dr. J. Boulva and Mr. D. Martin who made t h i s study leave possible for me. 1 General introduction Recent reviews have suggested that there are two d i s t i n c t planktonic food chains i n the oceans: a long food chain (the microbial loop) and a short food chain (the t r a d i t i o n a l or l i n e a r food chain) (Goldman 1988, Legendre and Le Fevre 1989, Cushing 1989). In the microbial lodp, small phytoplankton c e l l s (< 5 um) are exploited by several trophic l e v e l s (heterotrophic bacteria, protozoa, appendicularians). Most of the primary production i s respired within the euphotic zone and l i t t l e energy i s exported to large grazers such as copepods. In the t r a d i t i o n a l food chain, the production of large phytoplankton c e l l s (> 5 um) i s exportable from the euphotic zone, e i t h e r through sinking or grazing by large herbivores such as herbivorous copepods. Export production leading to the t r a d i t i o n a l food chain takes place p r i m a r i l y i n upwelling areas, t i d a l or density fronts and newly formed pycnoclines (Legendre and Le Fevre 1989). Massive production of herbivores, i n p a r t i c u l a r copepods, i s expected to occur when export production p e r s i s t s or when the large c e l l s are prevented from sinking to the bottom by mixing, upwelling or density gradients. Since copepod eggs and n a u p l i i are the main prey of a majority of po s t l a r v a l f i s h (e.g. Last 1980, Turner 1984), the spawning of dominant f i s h species i s expected to be 2 a s s o c i a t e d w i t h h y d r o g r a p h i c f e a t u r e s where a s h o r t f o o d c h a i n d e v e l o p s and copepods r e p r o d u c e p r o l i f i c a l l y . C u s h i n g (1989) s u g g e s t e d t h a t t h e g r e a t f i s h e r i e s o f t h e w o r l d a r e based on t h e s e h y d r o g r a p h i c f e a t u r e s . F r o n t a l a r e a s l o c a t e d between d i s t i n c t w a t e r masses r e p r e s e n t such f e a t u r e s . D u r i n g t h e l a s t decade, c o n s i d e r a b l e e f f o r t has been d i r e c t e d t o w a r d t h e s p a t i o - t e m p o r a l l o c a t i o n o f f r o n t a l a r e a s and t h e i r i n f l u e n c e on t h e d i s t r i b u t i o n o f c h e m i c a l and b i o l o g i c a l v a r i a b l e s (see r e v i e w s by H o l l i g a n 1981, L o d e r and P i a t t 1984, Le F e v r e 1986, Legendre et al. 1986). I n g e n e r a l , t h e s e s t u d i e s i n d i c a t e t h a t f r o n t s a r e o f t e n a r e a s o f h i g h p h y t o p l a n k t o n biomass and, o c c a s i o n a l l y , h i g h c o n c e n t r a t i o n s of z o o p l a n k t o n and f i s h l a r v a e . Our u n d e r s t a n d i n g of t h e mechanisms r e s p o n s i b l e f o r t h e enhancement i n p r i m a r y and s e c o n d a r y p r o d u c t i v i t y i s f a r from c o m p l e t e . F o r example, i t i s o f t e n u n c l e a r whether t h e a c c u m u l a t i o n o f biomass r e s u l t e d from t h e c r o s s - f r o n t a l c i r c u l a t i o n ( e . g . c o n v e r g e n c e , eddy m o t i o n s ) o r from a l o c a l improvement i n growth and/or s u r v i v a l r a t e s . There i s a l s o a p a r t i c u l a r l a c k o f d a t a i n d i c a t i n g t h e i m p o r t a n c e o f r i v e r i n e and e s t u a r i n e plume f r o n t s f o r p l a n k t o n i c f o o d c h a i n s . 3 Frontal area - Definition F r o n t a l a r e a i s a g e n e r a l t e rm encompassing many t y p e s o f i n t e r f a c e s . Denman and P o w e l l (1984) d e f i n e a f r o n t a l a r e a as a d i s c o n t i n u i t y i n t h e h o r i z o n t a l d i s t r i b u t i o n o f w a t e r mass p r o p e r t i e s on t h e s c a l e o f o b s e r v a t i o n . C o n v e n i e n t l y , t h e y may be c l a s s i f i e d i n t o s i x c a t e g o r i e s (Bowman and E s a i a s 1978): 1) f r o n t s o f p l a n e t a r y s c a l e , u s u a l l y a s s o c i a t e d w i t h t h e s u r f a c e Ekman t r a n s p o r t , 2) f r o n t s l o c a t e d a t t h e edge of major w e s t e r n boundary c u r r e n t s ( e . g . G u l f Stream, K u r o s h i o ) , 3) s h e l f b r e a k f r o n t s , formed a t t h e boundary of t h e s h e l f and s l o p e w a t e r , 4) u p w e l l i n g f r o n t s , formed a t t h e boundary o f u p w e l l e d w a t e r , 5) s h a l l o w sea f r o n t s ( o r t i d a l f r o n t s ) , formed a t t h e boundary of s h a l l o w t i d a l l y mixed n e a r - s h o r e w a t e r and s t r a t i f i e d , d e e p e r , o f f s h o r e w a t e r , and 6) r i v e r i n e and e s t u a r i n e plume f r o n t s , w h i c h a r e formed a t t h e edge o f r i v e r and e s t u a r i n e plumes. P h y s i c a l l y , f r o n t s b e l o n g i n g t o t h e f i r s t f i v e c a t e g o r i e s a r e g e n e r a l l y c h a r a c t e r i z e d by t h e r m a l g r a d i e n t s , w h i l e plume f r o n t s ( c a t e g o r y 6) a r e c h a r a c t e r i z e d by s t r o n g s a l i n i t y g r a d i e n t s . Frontal area - Primary production To d a t e , most o f t h e s t u d i e s on p h y t o p l a n k t o n have f o c u s e d on s h a l l o w sea f r o n t s ( t i d a l f r o n t s , c a t e g o r y 5) i n c o a s t a l w a t e r s c h a r a c t e r i z e d by s t r o n g t i d a l c u r r e n t s 4 ( P i n g r e e et al. 1974, Simpson and Hunter 1974, Bowman et al. 1983, Parsons et al. 1981, 1983, F o u r n i e r et al. 1984, H o l l i g a n et al. 1984a, 1984b, Savidge 1976, Savidge et al. 1984, R i c h a r d s o n et al. 1985). In t h i s case, a f r o n t a l zone i s formed between the v e r t i c a l l y s t a b l e ( g e n e r a l l y n u t r i e n t -poor) o f f s h o r e water and the v e r t i c a l l y mixed ( g e n e r a l l y n u t r i e n t - r i c h ) c o a s t a l water where f r i c t i o n produced from t i d a l c u r r e n t s on the bottom ( d e s t a b i l i z i n g f o r c e ) overcomes the s t a b i l i z i n g e f f e c t of s o l a r h e a t i n g . F o l l o w i n g t h i s model, the phytoplankton community l o c a t e d on the s t a b l e s i d e of the f r o n t i s n u t r i e n t - l i m i t e d w h i l e the community l o c a t e d i n the mixed s i d e e x p e r i e n c e s a low l i g h t regime. The t i d a l i n j e c t i o n of n u t r i e n t - r i c h water from the mixed s i d e i n t o the n u t r i e n t - p o o r s i d e r e s u l t s i n the f o r m a t i o n of a t r a n s i t i o n zone were n e i t h e r l i g h t nor n u t r i e n t s l i m i t p h y t o p l a n k t o n growth. In some cases, a d j a c e n t water masses may a l s o complement each o t h e r ' s n u t r i e n t d e f i c i e n c y (Savidge 1976, B e a r d a l l et al. 1982). However, t h e r e i s i n c r e a s i n g evidence t h a t a one-dimensional model as d e s c r i b e d above cannot f u l l y account f o r the p l a n k t o n c r o s s -f r o n t a l d i s t r i b u t i o n (Loder and P i a t t 1984, Le Fevre 1986) and t h a t c i r c u l a t i o n i s a l s o important. Accumulations of p l a n k t o n are o f t e n a s s o c i a t e d w i t h convergence zones which are t y p i c a l of f r o n t a l systems. An i n c r e a s e i n n u t r i e n t c o n c e n t r a t i o n s and p r i m a r y p r o d u c t i o n has a l s o been r e p o r t e d i n s h e l f break f r o n t s 5 (c a t e g o r y 3) o f f Nova S c o t i a ( F o u r n i e r et al. 1977, 1979), i n the C e l t i c Sea (Pingree 1979) and i n the Southeastern B e r i n g Sea (Ive r s o n et al. 1979). In t h i s type of f r o n t , the i n t e r a c t i o n between s u r f a c e wind s t r e s s and i n t e r n a l t i d e s r e s u l t s i n l o c a l l y enhanced v e r t i c a l mixing. T h i s r e s u l t s i n h i g h e r n u t r i e n t c o n c e n t r a t i o n s and phyt o p l a n k t o n p r o d u c t i v i t y . F i n a l l y , Yamamoto et al. (1988) a l s o observed t h a t the phy t o p l a n k t o n community l o c a t e d i n the Kuro s h i o C u r r e n t f r o n t ( c a t e g o r y 2) had a h i g h e r growth p o t e n t i a l than i n the su r r o u n d i n g a r e a s . Frontal area - Secondary and tertiary production In the marine environment, the p r o d u c t i o n of many comme r c i a l l y e x p l o i t e d f i s h are b e l i e v e d t o depend mai n l y on the s o - c a l l e d l i n e a r (or s h o r t ) food c h a i n (Cushing 1989). F o l l o w i n g t h i s scheme, v e r t i c a l s t a b i l i z a t i o n of n u t r i e n t -r i c h water promotes the growth of l a r g e p h y t o p l a n k t o n c e l l s (mainly diatoms) which c o n s t i t u t e s the bulk of the new (sensu Dugdale and Goering 1967) or e x p o r t a b l e (sensu Legendre and Le Fevre 1989) p r o d u c t i o n . High phytoplankton biomass promotes the r e p r o d u c t i o n and growth of h e r b i v o r o u s zooplankton (Runge 1984, 1985, Kiorboe and Johansen 1986) which, i n t u r n , c o n s t i t u t e s the p r i n c i p a l food of s e v e r a l p l a n k t o n i c f i s h l a r v a e (Richardson et al. 1986). For 6 copepods and p l a n k t o n i c f i s h l a r v a e , f r o n t a l areas may thus r e p r e s e n t advantageous environments. S i n c e p h y s i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s of f r o n t s v a r y w i t h time and space, the r e p r o d u c t i o n s t r a t e g i e s of f i s h e s may i n c l u d e a response t o these temporal and s p a t i a l s c a l e s of v a r i a b i l i t y . A survey of the l i t e r a t u r e i n d i c a t e s t h a t f r o n t a l areas may o r may not be a s s o c i a t e d w i t h an i n c r e a s e i n zooplankton biomass and/or p r o d u c t i v i t y . C h r i s t e n s e n et al. (1985) and Ki<|>rboe et al. (1988) r e p o r t e d an i n c r e a s e i n copepod egg p r o d u c t i o n r a t e s near a thermal f r o n t o f f the S c o t t i s h Coast and i n the North Sea r e s p e c t i v e l y . S i m i l a r l y , Ki<i>rboe and Johansen (1986) a l s o found h i g h e r copepod abundance and egg l a y i n g r a t e s i n a thermal f r o n t l o c a t e d i n the Buchan a r e a . During a second c r o s s i n g of the f r o n t , however, they found no i n c r e a s e i n copepod abundance a t the f r o n t . H o l l i g a n et al. (1984a, b) a l s o observed no i n c r e a s e i n copepod abundance i n a thermal f r o n t l o c a t e d i n the E n g l i s h Channel. The few s t u d i e s t h a t have c o n s i d e r e d zooplankton d i s t r i b u t i o n i n r e l a t i o n t o s a l i n i t y f r o n t s are e q u i v o c a l . Kahru e t al. (1986) r e p o r t e d zooplankton biomass t o peak w e l l away from a s a l i n i t y f r o n t i n the B a l t i c . On the o t h e r hand, h i g h c o n c e n t r a t i o n s of copepods have been measured i n a r i v e r i n e plume f r o n t (Krause e t al. 1986, Kahru et al. 1986). In these cases, the c o i n c i d e n c e of these maxima w i t h convergence zones suggests t h a t the accumulation was m a i n l y 7 under p h y s i c a l c o n t r o l . H i g h n a u p l i i and copepod c o n c e n t r a t i o n s have a l s o been measured i n r i v e r i n e f r o n t s l o c a t e d i n L i v e r p o o l Bay ( F l o o d g a t e e t al. 1981) and i n t h e N o r t h Sea ( R i c h a r d s o n 1985). As s t r e s s e d by R i c h a r d s o n e t al. ( 1 9 8 6 ) , i t seems l i k e l y t h a t t h e d u r a t i o n o f t h e o c c u r r e n c e o f e l e v a t e d p h y t o p l a n k t o n biomass i n t h e f r o n t a l r e g i o n w i l l d i c t a t e t h e i m p o r t a n c e t h a t p a t c h has t o s e c o n d a r y p r o d u c t i o n . The s p e c i f i c c o m p o s i t i o n o f t h e p h y t o p l a n k t o n community appears a l s o t o be i m p o r t a n t . H o l l i g a n e t al. (1984a, b) a t t r i b u t e d t h e low copepod c o n c e n t r a t i o n s i n t h e f r o n t t o t h e p r e s e n c e o f d i n o f l a g e l l a t e s p e c i e s i n s t e a d o f d i a t o m s . H i g h abundance o f p l a n k t o n i c l a r v a e o f f i s h have a l s o been o b s e r v e d i n f r o n t s (Munk e t al. 1986, Sakamoto and Tanaka 1986). F o r example, R i c h a r d s o n e t al. (1986) have d e m o n s t r a t e d a d i r e c t a s s o c i a t i o n between t h e d i s t r i b u t i o n o f h e r r i n g l a r v a e and a t h e r m a l f r o n t o f f t h e n o r t h e a s t c o a s t o f S c o t l a n d . They r e p o r t e d t h a t a r e a s o f h i g h e s t h e r r i n g abundance c o i n c i d e d w i t h a r e a s o f h i g h e s t s p e c i f i c z o o p l a n k t o n p r o d u c t i v i t y . H i g h c o n c e n t r a t i o n s o f f i s h l a r v a e have been a l s o r e p o r t e d i n t h e M i s s i s s i p p i R i v e r plume f r o n t (Govoni e t al. 1989) . Whether t h e g r e a t e r abundance o f f i s h l a r v a e i n t h e f r o n t per se r e s u l t s from a r e p r o d u c t i o n s t r a t e g y p r o m o t i n g t h e i r p a s s i v e t r a n s p o r t i n t h e f r o n t , o r from a b e t t e r s u r v i v a l o f l a r v a e e n d i n g up i n t h e f r o n t , remains u n c l e a r . 8 I n summary, t h e s e r e s u l t s i n d i c a t e t h a t f r o n t a l a r e a s may be o f prime i m p o r t a n c e f o r b i o l o g i c a l p r o d u c t i o n i n t h e mar i n e e n v i r o n m e n t . I t appears t h a t t h e i r e c o l o g i c a l i m p o r t a n c e f o r p r i m a r y p r o d u c t i o n and t h e h i g h e r t r o p h i c l e v e l s w i l l depend on t h e i n t e r a c t i o n between t h e i r s p a t i o -t e m p o r a l c h a r a c t e r i s t i c s and t h e r e s p o n s e t i m e o f t h e b i o l o g i c a l p r o c e s s e s (e.g. growth r a t e , r e p r o d u c t i o n r a t e , m i g r a t i o n ) . I n a d d i t i o n , t h e s t r e n g t h o f t h e l i n k between p r i m a r y p r o d u c t i o n and copepod and f i s h p r o d u c t i o n appears t o be r e l a t e d t o t h e t y p e of p h y t o p l a n k t o n ( H o l l i g a n et al. 1984a, b) and copepod community (Runge 1988). Objectives I n t h e i n t r o d u c t i o n , I have a t t e m p t e d t o o u t l i n e t h e d i v e r s i t y o f t h e f r o n t a l a r e a s and t h e i r p o t e n t i a l i m p o r t a n c e f o r t h e e n t i r e m a rine f o o d c h a i n . W h i l e some t y p e s o f f r o n t s have been e x t e n s i v e l y s t u d i e d (e.g. t i d a l and s h e l f b r eak f r o n t s ) , o t h e r s a r e l e s s w e l l documented. E s t u a r i n e plume f r o n t s a r e among t h e l a s t c a t e g o r y . T h i s t h e s i s a d d r e s s e s t h e f o l l o w i n g t h r e e o b j e c t i v e s : 1. To d e s c r i b e t h e v e r t i c a l and h o r i z o n t a l d i s t r i b u t i o n o f n u t r i e n t s and p h y t o p l a n k t o n i n an e s t u a r i n e plume f r o n t i n t h e G u l f o f S t . Lawrence. 9 2. To p r o v i d e a b e t t e r u n d e r s t a n d i n g o f t h e p h y s i o l o g i c a l r e s p o n s e s of t h e p h y t o p l a n k t o n community t o c r o s s - f r o n t a l c i r c u l a t i o n . 3. To a s s e s s t h e i m p o r t a n c e o f e s t u a r i n e plume f r o n t s f o r p l a n k t o n i c f i s h l a r v a e . The e s t u a r i n e plume f r o n t i n t h i s s t u d y i s l o c a t e d i n t h e n o r t h w e s t e r n p o r t i o n of t h e G u l f o f S t . Lawrence. A c o m p l e t e d e s c r i p t i o n o f t h e s t u d y a r e a i s g i v e n below. The main o b j e c t i v e o f t h i s r e s e a r c h was t o i n c r e a s e o u r knowledge of t h e i n f l u e n c e o f t h i s e s t u a r i n e plume f r o n t on n u t r i e n t and p h y t o p l a n k t o n d i s t r i b u t i o n s . To t h i s end, t h e s m a l l s c a l e h o r i z o n t a l (ca. 0.25 km) and v e r t i c a l ( c a . 2 m) d i s t r i b u t i o n o f t h e major i n o r g a n i c n u t r i e n t s ( n i t r a t e and s i l i c a t e ) and o f p h y t o p l a n k t o n community c o m p o s i t i o n and biomass were d e t e r m i n e d a c r o s s t h e f r o n t ( C h a p t e r s 1 and 2 ) . S i n c e t h e i n f l u e n c e of t h e f r o n t a l c i r c u l a t i o n i s l i k e l y t o v a r y d u r i n g t h e s e a s o n , t h e f r o n t a l a r e a was sampled s e v e r a l t i m e s d u r i n g s p r i n g and summer months and d u r i n g d i f f e r e n t y e a r s (1985, 1986 and 1987). The second main g o a l was t o p r o v i d e a b e t t e r u n d e r s t a n d i n g o f t h e i n f l u e n c e o f t h e c r o s s - f r o n t a l c i r c u l a t i o n upon t h e p h y s i o l o g i c a l s t a t u s o f t h e p h y t o p l a n k t o n community. F r o n t a l a r e a s a r e g e n e r a l l y c h a r a c t e r i z e d by s p e c i f i c n u t r i e n t c o n c e n t r a t i o n s and l i g h t r e g imes w h i c h f a v o r p h y t o p l a n k t o n growth. Depending on t h e 10 s p a t i o - t e m p o r a l s c a l e o f v a r i a b i l i t y of t h e f r o n t and t h e r e s p o n s e t i m e of t h e p h y t o p l a n k t o n , t h e c r o s s - f r o n t a l m i x i n g of t h e p h y t o p l a n k t o n community s h o u l d r e s u l t i n : 1) p h y s i o l o g i c a l a d j u s t m e n t s ( l e s s t h a n 1 day o r 1 km), 2) i n c r e a s e i n biomass (more t h a n 1 day o r 1 km), and 3) community s u c c e s s i o n (more t h a n 1 week o r 10 km). These s p a t i o - t e m p o r a l s c a l e s o f p h y t o p l a n k t o n r e s p o n s e s t o e n v i r o n m e n t a l changes have been summarized by H a r r i s ( 1 9 80). In r i v e r i n e and e s t u a r i n e plume f r o n t s w i t h s h a r p b o u n d a r i e s , t h e m i x i n g of t h e p h y t o p l a n k t o n communities i n t h e f r o n t i s most l i k e l y v e r y r a p i d ( l e s s t h a n one day) and s h o u l d r e s u l t i n measurable p h y s i o l o g i c a l changes. To t e s t t h i s h y p o t h e s i s , I measured s e v e r a l p h y s i o l o g i c a l - p a r a m e t e r s a c r o s s t h e f r o n t w h i c h were s e n s i t i v e t o t h e n u t r i e n t s t a t u s and p r e v i o u s l i g h t h i s t o r y of t h e c e l l s ( C h a p t e r s 1 and 2 ) . The t h i r d o b j e c t i v e of t h i s s t u d y was t o a s s e s s t h e i m p o r t a n c e o f t h e e s t u a r i n e plume f r o n t f o r p l a n k t o n i c f i s h l a r v a e . R i v e r s and e s t u a r i e s a r e i m p o r t a n t spawning a r e a s f o r many f i s h s p e c i e s . As a consequence, h i g h c o n c e n t r a t i o n s o f f i s h eggs and p l a n k t o n i c l a r v a e a r e o f t e n found i n r i v e r i n e and e s t u a r i n e plumes. The i n f l u e n c e on f i s h l a r v a e o f f r o n t s formed a t t h e b o u n d a r i e s o f t h e s e plumes i s p o o r l y known. The S t . Lawrence e s t u a r y i s an i m p o r t a n t spawning ground f o r sand l a n c e , c a p e l i n and h e r r i n g d u r i n g s p r i n g . I n June and J u l y , sand l a n c e and c a p e l i n l a r v a e a r e found i n abundance i n t h e Gaspe C u r r e n t . 11 During two of the f o u r surveys conducted d u r i n g t h i s study, the s m a l l s c a l e d i s t r i b u t i o n of the l a r v a e and of t h e i r p r e y (microzooplankton) was determined. Study area The f r o n t a l area under study i s l o c a t e d i n the northwestern p a r t of the G u l f of S t . Lawrence (Canada), a l o n g the Gaspe P e n i n s u l a ( F i g . 1.1). The f r o n t per se i s formed a t the i n t e r f a c e between the c y c l o n i c A n t i c o s t i Gyre and the Gaspe C u r r e n t . The A n t i c o s t i Gyre i s a t y p i c a l s h a l l o w sea (ca. 300 m) where wind p l a y s a dominant r o l e i n the hydrodynamics (Tang 1979). The phytoplankton annual c y c l e i s c h a r a c t e r i z e d by an e a r l y s p r i n g diatom bloom (April-May) which reduces n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s below 1 u.M. In summer, the phytoplankton biomass i s low and the community i s dominated by s m a l l f l a g e l l a t e s (Sevigny et al. 1979). No f a l l bloom has been r e p o r t e d . The Gasp§ Current i s a s t r o n g c o a s t a l j e t f l o w i n g seaward a l o n g the Gasp6 P e n i n s u l a (Tang 1983, Be n o i t et al. 1985, El-Sabh and Be n o i t 1985). T h i s c u r r e n t appears t o be d e n s i t y d r i v e n by the bouyancy i n f l u x from the S t . Lawrence E s t u a r y (Tang 1980b), i t s water coming from the upper 25 m of the S t . Lawrence E s t u a r y . I t has a mean width of 15 km, a mean t h i c k n e s s of 30 m and i t s p o s i t i o n i s r e l a t i v e l y c o n s t a n t throughout the summer except d u r i n g o c c a s i o n a l s h o r t - l i v e d events ,(.Tang 1980a). I t i s a r e l a t i v e l y s t a b l e 12 c o a s t a l c u r r e n t w i t h a mean s u r f a c e speed of 90 cm s~x d u r i n g the summer months, r e d u c i n g t o 60 cm s - 1 toward November. In w i n t e r , i t e i t h e r becomes v e r y weak o r t o t a l l y d i s a p p e a r s (Benoit et al. 1985, El-Sabh 1976). The boundary of the Gaspe Cu r r e n t d e f i n e s a f r o n t which g e n e r a l l y runs p a r a l l e l t o the c o a s t . The f r o n t i s c h a r a c t e r i z e d by s t r o n g changes i n both s a l i n i t y ( ca. 4 °/oc,) and c u r r e n t v e l o c i t y (up t o ca. 80 cm s - 1 ) . Thus, i t may be viewed as a h y b r i d between an e s t u a r i n e plume f r o n t and a c o a s t a l j e t f r o n t . Strong mixing occurs along the boundary of the s a l i n i t y wedge, as evidenced by the i n t e r l e a v i n g s t r u c t u r e of the temperature f i e l d (Tang 1983). S a t e l l i t e p i c t u r e s and f i e l d d a t a r e v e a l s the e x i s t e n c e of a band of c o l d water al o n g the f r o n t (Tang 1980a, b, 1982, 1985). T h i s c o l d band i s b e l i e v e d t o be caused by v e r t i c a l mixing and u p w e l l i n g below the f r o n t a l area (Tang 1980b, 1983). S a t e l l i t e p i c t u r e s i n d i c a t e the average width and l e n g t h of the u p w e l l i n g (or f r o n t a l ) zone t o be 10 and 150 km r e s p e c t i v e l y ( B e n o i t e t al. 1985). 13 CHAPTER 1. INFLUENCES OF THE CROSS-FRONTAL CIRCULATION UPON THE PHYSIOLOGY AND ECOLOGY OF PHYTOPLANKTON COMMUNITIES IN THE GASPE CURRENT ESTUARINE PLUME FRONT BACKGROUND On t h e c o n t i n e n t a l s h e l f , t h e i n j e c t i o n o f f r e s h w a t e r from r i v e r r u n o f f r e s u l t s i n t h e f o r m a t i o n o f d i f f e r e n t c a t e g o r i e s ' o f f r o n t s w h i c h have been c l a s s i f i e d as r i v e r i n e plume f r o n t s and e s t u a r i n e plume f r o n t s (Bowman and I v e r s o n 1978). R i v e r i n e plume f r o n t s o c c u r a t t h e b o u n d a r i e s of r i v e r i n e f l o w d i s c h a r g i n g i n t o c o a s t a l w a t e r (Denman and P o w e l l 1984). As t h e e f f l u e n t s p r e a d s o v e r t h e c o a s t a l w a t e r , i n t e r f a c i a l f r i c t i o n r e t a r d s t h e e x p a n s i o n o f t h e plume and r e s u l t s i n a s h a r p s a l i n i t y g r a d i e n t a t t h e l e a d i n g edge. Secondary f r o n t s , c a l l e d e s t u a r i n e plume f r o n t s , a l s o form o f f s h o r e and a r e c h a r a c t e r i z e d by r e l a t i v e l y s m a l l e r s a l i n i t y g r a d i e n t s . R i v e r i n e and e s t u a r i n e plumes a r e c h a r a c t e r i z e d by s t r o n g t i d a l and b u o y a n c y - d r i v e n c u r r e n t s . S t r o n g s h e a r s t r e s s i s t h u s e x p e c t e d t o o c c u r a t t h e i r b o u n d a r i e s . T h e i r c r o s s - f r o n t a l c i r c u l a t i o n i s o f t e n complex, w i t h c o n s i d e r a b l e h o r i z o n t a l and v e r t i c a l m i x i n g , and t h e f o r m a t i o n o f convergence o r d i v e r g e n c e zones (Kahru e t al. 1986). I n t h e s e t y p e s of f r o n t s , p h y s i c a l f o r c i n g i s e x p e c t e d t o p l a y a major r o l e i n t h e d i s t r i b u t i o n o f d i s s o l v e d and p a r t i c u l a t e m a t t e r . 14 R i v e r i n e and e s t u a r i n e plume f r o n t s a r e g e n e r a l l y narrow ( l e s s t h a n few km) and o f t e n go u n d e t e c t e d i n l a r g e s c a l e o c e a n o g r a p h i c s u r v e y s . F o r t h i s r e a s o n , t h e i r i n f l u e n c e upon p h y t o p l a n k t o n p h y s i o l o g y and e c o l o g y i s p o o r l y known and t h e i r o v e r a l l s i g n i f i c a n c e t o t h e ec o s y s t e m s t i l l needs t o be e v a l u a t e d . Recent s t u d i e s c o n d u c t e d i n Chesapeake Bay ( S e l i g e r e t al. 1981) and i n t h e German B i g h t ( K r a use e t al. 1986) i n d i c a t e t h a t e s t u a r i n e plume f r o n t s may be c h a r a c t e r i z e d by a p a r t i c u l a r n u t r i e n t regime and a p h y t o p l a n k t o n community d i s t i n c t from t h a t o f t h e two a d j a c e n t w a t e r masses. However, r e p o r t s on t h e e f f e c t s o f e s t u a r i n e plume f r o n t s upon p h y t o p l a n k t o n biomass d i s t r i b u t i o n s a r e c o n f l i c t i n g . I n Cheasapeake Bay, e s t u a r i n e plume f r o n t s were e i t h e r a s s o c i a t e d w i t h i n c r e a s e d ( T y l e r and S e l i g e r 1978, 1981) o r d e c r e a s e d ( S e l i g e r e t al. 1981) p h y t o p l a n k t o n biomass. I n t h e Hudson R i v e r plume f r o n t , Bowman and I v e r s o n (1978) o b s e r v e d a r a p i d d e c r e a s e of p h y t o p l a n k t o n biomass i n t h e f r o n t p e r s e . I n e s t u a r i n e plume f r o n t s i n t h e B a l t i c Sea, maximum p h y t o p l a n k t o n c o n c e n t r a t i o n s have been r e p o r t e d t o o c c u r e i t h e r i n t h e plume o r i n t h e f r o n t p e r se (Kahru e t al. 1986). These d a t a s u g g e s t t h a t t h e p h y s i c a l , c h e m i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s o f t h e f r o n t a l a r e a may change d r a s t i c a l l y w i t h t i m e . P a r t of t h i s v a r i a b i l i t y may be a t t r i b u t e d t o s e a s o n a l (due t o f r e s h w a t e r r u n o f f v a r i a t i o n s ) o r t i d a l v a r i a t i o n s i n s h e a r s t r e s s and m i x i n g i n t e n s i t y a c r o s s t h e f r o n t . The a c t u a l mechanisms r e s p o n s i b l e f o r c r o s s - f r o n t a l c i r c u l a t i o n and t h e i r i m p a ct on t h e d i s t r i b u t i o n and p h y s i o l o g y o f t h e p h y t o p l a n k t o n a r e s t i l l p o o r l y documented. I n t h e N o r t h w e s t e r n G u l f o f S t . Lawrence, a 150 km l o n g e s t u a r i n e j e t f r o n t i s formed d u r i n g s p r i n g and summer a t t h e edge o f t h e Gaspe C u r r e n t , a s t r o n g e s t u a r i n e j e t f l o w i n g e a s t w a r d a l o n g t h e s o u t h s h o r e of t h e Gaspe P e n i n s u l a . I n t h i s p a r t i c u l a r c a s e , t h e s t r o n g g r a d i e n t i n c u r r e n t v e l o c i t y a c r o s s t h e f r o n t i n i t i a t e s a complex c r o s s -c i r c u l a t i o n where m i x i n g ( v e r t i c a l and h o r i z o n t a l ) and u p w e l l i n g p l a y a major r o l e (Tang 1980, 1982, 1983, B e n o i t e t al. 1985). The g o a l o f t h i s s t u d y was t o d e t e r m i n e t h e e f f e c t o f t h e c r o s s - f r o n t a l c i r c u l a t i o n upon: 1) t h e s m a l l s c a l e d i s t r i b u t i o n of n u t r i e n t and p h y t o p l a n k t o n s p e c i e s and biomass, and 2) t h e p h y t o p l a n k t o n n u t r i e n t s t a t u s and p h o t o s y n t h e t i c c h a r a c t e r i s t i c s . 16 M A T E R I A L S AND METHODS F o u r c r u i s e s were c o n d u c t e d d u r i n g t h e y e a r s 1985 ( c r u i s e s A and B) and 1986 ( c r u i s e s C and D) , o f f Mont-L o u i s , i n t h e G u l f of S t . Lawrence ( F i g . 1.1). C r u i s e s A and C were c o n d u c t e d between May 30 and June 10, when t h e f r e s h w a t e r r u n o f f was s t i l l h i g h ( E l - S a b h 1979) and c r u i s e s B and D were c o n d u c t e d between June 20 and J u l y 30, when t h e f r e s h w a t e r r u n o f f r e a c h e d i t s summer minimum v a l u e . These two p e r i o d s a l s o c o r r e s p o n d e d r e s p e c t i v e l y t o t h e d i a t o m bloom and p o s t d i a t o m bloom p e r i o d i n t h e l o w e r e s t u a r y ( L e v a s s e u r e t al. 1984). A d e s c r i p t i o n o f each c r u i s e i s g i v e n i n T a b l e 1.1. To d e t e r m i n e t h e s m a l l - s c a l e c r o s s - f r o n t a l d i s t r i b u t i o n o f p h y t o p l a n k t o n and r e l a t e d p a r a m e t e r s , samples were c o l l e c t e d a t one d e p t h (3 m) a l o n g h o r i z o n t a l t r a n s e c t s . The d u r a t i o n o f t h e t r a n s e c t s v a r i e d from 8 t o 24 h. D u r i n g t h e t r a n s e c t , t h e s h i p was s a i l i n g a t ca. 3.5 k n o t s (6.5 km h - ! ) . Water was pumped c o n t i n u o u s l y from 3 m d e p t h and samples were c o l l e c t e d e v e r y 15 min. The f r e q u e n c y o f s a m p l i n g c o r r e s p o n d e d t o a d i s t a n c e between s t a t i o n s o f ca. 1.6 km. S a l i n i t y and t e m p e r a t u r e were c o n t i n u o u s l y r e c o r d e d w i t h an I n t e r O c e a n R Temp-Sal r e c o r d e r (model 5 4 1 ). D u r i n g c r u i s e s A and B, v e r t i c a l p r o f i l e s o f t e m p e r a t u r e were r e c o r d e d a l o n g t h e t r a n s e c t ( c a . each hour) u s i n g a XBT 17 F i g u r e 1.1. Map o f t h e n o r t h w e s t e r n G u l f o f S t . Lawrence showing s u r f a c e c i r c u l a t i o n ( f r o m E l - S a b h 1976), t h e a p p r o x i m a t e l o c a t i o n o f t h e f r o n t and t h e s t u d y a r e a (boxed a r e a ) . 18 T a b l e 1.1. C r u i s e i d e n t i f i c a t i o n , t y p e s o f s a m p l i n g ( h o r i z o n t a l and/or v e r t i c a l t r a n s e c t ) , and d a t e and t i d a l a m p l i t u d e a t w h i c h t h e c r u i s e s were c o n d u c t e d (s = s p r i n g t i d e , n = neap t i d e ) . C r u i s e Types o f s a m p l i n g Date Y e a r T i d a l a m p l i t u d e (m) A H o r i z o n t a l June 4-5 1985 -2.4 (±0.6) V e r t i c a l June 7-8 e1.7 (±0.5) B H o r i z o n t a l June 21 1985 s 1 . 8 (±0.6) V e r t i c a l June 24 "1.5 (±0.6) C H o r i z o n t a l June 3 1986 "1.4 ( ± 0 . 3 ) V - F I N R (1) June 3 "1.4 ( ± 0 . 3 ) V - F I N R (2) June 7 "2.1 V e r t i c a l June 5-6 "1.6 (±0.4) D H o r i z o n t a l J u l y 24 1986 *2.4 (±0.5) V e r t i c a l J u l y 26-27 = 1.9 ( ± 0 . 3 ) E<:L> V e r t i c a l J u l y 30 1987 -1.7 ( ± 0 . 3 ) <1> see C h a p t e r 2 f o r c r u i s e E d a t a 19 p r o b e . D u r i n g c r u i s e C, two h i g h r e s o l u t i o n v e r t i c a l c r o s s -s e c t i o n s o f t e m p e r a t u r e and s a l i n i t y were o b t a i n e d by t o w i n g a v e r t i c a l l y o s c i l l a t i n g CTD probe between 1 and 80 m (Endeco V-FIN system, model 1074). To d e t e r m i n e t h e v e r t i c a l d i s t r i b u t i o n o f n u t r i e n t s and p h y t o p l a n k t o n , v e r t i c a l p r o f i l e s were c o n d u c t e d on each s u r v e y a t s e l e c t e d s t a t i o n s a few days a f t e r t h e t r a n s e c t was c o m p l e t e d . S t a t i o n s a t w h i c h v e r t i c a l p r o f i l e s were c o n d u c t e d a r e d e s i g n a t e d by a l e t t e r ( c o r r e s p o n d i n g t o t h e c r u i s e , A t o C) and a number ( e . g . A l , A2, A3, e t c . ) . The v e r t i c a l d i s t r i b u t i o n of s a l i n i t y and t e m p e r a t u r e was f i r s t r e c o r d e d w i t h a CTD probe. N e x t , samples were c o l l e c t e d a t d i f f e r e n t d e p t h s ( u s u a l l y between 0 and 40 m) by r a i s i n g t h e i n t a k e hose o f a s u b m e r s i b l e pump from 40 m t o t h e s u r f a c e i n s t e p s o f 2 m, p a u s i n g f o r 2 min a t each i n t e r v a l t o e n s u r e t h a t t h e w a t e r c o l l e c t e d was from t h e c o r r e c t d e p t h ( c r u i s e A and C) o r by u s i n g a r o s e t t e e q u i p p e d w i t h 2.5 L N i s k i n b o t t l e s ( c r u i s e D). The v e r t i c a l s a m p l i n g r e s o l u t i o n v a r i e d f r om 2 t o 5 m, depending on t h e s a m p l i n g a p p a r a t u s (pump o r r o s e t t e ) used. W i t h t h e pump system, a c o m p l e t e v e r t i c a l s a m p l i n g t o o k ca. 50 min, w h i l e o n l y 15 min were n e c e s s a r y t o com p l e t e a v e r t i c a l p r o f i l e w i t h t h e r o s e t t e . 20 F i l t r a t i o n and chemical analyses From each water sample, a 250 ml subsample was c o l l e c t e d and f i l t e r e d through a Whatman GF/F f i l t e r f o r c h l o r o p h y l l a a n a l y s i s . Samples were e x t r a c t e d i n 90% acetone f o r 24 h, f o l l o w i n g the f l u o r o m e t r i c method of Yentsch and Menzel (1963) as m o d i f i e d by Holm-Hansen e t al. (1965). Another subsample (500 ml) was f i l t e r e d onto a combusted Whatman GF/F f i l t e r and s t o r e d f r o z e n i n a d e s i c c a t o r f o r l a t e r d e t e r m i n a t i o n of p a r t i c u l a t e o r g a n i c carbon and n i t r o g e n (POC and PON) u s i n g a P e r k i n E l m e r R e l e m e n t a l a n a l y s e r (Sharp 1974). An a d d i t i o n a l 30 ml subsample was f i x e d w i t h 1 ml of a c i d L ugol's s o l u t i o n f o r l a t e r enumeration of phytoplankton s p e c i e s and c e l l numbers u s i n g the Utermohl technique (Lund e t al. 1958). The f i l t r a t e which passed through a combusted Whatman GF/F f i l t e r and was f r o z e n f o r l a t e r d e t e r m i n a t i o n of n u t r i e n t s ( n i t r a t e , n i t r i t e and s i l i c a t e ) u s i n g a Technicon A u t o A n a l y s e r R (Parsons et al. 1984). Use of N:C r a t i o as i n d i c a t o r of nutrient d e f i c i e n c y The e l e m e n t a l composition of p a r t i c u l a t e matter has been w i d e l y used as an index of n u t r i t i o n a l s t a t u s of the phy t o p l a n k t o n community (Dortch et al. 1985, Sakshaug e t a i . 1986). A decrease i n the N:C r a t i o has been observed f o r s e v e r a l n i t r o g e n - ( M o r r i s 1981, Wheeler 1983) o r s i l i c a t e -21 l i m i t e d ( H a r r i s o n e t a l . 1977, S h i f r i n and C h i s h o l m 1981) p h y t o p l a n k t o n s p e c i e s . The s i g n i f i c a n c e o f t h i s i n d e x f o r a n a t u r a l p h y t o p l a n k t o n community i s l i m i t e d by: 1) t h e i n t e r -s p e c i e s v a r i a b i l i t y of t h e e l e m e n t a l c o m p o s i t i o n o f n u t r i e n t - s u f f i c i e n t c e l l s , and 2) t h e POC and PON c o n t r i b u t i o n o f n o n - a u t o t r o p h i c o r ganisms ( b a c t e r i a and m i c r o z o o p l a n k t o n ) and t h e p r e s e n c e o f d e t r i t u s (Banse 1974, Sakshaug and O l s e n 1986). I n h e t e r o t r o p h i c e n v i r o n m e n t s , t h e r e l a t i v e c o n t r i b u t i o n o f n o n - a u t o t r o p h i c organisms and d e t r i t u s t o POC and PON may be e s t i m a t e d by p l o t t i n g POC (and PON) a g a i n s t c h l o r o p h y l l a. When t h e c o r r e l a t i o n i s s i g n i f i c a n t , t h e p r o j e c t e d Y - i n t e r c e p t may be t a k e n as t h e amount o f POC ( o r PON) a t t r i b u t a b l e t o h e t e r o t r o p h s and d e t r i t u s (Sakshaug e t a l . 1983, L e v a s s e u r and T h e r r i a u l t 1987). T h i s approach t o c a l c u l a t i n g t h e n o n p h y t o p l a n k t o n b a s e l i n e assumes t h a t t h e c o n c e n t r a t i o n o f n o n p h y t o p l a n k t o n s e s t o n i s c o n s t a n t ( i . e . t h e r e g r e s s i o n between POC (PON) and c h l o r o p h y l l a must be l i n e a r ) . D u r i n g t h i s s t u d y , an in d e p e n d e n t r e g r e s s i o n a n a l y s i s was c o n d u c t e d between c h l o r o p h y l l a and POC (and PON) i n each p a r t of t h e f r o n t a l a r e a when t h e p h y t o p l a n k t o n community was d i f f e r e n t . F o r v e r t i c a l p r o f i l e s , a r e g r e s s i o n a n a l y s i s was c o n d u c t e d f o r each s t a t i o n . When t h e c o r r e l a t i o n was s i g n i f i c a n t , t h e p r o j e c t e d Y - i n t e r c e p t was s u b s t r a c t e d from t h e POC (and PON) v a l u e . I n t h i s c a s e , t h e N:C r a t i o s were r e f e r r e d t o as p h y t o p l a n k t o n N:C. D u r i n g c r u i s e D and i n t h e A n t i c o s t i G y r e, where c h l o r o p h y l l a c o n c e n t r a t i o n s were low and d e t r i t u s c o n c e n t r a t i o n s r e l a t i v e l y h i g h , c h l o r o p h y l l a and POC (and PON) were u n c o r r e l a t e d and t h i s c o r r e c t i o n method was not a p p l i e d . I n t h i s c a s e , N:C r a t i o s were r e f e r r e d t o as t h e N:C r a t i o o f s e s t o n . As s u g g e s t e d by Sakshaug e t al. ( 1 9 8 3 ) , o n l y v a l u e s of N:C below 0.10 were c o n s i d e r e d as s t r o n g e v i d e n c e of n u t r i e n t l i m i t a t i o n , g i v e n t h e pronounced i n t e r - s p e c i e s v a r i a b i l i t y i n b i o c h e m i c a l c o m p o s i t i o n . P h o t o s y n t h e t i c p a r a m e t e r s Carbon f i x a t i o n r a t e v e r s u s l i g h t ( P - I ) c u r v e s were c o n s t r u c t e d f o l l o w i n g t h e s h o r t i n c u b a t i o n t i m e method d e s c r i b e d by L e w i s and S m i t h (1983) f o r s m a l l c u l t u r e volumes. To a 30 ml sample o f s e a w a t e r , NaH^CO-j (New E n g l a n d N u c l e a r Corp.) was added t o g i v e a f i n a l s p e c i f i c a c t i v i t y o f 37 kBq ml-"'" (1 pCi. ml -"'"). The sample was d i v i d e d i n t o 20 a l i q u o t s o f 1 ml each i n s c i n t i l l a t i o n v i a l s ; 18 o f them were i n c u b a t e d f o r 20 min a t 9°C under -2 1 d i f f e r e n t i r r a d i a n c e s ( r a n g i n g f rom 0.7 t o 930 pE m s ). F o r t h e r e m a i n i n g 2 a l i q u o t s w h i c h were used as b l a n k s , 50 p i o f DCMU was added p r i o r t h e i n c u b a t i o n t o s t o p t h e a c t i v e i n c o r p o r a t i o n component of "^C (Legendre e t al. 1983). A c t i v i t y was measured on a LKB WALLAC Rack B e t a (model 1215-005) l i q u i d s c i n t i l l a t i o n c o u n t e r , u s i n g t h e c h a n n e l s - r a t i o method. D i s i n t e g r a t i o n s p e r minute (DPM) were c o n v e r t e d i n t o c a r b o n f i x a t i o n r a t e s u s i n g t h e e q u a t i o n o f P a r s o n s e t al. (1984). P h o t o s y n t h e t i c r a t e s were n o r m a l i z e d p e r u n i t 23 of c h l o r o p h y l l a. The f u n c t i o n a l r e l a t i o n s h i p between c a r b o n f i x a t i o n r a t e p e r u n i t c h l o r o p h y l l a (P ) and i r r a d i a n c e was d e s c r i b e d m a t h e m a t i c a l l y by t h e e q u a t i o n of J a s s b y and P i a t t (1976): P B ( I ) = P Bm [Tanh ( a I / P B m ) ] (1) where P B (mg C mg c h l a - 1 h - 1 ) i s t h e 1 4 C a s s i m i l a t i o n a t i r r a d i a n c e I , P Bm i s t h e maximum r a t e o f f i x a t i o n a t s a t u r a t i n g i r r a d i a n c e , and a B (mg C mg c h l a - 1 h - ^ [/JE m-^ s - 1 ] - - 1 - ) i s t h e i n i t i a l s l o p e o f t h e c u r v e . The pa r a m e t e r s P Bm and a B were e s t i m a t e d u s i n g an i t e r a t i v e l e a s t - s q u a r e s r e g r e s s i o n t e c h n i q u e (NOTEBOOK s o f t w a r e p a c k a g e ) . The pa r a m e t e r , 1^, w h i c h i s t h e minimum i r r a d i a n c e s u f f i c i e n t t o s a t u r a t e t h e p h o t o s y n t h e t i c a p p a r a t u s o f t h e c e l l s , was R / R c a l c u l a t e d as P m/a . 24 RESULTS Sampling year 1985 In 1985, two cruises were conducted i n the f r o n t a l area. The f i r s t cruise (cruise A) was conducted from May 30 to June 7, a period corresponding to the freshet i n the Lower St. Lawrence Estuary (El-Sabh 1979). The second cruise (cruise B) was conducted from June 20 to June 27, when lower freshwater runoff was expected. Both periods corresponded to the spring diatom bloom i n the Lower Estuary (Levasseur et al. 1984). Cruise A Horizontal distribution The positions of the stations during the 24 h transect are shown i n Figure 1.2. During the f i r s t part of the transect (stations 0 to 50), the ship t r a v e l l e d p a r a l l e l to the front, i n the A n t i c o s t i Gyre. Subsequently, the ship crossed the front perpendicularly (stations 51 to 61) and t r a v e l l e d p a r a l l e l to the front i n the l a s t part of the transect (stations 6 2 to 101). Variables measured at 3 m along the transect are presented i n Figure 1.3. The front, which can c l e a r l y be i d e n t i f i e d i n the s a l i n i t y transect (Fig. 1.3A; stations 55 to 6 2 and 8 2 to 100), was crossed twice during the transect. S a l i n i t y i n 25 • • * / * / » 40*30' ,' \ 1 1 / / / /' 0 10 I 1 km >A3 i / i / i > 30 20 10 1 \ \ 4oy ; v T(144) (1) 50/ 320m _ - 4 0. 2 0 # 100 90 8 0 7 0 / - - ' (360) 290m ... -: 40m"-V" ' d r b s - M b r r i e ' • M o n t - L o u i s RMe^nT^T^^ de la G r a n d e -Madeleine Vallee 6 5* 40" 65*30' 65*20' 65*10' Figure 1.2. Positions of the stations along the horizontal transects (numbers, 1 to 100) and where ve r t i c a l profiles were conducted (cruise letter/numbers, Al to A3) during cruise A. Fish larvae were sampled at the same time at a higher frequency along the transect (see chapter 3) . Numbers in brackets correspond to larval f i s h stations. 26 15 ~ 13 o w 11 <r D i 9 L U a 2 a ? 5 B 0 10 20 30 40 50 60 70 80 90 100 + GYRE +FRT+-GASPE- + -FRONT- + CURRENT STATIONS a o a o _ J x o 0 10 20 30 40 50 60 70 80 90 100 + GYRE +FRT+-GASPE-+ -FR0NT-+ CURRENT STATIONS Figure 1.3. Cruise A - Horizontal d i s t r i b u t i o n at 3 m of: (A) s a l i n i t y , (B) temperature, (C) the surface layer depth, (D) n i t r a t e , (E) s i l i c a t e , (F) chlorophyll a, (G) POC, (H) PON, (I) N:C r a t i o of phytoplankton (•) and seston (°)(dashed l i n e represents the Redfield r a t i o ) , (J) phytoplankton POC/CHL a r a t i o , (K) phytoplankton P0N/CHL a r a t i o , (L) P^, (M) a B , and (N) I* on a hor i z o n t a l transect across the f r o n t a l area of the Gasp6 Current i n the Gulf of St. Lawrence. (see F i g . 1.2 for p o s i t i o n of stations 0 to 100; FRT = front) 27 F i g u r e 1.3. ( c o n t i n u e d ) 28 the Gyre was ca. 29.8°/0«=» and temperature was ca. 7.5°C at 3 m depth ( F i g . 1.3A and B). In the f r o n t , s a l i n i t y decreased to 26.5 < = >/ 0 0 and remained a t t h i s v a l u e i n the Gaspe C u r r e n t ( s t a t i o n s 61 t o 80). Between s t a t i o n s 80 and 101, v a r i a t i o n s i n s a l i n i t y showed t h a t the f r o n t was sampled a g a i n even though the s h i p was t r a v e l l i n g p a r a l l e l t o the c o a s t , p r o b a b l y because of l a t e r a l a d v e c t i o n of the f r o n t . Temperature a t 3 m remained r e l a t i v e l y c o n s t a n t a t 7.5°C a c r o s s the f r o n t . The a n a l y s i s of the 30 XBT p r o f i l e s conducted r e g u l a r l y along the t r a n s e c t shows, however, t h a t the f r o n t per se was c h a r a c t e r i z e d by an u p l i f t i n g of the 6°C i s o t h e r m and f r e q u e n t temperature i n v e r s i o n s ( F i g . 1 . 4 ) . The depth of the s u r f a c e l a y e r , as d e f i n e d by the depth of the top of the thermocli'ney v a r i e d from 7.5 t o 20 m, w i t h the s m a l l e r v a l u e s measured i n the f r o n t ( F i g . 1.3 C). The presence of the f r o n t had a profound e f f e c t on the d i s t r i b u t i o n of most of the chemical and b i o l o g i c a l v a r i a b l e s measured ( F i g . 1.3D t o 1.3N). N u t r i e n t c o n c e n t r a t i o n s were low i n the Gyre and h i g h i n the Gasp6 Cu r r e n t and i n the f r o n t . N i t r a t e c o n c e n t r a t i o n s were below 1.0 (iM i n the Gyre ( s t a t i o n s 0 t o 50; F i g . 1.3D), i n c r e a s e d s h a r p l y t o c a . 4 uM i n the f r o n t ( s t a t i o n s 51 to 60, 80 t o 100) and decreased t o 1.5 uM, i n the Gaspe C u r r e n t (Stn 61 to 80). S i l i c a t e c o n c e n t r a t i o n s were a l s o low ( < 1.0 u.M) i n the Gyre ( F i g . 1.3E) and i n c r e a s e d t o c a . 5 u.M i n the f r o n t . 29 STATIONS F i g u r e 1 .4 . H o r i z o n t a l v a r i a t i o n s o f s a l i n i t y a t 3 m (A) and v e r t i c a l d i s t r i b u t i o n o f t e m p e r a t u r e (B) a l o n g t h e t r a n s e c t c o n d u c t e d d u r i n g c r u i s e A (see F i g . 1.3 f o r o t h e r v a r i a b l e s measured a t 3 m). Dots r e p r e s e n t XBT s t a t i o n s . 30 In c o n t r a s t t o n i t r a t e , s i l i c a t e c o n c e n t r a t i o n s remained r e l a t i v e l y h i g h (4 uM) i n t h e Gaspe C u r r e n t . H i g h v a l u e s o f c h l o r o p h y l l a (8 t o 35 ug l - 1 ) were l i m i t e d t o t h e d i l u t e d (26 t o 2 8 ° / 0 o ) s u r f a c e w a t e r s o f t h e Gasp6 C u r r e n t ( F i g . 1.3F). I n t h e s a l i n i t y g r a d i e n t ( s t a t i o n s 55 t o 62 and 82 t o 100), c h l o r o p h y l l a c o n c e n t r a t i o n s d e c r e a s e d as s a l i n i t y i n c r e a s e d ( F i g . 1.3A and F ) , s u g g e s t i n g a p a s s i v e c r o s s - f r o n t a l t r a n s p o r t o f t h e biomass. The p h y t o p l a n k t o n community was s i m i l a r i n t h e Gaspe C u r r e n t and i n t h e f r o n t . A t t h o s e s t a t i o n s , t h e community was dominated by t h e d i a t o m Thalassiosira nordenskioeldii and numerous u n i d e n t i f i e d f l a g e l l a t e s ( T a b l e 1.2). I n t h e Gy r e , t h e community was dominated by u n i d e n t i f i e d f l a g e l l a t e s , and two d i n o f l a g e l l a t e s , Prorocentrum balticum and Katodinium rotundatum. I m p o r t a n t v a r i a t i o n s o f t h e p h y t o p l a n k t o n b i o c h e m i c a l c o m p o s i t i o n were r e c o r d e d a c r o s s t h e f r o n t . I n t h e Gaspe C u r r e n t and i n t h e f r o n t , p h y t o p l a n k t o n N:C r a t i o s were g e n e r a l l y c l o s e t o , o r h i g h e r t h a n 0.15 ( d o t t e d l i n e i n F i g . 1.31), t h e v a l u e g e n e r a l l y found f o r n u t r i e n t - s u f f i c i e n t p h y t o p l a n k t o n ( R e d f i e l d 1963). I n t h e Gy r e , v a r i a t i o n s i n POC and PON were n ot s i g n i f i c a n t l y c o r r e l a t e d w i t h c h l o r o p h y l l a ( F i g . 1.3G and H, T a b l e 1.3) and, c o n s e q u e n t l y , o n l y t h e N:C r a t i o s o f t h e s e s t o n a r e p r e s e n t e d . N:C r a t i o s o f s e s t o n d e c r e a s e d down t o 0.05 as Table 1.2. Cruise A - Microplankton species and abundance (determined at 3 m depth along the transect) in the Gasp6 Current (station 75), the front per se (station 99) and in the Gulf Gyre (station 24). GaspS Current Front Gulf Gyre (103 cells l"1) Species BACILLARIOPHYCEAE Chaetoceros compressus -" convolutus -" debilis - - 56.8 " Jaciniosus . . . " septentrional is - - 0.9 ' similis - 8.8 " sp. 8.8 Eucampia groenlandica -Leptocylindrus danicus - - 0.9 " minimus - 17.6 1.9 Licmophora sp. - - -Havicola sp. 8.8 Nitzschia closteriun 8.8 " delicatissima 17.6 " seriata - - 12.7 • sp. 17.6 Skeletoneoa costatum - -Thalassiosira gravida 35.4 " nordensfcioeldii 1340.0 77.8 3.9 • sp. Unidentified pennates - 17.6 3.9 Unidentified centrales 17.6 DINOPHYCEAE Alexandrlun spp. -Gymnodinium lohmannii . . . " spp. 26.4 70.8 78.4 Gyrodinium grenlandicum . . . Katodinium rotundatum 35.4 124.0 160.0 Prorocentrum balticum - - 613.0 Oxitoxum spp. -Protoperidinium bipes . . . " ^depressum -Unidentified dinoflagellates . . . CHRYSOPHYCEAE Apedinella spinifera 44.2 35.4 Dinobiyon balticum - -CRYPTOPHYCEAE Leucociyptos marina 26.6 26.6 61.2 Table 1.2. (continued) Unidentified 212.0 451.0 4.9 PRASINOPHYCEAE Unidentified 17.6 EUGLENOPHYCEAE Unidentified 8.8 - 0.9 OTHERS Unidentified flagellates 610.0 628.2 833.0 CHOANOFLAGELLATES Calliacantha sp. - - 12.2 Monosiga sp. ' -CILIATES Helicostoaella subulata -Lohmnniella oviformis - - o.9 Salpingella sp. -Tintinnopsis sp. - 8.8 6.8 Unidentified ciliates -33 T a b l e 1.3. C r u i s e A - C o e f f i c i e n t s ( s l o p e s and Y - i n t e r c e p t ) o f t h e l i n e a r r e g r e s s i o n a n a l y s e s c o n d u c t e d between t h e i n d e p e n d e n t v a r i a b l e c h l o r o p h y l l a and t h e dependent v a r i a b l e s POC and PON i n each p a r t o f t h e f r o n t a l a r e a (n = number o f d a t a p o i n t s , r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = n o t s i g n i f i c a n t ) . A r e a Dependent s l o p e Y - i n t e r c e p t n r p v a r i a b l e (ug l - 3 - ) Gyre POC 205.6 167.6 53 0.11 n.s. PON 9.2 32.0 53 0.01 n.s. F r o n t & POC 40.4 173.6 26 0.82 0.01 C u r r e n t PON 8.4 27.6 26 0.88 0.01 34 s a l i n i t y i n c r e a s e d , i n d i c a t i n g a p r o g r e s s i v e impoverishment i n n i t r o g e n . Phytoplankton POC/CHL a and PON/CHL a r a t i o s were h i g h e r i n the f r o n t than i n the Gaspe C u r r e n t , s u g g e s t i n g t h a t the mixing of the phytoplankton community a c r o s s the f r o n t r e s u l t e d i n a decrease of the c h l o r o p h y l l a quotas ( F i g . 1.3J, K). V a r i a t i o n s of the p h o t o s y n t h e t i c parameters a c r o s s the f r o n t a re p r e s e n t e d i n F i g u r e 1.3L, M and N. Parameters P Bm and a B e x h i b i t e d a s i m i l a r p a t t e r n a c r o s s the f r o n t , w i t h low v a l u e s i n the Gaspe C u r r e n t and h i g h v a l u e s i n the f r o n t . The s i m i l a r i t i e s between these v a r i a t i o n s and those of POC/CHL a suggests t h a t the v a r i a t i o n s i n the p h o t o s y n t h e t i c parameters P Bm and a B r e s u l t e d from v a r i a t i o n s i n c h l o r o p h y l l a quotas a c r o s s the f r o n t . In the Gyre, carbon f i x a t i o n r a t e s a t s a t u r a t i n g l i g h t i n t e n s i t y (Pm) were below the d e t e c t i o n l i m i t of the method (< 1.5 ug C ug c h l a - 1 h _ 1 ) due to the low biomass, s h o r t i n c u b a t i o n time and r e l a t i v e l y low 1 4 C a c t i v i t y added t o the sample, lie v a l u e s v a r i e d from 75 t o 203 uE m - 2 s - 1 (mean ± S . D . = 148 ± 40), w i t h no obvious p a t t e r n r e l a t e d t o the f r o n t a l l o c a t i o n ( F i g . 1.3N). Vertical distribution The v e r t i c a l d i s t r i b u t i o n of s e l e c t e d v a r i a b l e s was determined i n each p a r t of the f r o n t two days a f t e r the t r a n s e c t ( F i g s . 1.5, 1.6 and 1.7). Along the c o a s t ( s t a t i o n 35 Salinity (%<>) 26 28 30 32 _| 1 1 1 L N0 3 or SIO4 0 2 4 6 8 Chlorophyll a (fxg.\ 0 4 8 12 22 - 1 i 0 oT2~oT4~oT6 0.8 Temperature (°C) P0 4 (/lM) F i g u r e 1.5. C r u i s e A - V e r t i c a l d i s t r i b u t i o n o f : (A) s a l i n i t y and t e m p e r a t u r e , (B) n i t r a t e , s i l i c a t e and phosphate and (C) c h l o r o p h y l l a a t s t a t i o n A l i n t h e Gasp6 C u r r e n t (see F i g . 1.2 f o r p o s i t i o n o f s t a t i o n A l ) . 36 S a l i n i t y (%<,) N 0 3 , S i 0 4 ( / i M ) C h l o r o p h y l l a (/ig . l " 1 ) 2 6 30 3 4 0 2 4 6 8 0 4 8 12 T e m p e r a t u r e (°C) P 0 4 (/iM) Figure 1.6. Cruise A - V e r t i c a l d i s t r i b u t i o n of: (A) s a l i n i t y and temperature, (B) n i t r a t e , s i l i c a t e and phosphate and (C) chlorophyll a at station A2 i n the front (see F i g . 1.2 for p o s i t i o n of s t a t i o n A2). 37 Salinity (%o) N 0 3 l S I 0 4 (plM) Chlorophyll a ( / x g . r 1 ) 28 32 36 0 2 4 6 8 0 4 8 12 Temperature (°C) P 0 4 ( / x M ) Figure 1.7. Cruise A - V e r t i c a l d i s t r i b u t i o n of: (A) s a l i n i t y and temperature, (B) n i t r a t e , s i l i c a t e and phosphate and (C) chlorophyll a at station A3 i n the Gulf Gyre (see F i g . 1.2 f o r p o s i t i o n of st a t i o n A3). 38 Table 1.4. Cruise A - Vertical distribution of microplankton abundance (103 cells l"1) at selected depths in the Gaspe" Current (Stn Al). Depths Species 0 2 4 6 8 10 12 BACILLARIOPHYCEAE Chaetoceros compressus - - - 4.9 - - -" convolutus - - - - - - -" debilis 45.0 55.9 25.5 16.7 19.6 16.6 3.9 " laciniosus 30.1 34.3 37.2 11.8 4.9 4.9 -" septentrionalis 2.0 - 12.2 0.9 - 0.9 -" similis - - - - - - -" sp. - - - - - - -Eucampia groenlandica - 3.9 0.9 - - 0.9 -Leptocylindrus danicus 4.9 13.7 7.8 14.7 8.8 15.6 0.9 ' minimus 7.8 3.9 16.7 14.7 0.9 7.8 0.9 Licroophora sp. - - - - 0.9 - -Uavicula sp. - - - - - - -Nitzschia closteiiw 5.9 9.8 6.8 9.8 4.9 3.9 1.9 delicatissioa 0.9 - - 0.9 0.9 - -seziata - - - - - - -sp. - - - - - - -SJteletonema costatim 2.9 - - 7.8 - - -Thalassiosiia gravida 7.8 37.2 12.7 14.7 2.9 - -nordensJtioeldii 2500.0 4320.0 2830.0 3360.0 1430.0 1490.0 527.0 sp. - - - - - - -Unidentified pennates 1.9 - 2.9 10.7 2.9 2.9 3.9 Unidentified centrales - - - - - - -DINOPHYCEAE Alexadriua spp. - - - - - -Gym/iodinium lohmannii - 0.9 0.9 - - - -" spp. 27.4 30.4 32.3 45.1 19.6 30.4 56.9 Gyrodinium grenlandicum 12.2 12.2 - 12.2 24.5 3.9 12.2 Xatodinium rotundatua 73.5 171.0 110.2 3.9 12.2 - -Oxitoxum spp. - - 0.9 0.9 - 1.9 0.9 Prorocentruin balticiui - - - - - - -ProtoperidJnium bipes 1.9 0.9 0.9 1.9 - 0.9 0.9 " depression - 0.9 - - - - -Unidentified dinoflagellates 0.9 1.9 0.9 0.9 • - -CHRYSOPHYCEAE Apedinella spinifera 36.8 24.5 49.0 24.5 12.2 - -Dinobcyon balticun 12.2 - - - - - -CRYPTOPHYCEAE Leucociyptos marina 49.0 24.5 49.0 73.5 - 12.2 -39 T a b l e 1.4. ( c o n t i n u e d ) U n i d e n t i f i e d 6.9 18.6 10.8 5.9 1.9 7.8 0.9 PRASINOPHYCEAE U n i d e n t i f i e d - 24.5 - - - - -EUGLENOPHYCEAE U n i d e n t i f i e d - 3.9 0.9 1.9 -OTHERS U n i d e n t i f i e d f l a g e l l a t e s 649.0 820.0 465.0 343.0 85.7 110.0 49.0 CHOANOFLAGELLATES Calliacantha s p . - 12.2 - - 24.5 - 24.5 Monosiqa s p . 40.0 36.8 61.2 12.2 24.5 - 24.5 C I L I A T E S Helicostomella subulata - 0.9 - -L o h m a n n i e l l a o v i f o r m i s 0.9 - 0.9 4.9 Salpingella s p . 0.9 - - - - - -T i n t i n n o p s i s s p . 2.9 1.9 - 0.9 -U n i d e n t i f i e d c i l i a t e s 4.9 3.9 5.9 1.9 2.9 1.9 4.9 40 Table 1.5. Cruise A - Vertical distribution of microplankton abundance (101 cells l"1) at selected depths in the front (Stn A2). Depths 0 6 8 10 12 14 22 Species BACILLARIOPHYCEAE Chaetocezos compressus - 7.9 8.8 " convolutus - - - - - - .-" debilis 6.8 5.9 11.7 4.9 2.9 - 5.8 " laciniosus 16.6 3.9 6.8 - - - -" septentzionalis - 1.9 1.9 - -" similis - - - - - - -sp. - - - - - - -Eucampia groenlandica 0.9 - - - -. - -Leptocylindrus danicus - - 0.9 0.9 0.9 - 56.8 • minimus 0.9 - 2.9 - 2.9 0.9 -Licmophora sp. - - - - 0.9 - -Navicula sp. - - - - - - -Mtzschia closterium 1.9 0.9 0.9 - 0.9 - -" delicatissima 0.9 - - 0.9 0.9 • - -" seiiata - - 1.0 - - 0.9 1.9 ' sp. - - - - - - -SJceletonema costatum - - - - - - -Thalassiosira gravida - - 5.9 4.9 - - -" nordensJtioeldii 147.0 208.0 1420.0 588.0 367.0 12.7 2.9 " sp. - - - - - - -Unidentified pennates - 0.9 - - - - -Unidentified centrales - - - - - - -DINOPHYCEAE Alexandrium spp. - - - - - - -Gynwodinium lohmnnii 2.9 0.9 - 2.9 - - 0.9 " spp. 107.0 66.6 67.7 58.8 41.2 21.6 27.4 Gyrodinium grenlandicum 12.2 61.2 1.9 8.8 12.2 12.2 -Katodinium rotundatum 36.8 110.0 61.2 24.5 12.2 0.9 -Prorocentrum balticum - - - - - - -Oxitoxum spp. - - - - - - 1.9 Protoperidinium bipes - - - - - - 1.9 depressum - 0.9 - - - - -pelliculum - 0.9 - 0.9 - - -Cladopyxis claytonii - 0.9 - - - - -Dinophysis acuminata - 0.9 - 2.9 - - 0.9 Unidentified dinoflagellates 2.9 1.9 0.9 - - - 0.9 CHRYSOPHYCEAE Apedinella spinifera - - - - • - - -Dinobiyon balticum - 12.2 49.0 12.2 12.2 1.9 -petiolatum - 0.9 - 0.9 - - -41 Table 1 .5. (continued) CRYPTOPHYCEAE Leucocryptos narlna 73.5 49.0 24.5 24.5 24.5 2.9 49.0 Unidentified 20.6 10.8 1.9 0.9 PRASINOPHYCEAE Unidentified - 12.2 - - - - -EUGLENOPHYCEAE Unidentified - - - - - - -OTHERS Unidentified f l age l la te sp. 294.0 355.0 355.0 171.0 85.7 24.5 12.2 Unidentified f lagel la tes 330.0 159.0 208.0 98.0 134.0 49.0 73.5 CHOANOFLAGELLATES Calliacantha sp. 12.2 - - - - - 24.5 Honosiga sp. 49.0 24.5 24.5 73.5 36.7 36.7 12.2 CILIATES Hel icos toael la subulata - - - - - . -Lob/nanniella oviformis - - - - - - -Salpingel la sp. - - 0.9 - - - -Tintinnopsis sp. 0.9 Unidentified c i l i a t e s 17.6 5.9 2.9 7.8 4.9 3.9 4.9 42 Table 1.6. Cruise A - Vertical distribution of microplankton abundance (103 cells l"1) at selected depths in the Gulf Gyre (Stn A3). Depths 0 10 20 22 26 28 34 Species BACILLARIOPHYCEAE chaetocezos compzessus ' convolutus - - - - - - -" debilis 56.8 49.9 367.0 808.0 65.6 73.5 31.3 " laciniosus - 3.9 - 1.9 4.9 1.9 " septentrionalis 0.9 1.9 7.8 24.5 12.2 36.7 12.2 " similis - - - - - - -sp. - - - - - - -" teres - - 3.9 - - - -Eucampia groenlandica - - - - - - -Leptocylindzus danicus 0.9 1.9 198.0 796.0 330.0 931.0 232.0 minimus 1.9 0.9 3.9 2.9 3.9 1.9 0.9 Licmophoza sp. - - - - - - -Savicula sp. - - - - - - -tfitzschia closteriua - - 0.9 - - - 0.9 " delicatissima - - - - - - -• seriata 12.2 4.9 16.6 15.7 4.9 11.7 4.9 • sp. - - - - - - -R/iizosolenia hebetata - 0.9 0.9 - - 0.9 -Thalassionema nitzschioides - 0.9 - - - - -Sfceletonema costatum - - - - - - -Thalassiosiza bioculata - - 4.9 - - - -" qzavida - - 6.8 - - - 1.9 " nordenstioeldii 3.9 - 17.6 4.9 1.9 11.7 17.6 " sp. - - - - - - -Unidentified pennates 3.9 0.9 - 0.9 1.9 -Unidentified centrales - - - - - - -DINOPHYCEAE Alexandrium spp. - - - - - - -Cladopyxis claytonii - 0.9 - - - - -Dinophysis acuminata 0.9 0.9 - - - - -Gymnodinium lohmannii - 0.9 1.0 - - - 0.9 " spp. 78.4 32.3 23.5 19.6 19.6 41.2 25.5 Gyzodinium grenlandicum - 12.2 12.2 12.2 3.9 36.7 12.2 Katodinium rotundatum 159.0 24.5 12.2 0.9 0.9 - -Oxitoxum spp. - - - - - - 1.9 Prorocentrum balticum 12.2 - - 12.2 - -Pzotopetidinium bipes " brevipes - 0.9 - - - - 1.9 0.9 - - - - - -depressum ovaturn pelliculum 43 Table 1.6. (continued) Unidentified dlnoflagellates 4.9 0.9 - - - 1.9 CHYSOPHYCEAE Apedinella spinifera - - - - - - -Dinobryon balticum 612.0 869.0 232.0 12.2 12.2 0 petiolatujn - - 24.5 - 12.2 CRYPTOPHYCEAE Leucocryptos marina 61.2 - 12.2 12.2 12.2 Unidentified 4.9 9.8 1.9 - - - 0.9 PRASINOPHYCEAE Unidentified EUGLENOPHYCEAE Unidentified 0.9 - 0.9 - 0.9 OTHERS Unidentified flagellate sp. 465.0 379.0 12.2 - 12.2 Unidentified flagellates 367.0 36.7 147.0 220.0 134.0 232.0 73.5 CHOANOFLAGELLATES Calliacantha sp. 12.2 - - - - - 24.5 Monosiga sp. - 24.5 19.6 12.2 12.2 CILIATES ffelicostoroella subulata - - - - - - -Lohioannlella oviformis 0.9 1.9 0.9 2.9 1.9 0.9 Salpingella sp. - 0.9 - 0.9 1.9 Tintinnopsis sp. - - - - - - -Unidentified ciliates 6.8 8.8 5.9 2.9 1.9 0.9 0.9 44 A l ) , h i g h v a l u e s o f c h l o r o p h y l l a (8 t o 22 ug l - 1 ) were l i m i t e d t o t h e d i l u t e d (26-28°/oc>) and warm (5-8°C) s u r f a c e w a t e r (above 10 m) of t h e Gasp6 C u r r e n t ( F i g . 1.5). As o b s e r v e d d u r i n g t h e t r a n s e c t , t h e p h y t o p l a n k t o n community was domin a t e d by Thalassioslra nordenskioeldii ( T a b l e 1.4). Below 10 m, low v a l u e s of c h l o r o p h y l l a (< 2 ug l - 1 ) were a s s o c i a t e d w i t h t h e c o l d (2-4°C), s a l i n e (28-32°/ O Q) Gyre w a t e r s u n d e r l y i n g t h e Gaspe C u r r e n t . N u t r i e n t ( N 0 3 , Si C U , PCu) c o n c e n t r a t i o n s were low i n t h e Gaspe C u r r e n t , i n c r e a s e d i m m e d i a t e l y under t h e p y c n o c l i n e (ca. 10 m) and d e c r e a s e d a g a i n below 22 m t o r e a c h c o n c e n t r a t i o n s t y p i c a l o f t h e Gyre s u r f a c e w a t e r s . I n t h e f r o n t ( s t a t i o n A 2 ) , c h l o r o p h y l l a c o n c e n t r a t i o n s were maximum (11 ug l - 1 ) a t 10 m, t h e d e p t h c o r r e s p o n d i n g t o t h e s h a r p s a l i n i t y g r a d i e n t a t t h e base of t h e Gasp6 C u r r e n t ( F i g . 1.6). C h l o r o p h y l l a c o n c e n t r a t i o n s were < 1 ug l - 1 b o t h i n t h e upper mixed l a y e r (0-8 m) and a t d e p t h s g r e a t e r t h a n 14 m. Thalassiosira nordenskioeldii dominated t h e community i n t h e upper mixed l a y e r and i n t h e s u b s u r f a c e c h l o r o p h y l l a maximum ( T a b l e 1.5). N i t r a t e and s i l i c a t e c o n c e n t r a t i o n s were low i n t h e f i r s t 18 m and n i t r a t e c o n c e n t r a t i o n s i n c r e a s e d t o 4.5 uM below 18 m. I n c o n t r a s t t o n i t r a t e , s i l i c a t e c o n c e n t r a t i o n s were l e s s t h a n 1.0 uM down t o 28 m. 45 I n t h e Gyre ( s t a t i o n A 3 ) , t h e upper mixed l a y e r was d e f i n e d by a s t r o n g t h e r m o - h a l o c l i n e a t 10 m ( F i g . 1.7). C h l o r o p h y l l a c o n c e n t r a t i o n s were low ( c a . 1 ug l - 1 ) i n t h e mixed l a y e r and t h e p h y t o p l a n k t o n community was d ominated by Dlnobryon balticum, f o l l o w e d by u n i d e n t i f i e d f l a g e l l a t e s and t h e d i n o f l a g e l l a t e , Katodinium rotundatum ( T a b l e 1.6). A dense bloom (up t o 11 n g l - 1 o f c h l o r o p h y l l a ) o c c u r r e d between 20 and 30 m and i t was dominated by t h e d i a t o m s Leptocylindrus danicus and Chaetoceros d e b i l i s . N i t r a t e and s i l i c a t e c o n c e n t r a t i o n s were low (< 1.0 u.M) i n t h e t o p 24 m. Below 26 m, t h e i r c o n c e n t r a t i o n s i n c r e a s e d t o 3.5 and 2.5 u.M f o r n i t r a t e and s i l i c a t e r e s p e c t i v e l y . Cruise B Horizontal d i s t r i b u t i o n The l o c a t i o n o f t h e s a m p l i n g s t a t i o n s f o r c r u i s e B a r e p r e s e n t e d i n F i g u r e 1.8. D u r i n g t h e 13 h t r a n s e c t , t h e s h i p t r a v e l l e d m o s t l y i n t h e f r o n t per se ( s t a t i o n s 4 t o 4 6 ) , p a r a l l e l t o t h e c o a s t , w i t h o n l y a few s t a t i o n s l o c a t e d i n t h e Gyre ( s t a t i o n s 1 t o 3) and i n t h e Gaspe C u r r e n t ( s t a t i o n s 47 t o 4 9 ) ( F i g s . 1.8 and 1.9A). T h i s s a m p l i n g s t r a t e g y was chosen t o maximize t h e number o f samples c o l l e c t e d i n t h e f r o n t . C r u i s e B was c a r r i e d o u t c a . 15 days a f t e r c r u i s e A. I n c o n t r a s t w i t h c r u i s e A, v a r i a t i o n s i n s a l i n i t y and t e m p e r a t u r e a t 3 m r e v e a l e d t h e p r e s e n c e of d i f f e r e n t w a t e r 46 _49,30' 49«20' eerso" 66*40' 66T30' F i g u r e 1.8. P o s i t i o n s o f the s t a t i o n s a l o n g the h o r i z o n t a l t r a n s e c t s (numbers 1 t o 49) and a t which v e r t i c a l p r o f i l e s were conduc ted ( c r u i s e l e t t e r / n u m b e r s , B l t o B17)) d u r i n g c r u i s e B . F i s h l a r v a e were sampled a t the same t ime a t a h i g h e r f r e q u e n c y a l o n g the t r a n s e c t (see C h a p t e r 3 ) . Numbers i n b r a c k e t s c o r r e s p o n d to l a r v a l f i s h s t a t i o n s . 47 Figure 1.9. Cruise B - Horizontal d i s t r i b u t i o n at 3 m of: (A) s a l i n i t y , (B) temperature, (C) the surface layer depth, (D) n i t r a t e , (E) s i l i c a t e , (F) chlorophyll a, (G) POC, (H) PON, (I) N:C r a t i o of phytoplankton (dashed l i n e represents the Redfield r a t i o ) , (J) phytoplankton POC/CHL a r a t i o , (K) phytoplankton PON/CHL a r a t i o , (L) PBm, (M) a B , and (N) I* on a h o r i z o n t a l transect across the Gyre (GY), front and Gasp6 Current (G. Current) i n the Gulf of St. Lawrence (see F i g . 1.8 f o r p o s i t i o n of stations 0 to 50). F i g u r e 1.9. ( c o n t i n u e d ) 49 masses i n t h e f r o n t p e r se ( s t a t i o n s 7 t o 3 9 ) . S t a t i o n s 4 t o 28 ( s a l i n i t i e s between 27.5 and 28.5 ° / o o ; i n t e r f a c e between t h e Gaspe C u r r e n t and t h e f r o n t ) were c h a r a c t e r i z e d by r e l a t i v e l y h i g h t e m p e r a t u r e s ( c a . 11-13°C). S t a t i o n s 29 t o 39 ( s a l i n i t y between 28.5 and 29.5 ° / 0 0 ; i n t e r f a c e between t h e Gyre and t h e f r o n t ) were c h a r a c t e r i z e d by l o w e r t e m p e r a t u r e s ( c a . 9.2°C). T h i s c o l d band o f w a t e r was l o c a t e d a t t h e o f f s h o r e edge o f t h e f r o n t , as i n d i c a t e d by t h e r e l a t i v e l y h i g h s a l i n i t y . Data from t h e 13 XBT p r o f i l e s c o n d u c t e d a l o n g t h e t r a n s e c t shows t h a t t h e 6 °C i s o t h e r m was c l o s e r t o t h e s u r f a c e i n t h e f r o n t (Fig» 1.10). Temperature i n v e r s i o n s were a l s o f r e q u e n t i n t h e f r o n t . The mixed l a y e r d e p t h was 15 m i n t h e Gyre, 10 m i n t h e Gaspe C u r r e n t and a p p r o x i m a t e l y 5 m i n t h e f r o n t ( F i g . 1.9C). D u r i n g c r u i s e B, t h e d i s t r i b u t i o n o f p h y t o p l a n k t o n biomass and n u t r i e n t s was a g a i n c l e a r l y a f f e c t e d by t h e f r o n t ( F i g . 1.9D t o 1.9H). I n c o n t r a s t w i t h c r u i s e A, c h l o r o p h y l l a c o n c e n t r a t i o n s were maximum (up t o 55 ug 1"^) i n t h e f r o n t p e r se ( s t a t i o n s 29 t o 39) r a t h e r t h a n i n t h e Gaspe C u r r e n t ( F i g . 1.9F). A bloom o f t h e d i a t o m Thalassiosira nordenskioeldii i n t h e c o l d w a t e r band d e s c r i b e d above ( T a b l e 1.7) was r e s p o n s i b l e f o r t h i s h i g h b iomass. These s t a t i o n s were c h a r a c t e r i z e d by t h e h i g h e s t n i t r a t e (5 JJM) and s i l i c a t e (4 uM.) c o n c e n t r a t i o n s measured a c r o s s t h e t r a n s e c t ( F i g . 1.9D, E ) . I n t h e Gaspe C u r r e n t and i n t h e warm p a r t o f t h e f r o n t , n i t r a t e and s i l i c a t e 50 [^GY.^ FRONT 4?"°>| 25T 1 1 1 1 1 1 1 1 1 1 0 10 20 30 40 50 40-} 1 1 1 1 1 . 1 1 1 0 10 20 30 40 50 STATIONS Figure 1.10. Horizontal variations i n s a l i n i t y at 3 m (A) and v e r t i c a l d i s t r i b u t i o n of temperature (B) along the transect conducted during cruise B (see F i g . 1.9 f o r other va r i a b l e s measured at 3 m; GY = gyre and G.C. = Gaspe" Current). Dots represent XBT stations. 51 c o n c e n t r a t i o n s were low (< 1 uM), and c h l o r o p h y l l a c o n c e n t r a t i o n s v a r i e d between 0.5 and 5.0 ug l - 1 . The p h y t o p l a n k t o n community was dominated by s m a l l f l a g e l l a t e s and a Chaetoceros sp. I n t h e G y r e , n u t r i e n t and c h l o r o p h y l l a c o n c e n t r a t i o n s were s t i l l low and t h e p h y t o p l a n k t o n community was dominated by s m a l l f l a g e l l a t e s and c r y p t o p h y t e s ( T a b l e 1.7). Marked changes i n t h e p h y t o p l a n k t o n b i o c h e m i c a l c o m p o s i t i o n were r e c o r d e d a c r o s s t h e f r o n t ( F i g . 1.91 t o 1.9N). P h y t o p l a n k t o n N:C r a t i o s were a p p r o x i m a t e l y 0.15 i n t h e Gaspe C u r r e n t , 0.12 i n t h e low t e m p e r a t u r e band w i t h i n t h e f r o n t and d e c r e a s e d below 0.10 i n t h e h i g h t e m p e r a t u r e p a r t o f t h e f r o n t ( F i g . 1.91). These r e s u l t s s u g g e s t t h a t t h e community l o c a t e d i n t h e h i g h t e m p e r a t u r e p a r t o f t h e f r o n t was n u t r i e n t - d e f i c i e n t . P h y t o p l a n k t o n POC/Chl a and PON/Chi a r a t i o s were h i g h i n t h e Gaspe C u r r e n t and d e c r e a s e d d r a s t i c a l l y t o ca. 25 and 3, r e s p e c t i v e l y , i n t h e f r o n t ( F i g . 1.9J, K ) . As o b s e r v e d f o r c r u i s e A, v a r i a t i o n s o f POC and PON were n o t c o r r e l a t e d w i t h c h l o r o p h y l l a i n t h e Gyre and, c o n s e q u e n t l y , t h e c o r r e c t i o n f o r d e t r i t u s was n o t a p p l i e d ( T a b l e 1.8). The p h o t o s y n t h e t i c p arameters a l s o e x h i b i t e d i m p o r t a n t v a r i a t i o n s a p p a r e n t l y a s s o c i a t e d w i t h t h e f r o n t . I n t h e h i g h t e m p e r a t u r e p a r t o f t h e f r o n t ( s t a t i o n s 5 t o 25 and 38 t o 4 6 ) , P Bm v a l u e s were g e n e r a l l y below t h e d e t e c t i o n l i m i t 52 Table 1.7. Cruise B - Microplankton species and abundance (103 cells l"1) (determined at 3 m along the transect) in the Gaspe" Current (station 47), the non-upwelling part of the front (station 20), the upwelling part of the front (station 36) and in the Gulf Gyre (station 1). Gasp£ Current Front Gulf Gyre Species non-upvelling upwelling BACILLARIOPHYCEAE Chaetocezos compressus • convolutus " debilis " laciniosus " septentrionalis " similis sp. Eucampia qzoenlandica Leptocylindrus danicus " minimus Licmophora sp. Navicula sp. Hitzschia closterium " delicatissima " seriata • sp. SJceletonema costatum Thalassiosiza gravida nordenskioeldii sp. Unidentified pennates Unidentified centrales 1240.0 26.6 26.6 44.2 132.0 372.0 26.6 44.2 26.6 79.6 35.4 1190.0 17.6 26.6 26.6 17.6 26.6 DINOPHYCEAE Alexandrium spp. Gymnodinium lohmannii " spp. Gyrodinium grenlandicum KatodinJum rotundatum Oxitoxum spp. Prorocentrum balticum Protoperidinium bipes " depressum Unidentified dinoflagellates 70.8 70.8 8.8 79.6 44.2 159.0 35.4 44.2 CHRYSOPHYCEAE Apedinella spinifera Dinobryon balticum 44.2 17.6 53 Table 1.7. CRYPTOPHYCEAE Leucocryptos marina 8.8 Unidentif ied 133.0 PRASINOPHYCEAE Unidentif ied EUGLENOPHYCEAE Unidentif ied 8.8 OTHERS Unidentif ied f lagel la tes 1740.0 CHOANOFLAGELLATES Calliacantha sp. Honosiga sp. -Unidentif ied 26.6 CILIATES Jfelicostomella subulata Lohmnnlella oviforms Salp ingel la sp. Tintinnopsis sp. Unidentif ied c i l i a t e s 26.6 (continued) 274.0 380.0 212.0 44.2 2070.0 513.0 442.0 8.8 - 8.8 8.8 35.4 8.8 8.8 54 Table 1.8. Cruise B - C o e f f i c i e n t s (slope and Y-intercept) of the l i n e a r regression analyses conducted between the independent v a r i a b l e chlorophyll a and the dependent var i a b l e s POC and PON i n each part of the f r o n t a l area (n = number of data points, r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = not s i g n i f i c a n t ) . Area Dependent slope Y-intercept n r p va r i a b l e (ug l - 1 ) Gyre POC PON 32.8 29.2 260.8 23.2 5 0.61 n.s 5 0.81 n.s Front POC PON 30.4 3.6 428.0 18 0.92 0.01 55.6 18 0.90 0.01 Upwelling POC PON 23.2 3.2 464.0 16 0.58 0.05 50.4 16 0.64 0.01 Current POC PON 125.6 26.8 132.8 0.0 5 5 0.89 0.89 0.05 0.05 55 e x c e p t a t 2 s t a t i o n s where v a l u e s o f 1.5 u.g C u.g c h l - 1 h _ 1 were measured ( F i g . 1.9L). P Bm and a B v a l u e s were s i g n i f i c a n t l y l o w e r i n t h e low t e m p e r a t u r e p a r t o f t h e f r o n t ( s t a t i o n s 26 t o 3 7 ) . I k v a l u e s were c o n s t a n t a t 33 ± 6 u.E m - 2 s - i a c r o s s t h e f r o n t ( F i g . 1.9N). The c a r b o n f i x a t i o n was n o t measured i n t h e Gasp§ C u r r e n t d u r i n g t h i s c r u i s e . I n t h e Gyre ( s t a t i o n 1 ) , c a r b o n f i x a t i o n r a t e was below t h e d e t e c t i o n l i m i t (< 1.5 ug C h - 3 - ) , as o b s e r v e d d u r i n g c r u i s e A ( F i g . 1.9L). Vertical distribution F o l l o w i n g t h e t r a n s e c t , 17 v e r t i c a l p r o f i l e s o f s a l i n i t y and t e m p e r a t u r e were c o n d u c t e d a c r o s s t h e f r o n t t o i n v e s t i g a t e t h e p h y s i c a l s t r u c t u r e o f t h e f r o n t and t h e o r i g i n o f t h e n u t r i e n t - r i c h , c o l d w a t e r s l o c a t e d a t t h e o f f s h o r e edge o f t h e f r o n t . A t t h e i n s h o r e edge o f t h e Gaspe C u r r e n t , t h e mixed l a y e r was d e f i n e d by a s t r o n g h a l o c l i n e l o c a t e d a t 15 m ( F i g . 1.11A). The d e p t h o f t h e mixed l a y e r d e c r e a s e d r a p i d l y t o 2 m and l e s s t o w a r d t h e f r o n t . As o b s e r v e d d u r i n g t h e t r a n s e c t , a band o f c o l d w a t e r ( c a . 7.5°C) was l o c a t e d a t t h e edge o f t h e f r o n t ( F i g . 1.11B). S a l i n i t y a t t h e s e s t a t i o n s was between 28 and 2 9 ° / O 0 n e a r t h e s u r f a c e . The T/S c h a r a c t e r i s t i c s o f t h e u p w e l l e d w a t e r mass c o r r e s p o n d e d t o w a t e r found a t t h e base of t h e Gasp6 C u r r e n t (12-15 m). 56 H*G.CURRENT*+«FRON"I*H GYRE - H STATIONS B1 B3 B5 B7 B9 B11 B13 B15 B17 DISTANCE FROM SOUTH SHORE (km) Figure 1.11. Cruise B - V e r t i c a l d i s t r i b u t i o n o f: (A) s a l i n i t y , and (B) temperature on a transect across the Gasp6 Current (Bl to B6), the front (B7 to B l l ) and the Gyre (B12 to B17) i n the Gulf of St. Lawrence (see F i g . 1.8 f o r p o s i t i o n s of the stations Bl to B17). 57 Sampling year 1986 Two a d d i t i o n a l c r u i s e s were c o n d u c t e d i n 1986 t o c o n f i r m t h e r e c u r r e n c e of t h e d i s t r i b u t i o n a l p a t t e r n s o b s e r v e d i n 1985. C r u i s e C was c o n d u c t e d a t t h e b e g i n n i n g of June (June 1 t o 1 1 ) , t h e same p e r i o d as c r u i s e A i n 1985. The second c r u i s e ( c r u i s e D) was c o n d u c t e d l a t e r i n t h e season ( J u l y 21-28) when n u t r i e n t l i m i t a t i o n was e x p e c t e d t o be more s e v e r e . Cruise C V-FIN* transects D u r i n g t h i s c r u i s e , h i g h r e s o l u t i o n c r o s s - s e c t i o n s o f t e m p e r a t u r e and s a l i n i t y were o b t a i n e d by t o w i n g a v e r t i c a l l y o s c i l l a t i n g CTD probe ( V - F I N R system) between 0-80 m (see F i g . 1.12 f o r p o s i t i o n o f t r a n s e c t s ) . The f i r s t t r a n s e c t was c o n d u c t e d s i m u l t a n e o u s l y w i t h t h e r e g u l a r h o r i z o n t a l t r a n s e c t ( F i g . 1.13A, B ) . The second one was c o n d u c t e d a t t h e end o f t h e c r u i s e t o a s s e s s t h e n e a p - s p r i n g v a r i a b i l i t y i n t h e s t r u c t u r e o f t h e f r o n t ( F i g . 1.13C, D). The p h y s i c a l c h a r a c t e r i s t i c s o f t h e f r o n t a l a r e a v a r i e d l i t t l e between t h e two t r a n s e c t s ( F i g . 1.13). I n each s e c t i o n , t h e wedge-shaped l a y e r o f low s a l i n i t y w a t e r c o v e r e d a p p r o x i m a t e l y 8 km a l o n g t h e s o u t h s h o r e . S t r o n g m i x i n g o c c u r r e d a l o n g t h e boundary of t h e wedge, as was 58 F i g u r e 1 . 1 2 . P o s i t i o n s o f t h e s t a t i o n s a l o n g t h e h o r i z o n t a l t r a n s e c t s (numbers) and a t wh i c h v e r t i c a l p r o f i l e s were c o n d u c t e d ( c r u i s e l e t t e r / n u m b e r s ) d u r i n g c r u i s e C. 59 STATIONS 1 < 12 II 24 34 2« 18 10 2 Distance from the South Shore (km) Figure 1.13. Cruise C - High re s o l u t i o n cross-sections of s a l i n i t y and temperature i n the study area. The transects were conducted June 3 (panels A and B) and June 7 (panels C and D). Data were obtained by towing a v e r t i c a l l y o s c i l l a t i n g CTD probe (V-FIN R system). The duration of the transects was 6 h (June 3) and 2 h (June 7). The f i r s t transect was conducted simultaneously with the regular h o r i z o n t a l sampling (data presented i n Figure 1.14). 60 Figure 1.13. (continued) 61 e v i d e n c e d by t h e i n t e r l e a v i n g s t r u c t u r e of t h e t e m p e r a t u r e f i e l d . The mixed zone was l o c a t e d between 6 and 8 km o f f s h o r e , where maximum c u r r e n t v e l o c i t i e s a r e found i n t h e Gaspe C u r r e n t ( E l - S a b h and B e n o i t 1985). G i v e n t h a t one t r a n s e c t was c o n d u c t e d i n neap t i d e s and t h e o t h e r one i n s p r i n g t i d e s ( T a b l e 1.1), t h e s i m i l a r i t y o f t h e s e c t i o n s s u g g e s t s t h a t t h e s t r u c t u r e s were o n l y m o d e r a t e l y a f f e c t e d by t h e f o r t n i g h t l y t i d a l c y c l e . Horizontal distribution D u r i n g t h i s c r u i s e , samples were c o l l e c t e d a t 3 m a l o n g an 8 h t r a n s e c t p e r p e n d i c u l a r t o t h e f r o n t ( F i g . 1.12). The p h y s i c a l c h a r a c t e r i s t i c s o f t h e f r o n t a l a r e a were s i m i l a r t o t h o s e o b s e r v e d d u r i n g t h e same p e r i o d o f t h e y e a r i n 1985 ( c r u i s e A ) ( F i g . 1.14A, B ) . S a l i n i t y v a r i e d from ca. 30° / o o i n t h e Gyre ( s t a t i o n s 1 t o 9) t o 2 4 ° / o o i n t h e Gaspe C u r r e n t ( s t a t i o n s 18 t o 2 4 ) . Temperature was c a . 11°C i n t h e Gyre and d e c r e a s e d t o 8°C i n t h e Gaspe C u r r e n t . A s h a r p band o f c o l d e r w a t e r was o b s e r v e d i n t h e Gaspe C u r r e n t a t s t a t i o n s 18 and 19. The s u r f a c e mixed l a y e r was deep i n t h e Gyre (15 m) and d e c r e a s e d p r o g r e s s i v e l y a c r o s s t h e f r o n t t o r e a c h 3 m i n t h e Gaspe C u r r e n t ( F i g . 1.14C). C r u i s e C and A were a l s o s i m i l a r w i t h r e g a r d t o p h y t o p l a n k t o n community and c r o s s - f r o n t a l d i s t r i b u t i o n o f t h e biomass. A dense bloom o f Thalassiosira nordenskioeldii 62 F i g u r e 1 . 1 4 . C r u i s e C - H o r i z o n t a l d i s t r i b u t i o n a t 3 m o f : (A) s a l i n i t y , (B) t e m p e r a t u r e , (C) t h e s u r f a c e l a y e r d e p t h , (D) n i t r a t e , (E) s i l i c a t e , (F) c h l o r o p h y l l a, (G) POC, (H) PON, ( I ) N:C r a t i o o f p h y t o p l a n k t o n (•) and s e s t o n (°)(dashed l i n e r e p r e s e n t s t h e R e d f i e l d r a t i o ) , ( J ) p h y t o p l a n k t o n POC/CHL a r a t i o , (K) p h y t o p l a n k t o n PON/CHL a r a t i o , ( L ) P Bm, (M) a B , and (N) I K on a h o r i z o n t a l t r a n s e c t a c r o s s t h e f r o n t a l a r e a o f t h e Gasp6 C u r r e n t i n t h e G u l f o f S t . Lawrence (see F i g . 1 . 1 2 f o r p o s i t i o n o f s t a t i o n s 0 t o 2 4 ) . 6 3 0 5 10 15 20 25 0 5 10 15 20 25 + GYRE + FRONT +GASPE CURRENT + GYRE + FRONT +GASPE CURRENT STATIONS STATIONS Figure 1.14. (continued) 64 and Chaetoceros lacinlosus was p r e s e n t i n t h e Gaspe C u r r e n t and i n t h e f r o n t ( T a b l e 1.9). The Gyre community was composed o f s m a l l f l a g e l l a t e s . C h l o r o p h y l l a c o n c e n t r a t i o n s were h i g h i n t h e Gasp6 C u r r e n t (up t o 30 ug l - 1 ) , d e c r e a s e d a c r o s s t h e f r o n t , and r e a c h e d v e r y low v a l u e s (< 0.6 ug l - 1 ) i n t h e Gyre ( F i g . 1.14F). N i t r a t e c o n c e n t r a t i o n s were l o w e r t h a n 0.5 uM a t a l l s t a t i o n s a c r o s s t h e t r a n s e c t , e x c e p t a t s t a t i o n s 18 and 19 ( t h e low t e m p e r a t u r e s t a t i o n s ) where s l i g h t l y h i g h e r c o n c e n t r a t i o n s (1 and 1.5 uM) were r e c o r d e d ( F i g . 1.14E). The d i s t r i b u t i o n o f s i l i c a t e was somewhat d i f f e r e n t , w i t h low c o n c e n t r a t i o n s ( c a . 1.5 uM) i n t h e Gyre and i n t h e Gasp6 C u r r e n t b u t h i g h e r c o n c e n t r a t i o n s ( c a . 3 uM) i n t h e f r o n t ( F i g . 1.14D). The c r o s s - f r o n t a l v a r i a t i o n s i n n u t r i e n t c o n c e n t r a t i o n s were p a r a l l e l e d by v a r i a t i o n s o f t h e n u t r i e n t s t a t u s o f t h e community. P h y t o p l a n k t o n N:C r a t i o s were g e n e r a l l y below 0.10 i n t h e n u t r i e n t i m p o v e r i s h e d Gasp£ C u r r e n t , i n d i c a t i n g p o s s i b l e n u t r i e n t d e f i c i e n c y ( F i g . 1.141). The r a t i o i n c r e a s e d g r a d u a l l y t o ca. 0.14 to w a r d t h e f r o n t , where h i g h e r n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s were measured. I n t h e G y r e , POC and PON v a l u e s were n o t s i g n i f i c a n t l y c o r r e l a t e d w i t h c h l o r o p h y l l a and no c o r r e c t i o n was made f o r t h e e f f e c t o f d e t r i t u s on t h e p h y s i o l o g i c a l p a r a m e t e r s ( T a b l e 1.10). A t t h e Gyre s t a t i o n s , p l a n k t o n N:C r a t i o s 65 Table 1.9. Cruise C - Microplankton species and abundance (determined at 3 m along the transect) in the Gasp£ Current (station 19), the front per se (station 12) and in the Gulf Gyre (station 2). Gaspi Current Front Gulf Gyre (103 cells l"1) Species BACILLARIOPHYCEAE Chaetoceros compressus ' -• convolutus . . . " debilis 24.5 " diadeaa 36.7 " furcelatus 12.2 • laciniosus 147.0 24.5 " septentiionalis 36.8 -" similis 12.2 • sp. 306.0 73.5 Eucampia groenlandica - -Leptocylindius danicus -• minimus - - -Licmophora sp. -Navicula sp. . . . tfitzschia closterium -delicatissima 36.7 12.2 longissima - 24.5 seriata . . . sp. -Sfceletonema costatum 49.0 Thalassiosiia gravida 24.5 " nordenskioeldii 2920.0 613.0 " pacifica 98.0 sp. - 73.5 Unidentified pennates 12.2 Unidentified centrales -DINOPHYCEAE Alexandiium spp. -Gymnodiniura lohmannii . . . " spp. - - 78.4 Gyrodinium grenlandicum . . . Katodinium rotundatum -Oxitoxum spp. . . . Prorocentrum balticum - - -Protoperidinium bipes . . . " depressum -Unidentified dinoflagellates 134.0 73.0 73.3 Table 1.9. (continued) CHRYSOPHYCEAE Apedlnella s p i n l f e r a 49.0 Dinobryon balticum - 12.5 CRYPTOPHYCEAE Leucoczyptos mazina -Unidentified 147.0 73.5 147.0 PRASINOPHYCEAE Unidentified 36.7 49.0 36.7 EUGLENOPHYCEAE Unidentified - - 12.2 OTHERS Unidentified flagellates 956.0 1300.0 380.0 Others 1050.0 625.0 270.0 CHOANOFLAGELLATES Calliacantha sp. -Honosiga sp. -Unidentified 73.5 85.7 40.0 CILIATES Welicostomella subulata -Lohmnniella oviformis . . . Saipingella sp. -Tintinnopsis sp. -Unidentified ciliates 24.5 - 12.2 67 T a b l e 1.10. C r u i s e C- C o e f f i c i e n t s ( s l o p e and Y - i n t e r c e p t ) o f t h e l i n e a r r e g r e s s i o n a n a l y s e s c o n d u c t e d between t h e i n d e p e n d e n t v a r i a b l e c h l o r o p h y l l a and t h e dependent v a r i a b l e s POC and PON i n each p a r t o f t h e f r o n t a l a r e a (n = number o f d a t a p o i n t s , r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = n o t s i g n i f i c a n t ) . A r e a Dependent s l o p e Y - i n t e r c e p t n r p v a r i a b l e (ug 1 _ 1 ) Gyre POC 285.9 -18.2 9 0.16 n.s. PON 14.8 15.3 9 0.01 n.s. F r o n t & POC 41.3 171.1 8 0.90 0.01 C u r r e n t PON 6.4 31.8 8 0.93 0.01 68 were v a r i a b l e , b u t g e n e r a l l y h i g h e r t h a n 0.12. The p h y t o p l a n k t o n POC/Chl a r a t i o was r o u g h l y c o n s t a n t a t 40 i n t h e f r o n t and i n c r e a s e d up t o 80 i n t h e Gaspe C u r r e n t ( F i g . 1. 1 4 J ) . The p h y t o p l a n k t o n PON/Chi a r a t i o was r e l a t i v e l y c o n s t a n t a t 6 a c r o s s t h e f r o n t a l a r e a ( F i g . 1.14K). Among t h e p h o t o s y n t h e t i c p a r a m e t e r s , o n l y 1^ e x h i b i t e d v a r i a t i o n s a s s o c i a t e d w i t h t h e f r o n t ( F i g . 1.14L, M, N). V a l u e s o f I k were s i g n i f i c a n t l y ( t w o - t a i l e d t - t e s t , p<0.05) h i g h e r i n t h e f r o n t (mean ± S.D. = 99 ± 3) t h a n i n t h e Gaspe C u r r e n t (mean ± S.D. = 60 ± 9 ) , s u g g e s t i n g t h a t t h e p h y t o p l a n k t o n community l o c a t e d i n t h e f r o n t per se was a d a p t e d t o a h i g h e r l i g h t regime t h a n i n t h e Gasp6 C u r r e n t . P Bm was c a . 2.5 ng C [ig c h l - 3 - h - 1 i n t h e f r o n t and i n t h e Gaspe C u r r e n t and a B v a r i e d from 0.02 t o 0.05 |ig C | i c h l - 1 ( i E _ x m 2 s. Carbon f i x a t i o n r a t e s were u n d e t e c t a b l e i n t h e Gyre ( F i g . 1.14L). Vertical distribution To i n v e s t i g a t e t h e i m p o r t a n c e o f t h e l a t e r a l a d v e c t i o n of t h e f r o n t (and a s s o c i a t e d changes i n t h e v e r t i c a l d i s t r i b u t i o n o f t h e c h e m i c a l and b i o l o g i c a l p a r a m e t e r s ) d u r i n g a t i d a l c y c l e , v e r t i c a l p r o f i l e s (0 t o 40 m) were c o n d u c t e d e v e r y 4 h f o r 24 h a t a f i x e d s t a t i o n l o c a t e d ca. 12 km o f f s h o r e ( F i g . 1.12; s t a t i o n C l ) . A n a l y s i s o f t h e s a l i n i t y d a t a i n d i c a t e d t h a t t h e s t a t i o n was l o c a t e d e i t h e r i n t h e f r o n t per se o r i n t h e Gaspe C u r r e n t ( F i g . 1.15A). FRONT GASPE CURRENT X CL LU Q 16:00 TIME (h) 24:00 08:00 16:00 C. NITRATE (/xM) D. SILICATE (^M) F i g u r e 1.15. C r u i s e C - V e r t i c a l d i s t r i b u t i o n o f : (A) s a l i n i t y , (B) t e m p e r a t u r e , (C) n i t r a t e , (D) s i l i c a t e , (E) c h l o r o p h y l l a, (F) s e s t o n N:C r a t i o and (G) p h y t o p l a n k t o n N:C r a t i o d u r i n g 24 h a t a f i x e d s t a t i o n l o c a t e d i n t h e f r o n t ( s e e F i g . 1.12 f o r p o s i t i o n o f s t a t i o n C I ) . FRONT GASPE CURRENT 16:00 TIME (h) 24:00 08:00 H 16:00 0 10 20 30 40-r lJsS 0.10 •J f \^0.12 -.16 >-O,14 0 5 ' < 0.10 t).1(r — M 0 1 G. N:C RATIO OF PHYTOPLANKTON (atoms Figure 1.15. (continued) 71 D u r i n g t h e f i r s t 8 h (16:00 t o 24:00 h ) , s u r f a c e s a l i n i t y was between 27.8 and 28.2°/ c, 0, v a l u e s c h a r a c t e r i s t i c o f t h e f r o n t . S u r f a c e s a l i n i t y d e c r e a s e d t o 2 5 . 7 ° / 0 0 between 24:00 and 04:00 h and remained below 2 6 . 2 ° / 0 0 f o r t h e l a s t 12 h (04:00 t o 16:00 h ) . The 7°C i s o l i n e was o n l y a t 6 m a t 16:00 h and sank t o ca. 25 m f o r t h e r e s t o f t h e s a m p l i n g p e r i o d ( F i g . 1.15B). V a r i a t i o n s i n near s u r f a c e n u t r i e n t s and c h l o r o p h y l l a were s i m i l a r t o t h o s e o b s e r v e d d u r i n g t h e t r a n s e c t a t 3 m. N i t r a t e c o n c e n t r a t i o n s were below 0.1 jiM i n t h e f i r s t 15 m, e x c e p t a t 16:00 h where s u r f a c e w a t e r was c o l d e r and n i t r a t e c o n c e n t r a t i o n s v a r i e d between 1.0 and 7 piM. N i t r a t e c o n c e n t r a t i o n s were a l s o s l i g h t l y h i g h e r (up t o 0.4 nM) a t t h e end o f t h e s a m p l i n g p e r i o d ( F i g . 1.15C). S i l i c a t e c o n c e n t r a t i o n s were below 1.0 uM i n t h e Gasp6 C u r r e n t s u r f a c e w a t e r and i n c r e a s e d t o 5.0 uM i n t h e f r o n t ( F i g . 1.15D). C h l o r o p h y l l a c o n c e n t r a t i o n s were v e r y h i g h (up t o 18 ug l - 1 ) i n t h e Gasp<§ C u r r e n t ( s u r f a c e s a l i n i t y < 26°/oo) and l o w e r t h a n 5.0 ng l - 1 i n t h e f r o n t ( F i g . 1.15E). I n g e n e r a l , t h e p h y t o p l a n k t o n biomass was c o n c e n t r a t e d i n t h e upper 15 m a t each s t a t i o n . As o b s e r v e d d u r i n g t h e t r a n s e c t , c h l o r o p h y l l a c o n c e n t r a t i o n s a t 3 m and s a l i n i t y were s i g n i f i c a n t l y c o r r e l a t e d ( r =-0.77, p < 0.01), s u g g e s t i n g t h a t t h e c r o s s - f r o n t a l d i s t r i b u t i o n o f t h e biomass r e s u l t e d m a i n l y from h o r i z o n t a l and v e r t i c a l m i x i n g . 72 V a r i a t i o n s i n n u t r i e n t c o n c e n t r a t i o n s d u r i n g t h e 24 h f i x e d s t a t i o n were p a r a l l e l e d by v a r i a t i o n s of s e s t o n and p h y t o p l a n k t o n N:C r a t i o s ( F i g . 1.15F, G). P l a n k t o n N:C v a l u e s were < 0.10 i n t h e n u t r i e n t poor w a t e r o f t h e Gaspe C u r r e n t and i n c r e a s e d t o > 0.15 i n t h e f r o n t . P h y t o p l a n k t o n N:C r a t i o s were e s t i m a t e d f o l l o w i n g t h e method d e s c r i b e d p r e v i o u s l y (see M a t e r i a l s and Met h o d s ) . I n t h i s c a s e , r e g r e s s i o n a n a l y s e s between c h l o r o p h y l l a and POC ( o r PON) were c o n d u c t e d a t each s t a t i o n ( T a b l e 1.11). P h y t o p l a n k t o n N:C r a t i o s were a l s o low (< 0.10) i n t h e Gaspe C u r r e n t and i n c r e a s e d t o > 0.14 i n t h e f r o n t . As o b s e r v e d d u r i n g t h e t r a n s e c t , low p h y t o p l a n k t o n N:C r a t i o s c o r r e s p o n d e d w i t h low s i l i c a t e c o n c e n t r a t i o n s . C r u i s e D Horizontal distribution D u r i n g t h i s c r u i s e , one 8 h t r a n s e c t p e r p e n d i c u l a r t o t h e s h o r e was co n d u c t e d ( F i g . 1.16). As d u r i n g t h e p r e v i o u s c r u i s e s , t h e l o c a t i o n o f t h e f r o n t can be e a s i l y i d e n t i f i e d a l o n g t h e s a l i n i t y t r a n s e c t . S a l i n i t y was c a . 29°/ o c > and 26.6°/ 0o i n t h e Gyre and i n t h e Gaspe C u r r e n t r e s p e c t i v e l y ( F i g . 1.17A). Temperatures were ca. 15°C a t 3 m i n t h e Gyre and d e c r e a s e d t o 11.5°C i n t h e Gasp6 C u r r e n t ( F i g . 1.17B). D u r i n g t h i s c r u i s e , t h e f r o n t per se was v e r y s h a r p and c l o s e t o t h e c o a s t . 73 Table 1.11. Cruise C - C o e f f i c i e n t s (slope and Y-intercept) of the l i n e a r regression analyses conducted between the independent v a r i a b l e chlorophyll a and the dependent va r i a b l e s POC and PON at each s t a t i o n across the f r o n t a l area during the transect of v e r t i c a l p r o f i l e s (n = number of data points, r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = not s i g n i f i c a n t ) . Station Dependent slope Y-intercept n r p v a r i a b l e (ug l - 1 ) CI POC 69 114 16 0.87 0.01 PON 10 17 16 0.92 0.01 C2 POC 166 50 16 0.91 0.01 PON 19 27 16 0.88 0.01 C3 POC 196 62 16 0.53 0.05 PON 48 13 16 0.53 0.05 C4 POC 85 103 15 0.98 0.01 PON 9 34 15 0.98 0.01 C5 POC 88 170 13 0.97 0.01 PON 7 46 13 0.98 0.01 C6 POC 251 95 16 0.98 0.01 PON 20 21 16 0.96 0.01 C7 POC 114 17 16 0.97 0.01 PON 13 5 16 0.98 0.01 74 i 1 I 49°30* 1 / / \ N •N _ i i \ \ 5 / / / t 1 K \ \ \ 49°20" i / s _ - - • •10 \ s. — V D10» • s ^ • 15 ^ ~ 290m - - " - _ . - - ' ' " • "" -250m-• '20' • i80tn--' • 24 - - - 40m -49o10>- ~ 'rTr?r:'•. •. "^V:-:- •• • v * •'* ;••* / • v' , -'iv dros -Morne ~ •' ' Mont- Louis 0 10 km 66°50' 66o,40' 66°30* F i g u r e 1.16. P o s i t i o n s o f t h e s t a t i o n s a l o n g t h e h o r i z o n t a l t r a n s e c t s (numbers, 1 t o 24) and a t w h i c h v e r t i c a l p r o f i l e s were c o n d u c t e d ( c r u i s e l e t t e r / n u m b e r s , DI t o D l l ) d u r i n g c r u i s e D. 75 Z i B no data • — • 10 15 20 25 5 10 15 20 25 GYRE +FR0NT + G.C.+ STATIONS 100 75 i_ I 5 0 z o a. 25 0.00 0 5 10 15 20 25 + GYRE + FR0NT+G. C.+ STATIONS F i g u r e 1.17. C r u i s e D - H o r i z o n t a l d i s t r i b u t i o n a t 3 m o f : (A) s a l i n i t y , (B) t e m p e r a t u r e , (C) n i t r a t e , (D) s i l i c a t e , (E) c h l o r o p h y l l a, (F) POC, (G) PON and (H) N:C r a t i o o f s e s t o n (dashed l i n e r e p r e s e n t s t h e R e d f i e l d r a t i o ) on a t r a n s e c t a c r o s s t h e Gyre, t h e f r o n t and t h e Gasp6 C u r r e n t (G.C.) i n t h e G u l f o f S t . Lawrence (see F i g . 1.16 f o r p o s i t i o n o f s t a t i o n s 1 t o 2 4 ) . 76 C o n t r a r y t o t h e p r e v i o u s c r u i s e s , n u t r i e n t c o n c e n t r a t i o n s were low a c r o s s t h e whole t r a n s e c t ( F i g . 1.17C, D) . N i t r a t e c o n c e n t r a t i o n s were g e n e r a l l y below 0.5 uM a t each s t a t i o n . Two s t a t i o n s l o c a t e d i n t h e Gaspe C u r r e n t e x h i b i t e d s l i g h t l y h i g h e r c o n c e n t r a t i o n s (1.7 uM) . S i l i c a t e c o n c e n t r a t i o n s v a r i e d from 0.5 t o 1.3 uM. C h l o r o p h y l l a c o n c e n t r a t i o n s were a l s o v e r y low (< 0.25 ug l - 1 ) a t a l l s t a t i o n s a c r o s s t h e t r a n s e c t ( F i g . 1.17E). C h l o r o p h y l l a c o n c e n t r a t i o n s were below 0.1 ug l - 1 i n t h e G y r e , i n c r e a s e d s l i g h t l y a c r o s s t h e f r o n t and r e a c h e d 0.2 ug l - 1 i n t h e Gasp6 C u r r e n t . These were t h e l o w e s t c h l o r o p h y l l a c o n c e n t r a t i o n s measured a c r o s s t h e f r o n t d u r i n g t h i s s t u d y . The p h y t o p l a n k t o n community was n u m e r i c a l l y d o m i n a t e d by s m a l l f l a g e l l a t e s a t a l l s t a t i o n s , f o l l o w e d by Leucocryptos marina and Gymnodinium spp. i n t h e G y r e , and u n i d e n t i f i e d p r a s i n o p h y t e s and Monosiga sp. i n t h e Gaspe C u r r e n t and i n t h e f r o n t ( T a b l e 1.12). Diatoms s p e c i e s were s l i g h t l y more abundant i n t h e Gasp§ C u r r e n t and i n t h e f r o n t . V a r i a t i o n s i n POC and PON were s m a l l and n o t r e l a t e d t o c h l o r o p h y l l a ( F i g . 1.17F, G). The N:C r a t i o of s e s t o n was c o n s t a n t a t 0.07 a c r o s s t h e t r a n s e c t ( F i g . 1.17H). 77 Table 1.12. Cruise 0 - Microplankton species and abundance (determined at 3 m along the transect) in the Gaspe" Current (station 24), the front per se (station 20) and in the Gulf Gyre (station 1). Gasp£ Current Front Gulf Gyre (103 cells l ' 1 ) Species BACILLARIOPHYCEAE Chaetoceios coapiessus -" constrictus -" convolutus 1.9 2.9 debilis 54.8 " diadema . . . " furcelatus " gracilis -" laciniosus 13.7 4.9 • septentrionalis -" similis . . . • sp. _ . -Eucampla groenlandica . . . Leptocylindrus danicus 1.9 • minimus 2.9 32.3 4.9 Licmophoia sp. -Havicula sp. . . . Nitzschia closterium - 2.9 0.9 " delicatissima 0.9 21.5 10.8 " longissima . . . " seriata " sp. -SJceletonema costatum 4.9 22.5 Thalassionema nitzschiodes - 0.9 Thalassiosiia anguste-lineata 1.9 gravida 9.8 - -nordenskioeldii 17.6 10.8 pacifica -spp. 15.7 - 2.9 Unidentified pennates 0.9 1.9 Unidentified centrales . . . DINOPHYCEAE Alexandrium spp. Ceratium longibipes - - 0.9 " triacantha - - 0.9 Gymnodiniuio lohmannii . . . spp. 41.1 39.2 26.5 Gyrodinium grenlandicua 24.5 2.9 Heterocapsa triguetra 1.9 8.8 6.9 Katodinium rotundatum - 12.2 Oxitoxum spp. -Table 1.12. (continued) Prorocentrum balticum Protoperidinium blpes " brevipes " depressum Sciippsiella tiochoidea Unidentified dinoflagellates CHRYSOPHYCEAE Apedinella spinifera Dinobryon balticum • petiolatua CRYPTOPHYCEAE Leucocryptos marina Unidentified PRASINOPHYCEAE Unidentified EUGLENOPHYCEAE Unidentified OTHERS Unidentified flagellates Others CHOANOFLAGELLATES Calliacantha sp. Monosiqa sp. Unidentified CILIATES Helicostooella subulata Lohmnniella oviformis Salpingella sp. Tintinnopsis sp. Unidentified ciliates 1.9 0.9 1.9 0.9 2.9 12.2 24.5 24.5 3.9 12.2 12.2 36.7 61.3 159.0 73.5 1.9 -220.0 330.0 184.0 24.5 85.7 49.0 25.5 2.9 5.9 5.9 0.9 79 Vertical distribution V e r t i c a l p r o f i l e s were c o n d u c t e d a t 11 s t a t i o n s l o c a t e d ca. 0.5 km a p a r t a l o n g a t r a n s e c t p e r p e n d i c u l a r t o t h e f r o n t ( F i g . 1.16). As o b s e r v e d d u r i n g t h e t r a n s e c t , s u r f a c e s a l i n i t y v a r i e d from 2 6 . 5 ° / 0 0 i n t h e Gaspe C u r r e n t t o 2 9 . 5 ° / 0 0 i n t h e Gyre ( F i g . 1.18A). S u r f a c e t e m p e r a t u r e s were h i g h (> 12.0°C) a l l a c r o s s t h e f r o n t and t h e upper l a y e r was d e f i n e d by a s t r o n g t h e r m o c l i n e l o c a t e d near 20 m i n t h e Gaspe C u r r e n t and i n t h e Gyre ( F i g . 1.18B). I n t h e f r o n t ( s t a t i o n s 4, 5 and 6) t h e d e p t h of t h e t h e r m o c l i n e d e c r e a s e d t o 10 m. The upper mixed l a y e r was i m p o v e r i s h e d i n , n i t r a t e (< 0.5 J J M) and s i l i c a t e (< 2.0 /JM) a c r o s s t h e whole t r a n s e c t ( F i g . 1.18C, D). I n t h e f r o n t , t h e d e p t h o f t h e n u t r i c l i n e d e c r e a s e d t o ca. 10 m. C h l o r o p h y l l a c o n c e n t r a t i o n s were a l s o low (< 0.5 pg 1~^) i n t h e upper mixed l a y e r a t a l l s t a t i o n s ( F i g . 1.18E). However, a deep d i a t o m bloom dominated by Chaetoceros debilis and Thalassiosira n o r d e n s k i o e l d i i was p r e s e n t ( T a b l e 1.13) i n t h e h a l o c l i n e where c h l o r o p h y l l a c o n c e n t r a t i o n s r e a c h e d 10 ug l - ^ . The N:C r a t i o of s e s t o n was g e n e r a l l y below 0.15 i n t h e t o p 8 m a l l a c r o s s t h e t r a n s e c t . I n t h e f r o n t , N:C r a t i o o f s e s t o n was between 0.15 and 0.20, s u g g e s t i n g t h a t t h e c e l l s were n u t r i e n t s u f f i c i e n t ( F i g . 1.18F). Due t o t e c h n i c a l p r o b l e m s , c h e m i c a l and b i o l o g i c a l v a r i a b l e s were not sampled below 10 m a t s t a t i o n s 3 and 4. 80 i ,| I P. SILICATE (fiM) 4 12 20 28 DISTANCE FROM SOUTH SHORE (km) Figure 1.18. Cruise D - V e r t i c a l d i s t r i b u t i o n of: (A) s a l i n i t y , (B) temperature, (C) n i t r a t e , (D) s i l i c a t e , (E) c h l o r o p h y l l a and (F) N:C r a t i o of seston on a transect across the Gasp6 Current (Dl to D7), the front (D8 to D9) and the Gyre (D10 to Dll ) i n the Gulf of St. Lawrence (see F i g . 1.16 f o r p o s i t i o n of stations Dl to D l l ) . L^GASPE CURRENT ^ F R O N J ^ G Y R E ^ STATIONS D1 D2 D3 D4 D5 D6 D7 D8 D9D10D11 <0.5 <0.15 F. NfeC RATIO (atoms) 4 12 20 28 DISTANCE FROM SOUTH SHORE (km) Figure 1.18. (continued) 82 Table 1.13. Cruise 0 - Vertical distribution of microplankton abundance (IO1 cells l"1) at selected depths in the front (Station D7). Depths Species 0 8 14 18 20 24 30 BACILLARIOPHYCEAE Chaetoceros compressus - 7.9 8.8 - - - -" convolutus 10.7 0.9 - 0.9 - 0.9 -" debilis - 35.2 478.0 466.0 441.0 23.5 588.0 • laciniosus - 1.9 17.6 21.5 18.6 1.9 8.8 " septentrionalis - - 12.2 12.2 12.2 - -• similis - - - - - - -sp. - - - - - - -Eucampia groenlandica 0.9 0.9 1.9 0.9 - - 4.9 Leptocylindrus danicus 0.9 - 2.9 11.7 0.9 - 1.9 ' minimus 29.4 11.7 3.9 11.7 16.6 15.7 -Licmophora sp. - - - - - - -Navicula sp. - - - - - - -tfitzschia closterium 0.9 1.9 1.9 7.8 5.9 1.9 2.9 • delicatissima 3.9 11.7 6.8 9.8 20.6 20.6 3.9 " seriata - - - - - - -• sp. - - - - - - -Skeletoned costatum 13.7 8.8 8.8 - 17.6 7.8 -Thalassiosira gravida - - 147.0 90.1 20.6 - 118.6 " nordenskioeldii 0.9 6.8 625.0 330.0 181.0 282.0 1250.0 " sp. ' - - - - - - -Unidentified pennates - 3.9 - 3.9 0.9 - 0.9 Unidentified centrales - - - - - - -DINOPHYCEAE Alexandrium spp. - - - - - - -Gymnodiniam lohmannil - 0.9 - - 0.9 - 0.9 ' spp. 11.7 22.5 11.7 19.6 13.7 10.8 11.8 Gyrodinium grenlandicum 0.3 24.5 24.5 12.3 12.3 24.5 12.2 Katodinium rotundatum 12.2 0.9 - • - - - 1.9 Oxitoxum spp. - - 0.9 - 0.9 - 1.9 Prorocentrum balticum - - - - - - -Protoperidinium bipes - - 0.9 - 0.9 - 1.9 " depressum - - - - - - -" pellloilua - - - - - - -Cladopyxls claytonli - - - - - - -Dinophysis acuminata - - - - - - -Unidentified dinoflagellates 2.9 2.9 0.9 2.9 1.9 1.9 6.8 CHRYSOPHYCEAE Apedinella spinifera 24.5 12.2 - - 12.2 - -Dinobryon balticum " petiolatum 5.9 - - - - 208.0 -0.9 - 12.2 - 12.2 -83 Table 1.13. (continued) CRYPTOPHYCEAE Leucociyptos marina 98.0 98.0 - 24.5 36.7 36.7 61.2 Unidentified 0 j PRASINOPHYCEAE Unidentified 24.5 - 12.2 - - 12.2 EUGIENOPHYCEAE Unidentified 0.9 0.9 1.9 0.9 - 0.9 1.9-OTHERS Unidentified flagellate spp. 392.0 355.0 306.0 306.0 318.0 245.0 355.0 CHOANOFLAGELLATES Calliacantha sp. 49.0 49.0 - - 24.5 Honosiga sp. 49.0 36.7 61.2 61.2 73.5 61.2 12.2 CILIATES Helicostomella subulata - - - 0.9 3.9 Lohmanniella oviformis - - - 0.9 -Salpingella sp. Tintinnopsis sp. 0.9 Unidentified ciliates 7.8 1.9 1.9 2.9 2.9 2.9 4.9 84 DISCUSSION P h y s i c a l c h a r a c t e r i s t i c s o f t h e f r o n t a l a r e a The f r o n t a l area l o c a t e d o f f s h o r e of Mont-Louis was surveyed 4 times between May and J u l y d u r i n g the ye a r s 1985 and 1986. The i n t e r f a c e between the Gaspe Cu r r e n t and the A n t i c o s t i Gyre was always d e f i n e d by a s t r o n g s a l i n i t y change of ca. 5 ° / 0 0 . The approximate h o r i z o n t a l width of the h i g h s a l i n i t y g r a d i e n t r e g i o n was es t i m a t e d f o r each t r a n s e c t u s i n g the s h i p speed, the sampling frequency and v a r i a t i o n s i n s a l i n i t y . The width of the f r o n t was 15, 9, 15 and 5 km d u r i n g c r u i s e s A, B, C and D r e s p e c t i v e l y . A l though p a r t of the v a r i a b i l i t y may be due t o the l a c k of s y n o p t i c i t y , these e s t i m a t e s are i n agreement wi t h the mean width of 10 km p r e v i o u s l y r e p o r t e d f o r t h i s p e r i o d of the year (see Study A r e a ) . The h i g h s a l i n i t y g r a d i e n t r e g i o n was g e n e r a l l y c h a r a c t e r i z e d by temperature i n v e r s i o n s and c o l d s u r f a c e (or sub s u r f a c e ) temperatures, r e s u l t i n g from e i t h e r v e r t i c a l m i x ing and/or u p w e l l i n g (Tang, 1980 a, b, 1982, 1983, B e n o i t et a i . 1985). B e n o i t et al. (1985) showed t h a t the c r o s s -f r o n t a l shear s t r e s s decreased w i t h the p r o g r e s s i o n of the season and t h a t t h i s decrease was accompanied by an o f f s h o r e d i s p l a c e m e n t of the l o c a t i o n of the maximum shear s t r e s s . The p h y s i c a l measurements a l s o r e v e a l e d these s e a s o n a l 85 v a r i a t i o n s w h i c h appear t o have a p r o f o u n d e f f e c t on t h e c h e m i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s of t h e f r o n t . I n J u n e , when c u r r e n t v e l o c i t y and s h e a r s t r e s s a r e maximum, t h e c r o s s - f r o n t a l c i r c u l a t i o n was e n t i r e l y d o m i n a t e d by t u r b u l e n t p r o c e s s e s . T h i s h y p o t h e s i s i s s u p p o r t e d by t h e s t r o n g c o r r e l a t i o n between s a l i n i t y and t e m p e r a t u r e a c r o s s t h e f r o n t i n e a r l y summer ( F i g . 1.19). The band o f c o l d w a t e r was l o c a t e d ca. 6-8 km o f f s h o r e , t h e same d i s t a n c e t h a t B e n o i t e t al. (1985) measured an a b r u p t d e c r e a s e i n c u r r e n t v e l o c i t y a t t h i s t i m e o f y e a r . C o n s i d e r a b l e s h e a r s t r e s s and t u r b u l e n t m i x i n g a r e t h u s e x p e c t e d t o o c c u r i n t h i s a r e a and may e x p l a i n t h e i n t r o d u c t i o n o f c o l d e r n u t r i e n t - r i c h w a t e r n e a r t h e s u r f a c e . R e s u l t s f rom t h e t h r e e s u c c e s s i v e t r a n s e c t s c o n d u c t e d i n J u l y ( c r u i s e C) show, however, t h a t t h e l o c a t i o n o f t h e t e m p e r a t u r e minimum may v a r y b o t h s p a t i a l l y ( e . g . d i s t a n c e from t h e s h o r e ) and w i t h r e s p e c t t o t h e h i g h s a l i n i t y g r a d i e n t r e g i o n . Over t h e 2-3 days i n t e r v a l s s e p a r a t i n g t h e t r a n s e c t s , t h e minimum t e m p e r a t u r e was a s s o c i a t e d w i t h s a l i n i t i e s o f 26, 28, and 2 6 ° / 0 0 . L a t e r i n t h e season ( c r u i s e s B and D), a w a t e r mass o f r e l a t i v e l y low t e m p e r a t u r e and h i g h s a l i n i t y was p r e s e n t i n t h e f r o n t ( F i g . 1.19), r e f l e c t i n g a d i f f e r e n t c r o s s - f r o n t a l c i r c u l a t i o n . T h i s change i n c i r c u l a t i o n p a t t e r n i s p r o b a b l y l i n k e d t o t h e d e c r e a s e i n c u r r e n t v e l o c i t y and s h e a r s t r e s s 86 12 10 i 8 12 10 26 27 28 29 SALINITY (0/00) 30 31 24 26 28 SALINITY (0/00) 30 32 25 26 27 28 29 SALINITY (0/00) 30 31 10-1 26 27 28 29 SALINITY (0/00) 30 31 Figure 1.19. Temperature vs s a l i n i t y relationships at 3 m during the transects conducted during: (A) cruise A, (B) cr u i s e B, (C) cruise C and (D) cr u i s e D. 87 p r e v i o u s l y r e p o r t e d d u r i n g summer ( B e n o i t e t al. 1985). The zone o f low s u r f a c e t e m p e r a t u r e , when p r e s e n t , was t h e n l o c a t e d more o f f s h o r e (ca. 18 km) and c o r r e s p o n d e d t o h i g h e r s a l i n i t i e s t h a n i n e a r l y June. D u r i n g c r u i s e B and E (see C h a p t e r 2 f o r d a t a on c r u i s e E ) , t h e band o f c o l d w a t e r (7°C) c o r r e s p o n d e d t o s a l i n i t i e s o f ca. 29.0°/oo and was o b s e r v e d a t t h e o f f s h o r e edge o f t h e f r o n t (see F i g . 1.9A, B and 1.11A, B ) . The w i d t h o f t h e c o l d w a t e r band was ca. 5 km. A l t h o u g h t h e p r e s e n c e o f t h i s c o l d w a t e r band has been p r e v i o u s l y a t t r i b u t e d t o u p w e l l i n g (see b e l o w ) , i t i s n o t e w o r t h y t h a t t h e a p p a r e n t o f f s h o r e d i s p l a c e m e n t of t h e c o l d w a t e r band d u r i n g t h e season c o i n c i d e d w i t h t h e p r e v i o u s l y r e p o r t e d d i s p l a c e m e n t o f t h e maximum c u r r e n t v e l o c i t y ( B e n o i t e t al. 1985). F i n a l l y , no anomaly i n t h e s u r f a c e t e m p e r a t u r e d i s t r i b u t i o n was o b s e r v e d i n c r u i s e D. F r o n t s a r e o f t e n c h a r a c t e r i z e d by l a r g e s c a l e v e r t i c a l m o t i o n such as d o w n w e l l i n g and u p w e l l i n g (Bowman and I v e r s o n 1978). I n s h a l l o w sea f r o n t s , f o r example, u p l i f t i n g o f t h e n e a r - s u r f a c e i s o t h e r m as w e l l as a narrow band o f minimum sea s u r f a c e t e m p e r a t u r e s was p r e v i o u s l y o b s e r v e d on t h e mixed s i d e o f t h e f r o n t (James 1978, K r a u s e e t al. 1986, Van H e i j s t 1986). A minimum i n sea s u r f a c e t e m p e r a t u r e has been a l s o o b s e r v e d a l o n g a s a l i n i t y f r o n t i n t h e B a l t i c Sea (Kahru e t al. 1986). In t h e case o f t h e Gaspe C u r r e n t f r o n t , Tang (1982) i n v e s t i g a t e d t h e h y p o t h e s i s t h a t t h e f r o n t a l c o l d w a t e r band r e s u l t e d from u p w e l l i n g . F o l l o w i n g 88 SEA SURFACE LATERAL • • • BOUNDARY • LAYER Figure 1.20. Schematic representation of the model of c r o s s - f r o n t a l c i r c u l a t i o n proposed by Tang (1980b) f o r the Gasp6 Current front. 89 Tang's model, w h i c h i s s c h e m a t i z e d i n F i g u r e 1.20, t h e u p w e l l i n g i s produced by a mechanism s i m i l a r t o t h a t g e n e r a t i n g c o a s t a l u p w e l l i n g ( A l l e n 1973). I n c o a s t a l u p w e l l i n g , t h e v e r t i c a l m o t i o n r e s u l t s from s u r f a c e w a t e r b e i n g t r a n s p o r t e d o f f s h o r e by t h e w i n d , c a u s i n g a co m p e n s a t i n g v e r t i c a l movement o f w a t e r t o t h e s u r f a c e t h r o u g h a l a t e r a l boundary l a y e r . I n t h e case o f a c o a s t a l j e t f r o n t , t h e s t r o n g v e r t i c a l s h e a r a c r o s s t h e i n t e r f a c e i n d u c e s an i n t e r f a c i a l Ekman t r a n s p o r t a l o n g t h e i n t e r f a c e and d i r e c t e d away from t h e f r o n t ( t oward t h e c o a s t i n t h e Gaspe C u r r e n t c a s e ) . A convergence zone i s t h u s c r e a t e d a t the edge o f t h e f r o n t and, i n o r d e r t o i n s u r e mass c o n t i n u i t y , a compensating f l o w i s drawn d i r e c t l y from below th e f r o n t . B oth u p w e l l i n g and d o w n w e l l i n g o f s u r f a c e w a t e r have been p r e d i c t e d f o r a f r o n t a l l a y e r p r o p a g a t i n g i n t o ambient s t i l l w a t e r ( A l l e n 1973, G a r v i n e 1974, 1979). As a l s o o b s e r v e d by Tang ( 1 9 8 2 ) , t h e T-S c h a r a c t e r i s t i c s o f t h e u p w e l l e d w a t e r i n d i c a t e t h a t t h e w a t e r came from beneath t h e Gasp6 C u r r e n t . To d a t e , i t i s s t i l l n o t c l e a r whether t h e minimum i n s u r f a c e t e m p e r a t u r e depends s o l e l y on v e r t i c a l m i x i n g o r on an u p w e l l i n g c i r c u l a t i o n . T e c h n i c a l l y , t h e p r e s e n c e o f s m a l l e r s c a l e t u r b u l e n t p r o c e s s e s a c r o s s t h e f r o n t makes i t more d i f f i c u l t t o d e t e c t r e l a t i v e l y weak v e r t i c a l m o t i o n s such as t h o s e a s s o c i a t e d w i t h u p w e l l i n g . However, my r e s u l t s have y i e l d e d t h e f o l l o w i n g new i n f o r m a t i o n : 1) t h e mechanisms r e s p o n s i b l e f o r t h e p r e s e n c e o f c o l d w a t e r i n t h e f r o n t a r e n o t always a c t i n g ( d a t a from c r u i s e D), 2) t h e p r e s e n c e o f s u r f a c e c o l d w a t e r may r e s u l t s o l e l y from v e r t i c a l m i x i n g ( d a t a from c r u i s e s A and C ) , 3) t h e c o l d w a t e r band may be a s s o c i a t e d w i t h s a l i n i t i e s v a r y i n g from 26°/oo ( c r u i s e s A and C) t o 2 9 ° / 0 0 ( c r u i s e s B and E; see C h a p t e r 2 f o r d a t a on c r u i s e E) and, 4) t h e l o c a t i o n o f t h e t e m p e r a t u r e minimum appears t o move o f f s h o r e d u r i n g t h e se a s o n . I n summary, my r e s u l t s i n d i c a t e t h a t i m p o r t a n t v a r i a t i o n s o f t h e p h y s i c a l c h a r a c t e r i s t i c s o f t h e f r o n t o c c u r d u r i n g t h e season. These v a r i a t i o n s appear t o be m o s t l y d r i v e n by l o n g t erm p r o c e s s e s such as t h e s e a s o n a l v a r i a t i o n s i n f r e s h w a t e r r u n o f f and a s s o c i a t e d v a r i a t i o n s i n c u r r e n t v e l o c i t y and s h e a r s t r e s s . D u r i n g s p r i n g , t h e c r o s s - f r o n t a l c i r c u l a t i o n i s e n t i r e l y d r i v e n by t u r b u l e n t m i x i n g and a zone o f maximum m i x i n g i s l o c a t e d n e a r t h e s h o r e . L a t e r d u r i n g t h e s e a s o n , t h e f r o n t i s more s t a b l e and a complex c r o s s - f r o n t a l c i r c u l a t i o n , p r o b a b l y c h a r a c t e r i z e d by u p w e l l i n g , t a k e s p l a c e . I n b o t h c a s e s , t h e c r o s s - f r o n t a l c i r c u l a t i o n r e s u l t s g e n e r a l l y i n t h e i n p u t o f c o l d n u t r i e n t - r i c h w a t e r (see below) i n t h e s u r f a c e w a t e r o f t h e f r o n t . 91 C h e m i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s o f t h e f r o n t a l a r e a S i m i l a r c h e m i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s were fo u n d i n t h e A n t i c o s t i Gyre d u r i n g a l l c r u i s e s c o n d u c t e d d u r i n g t h i s s t u d y . N i t r a t e and s i l i c a t e c o n c e n t r a t i o n s were low i n t h e upper l a y e r , as p r e v i o u s l y o b s e r v e d by S e v i g n y e t al. (1979) and T h e r r i a u l t and L e v a s s e u r (1985). The p h y t o p l a n k t o n community was dominated n u m e r i c a l l y by s m a l l f l a g e l l a t e s , and two d i n o f l a g e l l a t e s Prorocentrum balticum and Katodinium rotundata. C h l o r o p h y l l a c o n c e n t r a t i o n s were always l o w e r t h a n 1.0 pq i n t h e upper l a y e r . The low p h y t o p l a n k t o n biomass o b s e r v e d i n t h e Gyre i n J u n e - J u l y was e x p e c t e d s i n c e t h e a n n u a l d i a t o m bloom i s known t o o c c u r i n M a r c h - A p r i l i n t h i s p a r t o f t h e G u l f . D u r i n g c r u i s e A, a s u b s u r f a c e c h l o r o p h y l l a maximum was o b s e r v e d i n t h e Gyre between 20 and 30 m. The Gaspe C u r r e n t i s a h i g h l y a d v e c t i v e system and c o n s e q u e n t l y , v a r i a t i o n s i n i t s p r o p e r t i e s ( n u t r i e n t s and biomass c o n c e n t r a t i o n s ) r e f l e c t m a i n l y c o n d i t i o n s found u pstream, i n t h e l o w e r p o r t i o n o f t h e e s t u a r y . D u r i n g t h i s s t u d y , h i g h e s t c h l o r o p h y l l c o n c e n t r a t i o n s were always measured a t t h e end o f May o r t h e b e g i n n i n g o f June ( c r u i s e A and C ) , a p e r i o d c o r r e s p o n d i n g t o t h e l a t e s p r i n g d i a t o m bloom i n t h e l o w e r e s t u a r y ( L e v a s s e u r e t al. 1984). D u r i n g t h i s p e r i o d , c h l o r o p h y l l a c o n c e n t r a t i o n s were h i g h (up t o 92 35 ng l - 1 ) and homogeneously d i s t r i b u t e d i n t h e 10-20 m upper l a y e r . The p h y t o p l a n k t o n community was dominated by th e d i a t o m Thalassiosira nordenskioeldii, as p r e v i o u s l y o b s e r v e d a t t h i s t i m e df t h e y e a r i n t h e l o w e r p o r t i o n o f t h e e s t u a r y ( L e v a s s e u r e t al. 1984). R e l a t i v e l y low n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s (< 1-2 (iM) were measured. L a t e r d u r i n g t h e season ( c r u i s e s B and D), c h l o r o p h y l l a c o n c e n t r a t i o n s v a r i e d between 1 and 5 ng n u t r i e n t c o n c e n t r a t i o n s remained low, and t h e s p r i n g d i a t o m community was r e p l a c e d by a community dominated by f l a g e l l a t e s and d i n o f l a g e l l a t e s . The s m a l l s c a l e d i s t r i b u t i o n of n u t r i e n t s and p h y t o p l a n k t o n biomass a c r o s s t h e f r o n t e x h i b i t e d 3 d i s t i n c t p a t t e r n s w h i c h a r e s c h e m a t i z e d i n F i g u r e 1.21. These p a t t e r n s c o r r e s p o n d e d t o t h e s e a s o n a l v a r i a t i o n s i n t h e p h y s i c a l c h a r a c t e r i s t i c s o f t h e f r o n t o b s e r v e d p r e v i o u s l y . C r u i s e s A and C, w h i c h were c o n d u c t e d a t t h e b e g i n n i n g o f June ( l a t e s p r i n g ) e x h i b i t e d p a t t e r n A, w h i l e c r u i s e s B and D (and E, see C h a p t e r 2) w h i c h were c o n d u c t e d l a t e r i n t h e season (summer) e x h i b i t e d p a t t e r n B o r C. F o r t h e sake o f c l a r i t y , d a t a r e p r e s e n t a t i v e o f s p r i n g and summer c o n d i t i o n s s h a l l be d i s c u s s e d s e p a r a t e l y . 93 P A T T E R N 1 - SPRING P A T T E R N 2 - SUMMER C H L O R O P H Y L L NUTRIENTS O m H X 25m P A T T E R N 3 - SUMMER (upwelling) C H L O R O P H Y L L NUTRIENTS L 25m 25m F i g u r e 1.21. Schematic r e p r e s e n t a t i o n s o f t h e v e r t i c a l d i s t r i b u t i o n o f s a l i n i t y , n u t r i e n t s ( n i t r a t e o r s i l i c a t e ) and p h y t o p l a n k t o n biomass a c r o s s t h e f r o n t a l a r e a d u r i n g : (A) s p r i n g , (B) summer and (C) d u r i n g an u p w e l l i n g e v e n t ( H a t c h e d a r e a s r e p r e s e n t zone o f maximum c o n c e n t r a t i o n s ) . 9 4 L a t e s p r i n g c o n d i t i o n s The c r o s s - f r o n t a l d i s t r i b u t i o n o f n u t r i e n t s and p h y t o p l a n k t o n was s i m i l a r d u r i n g t h e two c r u i s e s c o n d u c t e d d u r i n g s p r i n g t i m e . N i t r a t e and s i l i c a t e c o n c e n t r a t i o n s were maximum i n t h e f r o n t and r e l a t i v e l y l ow i n t h e Gasp§ C u r r e n t and i n t h e Gyre. As d i s c u s s e d p r e v i o u s l y , t h e f r o n t was t h e n c h a r a c t e r i z e d by h i g h s h e a r s t r e s s r e s u l t i n g i n c o n s i d e r a b l e v e r t i c a l m i x i n g . S i n c e c o l d n u t r i e n t - r i c h w a t e r s were always p r e s e n t i n t h e f r o n t a t t h a t t i m e o f t h e y e a r , i t may be p o s t u l a t e d t h a t t h e f r o n t a l e n r i c h m e n t i n n u t r i e n t s i s c o n t i n u o u s d u r i n g s p r i n g t i m e . I n l a t e s p r i n g , a g r a d u a l o n s h o r e - o f f s h o r e d e c r e a s e i n p h y t o p l a n k t o n biomass was o b s e r v e d w i t h no n o t i c e a b l e change i n t h e p h y t o p l a n k t o n community s t r u c t u r e . These o b s e r v a t i o n s s u g g e s t t h a t t h e p h y t o p l a n k t o n p r o d u c e d i n t h e Gaspe C u r r e n t was p a s s i v e l y mixed and d i l u t e d w i t h t h e s u r f a c e w a t e r o f t h e Gyre. The i m p o r t a n c e of p h y s i c a l m i x i n g on t h e p h y t o p l a n k t o n d i s t r i b u t i o n a c r o s s t h e f r o n t was e s t i m a t e d by p l o t t i n g c h l o r o p h y l l a c o n c e n t r a t i o n s a g a i n s t s a l i n i t y ( F i g . 1.22). T h i s r e p r e s e n t a t i o n c l e a r l y shows t h e i m p o r t a n c e o f r a p i d t u r b u l e n t p r o c e s s e s on t h e p h y t o p l a n k t o n d i s t r i b u t i o n . F o r example, a s i g n i f i c a n t c o r r e l a t i o n c o e f f i c i e n t ( r = 0.85) between c h l o r o p h y l l a and s a l i n i t y i n d i c a t e s t h a t 72% o f t h e v a r i a b i l i t y i n t h e 95 3. >-X CL O or O _ i I 2 7 2 8 2 9 SALINITY (0/00) 30 31 =1 X a o or O _ J x u 2 7 SALINITY (0/00) F i g u r e 1.22. C h l o r o p h y l l a c o n c e n t r a t i o n s vs s a l i n i t y measured a t 3 m d u r i n g the t r a n s e c t s c o n d u c t e d d u r i n g : (A) c r u i s e A and (B) c r u i s e C. 96 d i s t r i b u t i o n o f c h l o r o p h y l l r e s u l t s from p h y s i c a l m i x i n g a c r o s s t h e f r o n t . The r e g r e s s i o n c o e f f i c i e n t between s a l i n i t y and c h l o r o p h y l l was s i g n i f i c a n t (p < 0.01) f o r b o t h c r u i s e s . L i n e a r r e g r e s s i o n s o f c h l o r o p h y l l a a g a i n s t s a l i n i t y i n d i c a t e t h a t 70 % ( c r u i s e A) and 60 % ( c r u i s e C) o f t h e v a r i a b i l i t y i n p h y t o p l a n k t o n biomass i s e x p l a i n e d by p h y s i c a l m i x i n g . F o r c r u i s e A, a d i f f e r e n t r e g r e s s i o n a n a l y s i s was used f o r t h e two d i f f e r e n t c r o s s i n g s o f t h e f r o n t ( v a l u e s used f o r t h e r e g r e s s i o n a r e p r e s e n t e d i n F i g . 1.23). These r e s u l t s a r e n o t s u r p r i s i n g s i n c e i t was shown p r e v i o u s l y t h a t t h i s p e r i o d o f t h e y e a r was c h a r a c t e r i z e d by h i g h f r e s h w a t e r r u n o f f , h i g h c u r r e n t v e l o c i t y and a h i g h l y t u r b u l e n t c r o s s - f r o n t a l c i r c u l a t i o n . B e n o i t et al. (1985) r e p o r t e d v e r y h i g h c u r r e n t v e l o c i t i e s (up t o 110 cm s - 1 ) and s h e a r s t r e s s i n t h e upper 40 m o f t h e Gaspe C u r r e n t i n June. V i g o r o u s and r a p i d m i x i n g a c r o s s t h e f r o n t i s t h u s e x p e c t e d . Plume f r o n t s have been p r e v i o u s l y a s s o c i a t e d w i t h a d e c r e a s e i n p h y t o p l a n k t o n biomass. Bowman and I v e r s o n (1978) r e p o r t e d a d e c r e a s e i n c h l o r o p h y l l a i n an e s t u a r i n e plume f r o n t o f t h e Hudson R i v e r . S i m i l a r l y , Kahru e t a l . (1986) o b s e r v e d l o w e r p h y t o p l a n k t o n biomass i n an e s t u a r i n e f r o n t i n t h e B a l t i c Sea. I n t h e s e c a s e s , a h i g h l y t u r b u l e n t c r o s s - f r o n t a l c i r c u l a t i o n may a l s o have been r e s p o n s i b l e f o r t h e p a s s i v e d i l u t i o n o f t h e biomass produced on t h e n u t r i e n t - e n r i c h e d s i d e o f t h e f r o n t . 97 CRUISE A CJ1 =1 X CL o cc o _i u 2 8 2 9 SALINITY ( 0 / 0 0 ) 3 0 Figure 1.23. Chlorophyll a concentrations vs s a l i n i t y measured at 3m during the two transects across the front conducted during c r u i s e A. 98 During c r o s s - f r o n t a l mixing, the chemical and p h y s i c a l d ata showed t h a t the phytoplankton community was s u b j e c t e d t o e n v ironmental changes (e.g. n u t r i e n t c o n c e n t r a t i o n s and l i g h t regime). I t i s w e l l known t h a t the response of ph y t o p l a n k t o n t o environmental v a r i a t i o n s i s s c a l e -dependent. As summarized by H a r r i s (1980), s h o r t term p e r t u r b a t i o n s w i l l l e a d t o p h y s i o l o g i c a l adjustment (e.g. b i o c h e m i c a l composition and p h o t o s y n t h e t i c parameters) w h i l e l o n g e r term p e r t u r b a t i o n s w i l l e v e n t u a l l y l e a d t o e c o l o g i c a l adjustment (e.g. change i n biomass and community s t r u c t u r e ) . During s p r i n g , the r e s u l t s of t h i s study i n d i c a t e t h a t the phy t o p l a n k t o n community of the Gaspe C u r r e n t was v i g o r o u s l y mixed w i t h the Gyre waters. No biomass accumulation o r changes i n the phytoplankton community were a s s o c i a t e d w i t h t h i s m i x i n g . These r e s u l t s suggest t h a t the h o r i z o n t a l m i xing between the two water masses oc c u r s over a time s c a l e s h o r t e r than those r e l e v a n t f o r phytoplankton s u c c e s s i o n (weeks) and biomass p r o d u c t i o n (days) ( H a r r i s 1980). S i n c e p h y s i o l o g i c a l adjustments t o new environmental c o n d i t i o n s may o c c u r over a time s c a l e of minutes t o hours ( H a r r i s 1980), p h y s i o l o g i c a l a d a p t a t i o n s t o the f r o n t a l l i g h t and n u t r i e n t c o n d i t i o n s are p r e d i c t e d . Among numerous s t u d i e s of f r o n t a l dynamics, evidence of p h y s i o l o g i c a l responses of the phyt o p l a n k t o n community t o the c r o s s - f r o n t a l c i r c u l a t i o n i s s p a r s e . T h i s l a c k of 99 i n f o r m a t i o n i s p a r t i c u l a r l y s u r p r i s i n g s i n c e , i n many c a s e s , t h e e c o l o g i c a l i m p a c t of t h e f r o n t i s assumed t o r e s u l t from some p h y s i o l o g i c a l improvements i n t h e community t o t h e new n u t r i e n t and l i g h t c o n d i t i o n s found i n t h e f r o n t per se ( P i n g r e e e t a l . 1975, 1977, 1978, 1982, 1983, Simpson and P i n g r e e 1978, P a r s o n s et al. 1983). I n g e n e r a l , t e m p o r a l and/or s p a t i a l v a r i a t i o n s i n t h e i n t e n s i t y o f v e r t i c a l m i x i n g and d e p t h o f t h e s u r f a c e l a y e r a r e b e l i e v e d t o r e s u l t i n v a r i a t i o n s o f l i g h t and n u t r i e n t s w h i c h , i n r e t u r n , w i l l a f f e c t p r i m a r y p r o d u c t i v i t y . I n s h a l l o w sea f r o n t s , f o r example, t h e h i g h p h y t o p l a n k t o n biomass found i n t h e f r o n t has been e x p l a i n e d by t h e f o r t n i g h t l y t i d a l e x c u r s i o n o f t h e f r o n t w h i c h c o n t r i b u t e s t o t h e p e r i o d i c r e p l e n i s h m e n t o f n u t r i e n t s i n t h e w a t e r on t h e s t a b l e s i d e o f t h e f r o n t (Legendre e t al. 1986). A l t h o u g h n e v e r r e p o r t e d , such v a r i a t i o n s i n t h e n u t r i e n t and l i g h t regime s h o u l d be f o l l o w e d by v a r i a t i o n s i n t h e n u t r i t i o n a l and p h o t o s y n t h e t i c c h a r a c t e r i s t i c s o f t h e p h y t o p l a n k t o n community. I t has a l s o been s u g g e s t e d t h a t t h e s e v a r i a t i o n s i n t h e p h y s i o l o g i c a l p a r a m e t e r s would p r o v i d e a more r e a l i s t i c a p p r e c i a t i o n o f t h e e x t e n t o f t h e f r o n t a l a r e a ( L o d e r and P i a t t 1984). D u r i n g c r u i s e A, p h y t o p l a n k t o n N:C r a t i o s were > 0.15 i n t h e Gasp<§ C u r r e n t and i n t h e f r o n t , s u g g e s t i n g t h a t t h e p h y t o p l a n k t o n community was n u t r i e n t s u f f i c i e n t . W h i l e n i t r a t e c o n c e n t r a t i o n s were low (< 1 \xK) i n t h e Gasp6 C u r r e n t d u r i n g t h i s c r u i s e , s i l i c a t e c o n c e n t r a t i o n s were 100 n e v e r l i m i t i n g (> 3 uM). On t h e Gyre s i d e o f t h e f r o n t , N:C r a t i o s o f s e s t o n d e c r e a s e d p r o g r e s s i v e l y , i n d i c a t i n g a g r a d u a l n i t r o g e n impoverishment of t h e s e s t o n . The i m p o r t a n c e of t h e c r o s s - f r o n t a l c i r c u l a t i o n on t h e n u t r i t i o n a l s t a t u s o f t h e p h y t o p l a n k t o n community was more a p p a r e n t d u r i n g c r u i s e C. D u r i n g t h i s c r u i s e , n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s were h i g h i n t h e f r o n t and low i n t h e Gaspe C u r r e n t . F o l l o w i n g t h e s e changes i n n u t r i e n t c o n c e n t r a t i o n s , p h y t o p l a n k t o n N:C r a t i o s i n c r e a s e d from 0.10 i n t h e Gaspe C u r r e n t t o 0.14 a c r o s s t h e f r o n t . These r e s u l t s s u g g e s t t h a t t h e m i x i n g o f t h e p h y t o p l a n k t o n community a c r o s s t h e f r o n t r e s u l t s i n an enhanced p h y t o p l a n k t o n n u t r i t i o n a l s t a t u s . As shown i n F i g u r e 1.24A, v a r i a t i o n s i n p h y t o p l a n k t o n N:C r a t i o s a c r o s s t h e f r o n t were s i g n i f i c a n t l y c o r r e l a t e d w i t h ambient s i l i c a t e ( r = 0.83, p < 0.01). T h i s r e s u l t s u g g e s t s t h a t t h e d i a t o m community was s i l i c a t e - d e f i c i e n t i n t h e Gaspe C u r r e n t and became p r o g r e s s i v e l y s i l i c a t e - s u f f i c i e n t i n t h e f r o n t . The i m p o r t a n c e o f s i l i c a t e as a l i m i t i n g n u t r i e n t f o r d i a t o m growth i n t h e S t . Lawrence system s h a l l be examined i n more d e t a i l i n C h a p t e r 2. The c r o s s f r o n t a l c i r c u l a t i o n ( m i x i n g , u p w e l l i n g , mixed l a y e r d epth) a l s o had c o n s i d e r a b l e e f f e c t s on t h e p h o t o s y n t h e t i c c h a r a c t e r i s t i c s of t h e p h y t o p l a n k t o n 101 in e o < or o 0.20 0.15 + 0.10 + 0.05 100 -C cn -C (J \ o O 0. SILICATE {p.q-ot Si T ') F i g u r e 1.24. (A) P h y t o p l a n k t o n N:C r a t i o s v s ambient s i l i c a t e and (B) p h y t o p l a n k t o n POC/CHL a vs ambient s i l i c a t e measured a t 3 m d u r i n g c r u i s e C. 102 community. I n t h i s s t u d y , t h e p h y t o p l a n k t o n p h o t o s y n t h e t i c c h a r a c t e r i s t i c s were e s t i m a t e d by t h e P-I c u r v e p a r a m e t e r s (P Bm , a B , I*) and t h e r a t i o s POC/chl a and PON/chl a. I t s h o u l d be mentioned t h a t p h o t o s y n t h e t i c p a r a m e t e r s P Bm and a B a r e known t o c o - v a r y d u r i n g t h e day. T h e i r d i u r n a l range i s c h a r a c t e r i z e d by a mid-morning o r noon maximum, a g r a d u a l a f t e r n o o n d e c r e a s e , and a minimum p r i o r t o t h e s t a r t o f t h e d a r k p e r i o d (Marra 1978, P r ^ z e l i n and L e y 1980, H a r d i n g e t a i . 1981a, 1981b, M i l l e r and Kamykowski 1986). A b s o l u t e v a l u e s o f P Bm and a.B d e t e r m i n e d a t d i f f e r e n t t i m e s o f t h e day, such as d u r i n g o ur t r a n s e c t , may c o n s e q u e n t l y r e f l e c t d i e l v a r i a t i o n s as w e l l as l i g h t a d a p t a t i o n s . F o r t h i s r e a s o n , o u r a t t e n t i o n s h a l l be d i r e c t e d m a i n l y on t h e para m e t e r I k w h i c h i s l e s s a f f e c t e d by d i e l v a r i a t i o n s o f p h o t o s y n t h e s i s ( H a r d i n g et al. 1987, P r e z e l i n et al. 1987) and i t r e p r e s e n t s a r e l i a b l e i n d i c a t o r o f t h e r e c e n t l i g h t r egime t o w h i c h t h e p h y t o p l a n k t o n community has a d a p t e d ( G o s s e l i n e t a i . 1985). D u r i n g c r u i s e A, p h y t o p l a n k t o n c e l l s l o c a t e d i n t h e Gasp6 C u r r e n t had s i g n i f i c a n t l y h i g h e r c h l o r o p h y l l a p e r u n i t c a r b o n and n i t r o g e n ( i . e . l o w e r p h y t o p l a n k t o n POC/chl a and PON/chl a r a t i o s ) t h a n t h o s e l o c a t e d i n t h e f r o n t . S i n c e t h e p h y t o p l a n k t o n community was s i m i l a r i n b o t h p a r t s of t h e f r o n t , t h e s e r e s u l t s i n d i c a t e t h a t t h e r e s i d e n c e t i m e o f t h e c e l l s i n t h e f r o n t was l o n g enough t o a l l o w a d a p t a t i o n t o a h i g h e r l i g h t r e gime. V a r i a t i o n s o f POC/CHL 103 a were s i g n i f i c a n t l y c o r r e l a t e d w i t h both the upper l a y e r depth ( r =0.74; p < 0.01) and c h l o r o p h y l l a i t s e l f ( r = 0.64; p < 0 . 0 1 ) ( F i g . 1.25A, B). Thus, the h i g h e r l i g h t regime i n the f r o n t may r e s u l t e i t h e r from a r e d u c t i o n i n the upper l a y e r depth ( t h i s study; Tang 1983) o r from a r e d u c t i o n of s e l f - s h a d i n g ( d i r e c t l y due to the d i l u t i o n of the biomass). The v a r i a t i o n i n the phytoplankton POC/chl a and PON/chl a r a t i o s a c r o s s the f r o n t were p a r a l l e l e d by v a r i a t i o n s i n the p h o t o s y n t h e t i c parameters P Bm and a B . T h i s r e s u l t suggests t h a t the v a r i a t i o n s of P Bm and a B r e s u l t e d from v a r i a t i o n s i n c h l o r o p h y l l a quotas. F i n a l l y , v a l u e s of I K were hi g h (mean = 148 uE m - 2 s - 1 ) and r e l a t i v e l y c o n s t a n t a c r o s s the f r o n t a l a r e a . During c r u i s e C, POC/chl a r a t i o s e x h i b i t e d an i n v e r s e p a t t e r n w i t h maximum and minimum v a l u e s i n the Gaspe C u r r e n t and i n the f r o n t , r e s p e c t i v e l y . In t h a t case, POC/CHL a r a t i o s and ambient s i l i c a t e c o n c e n t r a t i o n s were h i g h l y c o r r e l a t e d ( r = -0.84; p < 0.01), s u g g e s t i n g t h a t the i n c r e a s e i n POC/chl a r a t i o i n the C u r r e n t r e s u l t e d from the s i l i c a t e d e f i c i e n c y a l r e a d y suggested by the v a r i a t i o n s i n the N:C r a t i o (see F i g . 1.24A, B). An i n c r e a s e i n both carbon and n i t r o g e n per u n i t c h l o r o p h y l l a due t o s i l i c a t e l i m i t a t i o n has been observed i n l a b s t u d i e s f o r s e v e r a l diatoms ( H a r r i s o n et al. 1977). In the p r e s e n t case, the p h y t o p l a n k t o n PON/chl a r a t i o s d i d not appear t o be a f f e c t e d by s i l i c a t e d e f i c i e n c y . In c o n t r a s t w i t h c r u i s e A, the 104 TOO c i x u \ o o 0. 10 20 MIXED LAYER DEPTH (m) 100 c I o \ u o CL CHLOROPHYLL ( p g I " 1 ) Figure 1.25. Phytoplankton POC/CHL a ra t i o s vs (A) the mixed layer depth and (B) chlorophyll a measured at 3 m during c r u i s e A. 105 v a r i a t i o n s of the phytoplankton POC/chl a r a t i o were not accompanied by v a r i a t i o n s i n P sm and a B . F i n a l l y , I K v a l u e s were s l i g h t l y lower i n the Gaspe C u r r e n t (mean = 60 uE m - 2 s - 1 ) than i n the f r o n t (mean = 99 uE m~2 s - 1 ) . T h i s r e s u l t suggests t h a t the mixing of the community a c r o s s the f r o n t had some e f f e c t s on t h e i r p h o t o s y n t h e t i c c h a r a c t e r i s t i c s . The Pearson's c o e f f i c i e n t of l i n e a r c o r r e l a t i o n between I K and the upper l a y e r depth was not s i g n i f i c a n t ( F i g . 1.26). In summary, our r e s u l t s i n d i c a t e t h a t the r e s i d e n c e time of the phytoplankton c e l l s i n the f r o n t d u r i n g h i g h f r e s h w a t e r r u n o f f ( s p r i n g time) i s too s h o r t to r e s u l t i n e c o l o g i c a l adjustments but l o n g enough t o a l l o w p h y s i o l o g i c a l adjustments t o the l o c a l n u t r i e n t and l i g h t c o n d i t i o n s . Summer time c o n d i t i o n s D uring the summer, the decrease i n f r e s h w a t e r r u n o f f and shear s t r e s s r e s u l t e d i n a d i f f e r e n t , a p p a r e n t l y l e s s v i g o r o u s c r o s s - f r o n t a l c i r c u l a t i o n . On two o c c a s i o n s , a w e l l developed band of c o l d water was p r e s e n t a t the o f f s h o r e edge of the f r o n t . The f r o n t per se was then composed of two d i s t i n c t p a r t s - a non-upwelling and an u p w e l l i n g p a r t - c h a r a c t e r i z e d by s p e c i f i c chemical and b i o l o g i c a l p r o p e r t i e s . These two p a r t s w i l l be r e f e r r e d t o as the non-upwelling front and upwelling front. 106 CRUISE C 150 100 + CM LU 3. 5 10 15 MIXED LAYER DEPTH (m) 20 F i g u r e 1.26. P h o t o s y n t h e t i c parameter I * vs t h e m i x e d l a y e r d e p t h d u r i n g c r u i s e C. 107 The non-upwelling front I n summer, t h e n o n - u p w e l l i n g f r o n t formed a d i s t i n c t and s t a b l e environment where p h y t o p l a n k t o n growth and biomass a c c u m u l a t i o n o c c u r r e d . N i t r a t e and s i l i c a t e c o n c e n t r a t i o n s , w h i c h were s t i l l low i n t h e Gaspe C u r r e n t and i n t h e G y r e , were a l s o below 1.0 uM i n t h i s p a r t o f t h e f r o n t . The d e c r e a s e i n n u t r i e n t c o n c e n t r a t i o n s i n t h e f r o n t s u g g e s t s t h a t p h y t o p l a n k t o n growth o c c u r r e d i n t h i s w a t e r . The f r o n t was c h a r a c t e r i z e d by a s p e c i f i c p h y t o p l a n k t o n community dominated by t h e d i a t o m s T h a l a s s i o s i r a nordenskioeldil and Chaetoceros debllis. The e s t a b l i s h m e n t of a d i f f e r e n t p h y t o p l a n k t o n community i n t h e f r o n t p r o b a b l y r e s u l t e d from t h e l o w e r m i x i n g r a t e between t h e C u r r e n t and t h e Gyre. S p e c i f i c p h y t o p l a n k t o n communities a s s o c i a t e d w i t h r i v e r and e s t u a r i n e plume f r o n t s have been p r e v i o u s l y o b s e r v e d i n Chesapeake Bay ( S e l i g e r e t al. 1981) and i n t h e German B i g h t (Krause et al. 1986). I n Chesapeake Bay, B r a n d t et al. (1986) c o n c l u d e d t h a t t h e e s t u a r i n e f r o n t s were a c t i n g as a p h y s i c a l boundary l i m i t i n g c h e m i c a l and b i o l o g i c a l t r a n s p o r t . S i m i l a r c o n c l u s i o n s may be drawn from our summer d a t a i n t h e Gaspe C u r r e n t f r o n t . Between s p r i n g and summer, t h e l o c a t i o n o f t h e maximum i n p h y t o p l a n k t o n biomass moved from t h e Gaspe C u r r e n t t o t h e 108 front. Chlorophyll a concentrations were lower than 1.0 ug l - 1 in the Gyre and in the Gasp6 Current and increased up to 22 ng l - 1 in the front. The chlorophyll maximum formed a subsurface tongue which may (cruise E; see Fig. 2.3 in Chapter 2) or may not (cruises B and D) reach the surface. The development of the biomass in the front was explained by the relatively high nutrient concentrations found during the previous cruises (during spring time) in the sa l i n i t y gradient, as well as sufficient light. Secchi disk readings in the Gasp6 Current during summer were around 4 m, indicating that the 1 % incident light was at ca. 10 m. The sa l i n i t y gradient was generally shallower than 10 m. On the other hand, the accumulation of biomass was attributed to the lower advection rate (longer residence time) due to the seasonal decrease in current velocity (Benoit et al. 1985). These results also attested for the greater s t a b i l i t y of the front in summer. In this layer, the conditions were stable enough to result in nutrient exhaustion. Nitrate and s i l i c a t e exhaustion was observed in the non-upwelling front ( f i r s t 5 m) during cruises B and D. During cruise E, the area in the front that was depleted in nutrients, extended well below the Gaspe Current. These results indicate that the diatom community may be, at least temporally, nutrient deficient in the front. The analysis of the physiological parameters supports this hypothesis. During cruises B (and E), low N:C ratios were generally measured in the frontal nutrient impoverished layer. The undetectable 109 photosynthetic rates observed i n t h i s part of the front during cruise B may also be at t r i b u t e d to the poor p h y s i o l o g i c a l status of the community. During cruises B and D, the co-occurrence of low nutrient and biomass concentrations i n the front near the surface suggests that the diatom bloom had already started to sink. The upwelling front The upwelling c i r c u l a t i o n had an important e f f e c t on the nutrient d i s t r i b u t i o n . In summer, high near-surface n i t r a t e and s i l i c a t e concentrations (> 5 LIM) were only found i n the upwelling front, when present. In the cold band of water, n i t r a t e and s i l i c a t e concentrations were 10 times higher than i n the surrounding surface waters and i d e n t i c a l to those found at the base of the Gasp§ Current. Although the proposed model of upwelling c i r c u l a t i o n i s s t i l l conjectural, i t i s tempting to evaluate i t s p o t e n t i a l impact i n terms of nutrient supply rate to the photic zone. Upwelling v e l o c i t i e s associated with the c r o s s - f r o n t a l c i r c u l a t i o n s are usually low (less than 1 mm s - 3-; Van H e i j s t 1986). In the Gasp<§ Current front, Tang (1982) suggested that the v e r t i c a l upwelling v e l o c i t y may be as high as 40 mm s - 3-. Using Tang's proposed upwelling rate and the nutrient concentrations usually found at 12-15 m i n t h i s area ( N 0 3 = 6.0 aM; SiCU = 4.5 uM), the t h e o r e t i c a l v e r t i c a l fluxes of 110 n i t r a t e and s i l i c a t e a r e 8.6 and 6.4 umol 1 _ 1 h - x , r e s p e c t i v e l y . C o n s i d e r i n g an u p w e l l i n g a r e a o f 1500 km 2 (mean w i d t h o f 10 km and mean l e n g t h o f 150 km), t h e u p w e l l i n g c i r c u l a t i o n c o u l d b r i n g up 13 and 10 kmol h - 3- o f n i t r a t e and s i l i c a t e , r e s p e c t i v e l y . C h l o r o p h y l l a c o n c e n t r a t i o n s were v e r y h i g h i n t h e u p w e l l i n g zone. D u r i n g c r u i s e B, t h e y were 10 t i m e s h i g h e r t h a n i n t h e s u r r o u n d i n g s u r f a c e w a t e r s . The p h y t o p l a n k t o n communities found i n t h e u p w e l l i n g and n o n - u p w e l l i n g f r o n t were al w a y s dominated by t h e same d i a t o m genus (Thalassiosira nordenskioeldii and Thalassiosira sp. d u r i n g c r u i s e B, Thalassiosira debilis and Thalassiosira gravida d u r i n g c r u i s e E ) . The s i m u l t a n e o u s p r e s e n c e o f h i g h n u t r i e n t and c h l o r o p h y l l a c o n c e n t r a t i o n s i n t h e u p w e l l i n g zone i s somewhat p u z z l i n g . I t may r e s u l t e i t h e r from a c o n s t a n t n u t r i e n t e n r i c h m e n t (by v e r t i c a l m i x i n g , f o r example) o r from t h e p a s s i v e a c c u m u l a t i o n o f t h e biomass ( c o n v e r g e n t c i r c u l a t i o n o r s i n k i n g ) . One p o s s i b l e s c e n a r i o , w h i c h i s i n a c c o r d w i t h Tang's c i r c u l a t i o n model, i s t h a t t h e u p w e l l e d w a t e r was seeded by s i n k i n g c e l l s when i t was s t i l l l o c a t e d b e n eath t h e Gaspe C u r r e n t . The p h y s i o l o g i c a l s t a t u s o f t h e p o p u l a t i o n s l o c a t e d i n t h e u p w e l l i n g f r o n t seems a l s o t o r e f l e c t t h e i r d e p t h o r i g i n . P h y t o p l a n k t o n POC/CHL a and PON/ CHL a r a t i o s were low (25 and 3, r e s p e c t i v e l y ) , s u g g e s t i n g t h a t t h e c e l l s had I l l a h i g h c h l o r o p h y l l a q u o t a . L o r e n z e n (1967) o b t a i n e d a POC/chl a v a l u e o f 40 f o r a h e a l t h y p h y t o p l a n k t o n community i n an u p w e l l i n g a r e a . S i n c e a d a p t a t i o n t o h i g h l i g h t o c c u r s o v e r a t i m e s c a l e o f hours ( F a l k o w s k i 1984), t h e s e l o w POC/chl a and PON/chl a v a l u e s s u g g e s t t h a t t h e c e l l s have been exposed o n l y r e c e n t l y t o h i g h e r n e a r - s u r f a c e i r r a d i a n c e s . I k v a l u e s were low (ca. 30 L I E m - 2 s - 1 ) , a l s o i n d i c a t i n g t h a t t h e c e l l s were a d a p t e d t o a r e l a t i v e l y low l i g h t r e g i m e . T o g e t h e r , t h e s e r e s u l t s s u p p o r t t h e u p w e l l i n g c i r c u l a t i o n model proposed by Tang, and p r e c l u d e t h e p o s s i b l i t y t h a t t h e c o l d w a t e r was u p w e l l e d somewhere up s t r e a m (e.g. o f f s h o r e P o i n t e - d e s - M o n t s ) and a d v e c t e d by t h e a l o n g f r o n t c u r r e n t . Summary D u r i n g t h i s s t u d y , I documented v a r i o u s p h y t o p l a n k t o n r e s p o n s e s t o t h e c r o s s - f r o n t a l c i r c u l a t i o n . My r e s u l t s i n d i c a t e t h a t t h e t y p e s o f r e s p o n s e v a r y d e p ending on t h e c h a r a c t e r i s t i c s o f t h e c r o s s - f r o n t a l c i r c u l a t i o n . D u r i n g t h e h i g h f r e s h w a t e r p e r i o d , w a t e r from t h e Gasp§ C u r r e n t upper mixed l a y e r was mixed r a p i d l y w i t h s u r f a c e w a t e r from t h e G y re. D u r i n g t h i s p e r i o d , t h e p h y t o p l a n k t o n community i n t h e f r o n t was s i m i l a r t o t h e community i n t h e Gasp6 C u r r e n t , and t h e biomass was r a p i d l y d i l u t e d a c r o s s t h e f r o n t . These r e s u l t s i n d i c a t e t h a t t h e m i x i n g between t h e two w a t e r masses o c c u r r e d o v e r a t i m e s c a l e s h o r t e r t h a n 112 s c a l e s f o r p h y t o p l a n k t o n s u c c e s s i o n (week) and growth (days) ( H a r r i s 1980). On t h e o t h e r hand, t h e r e s i d e n c e t i m e o f t h e community i n t h e f r o n t may be l o n g enough ( h o u r s ; H a r r i s 1980) t o r e s u l t i n p h y s i o l o g i c a l a d j u s t m e n t t o t h e l o c a l n u t r i e n t c o n c e n t r a t i o n s and l i g h t i n t e n s i t y . These a d j u s t m e n t s r e s u l t e d i n v a r i a t i o n s o f t h e community b i o c h e m i c a l c o m p o s i t i o n (N:C, POC/chl a and PON/chl a) and p h o t o s y n t h e t i c c h a r a c t e r i s t i c s (P Bm, Q6B and I K ) . D u r i n g t h e low f r e s h w a t e r r u n o f f p e r i o d , t h e i n t e n s i t y o f h o r i z o n t a l and v e r t i c a l m i x i n g a c r o s s t h e Gasp6 C u r r e n t and t h e A n t i c o s t i Gyre d e c r e a s e d c o n s i d e r a b l y and a l e s s t u r b u l e n t c r o s s - f r o n t a l c i r c u l a t i o n t o o k p l a c e . T h e r e f o r e , t h e f r o n t r e p r e s e n t s a s t a b l e r e t e n t i o n zone where s u f f i c i e n t l i g h t and i n i t i a l n u t r i e n t c o n c e n t r a t i o n s h e l p t o i n i t i a t e a d i a t o m bloom. E v e n t u a l l y , t h e bloom becomes n i t r o g e n - o r s i l i c a t e - l i m i t e d . The e v o l u t i o n o f t h e f r o n t a l a r e a from a h i g h m i x i n g zone t o a s t a b l e r e t e n t i o n zone w i t h d e c r e a s i n g f r e s h w a t e r r u n o f f i s p r o b a b l y t y p i c a l o f most r i v e r i n e and e s t u a r i n e f r o n t s . On t h e o t h e r hand, t h e u p w e l l i n g c i r c u l a t i o n o b s e r v e d i n t h e Gasp6 C u r r e n t f r o n t i s p r o b a b l y more s p e c i f i c t o f r o n t s c h a r a c t e r i z e d by s t r o n g c u r r e n t v e l o c i t y g r a d i e n t s such as c o a s t a l j e t f r o n t . 113 CHAPTER 2. SILICATE AND NITROGEN DEFICIENCY OF A PHYTOPLANKTON COMMUNITY IN THE GASPE CURRENT ESTUARINE PLUME FRONT Background I n t h e marine e n v i r o n m e n t , p h y t o p l a n k t o n growth and biomass a c c u m u l a t i o n have been r e p o r t e d t o be l i m i t e d e i t h e r by n i t r o g e n (Thomas 1970a, b, R y t h e r and Dunstan 1971) o r s i l i c a t e (Dugdale 1972, Dugdale e t al. 1981, B r i n k e t al. 1981). I n many o c e a n i c and c o a s t a l r e g i o n s , n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s a r e s i m u l t a n e o u s l y low (< 1 JJM) o v e r e x t e n d e d p e r i o d s o f t i m e . S i m u l t a n e o u s e x h a u s t i o n of n i t r a t e and s i l i c a t e was o b s e r v e d , f o r example, i n t h e South P a c i f i c Ocean near t h e e q u a t o r ( Z e n t a r a and Kamykowski 1981, K i n g 1986), i n t h e A n t a r c t i c (Sommer 1986), i n t h e Canadian A r c t i c ( G o s s e l i n e t al. i n p r e s s ) , i n t h e I r i s h Sea ( P a r k e r e t al. 1988) and i n t h e S t . Lawrence e s t u a r y (Coote and Y e a t s 1979, L e v a s s e u r and T h e r r i a u l t 1987). The p h y s i o l o g i c a l r e s p o n s e o f t h e p h y t o p l a n k t o n community t o t h i s p o t e n t i a l c o - l i m i t a t i o n has y e t t o be i n v e s t i g a t e d d i r e c t l y i n t h e f i e l d . I t i s now r e c o g n i z e d t h a t v a r i a t i o n s i n t h e c h e m i c a l c o m p o s i t i o n o f p h y t o p l a n k t o n n a t u r a l communities may r e f l e c t more a d e q u a t e l y t h e i r a c t u a l n u t r i t i o n a l s t a t u s t h a n ambient 114 c o n c e n t r a t i o n s ( G l i b e r t and McCarthy 1984, D o r t c h et al. 1985). When n i t r o g e n - s u f f i c i e n t , many p h y t o p l a n k t o n s p e c i e s a c c u m u l a t e v a r i o u s forms o f i n t e r n a l i n o r g a n i c ( n i t r a t e , n i t r i t e , ammonium) and o r g a n i c n i t r o g e n (amino a c i d s , p r o t e i n s , RNA, pigments) (DeManche e t al. 1979, D o r t c h 1982, Thor e s e n e t al. 1982). F o l l o w i n g n i t r o g e n e x h a u s t i o n , t h e s e i n t r a c e l l u l a r p o o l s may be used as a l t e r n a t i v e n i t r o g e n s o u r c e s f o r s e v e r a l days ( D o r t c h 1982, D o r t c h e t al. 1984, A d m i r a a l e t al. 1986). As a r e s u l t , t h e n i t r o g e n c o n t e n t of t h e c e l l d e c r e a s e s as does t h e n i t r o g e n t o c a r b o n r a t i o (N:C) ( M o r r i s 1981, Wheeler 1983). The o c c u r r e n c e and r e l a t i v e s i z e o f t h e s e n i t r o g e n p o o l s may t h u s i n d i c a t e t h e n i t r o g e n s t a t u s o f t h e n a t u r a l p h y t o p l a n k t o n community and p o s s i b l y , i t s predominant n i t r o g e n ( n i t r a t e o r ammonium) s o u r c e ( C o l l o s and Slawyk 1976, D o r t c h e t al. 1985). Diatom s p e c i e s seem t o re s p o n d f a s t e r t o s i l i c a t e d e p l e t i o n t h a n n i t r o g e n d e p l e t i o n . The r a p i d d e c r e a s e i n p h o t o s y n t h e s i s due t o s i l i c a t e d e f i c i e n c y has been r e p o r t e d b o t h i n t h e l a b o r a t o r y ( V o l c a n i 1978) and i n n a t u r e (Malone e t al. 1980, Dugdale e t al. 1981). T h i s r e s p o n s e r e s u l t s from t h e g e n e r a l l y h i g h s i l i c a t e r e q u i r e m e n t o f d i a t o m s and t h e i r p oor a b i l i t y t o ac c u m u l a t e i m p o r t a n t i n t e r n a l r e s e r v e s of s i l i c a t e ( S u l l i v a n 1979). D u r i n g s i l i c a t e s t a r v a t i o n , t h e b i o c h e m i c a l pathways of p r o t e i n s y n t h e s i s a r e b l o c k e d e a r l i e r t h a n c a r b o h y d r a t e s y n t h e s i s ( S u l l i v a n and V o l c a n i 1981) and t h e N:C r a t i o d e c r e a s e s ( H a r r i s o n e t al. 1977, 115 S h i f r i n and C h i s h o l m 1981). A l t h o u g h v a r i a t i o n s i n N:C r a t i o s i n t h e f i e l d due t o s i l i c a t e l i m i t a t i o n have n e v e r been r e p o r t e d , t h e above mentioned r e s u l t s would i n d i c a t e t h a t changes i n t h e N:C r a t i o cannot d i s c r i m i n a t e between n i t r o g e n and s i l i c a t e d e f i c i e n c y . S e v e r a l d i a t o m s a l s o b u i l d up i n t e r n a l s i l i c a t e p o o l s when t h e y a r e s i l i c a t e -s u f f i c i e n t (Azam e t al. 1974, S u l l i v a n 1979). The p r e s e n c e of t h e s e s i l i c a t e p o o l s i n t h e n a t u r a l e n v i r o n m e n t , and t h e i r s i g n i f i c a n c e f o r t h e p h y t o p l a n k t o n community remain t o be a s c e r t a i n e d . I n t h i s c h a p t e r , I examined t h e e f f e c t s o f s i m u l t a n e o u s e x h a u s t i o n o f ambient n i t r a t e and s i l i c a t e on t h e n u t r i t i o n a l s t a t u s o f a n a t u r a l d i a t o m community from t h e Gasp<§ C u r r e n t j e t f r o n t . I n t e r n a l n u t r i e n t p o o l s ( n i t r a t e , ammonium, u r e a and s i l i c a t e ) , and s e s t o n and p h y t o p l a n k t o n N:C r a t i o s were used as an i n d e x o f t h e n u t r i t i o n a l s t a t u s of t h e community. I a l s o examined t h e r e l a t i o n s h i p s between th e s i n k i n g r a t e s o f t h e dominant d i a t o m s and t h e ambient and i n t e r n a l n u t r i e n t c o n c e n t r a t i o n s . 116 M a t e r i a l s and methods S a m p l i n g and l a b o r a t o r y p r o c e d u r e s S a m p l i n g was c a r r i e d out on a t r a n s e c t o f f s h o r e o f M o n t - L o u i s (Quebec) on J u l y 30, 1987 ( F i g . 2 .1). Ni n e s t a t i o n s ca. 4 km a p a r t , l o c a t e d on a t r a n s e c t p e r p e n d i c u l a r t o t h e c o a s t , were sampled between 0800 and 2200 h. A t each s t a t i o n , samples were c o l l e c t e d w i t h 5 1 N i s k i n b o t t l e s a t 0, 3, 8, 12, 20 and 40 m and t h e v e r t i c a l d i s t r i b u t i o n o f s a l i n i t y and t e m p e r a t u r e was d e t e r m i n e d w i t h a CTD prob e . From each w a t e r sample, t h e f o l l o w i n g v a r i a b l e s were measured as d e s c r i b e d i n C h a p t e r 1: c h l o r o p h y l l a, p a r t i c u l a t e o r g a n i c c a r b o n and n i t r o g e n (POC and PON) and p h y t o p l a n k t o n i d e n t i f i c a t i o n and e n u m e r a t i o n . N u t r i e n t s ( n i t r a t e , n i t r i t e , ammonium and phosphate) were d i r e c t l y measured on bo a r d u s i n g a T e c h n i c o n A u t o A n a l y s e r ( P a r s o n s e t al. 1984). A subsample o f t h e f i l t r a t e was f r o z e n f o r a n a l y s i s o f s i l i c a t e ( P a r s o n s e t al. 1984) and u r e a ( d i a c e t y l monoxime t h i o s e m i c a r b i z i d e t e c h n i q u e d e s c r i b e d by Aminot and K e r o u e l 1982 and P r i c e and H a r r i s o n 1987) upon r e t u r n t o t h e shore l a b o r a t o r y . Thawed n u t r i e n t samples were k e p t a t 4°C f o r 24 h b e f o r e s i l i c a t e was d e t e r m i n e d . I n t e r n a l p o o l s o f n i t r a t e , n i t r i t e , ammonium and u r e a ( I N - N 0 3 , IN-N0 2, IN-NH 4 and IN-urea) f o r t h e n a t u r a l s e s t o n community were measured u s i n g t h e b o i l i n g 117 r 1 1 1 1 r 67'Off 66* Off 65'Off Figure 2.1. Map of the northwestern Gulf of St. Lawrence showing surface c i r c u l a t i o n (from El-Sabh 1976) and l o c a t i o n of the sampling stat i o n s . Based on the s a l i n i t y d i s t r i b u t i o n , the transect was divided into three parts: (1) the Gasp6 Current (stations 6, 7 and 8); (2) the front (stations 2, 3, 4 and 5); and (3) the Gulf Gyre (stations 1 and 1') . 118 w a t e r e x t r a c t i o n methods d e s c r i b e d by Thorensen e t al. ( 1 9 8 2 ) . One l i t e r of seawater was f i l t e r e d t h r o u g h a p r e -combusted Whatman GF/F f i l t e r , w h i c h was t h e n r i n s e d w i t h 50 ml o f i s o t o n i c n u t r i e n t - f r e e a r t i f i c i a l s e a w a t e r . Then, 20 ml o f b o i l i n g d e i o n i z e d w a t e r was poured ont o t h e f i l t e r c o n t a i n i n g p h y t o p l a n k t o n c e l l s , and t h e f i l t r a t e was c o l l e c t e d i n a c i d - r i n s e d t u b e s . N u t r i e n t c o n c e n t r a t i o n s i n t h e f i l t r a t e were d e t e r m i n e d as d e s c r i b e d above. F o r a b l a n k d e t e r m i n a t i o n (two p e r s t a t i o n ) , a f i l t e r was r i n s e d s u c c e s s i v e l y w i t h 50 ml of a r t i f i c i a l s e a w a t e r and 20 ml o f b o i l i n g d e i o n i z e d w a t e r . The n u t r i e n t c o n c e n t r a t i o n s o f t h e f i l t r a t e from t h e r u p t u r e d c e l l s c o r r e c t e d f o r t h e b l a n k r e p r e s e n t e d t h e i n t r a c e l l u l a r n u t r i e n t p o o l f o r a n a t u r a l p l a n k t o n community c o n t a i n e d i n 1 1 of se a w a t e r . I n t e r n a l p o o l s o f s i l i c a t e ( I N - S i 0 4 ) were a l s o measured i n t h e f i l t r a t e s and i n t h e b l a n k s . Due t o t h e use o f g l a s s f i b e r f i l t e r s and b o r o s i l i c a t e g l a s s w a r e , b l a n k s were h i g h . However, t h e i r v a r i a b i l i t y was low (between d u p l i c a t e s C V . = 8%) and t h e s i g n a l / n o i s e r a t i o g e n e r a l l y h i g h e r t h a n 2 (and up t o 2 3 ) . Uptake r a t e s measurements Mid-day n i t r a t e , ammonium and u r e a u p t a k e r a t e s were e s t i m a t e d u s i n g t h e s t a b l e i s o t o p e 1 5 N t e c h n i q u e (Dugdale and G o e r i n g , 1967) a t 2 s t a t i o n s l o c a t e d i n t h e f r o n t ( s t a t i o n s 2 and 3) and one s t a t i o n l o c a t e d on t h e Gyre s i d e ( s t a t i o n 1; f i g . 2.1). W i t h i n 20 min of c o l l e c t i o n , 500 ml 119 samples of u n f i l t e r e d seawater from 3 m were t r a n s f e r r e d i n t o f o u r 500 ml Pyrex g l a s s b o t t l e s . The f i r s t t h r e e b o t t l e s were used f o r the d e t e r m i n a t i o n of n i t r o g e n uptake r a t e s and the f o u r t h was used t o determine the change i n PON d u r i n g the i n c u b a t i o n p e r i o d . Trace a d d i t i o n s of Na 1 5N03 (0.1 uM), 1 5NH 4C1 (0.05 uM) and CO ( 1 S N H 2 ) 2 (0.025 uM) were added i n the d i f f e r e n t b o t t l e s . Samples were i n c u b a t e d on deck f o r 4 h under n a t u r a l l i g h t , i n a water bath a t sea s u r f a c e temperatures. N e u t r a l d e n s i t y f i l t e r s were used t o decrease the i r r a d i a n c e to 60% of the s u r f a c e i r r a d i a n c e (ca. in situ i r r a d i a n c e a t 3 m). Incubations were t e r m i n a t e d by f i l t r a t i o n onto pre-combusted Whatman GF/F f i l t e r s . N i t r o g e n i n the p a r t i c u l a t e samples was conv e r t e d to N 2 (gas) by the micro-Dumas d r y combustion technique d e s c r i b e d by La Roche (1983), and then a n a l y z e d f o r 1 5 N enrichment w i t h a JASCO model N-150 e m i s s i o n spectrometer ( F i e d l e r and Proksch 1975). P a r t i c u l a t e matter (PON) from the f o u r t h u n e n r i c h e d b o t t l e was c o l l e c t e d a t the end of the i n c u b a t i o n onto a pre-combusted Whatman GF/F f i l t e r and s t o r e d f r o z e n i n a d e s i c c a t o r f o r PON a n a l y s i s u s i n g a P e r k i n Elmer e l e m e n t a l a n a l y s e r . N i t r o g e n s p e c i f i c uptake r a t e s and t r a n s p o r t r a t e s were c a l c u l a t e d f o l l o w i n g equations 2 and 3 of Dugdale and W i l k e r s o n (1986). Blank v a l u e s were e s t i m a t e d d u r i n g the c r u i s e as f o l l o w s : 500 ml of n a t u r a l seawater was f i l t e r e d onto a pre-combusted GF/F f i l t e r and the l a b e l l e d n i t r o g e n 120 added t o t h e l a s t 15 ml o f seawater i n o r d e r t o m i n i m i z e u p t a k e d u r i n g f i l t r a t i o n . The f i l t e r was t h e n r i n s e d w i t h 250 ml o f f i l t e r e d seawater t o s i m u l a t e more c l o s e l y t h e e f f e c t o f a normal 500 ml f i l t r a t i o n . F o l l o w i n g t h i s p r o t o c o l , we measured a mean b l a n k v a l u e ( i . e . n a t u r a l 1 S N c o n c e n t r a t i o n ) of 0.46 w i t h a c o e f f i c i e n t o f v a r i a t i o n o f 9% (n = 3 ) . T h i s e m p i r i c a l v a l u e was used i n s t e a d o f t h e l i t e r a t u r e v a l u e o f 0.37 i n t h e d e t e r m i n a t i o n o f t h e atom% 1 S N e x c e s s i n t h e sample ( 1 5 N x s , e q u a t i o n 1; Dugdale and W i l k e r s o n 1986). I n 84% o f t h e c a s e s , t h e added n i t r o g e n r e p r e s e n t e d l e s s t h a n 15% o f ambient n i t r o g e n c o n c e n t r a t i o n s . These uptake measurements were t a k e n as r e p r e s e n t a t i v e of in situ r a t e s w i t h o u t c o r r e c t i o n (McCarthy 1980). S i n c e t h e p r e s e n c e of d e t r i t a l n i t r o g e n may u n d e r e s t i m a t e t h e n i t r o g e n s p e c i f i c u p t a k e r a t e s , we c a l c u l a t e d i n s t e a d t h e c h l o r o p h y l l a s p e c i f i c u p t a k e r a t e ( n i t r o g e n t r a n s p o r t r a t e s / u g c h l o r o p h y l l a ) . S i l i c a t e u p t a k e r a t e s were e s t i m a t e d from t h e d e c r e a s e i n t h e s i l i c a t e c o n c e n t r a t i o n d u r i n g t h e 4 h i n c u b a t i o n p e r i o d . The i n c u b a t i o n s were c o n d u c t e d i n 500 ml p o l y c a r b o n a t e b o t t l e s . The c o l o r i m e t r i c method o c c a s i o n a l l y p r o v e d t o be i n a d e q u a t e t o measure s h o r t t i m e s i l i c a t e u p t a k e r a t e s due t o i t s l a c k o f s e n s i t i v i t y . However, d u r i n g t h i s s t u d y , p h y t o p l a n k t o n biomass was g e n e r a l l y h i g h enough t o measure a s i g n i f i c a n t n e t change i n s i l i c a t e o v e r 121 4 h. Changes i n s i l i c a t e over the incubation periods ranged from <0.20 nM (detection l i m i t ) to 1.56 uM (mean = 0.60 uM). Sinking rates At each of the other stations, (stations 1', 2, 4, 6 and 8), the s p e c i f i c sinking rates of the dominant diatom species were measured using the SETCOL technique (Bienfang 1981). At these stations, the s e t t l i n g columns were f i l l e d with gently mixed surface water. P a r t i c l e s were allowed to sink f o r 2 h i n the dark. The i n i t i a l temperature was maintained at ± 1.0 °C by c i r c u l a t i n g surface water. A f t e r the sinking period, samples were withdraw from the top and bottom parts of the column and phytoplankton were i d e n t i f i e d and enumerated. The sinking rates were then estimated using the equations of Bienfang (1981). 1 2 2 Results Temperature and s a l i n i t y data Surface s a l i n i t y varied between < 28 (inshore) to 31 ° / o o (offshore) (Fig. 2.2). Based on the s a l i n i t y d i s t r i b u t i o n , the transect was divided into three parts: (1) the Gaspe Current (stations 6, 7 and 8); (2) the front (stations 2, 3, 4 and 5); and (3) the Gulf Gyre (stations 1 and 1'). The Gaspe Current was approximately 10 km wide with a surface s a l i n i t y of ca. 27.5 ° / o o and a surface temperature of ca. 10°C. A strong h a l o c l i n e located around 25 m defined the v e r t i c a l extension of the upper layer (Fig. 2.2A) i n the Gaspe Current. The front was defined by a rapid horizontal s a l i n i t y change (28 to 30 °/ 0 0) ca. 10 km wide where the lowest surface temperatures (6°C) were found (Fig. 2.2B). The Gulf Gyre was characterized by surface s a l i n i t i e s > 30 ° / 0 0 and surface temperatures > 8.5°C. The upper layer i n the Gyre was delimited by a thermocline located around 20 m. B i o l o g i c a l data _3 High chlorophyll a concentrations (7 to 11 mg m ) were found i n the front (stations 2 to 5), surrounded by low — 3 values (< 1 mg m J) on both sides (Fig. 2.3A). Diatoms were the numerically dominant phytoplankton group i n each section of the sampling transect. The diatom community was dominated by the same four species at each s t a t i o n , but GASPE CURRENT FRONT GYRE STATIONS 0 6 12 18 24 DISTANCE FROM SOUTH SHORE (km) F i g u r e 2.2. V e r t i c a l d i s t r i b u t i o n o f (A) s a l i n i t y and (B t e m p e r a t u r e on a t r a n s e c t a c r o s s t h e f r o n t a l a r e a o f t h e Gasp6 C u r r e n t i n t h e G u l f o f S t . Lawrence. GASPE C U R R E N T FRONT STATIONS 20 40 A . Chlorophyll a (mg m~3) C . Thalassiosira gravida (10 6 cells l~1) D. Skeletonema costatum (10 6 cells I"1) 20 -"i r 6 12 18 DISTANCE FROM SOUTH SHORE (km) GYRE r 24 F i g u r e 2.3. V e r t i c a l d i s t r i b u t i o n s o f (A) c h l o r o p h y l l and o f c e l l abundance ( 1 0 s c e l l s . I - 3 - ) o f t h e dominant p h y t o p l a n k t o n s p e c i e s , (B) Chaetoceros d e b i l i s , (C) Thalassiosira gravida, (D) Skeletonema costatum, and (E Chaetoceros pelagicus on the t r a n s e c t a c r o s s t h e Gaspe C u r r e n t i n t h e G u l f o f S t . Lawrence. The shaded a r e a s i n d i c a t e t h e h i g h e s t c o n c e n t r a t i o n s . 125 t h e i r r e l a t i v e abundance v a r i e d a c r o s s t h e f r o n t a l a r e a . I n t h e Gaspe C u r r e n t , t h e community was dominated by Chaetoceros pelagicus, f o l l o w e d by Skeletonema costatum and Chaetoceros debilis. In t h e f r o n t , t h e d i a t o m C. debilis was t h e n u m e r i c a l l y dominant s p e c i e s f o l l o w e d by S. costatum, C. pelagicus and Thalassiosira gravida. I n t h e G y r e , t h e community was dominated by S. costatum ( F i g . 2.3D). The second most i m p o r t a n t p h y t o p l a n k t o n group was made up o f s m a l l (< 5 um) u n i d e n t i f i e d f l a g e l l a t e s , c o n c e n t r a t e d i n t h e f r o n t ( F i g . 2.4A), f o l l o w e d by c r y p t o p h y t e s . The two h e t e r o t h r o p h i c s p e c i e s Leucocryptos marina and Monosiga sp. were a l s o abundant i n t h e f r o n t , b u t t h e y had d i f f e r e n t v e r t i c a l d i s t r i b u t i o n s . Leucocryptos marina formed a s u b s u r f a c e c e l l maximum a t ca. 10-20 m, s l i g h t l y below t h e d i a t o m s u b s u r f a c e maximum ( F i g . 2.4B and 2.3). I n c o n t r a s t , Monosiga sp. c e l l s was c o n c e n t r a t e d i n t h e upper 15 m b o t h i n t h e Gaspe C u r r e n t and i n t h e f r o n t . N u t r i e n t s Ambient nutrient concentrations. The v e r t i c a l d i s t r i b u t i o n o f n u t r i e n t s a l o n g t h e s a m p l i n g t r a n s e c t r e f l e c t e d c l o s e l y t h e f r o n t a l s t r u c t u r e o b s e r v e d ( F i g . 2.5). I n g e n e r a l , t h e d i s t r i b u t i o n o f each n u t r i e n t was c h a r a c t e r i z e d by t h e p r e s e n c e o f two low c o n c e n t r a t i o n r e g i o n s on each s i d e o f 126 GASPE CURRENT FRONT GYRE E X t-Q- 20 HJ Q 40 J i _ _I L <0.1 <0.1 B.Leucocryptos marina (10* calls IT 1) 0 - p - , — i u 20-40 i , i 0.09 j , u -D.09-f0.07 ,0.01-•04 C.Monoslga sp. (10s cell* L"1) «0 .04 V 04" 6 12 0 18 24 DISTANCE FROM SOUTH SHORE (km) F i g u r e 2.4. V e r t i c a l d i s t r i b u t i o n s o f (A) u n i d e n t i f i e d f l a g e l l a t e s , (B) Leucocryptos marina and (C) Monosiga sp. on t h e t r a n s e c t a c r o s s t h e Gasp§ C u r r e n t i n t h e G u l f o f S t . Lawrence. The shaded a r e a s i n d i c a t e t h e h i g h e s t c o n c e n t r a t i o n s . 127 GASPE CURRENT FRONT GYRE STATIONS CL UJ o DISTANCE FROM SOUTH SHORE (km) F i g u r e 2.5. V e r t i c a l d i s t r i b u t i o n o f (A) n i t r a t e , (B) ammonium, (C) u r e a and (D) s i l i c a t e a c r o s s t h e f r o n t . The shaded a r e a s i n d i c a t e t h e l o w e s t c o n c e n t r a t i o n s . 128 t h e f r o n t and by t h e p r e s e n c e of h i g h c o n c e n t r a t i o n s i n t h e Gaspe C u r r e n t and i n t h e f r o n t p e r s e . N i t r a t e c o n c e n t r a t i o n s a v e r a g e d 2.3 uM i n t h e upper l a y e r o f t h e Gaspe C u r r e n t and i n t h e u p w e l l e d w a t e r a t s t a t i o n s 2 and 3 ( F i g . 2.5A). A s u b s u r f a c e t o n g u e - l i k e w a t e r mass w i t h low c o n c e n t r a t i o n s o f n i t r a t e ( s t a t i o n s 4, 5 and 6) s e p a r a t e d t h e f r o n t and t h e Gaspe C u r r e n t ( F i g . 2.5A). I n t h e Gyre ( s t a t i o n s 1 and 1'), n i t r a t e c o n c e n t r a t i o n s were u n d e t e c t a b l e i n t h e s u r f a c e l a y e r ( F i g . 2.5A) and i n c r e a s e d s h a r p l y t o 6 uM a t a d e p t h of 20 m. The d i s t r i b u t i o n s o f NH^ and u r e a were l e s s t i g h t l y c o u p l e d w i t h t h e f r o n t a l s t r u c t u r e o b s e r v e d i n F i g . 2.3 ( F i g . 2.5B, C ) . NH^ c o n c e n t r a t i o n s v a r i e d between 0.2 t o 1.9 pM, w i t h c o n c e n t r a t i o n s i n t h e upper 10 m b e i n g g e n e r a l l y l o w e r t h a n 1.0 pM.. Lowest NH^ c o n c e n t r a t i o n s (< 0.3 pM) were measured a t s t a t i o n 1 i n t h e Gyre and a t s t a t i o n 5 i n t h e f r o n t . Ambient u r e a c o n c e n t r a t i o n s ranged from 0.1 t o 2.3 pM (0.2 t o 4.6 ug-at.N l - 1 ) w i t h l o w e s t c o n c e n t r a t i o n s (< 0.25 JUM) a t s t a t i o n s 1 and 2 and a t t h e b ottom of t h e Gaspe C u r r e n t ( F i g . 2.5C). The v e r t i c a l s i l i c a t e d i s t r i b u t i o n was v e r y s i m i l a r t o t h e n i t r a t e d i s t r i b u t i o n o b s e r v e d a l o n g t h e t r a n s e c t ( F i g . 2.5D). Lowest c o n c e n t r a t i o n s (< 1.0 pM) were o b s e r v e d i n t h e Gyre ( s t a t i o n 1) and a t d e p t h i n t h e Gaspe C u r r e n t . 1 2 9 S i l i c a t e concentrations were high (> 2 LIM) i n the upwelling region of the front and highest (> 4 LIM) i n the Gaspe Current. Internal nutrients. Internal pools of n i t r a t e , n i t r i t e , ammonium and urea were observed i n the plankton at most of the stations (Fig. 2 . 6 ) . Together, they represented between 0 . 0 2 % to 6% of PON. When present, IN-N0 3 pools were the most important i n t e r n a l r e s e r v o i r of inorganic nitrogen, representing up to 97% of t o t a l inorganic nitrogen pools (IN-NG-3 + IN-NOs + IN-NH*). IN-N0 2 (not shown) never represented more than 0 .1% of PON, although i t occasi o n a l l y represented up to 28% of the t o t a l inorganic nitrogen pool. IN - N H 4 and IN-urea pool concentrations represented less than 1.0% and 0 .3% of PON respectively (Fig. 2 . 6 B , C). A s i m i l a r pattern of d i s t r i b u t i o n of i n t e r n a l pool concentrations (normalized to chlo r o p h y l l a) was observed for each nutrient (IN - N O 3 , IN-NH4, IN-urea and I N - S i 0 4 ) along the transect (Fig. 2 . 6 ) . Internal nutrient pool concentrations were generally high i n the Gaspe Current and i n the upwelling region of the front and low i n the Gyre and at stations 4 and 5. IN-NH4 and IN-urea concentrations were also low at near shore stations. In general, these patterns corresponded to the d i s t r i b u t i o n of ambient nutrient l e v e l s (Fig. 2 . 5 ) . Simultaneous absence of IN-NH4 and IN-N0 3 pools (along with low or undetectable ambient concentrations) was 130 GASPE CURRENT FRONT GYRE STATIONS 8 7 6 i i 5 4 3 2 i 1 V I X Q. ~t 1 1 r 6 12 18 24 DISTANCE FROM SOUTH SHORE (km) F i g u r e 2.6. V e r t i c a l d i s t r i b u t i o n o f p l a n k t o n i n t e r n a l n u t r i e n t p o o l c o n c e n t r a t i o n s a c r o s s t h e f r o n t : (A) n i t r a t e ( I N - N 0 3 ) , (B) ammonium ( I N - N H 4 ) f (C) u r e a ( I N - u r e a ) , and (D) s i l i c a t e ( I N - S i O * ) . The shaded a r e a s i n d i c a t e t h e l o w e s t c o n c e n t r a t i o n s . 131 o n l y o b s e r v e d a t s t a t i o n 1 (12 m d e e p ) . U n d e t e c t a b l e IN-u r e a p o o l s were measured a t s t a t i o n s 7 and 8 i n t h e Gaspe c u r r e n t and, i n t h e Gyre, a t s t a t i o n s 1 (20 m) and 1' (12 m) . IN-SiCU p o o l s were a l s o measured a t most of t h e s t a t i o n s a c r o s s t h e f r o n t a l a r e a ( F i g . 2.6D). T h e i r c o n c e n t r a t i o n s ( p e r u n i t c h l o r o p h y l l a) were i n t h e same range o r s l i g h t l y h i g h e r t h a n t h o s e o f t h e I N - N O 3 p o o l s . IN-SiCU p o o l c o n c e n t r a t i o n s were h i g h i n t h e Gaspe C u r r e n t , d e c r e a s e d by a f a c t o r 5 i n t h e f r o n t ( s t a t i o n s 2 t o 5; 0-8 m), and r e a c h e d u n d e t e c t a b l e c o n c e n t r a t i o n s i n t h e Gyre a t s t a t i o n 1'. A t s t a t i o n 1, however, v e r y h i g h IN-SiCU p o o l c o n c e n t r a t i o n s were measured, i n s p i t e o f t h e low ambient s i l i c a t e c o n c e n t r a t i o n s . The r e l a t i o n s h i p s between i n t e r n a l and e x t e r n a l n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s a r e p r e s e n t e d i n F i g . 2.7. The r e l a t i o n s h i p between i n t e r n a l and e x t e r n a l N 0 3 c o n c e n t r a t i o n s i n t h e upper 12 m was c h a r a c t e r i z e d by h i g h I N - N O 3 c o n c e n t r a t i o n s (> 4 x 1 0 - 3 umol ug c h l - 1 ) when ambient N 0 3 c o n c e n t r a t i o n s were h i g h e r t h a n 0.3 uM ( F i g . 2.7A). Below t h e 0.3 uM t h r e s h o l d , I N - N O 3 p o o l , c o n c e n t r a t i o n s d e c r e a s e d s h a r p l y . I n c o n t r a s t t o N0 3, no r e l a t i o n s h i p was o b s e r v e d between ambient and i n t e r n a l NH 4 c o n c e n t r a t i o n s and ambient and i n t e r n a l u r e a c o n c e n t r a t i o n s (not shown). I n t h e upper 3 m, an a p p a r e n t l i n e a r 1 2 Ambient N 0 3 (/iM) Ambient SI04 (11M) Figure 2.7. (A) i n t e r n a l n i t r a t e pool concentrations vs ambient n i t r a t e concentrations f o r a l l stations (0-12 m), and (B) i n t e r n a l s i l i c a t e pool concentrations vs ambient s i l i c a t e concentrations for a l l stations (0-3 m) ( s t a t i o n = o). 133 r e l a t i o n s h i p was found between i n t e r n a l and e x t e r n a l SiCU c o n c e n t r a t i o n s ( F i g . 2.7B). I f s t a t i o n 1 i s e x c l u d e d , t h e c o r r e l a t i o n between I N - S i 0 4 and ambient S i 0 4 c o n c e n t r a t i o n s i s s i g n i f i c a n t ( r = 0.68; p < 0.01). Nutrient uptake. T a b l e 2.1 compares t h e c h l o r o p h y l l a s p e c i f i c u p t a k e r a t e s o f N 0 3 / NH 4, u r e a and t o t a l n i t r o g e n ( N O 3 + NH 4 + ur e a ) i n t h e Gyre and i n t h e f r o n t . I n t h e f r o n t , a l l t h r e e n i t r o g e n s p e c i e s were s i m u l t a n e o u s l y u t i l i z e d by t h e p h y t o p l a n k t o n community. VN0 3 was h i g h (mean = 0.038 nmol ug c h l a~x h~x) and r e p r e s e n t e d ca. 73% of t o t a l n i t r o g e n u p t a k e . A t t h e s e s t a t i o n s , VNH 4 and Vure a were ca. 0.012 nmol ng c h l a - 1 h - 1 and 0.002 nmol ng c r x ± a ~ x h - 1 , and r e p r e s e n t e d , r e s p e c t i v e l y , 23% and 4% o f V n i t r o g e n . In t h e Gyre where ambient n i t r a t e was u n d e t e c t a b l e , VNH 4 was 0.064 \xmol ng c h l a~x h~x and r e p r e s e n t e d 87% o f V n i t r o g e n . V u r e a was 5 t i m e s h i g h e r t h a n i n t h e f r o n t (0.01 nmol ng c h l _ _ i h - i ) a n d r e p r e s e n t e d 13% o f V n i t r o g e n . When a l l n i t r o g e n s o u r c e s a r e c o n s i d e r e d , t h e c h l o r o p h y l l a s p e c i f i c u p t a k e r a t e o f n i t r o g e n was h i g h e r i n t h e Gyre (0.074 nmol ng c h l a - 1 h- 1) t h a n i n t h e f r o n t (0.052 nmol ng c h l a - 1 h - ) . Ambient s i l i c a t e c o n c e n t r a t i o n s , s i l i c a t e t r a n s p o r t r a t e s ( p S i 0 4 ) and upt a k e r a t e s ( V S i 0 4 ; n o r m a l i z e d p e r u n i t c h l o r o p h y l l a) measured a t 3 m, a r e p r e s e n t e d i n T a b l e 134 Table 2.1. Mid-day (10:00-15:00 h) chlor o p h y l l a s p e c i f i c uptake rates f o r n i t r a t e , ammonium, urea and t o t a l nitrogen (N0 3 + NH4 + urea) measured i n the Gyre and i n the front. (n.d. = non- detectable). Region Station VN0 3 VNH4 Vurea Vnitrogen (umol-ug c h l a-^-.h-1) Gyre 1 n.d. 0.064 0.010 0.074 Front 2 0.033 0.008 0.002 0.043 3 0.042 0.016 0.002 0.060 135 2.2. The h i g h e s t S i 0 4 t r a n s p o r t r a t e s were measured i n t h e Gaspe C u r r e n t w i t h r a t e s v a r y i n g between 0.05 and 0.39 uM h - 1 . These r a t e s a r e i n t h e same range as t h o s e measured by N e l s o n et al. (1981) i n t h e P e r u u p w e l l i n g u s i n g a s t a b l e i s o t o p e t e c h n i q u e (mean = 0.14 uM h - 1 , maximum = 0.55 uM h - 3 - ) . SiCU t r a n s p o r t r a t e s were ca. 3 t i m e s l o w e r (non-d e t e c t a b l e t o 0.18 uM h _ 1 ) i n t h e f r o n t i n s p i t e o f t h e g r e a t e r d i a t o m abundance. S i 0 4 t r a n s p o r t r a t e s were a l s o low (0.08 uM h - 1 ) o r u n d e t e c t a b l e i n t h e Gyre. The c h l o r o p h y l l a s p e c i f i c s i l i c a t e u p t a k e r a t e s ( V S i 0 4 ) were ca. 10 t i m e s g r e a t e r i n t h e Gasp<§ C u r r e n t (mean = 0.12 umol ug c h l a - 3- h - 1 ) t h a n i n t h e f r o n t and i n t h e Gyre (mean = 0.01 umol ug c h l a - 1 h - 1 ) . N:C r a t i o s N:C r a t i o s o f s e s t o n v a r i e d from 0.07 t o 0.15 a c r o s s t h e t r a n s e c t ( F i g . 2.8). N:C r a t i o s were h i g h e r (> 0.12) i n t h e Gaspe C u r r e n t and a t s t a t i o n s 2 and 3 where n i t r o g e n and s i l i c a t e o c c u r r e d i n h i g h c o n c e n t r a t i o n s . I n t h e n i t r a t e and s i l i c a t e i m p o v e r i s h e d p a r t o f t h e f r o n t and i n t h e G y r e , N:C r a t i o s were g e n e r a l l y l o w e r t h a n 0.10 n e a r t h e s u r f a c e . Lowest N:C r a t i o s (0.07) were measured a t s t a t i o n 1 i n t h e G y r e. V a r i a t i o n s o f t h e N:C r a t i o o f s e s t o n a c r o s s t h e f r o n t ( f o r t h e upper 3 m) were p l o t t e d a g a i n s t ambient i n o r g a n i c n i t r o g e n ( F i g . 2.9A), ambient i n o r g a n i c n i t r o g e n + u r e a 136 Table 2 .2 . Ambient s i l i c a t e c o n c e n t r a t i o n s , s i l i c a t e t r a n s p o r t r a t e s ( P S i 0 4 ) and c h l o r o p h y l l a s p e c i f i c s i l i c a t e u p t a k e r a t e s ( V S i 0 4 ) measured a t 3 m a c r o s s t h e f r o n t a l a r e a . ( n . d . = n o n - d e t e c t a b l e ) . R e g i o n S t a t i o n Ambient-SiCU P S i 0 4 V S i 0 4 (uM) (uM h- 1) (umol ug c h l - 1 h — ) Gyre 1 0.8 n.d. n.d. 1' 1.2 0.08 0.026 F r o n t 2 1.4 0.06 0.005 3 1.9 0.18 0.025 4 1.2 0.14 0.027 5 1.1 n.d. n.d. Gaspt% C u r r e n t 6 3.9 0.34 0.180 7 4.3 0.05 0.060 8 3.8 0.39 0.112 137 F i g u r e 2.8. V e r t i c a l d i s t r i b u t i o n o f t h e p a r t i c u l a t e n i t r o g e n : c a r b o n r a t i o (by atoms) of s e s t o n a c r o s s t h e f r o n t . 0.15-0.10 0.05 0 2 4 6 N0 3 +N0 2 +NH4 </lM) E o « 0.15-1 >• z 2 0.10-1 tn ui to u. O O 0.05 B • • • • • •• 2 T -4 6 N0 3 + N0 2 +NH4+UREA(/tM) 0.15-138 0.10-0.05- ~r-2 6 SIO4 (fLW Figure 2.9. N:C r a t i o s of seston vs A: t o t a l ambient inorganic nitrogen; B: t o t a l ambient inorganic nitrogen + urea; and C: ambient s i l i c a t e . Only values between 0 and 3 m are presented. 139 ( F i g . 2.9B) and s i l i c a t e c o n c e n t r a t i o n s ( F i g . 2.9C). F i g u r e 2.9 r e v e a l s a weak r e l a t i o n s h i p between N:C r a t i o s and ambient i n o r g a n i c n i t r o g e n c o n c e n t r a t i o n s , even though l o w e r N:C r a t i o s c o r r e s p o n d e d t o r e l a t i v e l y low (1.5 (iM) n i t r o g e n c o n c e n t r a t i o n s . When ambient u r e a c o n c e n t r a t i o n s a r e added t o t h e t o t a l i n o r g a n i c n i t r o g e n c o n c e n t r a t i o n s , no r e l a t i o n s h i p i s found between N:C r a t i o s and ambient n i t r o g e n . N:C r a t i o s i n c r e a s e d l i n e a r l y f o r s i l i c a t e c o n c e n t r a t i o n s v a r y i n g from 0.5 t o 1.5 LIM and remained h i g h and r e l a t i v e l y c o n s t a n t f o r s i l i c a t e c o n c e n t r a t i o n s above t h e 1.5 uM t h r e s h o l d . S i n k i n g r a t e s The r e l a t i o n s h i p between t h e s i n k i n g r a t e s o f t h e dominant d i a t o m s s p e c i e s (Chaetoceros debilis, Thalassiosira gravida, Skeletonema costatum and Chaetoceros pelagicus) and ambient and i n t e r n a l n u t r i e n t s ( n i t r a t e and s i l i c a t e ) c o n c e n t r a t i o n s a r e p r e s e n t e d i n F i g u r e s 2.10, 2.11, 2.12, and 2.13. Among t h e f o u r d i a t o m s , o n l y Chaetoceros debilis e x h i b i t e d an i n c r e a s e i n s i n k i n g r a t e w i t h d e c r e a s i n g n u t r i e n t c o n c e n t r a t i o n s ( F i g . 2.10). 140 1.0 o • 0.5 o • o • TJ E Ul cc o z z CO - J — 4 0 2 Ambient S I 0 4 IflM) 6 0 8 16 24 32 IN-SIO 4(10" 3 / imol- ttg chl a~1) 1.0 n B 0.5-o o 0 1 2 3 Ambient N 0 3 (/*M) i : 1 1 1 0 6 12 18 IN-NO 3 (10" 3 iimol-^ig chl a~1) F i g u r e 2.10. S i n k i n g r a t e s o f Chaetoceros d e b i l i s vs ambient (•) and i n t e r n a l (°) s i l i c a t e (A) and n i t r a t e (B) i n s u r f a c e (0-1 m) w a t e r s . 1 41 3 -i 2 -E ui oc o o • o 0 2 4 Ambient S I 0 4 (/AM) 6 I ! 1 1 1 0 8 16 24 32 IN-SI04(10" 3 itmol-fig chl a " 1 ) 3 -1 2 -•o I •o 1 -B i 2 0 1 Ambient N 0 3 (LlM) —i 3 r -0 —I 18 1 1 6 12 IN-NO 3 (10 - 3 fimol-/xg chl a - 1 ) Figure 2 . 1 1 . Sinking rates of T h a l a s s i o s i r a gravida vs ambient (•) and i n t e r n a l (°) s i l i c a t e (A) and n i t r a t e (B) i n surface ( 0 -1 m) waters. 0.3-0.2-0.1 cm _r* 0.0 •o E UJ r x O z z -o-e- —r~ 4 0 2 Ambient S10 4 (/xM) -1 6 0 8 16 24 32 IN-SIO4 (10-3/Amol-/ig chl a - 1 ) 0 .3 i B •o 0.2 0.1 0.0 o -o-0 1 2 3 Ambient N 0 3 (/XM) —r— 6 12 18 I N - N 0 3 ( 10"3ttmol-/ig chl a - 1) Figure 2.12. Sinking rates of Skeletonema costatum vs ambient (•) and i n t e r n a l (-) s i l i c a t e (A) and n i t r a t e (B) i n surface (0-1 m) waters. 143 TJ E LU DC O z 0.7 T o • A 0.4 0.3 - o • 0.2 0.1 O . O-lo—• r-0 2 -1 6 Ambient S I 0 4 (pM) i 1 1 1 1 — 0 8 16 24 32 IN-SI0 4 (10~ 3/imol-iig chl a " 1 ) w 7.0 B 0.4 0.3 •o 0.2 0.1 -0.0 f o — o T — • 1 2 3 0 1 Ambient N 0 3 (/iM) r-0 i 6 — i — 12 — i 18 IN-NO 3 (10" 3 fimol-fig chl a " 1 ) F i g u r e 2.13. S i n k i n g r a t e s o f Chaetoceros pelagicus vs ambient (•) and i n t e r n a l (°) s i l i c a t e (A) and n i t r a t e (B) i n s u r f a c e (0-1 m) w a t e r s . 144 Discussion N u t r i e n t S t a t u s of t h e P h y t o p l a n k t o n Community Nitrogen Nutrition D u r i n g t h i s s t u d y , t h e ambient and i n t e r n a l c o n c e n t r a t i o n s o f t h r e e d i f f e r e n t n i t r o g e n s p e c i e s ( N 0 3 , NH 4 and u r e a ) were d e t e r m i n e d a c r o s s t h e f r o n t a l a r e a , as w e l l as t h e i r u t i l i z a t i o n by t h e p h y t o p l a n k t o n community. N u t r i e n t u p t a k e e s t i m a t e s showed t h a t , when p r e s e n t , a l l t h r e e n i t r o g e n s o u r c e s were u t i l i z e d s i m u l t a n e o u s l y by t h e p h y t o p l a n k t o n community ( T a b l e 2.1). I n t h e Gaspe C u r r e n t and i n t h e u p w e l l i n g r e g i o n o f t h e f r o n t , t h e p r e s e n c e o f h i g h ambient and i n t e r n a l n i t r a t e c o n c e n t r a t i o n s , as w e l l as h i g h N:C r a t i o s , c l e a r l y i n d i c a t e t h a t t h e community was N-s u f f i c i e n t . I n t h e Gyre and i n t h e n i t r a t e i m p o v e r i s h e d p a r t s o f t h e f r o n t , t h e low o r u n d e t e c t a b l e IN-N0 3 p o o l s s u g g e s t t h a t t h e c e l l s had not been r e c e n t l y exposed t o n i t r a t e a t a r a t e e x c e e d i n g t h e a s s i m i l a t i o n r a t e ( C o l l o s and Slawyk 1976, D o r t c h e t al. 1985). T h i s c o n c l u s i o n i s s u p p o r t e d by t h e f a c t t h a t t h e dominant d i a t o m s p e c i e s were Chaetoceros debilis, Skeletonema costatum and Thalassiosira gravida, t h r e e d i a t o m s p e c i e s known t o form l a r g e i n t r a c e l l u l a r N0 3 p o o l s when N 0 3 - s u f f i c i e n t (DeManche e t al. 1979, D o r t c h 1982, D o r t c h et al. 1984). The r e l a t i o n s h i p between IN - N O 3 and ambient N0 3 showed t h a t IN - N O 3 p o o l s began t o d e c r e a s e o n l y when ambient NO3 r e a c h e d v a l u e s < 0.3 145 uM ( F i g . 2.7A), w h i c h c o r r e s p o n d s t o t h e h o u r l y N0 3 demand i n t h e f r o n t (mean = 0.33 uM h - 1 ) . T h e r e f o r e , i t seems t h a t t h e N O 3 s u p p l y r a t e by u p w e l l i n g ( o r l a t e r a l m i x i n g ) was n o t s u f f i c i e n t t o meet t h e n i t r a t e demand of t h e p h y t o p l a n k t o n community a t t h o s e s t a t i o n s . I n c o n t r a s t w i t h n i t r a t e , ambient ammonium c o n c e n t r a t i o n s and IN-NH 4 p o o l c o n c e n t r a t i o n s were alw a y s d e t e c t a b l e a c r o s s t h e f r o n t a l a r e a i n t h e upper 10 m, even a t t h o s e s t a t i o n s where n i t r a t e was e x h a u s t e d . These r e s u l t s s u g g e s t t h a t t h e p h y t o p l a n k t o n community was n e v e r s e v e r e l y n i t r o g e n s t r e s s e d . D o r t c h et al. (1985) o b s e r v e d t h a t N - s t a r v e d c e l l s had empty o r v e r y s m a l l IN-NH 4 p o o l s d u r i n g t h e i r s t u d y o f t h e n i t r o g e n n u t r i t i o n o f t h e p h y t o p l a n k t o n community i n Dabob Bay (W a s h i n g t o n ) . They a l s o c o n c l u d e d t h a t t h e p r e s e n c e o f i n t e r n a l NH 4 p o o l s i n any m e a s u r a b l e q u a n t i t y would i n d i c a t e t h a t t h e c e l l s a r e p r o b a b l y n i t r o g e n - s u f f i c i e n t . The h y p o t h e s i s t h a t c e l l s were n o t N - l i m i t e d i s a l s o s u p p o r t e d by t h e h i g h c h l o r o p h y l l a s p e c i f i c ammonium upta k e r a t e measured i n t h e Gyre a t 3 m. However, a t depths c o r r e s p o n d i n g t o t h e Skeletonema costatum bloom (10 t o 20 m) i n t h e Gyre, t h e absence o f IN-NH 4 p o o l s s u g g e s t s t h a t t h e community was n i t r o g e n - d e f i c i e n t . O r g a n i c forms o f n i t r o g e n such as u r e a have been shown t o be used as a n i t r o g e n s o u r c e by many p h y t o p l a n k t o n s p e c i e s i n c u l t u r e (Lund 1987, M o l l o y and S y r e t t 1988, P r i c e 146 and H a r r i s o n 1988) and i n t h e n a t u r a l e n vironment (Conover 1975, P r i c e e t al. 1985). Our u r e a uptake e s t i m a t e s d e m o n s t r a t e d t h a t u r e a c o n s t i t u t e d a s m a l l b u t s i g n i f i c a n t (up t o 13%; T a b l e 2.1) p a r t of t h e c h l o r o p h y l l a s p e c i f i c n i t r o g e n u p t a k e . C o n s e q u e n t l y , t h e p r e s e n c e of u r e a i n r e l a t i v e l y h i g h c o n c e n t r a t i o n i n t h e upper 10 m (mean = 1.0 uM) a c r o s s t h e f r o n t a l a r e a as w e l l as t h e p r e s e n c e o f d e t e c t a b l e u r e a i n t r a c e l l u l a r p o o l s a l s o s u g g e s t t h a t t h e communities were n o t n i t r o g e n - d e f i c i e n t . U n d e t e c t a b l e IN-u r e a p o o l s were o n l y o b s e r v e d i n t h e Gyre between 10 and 20 m ( t h e zone where Skeletonema costatum was maximum). The absence o f IN-urea p o o l s a t de p t h i n t h e Gyre p r o v i d e s f u r t h e r e v i d e n c e t h a t t h i s p a r t i c u l a r p o p u l a t i o n was n i t r o g e n - d e f i c i e n t . I n summary, i n s p i t e of t h e e x h a u s t i o n of n i t r a t e i n some p a r t s o f t h e f r o n t a l a r e a , t h e a n a l y s i s o f ambient c o n c e n t r a t i o n s , i n t e r n a l p o o l c o n c e n t r a t i o n s and upt a k e r a t e s o f ammonium and u r e a s u g g e s t t h a t p h y t o p l a n k t o n c o m m u n i t i t i e s were n e v e r s e v e r e l y n i t r o g e n s t r e s s e d i n t h e upper 10 m. Silicate Nutrition Ambient s i l i c a t e c o n c e n t r a t i o n s may have been l i m i t i n g f o r some d i a t o m s p e c i e s a c r o s s t h e f r o n t . N e l s o n e t al. (1981) c a l c u l a t e d a mean K s f o r s i l i c a t e o f 2.4 uM (S.D. = 1.4 uM) from v a r i o u s f i e l d and l a b o r a t o r y s t u d i e s . I n t h e 147 Gaspe C u r r e n t upper l a y e r , s i l i c a t e c o n c e n t r a t i o n s were h i g h (mean = 4.3 ± 1.1 uM) and t h e d i a t o m community was p r o b a b l y not s i l i c a t e - d e f i c i e n t . T h i s h y p o t h e s i s i s f u r t h e r s u p p o r t e d by t h e h i g h S i 0 4 u p t a k e r a t e s and IN-SiCU p o o l c o n c e n t r a t i o n s measured i n t h e Gaspe C u r r e n t ( T a b l e 2.2, F i g . 2.6D). I n t h e f r o n t and i n t h e G y r e , ambient s i l i c a t e c o n c e n t r a t i o n s were g e n e r a l l y below 2 uM and s i l i c a t e u p t a k e r a t e s ( p e r u n i t c h l o r o p h y l l a) were 10 t i m e s l o w e r t h a n i n t h e Gaspe C u r r e n t . These r e s u l t s s u g g e s t t h a t some d i a t o m s were s i l i c a t e - d e f i c i e n t . Conway and H a r r i s o n (1977) have shown t h a t Chaetoceros debilis, a n u m e r i c a l l y i m p o r t a n t s p e c i e s a t a l l s t a t i o n s a c r o s s t h e f r o n t , has a h i g h K s v a l u e (2.2 uM) f o r s i l i c a t e . E x t r a p o l a t i n g from t h e s e l a b o r a t o r y r e s u l t s t o t h e f i e l d , t h i s s p e c i e s was p r o b a b l y S i - d e f i c i e n t i n t h e f r o n t and i n t h e Gyre. O t h e r s p e c i e s such as Thalassiosira gravida and Skeletonema costatum, w i t h K s v a l u e s o f 0.3 and 0.7 uM s i l i c a t e r e s p e c t i v e l y (Conway and H a r r i s o n 1977), were p r o b a b l y n o t o r l e s s s e v e r e l y s i l i c a t e s t r e s s e d . The i n c r e a s e i n c h l o r o p h y l l a p e r c e l l due t o s i l i c a t e d e f i c i e n c y ( H a r r i s o n e t al. 1977) may a l s o be p a r t l y r e s p o n s i b l e f o r t h e l o w e r s i l i c a t e u p t a k e r a t e s ( p e r u n i t c h l o r o p h y l l a) measured i n t h e Gyre and i n t h e f r o n t . I n c o n t r a s t w i t h n i t r o g e n , t h e p r e s e n c e of s i l i c a t e i n t e r n a l p o o l s i n n a t u r e and t h e i r p o s s i b l e r e l a t i o n s h i p s w i t h t h e s i l i c a t e n u t r i e n t s t a t u s o f t h e d i a t o m community 148 a r e y e t t o be d e t e r m i n e d . When s i l i c a t e - s u f f i c i e n t , s e v e r a l d i a t o m s p e c i e s form i n t e r n a l p o o l s o f s i l i c a t e (Azam e t al. 1974, S u l l i v a n 1979). The f a t e o f t h e s e p o o l s d u r i n g a subsequent s t a r v a t i o n p e r i o d i s u n c l e a r , however. L a b o r a t o r y s t u d i e s s u g g e s t t h a t IN-SiCU p o o l c o n c e n t r a t i o n s may d e c r e a s e d u r i n g e x t e n d e d p e r i o d s o f s i l i c a t e l i m i t a t i o n . F o r i n s t a n c e , when Skeletonema costatum was grown f o r 5 days a t 0.025 h - 1 ( S i - l i m i t e d ) and s u b j e c t e d t o a s i l i c a t e p u l s e , s i l i c a t e was t a k e n up more r a p i d l y t h a n i t was a s s i m i l a t e d ( s u r g e u p t a k e ) , r e f l e c t i n g t h e r a p i d a c c u m u l a t i o n o f i n t e r n a l s i l i c a t e p o o l s ( D a v i s e t al. 1978). However, some di a t o m s such as Navlcula pelllculosa a r e a b l e t o m a i n t a i n a c o n s t a n t I N - S i 0 4 p o o l f o r 14-17 h f o l l o w i n g s i l i c a t e s t a r v a t i o n ( S u l l i v a n 1979). The s i z e o f t h e i n t e r n a l s i l i c a t e p o o l s was a l s o r e p o r t e d t o v a r y d u r i n g t h e c e l l d i v i s i o n c y c l e ( S u l l i v a n 1979). E x p e r i m e n t s c o n d u c t e d w i t h s y n c h r o n i z e d c u l t u r e s o f N. pelliculosa showed t h a t i n t e r n a l s i l i c a t e p o o l s i n c r e a s e d r a p i d l y t o 2.5X t h e i r p r e v i o u s s i z e d u r i n g t h e a c t i v e p e r i o d of s i l i c e o u s f r u s t u l e development. F o l l o w i n g c e l l d i v i s i o n , t h e p o o l s d e c r e a s e d t o t h e i r i n i t i a l v a l u e s . V a r i a t i o n s i n i n t e r n a l s i l i c a t e p o o l s due t o s y n c h r o n i z e d growth ( i f i t happens i n n a t u r e ) may c o n s e q u e n t l y o b s c u r e t h e r e l a t i o n s h i p between IN-SiCU p o o l s i z e and t h e a c t u a l s i l i c a t e n u t r i t i o n a l s t a t e o f t h e community. I n n a t u r e , however, s i l i c a t e u p t a k e r a t e s show l i t t l e o r no d i e l v a r i a b i l i t y , s u g g e s t i n g t h a t most d i a t o m s a r e n o t s y n c h r o n i z e d ( G o e r i n g e t al. 197 3, Azam and C h i s h o l m 149 1976). IN-SiCU p o o l c o n c e n t r a t i o n s should c o n s e q u e n t l y be r e l a t i v e l y c o n s t a n t , u n l e s s the c e l l s are s i l i c a t e - l i m i t e d . For a non-synchronized diatom community, v a r i a t i o n s of the IN-SiCU p o o l c o n c e n t r a t i o n s (as observed d u r i n g t h i s study) p r o b a b l y r e f l e c t v a r i a t i o n s i n the p r o p o r t i o n of a c t i v e l y d i v i d i n g c e l l s i n the community. Duri n g t h i s study, areas w i t h low ambient S i 0 4 and low SiO* uptake r a t e s were g e n e r a l l y c h a r a c t e r i z e d by s m a l l (or u n d e t e c t a b l e ) IN-SiCU p o o l s . In a d d i t i o n , a l i n e a r r e l a t i o n s h i p was observed between ambient SiCU c o n c e n t r a t i o n s and I N - S i 0 4 f o r samples c o l l e c t e d c l o s e t o the s u r f a c e (0-3 m) ( F i g . 2.7B). Although more s t u d i e s c o n c e r n i n g the dynamics of these p o o l s i s c l e a r l y needed, these r e s u l t s suggest t h a t the s i z e of the I N - S i 0 4 p o o l may be an i n d i c a t o r of s i l i c a t e d e f i c i e n c y . E f f e c t of N u t r i e n t S t r e s s on S i n k i n g Rate The s i n k i n g r a t e of diatoms i s i n f l u e n c e d by l i g h t ( B i e n f a n g et al. 1983) and n u t r i e n t s (Eppley et al. 1967, Smayda 1970, 1974, Bienfang et a l . 1982). Although the i n f l u e n c e of these v a r i a b l e s on the s i n k i n g r a t e s appears t o be s p e c i e s - s p e c i f i c , diatoms s i n k i n g r a t e s g e n e r a l l y i n c r e a s e w i t h n u t r i e n t s t a r v a t i o n , and p r i n c i p a l l y w i t h s i l i c a t e s t a r v a t i o n (Bienfang et al. 1982). In t h i s study, the s i n k i n g r a t e of the f o u r dominant diatoms v a r i e d by a f a c t o r of 33 (Chaetoceros debilis), 9 (Thalassiosira 150 gravida), 8 (Skeletonema costatum) and 23 (Chaetoceros pelagicus) a c r o s s the f r o n t . G e n e r a l l y , no r e l a t i o n s h i p was found between v a r i a t i o n s i n s i n k i n g r a t e and ambient o r i n t e r n a l n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s . Only Chaetoceros debilis e x h i b i t e d h i g h e r s i n k i n g r a t e s a t low e x t e r n a l and i n t e r n a l n u t r i e n t c o n c e n t r a t i o n s . Probably, t h i s r e s u l t r e f l e c t s the s i l i c a t e - d e f i c i e n t s t a t u s of t h i s s p e c i e s a c r o s s the f r o n t . E f f e c t of N u t r i e n t S t r e s s on N:C R a t i o V a r i a t i o n s of the N:C r a t i o of ses t o n a c r o s s the f r o n t suggest t h a t the p l a n k t o n i c community was n u t r i e n t - d e f i c i e n t a t some s t a t i o n s . In the Gaspe C u r r e n t and i n the u p w e l l i n g r e g i o n of the f r o n t , N:C r a t i o s were ca. 0.13 and 0.12 r e s p e c t i v e l y , i n the top 10 m ( F i g . 2.8). The comparison of these v a l u e s w i t h the R e d f i e l d r a t i o (N:C = 0.15:1) suggests t h a t these p o p u l a t i o n s were n u t r i e n t - s u f f i c i e n t . These v a l u e s are a l s o c l o s e t o the v a l u e of 0.14 o b t a i n e d by Sakshaug e t al. (1983) f o r n a t u r a l p h y t o p l a n k t o n communities under n i t r o g e n s a t u r a t i o n . In the n u t r i e n t impoverished r e g i o n of the f r o n t and i n the Gyre, N:C r a t i o s decreased below 0.10, i n d i c a t i n g p o s s i b l e n u t r i e n t d e f i c i e n c y (Sakshaug e t al. 1983). Lowest N:C r a t i o s (N:C = 0.07) were r e c o r d e d a t s t a t i o n 1 a t depths c o r r e s p o n d i n g t o the s u b s u r f a c e Skeletonema costatum p o p u l a t i o n . 151 I n t h e t o p 3 m, t h e N:C r a t i o o f s e s t o n d e c r e a s e d w i t h d e c r e a s i n g ambient s i l i c a t e c o n c e n t r a t i o n s ( F i g . 2.9C). Skeletonema costatum, Chaetoceros debilis and Thalassiosira gravida, t h e t h r e e dominant s p e c i e s i n t h e f r o n t , have been shown t o d e c r e a s e t h e i r N:C r a t i o when t h e y were grown as s i l i c a t e - d e f i c i e n t c u l t u r e s ( H a r r i s o n e t al. 1977). S i n c e a t l e a s t two o f t h e s e s p e c i e s may have been S i - l i m i t e d a t t h e ambient s i l i c a t e c o n c e n t r a t i o n s measured i n t h e f r o n t , t h e h y p o t h e s i s t h a t t h e v a r i a t i o n s i n N:C r a t i o s were d r i v e n by s i l i c a t e d e f i c i e n c y i s r e a l i s t i c . I n o r d e r t o d e t e r m i n e i f t h e v a r i a t i o n s o f N:C r a t i o o f s e s t o n r e p r e s e n t e d t h e a c t u a l n u t r i e n t s t a t u s o f t h e p h y t o p l a n k t o n community, N:C r a t i o s were c o r r e c t e d f o r non-a u t o t r o p h i c i n t e r f e r e n c e u s i n g t h e method d e s c r i b e d i n C h a p t e r 1. R e g r e s s i o n a n a l y s e s between POC (and PON) and c h l o r o p h y l l a were conducted a t each s t a t i o n ( T a b l e 2.3). When t h e c o r r e l a t i o n was s i g n i f i c a n t ( a l l s t a t i o n s e x c e p t s t a t i o n 1 ) , t h e Y - i n t e r c e p t was t a k e n as t h e amount of POC ( o r PON) n o t a t t r i b u t e d t o a u t o t r o p h i c o r g a n i s m s . The Y-i n t e r c e p t v a l u e was s u b s t r a c t e d from t h e POC (and PON) v a l u e and t h e N:C r a t i o r e c a l c u l a t e d . The r e l a t i v e i m p o r t a n c e o f d e t r i t u s a l w a y s i n c r e a s e d w i t h d e p t h . D e t r i t u s f o r t h e f r o n t r anged from 0% (near t h e s u r f a c e ) t o 78% ( a t 40 m) o f POC and 0% t o 63% o f PON. I n t h e Gaspe C u r r e n t , d e t r i t u s r a n g e d from 52% t o 100% of POC and 30% t o 100% o f PON. The r e l a t i o n s h i p s between t h e p h y t o p l a n k t o n N:C r a t i o ( c o r r e c t e d 152 Table 2.3. C o e f f i c i e n t s ( s l o p e and y - i n t e r c e p t ) of the l i n e a r r e g r e s s i o n a n a l y s e s conducted between the independent v a r i a b l e c h l o r o p h y l l a and the dependent v a r i a b l e s POC and PON a t each s t a t i o n a c r o s s the f r o n t a l a r e a . S t a t i o n s 6 and 7 were pool e d t o g e t h e r i n o r d e r t o i n c r e a s e the number of data p o i n t s . (n = number of d a t a p o i n t s , r = c o r r e l a t i o n c o e f f i c i e n t , p = p r o b a b i l i t y and n.s. = not s i g n i f i c a n t ) S t a t i o n Dependent Slope Y - i n t e r c e p t n r P v a r i a b l e (ug I-3-) 1' POC 122 242 4 0.40 n.s. PON 22 10 4 0.44 n.s. 1 POC 53 266 6 0.74 0.05 PON 11 20 6 0.80 0.05 2 POC 69 0 4 0.96 0.01 PON 12 10 4 0.99 0.01 3 POC 110 0 5 0.99 0.01 PON 22 0 5 0.99 0.01 4 POC 142 114 5 0.98 0.01 PON 21 1 5 0.98 0.01 5 POC 67 252 6 0.87 0.05 PON 11 14 6 0.80 0.05 6,7 POC 132 391 6 0.84 0.05 PON 32 42 6 0.90 0.05 8 POC 35 287 6 0.82 0.05 PON 8 23 6 0.87 0.05 153 v a l u e s ) and ambient s i l i c a t e c o n c e n t r a t i o n s a r e p r e s e n t e d i n F i g u r e 2.14. The d o t t e d l i n e s r e p r e s e n t t h e N:C r a t i o s r e p o r t e d f o r t h e t h r e e dominant d i a t o m s , Skeletonema costatum ( 0 . 1 9 ) , Chaetoceros debilis (0.31) and Thalassiosira gravida ( 0 . 3 7 ) , when t h e y a r e n u t r i e n t -s u f f i c i e n t ( P a r s o n s e t al. 1961, H a r r i s o n e t al. 1977). As o b s e r v e d f o r t h e N:C r a t i o o f s e s t o n , p h y t o p l a n k t o n N:C r a t i o s were h i g h f o r s i l i c a t e c o n c e n t r a t i o n s above ca. 2 LIM and d e c r e a s e d a b r u p t l y a t l o w e r c o n c e n t r a t i o n s . The p h y t o p l a n k t o n N:C r a t i o s were g e n e r a l l y w e l l below t h e N:C r a t i o s r e p o r t e d f o r t h e dominant s p e c i e s when n u t r i e n t s u f f i c i e n t . The n u m e r i c a l dominance o f Chaetoceros debilis a c r o s s t h e f r o n t , w i t h a K s of 2.2 uM f o r s i l i c a t e u p t a k e , was l i k e l y r e s p o n s i b l e of t h e r e l a t i o n s h i p o b s e r v e d i n F i g . 2.14. V a r i a t i o n s i n N:C r a t i o s o f s e s t o n a r e c u r r e n t l y used by l i m n o l o g i s t s and oceanographers t o d e t e r m i n e t h e n i t r o g e n s t a t u s o f n a t u r a l p h y t o p l a n k t o n communities (Goldman e t al. 1979, H e a l e y and H e n d z e l 1980, Sakshaug e t al. 1983, Sommer 1983). As mentioned p r e v i o u s l y , t h e s i g n i f i c a n c e o f t h i s i n d e x i s l i m i t e d by t h e c o n t r i b u t i o n o f n o n - a u t o t r o p h i c o r g a n i s m s and d e t r i t u s and t h e l a r g e i n t e r s p e c i f i c v a r i a b i l i t y o f t h e e l e m e n t a l c o m p o s i t i o n o f n u t r i e n t -s u f f i c i e n t c e l l s . The l a c k o f s p e c i f i c i t y o f t h i s i n d e x f o r n i t r o g e n r e p r e s e n t s a n o t h e r l i m i t a t i o n w h i c h i s r a r e l y t a k e n i n t o a c c o u n t . Among o t h e r f a c t o r s ( e . g . i r r a d i a n c e , 154 ~ 0.4 -, tn E o 03 n O Z O I-z < -J CL O h->-I CL 0.3 -0.2 -0.1 -T.g. C d . S.k. 2 -i 8 S i 0 4 (jiM) F i g u r e 2.14. N:C r a t i o s o f p h y t o p l a n k t o n vs ambient s i l i c a t e i n t h e t o p 3 m o f t h e water column a c r o s s t h e f r o n t . D o t t e d l i n e s r e p r e s e n t t h e N:C r a t i o f o r Skeletonema costatum (S.k.), Chaetoceros d e b i l i s (Cd.) and T h a l a s s i o s i r a gravida (T.g.) when n u t r i e n t - s u f f i c i e n t . 155 t e m p e r a t u r e ) , s i l i c a t e d e f i c i e n c y a l s o r e s u l t s i n a d e c r e a s e o f t h e N:C r a t i o . The p o s s i b l e e f f e c t of s i l i c a t e -d e f i c i e n c y on t h e r a t i o s h o u l d c o n s e q u e n t l y be t a k e n i n t o a c c o u n t , p a r t i c u l a r l y i n e n v i r o n m e n t s where b o t h n i t r o g e n and s i l i c a t e r e a c h low c o n c e n t r a t i o n s . As mentioned e a r l i e r , s i m u l t a n e o u s d e p l e t i o n o f n i t r a t e and s i l i c a t e has been r e p o r t e d i n numerous ma r i n e e n v i r o n m e n t s ( Z e n t a r a and Kamykowski 1981, Sommer 1986, P a r k e r et al. 1988, G o s s e l i n et al. i n p r e s s ) . 156 CHAPTER 3. DIATOM ABUNDANCE AND THE DISTRIBUTION OF FISH LARVAE AND THEIR FOOD IN A COASTAL JET FRONT BACKGROUND I n t e m p e r a t e s e a s , t h e spawning t i m e s and l o c a t i o n s o f many c o m m e r c i a l l y e x p l o i t e d f i s h e s a r e b e l i e v e d t o have e v o l v e d i n o r d e r t o maximize t h e t e m p o r a l and s p a t i a l c o r r e l a t i o n between f i r s t f e e d i n g f i s h l a r v a e and t h e i r f o o d ( C u s h i n g 1972). F i s h l a r v a e h a t c h o r a r e t r a n s p o r t e d by c u r r e n t s i n a r e a s where a l i n e a r ( s h o r t ) f o o d c h a i n o c c u r s ( S i n c l a i r 1988, C u s h i n g 1989). The l i n e a r f o o d c h a i n i s i n i t i a t e d by t h e v e r t i c a l s t a b i l i z a t i o n of n u t r i e n t - r i c h w a t e r w h i c h promotes t h e growth o f r - s e l e c t e d p h y t o p l a n k t o n ( m a i n l y d i a t o m s ; G u i l l a r d and K i l h a m 1977). The a c c u m u l a t i o n o f p h y t o p l a n k t o n biomass i s b e l i e v e d t o promote t h e r e p r o d u c t i o n o f h e r b i v o r o u s z o o p l a n k t o n (Runge 19 84, 1985, K i o r b o e and Johansen 1986) w h i c h i n t u r n c o n s t i t u t e s t h e p r i n c i p a l f o o d f o r s e v e r a l f i s h l a r v a e ( L a s t 1980, T u r n e r 1984, R i c h a r d s o n e t al. 1986). H y d r o g r a p h i c f e a t u r e s such as f r o n t s where p h y t o p l a n k t o n o f t e n a c c u m u l a t e s r e p r e s e n t i d e a l f e e d i n g grounds f o r f i s h l a r v a e . 157 I n t h e Gaspe C u r r e n t , t h e t h e r m o h a l i n e s t r a t i f i c a t i o n o f n u t r i e n t - r i c h w a t e r s from t h e S t . Lawrence e s t u a r y f a v o r s t h e p r o d u c t i o n o f d i a t o m s i n e a r l y June (C h a p t e r 1 ) . L a t e r d u r i n g t h e s e a s o n , t h e p r o d u c t i o n o f d i a t o m s i s c o n f i n e d t o t h e i n t e r f a c e between t h e Gaspe C u r r e n t and t h e A n t i c o s t i g y r e ( C h a p t e r 1 ) . Some r e s e a r c h e r s have h y p o t h e s i z e d t h a t t h e spawning and d i s p e r s i o n s t r a t e g i e s o f c a p e l i n , Mallotus villosus, and sand l a n c e , Ammodytes hexapterus, a r e c o n n e c t e d t o t h e h i g h p r o d u c t i v i t y o f t h e Gaspe C u r r e n t s u r f a c e w a t e r s ( P a r e n t and B r u n e i 1976, Jacquaz e t al. 1977, de L a f o n t a i n e e t al. 1984a, F o r t i e r e t al. 1987). I n t h i s C h a p t e r , I compare t h e d i s t r i b u t i o n o f e x p o r t p r o d u c t i o n , f i s h l a r v a e and t h e i r r e s o u r c e i n t h e f r o n t a l r e g i o n formed by t h e Gaspe C u r r e n t and t h e A n t i c o s t i g y r e . I n p a r t i c u l a r , I t e s t e d t h e f o l l o w i n g two h y p o t h e s e s : (A) t h a t t h e d i s t r i b u t i o n o f copepod eggs and n a u p l i i c o i n c i d e s s p a t i a l l y w i t h t h e p r o d u c t i o n of d i a t o m s ( e x p o r t p r o d u c t i o n ) , and (B) t h a t t h e c e n t e r s o f l a r v a l d i s t r i b u t i o n a r e a s s o c i a t e d w i t h t h e most abundant f o o d s u p p l y . 158 MATERIALS AND METHODS To d e s c r i b e the f i n e - s c a l e c r o s s - f r o n t a l d i s t r i b u t i o n of f i s h l a r v a e and t h e i r microzooplankton prey, a s e r i e s of hi g h s p a t i a l r e s o l u t i o n samples were c o l l e c t e d d u r i n g the h o r i z o n t a l t r a n s e c t s conducted d u r i n g c r u i s e s A and B. The sampler c o n s i s t e d i n a 1 m2 metal frame f i t t e d w i t h a 330 um mesh n e t , o u t s i d e and i n s i d e flow meters, a 3 L r i g i d l i v e -c a p t u r e codend w i t h 80 um mesh n e t t i n g . A b u i l t - i n d e p r e s s o r a l l o w e d the sampler t o move r a p i d l y t o a depth of 20 m d u r i n g a double o b l i q u e tow t h a t l a s t e d 80 s on average w h i l e the s h i p s a i l e d a t an average speed of 1.67 m s - 1 (3.1 k n o t s ) . The maximum sampling depth was f i x e d a t 20 m s i n c e the t a r g e t e d l a r v a l s p e c i e s ( c a p e l i n and sand l a n c e ) a r e known t o be c o n c e n t r a t e d i n the f i r s t 20 m of the water column ( L a c r o i x and Bergeron 1964, Jacquaz et al. 1977, Able 1978). Upon r e t r i e v a l , the net was t h o r o u g h l y r i n s e d and the codend r e p l a c e d i n l e s s than 150 s. Casts were rep e a t e d a t i n t e r v a l s of 240 s (4 min). Thus the s p a t i a l s e r i e s c o n s i s t e d of tows c o v e r i n g an average d i s t a n c e of 134 m (1.67 m s~x x 80 s) separated by an unsampled i n t e r v a l of 267 m (1.67 m s _ 1 x 160 s ) , g i v i n g an average s p a t i a l r e s o l u t i o n of 802 m (sampling frequency X 2). Pla n k t o n samples were p r e s e r v e d i n 4% f o r m a l i n s o l u t i o n . A l l l a r v a e were s o r t e d and the standard l e n g t h was measured. At eve r y 5 s t a t i o n s , up t o 10 l a r v a e were s e l e c t e d a t random f o r gut cont e n t i d e n t i f i c a t i o n . Water temperature, s a l i n i t y , s h i p 159 speed and d i r e c t i o n r e l a t i v e t o the water were r e c o r d e d a t 30 s i n t e r v a l s w i t h Aanderaa RCM-5 instruments suspended from the s i d e a t depths of 1, 5, and 10 m. Water was pumped from 3 m f o r the c o l l e c t i o n of water samples a t 15 min i n t e r v a l s f o r the d e t e r m i n a t i o n of n u t r i e n t s , POC, PON and c h l o r o p h y l l a (see Chapter 1). In 1985, two s e r i e s of 360 samples ( c r u i s e A) and 180 samples ( c r u i s e B) were o b t a i n e d on June 4 and 21 r e s p e c t i v e l y . The t r a j e c t o r y f o r the two s e r i e s a r e p r e s e n t e d i n F i g u r e s 1.2 and 1.8. C a l c u l a t i o n of l a r v a l f i s h food r e s o u r c e The e s t i m a t i o n of the r e l a t i v e abundance of l a r v a l r e s o u r c e was based on the r e t e n t i o n of microzooplankton by the 80 um mesh of the r i g i d cod end. For each sample, p a r t i c l e s i n the s i z e range 75 - 850 mm were enumerated u s i n g a C o u l t e r TA-II e l e c t r o n i c c o u n t e r . L a r v a l f i s h prey i n a known a l i q u o t of s e l e c t e d sample were enumerated and measured under the d i s s e c t i n g microscope. The l a r v a l f i s h p r e y c o n c e n t r a t i o n s r e p o r t e d i n t h i s study r e p r e s e n t a semi-q u a n t i t a t i v e e s t i m a t i o n ( a l l o w i n g f o r comparisons t o be made between s t a t i o n s and between c r u i s e s ) s i n c e most of the organisms escaped from the 330 um mesh net and o n l y a s m a l l f r a c t i o n was r e t a i n e d by the 80 um mesh of the cod end. 1 6 0 The d i e t of dominant l a r v a l species was determined by analysing the gut contents of 5 0 specimens of sand lance and re d f i s h i n transect A, and 5 0 specimens of capelin i n transect B. Cal c u l a t i o n of the actual resource of a given length class was based on the frequency of occurrence of the major prey i n the gut, the average weight of these prey and t h e i r abundance i n the environment ( F o r t i e r and Harris 1 9 8 9 ) . The resource R i i n mg m~3 for larvae of length 1 was defined as: R i = E A p F P i W p , f o r p = 1 , . . . ,n where n = number of d i f f e r e n t prey taxa i n the d i e t of the f i s h , A p = abundance of the p t h prey in s i t u , F p i frequency of the p t h l prey i n the gut of larvae of length 1 , and Wp,. i s the average weight of the p t h prey. The average weight of copepod prey was estimated by applying the weight-width r e l a t i o n s h i p of Pearre ( 1 9 8 0 ) to the measured width of 1 0 0 specimens. Average weight of non-copepod prey was estimated from volume values based on radius and length measurements and assuming a density of 1 . 0 2 8 ( F o r t i e r and Gagn6 i n press). 161 RESULTS Hydrography A d e t a i l e d d e s c r i p t i o n of t h e h y d r o g r a p h i c f e a t u r e s o f t h e f r o n t a l a r e a d u r i n g c r u i s e s A and B was g i v e n i n C h a p t e r 1. I n b r i e f , t h e Gaspe C u r r e n t formed a h a l f - l e n s of d i l u t e d w a t e r s f l o w i n g o v e r t h e more o c e a n i c w a t e r s o f t h e g y r e . A s t r o n g t h e r m o h a l i n e s t r a t i f i c a t i o n s t r e t c h e d from t h e s u r f a c e t o about 50 m, t h e d e p t h o f t h e i n t e r m e d i a t e g l a c i a l l a y e r ( F o r r e s t e r 1964). I n t h e A n t i c o s t i G y r e, a w e l l mixed s u r f a c e l a y e r 10 t o 15 m t h i c k was s e p a r a t e d from t h e i n t e r m e d i a t e g l a c i a l l a y e r a t 30 m by a s t r o n g t h e r m o c l i n e . The d e n s i t y b a r r i e r between t h e Gaspe C u r r e n t and t h e A n t i c o s t i Gyre reached t h e s u r f a c e between 5 and 11 km o f f - s h o r e , f o r m i n g a s t r o n g h o r i z o n t a l s a l i n i t y f r o n t . F o r t r a n s e c t A, t h e s h i p t r a v e l l e d p a r a l l e l t o t h e f r o n t i n t h e A n t i c o s t i g y r e , and s u b s e q u e n t l y c r o s s e d t h e f r o n t p e r p e n d i c u l a r l y and t h e n t r a v e l l e d p a r a l l e l t o t h e c o a s t f o r t h e l a s t p a r t of t h e t r a n s e c t ( F i g . 1.2). F o r c r u i s e B, t h e s h i p t r a v e l l e d most o f t h e t i m e i n t h e f r o n t per se, p a r a l l e l t o t h e c o a s t ( F i g . 1.8). Ichthyoplankton composition, length frequency and d i e t D u r i n g l a t e s p r i n g and e a r l y summer, t h e l a r v a e o f two i n s h o r e / e s t u a r i n e d e m e r s a l spawners a l t e r n a t e d as t h e dominant i c h t h y o p l a n k t o n s p e c i e s i n t h e a r e a ( T a b l e 3.1). 162 T a b l e 3.1. S p e c i f i c c o m p o s i t i o n and abundance (%) o f i c h t h y o p l a n k t o n i n t h e f r o n t a l a r e a o f t h e Gaspe C u r r e n t i n e a r l y June ( c r u i s e A) and a t t h e end o f June ( c r u i s e B) 1985. S p e c i e s 3-4 June 21-22 June Ammodytes hexapterus 10811 (73 •7) 338 ( 1 . 4) Mallotus v i l l o s u s 2100 (14 .3) 20768 (87 •7) Sebastes spp. 1408 (9- 6) 1754 (7 . 4) Stichaeus punctatus 267 ( 1 . 8) 71 (0. 3) Hippoglossoides platessoides 20 (0. 1) 28 (0. 1) Ulvaria subbifurcata 17 (0. 1) 166 (0. 7) Pseudopleuronectes americanus 15 (0. 1) -Clupea harengus harengus 10 (<0 •1) 401 ( 1 . 7) Myoxocephalus scorpius 5 (<0 •1) 2 (<0 •1) Enchelyopus cimbrius 4 (<0 •1) 35 (<0 •1) Aspidophoroides monopterygius 3 (<0 •1) 1 (<0 •1) Gadus morhua 2 (<0 •1) 34 (<0 •1) Pholis gunnellus 1 (<0 •1) 6 (<0 •1) Lumpenus lumpretaeformis 1 (<0 •1) 29 (<0 •1) Gasterosteus aculeatus 1 (<0 •1) -Cyclopterus lumpus 1 (<0 •1) -Liopsetta putnami - 29 (<0 •1) Limanda ferruginea - 4 (<0 •1) L i p a r i s sp. - 3 (<0 •1) Scomber scombrus - 2 (<0 •1) T o t a l 14666 23671 163 Sand l a n c e , Ammodytes hexapterus, l a r g e l y dominant (> 7 4 %) i n e a r l y June ( c r u i s e A ) , was r e p l a c e d by c a p e l i n Mallotus villosus (> 88 %) i n l a t e June ( c r u i s e B ) . The o v o v i v i p a r o u s r e d f i s h Sebastes marinus always r e p r e s e n t e d a s i g n i f i c a n t component (8-10%) o f t h e i c h t h y o p l a n k t o n assemblage. A r c t i c shanny Stichaeus punctatus and A t l a n t i c h e r r i n g Clupea harengus were sometimes c a p t u r e d i n numbers s u f f i c i e n t t o j u s t i f y f u r t h e r a n a l y s i s o f t h e i r d i s t r i b u t i o n . F o r t r a n s e c t A, two c o h o r t s o f sand l a n c e l a r v a e were p r e s e n t ( F i g . 3.1A). T h e i r s i z e d i s t r i b u t i o n was c h a r a c t e r i z e d by a peak a t 4-5 mm (65% of t o t a l c a p t u r e ) and a second s m a l l e r peak around 8 mm. C o n s i d e r i n g t h a t t h e l a r v a e h a t c h from eggs when t h e y a r e about 4 mm l o n g ( S c o t t and S c o t t 1988), t h e c o h o r t o f s m a l l l a r v a e r e p r e s e n t s a r e c e n t h a t c h i n g e v e n t . A s i n g l e c o h o r t of c a p e l i n (mean s i z e o f 7 mm), r e d f i s h (mean s i z e of 7 mm) and A r c t i c shanny (mean s i z e o f 14 mm) was p r e s e n t d u r i n g t h i s s u r v e y . F o r t r a n s e c t B, s e v e r a l c o h o r t s o f sand l a n c e l a r v a e were s t i l l p r e s e n t i n t h e f r o n t ( F i g . 3.1). I n c o n j u n c t i o n w i t h t h e d e c r e a s e i n abundance o f sand l a n c e l a r v a e , t h e mean s i z e of t h e n u m e r i c a l l y dominant c o h o r t i n c r e a s e d from 5 mm i n t r a n s e c t A t o 10-11 mm i n t r a n s e c t B. S e v e r a l c o h o r t s o f c a p e l i n l a r v a e were a l s o p r e s e n t d u r i n g t r a n s e c t B ( F i g . 3.1). T h e i r s i z e d i s t r i b u t i o n was c h a r a c t e r i z e d by a peak a t 7-8 mm and a s i g n i f i c a n t number o f l a r v a e l a r g e r t h a n 10 163a F i g u r e 3.1. S i z e c l a s s f r e q u e n c y o f t h e m a j o r f i s h l a r v a e c o l l e c t e d d u r i n g c r u i s e A (empty b a r ) and d u r i n g c r u i s e B ( f i l l e d b a r ) . 60 Ammedytu h«xapterua 30-ijllll ) * ! r f , — i — StlchMua punctata* 30-S1ZE CLASS (nun) 165 mm (up t o 19 mm). Larvae of r e d f i s h and A t l a n t i c h e r r i n g formed one s i n g l e c o h o r t c e n t e r e d a t 7 mm and 10 mm r e s p e c t i v e l y ( F i g . 3.1). Copepod eggs and n a u p l i i c o n s t i t u t e d the main d i e t of sand l a n c e , c a p e l i n and r e d f i s h l a r v a e (Table 3.2, 3.3 and 3.4). Copepodites were a l s o consumed by the 9-12 mm c a p e l i n l a r v a e i n t r a n s e c t B. In g e n e r a l , the d i e t s h i f t e d from copepod eggs t o n a u p l i i and copep o d i t e s as l a r v a l s i z e i n c r e a s e d . C r o s s - f r o n t a l d i s t r i b u t i o n o f p h y t o p l a n k t o n and copepod r e p r o d u c t i o n In t r a n s e c t A, phytoplankton biomass was low (< 0.5 ng l~x) i n the gyre ( F i g . 3.2) and the community was dominated by s m a l l f l a g e l l a t e s and d i n o f l a g e l l a t e s (Table 1.2). In the Gaspe C u r r e n t , phytoplankton biomass was h i g h (up t o 35 ng l - 1 ) and the community was dominated by l a r g e diatoms (Table 1.2). Diatom biomass decreased g r a d u a l l y a c r o s s the s a l i n i t y f r o n t s e p a r a t i n g the Gasp6 C u r r e n t from the gyre. Copepod eggs, n a u p l i i and copepodite c o n c e n t r a t i o n s were maximum i n the d i a t o m - r i c h waters of the Gasp6 C u r r e n t and f r o n t and minimum i n the Gyre where the phytoplankton biomass was low and the community was dominated by s m a l l f l a g e l l a t e s ( F i g . 3.2). The abundance of 203-500 nm p a r t i c l e s e x h i b i t e d a s i m i l a r t r e n d w i t h maximum v a l u e s (600 m~3) i n the Gaspe C u r r e n t and minimum v a l u e s (10 m - 3) i n the 166 T a b l e 3.2. P e r c e n t c o m p o s i t i o n o f l a r v a l c a p e l i n d i e t by l e n g t h c l a s s e s i n June 1985 ( c r u i s e B ) . L e n g t h c l a s s (mm) P r e y < 7 7-9 9-12 t o t a l Copepod eggs 96 .1 66.9 45.7 51.7 Copepod n a u p l i i 3 .8 24.4 21.7 21.4 C o p e p o d i t e s 0 .0 4.9 28.1 22.0 Copepods 0 .0 0.0 0.0 0.0 L a m e l l i b r a n c h l a r v a e 0 .0 0.0 0.0 0.0 P o l y c h a e t e l a r v a e 0 .0 0.0 0.0 0.0 Ot h e r 0 .0 3.8 4.4 4.9 T a b l e 3.3. P e r c e n t c o m p o s i t i o n o f l a r v a l sand l a n c e d i e t by l e n g t h c l a s s e s i n e a r l y June 1985 ( c r u i s e A ) . L e n g t h c l a s s (mm) P r e y < 8 8-10 10+ t o t a l Copepod eggs 40 .0 23.0 13.1 16.3 Copepod n a u p l i i 60 .0 74.4 84.4 81.3 C o p e p o d i t e s 0 .0 0.0 0.0 0.0 Copepods 0 .0 0.0 0.0 0.0 L a m e l l i b r a n c h l a r v a e 0 .0 0.0 1.6 1.2 P o l y c h a e t e l a r v a e 0 .0 0.0 0.0 0.6 O t h e r 0 .0 2.5 0.0 0.6 168 T a b l e 3.4 P e r c e n t c o m p o s i t i o n o f l a r v a l r e d f i s h d i e t by l e n g t h c l a s s e s i n e a r l y June 1985 ( c r u i s e A ) . L e n g t h c l a s s (mm) P r e y < 7 7-8 8+ t o t a l Copepod eggs 68.9 71.9 42.9 62.8 Copepod n a u p l i i 27.0 25.8 56.4 35.2 C o p e p o d i t e s 0.0 0.3 0.0 0.2 Copepods 0.0 0.0 0.0 0.0 L a m e l l i b r a n c h l a r v a e 4.0 0.6 0.0 0.9 P o l y c h a e t e l a r v a e 0.0 0.0 0.0 0.0 O t h e r 1.3 0.6 0.0 0.9 163 40 30 20 10 CHLOROPHYLL a (ju) L - 1) 100 SO COPEPOD EGGS (#/m3) nniiiiiinnniinnnnnj 0 50 100 ISO 200 250 300 350 + GYRE—+FRT+GASPE—+—FRONT—+ CURRENT STATION 500 400 H 300 200 100-PARTICLES 203-500 >m (#/m3) IPrinnnrinnnnflnnnnr 0 50 100 150 200 250 300 350 + GYRE +FRT+—GASPE—+—FRONT-+ CURRENT STATION Figure 3.2. Cruise A - Horizontal d i s t r i b u t i o n of s a l i n i t y (3m), c h l o r o p h y l l a (3m), and concentrations of copepod eggs (0-20 m), n a u p l i i (0-20 m), copepodites and small (< 4 mm) copepods (0-20 m) and 203-500 nm p a r t i c l e s (0-20 m) on a ho r i z o n t a l transect across the f r o n t a l area of the Gaspe Current i n the Gulf of St. Lawrence (see F i g . 1.2 f o r p o s i t i o n of stations 0-360). 170 gyre. Microscopic observation of the samples revealed that copepod eggs, n a u p l i i and copepodites constituted the p r i n c i p a l components of the 203-500 um size p a r t i c l e s . A s i g n i f i c a n t c o r r e l a t i o n ( r 2 = 0.69; p < 0.01) was found between p a r t i c l e abundance i n t h i s s i z e class and the t o t a l abundance of copepod eggs, n a u p l i i and copepodites ( Fig. 3.3A). Microzooplankton concentrations were p o s i t i v e l y correlated with fluorescence and negatively correlated with s a l i n i t y (Fig. 3.4). In transect B, at least two d i s t i n c t water masses were present i n the front per se. These two regions have been ref e r r e d to as the non-upwelling front and the upwelling front (see Chapter 1). Phytoplankton biomass at 3 m depth was generally low across the front, except i n the upwelling front (Fig. 3.5). On average, immature copepods were twice as abundant as during transect A. Their concentrations were maximum i n the non-upwelling front and minimum i n the gyre (Fig. 3.5). R e l a t i v e l y low concentrations were measured i n the Gaspe Current and i n the diatom-rich upwelling part of the front. Again, the abundance of 203-500 um p a r t i c l e s proved a good index of the abundance of copepod eggs, n a u p l i i and copepodites (Fig. 3.3B). No r e l a t i o n s h i p was found between the abundance of the 203-500 um p a r t i c l e s and chlor o p h y l l a at 3 m and s a l i n i t y (Fig. 3.6). 171 200 150 + 100 + 50 + A) r 2 = 0.69 9 • • e • • • # i 100 200 300 400 500 1500 1000 500 + B) r 2 = 0.45 • • • • • • • • W • • -• , 1 1 ; 500 1000 1500 PARTICLES 203-500 pun (#/m 3 ) 2000 Figure 3 . 3 . Copepod eggs, n a u p l i i and copepodite abundances vs concentration of 2 0 3 - 5 0 0 um p a r t i c l e s measured between 0 and 2 0 m i n transect A (A) and B (B). 172 600-400 + 200-. • • • •+- •+-1 2 3 4 5 FLUORESCENCE (relotive unit) 600-400 + 200 + .» • • • 26 27 28 29 SAUNITY (0/00) 30 31 F i g u r e 3.4. Abundance o f p a r t i c l e s 203-500 um (0-20 m) vs (A) f l u o r e s c e n c e (3 m) and (B) s a l i n i t y (3 m) d u r i n g t h e h o r i z o n t a l t r a n s e c t c o nducted d u r i n g c r u i s e A. 173 +—upweling—+ +—upweling + STATION STATION Figure 3.5. Cruise B - Horizontal d i s t r i b u t i o n of s a l i n i t y (3m), c h l o r o p h y l l a (3m), and concentrations of copepod eggs (0-20 m), n a u p l i i (0-20 m), copepodites and small (< 4 mm) copepods (0-20 m) and 203-500 um p a r t i c l e s (0-20 m) on a h o r i z o n t a l transect across the f r o n t a l area of the Gaspe Current i n the Gulf of St. Lawrence (see F i g . 1.8 f o r p o s i t i o n of sta t i o n s 1 to 180). 2000 rO I E a. o o m I rO O CM VI U J _ l O I— a: < CL 1500 + 1000 + 500-2000 1500 + 1000 + 500 + 26 _i 1 1 1 1 — 1 2 3 4 5 FLUORESCENCE (relative unit) 27 28 29 SALINITY (0/00) Figure 3.6. P a r t i c l e s 203-500 nm (0-20 m) vs (A) fluorescence (3 m) and (B) s a l i n i t y (3 m) during the horizontal transect conducted during cruise B. 175 C r o s s - f r o n t a l d i s t r i b u t i o n o f f i s h l a r v a e and t h e i r r e s o u r c e I n t r a n s e c t A, t h e f r o n t c l e a r l y d e l i n e a t e d t h e d i s t r i b u t i o n o f e s t u a r i n e (sand l a n c e and c a p e l i n ) and g y r e ( r e d f i s h and A r c t i c shanny) l a r v a l s p e c i e s ( F i g - 3.7). E a r l y p o s t l a r v a e (< 12 mm) o f sand l a n c e and c a p e l i n were found p r i n c i p a l l y i n t h e Gasp6 C u r r e n t and i n t h e s a l i n i t y g r a d i e n t w h i l e l a r v a e o f r e d f i s h and A r c t i c shanny were more abundant i n t h e Gyre s i d e of t h e f r o n t and a b s e n t from t h e Gaspe C u r r e n t . Both g y r e l a r v a l s p e c i e s , a l o n g w i t h l a r v a l sand l a n c e , e x h i b i t e d a s h a r p peak i n abundance n e a r t h e f r o n t , on t h e g y r e s i d e ( S t a t i o n s 165 t o 175). The t o t a l l a r v a l f o o d r e s o u r c e ( a l l s i z e c l a s s e s ) c a l c u l a t e d f o r each s p e c i e s ( e x c e p t f o r A r c t i c shanny) i s p r e s e n t e d a l o n g w i t h l a r v a l abundance i n F i g u r e 3.7. F o r a g i v e n s p e c i e s , l a r v a l f o o d r e s o u r c e was s i m i l a r f o r each s i z e c l a s s (same s i z e c l a s s e s as i n T a b l e s 3.2, 3.3 and 3.4). L a r v a l f o o d r e s o u r c e was h i g h l y c o n c e n t r a t e d i n t h e Gasp6 C u r r e n t and i n t h e f r o n t f o r each s p e c i e s . Sand l a n c e and c a p e l i n l a r v a e were c o n c e n t r a t e d where t h e i r r e s o u r c e was maximum w h i l e r e d f i s h l a r v a e were found i n r e l a t i v e l y r e s o u r c e - p o o r w a t e r s . As o b s e r v e d f o r e s t u a r i n e s e s t o n d u r i n g t h i s c r u i s e , p h y s i c a l m i x i n g p l a y e d an i m p o r t a n t r o l e i n t h e c r o s s -f r o n t a l d i s t r i b u t i o n o f e s t u a r i n e f i s h l a r v a e ( F i g . 3 . 8 ) . F o r s a l i n i t y between 27 and 30 c o n c e n t r a t i o n s o f sand 175a Figure 3.7. Cruise A - Horizontal d i s t r i b u t i o n of s a l i n i t y (3m), sand lance (Ammodytes hexapterus) larvae (0-20 m) and t h e i r resource, capelin (Mallotus villosus) larvae (0-20 m) and t h e i r resource, r e d f i s h (Sebastes spp.) larvae (0-20 m) and t h e i r resource, and A r c t i c shanny (Stichaeus punctatus) larvae (0-20 m) on a horizontal transect across the f r o n t a l area of the Gasp§ Current i n the Gulf of St. Lawrence (see F i g . 1.2 f o r p o s i t i o n of stations 0-360). 176 31 + GYRE +FKT+-QASPE-+—FRONT CURRENT SAUNTTY (o/oo) 160n Ammodytea haxoptaru* (#/100 m3) 80-RESOURCE 0>«/tOO m3) 50 n I ' I . I Uallotua vOloaua (•/100 m3) 25-REBOURCC lllll.l. I k l + GrrRE +FRT+-GASPE-+-FRONT CURRENT 60 Seboataa app. (i/IOOm3) ho 0 r« RESOURCE Cmt/IOOm3) 40 l l I I I | I I i I J I I I | I I • I | i l i l i l 204 Stlchaoua punototua (•/lOOm 3)! J U L 60 120 180 240 300 360 STATION *0 1J0 180 240 300 MO snnoN 177 3 0 0 -2 5 0 - • A m m o d y t e s hexapterus 200 •-o o £ 3 CJ 2 < a -z CD < 1 5 0 -100-5 0 -0 -6 0 -5 0 -4 0 -3 0 -2 0 -10-Mallotus villosus ' J ••••• . .* • • ~ I. • .... .Tt. -+-80-Sebastes spp. 60-4 0 -2 0 + o 1 h 6 M ^ r S ^ ; . * ^ . V , ; r . . . r t ^ - J L ) r ^ L -26 27 2B 29 30 31 SALINITY ( 0 / 0 0 ) F i g u r e 3 . 8 . R e l a t i o n s h i p between s a l i n i t y a t 3 m and t h e abundance o f sand l a n c e (Ammodytes hexapterus), c a p e l i n (Mallotus villosus) and r e d f i s h (Sebastes spp.) l a r v a e c o l l e c t e d between 0 and 20 m on a h o r i z o n t a l t r a n s e c t a c r o s s t h e f r o n t a l a r e a o f t h e Gaspe C u r r e n t i n t h e G u l f o f S t . Lawrence. 0 178 l a n c e and c a p e l i n l a r v a e were s i g n i f i c a n t l y c o r r e l a t e d w i t h s a l i n i t y ( r 2 = 0.61 and 0.68 r e s p e c t i v e l y ; t - t e s t p < 0.05)). The c o n c e n t r a t i o n of r e d f i s h l a r v a e g e n e r a l l y i n c r e a s e d w i t h s a l i n i t y (and d i s t a n c e from the f r o n t ) , i n d i c a t i n g t h a t they were a l s o d i l u t e d by the water from the Gasp6 C u r r e n t ( F i g . 3.8). The a n a l y s i s of the c r o s s - f r o n t a l d i s t r i b u t i o n of sand l a n c e l a r v a e by s i z e c l a s s i n d i c a t e d t h a t s m a l l l a r v a e (<6 mm) were c o n c e n t r a t e d i n the C u r r e n t ( s a l i n i t y < 28 ° / 0 0 ) w h i l e l a r g e r ones (6-10 mm and > 10 mm) were found a t h i g h e r s a l i n i t i e s (Fig. 3.9). T h i s t r e n d was p a r t i c u l a r l y obvious f o r l a r v a e > 10 mm which were found i n abundance on the gyre s i d e of the f r o n t . The i n s h o r e - o f f s h o r e i n c r e a s e i n l a r v a l s i z e seems t o r e f l e c t the ontogenic m i g r a t i o n of these l a r v a e through the f r o n t and suggests t h a t the e s t u a r i n e l a r v a e were r e t a i n e d i n the f r o n t . Larvae of c a p e l i n and r e d f i s h e x h i b i t e d no size-dependent v a r i a t i o n s i n t h e i r c r o s s - f r o n t a l d i s t r i b u t i o n (data not shown). In t r a n s e c t B, the c r o s s - f r o n t a l d i s t r i b u t i o n s of c a p e l i n and A t l a n t i c h e r r i n g l a r v a e were i d e n t i c a l ( F i g . 3.10), r e f l e c t i n g t h e i r s i m i l a r e s t u a r i n e o r i g i n (De L a f o n t a i n e et al. 1984a, b ) . The i n c r e a s e i n abundance of l a r v a l c a p e l i n and A t l a n t i c h e r r i n g i n the Gasp6 C u r r e n t i s c o n s i s t e n t w i t h the maximum spawning time f o r these s p e c i e s i n the lower e s t u a r y (Jacquaz et al. 1977, De L a f o n t a i n e et E 8 Q z 3 1501 1 0 0 -Ammodytes hexapterus 3 - 5 mm SO- • . . 150 -i Ammodytes hexapterus 5 - 6 mm 100 -50-. -1 * i—' • ' 150-, Ammodytes hexapterus 6—10 mm 100-50-179 50 I 40 • JO 20 -Ammodytes hexapterus ) 10 mm 10---4-26 27 28 29 SAUNITY (0/00) 30 31 F i g u r e 3 .9 . s i z e c l a s s e s R e l a t i o n s h i p between s a l i n i t y and d i f f e r e n t o f sand l a n c e l a r v a e d u r i n g c r u i s e A. 179a F i g u r e 3.10. C r u i s e B - H o r i z o n t a l d i s t r i b u t i o n of s a l i n i t y (3 m), sand lance (Ammodytes hexapterus) l a r v a e (0-20 m) and t h e i r resource, c a p e l i n (Mallotus villosus) l a r v a e (0-20 m) and t h e i r resource, r e d f i s h (Sebastes spp.) l a r v a e (0-20 m) and t h e i r resource, and A t l a n t i c h e r r i n g (Clupea harengus) l a r v a e (0-20 m) on a h o r i z o n t a l t r a n s e c t across the f r o n t a l area of the Gasp6 Current i n the Gulf of St. Lawrence (see F i g . 1.8 f o r p o s i t i o n of s t a t i o n s 0-180). 180 +OY+ FRONT +0. CURRENT 32 31 30-I 20 28 27 26 +—upasHIng—+• r14 32, 12 +GY+ SAUNITY (o/oo) TEMPERATURE — (°C) -FRONT +0. CURRENT +—upwalflng—+ 10 RESOURCE (mg/IOOn3) 1200 i I i i , • , I—L+l 600 30 15 1000 0 C)up«a horangvM (•/lOOm3) 0 30 60 90 120 150 160 STATIONS 800 ISO ISO SDUMN 181 al. 1984b, F o r t i e r and Gagne i n p r e s s ) . C a p e l i n and A t l a n t i c h e r r i n g l a r v a e were found i n h i g h c o n c e n t r a t i o n s i n t h e Gasp<§ C u r r e n t ( s t a t i o n s 175 t o 180) and a t t h e i n t e r f a c e between t h e Gaspe C u r r e n t and t h e f r o n t ( s t a t i o n s 73 t o 90, and 170 t o 175). Few l a r v a e were found i n t h e u p w e l l i n g p a r t o f t h e f r o n t and i n t h e Gyre. Sand l a n c e l a r v a e were v i r t u a l l y a b s e n t from t h e Gaspe C u r r e n t , i n d i c a t i n g t h a t t h e i r spawning p e r i o d i n t h e e s t u a r y was t e r m i n a t e d . They formed one s i n g l e h i g h d e n s i t y p a t c h i n t h e f r o n t ( s t a t i o n s 7 2 t o 85; s a l i n i t y o f 29 °/ 0 0) where numerous l a r g e l a r v a e (> 12 mm) were found ( F i g . 3.8). The v a r i a t i o n s i n s i z e o f t h e c a p e l i n l a r v a e a c r o s s t h e f r o n t i s shown i n F i g u r e 3.11. W h i l e s m a l l l a r v a e were found t h r o u g h o u t t h e f r o n t a l a r e a , l a r g e r ones (> 15 mm) were o n l y found i n t h e f r o n t a t a s a l i n i t y o f ca. 29 ° / 0 0 . T h i s t r e n d w h i c h has been o b s e r v e d f o r sand l a n c e l a r v a e d u r i n g c r u i s e A, s u g g e s t s t h a t l a r v a e (and s e s t o n i n g e n e r a l ) were t r a p p e d i n t h e f r o n t . I n c o n t r a s t w i t h t r a n s e c t A, r e d f i s h l a r v a e were found b o t h i n t h e g y r e and i n t h e f r o n t p e r se. Maximum c o n c e n t r a t i o n s were f o u n d i n t h e g y r e , as o b s e r v e d d u r i n g c r u i s e A, b u t a l s o i n t h e n o n - u p w e l l i n g f r o n t ( s t a t i o n s 4 t o 90) where t h e y formed 3-5 km p a t c h e s o f r e l a t i v e l y h i g h abundance. I n t r a n s e c t B, t h e r e s o u r c e of t h e t h r e e dominant f i s h l a r v a e was maximum i n t h e n o n - u p w e l l i n g f r o n t , r e l a t i v e l y low i n t h e Gaspe C u r r e n t and u p w e l l i n g f r o n t and minimum i n t h e Gyre ( F i g . 3.10). E s t u a r i n e l a r v a e ( c a p e l i n and sand 10001 800-Mollotus villosus 3 - 8 mm 600-400-200-0-1000-| 800--t- - f -Mollotus villosus 8-12 mm 600-400-200 • 0-I 80-, -+- -t-60--Mallotus villosus 12-15 mm 40-20-150-, -+-100-SO -Mallotus villosus ) 15 mm 26 27 • V " 28 29 SALINITY (0/00) 30 31 F i g u r e 3.11. R e l a t i o n s h i p between s a l i n i t y and d i f f e r e n t s i z e c l a s s e s o f c a p e l i n l a r v a e d u r i n g c r u i s e B. 183 lance) were less c l o s e l y associated with t h e i r resource than during transect A. It i s noteworthy that few larvae were found i n the r e l a t i v e l y resource-poor upwelling part of the front. In contrast with the previous cr u i s e , r e d f i s h larvae were abundant i n the non-upwelling front where t h e i r food resource was high. 184 DISCUSSION Copepod r e p r o d u c t i o n - t h e s h o r t f o o d c h a i n There i s i n c r e a s i n g e v i d e n c e t h a t copepod r e p r o d u c t i o n i s enhanced by t h e p r o d u c t i o n and a c c u m u l a t i o n o f l a r g e p h y t o p l a n k t o n c e l l s (Runge 1984, 1985, Kio>boe and Johansen 1986) w h i c h o c c u r m a i n l y a t h y d r o g r a p h i c f e a t u r e s (Legendre and Le F e v r e 1989). S i n c e immature copepod s t a g e s c o n s t i t u t e t h e main p r e y o f a m a j o r i t y o f f i s h p o s t l a r v a e ( L a s t 1980, T u r n e r 1984) and t h a t f o o d may l i m i t l a r v a l g r o w t h / s u r v i v a l ( L e g g e t t 1986, Anderson 1988, F o r t i e r and Gagne i n p r e s s ) , copepod r e p r o d u c t i o n r e p r e s e n t s t h e p o t e n t i a l l i n k between h y d r o g r a p h i c e v e n t s ( e . g . u p w e l l i n g and f r o n t s ) and t h e s u r v i v a l of f i s h l a r v a e (Runge 1988). I n e a r l y June, d i a t o m p r o d u c t i o n and biomass were h i g h i n t h e Gaspe C u r r e n t and e x t r e m e l y low i n t h e Gyre. Shear s t r e s s was h i g h between t h e C u r r e n t and t h e Gyre d u r i n g t h i s t i m e o f t h e y e a r and, as a r e s u l t , t h e c r o s s - f r o n t a l d i s p e r s i o n o f d i a t o m s was m o s t l y under p h y s i c a l c o n t r o l ( C h a p t e r 1 ) . The c r o s s - f r o n t a l d i s t r i b u t i o n o f immature copepod s t a g e s p a r a l l e l e d t h e d i s t r i b u t i o n o f d i a t o m s , as p r e d i c t e d by t h e s h o r t f o o d c h a i n model ( F i g . 3.2). S i n c e a d u l t copepods were not enumerated d u r i n g t h i s s t u d y , i t i s n o t p o s s i b l e , t o d e t e r m i n e i f t h e d i f f e r e n c e s between t h e abundance of immature copepods i n t h e Gyre and i n t h e Gaspe C u r r e n t r e s u l t e d from v a r i a t i o n s i n copepod abundance o r 185 p r o d u c t i v i t y . In the study area, however, Sevigny et al. (1979) reported no important v a r i a t i o n s i n zooplankton biomass between the Gaspe Current and the A n t i c o s t i Gyre. The estimated f i v e f o l d increase i n immature copepod abundance between the Gyre and the Current may r e s u l t from a food-induced increase i n copepod pr o d u c t i v i t y . An increase i n copepod p r o d u c t i v i t y due to higher phytoplankton biomass has been reported previously by Richardson ( 1 9 8 5 ) , Ki<j>rboe and Johansen ( 1986 ) and Kio>rboe et al. (1988). In the l a s t two studies, the authors observed a l i n e a r decrease i n copepod p r o d u c t i v i t y when chlorophyll concentrations f e l l below 1.5 u.g l~x. the chlorophyll concentations measured i n the Gyre side of the front (< 1.0 ng l - 1 ) are p o t e n t i a l l y l i m i t i n g f o r copepod production. A decrease i n zooplankton p r o d u c t i v i t y has been reported for chlorophyll a concentrations as high as 6 u.g l - 1 (Richardson 1985). It should be remembered that immature copepod concentrations reported here represent semi-quantitative estimation of the actual microzooplankton abundance since most of the organisms escaped from the 330 Lim mesh net and only a small f r a c t i o n was retained by the 80 um mesh of the cod end. Concentrations of copepod eggs, n a u p l i i and copepodites are roughly two orders of magnitude lower than those generally reported f o r productive coastal waters (Anderson 1988). This d i f f e r e n c e i s i n the same range as the r a t i o between the aperture diameter of the net and the codend diameter. 186 In the s a l i n i t y gradient, the abundance of immature copepods, as estimated by the abundance of p a r t i c l e s i n the 203-500 um f r a c t i o n , decreased l i n e a r l y with s a l i n i t y (Fig. 3 . 4 ) . This indicates that immature copepods were passively d i l u t e d across the front and that t h e i r co-variation with phytoplankton biomass resulted mainly from t h e i r s i m i l a r passive transport. Data from transect B provided information on the ef f e c t s of the summer s t a b i l i z a t i o n of the front and of the upwelling c i r c u l a t i o n on the c r o s s - f r o n t a l d i s t r i b u t i o n of immature copepods. The concentrations of immature copepod stages were high i n the non-upwelling front and r e l a t i v e l y low i n the upwelling front (Fig. 3 . 5 ) ) . In contrast with the previous transect, the abundance of immature copepod stages (0-20 m) and phytoplankton biomass (3 m) were then i n v e r s e l y r e l a t e d i n the front (Fig. 3 . 6 ) . This apparent paradox r e s u l t s p a r t l y from the poor co l o n i z a t i o n of the diatom-rich upwelled water by copepods. The fact that the phytoplankton biomass was determined only at one depth (3 m) may also explain t h i s negative r e l a t i o n s h i p . In the non-upwelling front, subsequent cruises demonstrated the presence of a strong subsurface (5-10 m) diatom maximum (Chapter 1) that was probably missed during t h i s transect. The presence of high concentrations of immature copepod stages i n t h i s part of the front may thus be associated with t h i s subsurface biomass maximum. 187 F i s h l a r v a e and p r e y r e l a t i o n s h i p s - t h e match/mismatch t h e o r y A l o n g - s t a n d i n g v i e w embedded i n t h e match/mismatch t h e o r y and i t s v a r i a n t s i s t h a t spawning s t r a t e g i e s i n r e l a t i o n t o l o c a l h y d r o g r a p h y f a c i l i t a t e t h e c o i n c i d e n c e i n t i m e and space of f i r s t f e e d i n g l a r v a e and t h e i r f o o d ( H j o r t 1926, Harden Jones 1968, C u s h i n g 1972). A l t h o u g h t h i s v i e w has been c h a l l e n g e d i n r e c e n t y e a r s ( l i e s and S i n c l a i r 1982, S i n c l a i r and Trembley 1984, 0'Boyle et al. 1984), numerous l a b s t u d i e s do i n d i c a t e t h a t f o o d l e v e l s g e n e r a l l y found i n n a t u r e a r e o f t e n t o o low t o s u p p o r t maximum l a r v a l growth (see L e g g e t t 19 86 and Anderson 1988 f o r r e v i e w s ) . I n t h a t r e s p e c t , i t may be i m p o r t a n t f o r t h e l a r v a e t o h a t c h and d r i f t i n r e s o u r c e - r i c h w a t e r masses, as o u t l i n e d by L o n g h u r s t (1981). I n t h e l o w e r e s t u a r y , t h e i c h t h y o p l a n k t o n community i s dominated f i r s t by sand l a n c e ( A p r i l / M a y ) w h i l e c a p e l i n and h e r r i n g l a r v a e become abundant l a t e r i n June. These p e r i o d s of maximum l a r v a l abundance c o i n c i d e w i t h t h e May/June a n n u a l maximum i n p r i m a r y p r o d u c t i v i t y and biomass ( L e v a s s e u r et al. 1984, T h e r r i a u l t and L e v a s s e u r 1985). S e v e r a l a u t h o r s have h y p o t h e s i z e d t h a t t h e spawning and d i s p e r s i o n s t r a t e g i e s o f c a p e l i n and sand l a n c e a r e r o o t e d i n t h e h i g h p r o d u c t i v i t y o f t h e Gasp6 C u r r e n t s u r f a c e w a t e r s (Jacquaz e t al. 1977, P a r e n t and B r u n e i 1976, De L a f o n t a i n e 188 et al. 1984a, F o r t i e r et al. 1987). In support of t h i s view, our r e s u l t s indicate that capelin and sand lance larvae were concentrated i n the Current where t h e i r resource was estimated to be more than ten times higher than i n adjacent waters i n e a r l y June (Fig. 3.7). It appears that the dispersion of the estuarine species i n r e l a t i o n to the l o c a l hydrography favours the e x p l o i t a t i o n of the resource-r i c h Gaspe Current by f i r s t - f e e d i n g postlarvae. The d i s t r i b u t i o n of the estuarine larvae and t h e i r resource i n the s a l i n i t y gradient varied between transects A and B, and these variat i o n s were apparently due to d i f f e r e n t types of c r o s s - f r o n t a l c i r c u l a t i o n . In transect A, estuarine larvae and prey concentrations decreased with increasing s a l i n i t y , suggesting that they were passively d i l u t e d with surface waters from the Gyre (Fig. 3.8). This behavior was expected since we have already demonstrated that p h y s i c a l mixing at the jet boundary was high during t h i s period of the year, r e s u l t i n g i n a rapid d i l u t i o n of the seston (phytoplankton and zooplankton). There i s consequently a physical co-dispersion across the front of the e a r l y postlarvae and t h e i r resource. As c r o s s - f r o n t a l mixing proceeded, a f r a c t i o n of the estuarine postlarvae spread to the resource-poor gyre waters. In transect B, no r e l a t i o n s h i p was found between l a r v a l concentrations and s a l i n i t y , suggesting that b i o l o g i c a l 189 r a t h e r than p h y s i c a l processes were c o n t r o l l i n g t h e i r c r o s s -f r o n t a l d i s t r i b u t i o n . L a r v a l c o n c e n t r a t i o n s were maximum i n the Gaspe Current and a t some s t a t i o n s i n the non-upwelling f r o n t ( F i g . 3.10). Highest l a r v a l food c o n c e n t r a t i o n s were measured i n the non-upwelling f r o n t . As p r e v i o u s l y shown f o r p h y t o p l a n k t o n (see Chapters 1 and 2), the decrease i n f r e s h w a t e r r u n o f f d u r i n g the two sampling p e r i o d s r e s u l t e d i n a l e s s v i g o r o u s c r o s s - f r o n t a l mixing and i n the e s t a b l i s h e m e n t of a s p e c i f i c p l a n k t o n i c f r o n t a l community. E s t u a r i n e l a r v a e were almost absent from the upwelled water. T h i s r e s u l t shows t h a t the e s t u a r i n e l a r v a e were not p r e s e n t at the o r i g i n a l depth (15-20 m) of the upwelled water and t h a t l i t t l e h o r i z o n t a l mixing o c c u r r e d between the upwelled and a d j a c e n t waters. In e a r l y June, r e d f i s h l a r v a e were c o n c e n t r a t e d on the gyre s i d e of the f r o n t . The a s s o c i a t i o n of the r e d f i s h l a r v a e w i t h more marine waters as compared t o e s t u a r i n e l a r v a e (e.g. c a p e l i n l a r v a e ) has been p r e v i o u s l y observed by De L a f o n t a i n e et al. (1984a). P r e v i o u s s t u d i e s i n d i c a t e d t h a t r e d f i s h l a r v a e and a d u l t s are v i r t u a l l y absent from the lower e s t u a r y ( S t e e l e 1957, Able 1978, De L a f o n t a i n e e t al. 1984b, S c o t t and S c o t t 1988). Thus, the l a r v a e found i n the gyre were p r o b a b l y spawned l o c a l l y and not advected from upstream. The a s s o c i a t i o n of r e d f i s h l a r v a e w i t h the gyre system has been p r e v i o u s l y r e p o r t e d by Sherman et al. (1984) i n the G u l f of Maine. Sherman et al. (1984) a l s o found t h a t 190 t h e l a r v a e were produced f o l l o w i n g t h e s p r i n g p u l s e o f copepod p r o d u c t i o n . I n t h e A n t i c o s t i g y r e , t h e mean s i z e o f t h e s i n g l e c o h o r t was 7 mm, a v a l u e c o r r e s p o n d i n g t o t h e l e n g t h a t e x t r u s i o n r e p o r t e d f o r t h i s s p e c i e s i n t h e N o r t h A t l a n t i c (6.2 t o 8.9 mm; Penney and Evans 1985). S i n c e some c o h o r t s o f t h i s s p e c i e s may e x h i b i t v e r y low growth r a t e s (< 0.10 mm d-3-) d u r i n g t h e f i r s t 10-15 days f o l l o w i n g e x t r u s i o n (Penney and Evans 1985), i t i s r e a l i s t i c t o assume t h a t t h e r e d f i s h c o h o r t has been spawned f o l l o w i n g t h e A p r i l / M a y p h y t o p l a n k t o n bloom. The p r e s e n c e o f t h e s t r o n g s u b s u r f a c e c h l o r o p h y l l a maximum i n t h e g y r e d u r i n g t h i s c r u i s e ( F i g . 1-7) a t t e s t s t o t h e r e c e n t s p r i n g bloom. S p a t i a l l y , r e d f i s h l a r v a e were n e g a t i v e l y c o r r e l a t e d w i t h t h e i r r e s o u r c e s i n t r a n s e c t A. The low e x p l o i t a t i o n o f t h e r e s o u r c e - r i c h Gaspe C u r r e n t s u g g e s t s t h a t t h e c u r r e n t was ou t - o f - b o u n d s f o r t h e a d u l t s . On t h e S c o t i a n S h e l f , 0 ' B oyle e t al. (1983) a l s o found t h a t peaks i n l a r v a l r e d f i s h p r o d u c t i o n d i d n o t appear i n any c o n s i s t e n t s y n c h r o n y w i t h t h e peaks o f z o o p l a n k t o n o v e r t h e e n t i r e banks. The mean s i z e o f t h e r e d f i s h s i n g l e c o h o r t remained t h e same between t h e two t r a n s e c t s , s u g g e s t i n g t h a t l i t t l e g r owth o c c u r r e d . A d e c l i n e i n growth r a t e i n newly e x t r u d e d r e d f i s h l a r v a e u n t i l 10-15 days o f age has been p r e v i o u s l y r e p o r t e d f o r s p e c i f i c c o h o r t s by Penney and Evans ( 1 9 8 5 ) . Such e a r l y growth r a t e d e c l i n e s have been a t t r i b u t e d by t h e s e a u t h o r s t o low p r e y c o n c e n t r a t i o n s . I n t r a n s e c t B, 191 r e d f i s h l a r v a e were n o t r e s t r i c t e d t o t h e g y r e s i d e of t h e f r o n t as o b s e r v e d d u r i n g t h e p r e v i o u s c r u i s e , b u t t h e y were a l s o abundant i n t h e r e s o u r c e - r i c h f r o n t . I t i s n o t p o s s i b l e t o d e t e r m i n e i f t h e l a r v a e found i n t h e f r o n t have been spawned l o c a l l y o r s u b s e q u e n t l y i n t r o d u c e d i n t o t h e f r o n t by h o r i z o n t a l m i x i n g . R e g a r d l e s s of t h e b i o l o g i c a l o r p h y s i c a l n a t u r e of t h e mechanism i n v o l v e d , i t d i d r e s u l t i n a b e t t e r match between t h e l a r v a e d i s t r i b u t i o n and t h e i r r e s o u r c e . I n t r a n s e c t A, A r c t i c shanny l a r v a e were a l s o c o n c e n t r a t e d on t h e r e s o u r c e - p o o r g y r e s i d e o f t h e f r o n t . I n c o n t r a s t w i t h r e d f i s h l a r v a e , shanny l a r v a e have been p r e v i o u s l y c o l l e c t e d i n t h e l o w e r e s t u a r y i n A p r i l / M a y (De L a f o n t a i n e et al. 1984b). I t i s p o s s i b l e t h a t t h e l a r v a e f o u n d i n t h e g y r e i n June were a d v e c t e d from upstream and i n t r o d u c e d i n t o t h e Gyre by t h e c r o s s - f r o n t a l c i r c u l a t i o n . The Gaspe C u r r e n t f r o n t as a r e t e n t i o n zone E s t u a r i e s r e p r e s e n t i m p o r t a n t spawning and n u r s e r y grounds f o r numerous f i s h s p e c i e s . I n t h e s e a d v e c t i v e e n v i r o n m e n t s , t h e r e t e n t i o n of t h e l a r v a e may r e s u l t from an i n t e r p l a y between t h e s p e c i f i c b e h a v i o r of t h e l a r v a e and t h e t w o - l a y e r e s t u a r i n e c i r c u l a t i o n o r from t h e p r e s e n c e o f h o r i z o n t a l l y d e f i n e d r e t e n t i o n zones ( F o r t i e r and L e g g e t t 1982, l i e s and S i n c l a i r 1982, F o r t i e r and Gagne i n p r e s s ) . R e c e n t l y , t h e d r i f t and r e t e n t i o n o f f i s h l a r v a e i n s u i t a b l e 192 n u r s e r y grounds has been h y p o t h e s i z e d to be of prime importance f o r the d e t e r m i n a t i o n and maintenence of p o p u l a t i o n i n t e g r i t y ( S i n c l a i r 1988). In the St. Lawrence system, hydrodynamical p r o c e s s e s have been shown to p l a y a major r o l e i n the r e t e n t i o n and s p a t i a l d i s t r i b u t i o n of major f i s h l a r v a e . In the upper e s t u a r y , F o r t i e r and Leggett (1983) demonstrated t h a t the seaward d r i f t of e a r l y p o s t l a r v a l A t l a n t i c h e r r i n g was l i m i t e d by t h e i r average v e r t i c a l p o s i t i o n i n the water column which i s c l o s e to the depth of zero net t r a n s p o r t . In a d d i t i o n , seaward d r i f t of the h e r r i n g p o s t l a r v a e appears t o be a l s o l i m i t e d by the s t r a t i f i c a t i o n f r o n t l o c a t e d between the upper and lower e s t u a r y ( F o r t i e r and Gagne i n p r e s s ) . By c o n t r a s t , c a p e l i n l a r v a e congregated to the s u r f a c e , r e s u l t i n g i n an a c c e l e r a t i o n of t h e i r seaward d r i f t . In the lower e s t u a r y , p r e v i o u s s t u d i e s i n d i c a t e d t h a t the hydrodynamic p r o p e r t i e s do not a l l o w the r e t e n t i o n of i c h t h y o p l a n k t o n i n summer. For example, De L a f o n t a i n e et al. (1984a) observed no i n c r e a s e i n c a p e l i n , sand l a n c e and h e r r i n g l a r v a l l e n g t h between A p r i l and August at a f i x e d s t a t i o n l o c a t e d o f f s h o r e from Rimouski. Rapid d r i f t of c a p e l i n l a r v a e patches i n the lower e s t u a r y has been observed a l s o by F o r t i e r and L e g g e t t (1985). The Gaspe C u r r e n t i s a h i g h l y a d v e c t i v e system and most of i t s i c h t h y o p l a n k t o n p o p u l a t i o n s are r a p i d l y advected eastward, toward the Magdalen Shallows (Cote et al. 1985). My r e s u l t s 193 i n d i c a t e d t h a t a p o r t i o n o f t h e l a r v a l p o p u l a t i o n o f t h e Gaspe C u r r e n t was t r a p p e d i n t h e f r o n t . As c r o s s - f r o n t a l m i x i n g p r o c e e d e d and p o s t l a r v a e s p r e a d t o t h e g y r e , t h e s e a w a r d c u r r e n t d e c r e a s e d ( B e n o i t e t al. 1985) a n d e s t u a r i n e l a r v a e a c c u m u l a t e d i n t h e a r e a . I n t h i s s t u d y , l a r g e r s a n d l a n c e and c a p e l i n l a r v a e were c o n c e n t r a t e d i n t h e f r o n t and on t h e g y r e s i d e o f t h e f r o n t d u r i n g b o t h t r a n s e c t s ( F i g s . 3.7 a n d 3.11). T h e s e r e s u l t s s u g g e s t t h a t e s t u a r i n e l a r v a e f r o m p r e v i o u s s p a w n i n g e v e n t s were i n t r o d u c e d and t r a p p e d i n t h e r e s o u r c e - r i c h s u r f a c e w a t e r s o f t h e f r o n t . A s i m i l a r l a r v a l r e t e n t i o n zone r e l a t e d t o a t e m p e r a t u r e f r o n t h a s b e e n o b s e r v e d by Ki<t>rboe e t al. (1988) i n t h e NW N o r t h S e a . I n t h i s c a s e , t h e y o b s e r v e d t h a t some h e r r i n g l a r v a l c o h o r t s were r e t a i n e d i n t h e f r o n t as t h e i r d r i f t s p e e d t e n d e d t o s l o w down as i t a p p r o a c h e d t h e f r o n t a l w a t e r s . As o b s e r v e d i n my s t u d y , t h e y a l s o f o u n d some s p a t i a l o v e r l a p b e t w e e n t h e c o h o r t s i n t h e f r o n t per se. J u v e n i l e and a d u l t c a p e l i n have b e e n f o u n d i n h i g h a b u n d a n c e i n t h e A n t i c o s t i g y r e ( B a i l e y e t al. 1 9 7 7 ) . A l t h o u g h some s p a w n i n g g r o u n d s e x i s t on t h e n o r t h s h o r e o f t h e g u l f , i t has b e e n s u g g e s t e d t h a t c a p e l i n l a r v a e h a t c h e d i n t h e u p p e r e s t u a r y a r e t r a n p o r t e d by c u r r e n t s t o t h e n o r t h w e s t e r n g u l f where t h e y a r e r e t a i n e d b y t h e c o u n t e r c l o c k w i s e A n t i c o s t i g y r e ( J a c q u a z e t al. 1 9 7 7 ) . L a t e r , De L a f o n t a i n e e t al. (1984a) s u g g e s t e d t h a t most o f t h e e s t u a r i n e l a r v a e were a d v e c t e d e a s t w a r d b y t h e Gaspe 194 C u r r e n t w i t h few l a r v a e remaining i n the gyre. In t h a t r e s p e c t , my r e s u l t s i n d i c a t e t h a t the mixing of the l a r v a e a c r o s s the f r o n t and the a s s o c i a t e d i n c r e a s e i n r e s i d e n c e time may r e p r e s e n t an important mechanism a l l o w i n g e s t u a r i n e l a r v a e to c o l o n i z e d the northwestern p o r t i o n of the g u l f . The same mechanism may be r e s p o n s i b l e f o r the presence of a r c t i c shanny l a r v a e i n the gyre d u r i n g the f i r s t c r u i s e . My r e s u l t s suggest t h a t a convergent c i r c u l a t i o n may a l s o be r e s p o n s i b l e f o r the accumulation of f i s h l a r v a e on the gyre s i d e of the f r o n t . T h i s view i s supported by the c o-occurrence of sand la n c e , r e d f i s h and shanny l a r v a e i n a patch l o c a t e d on the gyre s i d e of the f r o n t i n e a r l y June ( F i g . 3.7). The accumulation of l a r v a e from d i f f e r e n t o r i g i n s ( e s t u a r i n e and gyre) at t h i s l o c a t i o n , suggests t h a t a common, pr o b a b l y p h y s i c a l , mechanism was i n v o l v e d . The accumulation of s e s t o n i n f r o n t a l area has been p r e v i o u s l y a t t r i b u t e d to the p a r t i c u l a r c r o s s - f r o n t a l c i r c u l a t i o n , p r i n c i p a l l y a s u r f a c e convergence. In a r i v e r i n e plume f r o n t , accumulation of zooplankton due to convergence has been r e p o r t e d by Krause et al. (1986), Kahru et al. (1986) and Mackas and L o u t t i t (1988). F o l l o w i n g the model of c r o s s - f r o n t a l c i r c u l a t i o n proposed by Tang (1980b) f o r the Gaspe Cu r r e n t f r o n t , a convergence zone i s formed on the Gyre s i d e of the f r o n t ( F i g . 1.20). The h y p o t h e s i s t h a t l a r v a l accumulation r e s u l t e d from a convergent c i r c u l a t i o n assumes t h a t the downward movement of water at the f r o n t a l 195 i n t e r f a c e i s s l o w e r than t h e maximum swimming speeds of t h e l a r v a e (« 80 cm m i n - 1 ; Hunter 1981). T h u s , " l a r v a e a d v e c t e d h o r i z o n t a l l y i n t o t h e j e t f r o n t by t h e s u r f a c e c o n vergence can r e s i s t downward movement a t t h e f r o n t a l i n t e r f a c e and a c c u m u l a t e t h e r e . Such a mechanism has been p r o p o s e d by G o n o v i et al. (1989) as an e x p l a n a t i o n f o r t h e a c c u m u l a t i o n o f f i s h l a r v a e a t t h e boundary o f t h e M i s s i s s i p p i R i v e r plume. Conclusions I n summary, my r e s u l t s i n d i c a t e t h a t t h e b u l k o f e s t u a r i n e i c h t h y o p l a n k t o n found i n t h e s t u d y a r e a i n June e x p l o i t e d t h e massive p r o d u c t i o n o f d i a t o m s and immature copepods measured i n t h e Gaspe C u r r e n t and i n t h e f r o n t . On t h e o t h e r hand, t h e h i g h d i a t o m biomass found i n t h e u p w e l l i n g p a r t of t h e f r o n t seems t o have no immediate v a l u e f o r copepod and f i s h r e p r o d u c t i o n . The low c o n c e n t r a t i o n s o f e s t u a r i n e demersal f i s h s p e c i e s i n t h e u p w e l l i n g - f r o n t may be a t t r i b u t e d t o t h e low m i x i n g between t h e e s t u a r i n e and u p w e l l e d w a t e r . However, t h e u p w e l l i n g a r e a s h o u l d have been e x p l o i t e d by t h e o v o v i v i p a r o u s r e d f i s h . One e x p l a n a t i o n may be t h a t t h e b i o l o g i c a l c h a r a c t e r i s t i c s of t h e u p w e l l i n g a r e a a r e t o o ephemeral t o a l l o w m a s s i v e s e c o n d a r y p r o d u c t i o n and i t s c o l o n i z a t i o n by f i s h l a r v a e . I n c o n t r a s t w i t h c l a s s i c a l u p w e l l i n g , t h e h i g h d i a t o m c o n c e n t r a t i o n s found i n t h e u p w e l l i n g p a r t of t h e Gaspe C u r r e n t f r o n t a r e not produced in s i t u , but a r e u p w e l l e d 196 t o g e t h e r w i t h n u t r i e n t s ( C h a p t e r 1 and 2 ) . C o n s i d e r i n g t h e h i g h p h y t o p l a n k t o n biomass a l r e a d y p r e s e n t i n t h e u p w e l l e d w a t e r , t h e e x h a u s t i o n of ambient n u t r i e n t s and, c o n s e q u e n t l y , t h e s i n k i n g of p h y t o p l a n k t o n biomass may o c c u r r a p i d l y . The comparison between the ambient s u r f a c e n u t r i e n t c o n c e n t r a t i o n s ( n i t r a t e and s i l i c a t e ) i n t h e u p w e l l i n g f r o n t w i t h t h e i r r e s p e c t i v e t r a n s p o r t r a t e s by p h y t o p l a n k t o n d e t e r m i n e d i n C h a p t e r 2 (Table 3.5) s u p p o r t s t h i s h y p o t h e s i s . I t appears t h a t t h e u p w e l l e d n u t r i e n t s can s u p p o r t t h e p h y t o p l a n k t o n n u t r i e n t demand f o r l e s s t h a n one day. The r a p i d e x h a u s t i o n o f n u t r i e n t s and t h e a s s o c i a t e d i n c r e a s e i n d i a t o m s i n k i n g r a t e s (Chapter 2) may be r e p o n s i b l e f o r t h e a p p a r e n t l a c k o f u t i l i z a t i o n o f t h e u p w e l l e d biomass by p e l a g i c s econdary and t e r t i a r y p r o d u c e r s . T h i s p r o d u c t i o n i s p r o b a b l y d i r e c t l y e x p o r t e d t o d e p t h . 197 T a b l e 3.5. N i t r a t e and s i l i c a t e ambient c o n c e n t r a t i o n s , h o u r l y t r a n s p o r t r a t e s (mid-day) and e s t i m a t e d t i m e u n t i l n u t r i e n t e x h a u s t i o n (T) i n t h e u p w e l l i n g p a r t o f t h e f r o n t o f t h e Gaspe C u r r e n t ( d a t a a r e from c r u i s e E, C h a p t e r 2 ) . S t a t i o n NO 3 P N 0 3 TN0 3 S i 0 4 P S i O * TSiO* (uM) (uM h- 1) (h) (uM) (uM h- 1) (h) 2 4.1 0.4 12 1.4 0.1 23 3 1.5 0.3 5 1.9 0.2 11 198 GENERAL CONCLUSIONS In the marine environment, hydrodynamic features and b i o l o g i c a l processes are often t i g h t l y coupled. Examples of t h i s coupling are numerous i n the l i t e r a t u r e on primary production where phytoplankton blooms are often associated with physical processes a f f e c t i n g nutrient a v a i l a b i l i t y and the l i g h t regime (e.g. upwelling and wind mixing). More recently, f r o n t a l areas have been also recognized as e c o l o g i c a l l y important hydrodynamical systems. While our understanding of the actual mechanism(s) responsible for the observed v a r i a t i o n s i n primary production has improved considerably during the l a s t 10 years, the evidence for the e f f e c t of front on zooplankton and ichthyoplankton are s t i l l generally elusive. In many cases, one reason for the lack of a consistent pattern between phytoplankton, zooplankton and ichthyoplankton reported i n studies c a r r i e d out i n f r o n t a l regions, i s the transient nature of the fronts and/or t h e i r physical c h a r a c t e r i s t i c s . In Chapters 1 and 3, I determined how the spring and summer physical c h a r a c t e r i s t i c s of a coastal j e t front influenced the physiology and ecology of phytoplankton and how these e f f e c t s were transferred to the higher trophic l e v e l s . In Chapter 2, I demonstrated the pot e n t i a l importance of s i l i c a t e as a l i m i t i n g nutrient for diatom growth i n the Gulf of St. Lawrence. In t h i s concluding section, I w i l l emphasize four aspects of these r e s u l t s 199 w h i c h a r e d i r e c t l y r e l e v a n t t o c u r r e n t c o n c e p t s i n oceanography. The f i r s t one p e r t a i n s t o t h e c o u p l i n g between p h y s i c a l and b i o l o g i c a l p r o c e s s e s and t h e i m p o r t a n c e of s c a l e s i n t h i s c o u p l i n g . The second r e l a t e s t o t h e i m p o r t a n c e o f c i r c u l a t i o n i n s h a p i n g t h e b i o l o g i c a l c h a r a c t e r i s t i c s of a f r o n t a l a r e a . The t h i r d one i s r e l e v a n t t o f i s h l a r v a e and r e l a t e s t o t h e i m p o r t a n c e o f t h e r e t e n t i o n zones i n t h e d e f i n i t i o n o f s t o c k s and t h e d e t e r m i n a t i o n o f t h e i r s i z e . The f o u r t h one i s c o n c e r n e d w i t h t h e c a u s e s and consequences o f s i l i c a t e l i m i t a t i o n i n t h e S t . Lawrence system and i n marine e n v i r o n m e n t s i n g e n e r a l . P h y s i c a l / b i o l o g i c a l i n t e r a c t i o n s - t h e i m p o r t a n c e o f s c a l e s As s t a t e d by F r o n t i e r ( 1 9 8 6 ) , t h e p r o x i m a l f e a t u r e s o f f r o n t a l ecosystems depend on t h e i n t e r a c t i o n between s p a t i o -t e m p o r a l c h a r a c t e r i s t i c s of t h e p h y s i c a l regime o f t h e f r o n t and t h e r e s p o n s e t i m e of b i o l o g i c a l phenomena. F o r a f r o n t t o be p r o d u c t i v e , p h y s i c a l and b i o l o g i c a l s c a l e s must c o r r e s p o n d . S p a t i o - t e m p o r a l s c a l e s o f v a r i a b i l i t y r e l e v a n t f o r p h y t o p l a n k t o n have been summarized by H a r r i s ( 1 9 80). F o l l o w i n g h i s r e p r e s e n t a t i o n , t h e s p a t i o - t e m p o r a l c h a r a c t e r i s t i c s o f t h e p h y s i c a l regime o f a f r o n t s h o u l d exceed 1 day ( o r 1 km) t o be e c o l o g i c a l l y e f f e c t i v e i n t e r m o f p h y t o p l a n k t o n growth and biomass a c c u m u l a t i o n . R e s u l t s from t h i s s t u d y d e m o n s t r a t e d c l e a r l y t h e i m p o r t a n c e o f s c a l e s f o r p h y t o p l a n k t o n and how t e m p o r a l v a r i a t i o n s o f t h e 200 p h y s i c a l regime i n f l u e n c e the o v e r a l l e c o l o g i c a l i m p a c t of t h e f r o n t . In e a r l y June, when the f r e s h w a t e r r u n o f f was s t i l l h i g h , the c r o s s - f r o n t a l c i r c u l a t i o n was p r o b a b l y dominated by t u r b u l e n t p r o c e s s e s . The time s c a l e o f t h e c r o s s - f r o n t a l m i x i n g was then s u f f i c i e n t t o r e s u p p l y t h e n u t r i e n t s d e p l e t e d by b i o l o g i c a l p r o d u c t i o n and a l l o w p h y s i o l o g i c a l a d j u s t m e n t s , but d i d not a l l o w f o r t h e a c c u m u l a t i o n of biomass. In l a t e June o r J u l y , h o r i z o n t a l m i x i n g became l e s s i m p o r t a n t as r u n o f f d e c r e a s e d and t h e b i o l o g i c a l p r o p e r t i e s of t h e f r o n t were d e t e r m i n e d by in situ b i o l o g i c a l p r o c e s s e s . The f r o n t per se was then c h a r a c t e r i z e d by a p h y t o p l a n k t o n community d i s t i n c t f r om t h e a d j a c e n t a r e a s . F o r m a t e r i a l and energy t o be t r a n s f e r r e d t o h i g h e r t r o p h i c l e v e l s , p h y t o p l a n k t o n and z o o p l a n k t o n s c a l e s must c o r r e s p o n d . The r e p r o d u c t i o n of copepods w i l l be enhanced o n l y when the d u r a t i o n of t h e p r i m a r y p r o d u c t i o n ( o r h i g h biomass) c o r r e s p o n d s t o t h e z o o p l a n k t o n g e n e r a t i o n t i m e ( r e s o n a n c e ; Legendre and L e f e v r e 1989). In g e n e r a l , a l a g of 2-3 days may be e x p e c t e d between t h e i n c r e a s e i n p h y t o p l a n k t o n and z o o p l a n k t o n biomass ( P a r s o n s and K e s s l e r 1985, Runge 1985). Thus, th e minimum t e m p o r a l s c a l e of v a r i a b i l i t y of t h e p h y s i c a l regime of t h e f r o n t r e l e v a n t . t o z o o p l a n k t o n r e p r o d u c t i o n may be t e n t a t i v e l y s e t a t 4-5 days. In e a r l y June, t h e c r o s s - f r o n t a l m i x i n g was p r o c e e d i n g a t a h i g h e r r a t e (< 1 day) and t h u s , t h e o r e t i c a l l y p r e v e n t i n g any 201 b i o l o g i c a l l y - i n d u c e d v a r i a b i l i t y of copepod reproduction rates i n the front per se. In support of t h i s hypothesis, the c r o s s - f r o n t a l d i s t r i b u t i o n of eggs, n a u p l i i and copepodites was dominated by physical mixing during cruise A. However, the c r o s s - f r o n t a l mixing may had some ph y s i o l o g i c a l e f f e c t s (e.g. grazing a c t i v i t y and egg production) on the zooplankton community. The p o t e n t i a l e f f e c t s of the front on the ph y s i o l o g i c a l state of zooplankton should be tested i n the future i n order to complete our understanding of the b i o l o g i c a l importance of the f r o n t a l c i r c u l a t i o n . Later during the season, high concentrations of copepods eggs and n a u p l i i were found i n the non-upwelling front, suggesting a food-induced increase of copepod reproduction rates. On the other hand, the r e l a t i v e l y low abundance of immature copepods i n the upwelling front suggests that the dynamical properties of the divergent c i r c u l a t i o n was not appropriate to zooplankton scales. The low concentrations of immature copepods observed i n the upwelling part of the front have been at t r i b u t e d to the apparent ephemeral nature of the high biomass i n the upwelled water. The p o t e n t i a l responses of ichthyoplankton to hydrodynamical events d i f f e r from those of phytoplankton and zooplankton i n many ways. Except for s e r i a l spawners, spawning i s a unique event for an i n d i v i d u a l f i s h and the annual "timing" of spawning events with appropriate physical 202 and b i o l o g i c a l e v e n t s i s o f t e n c r i t i c a l . The i d e a o f " t i m i n g " i s c e n t r a l t o t h e match/mismatch t h e o r y ( C u s h i n g 1972). Depending on t h e i r mode of r e p r o d u c t i o n ( o v o v i v i p a r o u s o r d e m e r s a l ) , t h e a s s o c i a t i o n i n t i m e and space o f f i s h l a r v a e w i t h t h e i r r e s o u r c e may r e f l e c t d i f f e r e n t e v o l u t i o n a r y s t r a t e g i e s . L a r v a e may be spawned d i r e c t l y when and where t h e r e s o u r c e i s h i g h ( o v o v i v i p a r o u s s p e c i e s ) o r a d v e c t e d by t h e dominant c i r c u l a t i o n i n t o a r e s o u r c e - r i c h a r e a from more o r l e s s (0-100 km) remote spawning grounds ( o v i p a r o u s d e m e r s a l s p a w n e r s ) . F o r o v o v i v i p a r o u s s p e c i e s such as t h e r e d f i s h , t h e l a r v a e may be spawned d i r e c t l y when and where t h e r e s o u r c e i s abundant. Sherman e t al. (1984) r e p o r t e d a t i m i n g between t h e p r o d u c t i o n of r e d f i s h l a r v a e and t h e s p r i n g i n c r e a s e i n z o o p l a n k t o n abundance i n t h e G u l f of Maine. They c o n c l u d e t h a t r e d f i s h have a K r e p r o d u c t i v e s t r a t e g y , p r o d u c i n g eggs a t a t i m e t h a t i n c r e a s e s t h e p r o b a b i l i t y t h a t t h e i r l a r v a e w i l l e n c o u n t e r s u f f i c i e n t f o o d f o r growth and s u r v i v a l . I n t h e p r e s e n t s t u d y , r e d f i s h l a r v a e were w i d e s p r e a d i n t h e g y r e i n e a r l y June, p r o b a b l y e x p l o i t i n g t h e copepod r e p r o d u c t i o n t h a t f o l l o w e d t h e A p r i l / M a y bloom. D u r i n g t h i s p e r i o d , few l a r v a e were found i n t h e r e s o u r c e -r i c h Gasp§ C u r r e n t , s u g g e s t i n g t h a t t h e C u r r e n t was o u t - o f -bounds f o r t h e a d u l t s . I n l a t e J une, r e d f i s h l a r v a e were abundant i n t h e f r o n t per se, where t h e i r r e s o u r c e was h i g h . 203 Whether t h e i r p r e s e n c e i n t h e f r o n t r e s u l t e d from a l o c a l spawning e v e n t o r p h y s i c a l m i x i n g must s t i l l be d e t e r m i n e d . F o r e s t u a r i n e o v i p a r o u s spawners such as t h e sand l a n c e , c a p e l i n and h e r r i n g , t h e i n t r o d u c t i o n and r e t e n t i o n of t h e l a r v a e i n t h e f r o n t i m p l i e s some p h y s i c a l t r a n s p o r t . The d r i f t i n g and e v e n t u a l t r a p p i n g o f t h e f i s h l a r v a e i n t h e f r o n t s h o u l d r e s u l t from an e v o l u t i o n a r y a d a p t a t i o n t o l o c a l h y d r o g r a p h y . D u r i n g t h i s s t u d y , e s t u a r i n e f i s h l a r v a e were g e n e r a l l y c l o s e l y a s s o c i a t e d w i t h t h e i r r e s o u r c e . In e a r l y June, l a r v a e of sand l a n c e and c a p e l i n were c o n c e n t r a t e d i n t h e Gaspe c u r r e n t where copepod e g g s / n a u p l i i were abundant. A t t h i s t i m e , l a r v a e and t h e i r r e s o u r c e were g r a d u a l l y d i l u t e d a c r o s s t h e f r o n t , toward t h e g y r e . As a r e s u l t , a p o r t i o n o f t h e l a r v a l p o p u l a t i o n ended up i n t h e r e s o u r c e -p oor s u r f a c e w a t e r o f t h e g y r e , s u g g e s t i n g t h a t t h e c r o s s -f r o n t a l m i x i n g of t h e e s t u a r i n e l a r v a e r e p r e s e n t s a s i n k f o r t h e p o p u l a t i o n i n e a r l y June. T h i s h y p o t h e s i s c o u l d be t e s t e d by m e a s u r i n g a l a r v a l p h y s i o l o g i c a l i n d e x such as t h e RNA/DNA r a t i o ( B u c k l e y 1981, R o b i n s o n and Ware 1988) a c r o s s t h e f r o n t . A t t h e end o f June, t h e p r e s e n c e o f l a r g e r e s t u a r i n e l a r v a e i n t h e f r o n t i n d i c a t e d t h a t l a r v a e had been t r a p p e d by t h e f r o n t a l c i r c u l a t i o n . The importance of c i r c u l a t i o n I n t h e p r e v i o u s s e c t i o n , t h e b i o l o g i c a l p r o p e r t i e s o b s e r v e d i n t h e f r o n t a l a r e a were e x p l a i n e d w i t h one-2 d i m e n s i o n a l models i n v o l v i n g e s s e n t i a l l y s u i t a b l e c o m b i n a t i o n s o f l i g h t and n u t r i e n t s f o r p h y t o p l a n k t o n , f o o d r e s o u r c e s f o r h e r b i v o r o u s z o o p l a n k t o n and m i c r o z o o p l a n k t o n f o r f i s h l a r v a e . However, i n g e n e r a l f r o n t s a r e known f o r t h e i r complex c i r c u l a t i o n p a t t e r n s . Among t h e more f r e q u e n t l y c i t e d a r e s u r f a c e convergences and d i v e r g e n c e s , r e s i d u a l c i r c u l a t i o n p a r a l l e l t o t h e f r o n t and complex c r o s s - f r o n t a l exchanges t h r o u g h eddy m o t i o n s . As p o i n t e d o u t by L o d e r and P i a t t (1984) f o r a t i d a l f r o n t , t h e s e c i r c u l a t i o n p a t t e r n s may p l a y a dominant r o l e i n t h e c r o s s -f r o n t a l d i s t r i b u t i o n of p h y t o p l a n k t o n and a n i m a l s . A l l t h e c i r c u l a t i o n p a t t e r n s mentioned above have been o b s e r v e d i n t h e Gasp£ C u r r e n t f r o n t a l a r e a d u r i n g t h i s and p r e v i o u s s t u d i e s . T r a n s p o r t seems t o be a m ajor c h a r a c t e r i s t i c o f t h i s f r o n t , a c o n c l u s i o n t h a t may p r o b a b l y be e x t e n d e d t o any f r o n t c h a r a c t e r i z e d by a s t r o n g g r a d i e n t i n c u r r e n t v e l o c i t y . Divergence - One o f t h e most s p e c t a c u l a r c h a r a c t e r i s t i c s o f t h e Gaspe C u r r e n t f r o n t i s t h e p r e s e n c e o f t h e u p w e l l i n g zone l o c a t e d on i t s g y r e s i d e . I n c o n t r a s t w i t h t h e c l a s s i c a l u p w e l l i n g system where t h e u p w e l l e d w a t e r s a r e i n i t i a l l y n u t r i e n t - r i c h but p h y t o p l a n k t o n - p o o r , n u t r i e n t s and p h y t o p l a n k t o n were s i m u l t a n e o u s l y found i n h i g h c o n c e n t r a t i o n s i n t h e Gasp§ C u r r e n t f r o n t u p w e l l i n g . The c o - o c c u r r e n c e o f h i g h l e v e l s o f n u t r i e n t s and biomass 205 suggests t h a t most of the biomass was i n i t i a l l y p r e s e n t i n the newly upwelled water. Thus, c i r c u l a t i o n i n s t e a d of in situ growth appears to determine, a t l e a s t i n i t i a l l y , the b i o l o g i c a l p r o p e r t i e s of the f r o n t . M i c r o z o o p l a n k t o n and i c h t h y o p l a n k t o n c o n c e n t r a t i o n s were r e l a t i v e l y low i n the u p w e l l i n g f r o n t . O b v i o u s l y , d i r e c t spawning i n the upwelled water by r e d f i s h was l i m i t e d . As p r e v i o u s l y d i s c u s s e d , the low c o n c e n t r a t i o n s of microzooplankton and r e d f i s h l a r v a e i n the u p w e l l i n g p a r t of the f r o n t may r e f l e c t the i n c o m p a t a b i l i t y between the d u r a t i o n of the h i g h biomass and the g e n e r a t i o n time of copepods. Convergence - F r o n t a l systems are o f t e n c h a r a c t e r i z e d by convergent c i r c u l a t i o n which have a profound i n f l u e n c e on p l a n k t o n d i s t r i b u t i o n . In Chapter 3, I h y p o t h e s i z e d t h a t a convergent c i r c u l a t i o n was r e s p o n s i b l e f o r the accumulation of e s t u a r i n e and gyre f i s h l a r v a e a l o n g the gyre s i d e of the f r o n t . Although I r e c o g n i z e t h a t the e x i s t e n c e of the convergence i s s t i l l t o be demonstrated, a convergent c i r c u l a t i o n would be the more obvious way t o e x p l a i n t h i s a g g r e g a t i o n of the l a r v a e . Eddy motions - Eddy motions are commonly observed i n f r o n t a l areas (Yentsch and Phinney 1986). Due t o t h e i r l a r g e s i z e and u s u a l l y u n p r e d i c t a b l e n a t u r e , t h e i r r e l a t i v e importance f o r c r o s s - f r o n t a l exchange of p l a n k t o n i s d i f f i c u l t t o determine. To t h i s p o i n t , I have d i s c u s s e d the exchange 206 mechanisms t a k i n g p l a c e d u r i n g t h e u s u a l c r o s s - f r o n t a l c i r c u l a t i o n . Indeed, s a t e l l i t e p i c t u r e s and t h e o r e t i c a l c o n s i d e r a t i o n s from models i n d i c a t e t h a t t h e p o s i t i o n o f t h e f r o n t i s g e n e r a l l y s t a b l e d u r i n g t h e summer months and r u n s p a r a l l e l t o t h e c o a s t . On o c c a s i o n , t h e g e o s t r o p h i c b a l a n c e w h i c h d e t e r m i n e s t h e f r o n t a l c h a r a c t e r i s t i c s i s b r o k e n and t h e f r o n t t a k e s a w a v e l i k e form. These e p i s o d e s s t a r t w i t h an o f f s h o r e d i s p l a c e m e n t o f t h e c u r r e n t w h i c h may be due t o an i n c r e a s e i n d i s c h a r g e from t h e S t . Lawrence R i v e r (Tang 1980a). W h i l e t h e s u r f a c e c u r r e n t c o n t i n u e s t o f l o w e a s t w a r d , t h e bottom c u r r e n t r e v e r s e s . A f t e r 2-3 d a y s , a wave form s t a r t s t o d e v e l o p . The mechanism t h a t g e n e r a t e s t h e s e e v e n t s i s l i k e l y t o be i n s t a b i l i t y o f q u a s i -g e o s t r o p h i c f l o w s . A t some p o i n t , t h e c r e s t of t h e waves b r e a k and l a r g e p a r c e l s o f w a t e r from t h e s u r f a c e c u r r e n t a r e i n j e c t e d i n t o t h e A n t i c o s t i g y r e . A l t h o u g h t h i s c i r c u l a t i o n p a t t e r n appears t o be uncommon (two o b s e r v a t i o n s between J u l y and O c t o b e r 1978; Tang 1980a), i t may be o f c o n s i d e r a b l e e c o l o g i c a l i m p o r t a n c e c o n s i d e r i n g t h e s i z e o f t h e w a t e r masses i n v o l v e d ( w a v e l e n g t h o f c a . 50 km). Indeed, t h e f o r m a t i o n o f t h e s e overgrown waves w h i c h e v e n t u a l l y b r e a k , may r e p r e s e n t an i m p o r t a n t mechanism of t r a n s f e r o f e s t u a r i n e p l a n k t o n t o t h e g y r e . A b e t t e r u n d e r s t a n d i n g o f t h i s p h y s i c a l p r o c e s s i s t h u s n e c e s s a r y i n o r d e r t o a s s e s s t h e i m p o r t a n c e o f t h e c r o s s - f r o n t a l t r a n s p o r t . 207 Frontal areas as entrapment zones f o r f i s h larvae i n the St. Lawrence R e c e n t l y , i t has been h y p o t h e s i z e d t h a t t h e r e t e n t i o n o f f i s h and i n v e r t e b r a t e s l a r v a e i n h y d r o d y n a m i c a l systems (such as a f r o n t ) p l a y a d e t e r m i n a n t r o l e i n r e c r u i t m e n t ( l i e s and S i n c l a i r 1982, S i n c l a i r 1988). B a s i c a l l y , t h e s e a u t h o r s s u g g e s t e d t h a t t h e r e t e n t i o n o f f i s h l a r v a e i n s p e c i f i c h y d r o d y n a m i c a l systems ( " r e t e n t i o n zones") was r e s p o n s i b l e f o r t h e e x i s t e n c e of d i f f e r e n t f i s h s t o c k s . They a l s o h y p o t h e s i z e d t h a t t h e s i z e of t h e d i f f e r e n t s t o c k s was d e t e r m i n e d by t h e s i z e o f t h e i r s p e c i f i c r e t e n t i o n a r e a s , i n d e p e n d e n t l y of t h e i r f o o d abundance and/or q u a l i t y . The upper p o r t i o n o f t h e S t . Lawrence e s t u a r y ( o r i t s t r i b u t a r i e s ) i s an i m p o r t a n t spawning ground f o r r a i n b o w s m e l t (Osmerus mordax), c a p e l i n (Mallotus villosus), A t l a n t i c h e r r i n g (Clupea harengus) and tomcod (Microgadus tomcod), t h e dominant f i s h s p e c i e s i n t h e S t . Lawrence e s t u a r y and N o r t h w e s t e r n G u l f . P r e v i o u s s t u d i e s d e m o n s t r a t e d t h a t t h e d i s p e r s i o n and a c c u m u l a t i o n of t h e l a r v a e i n t h e m i d d l e e s t u a r y , o r t h e i r r a p i d a d v e c t i o n seaward, was t h e r e s u l t of a c o m b i n a t i o n of t h e l o c a l h y drodynamics and t h e b e h a v i o r o f t h e l a r v a e . Rainbow s m e l t l a r v a e a r e a p p a r e n t l y c o n f i n e d t o t h e upper e s t u a r y ( A b l e 1978) where t h e y t e n d t o a c c u m u l a t e i n t h e n o r t h e r n s e c t i o n ( O u e l l e t and Dodson 1985). O u e l l e t . and Dodson (1985) p r o p o s e d t h a t t h e v e r t i c a l d i s p l a c e m e n t s e x h i b i t e d by s m e l t 208 l a r v a e i n c o m b i n a t i o n w i t h t h e t w o - l a y e r c i r c u l a t i o n system of t h i s a r e a was r e s p o n s i b l e f o r t h e r e t e n t i o n of t h e l a r v a e . S i m i l a r l y , t h e seaward d i s p e r s i o n o f A t l a n t i c h e r r i n g l a r v a e w h i c h h a t c h i n t h e upper e s t u a r y i s bounded by a t i d a l f r o n t l o c a t e d between t h e upper and l o w e r e s t u a r y ( F o r t i e r and Gagne i n p r e s s ) . T h e i r p r e s e n c e i n deeper w a t e r a t n i g h t i s a l s o r e s p o n s i b l e f o r t h e i r f u r t h e r r e t e n t i o n i n t h e t i d a l f r o n t . The l o c a t i o n o f t h e s e r e t e n t i o n zones, a l o n g w i t h t h e Gasp§ C u r r e n t f r o n t , a r e p r e s e n t e d i n F i g u r e 4.1. I n c o n t r a s t w i t h t h e p r e v i o u s s p e c i e s , c a p e l i n l a r v a e w h i c h spawn i n t h e e s t u a r y t e n d t o s t a y i n t h e t o p 20 m o f t h e w a t e r column and, c o n s e q u e n t l y , a r e a d v e c t e d seaward ( F o r t i e r and L e g g e t t 1982, 1983, 1985). The absence of l a r v a e > 12 mm i n t h e l o w e r e s t u a r y (De L a f o n t a i n e e t al. 1984) and i n t h e Gaspe C u r r e n t ( t h i s s t u d y ) s u g g e s t s t h a t t h e a d v e c t i o n i s a l s o r a p i d i n t h e s e s e c t i o n s o f t h e S t . Lawrence sytem. However, t h e r e s u l t s of t h i s s t u d y show t h a t a p o r t i o n o f c a p e l i n (and sand l a n c e ) l a r v a e a d v e c t e d seaward i n t h e Gaspe C u r r e n t a r e t r a p p e d i n t h e e s t u a r i n e plume f r o n t l o c a t e d between t h e C u r r e n t and t h e A n t i c o s t i g y r e . Thus i t appears t h a t f o r l a r v a e w h i c h a r e c o n c e n t r a t e d i n s u r f a c e w a t e r s w i t h h i g h t r a n s p o r t r a t e s , t h e i r r e t e n t i o n depends on h o r i z o n t a l ( i n s t e a d of v e r t i c a l ) g r a d i e n t s i n c u r r e n t v e l o c i t y . F i n a l l y , i t i s i n t e r e s t i n g t o n o t e t h a t t h e r e s o u r c e f o r l a r v a e was h i g h i n b o t h t h e t i d a l and e s t u a r i n e plume f r o n t , a l l o w i n g l a r v a e t o s u r v i v e and grow. 209 F i g u r e 4.1. L o c a t i o n s o f f i s h l a r v a e r e t e n t i o n z o n e s a s s o c i a t e d w i t h f r o n t a l s y s t e m s i n t h e S t . L a w r e n c e e s t u a r y and G u l f o f S t . L a w r e n c e . Zone 1: r e t e n t i o n o f r a i n b o w s m e l t a n d tomcod l a r v a e a s s o c i a t e d w i t h a t u r b i d i t y f r o n t ; z o n e 2: r e t e n t i o n o f A t l a n t i c h e r r i n g l a r v a e a s s o c i a t e d w i t h a t i d a l f r o n t ; zone 3: r e t e n t i o n o f s a n d l a n c e and c a p e l i n l a r v a e a s s o c i a t e d w i t h a c o a s t a l j e t f r o n t . 210 I n f o r m a t i o n s on t h e i n f l u e n c e o f t h e Gaspe C u r r e n t and i t s f r o n t a l zone on t h e e c o l o g y o f t h e s o u t h w e s t e r n p o r t i o n o f t h e G u l f (eg. Magdalen S h a l l o w s ; F i g . 4.1) a r e s p a r s e (Bugden e t a l . 1982, Cote e t a l . 1986). There i s however a s u g g e s t i o n t h a t i n t e r a n n u a l v a r i a t i o n s i n S t . Lawrence f r e s h w a t e r r u n o f f a f f e c t y e a r - c l a s s s t r e n g t h o f some p o p u l a t i o n s o f f i s h and i n v e r t e b r a t e s i n t h i s p o r t i o n of t h e G u l f . A p o s i t i v e c o r r e l a t i o n was found between t h e s e a s o n a l d i s c h a r g e s o f t h e S t . Lawrence and t h e l o c a l l a n d i n g o f A m e r i c a n l o b s t e r and A t l a n t i c H a l i b u t ( S u t c l i f f e 1972, 1973) as w e l l as on cod p r o d u c t i v i t y and r e c r u i t m e n t (Bugden e t a l . 1982). The p o s s i b l e e x p l a n a t i o n s f o r t h e s e c o r r e l a t i o n s have been e x p l o r e d i n d e t a i l by Bugden e t a l . (1982). E x p l a n a t i o n s were m a i n l y c e n t e r e d around t h e arguments of v e r t i c a l m i x i n g and u p w e l l i n g c a used by t h e o u t f l o w and consequent e f f e c t s on l a r v a l s t a g e s , presumably by i n c r e a s e d p r i m a r y p r o d u c t i o n and/or s u r f a c e w a t e r t e m p e r a t u r e . S i n c e t h e r a t e of u p w e l l i n g a l o n g t h e Gaspe C u r r e n t f r o n t i s b e l i e y e d t o i n c r e a s e w i t h a l o n g f r o n t h o r i z o n t a l volume t r a n s p o r t (Tang 1980b), i t has been h y p o t h e s i z e d t h a t an i n c r e a s e i n f r e s h w a t e r r u n o f f w i l l i n c r e a s e t h e u p w e l l i n g c i r c u l a t i o n and n u t r i e n t e n r i c h m e n t of t h e Gaspe C u r r e n t (Bugden e t a l . 1982). I n t h a t r e s p e c t , my r e s u l t s i n d i c a t e t h a t t h e f r o n t a l u p w e l l i n g c i r c u l a t i o n may i n d e e d c o n t r i b u t e s s i g n i f i c a n t l y t o t h e n u t r i e n t budget of t h e G u l f . The huge amounts o f p h y t o p l a n k t o n and m i c r o z o o p l a n k t o n biomasses t r a n s p o r t e d by t h e C u r r e n t i n 211 J u n e / J u l y may however be of more d i r e c t e c o l o g i c a l v a l u e . A n n u a l v a r i a t i o n s i n s u r f a c e s a l i n i t y o v e r t h e Magdalen S h a l l o w s i n d i c a t e t h a t t h e e x t e n t o f t h e Gaspe C u r r e n t i n c r e a s e w i t h f r e s h w a t e r r u n o f f (Bugden e t a l . 1982). V a r i a t i o n s i n r i v e r r u n o f f may t h u s c o n t r o l t h e t o t a l amount of f i s h and i n v e r t e b r a t e s l a r v a e r e s o u r c e e x p o r t e d by t h e C u r r e n t t o t h e s o u t h w e s t e r n p o r t i o n o f t h e G u l f . S i l i c a t e l i m i t a t i o n i n the St. Lawrence - Causes and consequences Causes - I n t h e ma r i n e e n v i r o n m e n t , p h y t o p l a n k t o n p r o d u c t i o n was found t o be l i m i t e d by a v a i l a b l e n i t r a t e ( H a r r i s o n et al. 1981, Huntsman e t a l . 1981, Jones e t al. 1981) o r o c c a s i o n a l l y s i l i c a t e (Dugdale 1972, B r i n k e t al. 1981). Kamykowski (1974) was t h e f i r s t t o s u g g e s t t h a t t h e i n i t i a l l e v e l o f t h e s e two n u t r i e n t s i n a r e c e n t l y u p w e l l e d ( o r mixed) w a t e r mass p l a y e d a f u n d a m e n t a l r o l e i n t h e p h y t o p l a n k t o n dynamics by d e t e r m i n i n g w h i c h n u t r i e n t became e x h a u s t e d f i r s t . F u r t h e r s t u d i e s have e f f e c t i v e l y r e v e a l e d t h a t s i l i c a t e and n i t r a t e d e p l e t i o n s g e n e r a l l y found i n s u r f a c e w a t e r s o f t h e Pe r u and C a l i f o r n i a u p w e l l i n g r e g i o n s , r e s p e c t i v e l y , c o r r e s p o n d e d t o a l a t i t u d i n a l t r e n d i n t h e i r r e l a t i v e abundance ( Z e n t a r a and Kamykowski 1977). Moreover, L e v a s s e u r and T h e r r i a u l t (1987) showed t h a t t h e s p a t i o / t e m p o r a l s h i f t from s i l i c a t e t o n i t r a t e l i m i t a t i o n o f di a t o m s o b s e r v e d i n d i f f e r e n t u p w e l l i n g systems r e s u l t e d from v a r i a t i o n s i n t h e n i t r a t e t o s i l i c a t e r a t i o i n t h e newly u p w e l l e d w a t e r s . I n t h e S t . Lawrence e s t u a r y , 212 L e v a s s e u r and T h e r r i a u l t (1987) p r e v i o u s l y d e m o n s t r a t e d t h a t n i t r a t e and s i l i c a t e were d e p l e t e d s i m u l t a n e o u s l y f o l l o w i n g an u p w e l l i n g e v e n t . R e s u l t s from t h e p r e s e n t s t u d y i n d i c a t e t h a t n i t r a t e and s i l i c a t e were a l s o s i m u l t a n e o u s l y d e p l e t e d i n t h e n o r t h w e s t e r n p o r t i o n o f t h e G u l f . The i m p o r t a n c e of t h e i n i t i a l n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s i m p l i e s t h a t d i a t o m growth i s m a i n l y dependent on l o n g - t e r m r e g e n e r a t i o n p r o c e s s e s and on f a c t o r s c o n t r o l l i n g t h e i r a c c u m u l a t i o n i n s u b s u r f a c e w a t e r s ( C o d i s p o t i 1983). S i l i c a t e and n i t r a t e have d i f f e r e n t r e g e n e r a t i v e p r o c e s s e s w h i c h l e a d t o d i f f e r e n t v e r t i c a l d i s t r i b u t i o n . S i l i c a t e r e g e n e r a t i o n i s m o s t l y a d i s s o l u t i o n p r o c e s s w h i c h o c c u r s c o n t i n u o u s l y a t a l l d e p t h s i n t h e ocean. On t h e o t h e r hand, t h e major p a r t o f p e l a g i c n i t r o g e n r e g e n e r a t i o n o c c u r s n e a r t h e s u r f a c e , w h i c h r e f l e c t s i t s s t r o n g l i n k w i t h b i o l o g i c a l a c t i v i t y ( e . g . z o o p l a n k t o n e x c r e t i o n and b a c t e r i a l r e m i n e r a l i z a t i o n ) . As a r e s u l t o f t h e s e d i f f e r e n t p r o c e s s e s , t h e N0a/Si04 r a t i o i n o c e a n i c e n v i r o n m e n t s w i l l g e n e r a l l y d e c r e a s e w i t h d e p t h , and s t r o n g e r (deeper) u p w e l l i n g w i l l b r i n g w a t e r w i t h l o w e r r a t i o s t o t h e s u r f a c e . T h i s h y p o t h e s i s was p u t f o r w a r d by C o d i s p o t i (1983) and Dugdale (1983) t o e x p l a i n t h e t e m p o r a l s h i f t between n i t r a t e and s i l i c a t e l i m i t a t i o n i n t h e P e r u u p w e l l i n g . The e x a m i n a t i o n of t h e v e r t i c a l d i s t r i b u t i o n o f n i t r a t e and s i l i c a t e may p r o v i d e v a l u a b l e i n f o r m a t i o n on w h i c h element may e v e n t u a l l y l i m i t d i a t o m growth. T h i s 213 i n f o r m a t i o n i s p a r t i c u l a r l y i m p o r t a n t s i n c e d i a t o m s c o n s t i t u t e t h e b u l k o f t h e a n n u a l new p r o d u c t i o n w h i c h i s a v a i l a b l e t o h i g h e r t r o p h i c l e v e l s . I n t h e S t . Lawrence system, t h e w a t e r column i s formed of 3 l a y e r s i n t h e summer: a s u r f a c e l a y e r h e a t e d by s o l a r r a d i a t i o n w h i c h e x t e n d s down t o ca. 20 m, an i n t e r m e d i a t e c o l d l a y e r (formed d u r i n g w i n t e r t i m e ) l o c a t e d between 20 and 100 m, and a deep l a y e r o f A t l a n t i c w a t e r . A t y p i c a l example o f t h e a p p r o x i m a t e l o c a t i o n o f t h e s e l a y e r s and t h e v e r t i c a l d i s t r i b u t i o n o f n i t r a t e and s i l i c a t e a l o n g a t r a n s e c t e x t e n d i n g from t h e head o f t h e L a u r e n t i a n Channel t o Cabot S t r a i t a r e p r e s e n t e d i n F i g u r e 4.2 (adapted from Coote and Y e a t s 1979). F o r t h e purpose o f t h i s d i s c u s s i o n , two p o i n t s a r e o f i n t e r e s t : (1) t h e s i m i l a r i t y between t h e n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s ( N 0 3 / S i 0 4 r a t i o o f ca. 1) i n t h e i n t e r m e d i a t e c o l d l a y e r (20 and 100 m), and (2) t h e d r a s t i c i n c r e a s e i n s i l i c a t e c o n c e n t r a t i o n s a t g r e a t e r d e p t h . I n t h e i n t e r m e d i a t e c o l d l a y e r , n i t r a t e and s i l i c a t e c o n c e n t r a t i o n s were r o u g h l y t h e same a t 15-20 uM. N u t r i e n t c o n c e n t r a t i o n s i n t h i s l a y e r a r e o f p a r t i c u l a r i n t e r e s t s i n c e u p w e l l i n g and v e r t i c a l m i x i n g p r o b a b l y n e v e r exceed 100 m i n t h e S t . Lawrence ( T h e r r i a u l t and L a c r o i x 1976). I t may t h u s be p o s t u l a t e d t h a t NOs/SiCU r a t i o s i n newly u p w e l l e d o r mixed s u r f a c e w a t e r s w i l l r e f l e c t r a t i o s f o u n d i n t h i s l a y e r . Below 100 m, t h e c o n c e n t r a t i o n o f n i t r a t e and s i l i c a t e i n c r e a s e d g r a d u a l l y w i t h d e p t h . S i l i c a t e 214 B) SILICATE (LIM) C) NITRATE ifiM) Figure 4.2. C r o s s - s e c t i o n showing the approximate l o c a t i o n of the three main l a y e r s forming the water column i n summer (A), and v e r t i c a l d i s t r i b u t i o n of n i t r a t e (B) and s i l i c a t e (C) along a t r a n s e c t extending from the head of the Lau r e n t i a n Channel t o Cabot S t r a i t i n the S t . Lawrence (adapted from Coote and Yeats 1979). 215 exhibited the most d r a s t i c increase with concentrations reaching 48 uM at 300 m (N0 3/Si0 4 = 0.5). The important increase i n s i l i c a t e concentrations below the intermediate cold layer and the associated decrease of the NOa/SiCU r a t i o have been a t t r i b u t e d to the estuarine c i r c u l a t i o n and i t s a b i l i t y to trap s i l i c a t e more e f f i c i e n t l y than n i t r a t e . N i t r a t e i s flushed out r a p i d l y from the estuary due to i t s longer residence time i n the surface layer where most of the regenerative processes are occurring. The incorporation of s i l i c a t e into f a s t - s i n k i n g p a r t i c l e s (diatom theca and f e c a l p e l l e t s ) enables these p a r t i c l e s to r a p i d l y sink out of the highly advective surface waters and accumulate i n bottom waters where the residual c i r c u l a t i o n i s i n the upstream d i r e c t i o n . Due to t h i s accumulation of s i l i c a t e , n i t r a t e -l i m i t e d growth of diatoms i s expected i n unpolluted systems characterized by an estuarine c i r c u l a t i o n . In the St. Lawrence, the trapping e f f e c t for s i l i c a t e becomes s i g n i f i c a n t only i n deeper waters which are not re-introduced i n the photic zone during normal upwelling and mixing events. Nitrate and s i l i c a t e found near the surface o r i g i n a t e from the intermediate cold layer where they are found i n equal proportion. This seems to explain why simultaneous exhaustion of n i t r a t e and s i l i c a t e i s commmon i n the St. Lawrence system. Consequences - Due to the d r a s t i c e f f e c t s of s i l i c a t e l i m i t a t i o n on the phytoplankton community structure, and 216 d i a t o m p h y s i o l o g y and b i o c h e m i c a l c o m p o s i t i o n , s i l i c a t e -d e f i c i e n t systems may d i f f e r from n i t r o g e n - o r phosphorous-d e f i c i e n t ones i n a number of ways. O f f i c e r and R y t h e r (1980) s t r e s s e d t h e i m p o r t a n c e o f s i l i c a t e l i m i t a t i o n i n r e p l a c i n g a g e n e r a l l y d e s i r a b l e d i a t o m p o p u l a t i o n w i t h a f r e q u e n t l y u n d e s i r a b l e f l a g e l l a t e p o p u l a t i o n and consequent e u t r o p h i c a t i o n e f f e c t s . In a system w h i c h i s n i t r o g e n -d e f i c i e n t , t h e a d d i t i o n o f n i t r o g e n from sewage o r r a i n may f a v o r t h e development o f d i a t o m s , as l o n g as s i l i c a t e i s n o t e x h a u s t e d . I n a s i l i c a t e - d e f i c i e n t system, a s i m i l a r n i t r o g e n a d d i t i o n w i l l cause an i n c r e a s e i n f l a g e l l a t e s w i t h l i t t l e b e n e f i c i a l e f f e c t s f o r h i g h e r t r o p h i c l e v e l s . I n t h i s l a s t c a s e , t h e a d d i t i o n of n i t r o g e n i s more l i k e l y t o produce blooms of t o x i c d i n o f l a g e l l a t e s and oxygen d e p l e t i o n . G i v e n an i n c r e a s e o f a t m o s p h e r i c C 0 2 and t h e p r o p o s e d warming of t h e w o r l d c l i m a t e (greenhouse e f f e c t ) , numerous new s c i e n t i f i c programs have been d i r e c t e d t o w a r d t h e a n a l y s i s o f t h e c a r b o n c y c l e i n t h e oceans. A c e n t r a l aim o f t h e s e s t u d i e s i s t o d e t e r m i n e t h e f a t e ( s i n k o r l i n k ) o f t h e p h o t o s y n t h e t i c a l l y f i x e d c a r b o n . T h i s problem i s t h u s d i r e c t l y c o n n e c t e d t o t h e c o n c e p t s o f "new" vs " r e g e n e r a t e d " p r o d u c t i o n (Dugdale and G o e r i n g 1967), o r more p r e c i s e l y " e x p o r t " vs " r e c y c l e d " p r o d u c t i o n (Legendre and Le F e v r e 1989, Legendre and G o s s e l i n 1989). The t y p e o f n u t r i e n t l i m i t a t i o n ( n i t r o g e n vs s i l i c a t e ) may s i g n i f i c a n t l y 217 i n f l u e n c e t h e f a t e of t h e o r g a n i c c a r b o n . S i l i c a t e -d e f i c i e n t d i a t o m s t e n d t o s i n k f a s t e r t h a t n i t r o g e n -d e f i c i e n t o r h e a l t h y c e l l s ( B i e n f a n g et al. 1982). As a consequence, t h e s i n k i n g of t h e d i a t o m biomass may be f a s t e r i n s i l i c a t e - d e f i c i e n t s ystems, a l l o w i n g l e s s r e g e n e r a t i o n t o o c c u r i n t h e p h o t i c zone. S i l i c a t e l i m i t a t i o n a l s o a f f e c t s t h e b i o c h e m i c a l c o m p o s i t i o n of d i a t o m s ; s i l i c a t e - d e f i c i e n t d i a t o m s have more c a r b o n p e r c e l l and c o n s e q u e n t l y a l o w e r N:C r a t i o ( H a r r i s o n e t a J . 1977). The N:C r a t i o s of t h e s i n k i n g c e l l s w i l l t h u s be 50% l o w e r t h a n t h e R e d f i e l d v a l u e o f t e n u sed i n models of p h y t o p l a n k t o n - m e d i a t e d c a r b o n exchange between s u r f a c e and deep w a t e r s . Thus i t appears t h a t b o t h t h e new p r o d u c t i o n and t h e downward f l u x o f o r g a n i c c a r b o n o c c u r a t s p a t i o / t e m p o r a l s i n g u l a r i t i e s . D u r i n g t h e s e b r i e f o r s p a t i a l l y r e s t r i c t e d e v e n t s , t h e p h y t o p l a n k t o n communities a r e g e n e r a l l y n u t r i e n t - l i m i t e d and t h e i r b i o c h e m i c a l c o m p o s i t i o n s i g n i f i c a n t l y d i f f e r e n t from h e a l t h y c e l l s . The s p a t i o / t e m p o r a l v a r i a t i o n s i n new versus r e g e n e r a t e d p r o d u c t i o n , p h y t o p l a n k t o n b i o c h e m i c a l c o m p o s i t i o n and s i n k i n g r a t e s h o u l d be t a k e n i n t o c o n s i d e r a t i o n i n f u t u r e s t u d i e s of c a r b o n t r a n s p o r t i n t h e ocean. REFERENCES A b l e , K.A. : I c h t h y o p l a n k t o n o f t h e S t . Lawrence e s t u a r y : C o m p o s i t i o n , d i s t r i b u t i o n and abundance. J . F i s h . Res. B o a r d Can. 35: 1517-1531 (1978) A d m i r a a l , W., H. P e l e t i e r and R.W.P.M. 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