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Computer simulation of phytoplankton and nutrient dynamics in an enclosed marine ecosystem Carruthers, Alan Boyd 1981

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COMPUTER SIMULATION OF PHYTOPLANKTON AND NUTRIENT DYNAMICS IN AN ENCLOSED MARINE ECOSYSTEM by ALAN BOYD CARRUTHERS B.Sc. U n i v e r s i t y of Calgary 1976 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Departments of Zoology and Oceanography) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA 19 May 1981 © Alan Boyd C a r r u t h e r s , 1981 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e head o f my department o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f ^—00/0 The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date tn+r n, /rrt DE-6 (2/79). i i ABSTRACT Th i s t h e s i s presents a q u a n t i t a t i v e model of i n t e r a c t i o n s among phytoplankton, n u t r i e n t s , b a c t e r i a and g r a z e r s i n an enclosed marine ecosystem. The e n c l o s e d system was a 23 m deep, 9.6 m diameter column of s u r f a c e water i n Saanich I n l e t , B r i t i s h Columbia. Dynamics of l a r g e - and s m a l l - c e l l e d diatoms and f l a g e l l a t e s i n response to observed i r r a d i a n c e and zooplankton numbers and observed or simulated n i t r o g e n and s i l i c o n c o n c e n t r a t i o n s were modelled over a simulated 76-day p e r i o d between J u l y 12 and September 26. The model's p r e d i c t i o n s p o o r l y matched the observed events i n C o n t r o l l e d Experimental Ecosystem 2 (CEE2), but n e v e r t h e l e s s p r o v i d e d some important i n s i g h t s i n t o system b e h a v i o r . C i l i a t e g r a z i n g probably prevented s m a l l - c e l l e d phytoplankton from i n c r e a s i n g to l a r g e c o n c e n t r a t i o n s i n CEE2. By v i r t u e of t h e i r tremendous numbers, c o l o u r l e s s f l a g e l l a t e s were p o t e n t i a l l y the most important g r a z e r s on b a c t e r i a , much more important than larvaceans or metazoan l a r v a e . Whereas s m a l l - c e l l e d phytoplankton were l i m i t e d by g r a z e r s , l a r g e phytoplankton dynamics were not markedly a f f e c t e d by g r a z i n g . The average observed r a t e of 1 4 C f i x a t i o n i n the s u r f a c e 8 m was roughly c o n s i s t e n t with an i n t e r p r e t a t i o n i n which a r t i f i c i a l a d d i t i o n s of n i t r o g e n c o n t r i b u t e d 62% of i n f e r r e d net uptake of n i t r o g e n by phytoplankton, mixing from subsurface water c o n t r i b u t e d 18%, 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 12%, and zooplankton e x c r e t i o n 9%. However, independent o b s e r v a t i o n s of r a p i d a c t i v i t y by microheterotrophs (presumably b a c t e r i a ) suggested that 1*C f i x a t i o n c o n s i d e r a b l y underestimated net primary p r o d u c t i o n . T h i s y i e l d e d an a l t e r n a t i v e i n t e r p r e t a t i o n i n which n u t r i e n t a d d i t i o n s c o n t r i b u t e d 46% of i n f e r r e d net uptake of n i t r o g e n i n the s u r f a c e l a y e r , mixing 13%, b a c t e r i a 35%, and zooplankton 7%. D i s s o l u t i o n of s i l i c a was r e s p o n s i b l e f o r the observed accumulation of s i l i c i c a c i d below 8 m depth i n CEE2, but the importance of s i l i c a d i s s o l u t i o n as a source of S i f o r diatom growth i n the s u r f a c e 8 m i s u n c e r t a i n . The model's major f a i l i n g was i t s assumption of unchanging maximum growth r a t e s of phytoplankton, and unchanging r a t e s of exudation, s i n k i n g , and r e s p i r a t i o n . P h y s i o l o g i c a l parameter val u e s which accounted f o r the huge bloom of Stephanopyxis i n CEE2 c o u l d not account f o r the ensuing c o l l a p s e . T r a d i t i o n a l m o d e l l i n g assumptions of slowly changing i n t e r n a l p h y s i o l o g y , although adequate f o r marine systems dominated by p h y s i c a l f a c t o r s such as s e a s o n a l i t y or water movement, cannot capture the behavior of b i o l o g i c a l l y dominated systems l i k e the e n c l o s e d system c o n s i d e r e d here. i v TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS . x i i i 1. INTRODUCTION .. . 1 2. THE FOOD WEB I EXPERIMENT 5 2.1 Experimental Design 5 2.2 Sampling P r o t o c o l and Accuracy 6 2.3 Data Sources 8 2.4 Observed Events i n CEE2 9 3. THE COMPONENTS OF THE SYSTEM 39 . 3.1 Overview of the Model 39 3.2 L i g h t 41 3.2.1 Surface i r r a d i a n c e 41 3.2.2 E x t i n c t i o n of l i g h t w i t h i n the water column .. 43 3.3 Mixing 47 3.4 Phytoplankton 50 3.4.1 Species and c e l l s i z e s 50 3.4.2 Chemical composition of phytoplankton 51 3.4.3 Photosynthesis 57 3.4.4 Phytoplankton r e s p i r a t i o n 71 V 3.4.5 Phytoplankton exudation 72 3.4.6 Nutrient l i m i t a t i o n of phytoplankton growth .. 73 3.4.7 Phytoplankton sinking 76 3.5 Zooplankton 77 3.5.1 Numbers, species, and body weight 77 3.5.2 Zooplankton grazing 78 3.5.3 Zooplankton excretion 94 3.6 Bacteria 98 3.7 Inorganic Nutrients 103 3.7.1 Nutrient uptake 103 3.7.2 Nutrient sources 103 3.7.3 Dissolution of pa r t i c u l a t e s i l i c a 104 3.8 F i n a l Comments 108 4. COMPUTER IMPLEMENTATION 112 4.1 Approach 112 4.2 Nutrient Interpolation 113 4.3 Accuracy of the F i n i t e Difference Approximation ...115 5. RESULTS AND DISCUSSION 116 5.1 The Reference Simulation 117 5.1.1 Large diatoms 117 5.1.2 Large f l a g e l l a t e s 127 5.1.3 Small diatoms and small f l a g e l l a t e s 134 5.1.4 Bacteria 137 5.1.5 Nitrogen c y c l i n g 142 v i 5.1.6 S i l i c o n 160 5.2 S i l i c o n S i m u l a t i o n s ..165 5.2.1 Si m u l a t i o n S i - 1 . Comparison with r e f e r e n c e run .165 5.2.2 Si m u l a t i o n Si-2 and S i ~ 3 . L i m i t a t i o n of diatom growth 168 5.2.3 Si m u l a t i o n Si-4 and S i - 5 . V a r i a b l e C/Si r a t i o 171 5.2.4 S i m u l a t i o n S i - 6 . S i l i c o n uptake l i n k e d to net phot o s y n t h e s i s 179 5.2.5 S i m u l a t i o n S i - 7 . D i s s o l u t i o n of s i l i c a ....'...18 0 5.2.6 Con c l u s i o n s r e g a r d i n g S i dynamics 187 5.3 Phytoplankton Growth and Loss ....188 5.3.1 Have maximum r a t e s of primary p r o d u c t i o n been underestimated? 189 5.3.2 Have l o s s e s or growth l i m i t a t i o n been underestimated? 196 6. WHAT WENT WRONG? 201 6.1 The Bloom and C o l l a p s e of L a r g e - C e l l e d Diatoms ....204 6.2 Parameter E s t i m a t i o n Technique 204 6.3 R e s u l t s and D i s c u s s i o n 209 6.3.1 The diatom bloom, days 13 to 25 209 6.3.2 The diatom c o l l a p s e , days 25 to 51 213 6.4 Co n c l u s i o n s Regarding P h y s i o l o g i c a l V a r i a b i l i t y ...222 7. FINAL CONCLUSIONS 227 REFERENCES 231 APPENDIX 1 256 APPENDIX 2 261 LIST OF TABLES Table I. L i t e r a t u r e estimates of volume of water swept c l e a r by c i l i a t e s g r a z i n g b a c t e r i a or other small p a r t i c l e s 88 Table I I . L i t e r a t u r e estimates of the volume of water c l e a r e d by gastropod and pelecypod l a r v a e g r a z i n g on small phytoplankton 90 Table I I I . L i t e r a t u r e estimates of s p e c i f i c d i s s o l u t i o n r a t e of s i l i c a i n l i v i n g and dead phytoplankton .106 Table IV. Summary of parameter v a l u e s used i n the main s i m u l a t i o n model. 109 Table V. Sum of squared d e v i a t i o n s between p r e d i c t e d and observed large-diatom biomass i n the s u r f a c e of CEE2 for d i f f e r e n t parameter val u e s 210 v i i i LIST OF FIGURES F i g u r e 1. P h o t o s y n t h e t i c a l l y a c t i v e quanta immediately below the s u r f a c e of CEE2 10 F i g u r e 2. Separate biomass of four groups of phytoplankton observed i n the s u r f a c e 8 m of CEE2 13 F i g u r e 3. Accumulative biomass of four groups of phytoplankton observed i n the s u r f a c e 8 m of CEE2 18 F i g u r e 4. Observed and i n t e r p o l a t e d c o n c e n t r a t i o n s of d i s s o l v e d n i t r o g e n and s i l i c o n i n the s u r f a c e 8 m of CEE2 20 F i g u r e 5. Biomass of copepods observed i n the s u r f a c e 8 m of CEE2. 24 F i g u r e 6. Biomass of ctenophores and chaetognaths observed i n the s u r f a c e 20 m of CEE2 26 F i g u r e 7. Biomass of c i l i a t e s observed i n the s u r f a c e 8 m of CEE2 28 F i g u r e 8. Biomass of metazoan l a r v a e observed i n the s u r f a c e 8 m of CEE2. " 30 F i g u r e 9. Biomass of l a r v a c e a n s observed i n the s u r f a c e 20 m of CEE2 32 F i g u r e 10. Biomass of c o l o u r l e s s f l a g e l l a t e s observed i n the s u r f a c e 8 m of CEE2 34 F i g u r e 11. Biomass of b a c t e r i a observed i n the s u r f a c e 8 m of CEE2 37 F i g u r e 12. A t t e n u a t i o n c o e f f i c i e n t of p h o t o s y n t h e t i c a l l y a c t i v e quanta vs. phytoplankton carbon i n CEE2 45 F i g u r e 13. Observed phytoplankton carbon vs. p a r t i c u l a t e o r g a n i c carbon i n CEE2 53 F i g u r e 14. P a r t i c u l a t e organic carbon and n i t r o g e n i n CEE2 55 F i g u r e 15. S p e c i f i c carbon f i x a t i o n r a t e vs. average p h o t o s y n t h e t i c a l l y a c t i v e i r r a d i a n c e d u r i n g 4-hour midday i n c u b a t i o n s 60 F i g u r e 16. H y p o t h e t i c a l depth p r o f i l e of I r k i n CEE2 63 F i g u r e 17. Modelled e x c r e t i o n r a t e s of c i l i a t e s and c o l o u r l e s s f l a g e l l a t e s , ctenophores, and other zooplankton 99 F i g u r e 18. Biomass of l a r g e - c e l l e d diatoms i n the s u r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run 118 F i g u r e 19. Modelled gain and l o s s r a t e s s p e c i f i c to p r e d i c t e d l a r g e diatom biomass; r e f e r e n c e run; s u r f a c e 8 m 121 F i g u r e 20. P r e d i c t e d s p e c i f i c g r a z i n g l o s s of l a r g e diatoms; r e f e r e n c e run; s u r f a c e 8 m 124 F i g u r e 21. P r e d i c t e d biomass of l a r g e diatoms i n the su r f a c e and deep l a y e r s ; r e f e r e n c e run 128 X F i g u r e 22. Biomass of l a r g e - c e l l e d f l a g e l l a t e s i n the su r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run 130 F i g u r e 23. Modelled gain and l o s s r a t e s s p e c i f i c t o p r e d i c t e d l a r g e f l a g e l l a t e biomass; r e f e r e n c e run; s u r f a c e 8 m .132 F i g u r e 24. P r e d i c t e d s p e c i f i c g r a z i n g l o s s of small diatoms; r e f e r e n c e run; su r f a c e 8 m 135 F i g u r e 25. Biomass of b a c t e r i a i n the s u r f a c e 8 m p r e d i c t e d by the re f e r e n c e run 138 F i g u r e 26. Co n c e n t r a t i o n of t o t a l d i s s o l v e d i n o r g a n i c n i t r o g e n ( n i t r a t e + n i t r i t e + ammonium) i n the su r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run 143 F i g u r e 27. P r e d i c t e d e x c r e t i o n of ammonium-N by zooplankton; r e f e r e n c e run; s u r f a c e 8 m 146 F i g u r e 28. P r e d i c t e d and observed i n g e s t i o n r a t e by a d u l t female Pseudocalanus 157 F i g u r e 29. Con c e n t r a t i o n of d i s s o l v e d s i l i c o n i n the s u r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run 161 F i g u r e 30. Sim u l a t i o n S i - 1 . P r e d i c t e d l a r g e diatom biomass and s i l i c i c a c i d c o n c e n t r a t i o n i n the sur f a c e 8 m 166 F i g u r e 31. Si m u l a t i o n S i - 2 . P r e d i c t e d l a r g e diatom biomass and s i l i c i c a c i d c o n c e n t r a t i o n i n the s u r f a c e 8 m 169 F i g u r e 32. S i m u l a t i o n S i - 3 . P r e d i c t e d l a r g e diatom biomass and s i l i c i c a c i d c o n c e n t r a t i o n i n the sur f a c e 8 m 172 F i g u r e 33. S i m u l a t i o n S i - 4 . P r e d i c t e d l a r g e diatom biomass and s i l i c i c a c i d c o n c e n t r a t i o n i n the s u r f a c e 8 m. 174 F i g u r e 34. S i m u l a t i o n S i - 5 . P r e d i c t e d l a r g e diatom biomass and s i l i c i c a c i d c o n c e n t r a t i o n i n the s u r f a c e 8 m 177 F i g u r e 35. S i m u l a t i o n S i - 6 . P r e d i c t e d l a r g e diatom biomass and s i l i c i c a c i d c o n c e n t r a t i o n i n the s u r f a c e 8 m 181 F i g u r e 36. S i m u l a t i o n S i - 7 . P r e d i c t e d l a r g e diatom biomass and s i l i c i c a c i d c o n c e n t r a t i o n i n the s u r f a c e 8 m 183 F i g u r e 37. S i m u l a t i o n S i - 7 . P r e d i c t e d c o n c e n t r a t i o n of s i l i c i c a c i d i n the 8-20 m l a y e r 185 F i g u r e 38. S p e c i f i c net growth r a t e of l a r g e f l a g e l l a t e carbon i n the s u r f a c e 8 m of CEE2 190 F i g u r e 39. S p e c i f i c net growth r a t e s of l a r g e diatoms and l a r g e f l a g e l l a t e s i n the s u r f a c e 8 m p r e d i c t e d by a s i m u l a t i o n i n which a l l s t a t e v a r i a b l e s were i n t e r p o l a t e d between t h e i r observed c o n c e n t r a t i o n s . ...193 F i g u r e 40. P r e d i c t e d l i m i t a t i o n of l a r g e diatom growth i n the s u r f a c e 8 m due to l i g h t , d i s s o l v e d n i t r o g e n and s i l i c o n 198 F i g u r e 41. Biomass of l a r g e .diatoms p r e d i c t e d by a s i m u l a t i o n i n which only the dynamics of l a r g e diatoms were enabled 202 F i g u r e 42. Observed c o n c e n t r a t i o n of v e g e t a t i v e c e l l s and r e s t i n g spores, of l a r g e diatoms i n CEE2 205 F i g u r e 43. SSQ su r f a c e f o r p e r i o d of l a r g e diatom bloom, days 13 to 25. S=0.106 m day" 1 211 Fi g u r e 44. SSQ su r f a c e f o r p e r i o d of l a r g e diatom bloom, days 13 to 25. KrN=0.242 ug-atom N l " 1 . 214 Fi g u r e 45. Con c e n t r a t i o n of l a r g e diatoms i n s u r f a c e 8 m p r e d i c t e d with Prmax = 1.63 day" 1, K rN = 0.242 ug-atom N 1 _ 1 , S = 0.106 m day" 1; days 13. to 25. . 216 Fi g u r e 46. Co n c e n t r a t i o n of l a r g e diatoms i n s u r f a c e 8 m p r e d i c t e d with Prmax = 1.63 day" 1, K rN = 0.242 ug-atom N l " 1 , S = 0.106 m day" 1; days 25 to 51. 218 Fi g u r e 47. Con c e n t r a t i o n of l a r g e diatoms i n s u r f a c e 8 m p r e d i c t e d with Prmax = 2.00 day" 1, K rN = 0.783 ug-atom N l " 1 , s = 3.97 m day" 1? days 25 to 51 220 x i i i ACKNOWLEDGEMENTS I wish to thank F. Azam, P. K. Bienfang, C. 0. Davis, G. D. G r i c e , R. P. H a r r i s , J . F. Heinbokel, J . T. H o l l i b a u g h , K. R. King, I. Koike, R. S. M e r c i e r , P. P a r s l e y , M. R. Reeve, M. Takahashi and P.. J . l e B . W i l l i a m s who c o n t r i b u t e d experimental data, and a l l members of the CEPEX s t a f f who p a r t i c i p a t e d i n the c o l l e c t i o n and a n a l y s i s of f i e l d data. I have a p p r e c i a t e d having i n f o r m a l d i s c u s s i o n s with A. T. Chan, P. J . H a r r i s o n , R. H i l b o r n , J . Parslow, R. I. Perry, and N. C. Sonntag. I a l s o thank my s u p e r v i s o r T. R. Parsons f o r h i s support, encouragement and forbearance over the past 4 years. T h i s r e s e a r c h was supported by N a t i o n a l Research C o u n c i l Postgraduate S c h o l a r s h i p s and a U n i v e r s i t y of B r i t i s h Columbia Graduate F e l l o w s h i p to the author. ' L i f e t r i c k e d so shamelessly. I t was enough to make men laugh or weep. A man c o u l d l i v e , l e t t i n g h i s senses have f r e e r e i n , sucking h i s f i l l at the b r e a s t s of Eve, h i s m o ther—and then, though he might r e v e l and enjoy, there was no p r o t e c t i o n a g a i n s t her t r a n s i e n c e , and so, l i k e a t o a d s t o o l i n the woods, he shimmered today i n the f a i r e s t c o l o u r s , tomorrow r o t t e d , and f e l l to dust. 'Or he c o u l d set up h i s defenses a g a i n s t l i f e , l o c k h i m s e l f i n t o a workshop, and seek to b u i l d a monument beyond time. And then l i f e h e r s e l f must be renounced; the man was nothing but her instrument: though he might serve e t e r n i t y he withered, he l o s t h i s freedom, f u l l n e s s , and joy of days.... 'And yet our days had only a meaning i f both these goods c o u l d be achieved, and l i f e h e r s e l f had not been c l e f t by the barren d i v i s i o n of a l t e r n a t i v e s . To work and yet not pay l i f e ' s p r i c e fo r working: to l i v e , yet not renounce the work of c r e a t i o n . Could i t ever be done?' — Hermann Hesse, N a r z i s s and Goldmund t r a n s l a t e d by G e o f f r e y Dunlop 1 1. INTRODUCTION Conceptual models of marine p r o d u c t i o n have changed g r e a t l y i n t h i s c e n t u r y . The a g r i c u l t u r a l model of marine p r o d u c t i o n (Johnstone 1908) a s s e r t e d that algae were produced u n t i l the n u t r i e n t i n l e a s t supply was exhausted, and then the algae were eaten by h e r b i v o r e s . In 1935, Harvey et a l . r e p o r t e d that the decrease i n phosphate c o n c e n t r a t i o n o f f Plymouth du r i n g the second h a l f of the s p r i n g bloom of alg a e , when converted to an e q u i v a l e n t amount of a l g a l biomass, represented 30 to 40 times the stock of algae p r e s e n t . Harvey et a l . (1935) argued that the s p r i n g outburst was c o n t r o l l e d not by n u t r i e n t exhaustion but by g r a z i n g and that zooplankton consumed p r a c t i c a l l y a l l of the p r o d u c t i o n . They s p e c u l a t e d t h a t the reappearance of phosphate l a t e i n the summer was caused by delayed b a c t e r i a l breakdown of organic phosphorus d e r i v e d from broken or p a r t i a l l y d i g e s t e d diatoms. Subsequently, Gardiner (1937) demonstrated that zooplankton c o u l d regenerate phosphate i n t o the sea, and much l a t e r , H a r r i s (1959) measured the re g e n e r a t i o n of ammonia by zooplankton. I t was r e a l i z e d that zooplankton, i n a d d i t i o n to t h e i r t r a d i t i o n a l r o l e as g r a z e r s of phytoplankton, c o u l d resupply much of the n i t r o g e n r e q u i r e d f o r a l g a l p r o d u c t i o n . The p r o p o r t i o n of n i t r o g e n r e q u i r e d by phytoplankton which i s s u p p l i e d by zooplankton e x c r e t i o n i s q u i t e v a r i a b l e : H a r r i s (1959) estimated that zooplankton e x c r e t e d 55% of n i t r o g e n r e q u i r e d i n Long I s l a n d Sound; 90% of requirement was e x c r e t e d i n the Columbia R i v e r plume o f f s h o r e and 36% i n oceanic water 2 o f f the Oregon coast (Jawed 1973); 70% i n the c e n t r a l gyre of the North P a c i f i c (Eppley et a l . 1973); 25% i n the c o a s t a l u p w e l l i n g • o f f northwest A f r i c a (Smith and Whitledge 1977); and 5% i n Narragansett Bay (Vargo 1979). U n t i l r e c e n t l y , e x c r e t i o n by nekton or v e r t i c a l a d v e c t i o n or d i f f u s i o n were, by d e f a u l t , g e n e r a l l y assumed to provide the remaining n i t r o g e n r e q u i r e d by a l g a l p r o d u c t i o n i n the s u r f a c e l a y e r . However, there i s a growing b e l i e f , as yet l a r g e l y unsupported by f i r m evidence, that microzooplankton (Heinbokel and Beers 1979) and e s p e c i a l l y b a c t e r i a (Pomeroy 1974; S i e b u r t h 1976) may be important consumers of a l g a l p r o d u c t i o n and regenerators of n u t r i e n t s . The present study sought to develop a coherent model of phytoplankton growth as a f u n c t i o n of micro- and macrozooplankton g r a z i n g and n u t r i e n t r e g e n e r a t i o n by both zooplankton and b a c t e r i a in an e x t e n s i v e l y s t u d i e d marine ecosystem. The ecosystem chosen was the 1300 m3 enclosed water column of the 1978 C o n t r o l l e d Ecosystem P o p u l a t i o n s Experiment (CEPEX), a l s o termed the Foodweb I experiment ( G r i c e et a l . 1980). Three such e n c l o s u r e s , t r a p p i n g 23 metre deep columns of s u r f a c e water i n Saanich I n l e t , B r i t i s h Columbia, o f f e r e d two u n p a r a l l e l e d advantages f o r s i m u l a t i o n m o d e l l i n g . F i r s t , these microcosms are probably the most i n t e n s i v e l y s t u d i e d p a r c e l s of seawater i n h i s t o r y : d e t a i l e d time s e r i e s of i r r a d i a n c e , i n o r g a n i c n u t r i e n t s , standing stocks of i n d i v i d u a l s p e c i e s of phytoplankton and zooplankton, b a c t e r i a l abundance, and primary p r o d u c t i v i t y were a v a i l a b l e . Second, because the water columns 3 were en c l o s e d , v e r t i c a l and h o r i z o n t a l a d v e c t i o n were e l i m i n a t e d , thus p e r m i t t i n g r e p e t i t i v e sampling of the same p o p u l a t i o n s . I t was hoped that a s i m u l a t i o n model of the dynamics of diatoms and f l a g e l l a t e s , n i t r o g e n and s i l i c o n , and b a c t e r i a c o u l d be b u i l t to account f o r the observed behavior of these v a r i a b l e s i n one of the CEPEX e n c l o s u r e s . The s i m u l a t i o n model developed in t h i s t h e s i s r e l i e d on t h e o r e t i c a l r e l a t i o n s h i p s used i n e a r l i e r m o d e l l i n g s t u d i e s by S t e e l e (1974), S t e e l e and F r o s t (1977), Kremer and Nixon (1978), and o t h e r s , with one important e x t e n s i o n . E a r l i e r models of marine plankton systems had, without e x c e p t i o n , e i t h e r completely ignored b a c t e r i a or i m p l i c i t l y i n c l u d e d them in some ge n e r a l term f o r n u t r i e n t r e g e n e r a t i o n . C i r c u m s t a n t i a l evidence suggested that b a c t e r i a might be p r i m a r i l y r e s p o n s i b l e f o r ammonia re g e n e r a t i o n i n the CEPEX e n c l o s u r e s (Hol.libaugh et a l . 1980). I t t h e r e f o r e seemed h i g h l y d e s i r e a b l e to e x p l i c i t l y model b a c t e r i a l growth and metabolism. Having s u c c e s s f u l l y p r e d i c t e d the observed changes in phytoplankton and n u t r i e n t c o n c e n t r a t i o n s (so i t was hoped), i t would then be a simple matter to d e r i v e the r e l a t i v e c o n t r i b u t i o n of micro- and macrozooplankton and b a c t e r i a to n u t r i e n t c y c l i n g , a d e r i v a t i o n c o n s i s t e n t with (but not n e c e s s a r i l y e n t a i l e d by) a v a i l a b l e o b s e r v a t i o n s . T h i s p l a n of r e s e a r c h never m a t e r i a l i z e d . Although the model developed here d i d p r o v i d e many i n s i g h t s i n t o the events which o c c u r r e d i n Foodweb I, i t was completely unable to reproduce the observed bloom and c o l l a p s e of Stephanopyxis or 4 the delayed surge i n C e r a t i u m — e v e n t s o f major s i g n i f i c a n c e i n one of the enclosed water columns. The f a i l u r e was fundamental and l e d to a reexamination of the t r a d i t i o n a l approach to mo d e l l i n g marine ecosystems. The design of the 1978 CEPEX experiment and a summary of observed events are given i n the next s e c t i o n of t h i s t h e s i s . The t h i r d s e c t i o n p r o v i d e s a d e t a i l e d d e s c r i p t i o n of the s i m u l a t i o n model and a review of the l i t e r a t u r e u n d e r l y i n g i t . 1 The f o u r t h s e c t i o n o u t l i n e s the computer implementation of the model. S i m u l a t i o n r e s u l t s are presented and d i s c u s s e d i n the f i f t h s e c t i o n , and the model's f a i l u r e i s i n v e s t i g a t e d i n s e c t i o n s i x . The f i n a l s e c t i o n a n a l y z e s the performance of two commonly c i t e d s i m u l a t i o n s t u d i e s i n l i g h t of the f a i l u r e r e p o r t e d here. 1It i s suggested on f i r s t reading that the reader s k i p a l l of s e c t i o n 3 except the overview i n 3.1; d e t a i l e d s u b s e c t i o n s can be c o n s u l t e d l a t e r f o r c l a r i f i c a t i o n . 5 2. THE FOODWEB I EXPERIMENT 2.1 Experimental Design The Foodweb I experiment was designed to see i f markedly d i f f e r e n t phytoplankton communities c o u l d be encouraged to develop i n two enclosed water columns by manipulating l i g h t , mixing and n u t r i e n t regimes. An o v e r a l l account of the experiment i s given by G r i c e et a l . (1980). The experiments i n v o l v e d the simultaneous capture of 1300 m3 water columns d e s c r i b e d by Case (1978), G r i c e et a l . (1977) and Menzel and Case (1977). The c o n t r o l l e d experimental ecosystems (CEEs) c o n s i s t e d of p o l y e t h y l e n e bags 9.6 m i n diameter and 23.5 m deep, open at the sea s u r f a c e and t a p e r i n g to a c l o s e d cone below 16 m. On J u l y 9, 1978 (day 1), three CEEs were simultaneously r a i s e d from a depth of about 30 m and a t t a c h e d to s t e e l f l o a t a t i o n r i n g s moored at 123° 29.1' W, 48° 39.6' N i n P a t r i c i a Bay, part of Saanich I n l e t , B r i t i s h Columbia. The three e n c l o s u r e s were desi g n a t e d CEE2, CEE3 and CEE4. T h i s t h e s i s i s concerned with the events i n CEE2 which was manipulated i n a manner thought to encourage the dominance of diatoms. CEE2 was g e n t l y mixed by p e r i o d i c bubbling above 8.5 m depth to r e t a r d the s i n k i n g of diatoms. To prevent r a p i d n u t r i e n t d e p l e t i o n , diatom growth was slowed by means of a white s a i l c l o t h p l a c e d over CEE2 between days 3 and 54. T h i s l i g h t screen reduced i n c i d e n t l i g h t by about 40%. N i t r a t e , s i l i c a t e and phosphate were added to the upper 8 m on eleven occasions to e l e v a t e t h e i r r e s p e c t i v e c o n c e n t r a t i o n s by 7.6, 14 6 and 1.1 uM on each a d d i t i o n . 1 Each time n u t r i e n t s were added about 800 l i t r e s of sediment from the bottom of the CEE was pumped t o the sur f a c e to r e c y c l e diatom spores and n u t r i e n t s . A second CEE (CEE3) was not mixed, and r e c e i v e d p e r i o d i c a d d i t i o n s of n i t r a t e and phosphate. No sediment was r e c y c l e d and t h e r e was no l i g h t s c r e e n . These c o n d i t i o n s were designed to l i m i t diatom growth and encourage f l a g e l l a t e s . CEE4 was used e x c l u s i v e l y to i n v e s t i g a t e the p h y s i c s and chemistry of the enclo s e d water column. 2.2 Sampling P r o t o c o l and Accuracy Net or pump samples were r o u t i n e l y taken two or three times a week. Zooplankton were sampled once a week a f t e r day 70, and larvaceans were sampled d a i l y u n t i l day 63. Water was removed from f i v e depth i n t e r v a l s (0-4, 4-8, 8-12, 12-16, 16-20 m) by a p e r i s t a l t i c pump f o r measurement of n u t r i e n t s , c h l o r o p h y l l - a , p a r t i c u l a t e carbon and n i t r o g e n , 1 4 C primary p r o d u c t i v i t y and phytoplankton and b a c t e r i a l abundance as d e s c r i b e d i n G r i c e et a l . (1980), Davis (1980), and H o l l i b a u g h et a l . (1980). Primary p r o d u c t i o n was determined i n 4-hour i_n s i t u i n c u b a t i o n s between about 1000 and 1400 hours P a c i f i c Standard Time. A l g a l carbon was estimated from c e l l volumes of phytoplankton i n u n f i l t e r e d water samples f i x e d with xThe symbols "urn", "ug", "uM", " u l " , and "uEin" r e s p e c t i v e l y r e p r e s e n t "micrometre", "microgram", "micromolar", " m i c r o l i t r e " , and " m i c r o E i n s t e i n " . 7 Lugol's s o l u t i o n and c o n c e n t r a t e d by sedimentation. B a c t e r i a l carbon was d e r i v e d from v i s u a l estimates of c e l l s i z e d i s t r i b u t i o n i n samples s t a i n e d with a c r i d i n e orange. Crustaceans, protozoans and other small zooplankton were c o l l e c t e d from 4-metre depth i n t e r v a l s with a diaphragm pumping system d e s c r i b e d by Beers et a l . (1977). C i l i a t e s i n unconcentrated water samples were f i x e d and examined s i m i l a r l y to phytoplankton. A p p r o p r i a t e mesh s i z e s were used to c o n c e n t r a t e other zooplankton. V e r t i c a l tows with a v a r i e t y of nets were used to sample ctenophores, chaetognaths and l a r v a c e a n s ( H a r r i s et a l . 1980; King et a l . 1980). Zooplankton numbers were converted to biomass of carbon using the body weights r e p o r t e d i n Appendix 2. How p r e c i s e were i n d i v i d u a l o b s e r v a t i o n s of standing st o c k s to which the s i m u l a t i o n r e s u l t s were compared? R e s u l t s from r e p l i c a t e samplings of the enclosed water columns were not a v a i l a b l e f o r any b i o l o g i c a l ' o r chemical parameter observed i n Foodweb I, and t h e r e f o r e the sampling p r e c i s i o n c o u l d not be d i r e c t l y estimated. Lawson and G r i c e (1977) r e p e a t e d l y towed a Bongo net v e r t i c a l l y through a 68 m3 CEE to determine the p r e c i s i o n of sampling macrozooplankton. Using log-transformed data they reported 95% c o n f i d e n c e l i m i t s of 41 - 238% of a s i n g l e o b s e r v a t i o n . T h i s degree of u n c e r t a i n t y i s s i m i l a r to that found by Wiebe and H o l l a n d (1968) who reviewed sampling v a r i a b i l i t y of net tows under t y p i c a l f i e l d c o n d i t i o n s . P i a t t et a l . (1970) s t u d i e d the sampling v a r i a b i l i t y of c h l o r o p h y l l and phosphate at s e v e r a l s t a t i o n s i n St. Margaret's Bay. The 8 average w i t h i n - s t a t i o n v a r i a b i l i t y was represented by co n f i d e n c e l i m i t s of 66 - 152% of a s i n g l e c h l o r o p h y l l o b s e r v a t i o n ; the average l o g a r i t h m i c c o e f f i c i e n t of v a r i a t i o n of phosphate measurements ( i n c l u d i n g between-station v a r i a b i l i t y ) was 62% of that of c h l o r o p h y l l measurements. The s u b s t a n t i a l p a t c h i n e s s of plankton w i t h i n CEEs (e.g. F i g . 10 of Takahashi et a l . 1975) may f u r t h e r degrade sampling accuracy. In summary, the a c t u a l accuracy or p r e c i s i o n of the r e s u l t s r e p o r t e d f o r Foodweb I i s unknown, but one can reasonably guess that true n u t r i e n t concentrations' were w i t h i n roughly 75 - 130% of r e p o r t e d v a l u e s , true phytoplankton and microzooplankton biomass to w i t h i n 55 - 180%, and t r u e macrozooplankton numbers t o w i t h i n 40 - 250%. Reported b a c t e r i a l biomass i s probably no more ac c u r a t e than that of macrozooplankton. 2.3 Data Sources Data c o l l e c t e d d u r i n g the Foodweb I experiment were obtained from s e v e r a l p r i n c i p a l i n v e s t i g a t o r s . The sampling schedule, c h l o r o p h y l l and n u t r i e n t c o n c e n t r a t i o n s , 1 4 C p r o d u c t i v i t y , water temperature, i n c i d e n t r a d i a t i o n and l i g h t p e n e t r a t i o n are documented in the main data r e p o r t of the C o n t r o l l e d Ecosystem P o p u l a t i o n s Experiment f o r Foodweb I, a v a i l a b l e from G. D. G r i c e , Woods Hole Oceanographic I n s t i t u t i o n . Zooplankton numbers are l i s t e d i n CEPEX Report 3 of March 1979; phytoplankton numbers, volume and carbon are on magnetic tape; both are a l s o a v a i l a b l e from G r i c e . F u r t h e r sources of i n f o r m a t i o n were: 9 • 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 : F. Azam, I n s t i t u t e of Marine Resources, S c r i p p s I n s t i t u t i o n of Oceanography • b a c t e r i a l numbers, carbon, a c t i v i t y : F. Azam • c i l i a t e s : J . F. Heinbokel, Chesapeake Bay I n s t i t u t e , Johns Hopkins U n i v e r s i t y • ctenophores, chaetognaths: M. R. Reeve, R o s e n s t i e l School of Marine and Atmospheric Science, U n i v e r s i t y of Miami • l a r v a c e a n s : K. R. King and K. Banse, Department of Oceanography, U n i v e r s i t y of Washington Unless otherwise s t a t e d , a l l a nalyses of raw data c o l l e c t e d d u r i n g Foodweb I and r e p o r t e d i n t h i s t h e s i s are the author's. 2.4 Observed Events i n CEE2 G r i c e et a l . . (1980) provide an overview of the observed events i n Foodweb I. B r i e f l y , these were as f o l l o w s : I r r a d i a n c e — T h e f l u c t u a t i o n of s u r f a c e i r r a d i a n c e ( c o r r e c t e d as d e s c r i b e d in s e c t i o n 3.2.1) over the f i r s t 80 days of Foodweb I i s shown in F i g . 1. The weather was mainly dry and sunny u n t i l day 33 (August 10) and then was q u i t e cloudy and wet u n t i l the end of the experiment. P h y t o p l a n k t o n — T h e r e were two major blooms of 10 F i g u r e 1. P h o t o s y n t h e t i c a l l y a c t i v e quanta immediately below the s u r f a c e of CEE2 between J u l y 9, 1978 (day 1) and September 26 (day 80). Observed i r r a d i a n c e s were c o r r e c t e d as d e s c r i b e d i n s e c t i o n 3.2.1 before p l o t t i n g . L i g h t screen i n p l a c e from day 3 to 54. 11 Surface Irradiance (uEin i n " 2 s " ' ) a 2 0 0 0 oo J C D 12 phytoplankton i n CEE2 ( F i g s . 2, 3). L a r g e - c e l l e d (>15 um e q u i v a l e n t s p h e r i c a l diameter (>15 um ESD)) diatoms i n c r e a s e d to 687 ug C l " 1 in the s u r f a c e 8 m by day 25 with Stephanopyxis t u r r i s comprising 99% of the biomass at the height of the bloom. The l a r g e diatoms then c o l l a p s e d to 5 ug C l " 1 by day 51, i n c r e a s e d again to >76 ug C l " 1 between days 65 and 75, then f e l l to 14 ug C l " 1 by day 79-. The other major bloom was that of l a r g e - c e l l e d f l a g e l l a t e s 2 l a t e r i n the experiment. Large f l a g e l l a t e s remained below 67 ug C l " 1 i n the s u r f a c e 8 m u n t i l day 55 and then r a p i d l y i n c r e a s e d to over 1000 ug C l " 1 by day- 72. T h i s bloom was dominated by Ceratium  f u s i s . The t o t a l biomass of other ( s m a l l - c e l l e d ) n o n - c o l o u r l e s s phytoplankton f l u c t u a t e d between 16 and 132 ug C l " 1 i n the upper 8 m except on day 55 when small diatoms peaked at 221 ug C 1 _ 1 . Below 8 m phytoplankton c o n c e n t r a t i o n s were low and p a s s i v e l y f o l l o w e d c y c l e s of abundance i n the s u r f a c e 8 m; l i t t l e 1 4 C f i x a t i o n o c c u r r e d at depth. N u t r i e n t s — A s the l a r g e diatom bloom peaked on day 25, NOr3~ (see footnote 3 below) and NOr2~ were each s t r i p p e d to <0.05 ug-atom N l " 1 , and NH r4 + to 0.2 ug-atom N l " 1 , i n the su r f a c e 8 m ( F i g . 4a). A f t e r day 36 d i s s o l v e d i n o r g a n i c 2 L a r g e - c e l l e d f l a g e l l a t e s i n c l u d e >15 um ESD d i n o f l a g e l l a t e s and the chrysophyte Distephanus speculum. 3A s p e c i a l nomenclature f o r s u b s c r i p t s i s used i n t h i s t h e s i s : "NOr3~ " r e f e r s to n i t r a t e , " S i(OH) r4" to o r t h o - s i l i c i c a c i d , "Prmax" to the maximum gross p h o t o s y n t h e t i c r a t e , e t c . 13 F i g u r e 2. Observed carbon biomass of phytoplankton i n the s u r f a c e 8 m of CEE2. Note d i f f e r e n t biomass s c a l e s . a) l a r g e - c e l l e d diatoms (>15 um diameter) b) s m a l l - c e l l e d diatoms (<15 um diameter) c) l a r g e - c e l l e d f l a g e l l a t e s (>15 um diameter) d) s m a l l - c e l l e d f l a g e l l a t e s (<15 um diameter) Small Diatoms (ug C 1"'; 0-8 m) 0 2 0 0 i—i . - j ' 1 1 ' 1 1 ! 1 1 1 CD Small F lage l la tes (ug C 0-8 m) a 2QQ i— i i i i i i i 18 Fi g u r e 3. Accumulative carbon biomass of the four phytoplankton groups p l o t t e d s e p a r a t e l y i n F i g . 2. 1 - l a r g e diatoms 2 - sm a l l diatoms 3 - l a r g e f l a g e l l a t e s 4 - small f l a g e l l a t e s Phytoplankton (ug C I" 1; 0-8 m) a i5ao CD— I 1 1 1 1 1 1 1 1 1 I C D 20 F i g u r e 4. Observed and i n t e r p o l a t e d c o n c e n t r a t i o n s of d i s s o l v e d i n o r g a n i c n i t r o g e n an s i l i c o n i n the s u r f a c e 8 m of CEE2. I n t e r p o l a t e d n u t r i e n t s ( s o l i d l i n e ) i n c l u d e the expected i n c r e a s e s from n u t r i e n t a d d i t i o n s to the s u r f a c e l a y e r . Observed c o n c e n t r a t i o n s are p l o t t e d as c i r c l e s . a) n i t r o g e n ( n i t r a t e + n i t r i t e + ammonium) b) s i l i c o n 23 n i t r o g e n ( n i t r a t e + n i t r i t e + ammonium) had i n c r e a s e d above 2 ug-atom N l " 1 and was f r e q u e n t l y >6 ug-atom N l " 1 . S i ( O H ) r4 f e l l below 4 ug-atom S i l " 1 o n l y on day 6 and exceeded 9 ug-atom S i l " 1 a f t e r day 20 ( F i g . 4b). P O r 4 3 _ d i d not drop below 1.8 ug-atom P 1'1 a f t e r day 8 and was u s u a l l y above 3 ug-atom P l " 1 i n the s u r f a c e 8 m of CEE2. N u t r i e n t c o n c e n t r a t i o n s were hig h below 8 m, ranging from 10.7 - 20.8 ug-atom n i t r a t e - N l ~ l , 0.2 - 13.7 ug-atom ammonium-N l " 1 , 1.0 4.8 ug-atom phosphate-P l - 1 , 23.7 - 39.2 ug-atom s i l i c a t e - S i l " 1 . N i t r i t e remained below 0.5 ug-atom N l " 1 i n the s u r f a c e and deep l a y e r s . Z o o p l a n k t o n — H e r b i v o r o u s copepods i n c r e a s e d to 107 ug C l " 1 by day 18 i n the upper 8 m of CEE2, but r a p i d l y f e l l to <28 ug C l " 1 a f t e r day 25 f o l l o w i n g a bloom of ctenophores ( F i g . 5). Ctenophores peaked i n c o n c e n t r a t i o n on day 27 at 7.2 ug C l " 1 and d e c l i n e d slowly t h e r e a f t e r ( F i g . 6). The peak c o n c e n t r a t i o n of chaetognaths o c c u r r e d on day 20 at 4.8 ug C l " 1 ( F i g . 6). As copepods d e c l i n e d in abundance, other h e r b i v o r e s — c i l i a t e s ( F i g . 7) and metazoan l a r v a e ( F i g . 8 ) — c o m p r i s e d a l a r g e r f r a c t i o n of the g r a z i n g community. Larvaceans ( F i g . 9) b r i e f l y i n c r e a s e d i n biomass at the s t a r t of the experiment, d e c l i n e d to low l e v e l s by day 15 and recovered again a f t e r day 40. C o l o u r l e s s f l a g e l l a t e s reached higher c o n c e n t r a t i o n s than herbivorous copepods, peaking at 133 ug C 1" 1 on day 9 ( F i g . 10). 24 F i g u r e 5. Accumulative carbon biomass of copepods observed i n the s u r f a c e 8 m of CEE2. 1 - h a r p a c t i c o i d s , non-feeding Nl and N2 n a u p l i i , and s p e c i e s c o n s i d e r e d to be c a r n i v o r o u s (see s e c t i o n 3.5.2) 2 - herbivorous c a l a n o i d s and c y c l o p o i d s 26 F i g u r e 6. Observed biomass and B o l i n o p s i s ( s o l i d elegans (dashed l i n e ) of the ctenophores P l e u r o b r a c h i a l i n e ) and the.chaetognath S a g i t t a in CEE2 (0-20 m depth). 28 F i g u r e 7. Observed biomass of c i l i a t e s ( e x c l u d i n g Mesodinium  rubrum) i n the su r f a c e 8 m of CEE2. 30 F i g u r e 8. Observed biomass of metazoan l a r v a e in the s u r f a c e 8 m of CEE2. Metazoan l a r v a e i n c l u d e gastropod v e l i g e r s , cyphonautes l a r v a e , trochophores, pelecypod and polychaete l a r v a e . 32 F i g u r e 9. Observed biomass of the larvacean Oikopleura d i o i c a i n CEE2 (0-20 m). Larvaceans (ug C 0-20 m) _1 I ' I I I I I I r o C D 34 F i g u r e 10. Observed biomass of c o l o u r l e s s f l a g e l l a t e s i n the s u r f a c e 8 m of CEE2. 36 B a c t e r i a — B a c t e r i a reached t h e i r h i g h e s t biomass in the s u r f a c e 8 m of CEE2 d u r i n g the p e r i o d of the large-diatom c o l l a p s e : 72 ug C l " 1 on day 44 ( F i g . 11). A s u b s i d i a r y peak of b a c t e r i a occurred on day 18 (while l a r g e diatoms were i n c r e a s i n g s u b s t a n t i a l l y i n c o n c e n t r a t i o n ) at 51 ug C l ~ l . 37 F i g u r e 11. Observed b a c t e r i a l biomass i n the s u r f a c e 8 m of CEE2. 39 3. THE COMPONENTS OF THE SYSTEM 3.1 Overview of the Model Four groups of phytoplankton i n CEE2 were modelled: l a r g e -and s m a l l - c e l l e d diatoms, and l a r g e - and s m a l l - c e l l e d f l a g e l l a t e s . The growth of each phytoplankton group over s u c c e s s i v e day-night c y c l e s was modelled by t a k i n g a f i x e d maximum gross p h o t o s y n t h e t i c r a t e , then reducing i t to the extent that whichever f a c t o r — n i t r o g e n , s i l i c o n or l i g h t — w a s l i m i t i n g . (The a l t e r n a t i v e f o r m u l a t i o n i n which gross p h o t o s y n t h e s i s i s l i m i t e d by some s o r t of m u l t i p l i c a t i v e i n t e r a c t i o n among n i t r o g e n , s i l i c o n and l i g h t was not considered.) Phosphorus was assumed to be n o n - l i m i t i n g , and the e f f e c t of changing temperature on growth was ignored. Net p h o t o s y n t h e s i s was determined by s u b t r a c t i n g a constant r e s p i r a t i o n r a t e from gross p h o t o s y n t h e s i s . T h e r e a f t e r , a constant f r a c t i o n of net p r o d u c t i o n was exuded as d i s s o l v e d o r g a n i c m a t e r i a l , and the phytoplankton biomass was f u r t h e r grazed by zooplankton or l o s t by s i n k i n g . To c l o s e the model the p o p u l a t i o n dynamics of zooplankton were not e x p l i c i t l y s i m u l a t e d . Instead, zooplankton numbers were i n t e r p o l a t e d between t h e i r observed d e n s i t i e s ; t h e i r e x c r e t i o n of i n o r g a n i c and o r g a n i c n i t r o g e n was h e l d at a f i x e d r a t e per i n d i v i d u a l . G r a z i n g r a t e per copepod was a f u n c t i o n of phytoplankton c o n c e n t r a t i o n , but other zooplankton f i l t e r e d water at a constant r a t e per i n d i v i d u a l . Zooplankton grazed only phytoplankton and b a c t e r i a ; p a r t i c u l a t e d e t r i t u s was not 40 modelled and t h e r e f o r e was not grazed. B a c t e r i a were assumed to absorb a l l d i s s o l v e d organic n i t r o g e n r e l e a s e d i n t o the water, e i t h e r from phytoplankton .exudation or from zooplankton e x c r e t i o n . B a c t e r i a l r e s p i r a t i o n was modelled as the sum of a b a s a l metabolic r a t e and a f i x e d f r a c t i o n of organic uptake. Inorganic n i t r o g e n r e c y c l e d back to the water column was p r o p o r t i o n a l to b a c t e r i a l + zooplankton r e s p i r a t i o n . N i t r a t e , n i t r i t e and ammonium were not d i s t i n g u i s h e d in the model but were lumped together as d i s s o l v e d i n o r g a n i c n i t r o g e n . N itrogen was removed from the water column in p r o p o r t i o n to the gross primary p r o d u c t i o n of each a l g a l group, whereas s i l i c o n was removed only by diatoms. The p e r i o d i c a d d i t i o n s of n i t r o g e n and s i l i c o n t o the s u r f a c e of CEE2 were accounted f o r . Even i g n o r i n g n u t r i e n t a d d i t i o n s , n i t r o g e n was not conserved in the model because zooplankton e x c r e t i o n was not l i n k e d to zooplankton i n g e s t i o n . Once absorbed by phytoplankton, s i l i c o n was u s u a l l y c o n s i d e r e d to be l o s t from the system, but i n some model runs the d i s s o l u t i o n of s i l i c a was s i m u l a t e d . Because CEE2 was bubbled to 8.5 m depth, and n u t r i e n t s were added to the top 8 m only, a d i s c o n t i n u i t y i n n u t r i e n t c o n c e n t r a t i o n s was o f t e n observed at 8 m. For t h i s reason, and a l s o to take i n t o account the g r e a t l y reduced i r r a d i a n c e below 8 m, two depth l a y e r s were modelled: 0 - 8 m, and 8 - 20 m. A l l system components were fo l l o w e d s e p a r a t e l y i n each l a y e r . The two l a y e r s were l i n k e d by mixing a c r o s s the 8 m i n t e r f a c e , as 41 w e l l as by s i n k i n g of phytoplankton from the top to bottom l a y e r . A l s o , phytoplankton i n the s u r f a c e 8 m shaded c e l l s i n the bottom l a y e r . M a t e r i a l which s e t t l e d below 20 m was assumed to be l o s t from the system. 3.2 L i g h t 1 3.2.1 Surface i r r a d i a n c e Quantum s c a l a r i r r a d i a n c e (400 - 700 nm wavelength; Booth 1976) on land 3 km from the CEPEX s i t e was i n t e g r a t e d over 1-hour i n t e r v a l s d u r i n g the experiment. Two s e r i o u s problems were apparent i n the Foodweb I da t a : 1) maximum rep o r t e d i r r a d i a n c e s were about double the maximum expected f o r the l a t i t u d e of the CEPEX s i t e , and 2) i r r a d i a n c e s were t r u n c a t e d at an upper r e p o r t e d l i m i t of 7.75»10 2 0 quanta cm" 2 h " 1 . On c l e a r days t h i s r e s u l t e d i n a f l a t - t o p p e d i r r a d i a n c e v_s. time of day curve. To p a r t i a l l y salvage the data, i t was decided to ob t a i n an independent estimate of peak i r r a d i a n c e at s o l a r noon at P a t r i c i a Bay, then reduce a l l rep o r t e d i r r a d i a n c e s by a constant f a c t o r i n order to match the tr u n c a t e d maximum to the expected maximum. An independent estimate of peak i r r a d i a n c e was obtained x I t i s suggested that on f i r s t r eading the reader s k i p the remainder of s e c t i o n 3; su b s e c t i o n s which f o l l o w can be c o n s u l t e d l a t e r f o r d e t a i l s . 42 from hour l y r a d i a t i o n recorded f o r Departure Bay, Nanaimo (49° 13' N, 123° 57' W) and the U n i v e r s i t y of B r i t i s h Columbia, Vancouver (49° 15' N, 123° 15' W) (Environment Canada 1978a). The l a r g e s t s i n g l e amount of t o t a l s o l a r r a d i a t i o n r e c e i v e d i n a 1-hour p e r i o d i n each of the months of J u l y , August and September 1978 was e x t r a c t e d f o r each s t a t i o n , y i e l d i n g s i x values f o r peak i r r a d i a n c e . These values were then c o r r e c t e d by c o n s i d e r i n g the change i n the sun's e l e v a t i o n i n the sky at l o c a l apparent noon with change i n date and l a t i t u d e . For the purpose of t h i s c o r r e c t i o n i t was assumed that a l l the energy r e c e i v e d was d i r e c t . The peak i r r a d i a n c e on a h o r i z o n t a l s u r f a c e at a l a t i t u d e of L degrees then v a r i e s as the s i n e of 90° - L - D, where D i s the d e c l i n a t i o n of the sun f o r 0 hours Ephemeris Time. The sun's d e c l i n a t i o n i s t a b u l a t e d i n the A stronomical Ephemeris (1978). The mean of the peak i r r a d i a n c e s c o r r e c t e d to the CEPEX s i t e , J u l y 9, 1978, was 3.368 MJ n r 2 h~1. Using the approximate c o n v e r s i o n from t o t a l energy to p h o t o s y n t h e t i c a l l y a c t i v e quanta given in J i t t s et a l . (1976) t h i s i s 1827 uEin m~2 s " 1 . T h i s value was then reduced by 7% to roughly account f o r r e f l e c t i o n of l i g h t a t the sea s u r f a c e ( H o j e r s l e v 1978: p. 140). Thus, 1699 uEin n r 2 s- - 1 was the peak p h o t o s y n t h e t i c a l l y a v a i l a b l e i r r a d i a n c e expected on a c l e a r day on J u l y 9 immediately below the water's s u r f a c e . Because the c u t o f f of r e p o r t e d i r r a d i a n c e s o c c u r r e d at 3575 uEin n r 2 s " 1 , a s c a l i n g f a c t o r of 1699 / 3575 = 0.475 was a p p l i e d to the Foodweb I l i g h t data. A f u r t h e r c o r r e c t i o n was a p p l i e d on a day by day b a s i s to mimic the sun's i n c r e a s i n g d e c l i n a t i o n with 43 season. By day 80 (September 26), peak i r r a d i a n c e would be expected to be only 72% of that on day 1 ( J u l y 9). Between days 3 and 54, when the l i g h t - s c r e e n was i n p l a c e over CEE2, i r r a d i a n c e was reduced by 40%. 3.2.2 E x t i n c t i o n of l i g h t w i t h i n the water column The a t t e n u a t i o n of l i g h t through the water column was represented by the f o r m u l a t i o n of P i a t t et a l . (1977: p. 819): I ( z ) = I r 0 exp{-[k rw z + k r s (B (X ) dx]} (1) where I ( z ) = i r r a d i a n c e at depth z, I r 0 = i r r a d i a n c e immediately below the sea s u r f a c e , k [-w = a t t e n u a t i o n c o e f f i c i e n t of water in the absence of c h l o r o p h y l l ( u n i t s : nr 1 ) , k r s = a t t e n u a t i o n c o e f f i c i e n t per u n i t . of c h l o r o p h y l l - a ( u n i t s : m2 (mg C h l - a ) " 1 ) , B(x) = c o n c e n t r a t i o n of c h l o r o p h y l l - a at depth x ( u n i t s : mg Chl-a n r 3 ) . The term JB(x) dx i s r e f e r r e d to by P i a t t et a l . (1977) as "cumulative phytoplankton cover." A p p r o p r i a t e values of krw and k r s were found by f i r s t m a n i p u l a t i n g Eq. 1 to give In [I (z r l ) / I (z r2) ] / (z r2 - z r l ) = krw + k r s |B(x) dx, (2) where z r l and z r2 are two depths, z r 2 > z r l , with z being p o s i t i v e downward. Eq. 2 d e s c r i b e s the a t t e n u a t i o n of l i g h t per metre through a depth i n t e r v a l from z r l to z r2 metres. R e l a t i v e i r r a d i a n c e was measured at 0, 1, 3, 5, 7, 10, 15 44 and 20 m i n the water column, whereas c h l o r o p h y l l - a and phytoplankton carbon were determined as i n t e g r a l s over the depth i n t e r v a l s 0-4, 4-8, 8-12, 12-16 and 16-20 m. These i n t e g r a t e d phytoplankton c o n c e n t r a t i o n s correspond to the i n t e g r a l i n Eq. 2, with z r l and z r2 the endpo.ints of each depth i n t e r v a l . U n f o r t u n a t e l y these endpoints were not the same depths at which i r r a d i a n c e was measured. I r r a d i a n c e was t h e r e f o r e i n t e r p o l a t e d to 4, 8, 12 and 16 m by f i t t i n g a c u b i c polynomial through the i r r a d i a n c e s measured at the four depths nearest to the i n t e r p o l a t i o n depth. Thus, f o r every depth i n t e r v a l on each day f o r which data were a v a i l a b l e , the l e f t hand s i d e of Eq. 2 was p l o t t e d on the Y-axis y_s. the i n t e g r a t e d phytoplankton c o n c e n t r a t i o n on the X - a x i s . I f the a t t e n u a t i o n of l i g h t i n CEE2 f o l l o w e d Eq. 1 e x a c t l y , the p l o t would be a s t r a i g h t l i n e with Y - i n t e r c e p t of krw and slope k r s . F i g . 12 shows the s c a t t e r using phytoplankton carbon; c h l o r o p h y l l - a was no b e t t e r . The geometric mean estimate of the f u n c t i o n a l r e g r e s s i o n of Y on X (R i c k e r 1973) gave a b e s t - f i t t i n g l i n e with krw = 0.168 nr 1 and k r s = 0.0207 m2 (mg C h l - a ) " 1 using c h l o r o p h y l l - a to measure phytoplankton c o n c e n t r a t i o n . The d i f f u s e a t t e n u a t i o n c o e f f i c i e n t of CEE2 water i n the absence of c h l o r o p h y l l , 0.168 n r 1, was c o n s i d e r a b l y higher than what i s t y p i c a l of c l e a r ocean water (e.g. Sargasso Sea: krw = 0.027 n r 1; Smith and Baker 1978). The s p e c i f i c a t t e n u a t i o n by c h l o r o p h y l l , 0.021 m2 (mg C h l - a ) " 1 , was s l i g h t l y l a r g e r than the value of 0.016 ± 0.003 f o r c h l o r o p h y l l - l i k e pigments ( i n c l u d i n g 45 F i g u r e 12. A t t e n u a t i o n c o e f f i c i e n t of p h o t o s y n t h e t i c a l l y a c t i v e quanta (Y a x i s ) vs. phytoplankton carbon i n CEE2 (X a x i s ) . F i t t e d l i n e i s geometric mean r e g r e s s i o n . Equation i s : a t t e n u a t i o n (rrr 1) = 0.148 + 5.11 • 10" 4 • phytoplankton carbon (mg C m" 3). 47 phaeopigments) from v a r i o u s oceanic and c o a s t a l waters (Smith and Baker 1978). T h i s d i s c r e p a n c y was probably due to the presence of p a r t i c u l a t e o rganic m a t e r i a l which c o v a r i e d with c h l o r o p h y l l . That the values of krw and k r s d e v i a t e d i n the expected d i r e c t i o n from Smith and Baker's val u e s i n c r e a s e s c o n f i d e n c e i n analogous estimates u s i n g phytoplankton carbon. Since carbon, r a t h e r than c h l o r o p h y l l , was used to q u a n t i f y phytoplankton abundance in the s i m u l a t i o n model, i t was these l a t t e r e stimates that were used to p r e d i c t l i g h t e x t i n c t i o n i n CEE2. They were: k rw = 0.148 i r r 1 , and k r s = 5.11 • 10 _ 4 m2 (mg C ) " 1 ( F i g . 12). _3._3 Mixing Two l a y e r s were modelled i n the CEE2 water column: an upper bubbled l a y e r from 0 to 8 m, and a lower l a y e r from 8 to 20 m. Within each l a y e r , c o n c e n t r a t i o n s of phytoplankton, n u t r i e n t s , and b a c t e r i a were assumed to be homogeneously d i s t r i b u t e d . I t was t h e r e f o r e necessary to c o n s i d e r only the mixing of m a t e r i a l a c r o s s the 8 m i n t e r f a c e . 2 Mixing r a t e s were i n d i r e c t l y estimated by the r a t e of v e r t i c a l spreading of p u l s e s of t r i t i u m i n t r o d u c e d at 8 m i n CEEs 2, 3 and 4 (Mercier and Farmer 1980). To d e s c r i b e the spreading of t r i t i u m , Mercier and Farmer used the one-dimensional d i f f u s i o n equation f o r a c o n s e r v a t i v e t r a c e r of c o n c e n t r a t i o n C, 2 M i x i n g between the 8-20 m l a y e r and sediments i n the bottom of CEE2 was much slower and was ignored i n the model. 48 Ic/at = d/qz (A(z) • oC/bz), (3) to f i t an assumed v e r t i c a l p r o f i l e of the apparent v e r t i c a l eddy d i f f u s i v i t y , A ( z ) , to the o b s e r v a t i o n s . An adequate f i t to the observed spread of t r a c e r i n CEE 3 and 4 was obtained by M e r c i e r (pers. comm.) when A(z) = 0.092 exp(-0.11 z ) , (4) where z i s depth i n metres and A i s apparent eddy d i f f u s i v i t y i n cm 2 s " 1 . Mercier and Farmer (1980) suggest that Eq. 4 overestimates d i f f u s i v i t y by at l e a s t 60%. In any event, mixing was much more r a p i d at the su r f a c e of the unbubbled CEEs than below 10 m where M e r c i e r and Farmer estimated that d i f f u s i v i t y was only one or two orders of magnitude gre a t e r than the molecular l i m i t . M e r c i e r (pers. comm.) f e l t that below the depth of bubbling there was no reason to b e l i e v e eddy d i f f u s i v i t y i n CEE2 to be any d i f f e r e n t than that i n CEE 3 or 4. 3 How f a s t was CEE2 mixed by bubbling? M e r c i e r (pers. comm.) found that a pulse of t r i t i u m i n j e c t e d at 8 m was completely mixed throughout the bubbled l a y e r between 12 and 48 hours 3 T h i s i s why d i f f u s i o n of regenerated n u t r i e n t s from the sediments i n t o the o v e r l y i n g water was ne g l e c t e d . Although ammonium c o n c e n t r a t i o n was s l i g h t l y higher i n the 16-20 m sampling i n t e r v a l than i n shallower depths, the c o n c e n t r a t i o n g r a d i e n t above 16 m was not steep enough to allow sediment-derived n u t r i e n t s to pene t r a t e the 8 m i n t e r f a c e . Accumulation of ammonium i n the 8-20 m l a y e r was s l i g h t l y under es,t imated. 49 l a t e r . To determine a p p r o p r i a t e v a l u e s of A(z) i n the s u r f a c e l a y e r , Eq. 3 was n u m e r i c a l l y s o l v e d using Crank-Nicholson f i n i t e d i f f e r e n c i n g (Carnahan et a l . 1969: pp. 440-442, 451) for a water column 20 m deep. A time-step of 0.05 day and depth increment of 0.4 m gave s t a b l e and smooth time behavior. Numerical runs were s t a r t e d with a pulse of c o n s e r v a t i v e substance at 8 m, and by t r i a l and e r r o r a depth p r o f i l e of A(z) was found which r e s u l t e d i n an e s s e n t i a l l y homogeneous d i s t r i b u t i o n of the d i f f u s i n g substance i n the top 8 m a f t e r 24 hours of simulated time. Having developed a f i n e - g r i d model of mixing i n CEE2, the expected r a t e of exchange between the 0-8 m and 8-20 m l a y e r s i n the main model was f i n a l l y parameterized. T h i s was done by i n i t i a l i z i n g the f i n e - g r i d model with v a r i o u s depth p r o f i l e s of d i f f u s i n g substance, then f o l l o w i n g the f l u x a c r o s s the 8 m i n t e r f a c e as a f u n c t i o n of the d i f f e r e n c e i n average c o n c e n t r a t i o n between 0-8 and 8-20 m. T h i s d i f f e r e n c e in average c o n c e n t r a t i o n can be viewed as the c o n c e n t r a t i o n g r a d i e n t down which turbulence moves suspended and d i s s o l v e d substances between l a y e r s i n the main two-layer model. Flux a c r o s s the 8 m i n t e r f a c e was e r r a t i c d u r i n g the f i r s t 5 days of simulated time because of the s t r o n g l y d i s c o n t i n u o u s c o n c e n t r a t i o n p r o f i l e s used to s t a r t the runs. Ignoring t h i s i n i t i a l 5-day r e l a x a t i o n p e r i o d , f l u x was very n e a r l y a l i n e a r f u n c t i o n of c o n c e n t r a t i o n g r a d i e n t , and exchange between the top and bottom l a y e r s was approximately given by 50 r)C rtop/dt = - ( C r t o p - C rbottom) • b / D rtop, (5) dC rbottom/dt = ( C r t o p - C rbottom) • b / D (-bottom, where C = c o n c e n t r a t i o n ( a r b i t r a r y u n i t s ) , D = t h i c k n e s s of l a y e r (m), b = exchange c o e f f i c i e n t (m h " 1 ) . b was found to l i e i n the range of 0.005 to 0.010 m h " 1 , r e g a r d l e s s of the s t a r t i n g c o n c e n t r a t i o n p r o f i l e . Since eddy d i f f u s i v i t y was l i k e l y overestimated, the value of b = 0.005 m h " 1 was used i n main model runs. Thus, given t h a t D r t o p = 8 m, i f the c o n c e n t r a t i o n of a substance i n the top l a y e r was, say, double that i n the bottom l a y e r , the c o n c e n t r a t i o n i n the top l a y e r would i n i t i a l l y f a l l a t a r a t e of only 0.8% per day. Bubbling the s u r f a c e l a y e r e v i d e n t l y had l i t t l e e f f e c t on the exchange of m a t e r i a l between 0-8 and 8-20 m i n CEE2. J3.4 Phytoplankton 3_. 4 .1_ Species and c e l l s i z e s To t r y to understand why the l a r g e - c e l l e d diatom Stephanopyxis t u r r i s and l a r g e - c e l l e d f l a g e l l a t e Ceratium f u s i s bloomed to huge c o n c e n t r a t i o n s i n CEE2, whereas s m a l l - c e l l e d phytoplankton d i d not, four groups of phytoplankton were modelled: l a r g e - c e l l e d (>15 um e q u i v a l e n t s p h e r i c a l diameter (>15 um ESD) ) and s m a l l - c e l l e d (<15 um ESD) diatoms arid l a r g e -and s m a l l - c e l l e d f l a g e l l a t e s . C o l o u r l e s s f l a g e l l a t e s - were t r e a t e d as a s u b c l a s s of "zooplankton" and are d i s c u s s e d l a t e r . 51 Diatom spores or r e s t i n g c y s t s of d i n o f l a g e l l a t e s never exceeded 25 ug C l " 1 c o n c e n t r a t i o n i n e i t h e r the top or bottom l a y e r s of CEE2, and thus were not modelled. Mesodinium rubrum, a p h o t o s y n t h e t i c c i l i a t e (Smith and Barber 1979), had a c o n c e n t r a t i o n <<15 ug C l ~ l i n CEE2, and i t too was ignored i n the main model. Standing stock of phytoplankton was expressed as ug C l " 1 . Appendix 1 give s the sp e c i e s composition of each group as w e l l as the c e l l diameter, c e l l biomass, and average abundance of each s p e c i e s i n CEE2. Weighted a c c o r d i n g to average species abundance, the mean c e l l diameter / c e l l biomass of each phytoplankton group was: l a r g e diatoms 51.4 um/2127 pg C, small diatoms 10.6 um/50.7 pg C, l a r g e f l a g e l l a t e s 36.3 um/2283 pg C, and s m a l l f l a g e l l a t e s 8.0 um/66.4 pg C. 3^.4.2 Chemical composition of phytoplankton I t was necessary to c o n s i d e r the n i t r o g e n (N) and s i l i c o n ( S i ) composition of phytoplankton when c a l c u l a t i n g a l g a l uptake of n u t r i e n t s . S t r i c t l y speaking, the C/N and C/Si r a t i o s employed i n the model r e f e r to newly s y n t h e s i z e d a l g a l biomass. N i t r o g e n — V i g o r o u s l y growing phytoplankton with excess n i t r a t e i n the water would be expected t o have C/N mass r a t i o s as low as 3, while unhealthy c e l l s i n n i t r a t e - d e p l e t e d water c o u l d have r a t i o s as high as 15 (Parsons et a l . 1977: p. 52). C/N was not determined f o r phytoplankton i n CEE2, but the C and 52 N content of p a r t i c u l a t e s r e t a i n e d on 984H Reeve Angel g l a s s f i b r e f i l t e r s was measured. Phytoplankton carbon as estimated from c e l l volume c o n s t i t u t e d a s i z e a b l e p r o p o r t i o n of p a r t i c u l a t e organic carbon (POC) on numerous oc c a s i o n s ( F i g . 13). The C/N r a t i o of p a r t i c u l a t e s (POC/PON) was assumed to be r e p r e s e n t a t i v e of the C/N r a t i o of phytoplankton. The mean C/N mass r a t i o i n CEE2 was 5.69 (range 1.37 - 8.13, F i g . 14), c l o s e to the value of 6 suggested by S t r i c k l a n d (1960: p. 12) as r e p r e s e n t a t i v e of phytoplankton. Very high or very low C/N r a t i o s were o c c a s i o n a l l y observed, but these bore no c o n s i s t e n t r e l a t i o n to the amount of i n o r g a n i c N present i n the water. On the b a s i s of these o b s e r v a t i o n s , the C/N mass r a t i o of phytoplankton biomass was modelled as a constant value of 6. . S i l i c o n — P a r t i c u l a t e s i l i c o n was not measured, so recourse was made to l i t e r a t u r e e stimates of C/Si r a t i o s of diatoms. (Because s i l i c o f l a g e l l a t e s were absent i n CEE2, f l a g e l l a t e s were assumed to be devoid of S i . ) V i g o r o u s l y growing diatoms have C/Si mass r a t i o s ranging from 0.85 to 4.2 (P. J . H a r r i s o n et a l . 1977; Paasche 1980; Parsons et a l . 1961). S t r i c k l a n d (1960: p. 19) concludes that the C/Si r a t i o of n a t u r a l marine diatoms i s probably near to 1.25 i n areas where S i does not l i m i t growth. T h i s was the value chosen f o r l a r g e and small diatoms i n the standard model run s i n c e S i c o n c e n t r a t i o n i n the top 8 m of CEE2 seldom dropped below 4 ug-atom l " 1 . However, a v a r i a b l e C/Si r a t i o was adopted on some l a t e r runs a f t e r 53 F i g u r e 13. Observed phytoplankton carbon ( i n c l u d i n g c o l o u r l e s s f l a g e l l a t e s ) vs. p a r t i c u l a t e organic carbon i n CEE2. S t r a i g h t l i n e i n d i c a t e s e q u a l i t y of phytoplankton carbon and POC. 54 55 F i g u r e 14. P a r t i c u l a t e organic carbon and n i t r o g e n i n CEE2. S t r a i g h t l i n e i n d i c a t e s C/N mass r a t i o of 6. POC (UG / L) 0 - 150 300 450 600 750 900 1050 ]200 1350 1500 ° I I I I I I I I I i i i i I I I I I I i I 57 n o t i c i n g that simulated S i c o n c e n t r a t i o n s were very low. P. J . H a r r i s o n et a l . (1977) c i t e s e v e r a l r e f e r e n c e s which demonstrate that n u t r i e n t uptake r a t i o s or c e l l u l a r chemical composition can be a l t e r e d by c e l l s i z e , age of c u l t u r e , l i g h t i n t e n s i t y , temperature and type of n u t r i e n t l i m i t a t i o n . Only the impact of n u t r i e n t l i m i t a t i o n was t r e a t e d here. P. J . H a r r i s o n et a l . (1977) found t h a t S i - l i m i t e d or s t a r v e d c u l t u r e s had C/Si r a t i o s 1.3 to 6.4 times higher than f o r n o n - l i m i t e d diatoms. These r e s u l t s suggested the f o l l o w i n g scheme: C/Si (by mass) i n c r e a s e d l i n e a r l y from 1.25 with no S i - l i m i t a t i o n to 5 • 1.25 = 6.25 at complete l i m i t a t i o n . The degree of l i m i t a t i o n was d e f i n e d by the f r a c t i o n [ S i ] / ( K r S i + [ S i ] ) , where [ S i ] i s the ambient S i c o n c e n t r a t i o n and K r S i i s the h a l f - s a t u r a t i o n constant of gross p h o t o s y n t h e s i s of the p a r t i c u l a r diatom group (see s e c t i o n 3.4.6). 3_.4.3 Photosynthesis The growth of phytoplankton was modelled by f i r s t assuming fo r each group a f i x e d maximum gross p h o t o s y n t h e t i c r a t e , Prmax ( u n i t s : h ~ 1 ) , s p e c i f i c t o phytoplankton carbon. T h i s maximum ra t e was then reduced to a r e a l i z e d r a t e , P r g , by the degree of l i g h t or n u t r i e n t l i m i t a t i o n , whichever was most severe. The f r a c t i o n a l l i m i t a t i o n of gross p h o t o s y n t h e s i s due to l i g h t was expressed by Smith's (1936) e q u a t i o n : P rg/P rmax = I • (I r k 2 + 1 2) L-0. 5 (see footnote 4 ) , (6) 58 where I i s the p h o t o s y n t h e t i c a l l y a c t i v e quantum i r r a d i a n c e and I r-k ( T a i l i n g 1957) i s the i r r a d i a n c e at which P rg = 2 L-0.5 • Prmax = 0.707 P rmax. Smith's equation does not provide f o r b r i g h t - l i g h t i n h i b i t i o n of p h o t o s y n t h e s i s . To summarize the d i s c u s s i o n which f o l l o w s , a P vs. I curve was c o n s t r u c t e d from Foodweb I data to see i f the lack of p h o t o i n h i b i t i o n was reasonable, and rough values of I r k were a l s o e x t r a c t e d . C e l l - s i z e - d e p e n d e n t estimates of Prmax f o r diatoms and f l a g e l l a t e s were d e r i v e d from the l i t e r a t u r e . F i n a l l y , Eq. 6 was i n t e g r a t e d over depth to pr o v i d e an estimate of average l i g h t - l i m i t a t i o n i n the model's two depth l a y e r s . P h otosynthesis vs. i r r a d i a n c e — A P vs. I curve f o r phytoplankton was d e r i v e d from 1 4 C f i x a t i o n , i r r a d i a n c e , and phytoplankton carbon measured i n Foodweb I. To i n c r e a s e the sample s i z e , data from both CEE 2 and 3 were analyzed. The raw data c o n s i s t e d of; 1) 1 4 C f i x a t i o n r a t e s i n u n f i l t e r e d water removed from 0-4, 4-8, 8-16 m depth and incubated _in s i t u from lOOOh - 1400h PST at 2, 6 and 12 m depth; 2) quantum i r r a d i a n c e r e l a t i v e to sea su r f a c e i r r a d i a n c e measured at 0, 1, 3, 5, 7, 10, 15 and 20 m depth; 3) c o r r e c t e d quantum i r r a d i a n c e immediately below the sea su r f a c e ( c f . s e c t i o n 3.2.1); 4A s p e c i a l nomenclature i s used i n t h i s t h e s i s f o r exponents which cannot be expressed as sums of whole numbers: "Y L2" denotes Y 2, "Wi—0.5" denotes the r e c i p r o c a l of the square root of W, "(A + B ) L c " denotes the sum A + B r a i s e d to the power c, e t c . 59 4) phytoplankton carbon i n 0-4, 4-8, 8-16 m depth; 5) ambient water temperature and n u t r i e n t c o n c e n t r a t i o n s . P h o t o s y n t h e s i s was expressed as a s p e c i f i c r a t e by d i v i d i n g 1 4 C f i x a t i o n (ug C l " 1 h" 1) by s t a n d i n g crop of phytoplankton (ug C . I"1). S p e c i f i c p h o t o s y n t h e s i s was then normalized to 15 °C using the temperature c o r r e c t i o n of Eppley (1972, Q r10 of 1.88). The e f f e c t of p o t e n t i a l n u t r i e n t l i m i t a t i o n of p h o t o s y n t h e s i s was removed by r e j e c t i n g samples having <4 ug-atom n i t r a t e - N l " 1 . M u l t i p l y i n g the average s u r f a c e i r r a d i a n c e d u r i n g the 4-hour i n c u b a t i o n by the r e l a t i v e subsurface i r r a d i a n c e s gave a depth p r o f i l e of a b s o l u t e i r r a d i a n c e . I r r a d i a n c e at the depth of i n c u b a t i o n was i n t e r p o l a t e d from the depth p r o f i l e of i r r a d i a n c e u sing a c u b i c polynomial f i t t e d through the i r r a d i a n c e s at the 4 nearest depths. The r e s u l t i n g p l o t s of s p e c i f i c 1 4 C f i x a t i o n vs. i r r a d i a n c e are given i n F i g . 15. No c l e a r P vs. I r e l a t i o n s h i p was evident i n the data from e i t h e r CEE 2 or 3 due to the l a r g e s c a t t e r of p o i n t s . (The s c a t t e r was j u s t as bad when using c h l o r o p h y l l - a r a t h e r than carbon as a measure of standing crop.) There i s some suggestion that p h o t o s y n t h e s i s i n CEE3 s a t u r a t e d somewhere between 100 and 200 uEin n r 2 s" 1 . B r i g h t - l i g h t i n h i b i t i o n was not c o n s i s t e n t l y p r esent, and t h e r e f o r e the absence of p h o t i n h i b i t i o n in Eq. 6 was c o n s i d e r e d a c c e p t a b l e . I_ rk—A value of I rk equal to 200 uEin m"2 s~1 was a r b i t r a r i l y chosen f o r a l l phytoplankton groups. Although 60 F i g u r e 15. S p e c i f i c carbon f i x a t i o n r a t e of u n f i l t e r e d water vs. average p h o t o s y n t h e t i c a l l y a c t i v e i r r a d i a n c e d u r i n g 4-hour midday i n c u b a t i o n s . Incubations i n water with <4 ug-atom n i t r a t e - N l " 1 were r e j e c t e d to e l i m i n a t e p o t e n t i a l l y n i t r o g e n l i m i t e d p o p u l a t i o n s . 1 4 C f i x a t i o n r a t e s normalized to 15 °C using temperature c o r r e c t i o n of Eppley (1972). c i r c l e s - diatoms comprise >80% of t o t a l p h o t o s y n t h e t i c biomass t r i a n g l e s - f l a g e l l a t e s comprise >80% of biomass c r o s s e s - n e i t h e r diatoms nor f l a g e l l a t e s comprise >80% of biomass 61 PRODUCTIVITY (G C / G C / H) 0.0 0.1 0.2 0.3 0.4 0.5 ° J , J I I I I I I I I I G o m o TO -A o m O x X ro cog o G O o m m CO PRODUCTIVITY (G C / . G C / H) 0.0 0.1 0.2 0.3 0.4 0.5 ° A 1 1 1 1 1 1 1 L I I 3 g . D o o -z. r> m ' n TO o CO —I m x x o m m 62 Ryther (1956) observed marked d i f f e r e n c e s between diatoms and d i n o f l a g e l l a t e s i n the i r r a d i a n c e which s a t u r a t e d p h o t o s y n t h e s i s , n e i t h e r Dunstan (1973) nor Chan (1978a) observed any d i f f e r e n c e between the two a l g a l groups. I r k v a r i e d s e a s o n a l l y i n the model ( c f . Hameedi 1977) i n step with peak i r r a d i a n c e ( s e c t i o n 3.2.1), so that I r k d e c l i n e d from 200 uEin n r 2 s " 1 on day 1 to 144 uEin n r 2 s " 1 by day 80. In an attempt to model a d a p t a t i o n to dim l i g h t , I rk was f u r t h e r assumed to decrease with i n c r e a s i n g depth i n the water column. F o l l o w i n g Jamart et a l . (1977), I rk might be expected to decrease e x p o n e n t i a l l y with depth -to h a l f i t s s u r f a c e value at the depth of 1% s u r f a c e l i g h t , p r o v i d e d that the water column was unmixed. Because the time r e q u i r e d to completely mix the s u r f a c e 8 m of CEE2 (1/2 to 2 days) i s comparable to a d a p t a t i o n times f o r I r k (2 to 6 days; Steeman N i e l s e n et a l . 1962; Steeman N i e l s e n and Park 1964), I rk would be expected to be constant with depth in the bubbled l a y e r of CEE2. As a compromise, I r k was assumed to be constant i n the surface 8 m but equal to the depth-average of the e x p o n e n t i a l l y d e c r e a s i n g value i t would of had had the s u r f a c e l a y e r been unmixed ( F i g . 16). I f we l e t k r s u r f = a t t e n u a t i o n c o e f f i c i e n t of l i g h t i n the s u r f a c e 8 m (nr 1 ) , k rdeep = a t t e n u a t i o n c o e f f i c i e n t of l i g h t i n 8-20 m l a y e r (nr 1) , z r.01 = depth of 1% s u r f a c e l i g h t (m), I r k s u r f = value of I r k i n the surface 8 m of CEE2, ure 16. H y p o t h e t i c a l depth p r o f i l e of I r k i n CEE2. Curved d o t t e d l i n e shows depth behavior of I r k i n the absence of mixing. With mixing, I r k i n the bubbled, l a y e r i s uniform and equals the depth-average of the e x p o n e n t i a l decay curve i n that l a y e r . 64 'k L bubbled layer depth of 1% surface l i g h t 20 J 6 5 l r k ( 0 ) = value of I rk expected at 0 m depth in the absence of mixing, then z r.01 = A -ln(.01)/k rsurf, k rsurf > -ln(.01)/8 8 - [ln(.Ol) + 8k rsurf]/k rdeep, k rsurf < -ln( .OD/8. (7) Furthermore, I r k s u r f l r k ( 0 ) • [1 - exp(-8d) ]/8d, z r.01 > 8 l r k ( 0 ) • {[1 - exp(-z r.01 d ) ] / d - 0.5z r.01 + 4}/8, z.i-.Ol < 8, (8) where d = I n ( 0 . 5 ) / z r . 0 1 . Below the 8 m s u r f a c e l a y e r , I r k ( z ) l r k ( 0 ) exp(-d z ) , z < z r.01 0.5 I r k ( 0 ) , z > z r.01. (9) To i l l u s t r a t e how I r k was modelled, suppose t h a t on August 8 (day 31) the model p r e d i c t e d a moderate bloom of phytoplankton, say, 500 ug C l " 1 ( a l l four a l g a l groups combined) in both the top and bottom l a y e r s of CEE2. By day 31, l r k ( 0 ) would have s e a s o n a l l y d e c l i n e d to 188 uEin m; 2 s" 1 from a value of 200 uEin n r 2 s " 1 on day 1. From s e c t i o n 3.2.2, k r s u r f = k rdeep = 0.148 + 5.11 •10' 4 • 500 = 0.404 n r 1. The 1% l i g h t depth, Z|-.01, equals 11.4 m. The value of I rk i n the bubbled s u r f a c e l a y e r , I r k s u r f , i s 149 uEin n r 2 s " 1 . Immediately below the bubbled l a y e r , at 8 m depth, I r k would equal 116 uEin n r 2 s~ 1 . P r m a x — M i n d f u l of the u n r e l i a b l e estimates 1 4 C f i x a t i o n 66 may g i v e of primary p r o d u c t i o n (e.g. Gieskes et a l . 1979), i t was decided to o b t a i n independent estimates of P rmax from Chan (1978b). Chan measured the growth r a t e of f i v e diatom and four d i n o f l a g e l l a t e s p e c i e s under continuous and 12h:12h l i g h t : d a r k c y c l e s at 21 °C. Chan's Table 13 (p. 57 of Chan 1978b) p r o v i d e s r e g r e s s i o n equations of maximum growth rate as a f u n c t i o n of c e l l p r o t e i n of the form G ( d i v i s i o n s d a y - 1 ) = a WLb (see footnote 4 on p. 58), ( 1 0 ) where W i s ng p r o t e i n per c e l l . Taking the average of 'a' measured under continuous l i g h t and 12L:12D, and the average of 'b', then c o r r e c t i n g growth to 13.5 °C (the mean water temperature i n ' CEE2) using Eppley (1972) and e x p r e s s i n g i t as s p e c i f i c growth per day, g i v e s diatoms: G ( d a y 1 ) = 0.8815 WL- . 070 , (11a) d i n o f l a g e l l a t e s : G ( d a y 1 ) = 0.3390 W--. 135. ( l i b ) Chan (1978b, h i s Table 14, p. 69) found p r o t e i n / c a r b o n r a t i o s ranging from 0.73 to 0.97 f o r 3 s p e c i e s of diatoms, and from 0.78 to 1.04 f o r 3 s p e c i e s of d i n o f l a g e l l a t e s . Parsons et a l . (1961) determined r a t i o s of 0.88 to 1.38 f o r 4 diatom s p e c i e s and 0.69, 0.70 f o r 2 d i n o f l a g e l l a t e s p e c i e s . Assuming a p r o t e i n / c a r b o n r a t i o of 0.85, Eqs. 11a,b were converted to diatoms: G ( d a y 1 ) = 1.446 W (pg C) L-.070, (12a) d i n o f l a g e l l a t e s : G ( d a y 1 ) = 0.880 W (pg C) L - . 1 3 5 . (12b) Eq. 12b was assumed to a l s o r e p r e s e n t f l a g e l l a t e s other than d i n o f l a g e l l a t e s . D e r i v i n g maximum gross p h o t o s y n t h e t i c r a t e s from maximum 67 growth r a t e s s p e c i f i e d by Eq. 12 was d i f f i c u l t because r e s p i r a t o r y and exudatory l o s s e s had to be allowed f o r , as w e l l as d i u r n a l l y v a r y i n g i r r a d i a n c e . The d e r i v a t i o n which was employed hinged upon the assumption that the growth of a s u r f a c e - d w e l l i n g phytoplankter ( i n the absence of any growth l i m i t a t i o n other than l i g h t ) d u r i n g a c y c l e of n a t u r a l l y v a r y i n g s u n l i g h t on a c l e a r summer day w i l l have the same average r a t e of growth as maximal r a t e s r e p o r t e d by Chan f o r phytoplankton c u l t u r e d i n continuous l i g h t or a 12L:12D c y c l e . T h i s assumption i s supported by Paasche (1967), who found that C o c c o l i t h u s h u x l e y i c u l t u r e d i n a regime of 16L:8D grew as f a s t as c o n t i n u o u s l y i l l u m i n a t e d c e l l s r e g a r d l e s s of the i r r a d i a n c e . In a regime of 10L:14D growth was about 70% of that under continuous l i g h t . S i m i l a r r e s u l t s are given by Paasche (1968) f o r D i tylum b r i q h t w e l l i i and N i t z s c h i a t u r g i d u l a . (In c o n t r a s t , Tamiya et a l . (1955) found that growth of C h l o r e l l a e l l i p s o i d e a was almost p r o p o r t i o n a l to the d u r a t i o n of the photoperiod.) Since maximal growth would be expected of phytoplankton at 0 m depth in the absence of mixing (assuming excess n u t r i e n t s and no p h o t o i n h i b i t i o n ) , Eq. 6 was used as i s without c o n s i d e r i n g a complicated d e p t h - i n t e g r a l . The s u r f a c e i r r a d i a n c e , I, on a c l e a r day i n J u l y at P a t r i c i a Bay i s c l o s e l y d e s c r i b e d by 68 I = 0,. t < 12-D/2 or t > 12+D/2 I rmax cos[TT(t - 12)/d], 12-D/2 < t < 12+D/2, where t i s time of day (h), D i s daylength (14.9 h ) , and I rmax i s the maximum i r r a d i a n c e at l o c a l noon (1699 uEin n r 2 s" 1 ). R e s p i r a t i o n was assumed to be 10% of maximum instantaneous gross p h o t o s y n t h e s i s (10% of P rmax) and exudation was set to 10% of l i g h t - l i m i t e d gross p h o t o s y n t h e s i s (10% of P r g ) . The net rat e of growth over a 24-hour l i g h t - d a r k c y c l e i s then G (day" 1) = (gross p r o d u c t i o n - exudation) - r e s p i r a t i o n X(Pr9 " 0 , 1 P r 9 ) d t " 0 , 1 p r m a x - < 1 4) But, from Eq. 6, P rg = Prmax I ( I r k 2 + 1 2) L-0.5, and I v a r i e s as given i n Eq. 13. Eq. 14 can be rearranged to P rmax = G • {0.9 [I • ( I r k 2 + I 2 ) L - 0 . 5 ] dt - 0.1}- x . (15) The i n t e g r a l i n Eq. 15 was ev a l u a t e d n u m e r i c a l l y using Simpson's r u l e (Carnahan et a l . 1969: pp. 71-79). The m u l t i p l i e r of G i n Eq. 15, equal to 2.401 with I r k = 200 uEin n r 2 s " 1 , was a p p l i e d to the r i g h t hand s i d e of Eqs. 12a,b to y i e l d e x p r e s s i o n s of the maximum instantaneous gross p h o t o s y n t h e t i c r a t e of diatoms and f l a g e l l a t e s : diatoms: Prmax (day" 1) = 3.471 W (pg C) L-.070 d i n o f l a g e l l a t e s : Prmax (day" 1) = 2.114 W (pg C) L-.135. (16) L a s t l y , to use Eqs. 16a,b to estimate Prmax of the four phytoplankton groups, some measure of average c e l l biomass of 69 each group was needed. The average for a l g a l group j was c a l c u l a t e d as B r j = JZ(xri W ri4>) / I x ri, j = 1 to 4, ' i (17) where x r i = mean observed c o n c e n t r a t i o n (ug C l " 1 ) of s p e c i e s i belonging to a l g a l group j i n CEE2 (Appendix 1), W ri = carbon biomass of one c e l l of s p e c i e s i (pg C; Appendix 1), b = -0.070 f o r diatoms, -0.135 f o r f l a g e l l a t e s . From Eq. 16, Prmax of a l g a l group j was 3.471 B r j f o r diatoms, 2.114 B r j f o r f l a g e l l a t e s . Thus, Prmax = 2.086 day" 1 = 0.0869 h " 1 , l a r g e diatoms = 2.648 day" 1 = 0.1104 h r 1 , small diatoms = 0.748 day" 1 = 0.0312 h r 1 , l a r g e f l a g e l l a t e s = 1.336 day" 1 = 0.0556 t r 1 , small f l a g e l l a t e s . (18) I t i s s u p r i s i n g that these r a t e s are lower than the maximum s p e c i f i c 1*C f i x a t i o n r a t e s observed i n CEE 2'and 3 ( F i g . 15). E i t h e r the d e r i v e d r a t e s are u n r e a l i s t i c a l l y low, or the r a t e s of s p e c i f i c 1 4 C f i x a t i o n are u n r e a l i s t i c a l l y h i gh. T h i s q u e s t i o n w i l l be d i s c u s s e d l a t e r i n s e c t i o n 5.3.1. Notwithstanding t h i s ambiguity, Prmax values d e r i v e d from Chan's work were used in the model. Depth i n t e g r a l of p h o t o s y n t h e s i s — A t each time step, the f r a c t i o n a l l i g h t - l i m i t a t i o n of gross photosynthesis i n the s u r f a c e and bottom l a y e r s was c a l c u l a t e d . The f r a c t i o n a l l i m i t a t i o n , P rg/P rmax, was given by the d e p t h - i n t e g r a l of 70 Eq. 6, t a k i n g i n t o account the v a r i a t i o n of I r k with depth. In a l a y e r of water z r l metres deep at i t s upper boundary and z r 2 metres deep at i t s lower boundary, the mean P rg/P rmax i s P rg/P rmax = x / ( l + x 2 ) L 0 . 5 , k = d or z r l = z r2 ln{[x + (1 + x 2 ) L 0 . 5 ] / [ b + (1 + b 2 ) L 0 . 5 ] } • [ z r 2 - z r l ) ( k - d ) ] " 1 , k*d, z r l * z r 2 , (19) where d = r a t e of decay of I r k with depth as d e f i n e d f o r Eq. 8. In a bubbled l a y e r , d = 0, x = I ( z r l ) / l r k ( z r l ) , k = a t t e n u a t i o n c o e f f i c i e n t of l i g h t i n the l a y e r , b = x • exp[(d - k ) ( z r 2 - z r l ) ] . For the bubbled l a y e r of CEE2, I r k = I r k s u r f (Eq. 8). The mean P rg/P rmax i s thus a f u n c t i o n of d and k w i t h i n a l a y e r bounded at z r l and z r2 metres. That i s , Prg/Prmax = F(z r l , z r2,k,d), (20) where F i s the f u n c t i o n d e f i n e d by Eq. 19. Adopting t h i s n o t a t i o n , i n the s u r f a c e 8 m of CEE2, P rg/P rmax = F ( 0 , 8 , k r s u r f , 0 ) . In the 8-20 m l a y e r P rg/P rmax = F(8,20,k rdeep,0) , z r.01 < 8 m [ ( z r . 0 1 - 8) • F(8,z r.01,k rdeep,d) + (20-z r.01) • F(z r.01,20,k rdeep,0)]/12, 8 < z r. 01 < 20 m F(8,20,k rdeep,d) , z r.01 > 20 m, (21) 71 where z r.01 i s the depth of 1% su r f a c e l i g h t (Eq. 7). 2.4.4 Phytoplankton r e s p i r a t i o n Banse (1976) c o u l d f i n d no s i g n i f i c a n t r e l a t i o n s h i p between r e s p i r a t i o n expressed as a f r a c t i o n of growth and the s i z e of an a l g a l c e l l . R e s p i r a t i o n was t h e r e f o r e modelled as a constant percentage of maximal gross p h o t o s y n t h e s i s f o r a l l c e l l s i z e s . P h o t o r e s p i r a t i o n ( T o l b e r t 1974) was ignored. There i s c o n s i d e r a b l e u n c e r t a i n t y r e g a r d i n g d i f f e r e n c e s between the r a t e of r e s p i r a t i o n of diatoms and f l a g e l l a t e s . Parsons et a l . (1977: p. 70) s t a t e that when p h y t o f l a g e l l a t e s are abundant the r a t i o of r e s p i r a t i o n (R) to Prmax should change from n l 0 % 5 ( t y p i c a l of diatom assemblages) because high r e s p i r a t i o n r a t e s of 35 to 60% of Prmax were observed i n s e v e r a l f l a g e l l a t e s c u l t u r e d by Moshkina (1961). Other workers, however, re p o r t no systematic d i f f e r e n c e s between the r e s p i r a t i o n r a t e s of diatoms and f l a g e l l a t e s . Humphrey (1975, h i s T able I, p. 115) c i t e s R/Prmax r a t i o s ranging from 6.9 - 48% f o r diatoms and 5.5 - 67% f o r f l a g e l l a t e s . Laws and Caperon (1976) found that the dark r e s p i r a t i o n r a t e of Monochrysis l u t h e r i c u l t u r e d at i t s maximum r a t e of growth was 10.5% of gross carbon p r o d u c t i o n . P i a t t and Jassby (1976) c i t e R/Pr-max r a t i o s f o r c u l t u r e d diatoms ranging from 8 - 100%, and fo r c u l t u r e d f l a g e l l a t e s 10% and 37%. M c A l l i s t e r et a l . (1964) 5The symbol "a" denotes "approximately". 72 found r a t i o s of 1.5 - 20% f o r two c u l t u r e d f l a g e l l a t e s p e c i e s , and r a t i o s of 9 - 16% f o r Skeletonema costatum. Turning to n a t u r a l plankton assemblages, measurements of whole-community r e s p i r a t i o n as a f r a c t i o n of maximum 1 4 C uptake ( P i a t t and Jassby 1976; Devol and Packard 1978; Steeman N i e l s e n and Hansen 1959) or as a f r a c t i o n of gross p h o t o s y n t h e s i s i n the euphotic zone over a d i e l c y c l e (Packard 1979; S e t c h e l l and Packard 1979) g i v e r a t i o s which g e n e r a l l y l i e i n the range of 5 - 25%. A value of 10% of Prmax was chosen here as the r e s p i r a t i o n r a t e of s u r f a c e - d w e l l i n g phytoplankton of a l l groups, day and n i g h t . F o l l o w i n g Winter et a l . (1975) and Jamart et a l . (1977), r e s p i r a t i o n was made a f u n c t i o n of depth i n a manner analogous to that f o r I r k (see Eqs. 8, 9), with one e x c e p t i o n : r e s p i r a t i o n d i d not change s e a s o n a l l y . 3_.4.5 Phytoplankton exudation In g e n e r a l , the r e l e a s e of s o l u b l e o r g a n i c s from a c t i v e l y growing phytoplankton amounts to 10 - 20% or l e s s of the t o t a l carbon f i x e d (p. 117 of Parsons et a l . 1977; p. 147 of Wangersky 1978), although c o n t r o v e r s y surrounds t h i s issue (Sharp 1977; Fogg 1977). In the model, a l l phytoplankton groups ex c r e t e d 10% of t h e i r gross primary p r o d u c t i o n as organic matter. Thus, when primary p r o d u c t i o n was s e v e r e l y l i m i t e d by d e p l e t e d n u t r i e n t s , modelled exudation was very low as w e l l . At n i g h t , exudation was zero because gross p h o t o s y n t h e s i s was zer o . T h i s scheme of m o d e l l i n g a l g a l exudation i s a d m i t t e d l y 73 s u p e r f i c i a l . I t ignores the a c c e l e r a t e d r e l e a s e of o r g a n i c s expected of p o p u l a t i o n s i n s t a t i o n a r y growth ( G u i l l a r d and Wangersky 1958; Marker 1965). A l s o , any environmental c o n d i t i o n which i n h i b i t s c e l l m u l t i p l i c a t i o n but permits p h o t o a s s i m i l a t i o n to continue g e n e r a l l y r e s u l t s i n high exudation r a t e s ( H e l l e b u s t 1974). •3.4.6 N u t r i e n t l i m i t a t i o n of phytoplankton growth Recent l i t e r a t u r e has emphasized that the growth of n u t r i e n t - l i m i t e d phytoplankton i n an unsteady environment i s dependent upon i n t e r n a l n u t r i e n t p o ols r a t h e r than c o n c e n t r a t i o n s of n u t r i e n t s e x t e r n a l to the c e l l (e.g. Burmaster 1979; Davis et a l . 1978; Grenney et a l . 1973; Droop 1973). Furthermore, the parameters used to d e s c r i b e the Michaelis-Menten uptake of n u t r i e n t s — t h e s a t u r a t e d uptake rate and the h a l f - s a t u r a t i o n "constant"—may vary tremendously depending on the n u t r i e n t h i s t o r y of the c e l l s (Conway et a l . 1976; Conway and H a r r i s o n 1977). U n f o r t u n a t e l y , no estimates of c e l l u l a r n u t r i e n t quotas or of the changing p h y s i o l o g i c a l parameters of n u t r i e n t l i m i t a t i o n were obtained f o r the e n c l o s e d phytoplankton i n Foodweb I. I t i s not. even c l e a r that meaningful estimates c o u l d have been obtained f o r the d i v e r s e assemblages of s m a l l - c e l l e d phytoplankton. For these reasons, the p h o t o s y n t h e t i c f i x a t i o n of carbon and uptake of n u t r i e n t s were d i r e c t l y coupled i n the model, and the extent of n u t r i e n t l i m i t a t i o n by n i t r o g e n and s i l i c o n was d e s c r i b e d by simple Michaelis-Menten k i n e t i c s . 74 Under c e r t a i n c o n d i t i o n s ammonium i s taken up p r e f e r e n t i a l l y to n i t r a t e (Eppley et a l . 1969b; Packard and Blasco 1974). Takahashi et a l . (1980) found that m i c r o f l a g e l l a t e s i n Foodweb I p r e f e r e n t i a l l y absorbed ammonium whereas c e n t r i c diatoms showed no such p r e f e r e n c e . N i t r a t e and ammonium were not d i s t i n g u i s h e d i n the model because these n u t r i e n t s g e n e r a l l y c o v a r i e d i n the upper 8 m (except immediately a f t e r p e r i o d i c n i t r a t e a d d i t i o n s ) and because the added complexity of mo d e l l i n g the i n t e r c o n v e r s i o n of n i t r a t e and ammonium would have been f o r m i d a b l e . The f r a c t i o n a l l i m i t a t i o n of gross p h o t o s y n t h e s i s due to n i t r o g e n l i m i t a t i o n was given by P rg/P rmax = [N] / (K rN + [N]), (22) where K rN i s the h a l f - s a t u r a t i o n constant and [N] the n i t r o g e n c o n c e n t r a t i o n . S i m i l a r l y , f o r s i l i c a t e l i m i t a t i o n P rg/P rmax = [ S i ] / (K r S i + [ S i ]) . (23) Although there i s disagreement about the dependence of the h a l f - s a t u r a t i o n constant on c e l l s i z e (Parsons and Takahashi 1973, 1974; Hecky and Kilham 1974), S t e e l e and F r o s t (1977) was fo l l o w e d by making K rN (and K r S i ) l i n e a r l y p r o p o r t i o n a l to c e l l diameter. S t e e l e and F r o s t ' s minimum estimate of K rN was K j-N (ug-atom N l " 1 ) = 0.015 D, (24) where D i s c e l l diameter i n um. The s i z e dependence of S i l i m i t a t i o n was analogously d e f i n e d here as 75 K r S i (ug-atom S i l ' 1 ) = 0.04 D (um). (25) Using the mean e q u i v a l e n t s p h e r i c a l diameter of each phytoplankton group ( s e c t i o n 3.4.1) f o r D i n Eqs. 23 and 24 gi v e s K rN K r S i l a r g e diatoms: 0.77 2.06 ug-atom 1~1 small diatoms: 0.16 0.42 l a r g e f l a g e l l a t e s : 0.54 small f l a g e l l a t e s : 0.12 L i t e r a t u r e estimates of K r S i t y p i c a l l y range between 0.2 - 3.4 ug-atom S i 1 _ 1 (Nelson et a l . 1976; Thomas and Dodson 1975; Goering et a l . 1973; G u i l l a r d et a l . 1973; Paasche 1973a,b). The K rN valu e s adopted above are compatible with a K rN of 0.6 ug-atom n i t r a t e - N 0 l ' 1 measured by H a r r i s o n and Davis (pers. comm. c i t e d by Parsons et a l . 1978) f o r an assemblage of l a r g e diatoms from P a t r i c i a Bay. W. G. H a r r i s o n et a l . (.1977) found that phytoplankton i n c o p p e r - t r e a t e d CEPEX e n c l o s u r e s had average h a l f - s a t u r a t i o n constants of 1.1 and 1.2 ug-atom N 1'1 f o r n i t r a t e and ammonium a s s i m i l a t i o n , r e s p e c t i v e l y . In summary, the gross p h o t o s y n t h e t i c r a t e (Prg) ot each phytoplankton group as a f r a c t i o n of maximum gross p h o t o s y n t h e s i s (P rmax) was given by the minimum of Eqs. 20-23 at each time step. 76 3.4.7 Phytoplankton s i n k i n g Bienfang (1980) measured the s i n k i n g r a t e s of phytoplankton assemblages removed from CEE 2 and 3 d u r i n g Foodweb I. He r e p o r t s that the mean s i n k i n g r a t e of phytoplankton in undisturbed water ranged from 0.32 to 1.69 m d a y - 1 under a wide range of environmental c o n d i t i o n s . Bienfang notes that although s i n k i n g r a t e s were not s t a t i s t i c a l l y c o r r e l a t e d with measured n u t r i e n t s , water temperature, i r r a d i a n c e , c h l o r o p h y l l c o n c e n t r a t i o n • or primary p r o d u c t i v i t y , there were cases of higher s i n k i n g r a t e s i n p o p u l a t i o n s dominated by l a r g e phytoplankton c e l l s . The p r e c o n d i t i o n i n g h i s t o r y of the phytoplankton as i t a f f e c t e d s i n k i n g r a t e was ignored i n the model. The i n f l u e n c e of c e l l s i z e was c o n s i d e r e d , however. Bienfang notes that on day 6, p o p u l a t i o n s i n CEE3 were dominated by small diatoms ( n i l um ESD) that had s i n k i n g r a t e s of 0.4 to 0.7 m d a y - 1 . On day 16, enhanced c o n c e n t r a t i o n s of Thalassionema n i t z s c h i o d e s i n CEE2 (18 um c e l l u l a r ESD; C o u l t e r Counter peak at n60 um) had a s i n k i n g r a t e of 0.9 m d a y - 1 . Two days l a t e r t h i s predominance continued and p o p u l a t i o n s i n k i n g r a t e s had r i s e n to 1.7 m d a y - 1 . Bienfang a l s o determined that the s i n k i n g r a t e of the 8-53 um f r a c t i o n , dominated by the l a r g e f l a g e l l a t e Ceratium f u s i s , ranged from 1.1 t o 1.5 m d a y - 1 a f t e r day 60 i n CEE2. In the model, a constant s i n k i n g r a t e of 1.3, m d a y - 1 was a s s i g n e d to l a r g e diatoms and l a r g e f l a g e l l a t e s , and 0.5 m d a y - 1 to small diatoms and s m a l l f l a g e l l a t e s . These are s i n k i n g r a t e s expected i n an unmixed water column. The 77 e f f e c t i v e r a t e of s i n k i n g i n the s u r f a c e bubbled l a y e r of CEE2 would be l e s s . E f f e c t i v e s i n k i n g r a t e s i n the bubbled l a y e r were a r b i t r a r i l y assumed to be 25% of undisturbed r a t e s . U n f o r t u n a t e l y , observed changes i n the depth of the c h l o r o p h y l l or phytoplankton-carbon maximum i n CEE2 were i r r e g u l a r and p r o v i d e d no i n d i c a t i o n of the a c t u a l s i n k i n g r a t e s expected of p o p u l a t i o n s i n the s u r f a c e 8 m. Since only two l a y e r s were modelled, the s e t t l i n g of c e l l s through the water column c o u l d not be e x p l i c i t l y f o l l o w e d . I n s t e a d , the s i n k i n g of biomass from a l a y e r was parameterized by d i v i d i n g the s i n k i n g r a t e i n that l a y e r (m d a y - 1 ) by the t h i c k n e s s of the l a y e r i n metres, y i e l d i n g a s p e c i f i c r a t e of l o s s per day. 2-5 Zooplankton 2.5.1 Numbers, s p e c i e s , and body weight Zooplankton standing stock was not dynamically modelled. Instead, zooplankton numbers i n each of 134 taxa (Appendix 2) were l i n e a r l y i n t e r p o l a t e d between observed d e n s i t i e s . In the s p e c i a l case of larvaceans and c i l i a t e s where o b s e r v a t i o n s were a v a i l a b l e only u n t i l day 63 and 73, r e s p e c t i v e l y , numbers were simply h e l d f i x e d at the l a s t observed d e n s i t y u n t i l the end of the s i m u l a t i o n . Whenever d e t a i l e d depth d i s t r i b u t i o n s were a v a i l a b l e , zooplankton numbers i n the 0-8 and 8-20 m l a y e r s were s e p a r a t e l y i n t e r p o l a t e d . However, only the average p o p u l a t i o n d e n s i t y over 0-20 m depth was repo r t e d f o r copepod 78 n a u p l i i , l a r v a c eans, chaetognaths and ctenophores. For these taxa the same i n t e r p o l a t e d numbers were used i n both depth l a y e r s . The carbon biomass of i n d i v i d u a l z o oplankters was r e q u i r e d to determine the e x c r e t i o n r a t e and s i z e - s e l e c t i v e g r a z i n g r a t e of i n d i v i d u a l s i n each taxon. These body weights were obtained at the CEPEX s i t e or from the l i t e r a t u r e as d e t a i l e d i n Appendix 2. 3.5.2 Zooplankton g r a z i n g G r a z i n g on phytoplankton and b a c t e r i a was modelled by s p e c i f y i n g g r a z i n g r a t e s per i n d i v i d u a l f o r each zooplankton taxon, then m u l t i p l y i n g by the number of i n d i v i d u a l s of each taxon observed i n CEE2. D e t a i l e d g r a z i n g r a t e s of zooplankton were not a v a i l a b l e f o r Foodweb I, and thus a s y n t h e s i s of o f t e n c o n t r a d i c t o r y r e s u l t s from the l i t e r a t u r e was undertaken. I t was decided to make copepod g r a z i n g r a t e s dependent upon food c o n c e n t r a t i o n , s a t u r a t i n g at a maximum r a t i o n which was a f u n c t i o n of body weight. Other zooplankton groups., however, were assumed to f i l t e r water at a constant r a t e r e g a r d l e s s of food c o n c e n t r a t i o n . Copepod g r a z i n g — T w o q u e s t i o n s were asked of g r a z e r s : 1) What determines the amount of food ingested?, and 2) How was the i n g e s t e d food s e l e c t e d ? Addressing the f i r s t q u e s t i o n , a r e c t i l i n e a r model (F r o s t 1972) was used to d e s c r i b e i n g e s t i o n 79 as a f u n c t i o n of food c o n c e n t r a t i o n . That i s , the r a t e of i n g e s t i o n (ug C copepod" 1 h _ 1 ) i n c r e a s e d l i n e a r l y from zero i n g e s t i o n at a prey t h r e s h o l d c o n c e n t r a t i o n of P r t h r (ug C l " 1 ) , to maximum i n g e s t i o n at the c r i t i c a l food c o n c e n t r a t i o n of P r c r i t (ug C l " 1 ) . The e x i s t e n c e of a non-zero food c o n c e n t r a t i o n below which fee d i n g ceases i s u n s e t t l e d . Parsons et a l . (1969) found that mixed zooplankton i n water samples from the S t r a i t of Georgia began to graze at t h r e s h o l d phytoplankton c o n c e n t r a t i o n s of n50 - 90 ug C l " 1 . Adams and S t e e l e (1966) observed a P r t h r of n70 ug C l ' 1 f o r Calanus  f i n m a r c h i c u s i n water from the northern North Sea. In c o n t r a s t , P a f f e n h o f e r (1971) and Corner et a l . (1972) found that Calanus may feed' on l a r g e c e l l s at c o n c e n t r a t i o n s l e s s than 20 ug C l " 1 u s i n g c u l t u r e d algae as food r a t h e r than n a t u r a l plankton assemblages. The feed i n g r a t e of Calanus p a c i f i c u s on T h a l a s s i o s i r a f l u v i a t i l i s i s depressed at c o n c e n t r a t i o n s below 27 ug C l " 1 , but C. p a c i f i c u s i s able to graze down an a l g a l suspension to as low as 15 ug C l ' 1 ( F r o s t 1975). A feeding t h r e s h o l d of zero was used i n the model. The e s t i m a t i o n of a c r i t i c a l food c o n c e n t r a t i o n , P r c r i t , was e q u a l l y u n c e r t a i n . Conover (1978a) and Mayzaud and Poulet (1978) even a s s e r t that s a t u r a t i o n of g r a z i n g r a r e l y occurs i n nature because grazers normally a c c l i m a t e to the ambient c o n c e n t r a t i o n of food. Yet Gamble (1978) observed constant i n g e s t i o n r a t e s at c o n c e n t r a t i o n s of suspended p a r t i c u l a t e s above 300 ug C l " 1 f o r copepods captured from and incubated i n water samples removed from the North Sea. A major c o m p l i c a t i n g 80 f a c t o r i s that P r c r i t depends on the r a t i o of phytoplankter s i z e to copepod s i z e . For example, when feeding on the diatom T h a l a s s i o s i r a f l u v i a t i l i s , an a d u l t female Pseudocalanus achieves i t s maximal r a t i o n at about one t h i r d the c e l l d e n s i t y r e q u i r e d by the l a r g e r a d u l t female Calanus p a c i f i c u s (Frost 1974). The problem with t h i s c o m p l i c a t i o n i s that i t confounds e f f e c t s due to a changing r a t i o of copepod mass/phytoplankton s i z e with e f f e c t s due to the f o o d - s i z e s e l e c t i v i t y of' copepods. Moreover, the q u e s t i o n of what c r i t i c a l food c o n c e n t r a t i o n i s a p p r o p r i a t e when gr a z e r s are c o n f r o n t e d with a mixture of phytoplankton of v a r y i n g s i z e s i s unanswered. The approach taken here was to adopt the same c r i t i c a l c o n c e n t r a t i o n f o r any s i z e of copepod f e e d i n g on any s i z e of p h y t o p l a n k t e r . T h i s approach, combined with the s i z e - s e l e c t i v e f e e d i n g d e s c r i b e d l a t e r , mimicked F r o s t ' s (1974) r e s u l t s . F r o s t (1972) found P r c r i t v a l u e s ranging from 124 to 318 ug C 1 _ 1 f o r Calanus p a c i f i c u s f e e d i n g on d e c r e a s i n g s i z e s of c u l t u r e d diatoms. Parsons et a l . (1969) found that g r a z i n g by mixed zooplankton s a t u r a t e d at n400 ug C l " 1 i n a n a t u r a l assemblage of phytoplankton. Gamble's (1978) f i g u r e of 300 ug C 1 _ 1 mentioned e a r l i e r was used as the value of P r c r i t i n the model. Having chosen values of P r t h r and P r c r i t f o r the r e c t i l i n e a r model, only the maximal r a t i o n (R rmax, ug C co p e p o d - 1 d a y 1 ) remained. To gi v e some idea of the v a r i a b i l i t y of e s timates i n the l i t e r a t u r e , c o n s i d e r the maximum r a t i o n of ad u l t Pseudocalanus spp. having a body weight of nlO - 20 81 ug C. The w e i g h t - s p e c i f i c Rrmax (ug C (ug C ) " 1 day" 1) has been v a r i o u s l y determined as 0.17 (Poulet 1973), 0.45 (Parsons et a l . 1969) and 0.55 (Poulet 1974), while P a f f e n h o f e r and H a r r i s (1976) found an Rrmax of 1.5 d a y 1 f o r P. elongatus feeding on c u l t u r e d T h a l a s s i o s i r a r o t u l a . F r o s t ' s (1972) work was used as the b a s i s f o r the model estimate of Rrmax. F r o s t c a l c u l a t e d that unstarved Calanus p a c i f i c u s females with an average body weight of 68 ug C l " 1 had an Rrmax of 27 ug C cop e p o d - 1 day" 1 (= 0.39 d a y - 1 ) . The r e l a t i o n s h i p between body weight (W, ug C) and r a t i o n (R, ug C c o p e p o d - 1 day" 1) can be d e s c r i b e d as R = c WLd. (26) Sushchenya and Khmeleva (1967, c i t e d i n Conover 1978b) found an average exponent, d, of 0.80 f o r cr u s t a c e a n s . Conover (1978b), using data from P a f f e n h o f e r (1971), c a l c u l a t e d an exponent of 0.74 f o r j u v e n i l e Calanus h e l g o l a n d i c u s (= C. p a c i f i c u s ) . P u t t i n g d = 0.75, and assuming Rrmax = 0.4 day" 1 f o r a body weight of 68 ug C, gi v e s Rrmax ( U g C copepod" 1 day" 1) = 1.149 W-0.75. (27) Eq. 27 p r e d i c t s that an a d u l t female Pseudocalanus (w = 10.8 ug C) would have Rrmax = 0.63 day" 1, whereas the stage III n a u p l i u s • (W = 0.0432 ug C) would have Rrmax = 2.5 day" 1. T h i s represents the s a t u r a t e d r a t e of i n g e s t i o n of phytoplankton o_f optimal s i z e . T h i s leads to the 0 second q u e s t i o n asked of g r a z e r s : How was. the ingested food s e l e c t e d ? " S e l e c t i v e f e e d i n g " i s here d e f i n e d to mean any removal of 82 p a r t i c l e s i n d i f f e r e n t s i z e - c l a s s e s not i n p r o p o r t i o n to t h e i r r e l a t i v e abundance i n the water. Two b a s i c s e l e c t i o n " s t r a t e g i e s " have been i d e n t i f i e d : g r a z e r s can s e l e c t l a r g e p a r t i c l e s ( M u l l i n 1963; Richman and Rogers 1969; F r o s t 1977) or p a r t i c l e s i n s i n g l e or m u l t i p l e peaks of biomass c o n c e n t r a t i o n i n the p a r t i c l e - s i z e spectrum (Poulet 1973, 1974; Donaghay and Small 1979a). The s i z e spectrum may a l s o be a l t e r e d by breaking long c h a i n s of phytoplankton i n t o s m a l l e r , more manageable p i e c e s (Donaghay and Small 1979b). T h i s extreme f l e x i b i l i t y of b e h a v i o r a l response was not modelled. Instead, a s e l e c t i v i t y f u n c t i o n was " f r o z e n " at a shape c h a r a c t e r i s t i c of each copepod body weight. That i s , a copepod of a given weight was assumed to be unable to d y n a m i c a l l y a l t e r i t s s e l e c t i v i t y f o r d i f f e r e n t phytoplankton s i z e s i n response to changing s i z e s p e c t r a . The s e l e c t i v i t y f u n c t i o n chosen here was that of Steele, and F r o s t (1977: pp. 502-503 and t h e i r F i g . 1 4 ( b ) ) : S re = exp{-[ ln(WL-l/3 D / D rm)] 2 / u} , (28) where S re = s e l e c t i v i t y , a d i m e n s i o n l e s s f r a c t i o n , (0 < S re < 1), D = e q u i v a l e n t s p h e r i c a l diameter of food p a r t i c l e (um), Drm = p a r t i c l e diameter (um) at which S re = 1 f o r a copepod of 1 ug C weight, W = weight of copepod (ug C), 83 u = measure of spread of s e l e c t i o n curve. Eq. 28 "can be i n t e r p r e t e d as assuming t h a t , f o r any animal, the a b i l i t y to c a t c h c e l l s i s log-normally d i s t r i b u t e d so that a c e l l with h a l f the optimal diameter has the same s e l e c t i v i t y as one with double the optimum diameter," ( S t e e l e and F r o s t 1977). S t e e l e and F r o s t put Drm = 10 um and u = 2.5. These values p r e d i c t more of an o v e r l a p between the s e l e c t i v i t y curves of A c a r t i a c l a u s i a d u l t and copepodite I stages than was estimated by N i v a l and N i v a l (1976) from i n t e r - s e t u l e d i s t a n c e s on the m a x i l l a e . The f i t was b e t t e r when u = 1. Values of Drm = 10 um and u = 1 were used i n f u r t h e r c a l c u l a t i o n s . Eq. 28 p r e d i c t s the s e l e c t i v i t y of a copepod f o r c e l l s of a c e r t a i n diameter. In the model i t was necessary to d e f i n e the s e l e c t i v i t y of a copepod f o r each of four phytoplankton assemblages. ( I t was assumed that copepods c o u l d not f i l t e r b a c t e r i a . ) Although the c e l l diameters of i n d i v i d u a l s p e c i e s comprising each group were measured (Appendix 1), g r a z e r s presumably r e a c t e d to the l e n g t h of c h a i n s of c e l l s r a ther than c e l l diameters. Chain lengths i n Foodweb I were u n a v a i l a b l e , however, and i n any case these l e n g t h s would vary g r e a t l y with time. Consequently, only c e l l diameters were used to d e r i v e composite s e l e c t i v i t i e s of g r a z i n g on the four a l g a l groups. T h i s was done f o r each copepod taxon by computing Sre f o r each phytoplankton s p e c i e s comprising each a l g a l group, then weighting the S re v a l u e s a c c o r d i n g to the observed mean abundance of a l g a l s p e c i e s i n CEE2 (Appendix 1 ) . Four composite s e l e c t i v i t i e s of g r a z i n g on the four a l g a l groups were obtained 84 i n t h i s way f o r each type of copepod; these s e l e c t i v i t i e s are l i s t e d i n Appendix 2. Some copepod taxa have no g r a z i n g s e l e c t i v i t i e s l i s t e d i n Appendix 2. N a u p l i a r stages I and II do not feed ( M a r s h a l l and Orr 1956; p. 112 of C o r k e t t and McLaren 1978). Tortanus  discaudatus i s c a r n i v o r o u s (Anraku and Omori 1963; Amber and F r o s t 1974; M u l l i n 1979) and was not i n c l u d e d as a phytoplankton g r a z e r . The copepodite V and a d u l t stages of E p i l a b i d o c e r a were assumed to be c a r n i v o r o u s i n analogy to Labidocera (Landry 1978). Although most copepods are omnivorous (e.g. A c a rt i a , Centropages (Anraku and Omori 1963; Lonsdale et a l . 1979), Oithona ( P e t i p a et a l . 1970; Lampitt 1978)), they were c o n s i d e r e d to graze p l a n t food j u s t as e f f i c i e n t l y as the more herbivorous Pseudocalanus or Calanus. F i n a l l y , to combine i n g e s t i o n and s e l e c t i v i t y i n t o a coherent model of zooplankton g r a z i n g , the approach of Sonntag and Greve (1977: p. 2300) was used. For a given copepod, the c o n c e n t r a t i o n of each of the four phytoplankton groups was f i r s t m u l t i p l i e d by that copepod's r e s p e c t i v e g r a z i n g s e l e c t i v i t i e s on the four groups. T h i s gave four " r e a l i z e d " prey c o n c e n t r a t i o n s which were then summed. T h i s sum determined the o v e r a l l i n g e s t i o n r a t e of the copepod as given by i t s r e c t i l i n e a r i n g e s t i o n vs. ( r e a l i z e d ) food c o n c e n t r a t i o n curve. L a s t l y , t h i s t o t a l i n g e s t i o n was s p l i t among the four a l g a l groups i n p r o p o r t i o n to t h e i r r e a l i z e d c o n c e n t r a t i o n s . An example may c l a r i f y t h i n g s . Consider Paracalanus copepodite I f e e d i n g on a mixture of l a r g e and small diatoms and l a r g e and 85 small f l a g e l l a t e s , with each group having a c o n c e n t r a t i o n of 100 ug C l " 1 . T h i s copepod's r e s p e c t i v e feeding s e l e c t i v i t i e s on the four groups i s 0.038, 0.632, 0.027 and 0.771 (Appendix 2). The r e a l i z e d a l g a l c o n c e n t r a t i o n s are t h e r e f o r e 3.8, 63.2, 2.7 and 77.1 ug C l " 1 ; t h e i r sum i s 146.8 ug C l " 1 . The copepod's body weight i s 0.152 ug C, so i t s maximum r a t i o n i s 1.149 • (0.152)1-0.75 = 0.280 ug C day" 1. The a c t u a l r a t i o n i s 1 4 6 . 8 / P r c r i t • 0.280 ug C day" 1 = 0.137 ug C day" 1, s i n c e P r c r i t = 300 ug C l " 1 . Of t h i s r a t i o n , 3.8/146.8 • 100 = 2.6% came from l a r g e diatoms, 43.1% from small diatoms, and 1.8 and 52.5% from l a r g e and s m a l l f l a g e l l a t e s . T h i s example a l s o p o i n t s out that the s e l e c t i v i t i e s of very small copepods were low on a l l phytoplankton groups. The most extreme example of t h i s i s Oithona n a u p l i u s III with an estimated body weight of 14.3 ng C. I t ' s h i g h e s t s e l e c t i v i t y was 0.375 on small f l a g e l l a t e s . T h i s was a consequence of using the s e l e c t i v i t y f u n c t i o n of Eq. 28 which p r e d i c t e d an optimum food diameter of 2.4 um, smaller than the s m a l l e s t m i c r o f l a g e l l a t e enumerated. These low s e l e c t i v i t i e s c o u l d have c o n c e i v a b l y r e s u l t e d i n the underestimation of copepod g r a z i n g p r e s s u r e . To t e s t t h i s , a s i m u l a t i o n was run with s e l e c t i v i t i e s s c a l e d upward so that the " p r e f e r r e d " food of each copepod taxon had a s e l e c t i v i t y of u n i t y . I t was found that s c a l i n g had n e g l i g i b l e e f f e c t , so s e l e c t i v i t i e s were l e f t unchanged i n other runs. C i l i a t e g r a z i n g — T h e rate of g r a z i n g by c i l i a t e s on phytoplankton and b a c t e r i a was modelled as a constant volume of 86 water swept c l e a r , i n s p i t e of evidence of s a t u r a t i o n of gr a z i n g at high food c o n c e n t r a t i o n s (Fenchel 1980a; Heinbokel 1978a,b). Maximum re p o r t e d f i l t e r i n g r a t e s were used r a t h e r than s a t u r a t i o n r a t e s to see which zooplankton groups might have had s i g n i f i c a n t impact on phytoplankton dynamics i n CEE2. The p u b l i s h e d r e s u l t s most r e l e v a n t to what would be expected of c i l i a t e s i n the n a t u r a l environment are those of Heinbokel (1978a,b), S p i t t l e r (1973) and Blackbourn (1974), a l l studying the g r a z i n g of t i n t i n n i d s on m i c r o f l a g e l l a t e s , corn s t a r c h or yea s t . Maximal c l e a r a n c e r a t e s per c i l i a t e were 2 - 4.7 u l h" 1 (Heinbokel 1978a), n8 u l h " 1 (Heinbokel 1978b), 0.5 - 8.5 u l h" 1 ( S p i t t l e r 1973) and 4.2 u l h " 1 (Blackbourn 1974). Data on c i l i a t e abundance i n CEE2 c o n s i s t e d of numbers of c i l i a t e s which fed predominantly on p a r t i c l e s of <3, 3 - 15, and >15 um diameter, and c i l i a t e s of unassigned f e e d i n g p r e f e r e n c e ( J . Heinbokel, p e r s . comm.). A c l e a r a n c e r a t e of 4 u l c i l i a t e " 1 h " 1 , midrange of the maximum r a t e s c i t e d above, was assig n e d to those c i l i a t e s feeding predominantly on p a r t i c l e s of 3 - 15 um diameter. In the model these c i l i a t e s fed e x c l u s i v e l y on small diatoms and small f l a g e l l a t e s with a s e l e c t i v i t y of u n i t y . The biomass of algae grazed per c i l i a t e per u n i t time was simply the product of c l e a r a n c e r a t e ( l i t r e s c i l i a t e - 1 h " 1 ) , a l g a l c o n c e n t r a t i o n (ug C l " 1 ) , and s e l e c t i v i t y ( d i m e n s i o n l e s s ) . For those c i l i a t e s having a feeding p r e f e r e n c e f o r p a r t i c l e s >15 um diameter, an upper s i z e - l i m i t of grazed 87 p a r t i c l e s was s p e c i f i e d so that t h e i r f e e d i n g s e l e c t i v i t y on l a r g e diatoms and f l a g e l l a t e s c o u l d be estimated. Blackbourn (1974, h i s F i g . 5, p. 54) observed that the maximum food s i z e of the l a r g e s t t i n t i n n i d captured was 42 um ESD. The data of Beers and Stewart (1970) and Taniguchi (1977) suggest that 42 um i s extreme. An upper l i m i t of 25 um was used here. The mean biomass of 15 - 25 um phytoplankton as a p r o p o r t i o n of l a r g e diatom and l a r g e f l a g e l l a t e biomass was 0.134 and 0.019, r e s p e c t i v e l y , i n CEE2. The g r a z i n g s e l e c t i v i t y of c i l i a t e s p r e f e r r i n g >15 um p a r t i c l e s was t h e r e f o r e taken as 0.134 on l a r g e diatoms and 0.019 on l a r g e f l a g e l l a t e s . The g r a z i n g s e l e c t i v i t y was set to zero on groups with <15 um ESD, i . e . small diatoms, small f l a g e l l a t e s and b a c t e r i a . A r e p r e s e n t a t i v e g r a z i n g r a t e of c i l i a t e s on b a c t e r i a was d i f f i c u l t to estimate because p u b l i s h e d r e s e a r c h has t y p i c a l l y used c u l t u r e d r a t h e r than n a t u r a l b a c t e r i a as prey. T h i s c o u l d be a s e r i o u s source of e r r o r s i n c e the average volume of marine b a c t e r i a i s l e s s than 10% of the volume of c u l t u r e d b a c t e r i a (Watson 1978). Some p u b l i s h e d c l e a r a n c e r a t e s are l i s t e d i n Table I. Even i g n o r i n g the c r i t i c i s m of b a c t e r i a l c e l l s i z e , the a p p l i c a b i l i t y of most of these c l e a r a n c e r a t e s to the n a t u r a l marine environment i s dubious. C i l i a t e s , when c u l t u r e d , were maintained by Cox (1967) and Curds and Cockburn (1968) i n conc e n t r a t e d proteose-peptone s o l u t i o n s . C i l i a t e d e n s i t i e s i n fee d i n g experiments were as high as 1.6 • 10' c i l i a t e s l " 1 (Berk et a l . 1976) and b a c t e r i a l d e n s i t i e s as high as 1.2 • 1 0 1 2 l " 1 (Laybourn and Stewart 1975). Clearance r a t e s on Table I. L i t e r a t u r e estimates of the volume of water swept clear by c i l i a t e s grazing b a c t e r i a or other small p a r t i c l e s . ., . . clearance rate , c i l i a t e species prey reference (n l h X) Colpidium campylum Moraxella sp. 42 Laybourn & Stewart 1975 Colpoda s t e i n i i E s c h e r i c h i a c o l i 45 Proper & Garver 1966 Epidinium spp. B a c i l l u s megasterium 0.06-7 Coleman & Laurie 1974 Tetrahymena pyriformis carbon p a r t i c l e s 0.60 Rasmussen et a l . 1975 Tetrahymena pyriformis carbon p a r t i c l e s 5.9-10.3 Cox 1967 Tetrahymena vorax carbon p a r t i c l e s 0.94 Rasmussen et a l . 1975 Uronema nigricans V i b r i o sp.; 0.5 Berk et a l . 1976 B a c i l l u s sp. 0.15-0.19 14 c i l i a t e species^" latex beads, bakers 2 33^ Fenchel 1980b yeast, l i v e c i l i a t e s J 5 0 A 720 Clearance rates are expressed as nl/h per i n d i v i d u a l 1. Quoted clearance rates are taken from Fenchel's l i n e a r f i t to log clearance rate vs. log optimal p a r t i c l e s i z e (Fenchel 1980b: p. 16) assuming a C/wet wt. of c i l i a t e protoplasm of 0.06 and s p e c i f i c gravity of 1. 2. For c i l i a t e of 0.15 ng C body weight feeding on 1 um diameter p a r t i c l e s . 3. For 0.3 ng C c i l i a t e feeding on 3 um p a r t i c l e s . 4. For 0.3 ng C c i l i a t e feeding on 15 um p a r t i c l e s . b a c t e r i a are 2 - 3 orders of magnitude l e s s than c l e a r a n c e r a t e s on phytoplankton. A c l e a r a n c e rate of 20 n l c i l i a t e - 1 h" 1 was assumed f o r c i l i a t e s having a preference f o r p a r t i c l e s <3 um diameter. ( E q u i v a l e n t l y , c i l i a t e s had a c l e a r a n c e r a t e of 4 u l h ~ x and a s e l e c t i v i t y of 0.005 on b a c t e r i a . ) T h i s group of c i l i a t e s was assumed not to graze phytoplankton. Those c i l i a t e s which were unassigned by Heinbokel as to f e e d i n g p r e f e r e n c e were a s s i g n e d s e l e c t i v i t i e s of 0.134, 1, 0.019, 1 and 0.005 on l a r g e and small diatoms, l a r g e and small f l a g e l l a t e s , and b a c t e r i a , r e s p e c t i v e l y , with a uniform c l e a r a n c e r a t e of 4 u l c i l i a t e " 1 h ~ 1 . G r a z i n g by gastropod v e l i g e r s and pelecypod  l a r v a e — T a b l e II l i s t s l i t e r a t u r e estimates of the g r a z i n g r a t e s of pelecypod l a r v a e and mud s n a i l v e l i g e r s on phytoplankton (mostly naked f l a g e l l a t e s ) . Feeding by l a r v a e has been r e p o r t e d to be continuous (Ukeles and Sweeney 1969); Wilson (1979) found t h a t l a r v a e d i d not s t o p f e e d i n g at h i g h c e l l c o n c e n t r a t i o n s but i n s t e a d r e j e c t e d a high p r o p o r t i o n of captured food. Conversely, F r e t t e r and Montgomery (1968) report that prosobranch v e l i g e r s stopped feeding once the gut was f u l l , and Pechenik and F i s h e r (1979) observed s a t u r a t i o n of i n g e s t i o n at high food d e n s i t i e s i n mud s n a i l l a r v a e . Grazing by pelecypod l a r v a e and gastropod v e l i g e r s was modelled as a constant volume of 30 u l l a r v a - 1 h _ 1 swept c l e a r when feeding on small diatoms and f l a g e l l a t e s . Although both Calabrese and Davis (1970) and Walne (1963) confirmed e a r l i e r f i n d i n g s that Table I I . L i t e r a t u r e estimates of the volume of water cleared by gastropod and pelecypod larvae grazing on small phytoplankton. l a r v a l species prey clearance rate (nl h" 1) reference Pelecypods Mytilus edulis  Ostrea edulis Ostrea edulis Ostrea edulis Isochrysis galbana 5 . 8 - 2 5 Isochrysis galbana, 2 . 2 - 1 3 Phaeodactylum tricornutum Isochrysis galbana 1 8 - 2 0 " f l a g e l l a t e s " 27 Bayne 1 9 6 5 Wilson 1 9 7 9 Walne 1 9 5 6 Jorgensen 1 9 5 2 Mud s n a i l Nassarius obsoletus D u n a l i e l l a t e r t i o l e c t a , 5 0 - 7 0 , T h a l a s s i o s i r a pseudonana, 1 0 - 30 ' ' Isochrysis galbana Pechenik & Fisher 1 9 7 9 Clearance rates are expressed as nl/h per i n d i v i d u a l 1 . Low food d e n s i t i e s . 2 . More than 6 X 1 0 7 c e l l s l " 1 . 91 naked f l a g e l l a t e s are more e a s i l y d i g e s t e d than algae with c e l l w a l l s , t h i s may be i r r e l e v a n t to the r e l a t i v e r a t e s of i n g e s t i o n of diatoms and f l a g e l l a t e s : Pechenik and F i s h e r (1979) found very s i m i l a r s a t u r a t e d r a t e s of i n g e s t i o n of two naked f l a g e l l a t e s and a small c e n t r i c diatom grazed by mud s n a i l l a r v a e . What i s a reasonable upper s i z e - l i m i t of p a r t i c l e s eaten by v e l i g e r s and pelecypod l a r v a e ? Ukeles and Sweeney (1969) s t a t e that the p o t e n t i a l d i e t of s t r a i g h t - h i n g e l a r v a e i s r e s t r i c t e d by the s i z e of the mouth which i s l e s s than 10 um wide i n C r a s s o s t r e a v i r g i n i c a l a r v a e of 75 um s h e l l l e n g t h . Ostrea e d u l i s l a r v a e (165 - 200 um s h e l l length) have an esophagus of n20 um diameter (Yonge 1926). F r e t t e r and Montgomery (1968) found that T h a l a s s i o s i r a c e l l s 30 um long were too l a r g e f o r the younger prosobranch v e l i g e r s . An upper l i m i t of 30 um was assumed, y i e l d i n g s e l e c t i v i t i e s ( i n a manner analogous 'to those c a l c u l a t e d f o r c i l i a t e s g r a z i n g 15 - 25 um c e l l s ) of 0.178 f o r l a r g e diatoms and 0.019 f o r l a r g e f l a g e l l a t e s . The author i s unaware of any p u b l i s h e d r a t e s of f i l t r a t i o n of b a c t e r i a by v e l i g e r s or mollusk l a r v a e , although P i l k i n g t o n and F r e t t e r (1970) r e p o r t that b a c t e r i a >0.46 um diameter are i n g e s t e d . A s e l e c t i v i t y of 0.01 f o r b a c t e r i a was assumed. In summary, gastropod v e l i g e r s and pelecypod l a r v a e were assumed to f i l t e r water at a constant r a t e of 30 u l l a r v a - 1 h - 1 with f e e d i n g s e l e c t i v i t i e s of 0.178, 1, 0.019, 1 and 0.01 on l a r g e and s m a l l diatoms, l a r g e and small f l a g e l l a t e s and b a c t e r i a , 92 r e s p e c t i v e l y . G r a z i n g by larvaceans—Numbers of Oikopleura d i o i c a i n 19 s i z e c l a s s e s and i n g e s t i o n r a t e as a f u n c t i o n of s i z e was measured by King et a l . (1980) i n Foodweb I. King et a l . (1980) m u l t i p l i e d the i n g e s t i o n r a t e by 1.2 to g i v e a c l e a r a n c e r a t e which i n c l u d e d p a r t i c l e s which adhered to the l a r v a c e a n ' s house but were not ing e s t e d : F = 1.3130 • 1 0 ~ 1 2 • TL 1-3.1492, (29) where F = c l e a r a n c e rate ( l i t r e s l a r v a c e a n " 1 h " 1 ) , TL = trunk length (um). Assuming that trunk l e n g t h s of i n d i v i d u a l s f a l l i n g w i t h i n a given l e n g t h i n t e r v a l are u n i f o r m l y d i s t r i b u t e d throughout that i n t e r v a l , Eq. 29 p r e d i c t s c l e a r a n c e r a t e s per l a r v a c e a n ranging from 5.508 u l h _ 1 f o r i n d i v i d u a l s of 100 - 150 um trunk l e n g t h , to 4291 u l h " 1 f o r >950 um trunk l e n g t h (1050 um used f o r TL i n Eq. 29) and 6535 u l h " 1 f o r "spent" l a r v a c e a n s (1200 um trunk l e n g t h ) . Although Paffenhofer (1976) s t a t e s that the f i l t e r i n g r a t e of 0. d i o i c a i n c r e a s e s with d e c r e a s i n g food c o n c e n t r a t i o n at n a t u r a l l y o c c u r r i n g phytoplankton d e n s i t i e s , King et a l . (1980) found l i t t l e v a r i a t i o n of f i l t r a t i o n r a t e w i t h i n the range of b a c t e r i a l c o n c e n t r a t i o n s found i n Foodweb I. A l s o , King ( p e r s . comm.) found c l e a r a n c e r a t e s on nanophytoplankton to be e q u i v a l e n t to those on b a c t e r i a . King noted that small 93 la r v a c e a n s were l i m i t e d to f i l t e r i n g p a r t i c l e s <5 um diameter, and l a r g e l a r v a c e a n s to <25 um, approximately. These estimates permit the c a l c u l a t i o n of fee d i n g s e l e c t i v i t i e s on the four modelled phytoplankton groups. Those i n d i v i d u a l s with a trunk l e n g t h of 100 - 150 um, the s m a l l e s t r e p o r t e d s i z e , were as s i g n e d an upper food s i z e l i m i t of 5 um. I n d i v i d u a l s i n the two l a r g e s t c a t e g o r i e s ("950+ um" and "spent") were assigned an upper l i m i t of 25 um, and i n d i v i d u a l s of in t e r m e d i a t e l e n g t h were a s s i g n e d l i n e a r l y i n t e r p o l a t e d s i z e l i m i t s . Feeding s e l e c t i v i t i e s were then c a l c u l a t e d analogously to c i l i a t e s . B a c t e r i a were grazed by l a r v a c e a n s with a s e l e c t i v i t y of 1 (Appendix 2). Grazing by other z o o p l a n k t o n — P o l y c h a e t e l a r v a e , trochophores and cyphonautes l a r v a e were assumed to f i l t e r water at the same r a t e and have the same feeding s e l e c t i v i t i e s as pelecypod l a r v a e and v e l i g e r s . I t was a l s o assumed t h a t h a r p a c t i c o i d copepods were g r a z i n g the f o u l i n g community on CEE2's w a l l r a t h e r than phytoplankton or b a c t e r i o p l a n k t o n . C o l o u r l e s s f l a g e l l a t e s were t r e a t e d as zooplankton i n the model. They comprised a group of s e v e r a l s p e c i e s of d i n o f l a g e l l a t e s and the c h o a n o f l a g e l l a t e s Salpingoea spp. and Diaphanoeca sp. Haas and Webb (1979) have shown that c u l t u r e d , non-pigmented m i c r o f l a g e l l a t e s i n g e s t b a c t e r i a , but Parsons et a l . ( i n press) c o u l d .find no evidence of microf l a g e l l a t e p r e d a t i o n on b a c t e r i a , i n enclosed water columns. I t was assumed here that c o l o u r l e s s f l a g e l l a t e s f i l t e r e d b a c t e r i a at a r a t e of 94 20 n l h ~ 1 per f l a g e l l a t e , equal to the assumed c l e a r a n c e r a t e of c i l i a t e s g r a z i n g b a c t e r i a . 3.5.2 Zooplankton e x c r e t i o n The e x c r e t i o n of n i t r o g e n by zooplankton was modelled as a f u n c t i o n of body carbon only, although e x c r e t i o n r a t e i s s i g n i f i c a n t l y a f f e c t e d by food supply, a c t i v i t y , r e p r o d u c t i v e c o n d i t i o n , and amount and type of s t o r e d reserves (Conover 1978b: p. 376). E x c r e t i o n of n i t r o g e n (N) i n the model was independent of food supply. Compounding the u n c e r t a i n t y of N e x c r e t i o n as a f u n c t i o n of i n g e s t e d food with the l a r g e u n c e r t a i n t y of amount of m a t e r i a l i n g e s t e d was thought to be more prone to s e r i o u s e r r o r than the adoption of a constant r a t e of e x c r e t i o n per i n d i v i d u a l . Three separate f o r m u l a t i o n s of e x c r e t i o n r a t e vs. body weight were d e r i v e d : one f o r c i l i a t e s and c o l o u r l e s s f l a g e l l a t e s , one f o r ctenophores, and one f o r a l l other zooplankton. E x c r e t i o n by c i l i a t e s and c o l o u r l e s s f l a g e l l a t e s — D e w e y and Banse have surveyed l i t e r a t u r e on r e s p i r a t i o n r a t e s of non-feeding, b a c t e r i a - f r e e protozoa and f l a g e l l a t e s (Dewey 1976). They c o r r e c t e d r e s p i r a t i o n to 20 °C f o l l o w i n g Krogh (1916) and used a C/dry weight r a t i o of 0.4. They r e p o r t r e s p i r a t i o n as (p. 118 of Dewey 1976): r e s p i r a t i o n ( p i c o l i t r e s 0 r2 h" 1) = 13.6836 wL-0.7354, (30) 95 where W i s ng dry weight per i n d i v i d u a l . To convert to N e x c r e t i o n , an a p p r o p r i a t e 0/N r a t i o was r e q u i r e d . No such p u b l i s h e d r a t i o f o r protozoa was found, so l i t e r a t u r e on copepod and mixed zooplankton e x c r e t i o n was c o n s u l t e d . H a r r i s (1959) found 0/N atomic r a t i o s averaging 7.7 f o r Long I s l a n d Sound zooplankton. Corner et a l . (1965) obtained r a t i o s between 9.8 and 15.6 f o r Calanus h e l g o l a n d i c u s and C. f i n m a r c h i c u s . Conover and Corner (1968) found that O/N r a t i o s change s e a s o n a l l y . During the s p r i n g bloom, 0/N r a t i o s of C. hyperboreus were g e n e r a l l y between 10 - 30, then i n c r e a s e d past 150 immediately a f t e r the bloom using only s t o r e d f a t as an energy source. During the summer 0/N f e l l to n20 - 30 a g a i n . For C. f i n m a r c h i c u s and M e t r i d i a longa r a t i o s were l e s s v a r i a b l e , f a l l i n g from n30 - 55 j u s t a f t e r the s p r i n g bloom to n20 by the end of September. Using an 0/N atomic r a t i o of 20, a C/dry weight r a t i o of 0.4, and c o r r e c t i n g (Krogh 1916, h i s F i g . 28, p. 96) to the mean temperature of water i n CEE2, Eq. 30 was transformed to E = 0.01070 WMD.7354, (31) where E i s e x c r e t i o n of i n o r g a n i c n i t r o g e n (ng-atom N h~ l) and W i s body weight in ug C. Assuming that 20% of e x c r e t e d N was organic (see d i s c u s s i o n of copepod e x c r e t i o n ) , the t o t a l e x c r e t i o n of N, both organic and i n o r g a n i c , was expected to equal E (ng-at N h" 1) = 0.01337 W-0.7354. (32) The t o t a l e x c r e t i o n of c i l i a t e s and c o l o u r l e s s f l a g e l l a t e s was 96 simply the product of observed numbers per u n i t volume and e x c r e t i o n r a t e per i n d i v i d u a l as d e f i n e d in Eq. 32, t a k i n g i n t o account the d i f f e r e n t weights of body carbon of the v a r i o u s zooplankton taxa. E x c r e t i o n by ctenophores—Kremer (1977) r e p o r t e d that e x c r e t i o n r a t e s of Mnemiopsis l e i d y i were a l i n e a r f u n c t i o n of organism dry weight, r a t h e r than a power f u n c t i o n t y p i c a l of other groups. L i n e a r dependence was a l s o found by Biggs (1977), H i r o t a (1972) and W i l l i a m s and B a p t i s t (1966, c i t e d by Kremer 1977) f o r s e v e r a l ctenophore s p e c i e s . However, Kremer noted that M i l l e r (1970) c a l c u l a t e d metabolic exponents of 0.82 - 0.87 f o r M. mccradyi. An exponent of u n i t y ( l i n e a r dependence) was assumed here. Kremer (1977) measured a mean e x c r e t i o n r a t e of 14 ug-atom ammonium-N (g dry wt . ) " 1 d a y - 1 f o r unfed M. l e i d y i at 10 - 18 °C. 56% of excreted N was o r g a n i c . Jawed (1973) re p o r t e d r a t e s of 20 - 60 ug-atom ammonium-N (g dry wt . ) " 1 day" 1 f o r j e l l y f i s h and P l e u r o b r a c h i a at 13 °C. Kremer (1976) give s an e x c r e t i o n r a t e of 25 ug-atom ammonium-N and 20 ug-atom organic-N (g dry w t . ) " 1 day" 1 f o r M. l e i d y i at n22 °C. I t i s u n c e r t a i n whether a temperature c o r r e c t i o n i s necessary f o r comparison of r e s u l t s . Biggs (1977) found that the oxygen consumption of g e l a t i n o u s zooplankton d i d not markedly change when a c c l i m a t e d to temperatures lower than t h e i r n a t u r a l environment, whereas Kremer (1977) found a Q r10 of 3.67 f o r M. l e i d y i s t u d i e d at ambient temperatures of 10 - 24 °C. 97 Assuming a t o t a l n i t r o g e n e x c r e t i o n r a t e of 40 ug-atom N (g dry w t . ) - 1 day" 1, and a C/dry weight r a t i o of 2% (Kremer 1976: p. 354), g i v e s E (ng-at N h" 1) = 0.0833 W (ug C ) . (33) In the model 50% of n i t r o g e n e x c r e t e d by ctenophores was assumed to be o r g a n i c . E x c r e t i o n by other z o o p l a n k t o n — I k e d a (1974) measured the e x c r e t i o n of ammonium by v a r i o u s s u b t r o p i c a l , t r o p i c a l , temperate and b o r e a l zooplankton as a f u n c t i o n of body weight and h a b i t a t temperature. H i s equation (5) as m o d i f i e d by Ikeda and Motoda (1978) i s l o g r 1 0 E = (-0.00941 T + 0.8338) • l o g r 1 0 W + 0.02865 T - 1.2802, (34) where E = e x c r e t i o n (ug ammonium-N a n i m a l " 1 h " 1 ) , W = body dry weight (mg), T = h a b i t a t temperature (°C). c Taking a water temperature of 13.5 °C, and assuming a C/dry weight r a t i o of 0.32 (Weibe et a l . 1975), Eq. 34 i s converted to E (ng-at ammonium-N h" 1) = 0.1548 WL0.7068, (35) where W i s body weight i n ug C. What f r a c t i o n of e x c r e t e d N would be expected to be 98 o r g a n i c ? Organic N comprises 15 - 30% (Mayzaud 1973), 18 - 24% (Jawed 1969), 22% ( B u t l e r et a l . 1969) and 25% (Corner and Newell 1967) of the t o t a l N e x c r e t e d . Assuming a value of 20%, the e x c r e t i o n of inorganic+organic n i t r o g e n by copepods, l a r v a l metazoans, larvaceans and S a g i t t a was modelled as E (ng-at N h" 1 ) = 0.1935 VJL0.7068, (36) where W i s ug C body weight. To summarize, the e x c r e t i o n of organic and i n o r g a n i c n i t r o g e n by protozoans, ctenophores, and other zooplankton, was d e s c r i b e d by Eqs. 32, 33 and 36, r e s p e c t i v e l y . These f u n c t i o n s are p l o t t e d i n F i g . 17. 3.6 B a c t e r i a B a c t e r i a i n the model r e c y c l e d i n o r g a n i c n i t r o g e n by m i n e r a l i z i n g d i s s o l v e d organic matter (DOM) exuded from phytoplankton or e x c r e t e d by zooplankton. P a r t i c u l a t e organic matter (POM) was c o n s i d e r e d to be an i n s i g n i f i c a n t food source f o r b a c t e r i a . T h i s accords with Azam and Hodson (1977) who found t h a t about 90% of the h e t e r o t r o p h i c uptake of glucose, s e r i n e or a c e t a t e was by b a c t e r i a p a s s i n g a 1 um-pore f i l t e r . Fuhrman and Azam (1980) r e p o r t that >90% of thymidine was taken up by <1 um b a c t e r i a . These b a c t e r i a , being unattached to p a r t i c l e s , should have been unable to r a p i d l y degrade p a r t i c u l a t e s . B a c t e r i a were assumed not to sink because they were f r e e - l i v i n g and of such small s i z e . Inorganic n i t r o g e n was assumed not to be taken up by ' 99 F i g u r e 17. Modelled e x c r e t i o n r a t e s of c i l i a t e s and c o l o u r l e s s f l a g e l l a t e s , ctenophores, and other zooplankton. Of the t o t a l n i t r o g e n e x c r e t e d by ctenophores, 50% was assumed to be o r g a n i c . Protozoans and other zooplankton excreted 20% organic n i t r o g e n . 101 b a c t e r i a . Thayer (1974) has suggested that h e t e r o t r o p h i c b a c t e r i a compete with algae f o r a n i t r o g e n source when m e t a b o l i z i n g compounds with l a r g e C/N r a t i o s . Parsons et a l . ( i n press) give i n d i r e c t evidence of b a c t e r i a out-competing phytoplankton i n the uptake of n i t r a t e i n water columns en r i c h e d with g l u c o s e . T h i s p o s s i b i l i t y was not e n t e r t a i n e d here p r i m a r i l y because a C/N mass r a t i o of 6 was adopted f o r phytoplankton biomass. To maintain t h i s constant C/N r a t i o while exuding DOM r e q u i r e d that the C/N r a t i o of exuded DOM be 6 a l s o , w e l l below r a t i o s which r e s u l t i n net uptake of i n o r g a n i c n i t r o g e n by b a c t e r i a ( H o l l i b a u g h 1978). Organics excreted by zooplankton were a l s o assumed to have a C/N mass r a t i o of 6. I t i s impossible at present to estimate what p r o p o r t i o n of phytoplankton exudation products c o n s i s t s of s m a l l molecules r e a d i l y a v a i l a b l e to heterotrophs (Nalewajko 1977). A l l of the DOM r e l e a s e d by algae or zooplankton was t h e r e f o r e c o n s i d e r e d to be a v a i l a b l e f o r b a c t e r i a l u t i l i z a t i o n . Furthermore, the b a c t e r i a were assumed to immediately absorb a l l of the organic matter r e l e a s e d i n t o the water column, thus removing the n e c e s s i t y of s p e c i f y i n g uptake k i n e t i c s or s e p a r a t e l y f o l l o w i n g a pool of DOM. T h i s simple approach was used because i t i s not known what k i n e t i c s a p p r o p r i a t e l y d e s c r i b e the uptake of a complex mixture of organic s u b s t r a t e s by a d i v e r s e community of h e t e r o t r o p h s . Parsons and S t r i c k l a n d (1962) and Wright and Hobbie (1965, 1966) d i s c o v e r e d t h a t h e t e r o t r o p h i c uptake of s i n g l e o r g a n i c compounds by n a t u r a l assemblages of plankton 102 c o u l d be d e s c r i b e d by Michaelis-Menten k i n e t i c s , but i n some cases s e r i o u s e r r o r s can r e s u l t when a p p l y i n g these k i n e t i c s to the uptake of a s i n g l e s u b s t r a t e by a heterogeneous community of plankton ( W i l l i a m s 1973). The immediate a b s o r p t i o n of DOM by modelled b a c t e r i a i s not unreasonable when one r e a l i z e s that as much as 45%, 100% and 40% of the pools of gl u c o s e , l e u c i n e and thymidine r e s p e c t i v e l y i n CEE2 water was h e t e r o t r o p h i c a l l y a s s i m i l a t e d per hour. (Admittedly these are maximum uptake r a t e s of e a s i l y u t i l i z e d o r ganic s u b s t r a t e s . ) W i l l i a m s and Gray (1970) found an immediate r i s e i n the r a t e of s u b s t r a t e o x i d a t i o n by e s t u a r i n e plankton a f t e r the amino a c i d c o n c e n t r a t i o n was a r t i f i c i a l l y i n c r e a s e d . Some percentage of organic matter taken up by b a c t e r i a i s immediately r e s p i r e d . For i n d i v i d u a l amino a c i d s added to n a t u r a l plankton assemblages, the percentage r e s p i r e d of the t o t a l carbon taken up ranges from 8 to 61% (Hobbie and Crawford 1969; Crawford et a l . 1974). 21 - 50% of carbon i n added mixtures of amino a c i d s and 33 - 65% of added glucose-carbon was r e s p i r e d ( Williams 1970; W i l l i a m s and Yentsch 1976; Herbland 1978). The model assumed that 40% of d i s s o l v e d organic carbon taken up by b a c t e r i a was immediately r e s p i r e d . I t was a l s o assumed that i n o r g a n i c n i t r o g e n was simultaneously r e l e a s e d i n the p r o p o r t i o n of C r e s p i r e d / N m i n e r a l i z e d = 6 (by mass). In a d d i t i o n to the immediate r e s p i r a t i o n of some p r o p o r t i o n of absorbed s u b s t r a t e , there i s presumably a maintenance metabolism. Novitsky and M o r i t a (1977) found that 103 the endogenous r e s p i r a t i o n of a p s y c h r o p h i l i c marine v i b r i o maintained a t 5 °C was 0.9% of the t o t a l c e l l u l a r carbon per hour at the beginning of s t a r v a t i o n . Assuming a Q r10 of 2 to c o r r e c t N o v i t s k y and M o r i t a ' s r e s u l t to 13.5 °C, maintenance metabolism was modelled as a constant s p e c i f i c l o s s r a t e of 0.016 h""1 of b a c t e r i a l carbon and n i t r o g e n . 3.1. Inorganic N u t r i e n t s 3.7_.l N u t r i e n t uptake Uptake of d i s s o l v e d i n o r g a n i c n i t r o g e n (N) and s i l i c o n ( S i ) was d i r e c t l y coupled to gross p h o t o s y n t h e s i s by phytoplankton. Nitrogen was taken up i n the r a t i o of 6 g C p h o t o s y n t h e t i c a l l y f i x e d to 1 g N removed from s o l u t i o n . In some s i m u l a t i o n s the r a t i o of S i taken up to C f i x e d v a r i e d a c c o r d i n g to the s e v e r i t y of S i - l i m i t a t i o n ( s e c t i o n 3.4.2). N u t r i e n t uptake was assumed not to occur at n i g h t when gross p h o t o s y n t h e s i s was z e r o . 3.1_.2 N u t r i e n t sources N i t r a t e was added to the top 8 m of CEE2 on nine o c c a s i o n s , and s i l i c a t e on seven o c c a s i o n s before day 80. These a d d i t i o n s were q u a n t i t a t i v e l y accounted f o r i n the model. About 800 l i t r e s of sediment was a l s o pumped back to the s u r f a c e of CEE2 immediately a f t e r each n u t r i e n t a d d i t i o n . G e n e r a l i z i n g from the c o n c e n t r a t i o n of sediment n u t r i e n t s measured only once 104 (C. D avis, p e r s . comm.), about 0.55 ug-atom S i l " 1 and 1.11 ug-atom N l ' 1 was i n t r o d u c e d i n t o the s u r f a c e 8 m of CEE2 a f t e r each pumping. T h i s source was a l s o modelled. Inorganic n i t r o g e n was returned to the water v i a phytoplankton and b a c t e r i a l r e s p i r a t i o n and zooplankton e x c r e t i o n . N was r e l e a s e d i n p r o p o r t i o n to C r e s p i r e d so as to maintain a C/N mass r a t i o of 6 i n phytoplankton and b a c t e r i a l protoplasm. Once S i was taken up by diatoms i t never r e t u r n e d to the water, except i n those runs where the d i s s o l u t i o n of s i l i c a was modelled ( s e c t i o n 3.7.3). R a i n f a l l was an unimportant source of n u t r i e n t s . The t o t a l p r e c i p i t a t i o n at V i c t o r i a I n t e r n a t i o n a l A i r p o r t , 5 km from the CEPEX s i t e , was 10.65 cm between J u l y 9 (day 1) and September 26 (day 80) (Environment Canada 1978b). Any n u t r i e n t s i n t h i s r a i n f a l l would have been d i l u t e d i n the top 8 m of CEE2 to 1.3% of t h e i r former c o n c e n t r a t i o n . I f we accept Junge's (1958) average of 8.3 ug-atom ammonium-N l " 1 and 4.7 ug-atom n i t r a t e - N l " 1 i n c o a s t a l r a i n f a l l as r e p r e s e n t a t i v e of the r a i n f a l l d uring Foodweb I, t h i s would imply an i n c r e a s e of only 0.17 ug-atom N l'1. There i s l i t t l e atmospheric c o n t r i b u t i o n of s i l i c o n from r a i n or dust (Parsons and H a r r i s o n , unpubl.). 3.7.2 D i s s o l u t i o n of p a r t i c u l a t e s i l i c a In view of the extreme shortage of d i s s o l v e d s i l i c o n p r e d i c t e d i n e a r l y s i m u l a t i o n runs, some mechanism f o r r e c y c l i n g S i back to the water was thought necessary. S i 105 r e g e n e r a t i o n was modelled simply as a constant rate of d i s s o l u t i o n of s i l i c a present in l i v i n g diatoms and d e t r i t u s . D i s s o l u t i o n r a t e s r e p o r t e d f o r diatom c u l t u r e s or n a t u r a l plankton assemblages u n t r e a t e d with a c i d s are l i s t e d i n Table I I I . A s p e c i f i c d i s s o l u t i o n r a t e of 0.005 h" 1 was chosen fo r both d e t r i t a l and n o n - d e t r i t a l s i l i c a , near the upper l i m i t of r e p o r t e d d i s s o l u t i o n r a t e s . ( D i s s o l u t i o n of s i l i c a was suspended in the model i f the bulk C/Si mass r a t i o of an a l g a l group reached i t s c e i l i n g of 6.25.) Lewin (1961) d e t e c t e d l i t t l e or no d i s s o l u t i o n of s i l i c a from l i v i n g diatoms, but she r e c o g n i z e d that simultaneous uptake of S i c o u l d have masked d i s s o l u t i o n . Nelson et a l . (1976) found that the r a t e of s i l i c a d i s s o l u t i o n from e x p o n e n t i a l l y growing T h a l a s s i o s i r a pseudonana c e l l s was s i m i l a r to t h a t found by Paasche (1973b) f o r c e l l s k i l l e d by f r e e z i n g , a process which leaves the c e l l s ' p r o t e c t i v e organic c o a t i n g i n t a c t (Lewin 1961). T h i s p r o v i d e s some j u s t i f i c a t i o n f o r assuming an equal d i s s o l u t i o n r a t e of s i l i c a d e p o s i t e d in l i v i n g diatoms and d e t r i t a l s i l i c a . However, d e t r i t a l s i l i c a i n c l u d e s not only s i l i c a found w i t h i n i n t a c t , dead diatoms, but a l s o p a r t i c u l a t e s i l i c a i n zooplankton f e c a l p e l l e t s , p u l v e r i z e d diatom f r u s t u l e s , e t c . These l a t t e r forms of d e t r i t a l s i l i c a would d i s s o l v e more r a p i d l y than i n t a c t s i l i c a (see r e f e r e n c e s c i t e d i n G r i l l 1970). N e v e r t h e l e s s , in the i n t e r e s t s of s i m p l i c i t y these d i f f e r e n t forms of d e t r i t a l s i l i c a were not d i s t i n g u i s h e d i n the model. I n f l u x of S i i n t o the n o n - d e t r i t a l s i l i c a pool o c c u r r e d Table I I I . L i t e r a t u r e estimates of s p e c i f i c d i s s o l u t i o n rate of s i l i c a i n l i v i n g and dead phytoplankton. s p e c i f i c d i s s o l u t i o n rate (per 1000 hrs.) corrected measured to 13.5 C reported temperature (c e l s i u s ) nature of sample references up to 60 3.4 9.4 2.0-8.5 0.2-0.3 0.71 1.5 0.83 0.59 0.44 0.25 1.6 7.9J 5.5 1.2-5.0 0.1-22 0.88 0.46 0.38 0.38 0.39 0.30 0.43] 20 20 20 11 28 23 19 15 11 30 natural phytoplankton assemblage, upwelling region off northwest A f r i c a ashed f r u s t u l e s suspended at 110 m i n Lake Michigan f r e e z e - k i l l e d , Si-depleted Skeletonema costatum untreated T h a l a s s i o s i r a pseudonana natural phytoplankton assemblages, Japanese coast natural phytoplankton assemblage c o l l e c t e d at 100 m from S t r a i t of Juan de Fuca, then placed i n dark natural phytoplankton assemblage from surface of Kaneohe Bay, Hawaii, then placed i n dark untreated T h a l a s s i o s i r a decipiens kept i n dark, no d i s s o l u t i o n was evident during f i r s t 10 days Nelson & Goering 1977 Parker et a l . 1977 Paasche & Ostergren 1980 Nelson et a l . 1976 Kamatani & Ueno 1980 G r i l l & Richards 1964 Lawson et a l . 1978 Kamatani & R i l e y 1979 D i s s o l u t i o n was corrected to 13.5 C, the mean water temperature of CEE2 between days 1 and 80 1. Rate correction follows Lawson et a l . 1978. 2. Rate correction follows Kamatani & Ueno 1980. 107 v i a uptake of d i s s o l v e d S i . The model assumed that a l l S i taken up by diatoms was i n c o r p o r a t e d i n t o the c e l l w a l l , and that i n t r a c e l l u l a r pools of m e t a b o l i c a l l y a c t i v e S i were n e g l i g i b l e (Paasche 1980; Davis et a l . 1978). S i l i c a w i t h i n l i v i n g c e l l s was t r a n s f e r r e d to the d e t r i t a l f r a c t i o n whenever phytoplankton were grazed. No d i g e s t i v e delay i n grazers was modelled duri n g t r a n s f e r to the d e t r i t a l p o o l . The other avenue of t r a n s f e r o c c u r r e d when phytoplankton r e s p i r a t i o n or exudation of carbon reduced the bulk C/Si mass r a t i o of an a l g a l group to i t s lower l i m i t of 1.25. Further r e s p i r a t i o n r e s u l t e d i n the simultaneous t r a n s f e r of s i l i c a to the d e t r i t a l pool so as to maintain a bulk r a t i o of 1.25. Although i n r e a l i t y r e s p i r a t i o n or exudation i s not d i r e c t l y l i n k e d with Si l o s s i n t h i s manner, the modelled process can be i n t e r p r e t e d as death or l y s i s of a subset of the diatom p o p u l a t i o n . G r a z i n g - and r e s p i r a t i o n - i n d u c e d l o s s e s of n o n - d e t r i t a l s i l i c a from l a r g e 'and s m a l l diatoms entered the same pool of d e t r i t a l s i l i c a . D e t r i t a l s i l i c a was mixed between l a y e r s and sank at a r a t e of 20 m day" 1 i n the u n d i s t u r b e d 8-20 m l a y e r and 5 m day" 1 i n the bubbled 0-8 m l a y e r . Bienfang ( i n press) found that f e c a l p e l l e t s of Calanus from CEE2 had s i n k i n g r a t e s of 70 - 171 m day" 1 when f e e d i n g mainly on diatoms. These very f a s t r a t e s would be a p p l i c a b l e only to that f r a c t i o n of s i l i c a e n c l o s e d w i t h i n f e c a l p e l l e t s of l a r g e copepods. The slower s i n k i n g r a t e used in the model recognized the d i v e r s e o r i g i n s of d e t r i t a l s i l i c a i n CEE2. 108 3.8 F i n a l Comments Values of the major parameters of the model are summarized i n Table IV. A s i m p l i f i c a t i o n was noted i n the overview of the model ( s e c t i o n 3.1) that m e r i t s comment. T h i s was the assumption that n e i t h e r phosphorus nor temperature l i m i t e d phytoplankton growth i n Foodweb I. Phosphate c o n c e n t r a t i o n i n the s u r f a c e 8 m of CEE2 was 3.75 ± 1.67 ug-atom P 1 _ 1 (mean + s.dev., n=32) and 3.03 ± 0.94 ug-atom P l " 1 (n=32) i n the 8-20 m l a y e r . On only two occasions a f t e r day 11 d i d phosphate drop below 3 ug-atom P l ~ l i n the s u r f a c e l a y e r , and below 2 ug-atom P l " 1 i n the deep l a y e r . These c o n c e n t r a t i o n s were g e n e r a l l y w e l l above h a l f - s a t u r a t i o n c onstants of phosphate uptake or p h o s p h a t e - l i m i t e d growth r e p o r t e d i n the l i t e r a t u r e (e.g. 0.38 - 0.63 ug-atom P l " 1 , Monochrysis l u t h e r i , Burmaster and Chisholm 1979; 0.02 - 2.8 ug-atom P 1 _ 1 , A s t e r i o n e l l a formosa, C y c l o t e l l a meneghiniana, Tilman and Kilham 1976; 0.6 ug-atom P l " 1 , Scenedesmus sp., Rhee 1973). The n u t r i e n t which i s most o f t e n c i t e d as l i m i t i n g phytoplankton growth i n the sea i s n i t r o g e n r a t h e r than phosphorus (e.g. Eppley et a l . 1969a; Dugdale and Goering 1967). Phosphorus was t h e r e f o r e c o n s i d e r e d not to l i m i t phytoplankton growth i n CEE2, hence i t s dynamics were not p o r t r a y e d . The e f f e c t of temperature on phytoplankton growth was a l s o ignored. Water temperature at zero metres depth i n c r e a s e d from 15.0 °C at the s t a r t of the experiment to 18.0 °C by day 30 (August 7), then f e l l t o 12.7 °C by day 79. The temperature of water 20 m deep i n c r e a s e d from 10.2 °C on Table IV. Summary of parameter values used i n the main simulation model, symbol value d e f i n i t i o n and comments k 0.148 m attenuation c o e f f i c i e n t of water i n the absence of phytoplankton max ,-4 2 „ N-1 w k 5.11 lO -"* m*" (mg C) x attenuation c o e f f i c i e n t per unit of phytoplankton carbon S -1 b 0.005 m h exchange c o e f f i c i e n t f o r mixing across 8 m int e r f a c e C/N 6 C/N mass r a t i o of phytoplankton C/Si 1.25 C/Si mass r a t i o of diatoms, reference run 1.25-6.25 v a r i a b l e C/Si mass r a t i o of diatoms -2 -1 I 200 uEin m s saturation irradiance of surface-dwelling phytoplankton on July 9 declines seasonally and with depth P 0.087 h ^ maximum gross photosynthetic rate, large diatoms 0.110 small diatoms 0.031 large f l a g e l l a t e s 0.056 small f l a g e l l a t e s R 0.1 r e s p i r a t i o n of surface-dwelling phytoplankton as a f r a c t i o n of P declines with depth X 0.1 phytoplankton exudation as a f r a c t i o n of r e a l i z e d gross photosynthesis 0.77 ug-atom N 1 ^ h a l f - s a t u r a t i o n constant of N-limited gross photosynthesis, large diatoms 0.16 small diatoms 0.54 large f l a g e l l a t e s 0.12 small f l a g e l l a t e s Kg^ 2.06 ug-atom S i 1 ^ h a l f - s a t u r a t i o n constant of S i - l i m i t e d gross photosynthesis, large diatoms 0.42 small diatoms 1.3 m day ^ large diatom and large f l a g e l l a t e sinking rate i n 8-20 m layer of CEE2 -1 i n surface bubbled layer, S = 0.325 m/day 0.5 m day * small diatom and small f l a g e l l a t e sinking rate i n 8-20 m layer of CEE2 i n surface bubbled layer, S = 0.125 m/day continued on next page Table IV. (continued) symbol value d e f i n i t i o n and comments thr c r i t R max 'Si 5 S i 0 ug C 1 -1 -1 300 ug C 1 0.4 day" 1 4 u l h " 1 0.02 30 0.3 5.5-6500 (see F i g . 17) -1 0.005 h 20 m day -1 copepod grazing threshold c r i t i c a l phytoplankton concentration which saturates copepod grazing maximum s p e c i f i c r a t i o n of a copepod with a body weight of 68 ug C clearance rate clearance rate clearance rate clearance rate clearance rate b a c t e r i a and of c i l i a t e s f i l t e r i n g phytoplankton of optimal s i z e of c i l i a t e s and colourless f l a g e l l a t e s f i l t e r i n g b a c t e r i a of metazoan larvae f i l t e r i n g phytoplankton of optimal s i z e of metazoan larvae f i l t e r i n g b a c t e r i a of larvaceans of increasing trunk length f i l t e r i n g phytoplankton of optimal s i z e excretion rate of zooplankton s p e c i f i c d i s s o l u t i o n rate of s i l i c a s i n k i n g rate of d e t r i t a l s i l i c a i n 8-20 m layer of CEE2 i n surface bubbled layer, S = 5 m/day I l l day 1 to 14.1 °C by day 30, then f e l l to 10.2 °C by day 79. Temperature changed g r a d u a l l y over depth and time; S t e e l e et a l . (1977) found that l a r g e p l a s t i c e n c l o s u r e s s i m i l a r to CEE2 s t r o n g l y damped e x t e r n a l temperature f l u c t u a t i o n s with p e r i o d s l e s s than one day. I t was thought, t h e r e f o r e , that captured p o p u l a t i o n s would have f u l l y a c c l i m a t e d to the changing temperature regime, at l e a s t over the modelled p e r i o d of 80 days. Previous s e c t i o n s have d e s c r i b e d i n d e t a i l how the s i m u l a t i o n model was c o n s t r u c t e d from data gathered i n Foodweb I and from the s c i e n t i f i c l i t e r a t u r e . V a r i a t i o n of s u r f a c e i r r a d i a n c e over time, l i g h t a t t e n u a t i o n i n the water column, s a t u r a t i o n i r r a d i a n c e of p h o t o s y n t h e s i s , s i n k i n g , mixing and standing s t o c k s of zooplankton were based l a r g e l y or e n t i r e l y on Foodweb I data. Other model components such as maximum gross p h o t o s y t h e s i s , phytoplankton r e s p i r a t i o n and exudation, k i n e t i c s of n u t r i e n t l i m i t a t i o n , zooplankton g r a z i n g and e x c r e t i o n , b a c t e r i a l m i n e r a l i z a t i o n ' a n d s i l i c a d i s s o l u t i o n , were based on p l a u s i b l e l i t e r a t u r e e s t i m a t e s . The goal was to c o n s t r u c t a coherent and c o n s i s t e n t model of how phytoplankton and n u t r i e n t dynamics were expected to behave, at l e a s t approximately, i n CEE2. The next s e c t i o n d e s c r i b e s how t h i s model was implemented on the computer. 112 4. COMPUTER IMPLEMENTATION 4.1 Approach The v a r i o u s r a t e p r o c e s s e s d i s c u s s e d as system components in s e c t i o n 3 were expressed as a set of simultaneous o r d i n a r y d i f f e r e n t i a l equations f o r each of two depth l a y e r s — 0 to 8 and 8 to 20 m — i n CEE2. The two l a y e r s were coupled to simulate mixing of d i s s o l v e d and suspended m a t e r i a l between l a y e r s , s i n k i n g of l a r g e p a r t i c l e s from top to bottom l a y e r s , and shading of the bottom l a y e r by s u r f a c e phytoplankton. The computer implementation was designed to allow any of the s t a t e v a r i a b l e s to tra c k observed c o n c e n t r a t i o n s at the d i s c r e t i o n of the a n a l y s t . For example, n i t r o g e n c o u l d be c o n s t r a i n e d to f o l l o w observed c o n c e n t r a t i o n s r e g a r d l e s s of the modelled uptake of n i t r o g e n by phytoplankton or re g e n e r a t i o n by b a c t e r i a . S i m u l a t i o n runs began at t=3.5 days (noon on day 4; t=0 equals 00.00 hours on J u l y 9, 1978, the e a r l y morning of day 1) with a l l s t a t e v a r i a b l e s i n i t i a l i z e d at t h e i r observed v a l u e s . S i m u l a t i o n runs d i d not begin at the time of CEE deployment, t=0.35 day, because d e t a i l e d o b s e r v a t i o n s of zooplankton abundance were not made u n t i l day 4. 113 4.2 N u t r i e n t I n t e r p o l a t i o n When observed time s e r i e s were used i n the model (as was always the case f o r zooplankton numbers), l i n e a r i n t e r p o l a t i o n between o b s e r v a t i o n s was performed f o r those time steps not c o i n c i d i n g with sampling t i m e s . 1 T h i s method of i n t e r p o l a t i o n c o u l d be f a u l t e d i n the s p e c i a l case of d i s s o l v e d n i t r o g e n and s i l i c o n . Water samples f o r n u t r i e n t analyses were taken immediately before n u t r i e n t a d d i t i o n s , but not immediately a f t e r . In s e v e r a l cases there was no apparent i n c r e a s e i n n i t r o g e n or s i l i c o n i n water sampled 2 or 3 days a f t e r n u t r i e n t a d d i t i o n , and i n no case d i d c o n c e n t r a t i o n s i n c r e a s e by the amount expected i f no uptake were to occur. Thus by l i n e a r l y i n t e r p o l a t i n g between observed n u t r i e n t c o n c e n t r a t i o n s t r a n s i e n t i n c r e a s e s f o l l o w i n g a d d i t i o n s were unaccounted f o r or underestimated. To see i f t h i s o v e r s i g h t might have s i g n i f i c a n t l y a f f e c t e d phytoplankton dynamics i n model runs i n which n u t r i e n t s were c o n s t r a i n e d t o fo l l o w t h e i r l i n e a r l y i n t e r p o l a t e d c o n c e n t r a t i o n s , a s p e c i a l i n t e r p o l a t i o n technique was t e s t e d on one run. In t h i s s p e c i a l run, n i t r o g e n and s i l i c o n c o n c e n t r a t i o n s were l i n e a r l y i n t e r p o l a t e d as be f o r e , except that t h e i r c o n c e n t r a t i o n i n the s u r f a c e 8 m was i n c r e a s e d by the expected amount at the time of each n u t r i e n t a d d i t i o n , and then was l i n e a r l y decreased to the next observed value ( F i g . 4 ) . The two methods of i n t e r p o l a t i o n were compared x F o r the purpose of i n t e r p o l a t i o n , o b s e r v a t i o n s were c o n s i d e r e d as having been made at 12 noon of a sampling day. 114 by s i m u l a t i n g the p e r i o d between t=17 and t=33 days when observed n i t r o g e n c o n c e n t r a t i o n s remained low d e s p i t e two n u t r i e n t a d d i t i o n s . Only phytoplankton dynamics were a c t i v a t e d . There was no q u a l i t a t i v e d i f f e r e n c e between phytoplankton c o n c e n t r a t i o n s p r e d i c t e d by the two runs. By t=33 days, the simple and s p e c i a l i n t e r p o l a t i o n schemes r e s p e c t i v e l y p r e d i c t e d 1701 and 1816 ug C l ' 1 f o r l a r g e diatoms, 0.1906 and 0.1049 pg C l " 1 f o r small diatoms, 17.51 and 16.35 ug C l " 1 f o r l a r g e f l a g e l l a t e s , and 0.2540 and 0.1982 pg C l " 1 f o r s m a l l f l a g e l l a t e s . The maximum abs o l u t e d i f f e r e n c e between p r e d i c t i o n s occurred at t=25.83 days, when l a r g e diatom biomass was 1085 ug C 1 _ 1 i n the run using s p e c i a l n u t r i e n t i n t e r p o l a t i o n and only 754 ug C l ' 1 i n the other run, a d i f f e r e n c e of 331 ug C 1" 1. Comparisons of the two i n t e r p o l a t i o n schemes on other s i m u l a t i o n runs confirmed that there was q u a l i t a t i v e agreement between p r e d i c t i o n s . I t was d e c i d e d to use the more ac c u r a t e n u t r i e n t i n t e r p o l a t i o n scheme in s i m u l a t i o n runs r e p o r t e d i n s e c t i o n 5 (whenever n i t r o g e n or s i l i c o n dynamics were d i s a b l e d ) . For reasons of expediency, the work r e p o r t e d i n s e c t i o n 6.3, i n which only the dynamics of l a r g e diatoms were a c t i v a t e d , was not repeated using s p e c i a l i n t e r p o l a t i o n . The author i s c o n f i d e n t that none of the c o n c l u s i o n s of s e c t i o n 6 are s i g n i f i c a n t l y a f f e c t e d or i n any way i n v a l i d a t e d by the use of the simpler scheme of n u t r i e n t i n t e r p o l a t i o n . 115 4.3 Accuracy of the F i n i t e D i f f e r e n c e Approximation I t was of course impossible to determine the a b s o l u t e accuracy of the f i n i t e - d i f f e r e n c e scheme when the exact s o l u t i o n to the system of d i f f e r e n t i a l equations was unknown. Instead, the model was run with a s e r i e s of s u c c e s s i v e l y l e s s a c c urate f i n i t e - d i f f e r e n c e schemes whose output was compared to a "base run" of highest accuracy. A l l runs began with the same i n i t i a l c o n d i t i o n s at t=3.5 days and ran u n t i l t=40 days. The base run of highest accuracy used d o u b l e - p r e c i s i o n a r i t h m e t i c , the second-order Midpoint method of f i n i t e d i f f e r e n c e s (p. 242 of Burden et a l . 1978), and a time step of 1 hour. The v a l u e s of the s t a t e v a r i a b l e s at the end of the base run were then compared to those p r e d i c t e d by f i r s t - o r d e r E u l e r f i n i t e d i f f e r e n c e s u s ing s i n g l e - p r e c i s i o n a r i t h m e t i c and a 4-hour time ste p . The a b s o l u t e e r r o r r e l a t i v e to the base run was n e g l i g i b l e : l e s s than 1.3 pg C l " 1 e r r o r f o r any phytoplankton group. Because the E u l e r method using s i n g l e - p r e c i s i o n a r i t h m e t i c and a 4-hour time step, produced simulated time streams which were v i r t u a l l y i n d i s t i n g u i s h a b l e from the much more c o s t l y Midpoint method, the E u l e r method was adopted f o r a l l subsequent runs and was c o n s i d e r e d to a c c u r a t e l y s o l v e the model's system of equations. 116 5. RESULTS AND DISCUSSION 'Agathon: But i t was you who proved t h a t death doesn't e x i s t . A l l e n : Hey, l i s t e n — I ' v e proved a l o t of t h i n g s . That's how I pay my r e n t . T h e o r i e s and l i t t l e o b s e r v a t i o n s . A p u c k i s h remark now and then. O c c a s i o n a l maxims. I t beats p i c k i n g o l i v e s , but l e t ' s not get c a r r i e d away. Agathon: But you have proved many times t h a t the s o u l i s immortal. A l l e n : And i t i s ! On paper.' — Woody A l l e n , My_ Apology, 1980 '... i f we take A = -1 m i . / s e c , then ... T h i s means that f o r every pound of matter r e t u r n i n g [from the moon] a m i l l i o n tons would have to s t a r t out [from the e a r t h ] , and i f we s p e c i f y mr2 = 10 6 l b . = 500 tons, then mr0 = 2.0 • 1 0 1 5 l b . . . . Hence, i f mr0 were d i s t r i b u t e d as a homogeneous sphere of m a t e r i a l of d e n s i t y about t h a t of s u r f a c e rock ... [then the spaceship] would thus be over f i v e m i l e s i n diameter and so would be almost as massive as Mt. E v e r e s t ! ' — J . W. Campbell, 1941, Rocket f l i g h t to the moon, The P h i l o s o p h i c a l Magazine, V o l . 31 (Seventh S e r i e s ) , pp. 24-34. Previo u s s e c t i o n s have o u t l i n e d the observed events i n CEE2, the c o n s t r u c t i o n of a model to simulate phytoplankton dynamics and n u t r i e n t c y c l i n g , and the computer implementation of that model. In t h i s s e c t i o n the model's p r e d i c t i o n s w i l l be p i t t e d a g a i n s t what was a c t u a l l y observed, accompanied by i n t e r p r e t a t i o n and d i s c u s s i o n — a blow-by-blow commentary on the model's performance. 117 5.1 The Reference S i m u l a t i o n In the r e f e r e n c e s i m u l a t i o n the dynamics of a l l s t a t e v a r i a b l e s were a c t i v a t e d and the parameter v a l u e s i n Table IV were used. The C/Si r a t i o of diatoms was not v a r i e d and s i l i c a d i s s o l u t i o n was not modelled. There was acute disagreement between p r e d i c t e d and observed events i n CEE2. L a r g e - c e l l e d diatoms were s e v e r e l y l i m i t e d i n the r e f e r e n c e run by exhaustion of s i l i c o n i n the s u r f a c e l a y e r ; d i s s o l v e d i n o r g a n i c n i t r o g e n accumulated. S m a l l - c e l l e d diatoms and f l a g e l l a t e s were q u i c k l y grazed to e x t i n c t i o n . L a r g e - c e l l e d f l a g e l l a t e s grew slowly throughout the s i m u l a t i o n to high c o n c e n t r a t i o n s i n the s u r f a c e 8 m by day 80. The f o l l o w i n g s e c t i o n s document these p r e d i c t i o n s i n d e t a i l . 5.1.1 Large diatoms Simulated l a r g e diatoms peaked to n400 ug C 1 _ 1 then f e l l on s e v e r a l occasions d u r i n g the f i r s t 50 days i n the s u r f a c e 8 m ( F i g . 18). The beginning and end of each bloom c o i n c i d e d with the r e s p e c t i v e a d d i t i o n and exhaustion of s i l i c o n i n the s u r f a c e l a y e r . (The jagged i n c r e a s e of biomass i n each bloom i s due to the absence at night of any p h o t o s y n t h e s i s to counteract r e s p i r a t o r y , g r a z i n g , s i n k i n g and mixing l o s s e s . ) Once s i l i c o n was exhausted in the s u r f a c e l a y e r i t remained below 0.3 ug-atom S i 1 _ 1 u n t i l r e p l e n i s h e d by f u r t h e r a d d i t i o n s . These low c o n c e n t r a t i o n s of S i prevented s i g n i f i c a n t gross p h o t o s y n t h e s i s by diatoms and t h e r e f o r e the l a r g e diatoms 118 F i g u r e 18. Biomass of l a r g e - c e l l e d diatoms i n the s u r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run. Octagons are c o n c e n t r a t i o n s observed i n CEE2. 120 d e c l i n e d p r e c i p i t o u s l y a f t e r S i was exhausted. I t i s of i n t e r e s t that the l a r g e diatoms d i d not bloom when s i l i c o n was added on day 62 but i n s t e a d i n c r e a s e d slowly to 95.1 ug C l " 1 over the next 18 days. The reason they d i d not r a p i d l y i n c r e a s e in abundance as they had a f t e r e a r l i e r a d d i t i o n s was because the high c o n c e n t r a t i o n of l a r g e - c e l l e d f l a g e l l a t e s — o v e r 1000 ug C l " 1 — s t r o n g l y attenuated the ambient l i g h t . At the time of the f i n a l s i l i c o n a d d i t i o n the 1% s u r f a c e l i g h t depth was 6.4 m and ambient l i g h t l i m i t e d the gross p h o t o s y n t h e t i c r a t e of l a r g e diatoms in the s u r f a c e 8 m to <37% of t h e i r maximum r a t e . The growth of l a r g e f l a g e l l a t e s was l i m i t e d t o the same extent by the low l i g h t , but having a t t a i n e d such a l a r g e c o n c e n t r a t i o n they e f f e c t i v e l y prevented any other a l g a l group from c h a l l e n g i n g t h e i r dominance. T h i s c o u l d e x p l a i n why l a r g e diatoms i n CEE2 f a i l e d to i n c r e a s e above 94.6 ug C l " 1 i n the s u r f a c e 8 m once Ceratium had exploded in biomass a f t e r day 65, d e s p i t e observed c o n c e n t r a t i o n s of s i l i c o n and i n o r g a n i c n i t r o g e n i n excess of 19.1 and 1.6 ug-atom 1' 1 r e s p e c t i v e l y , and l i t t l e g r a z i n g by h e r b i v o r e s . The simulated changes i n l a r g e diatom carbon i n the top 8 m were determined by the changing balance of gain and l o s s 121 F i g u r e 19. Modelled gain and l o s s r a t e s s p e c i f i c to p r e d i c t e d l a r g e diatom biomass; r e f e r e n c e run; s u r f a c e 8 m. Rates averaged over s u c c e s s i v e 24-hour i n t e r v a l s . Mixing l o s s e s are n e g l i g i b l e and are not p l o t t e d . 1 - gross p h o t o s y n t h e s i s 2 - r e s p i r a t i o n 3 - exudation 4 - s i n k i n g 5 - g r a z i n g 122 Spec i f i c Gain/Loss Rate (day') o A 123 r a t e s shown i n F i g . 19. 1 S p e c i f i c r e s p i r a t i o n d e c l i n e d s l i g h t l y from 0.18 day" 1 on day 4 to 0.14 day" 1 on day 80. There was a constant s i n k i n g l o s s of 0.04 day" 1, and mixing l o s s was n e g l i g i b l e (<0.014 d a y " 1 ) . Exudation was modelled as 10% of the gross p h o t o s y n t h e t i c r a t e ( s e c t i o n 3.4.5). Gr a z i n g pressure s u b s t a n t i a l l y decreased over the s i m u l a t i o n , accounting f o r 45% of the t o t a l l o s s from l a r g e diatoms at noon on day 13, but only 15% of t o t a l l o s s at noon on day 80. T h i s r e d u c t i o n i n g r a z i n g i s a t t r i b u t a b l e to the fewer numbers of herbivorous copepods and c i l i a t e s (able to f i l t e r l a r g e p a r t i c l e s ) l a t e r i n the experiment. What i s s u p r i s i n g i s that the model p r e d i c t e d that c i l i a t e s were the most important g r a z e r s of l a r g e diatoms, ra t h e r than copepods as i s c o n v e n t i o n a l l y supposed ( F i g . 20). Even when copepods were most numerous, c i l i a t e s were p r e d i c t e d to be r e s p o n s i b l e f o r 66% of the t o t a l g r a z i n g l o s s . When c o n s i d e r i n g the c r e d i b i l i t y of t h i s p r e d i c t i o n i t i s important to remember that c i l i a t e s were assumed to be capable of f i l t e r i n g c e l l s no l a r g e r than 25 um e q u i v a l e n t s p h e r i c a l diameter (p. 87 of s e c t i o n 3.5.2). Diatoms between 15 and 25 um ESD averaged 13% of the l a r g e diatom biomass observed in CEE2, t h e r e f o r e c i l i a t e s capable of f i l t e r i n g l a r g e p a r t i c l e s were 1 I n F i g . 19 the modelled r a t e s of change have been averaged over s u c c e s s i v e 24-hour i n t e r v a l s to e l i m i n a t e the r a p i d d i u r n a l o s c i l l a t i o n s of gross p h o t o s y n t h e s i s and exudation. Thus, peak gross p h o t o s y n t h e t i c r a t e s of n l day" 1 ( s p e c i f i c to modelled l a r g e diatom carbon) recorded i n F i g . 19 immediately a f t e r the f i r s t three s i l i c o n a d d i t i o n s are l e s s than the peak instantaneous r a t e s of 1.7 day" 1 experienced at midday. D a i l y averages provide a c l e a r e r p i c t u r e of gain or l o s s r e a l i z e d over d i e l c y c l e s . 124 F i g u r e 20. P r e d i c t e d s p e c i f i c g r a z i n g l o s s of l a r g e diatoms; r e f e r e n c e run; surface 8 m. 1 - l a r v a c e a n s 2 - metazoan l a r v a e 3 - c i l i a t e s 4 - copepods 125 126 assumed to have a constant s e l e c t i v i t y of 0.13 f o r l a r g e diatoms. Had the r e f e r e n c e run reproduced the observed e a r l y bloom of l a r g e diatoms which was dominated by 60 um c e l l s , i t would have erroneously p r e d i c t e d a (presumably nonexistent) g r a z i n g l o s s to c i l i a t e s . Conversely, i t would have underestimated the impact of c i l i a t e s l a t e r when l a r g e - c e l l e d diatoms of <25 um ESD c o n s t i t u t e d much more than 13% of l a r g e diatom biomass. (Of course the formation of diatom chains would s i g n i f i c a n t l y decrease g r a z i n g l o s s to c i l i a t e s . ) What can be reasonably concluded i s that the dominant g r a z e r of c e l l s s m a l l e r than n25 um c o u l d have been c i l i a t e s . Whereas the importance of marine benthic c i l i a t e s as g r a z e r s of diatoms and d i n o f l a g e l l a t e s has been e x t e n s i v e l y documented (Fenchel 1968), p e l a g i c c i l i a t e s have not yet been accorded the same r e c o g n i t i o n . Below 8 m the r e s p i r a t i o n of a l l a l g a l groups exceeded t h e i r gross p h o t o s y n t h e s i s , y i e l d i n g an average net p h o t o s y t h e t i c rate of -3.2 ug C 1 _ 1 d a y 1 over the 76.5 day simulated time p e r i o d . As might be expected, t h i s p r e d i c t e d r a t e of net ph o t o s y n t h e s i s i n the 8-20 m l a y e r was l e s s than the average 1 4 C f i x a t i o n of 3.1 ug C l " 1 observed i n 4-hour i n  s i t u i n c u b a t i o n s at the r e l a t i v e l y shallow depth of 12 m. The modelled phytoplankton d i d not a c t i v e l y grow i n the deep l a y e r but p a s s i v e l y changed c o n c e n t r a t i o n i n phase with the f l u x of carbon s i n k i n g from the s u r f a c e l a y e r . Because the model assumed that phytoplankton sank at constant r a t e s , a l g a l carbon i n the deep l a y e r peaked simultaneously (but at g r e a t l y 127 d i m i n i s h e d c o n c e n t r a t i o n ) with blooms in the s u r f a c e l a y e r . T h i s i s best i l l u s t r a t e d by modelled l a r g e diatom biomass i n the s u r f a c e and deep l a y e r s ( F i g . 21). T h i s same e f f e c t was observed f o r a l l four a l g a l groups i n CEE2: there was no apparent l a g between i n c r e a s e s i n phytoplankton carbon above and below 8 m. 5.1.2 Large f l a g e l l a t e s In c o n t r a s t to the c y c l i c growth and c o l l a p s e of l a r g e diatoms i n the s u r f a c e 8 m, the growth of l a r g e f l a g e l l a t e s was s u s t a i n e d u n t i l the end of the s i m u l a t i o n ( F i g . 22). The l a r g e f l a g e l l a t e s peaked at 1400 ug C l " x on day 72, c l o s e to the peak of 1250 ug C 1 _ 1 observed on day 74. The l a r g e f l a g e l l a t e s grew r e l a t i v e l y slowly compared to the r a p i d b u r s t s of l a r g e diatom biomass p r e d i c t e d by the r e f e r e n c e run. T h i s was due to the assumption that the maximum gross p h o t o s y n t h e t i c r a t e of l a r g e f l a g e l l a t e s was only 36% of that of l a r g e diatoms (Eq. 18, p. 69); the l a r g e s t p r e d i c t e d s p e c i f i c r a t e of gross p h o t o s y n t h e s i s of l a r g e f l a g e l l a t e s was 0.63 d a y 1 at noon on day 12 compared to 1.73 d a y 1 f o r l a r g e diatoms on day 13. The growth of l a r g e f l a g e l l a t e biomass slowed and f i n a l l y stopped l a t e i n the s i m u l a t i o n , not because of i n c r e a s i n g l o s s r a t e s — g r a z i n g pressure decreased markedly a f t e r day 2 0 — b u t because gross p h o t o s y n t h e s i s became more and more l i g h t l i m i t e d through s e l f - s h a d i n g by the huge c o n c e n t r a t i o n of l a r g e f l a g e l l a t e s ( F i g . 23). Grazing pressure s p e c i f i c to l a r g e f l a g e l l a t e biomass was 128 F i g u r e 21. P r e d i c t e d biomass of l a r g e diatoms i n the s u r f a c e ( s o l i d l i n e ) and deep (dashed l i n e ) l a y e r s ; r e f e r e n c e run. 130 Fi g u r e 22. Biomass of l a r g e - c e l l e d f l a g e l l a t e s i n the s u r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run. Octagons are c o n c e n t r a t i o n s observed i n CEE2. 132 F i g u r e 23. Modelled gain and l o s s r a t e s s p e c i f i c to p r e d i c t e d l a r g e f l a g e l l a t e biomass; r e f e r e n c e run; s u r f a c e 8 m. Rates averaged over s u c c e s s i v e 24-hour i n t e r v a l s . Mixing l o s s e s are n e g l i g i b l e and are not p l o t t e d . 1 - gross p h o t o s y n t h e s i s 2 - r e s p i r a t i o n 3 - exudation 4 - s i n k i n g 5 - g r a z i n g 1 3 3 C D -Spec i f i c Gain/Loss Rate (day"') - 0 . 5 v 0.5 134 never more than 58% of the s p e c i f i c g r a z i n g on l a r g e diatoms. The much lower g r a z i n g pressure on l a r g e f l a g e l l a t e s was due to the very small p r o p o r t i o n of f l a g e l l a t e c e l l s between 15 and 25 um e q u i v a l e n t s p h e r i c a l diameter. F l a g e l l a t e s with c e l l s i z e s l a r g e r than 25 um were s h i e l d e d from c i l i a t e g r a z i n g : c i l i a t e s accounted f o r 80 and 46% of the r e s p e c t i v e g r a z i n g l o s s from l a r g e diatoms and l a r g e f l a g e l l a t e s . 5.1.2 Small diatoms and small f l a g e l l a t e s S m a l l - c e l l e d phytoplankton i n the r e f e r e n c e run were r a p i d l y removed by c i l i a t e g r a z i n g . Small f l a g e l l a t e biomass dropped below 1 ug C 1 _ 1 by day 9, and small diatom biomass dropped below 1 ug C 1 _ 1 by day 10. The g r a z i n g pressure was e s p e c i a l l y intense d u r i n g the f i r s t 40 days when the s p e c i f i c g r a z i n g l o s s ranged from 0.9 to 2.3 d a y - 1 ( F i g . 24 shows g r a z i n g on small diatoms; g r a z i n g on small f l a g e l l a t e s was v i r t u a l l y i d e n t i c a l ) . On the average, g r a z i n g accounted fo r 76 and 83% of the r e s p e c t i v e carbon l o s s from s m a l l diatoms and f l a g e l l a t e s , and c i l i a t e s were r e s p o n s i b l e f o r 88% of the g r a z i n g l o s s . E i t h e r the r e f e r e n c e run had overestimated g r a z i n g on small phytoplankton, or net p h o t o s y n t h e s i s had been underestimated. T h i s c o n c l u s i o n p e r t a i n s e s p e c i a l l y to the p e r i o d between days 48 and 55, when small diatoms were observed to i n c r e a s e i n c o n c e n t r a t i o n from 4.49 ug C l " 1 to 221 ug C l ' 1 i n the top 8 m of CEE2 ( F i g . 2b), an average net r a t e of growth of 0.56 day" 1. A s i m u l a t i o n run i n which phytoplankton 135 F i g u r e 24. P r e d i c t e d s p e c i f i c g r a z i n g l o s s of small diatoms; r e f e r e n c e run; s u r f a c e 8 m. 1 - l a r v a c e a n s 2 - metazoan l a r v a e 3 - c i l i a t e s 4 - copepods 137 were c o n s t r a i n e d to f o l l o w t h e i r observed c o n c e n t r a t i o n s p r e d i c t e d an average s p e c i f i c g r a z i n g l o s s from small diatoms of 0.51 d a y 1 during the same p e r i o d . N e g l e c t i n g l o s s e s other than g r a z i n g , small diatoms would have needed a s p e c i f i c net p h o t o s y n t h e t i c r a t e of 0.56 + 0.51 = 1.07 d a y 1 (1.54 doublings per day) to y i e l d t h e i r observed i n c r e a s e i n c o n c e n t r a t i o n . T h i s d o u b l i n g r a t e i s c l o s e to the maximal growth r a t e of 1.59 doublings per day expected on the b a s i s of Chan's (1978b) work ( d e r i v e d f o r the small diatom assemblage of CEE2 by combining B r j from Eq. 17, p. 69 with Eq. 11a, p. 66). 5.1.4 B a c t e r i a Although the average c o n c e n t r a t i o n of b a c t e r i a p r e d i c t e d by the r e f e r e n c e run f o r the s u r f a c e l a y e r , 22.0 ug C l " 1 , agreed with the average c o n c e n t r a t i o n of 22.6 ug C l " 1 observed over the same p e r i o d i n CEE2, there was l i t t l e resemblance between p r e d i c t e d and observed time s e r i e s of b a c t e r i a l carbon ( F i g . 25). In the deep l a y e r the average p r e d i c t e d c o n c e n t r a t i o n of b a c t e r i a was 4.2 ug C l " 1 , only 35% of the observed average of 11.9 ug C l " 1 . The p r e d i c t e d peaks i n b a c t e r i a l carbon c o i n c i d e d with p r e d i c t e d s p u r t s i n l a r g e - d i a t o m exudation. I t was noted e a r l i e r ( s e c t i o n 2.4, p. 36) that the major peak of b a c t e r i a l carbon in the s u r f a c e 8 m of CEE2 occu r r e d d u r i n g the l a s t h a l f of the c o l l a p s e of l a r g e diatoms ( c f . F i g s . 2a and 11). I t i s not unusual f o r exudation by n a t u r a l phytoplankton p o p u l a t i o n s to i n c r e a s e d u r i n g the c o l l a p s e of blooms (e.g. H e l l e b u s t 1965), and t h i s 138 Fi g u r e 25. Biomass of b a c t e r i a i n the s u r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run. Octagons are c o n c e n t r a t i o n s observed i n CEE2. 140 c o u l d have accounted f o r the main peak i n b a c t e r i a l biomass. Vinogradov et a l . (1972, 1973) p r e d i c t e d that b a c t e r i a would peak i n abundance s h o r t l y a f t e r peak phytoplankton abundance when d e t r i t u s from the c o l l a p s i n g bloom was a s u b s t a n t i a l source of food f o r b a c t e r i a . I t i s not known how d e t r i t u s changed i n abundance i n CEE2. Fuhrman et a l . (1980) found, t h a t b a c t e r i a l growth r a t e s i n the Southern C a l i f o r n i a Bight were s i g n i f i c a n t l y c o r r e l a t e d with c h l o r o p h y l l , but not with primary p r o d u c t i o n . They s p e c u l a t e that b a c t e r i a l growth was s t i m u l a t e d by o r g a n i c s r e l e a s e d d u r i n g zooplankton g r a z i n g r a t h e r than by or g a n i c s leaked from a c t i v e l y p h o t o s y n t h e s i z i n g c e l l s . T h i s i n t e r p r e t a t i o n would seem to be c o n s i s t e n t with the tim i n g of the main peak of b a c t e r i a l biomass i n CEE2. I n d i r e c t evidence to be presented i n s e c t i o n 6 a l s o suggests that p e r i o d s of a c t i v e a l g a l growth and l o s s of o r g a n i c s were out of phase i n CEE2. The g r e a t e s t l o s s of biomass from b a c t e r i a was v i a r e s p i r a t i o n r a t h e r than g r a z i n g . In the su r f a c e l a y e r , r e s p i r a t i o n , g r a z i n g and mixing r e s p e c t i v e l y accounted f o r 85%, 14% and 1% of the t o t a l l o s s of b a c t e r i a l carbon d u r i n g the re f e r e n c e run. The average growth e f f i c i e n c y of b a c t e r i a , d e f i n e d as 1 - (carbon r e s p i r e d / c a r b o n a s s i m i l a t e d ) , was p r e d i c t e d to be 0.15 i n the su r f a c e l a y e r and -0.01 i n the deep l a y e r , much l e s s than the e f f i c i e n c y of 0.6 which i s t y p i c a l of c u l t u r e d b a c t e r i a i n t h e i r growth phase (Calow 1977). The p r e d i c t e d growth e f f i c i e n c i e s were much l e s s than. 0.6 because endogenous r e s p i r a t i o n exceeded the immediate r e s p i r a t o r y l o s s 141 of 40% of a s s i m i l a t e d carbon. There are three p o s s i b l e e x p l a n a t i o n s of t h i s d i s c r e p a n c y between p r e d i c t e d and t y p i c a l growth e f f i c i e n c i e s : 1) the growth e f f i c i e n c y of c u l t u r e d b a c t e r i a i s not a p p l i c a b l e t o n a t u r a l b a c t e r i a ; or 2) one or both components of b a c t e r i a l r e s p i r a t i o n have been overestimated; or 3) the i n f l u x of carbon to b a c t e r i a has been g r e a t l y underestimated, p r e v e n t i n g a s s i m i l a t i o n - r e l a t e d r e s p i r a t i o n from dominating t o t a l r e s p i r a t i o n . The i m p l i c a t i o n s of the t h i r d p o s s i b i l i t y are e s p e c i a l l y i n t e r e s t i n g and they w i l l be c o n s i d e r e d i n s e c t i o n 5.1.5. C o l o u r l e s s f l a g e l l a t e s , l a r v a c e a n s , c i l i a t e s and metazoan l a r v a e were p r e d i c t e d to r e s p e c t i v e l y account f o r 80.9, 13.6, 5.2 and 0.3% of the t o t a l b a c t e r i a l carbon grazed i n the sur f a c e 8 m. ( S i m i l a r percentages were p r e d i c t e d when b a c t e r i a l carbon was c o n s t r a i n e d to f o l l o w observed c o n c e n t r a t i o n s . ) The r e l a t i v e importance of these p o t e n t i a l b a c t e r i o v o r e s p r e d i c t e d by the model agrees with King et a l . (1980) who c a l c u l a t e d that l a r v a c e a n s had l i t t l e impact on b a c t e r i o p l a n k t o n i n CEE2. King et a l . (1980) suggested that phagotrophic f l a g e l l a t e s c o u l d be the p r i n c i p l e consumers of b a c t e r i a . T h i s would d e f i n i t e l y be the case i f they d i d i n f a c t f i l t e r b a c t e r i a at the r a t e of 20 n l h" 1 per i n d i v i d u a l assumed i n the model. T h i s c l e a r a n c e r a t e e q u a l l e d the assumed c l e a r a n c e r a t e of c i l i a t e s but was much l e s s than l a r v a c e a n c l e a r a n c e r a t e s (per i n d i v i d u a l ) ; the overwhelming importance of c o l o u r l e s s f l a g e l l a t e s as p o t e n t i a l p r e d a t o r s on b a c t e r i a was simply due to t h e i r tremendous numbers—as many as 1500 per ml. 142 5.^.5 N i t r o g e n c y c l i n g S i m u l a t i o n r e s u l t s , s u r f a c e l a y e r — D i s s o l v e d i n o r g a n i c n i t r o g e n was p r e d i c t e d t o accumulate to c o n c e n t r a t i o n s as h i g h as 25 ug-atom N l ' 1 i n the s u r f a c e l a y e r , much higher than observed c o n c e n t r a t i o n s ( F i g . 26). To examine the model's performance more c l o s e l y , only the p e r i o d p r i o r to noon on day 62 was c o n s i d e r e d . 2 D u r i n g . t h i s p e r i o d from t=3.5 to t=61.5 (58 days), the r e f e r e n c e run p r e d i c t e d the accumulation of 15.7 ug-atom N l " 1 i n the s u r f a c e 8 m. The observed accumulation was 1.8 ug-atom N l~x». On the average, t h e r e f o r e , the r e f e r e n c e run e i t h e r underestimated net phytoplankton uptake of n i t r o g e n or overestimated n i t r o g e n r e g e n e r a t i o n from b a c t e r i a or zooplankton i n the s u r f a c e l a y e r . Of the v a r i o u s sources of n i t r o g e n i n the s u r f a c e l a y e r , a r t i f i c i a l a d d i t i o n s of NH r4 + ( v i a sediment pumping) and N0 r3" were p r e d i c t e d to account f o r 70% of t o t a l supply. In descending importance, 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 accounted f o r 14% of t o t a l supply, zooplankton e x c r e t i o n 10%, and mixing from 2 T h i s p e r i o d was s e l e c t e d because larvaceans were not sampled a f t e r day 63; u n c e r t a i n t y i n t h e i r numbers a f t e r day 63 would have been an a d d i t i o n a l (though presumably small) source of e r r o r when e s t i m a t i n g zooplankton e x c r e t i o n . Day 62 was s e l e c t e d r a t h e r than day 63 because n u t r i e n t c o n c e n t r a t i o n s were observed on day 62. 3Input v i a mixing has probably been underestimated i n the r e f e r e n c e run because p r e d i c t e d s u r f a c e c o n c e n t r a t i o n s were higher than observed, r e s u l t i n g i n an underestimated c o n c e n t r a t i o n g r a d i e n t between the s u r f a c e and deep l a y e r s . 143 F i g u r e 26. C o n c e n t r a t i o n of n i t r a t e + n i t r i t e + ammonium i n the s u r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run. Octagons are c o n c e n t r a t i o n s observed i n CEE2. Nitrogen (ug-at N I"') 0 30 C D 145 the deep l a y e r 6%.3 Copepods were r e s p o n s i b l e f o r most of the zooplankton e x c r e t i o n e a r l y i n the s i m u l a t i o n , but metazoan l a r v a e (of p o l y c h a e t e s , o l i g o c h a e t e s , gastropods, pelecypods and bryozoans) dominated e x c r e t i o n a f t e r day 40 ( F i g . 27). T h i s r a i s e s an embarrassing q u e s t i o n : How d i d copepods and metazoan l a r v a e c o n t r i b u t e most of the n i t r o g e n e x c r e t e d when they were not the most v o r a c i o u s g r a z e r s of phytoplankton or b a c t e r i a l biomass ( c f . F i g s . 20, 24 and 27)? U n f o r t u n a t e l y , the model d i d not e x p l i c i t l y couple zooplankton e x c r e t i o n with g r a z i n g . To e l i m i n a t e any b i a s e s caused by u n r e a l i s t i c p r e d i c t i o n s of phytoplankton and b a c t e r i a l biomass by the r e f e r e n c e run, a f u r t h e r s i m u l a t i o n was executed i n which phytoplankton and b a c t e r i a were c o n s t r a i n e d to f o l l o w t h e i r observed c o n c e n t r a t i o n s . In t h i s run, again between t=3.5 and 61.5 days, copepods e x c r e t e d 114% of the n i t r o g e n they grazed i n the s u r f a c e l a y e r , i . e . they e x c r e t e d more than they a t e ! Metazoan l a r v a e e x c r e t e d 182% of grazed n i t r o g e n ; c i l i a t e s — t h e dominant g r a z e r s of phytoplankton carbon—-excreted o n l y 1.6% of the n i t r o g e n they grazed. Since modelled g r a z i n g and e x c r e t i o n r a t e s were independently d e r i v e d , i t i s h a r d l y s u p r i s i n g that these two parameters were so p o o r l y c o r r e l a t e d i n i n d i v i d u a l groups of zooplankton. Considered as a whole, zooplankton e x c r e t e d 16% of the n i t r o g e n grazed (phytoplankton and b a c t e r i a l dynamics d i s a b l e d ) . The r e f e r e n c e run p r e d i c t e d 44%. (Over the e n t i r e water column, the two runs r e s p e c t i v e l y p r e d i c t e d e x c r e t i o n of 25 and 73% of the n i t r o g e n grazed.) I n v e r t e b r a t e s t y p i c a l l y e x c rete 30 - 70% of the organic 146 F i g u r e 27. P r e d i c t e d e x c r e t i o n of i n o r g a n i c n i t r o g e n by zooplankton; r e f e r e n c e run; s u r f a c e 8 m. 1 - chaetognaths 2 - ctenophores 3 - c o l o u r l e s s f l a g e l l a t e s 4 - larv a c e a n s 5 - metazoan l a r v a e 6 - c i l i a t e s 7 - copepods Zooplankton Excret ion (ng-at N 1~; day"1) Q 5 0 0 i—]._| ,i i i i i i i i i i CD 148 m a t e r i a l they i n g e s t (Calow 1977). What can be concluded i s that over 0-20 m simulated zooplankton e x c r e t i o n as a whole was not an unreasonable f r a c t i o n of the m a t e r i a l grazed, but that the f r a c t i o n e x c r e t e d was too small above 8 m and too l a r g e below 8 m. For i n d i v i d u a l zooplankton groups, the balance between g r a z i n g and e x c r e t i o n was c l e a r l y u n r e a l i s t i c . S i m u l a t i o n r e s u l t s , deep l a y e r — I n the deep l a y e r , the r e f e r e n c e run p r e d i c t e d an accumulation of 5.1 ug-atom N 1 _ 1 between t=3.5 and 61.5 days. The observed accumulation was 5.0 ug-atom N l " 1 , which can be f u r t h e r r e s o l v e d i n t o a net i n c r e a s e of 13.3 ug-atom ammonium-N 1 _ 1 and a net drop of 8.3 ug-atom n i t r a t e - N 1 _ 1 (NO r2~ c o n c e n t r a t i o n f l u c t u a t e d between 0.17 and 0.42 ug-atom N l " 1 ) . The agreement between the observed and p r e d i c t e d accumulation of n i t r o g e n i n the deep l a y e r i s c o i n c i d e n t a l . The l o s s of n i t r o g e n v i a mixing i n t o the s u r f a c e l a y e r , 3.3 ug-atom N 1 _ 1 over 58 days, was probably underestimated f o r reasons noted e a r l i e r ( f o o t n o t e 3 , p. 142). One can roughly c a l c u l a t e a more r e a l i s t i c mixing l o s s by employing the exchange c o e f f i c i e n t developed i n s e c t i o n 3.3 (Eq. 5b, p. 49) i n c o n j u n c t i o n with the observed d i f f e r e n c e i n NO [-3" and NH r4 + c o n c e n t r a t i o n between the s u r f a c e and deep l a y e r s on each of 25 sampling days between t=3.5 and 61.5. T h i s y i e l d s an estimated l o s s of 7.7 ug-atom n i t r a t e - N l " 1 and 3.5 ug-atom ammonium-N l " 1 from the deep l a y e r . Thus the observed drop of 8.3 ug-atom n i t r a t e - N l " 1 may have simply been due to mixing i n t o the s u r f a c e l a y e r together with some phytoplankton 149 uptake. The r a p i d i t y of ammonium b u i l d u p observed i n the deep l a y e r between days 4 and 62 was unexpected. B a c t e r i a and zooplankton were p r e d i c t e d to r e m i n e r a l i z e 7.8 ug-atom N l " 1 (as ammonium) i n the ref e r e n c e run du r i n g the 58-day p e r i o d ; phytoplankton r e s p i r e d 6.5 ug-atom N l " 1 (see footnote 4 ) ; 3.5 ug-atom ammonium-N I' 1 would be l o s t by mixing. The expected net gain of ammonium i n the deep l a y e r was . t h e r e f o r e 7.8 + 6.5 - 3.5 = 10.8 ug-atom N l " 1 , s hort of the observed 13.3 ug-atom N l " 1 g a i n . Moreover, phytoplankton uptake has not been c o n s i d e r e d . I t must be concluded that r e m i n e r a l i z a t i o n of ammonia i n the deep l a y e r of CEE2 was underestimated i n the ref e r e n c e run. I t i s u n l i k e l y that zooplankton e x c r e t i o n has been underestimated ( i t being a l r e a d y an uncomfortably high p r o p o r t i o n of the organic n i t r o g e n grazed); i n s t e a d , 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 i n the deep l a y e r was probably underestimated. In the pr e v i o u s s e c t i o n (p. 14,0) i t was noted that b a c t e r i a were p r e d i c t e d to r e s p i r e more m a t e r i a l than was absorbed from the deep l a y e r , and consequently b a c t e r i a l r e s p i r a t i o n was e i t h e r too high or i n f l u x of o r g a n i c s too low. These separate c o n c l u s i o n s can be r e c o n c i l e d only i f the i n f l u x of o r g a n i c s to b a c t e r i a was indeed underestimated. T h i s s h o r t f a l l of organic supply to b a c t e r i a c o u l d be made up i n many ways: a c c e l e r a t e d *The model assumed that N was r e l e a s e d p r o p o r t i o n a l l y to r e s p i r e d C to maintain a constant C/N r a t i o even though the r e l e a s e of N would not i n f a c t be d i r e c t l y l i n k e d to r e s p i r a t i o n ; f o r the sake of argument t h i s " r e s p i r e d " n i t r o g e n was assumed to be ammonium-N. 150 exudation or l y s i s of phytoplankton which s e t t l e d below the p h o t i c zone, i n c r e a s e d s i n k i n g from the upper l a y e r , or decomposition of sediment below 20 m. Simple n i t r o g e n budget f o r CEE2—Two d r a m a t i c a l l y d i f f e r e n t p i c t u r e s of n i t r o g e n c y c l i n g i n CEE2 can be c o n s t r u c t e d depending on whether or not one accepts the measured 1 4 C f i x a t i o n r a t e s as r e p r e s e n t a t i v e of the true net pro d u c t i o n of carbon and the simulated r e g e n e r a t i o n of n i t r o g e n by b a c t e r i a and zooplankton as being r e a l i s t i c . I f we accept these premises, do they l e a d to a c o n s i s t e n t p i c t u r e of n i t r o g e n c y c l i n g ? The i n f e r r e d net phytoplankton uptake of n i t r o g e n ( i n f e r r e d NPUN) i n the s u r f a c e 8 m can be d e f i n e d as NPUN = a r t i f i c i a l N a d d i t i o n s + N i n f l u x v i a mixing + b a c t e r i a l and zooplankton r e g e n e r a t i o n of N observed i n c r e a s e i n N c o n c e n t r a t i o n . (37) Co n s i d e r i n g as before only the p e r i o d from t=3.5 to 61.5, the terms i n Eq. 37 can be f i l l e d i n as f o l l o w s : NPUN (ug-atom N l - l / 5 8 days) = 58.8 + (16.8) + (20.4) - 1.8 = 94.1. (38) The bracketed terms i n Eq. 38 are u n c e r t a i n : the mixing i n f l u x of 16.8 ug-atom N l " 1 was roughly estimated from observed NO r3" and NHr4* c o n c e n t r a t i o n s as d e s c r i b e d e a r l i e r (p. 148), and the re g e n e r a t i o n of 20.4 ug-atom N l " 1 i s that p r e d i c t e d by the re f e r e n c e run. The NPUN value of 94.1 ug-atom N l " 1 / 5 8 days can be converted to an expected average r a t e of net 151 p r o d u c t i o n of 136 ug C l " 1 day" 1 assuming a C/N mass r a t i o of 6. The observed 1 4 C f i x a t i o n r a t e i n the top 4 m of CEE2 duri n g 4-hour midday i n c u b a t i o n s was 58.8 ± 40.3 ug C l " 1 (mean ± s.dev., n=25) and 20.0 ± 15.7 ug C l " 1 (n=26) in 4-8 m, again d u r i n g the same 58 day p e r i o d . The average observed net primary p r o d u c t i o n was t h e r e f o r e roughly (58.8 + 20.0)/2 = 39.4 ug C 1 " V 4 hours 2.4 ( f o o t n o t e 5) = 95 ug C l " 1 day" 1. (Nighttime r e s p i r a t i o n of f i x e d carbon has not been subtracted.) C o n s i d e r i n g the roughness of these c a l c u l a t i o n s and the l a r g e v a r i a b i l i t y of 1 4 C f i x a t i o n r a t e s , the agreement between the i n f e r r e d c a r b o n - e q u i v a l e n t NPUN of 136 ug C l " 1 day" 1 and the observed 1 4 C p r o d u c t i v i t y of 95 ug C l " 1 day" 1 i s remarkably good. Thus the simulated rate of b a c t e r i a l and zooplankton r e g e n e r a t i o n •of n i t r o g e n i s c o n s i s t e n t with observed 1 4 C primary production i n the bubbled l a y e r of CEE2. The bubbled l a y e r of CEE2 i s analogous to the s u r f a c e mixed l a y e r of a h i g h l y e u t r o p h i c upwelling zone. N i t r a t e , s i l i c a t e and phosphate were added to the bubbled l a y e r at weekly i n t e r v a l s ; s i m i l a r l y , t r a n s i e n t i n j e c t i o n s of n u t r i e n t s i n t o the mixed l a y e r from below the euphotic zone d u r i n g storms or the onset of c o a s t a l u p w e l l i n g have been r e p o r t e d (Walsh et a l . 1974, 1978). Eppley et a l . (1979a) have shown that the i n j e c t i o n of n i t r a t e i n t o the euphotic zone of southern 5 2.-4 = i n s o l a t i o n over f u l l d i u r n a l c y c l e d i v i d e d by i n s o l a t i o n between 1000 and 1400 hours PST ( i n c u b a t i o n p e r i o d ) , f o r a c l e a r , sunny day at 48° N d u r i n g Foodweb I. 152 C a l i f o r n i a c o a s t a l waters not only i n c r e a s e s phytoplankton p r o d u c t i o n , but a l s o a c c e l e r a t e s h e t e r o t r o p h i c r e m i n e r a l i z a t i o n . How d i d the heavy n i t r a t e l o a d i n g of the sur f a c e l a y e r of CEE2 a f f e c t h e t e r o t r o p h i c a c t i v i t i e s i n Foodweb I? T h i s q u e s t i o n can be approached by c o n s i d e r i n g the amount of "new" and "regenerated" p r o d u c t i o n in CEE2. Dugdale and Goering (1967) d e f i n e d new p r o d u c t i o n as primary p r o d u c t i o n a s s o c i a t e d with N0 r3" or N r2 newly i n c o r p o r a t e d by phytoplankton i n the euphotic zone, and regenerated p r o d u c t i o n to be a s s o c i a t e d with r e c y c l e d ammonium-N or d i s s o l v e d organic N. I m p l i c i t i n t h i s approach i s the assumption that iri s i t u b a c t e r i a l n i t r i f i c a t i o n or d e n i t r i f i c a t i o n , or n i t r a t e r e d u c t i o n by phytoplankton or b a c t e r i a , i s n e g l i g i b l e ; N r2 f i x a t i o n or phytoplankton uptake of urea or other sources of organ i c n i t r o g e n have a l s o been n e g l e c t e d here. The i n f e r r e d r a t i o of new to regenerated p r o d u c t i o n i n the e n t i r e water column i s then (again assuming that simulated r e m i n e r a l i z a t i o n of n i t r o g e n by b a c t e r i a and zooplankton i s r e a l i s t i c ) : New/Regenerated = NO r3" added - observed AN0 r3'  NH r4 + r e m i n e r a l i z e d + NH r4 + from sediment - observed ANH r4 + 20.4 + 4.8 = 2.4. (39) 12.8 + 6.5 - 8.7 The i n d i v i d u a l terms in Eq. 39 have u n i t s of ug-atomN l _ 1 / 5 8 days. They were d e r i v e d by c a l c u l a t i n g f l u x e s s e p a r a t e l y f o r 0-8 and 8-20 m, then r e s p e c t i v e l y weighting the two l a y e r s by 0.4 and 0.6 to r e f l e c t t h e i r d i f f e r e n t volumes (e.g. NOr3~ 153 a d d i t i o n to the top 8 m was 51.0 ug-atom N 1'1 = an average a d d i t i o n of 51.0 • 0.4 = 20.4 ug-atom N l " 1 over 0-20 m). NH [-4 * from sediments i n c l u d e d the expected a d d i t i o n of 7.8 ug-atom N 1 _ 1 to the s u r f a c e 8 m from sediment pumping p l u s an unmeasured f l u x i n t o the deep l a y e r from sediments. T h i s l a t t e r f l u x was estimated as the observed b u i l d u p of NH r4 + i n the deep l a y e r p l u s the mixing l o s s i n t o the s u r f a c e l a y e r , minus the simulated r e m i n e r a l i z a t i o n by b a c t e r i a and zooplankton. The net uptake of ammonium by phytoplankton i n the deep l a y e r was assumed to be zero. Expressed d i f f e r e n t l y , new pro d u c t i o n as a percentage of t o t a l p r o d u c t i o n was 70%, pro v i d e d that r e m i n e r a l i z a t i o n r a t e s were r e a l i s t i c a l l y s i m u l a t e d . T h i s percentage was s u b s t a n t i a l l y l a r g e r than the 50% c o n s i d e r e d t y p i c a l of e u t r o p h i c areas ( n e g l e c t i n g the c o n t r i b u t i o n of urea to regenerated p r o d u c t i o n ; Dugdale 1976). In c o n c l u s i o n , i f the simulated r a t e of n i t r o g e n r e m i n e r a l i z a t i o n i s accepted as being approximately c o r r e c t , we produce a p i c t u r e of n i t r o g e n c y c l i n g i n CEE2 which i s dominated by a r t i f i c i a l a d d i t i o n s of n i t r a t e to the bubbled l a y e r . The average measured r a t e of 1*C p r o d u c t i o n i n CEE2 i s compatible with t h i s p i c t u r e . U n f o r t u n a t e l y , t h i s simple n i t r o g e n budget, although i n t e r n a l l y c o n s i s t e n t , cannot be r e c o n c i l e d with two cogent l i n e s of o b s e r v a t i o n . A l t e r n a t i v e n i t r o g e n budget f o r C E E 2 — I t was demonstrated on pages 148 and 150 that a net phytoplankton uptake of 154 n i t r o g e n (NPUN) of 94.1 ug-atom N l " 1 (=136 ug C l ' 1 d a y 1 ) during the p e r i o d from t=3.5 to 61.5 days was r e q u i r e d to y i e l d the observed accumulation of n i t r o g e n i n the s u r f a c e 8 m of CEE2 _if_ r e m i n e r a l i z a t i o n proceeded at the simulated r a t e . The r e a l i s m of t h i s p r o v i s o i s s e r i o u s l y undermined by the r e s u l t s of P. J . l e B . W i l l i a m s (1980; pers. comm.). Wi l l i a m s used p r e c i s e Winkler t i t r a t i o n to measure the r e s p i r a t i o n and phot o s y n t h e s i s of d i f f e r e n t s i z e f r a c t i o n s of p a r t i c l e s i n water from the upper 5 m of CEE2. On day 9 when few m i c r o f l a g e l l a t e s were present, the <5 um f r a c t i o n had a r e s p i r a t i o n r a t e of 214 ug Op2 l " 1 d a y 1 ; i t s ph o t o s y n t h e t i c r a t e was 48 ug 0 r2 l " 1 d a y 1 , only 7% of the p h o t o s y n t h e t i c r a t e of u n f i l t e r e d water. On other sampling days W i l l i a m s c o u l d not f r a c t i o n a t e microheterotrophs from autotrophs. I f we take 214 ug O r2 I " 1 d a y 1 as r e p r e s e n t a t i v e of b a c t e r i a l r e s p i r a t i o n i n CEE2, the b a c t e r i a were r e s p i r i n g 64.3 ug C l " 1 d a y 1 , assuming an RQ of 0.8. (The b a c t e r i a l r e s p i r a t i o n r a t e p r e d i c t e d by the r e f e r e n c e run averaged 16.9 ug C l " 1 d a y 1 between t = 3.5 and 61.5 days (16.3 ug C l " 1 d a y 1 over e n t i r e s i m u l a t i o n ) . ) The i n c r e a s e d r e m i n e r a l i z a t i o n r a t e by b a c t e r i a would r e q u i r e an NPUN of 127 ug-atom N . l _ 1 / 5 8 days (=183 ug C l " 1 d a y 1 ) to prevent n i t r o g e n from accumulating f a s t e r than observed. T h e r e f o r e , i f W i l l i a m s ' r e s u l t s r e f l e c t the t r u e r a t e of 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 then the r e f e r e n c e run has underestimated net primary p r o d u c t i v i t y i n the s u r f a c e 8 m of CEE2 by 46%. The p r e d i c t e d f l u x of organic carbon i n t o b a c t e r i a must 155 a l s o have been underestimated. Fuhrman and Azam (1980) estimated b a c t e r i a l growth in CEE2 from thymidine i n c o r p o r a t i o n r a t e s and from i n c r e a s e of b a c t e r i a l numbers over time i n 3 um f i l t r a t e s . The former method estimated growth of 11 - 71 ug C l " 1 d a y - 1 on day 27, and the l a t t e r method 29 ug C l " 1 day" 1 on day 32 and 5 - 1 2 ug C l " 1 day" 1 on day 69. Thus the i n f l u x of carbon needed to support b a c t e r i a l r e s p i r a t i o n (64 ug C l " 1 day" 1) and growth (5 - 71 ug C l " 1 day" 1) i s much gr e a t e r than the average simulated i n f l u x of 19 ug C l " 1 day" 1 i n the s u r f a c e 8 m between days 4 and 62. T h i s i n f l u x of organic carbon to b a c t e r i a must have come from; 1) phytoplankton, d i r e c t l y v i a exudation or l y s i s or i n d i r e c t l y v i a zooplankton s p o l i a t i o n or e x c r e t i o n of grazed phytoplankton carbon; or from 2) other b a c t e r i a , d i r e c t l y or i n d i r e c t l y . However, s i n c e the carbon r e s p i r e d by b a c t e r i a cannot be d i r e c t l y re-absorbed by b a c t e r i a , t h a t f r a c t i o n of organic i n f l u x to b a c t e r i a which i s ' r e s p i r e d must u l t i m a t e l y come from phytoplankton i f b a c t e r i a l p r o d u c t i v i t y i s to be s u s t a i n e d . Thus the i n f l u x of o r g a n i c carbon to b a c t e r i a i n the s u r f a c e l a y e r r e q u i r e d at l e a s t 64.3/183 • 100 = 35% of estimated net primary p r o d u c t i o n . I f Fuhrman and Azam's b a c t e r i a l growth r a t e of 71 ug C l " 1 day" 1 i s a l s o i n c l u d e d , then (64.3 + 71)/183 • 100 = 74% of net primary p r o d u c t i o n would be r e q u i r e d by b a c t e r i a . C o n s i d e r i n g the number of u n t e s t e d assumptions u n d e r l y i n g these c a l c u l a t i o n s , the d e r i v e d percentages of 35% and 74% should only be c o n s i d e r e d suggestive of the importance of b a c t e r i a i n Foodweb I. 156 Could phytoplankton have supported such a steep requirement of organic carbon f o r b a c t e r i a l r e s p i r a t i o n and growth? A s i m u l a t i o n run i n which phytoplankton were i n t e r p o l a t e d between t h e i r observed c o n c e n t r a t i o n s p r e d i c t e d an average s i n k i n g l o s s of 8.9 ug C 1'1 d a y 1 from the s u r f a c e l a y e r between days 4 and 62. The p r e d i c t e d mixing l o s s was 1.8 ug C l " 1 d a y 1 , and 95.1 ug C l " 1 d a y 1 was grazed. T h i s g r a z i n g l o s s was very l i k e l y o v erestimated. H a r r i s et a l . (1980) measured an average i n g e s t i o n r a t e by a d u l t female Pseudocalanus ( f e e d i n g i n water removed from 4-8 m of CEE2) of 1.7 ug C c o p e p o d - 1 d a y 1 ; the simulated r a t e averaged 3.5 ug C copepod" 1 d a y 1 ( F i g . 28). A d u l t female Calanus i n g e s t e d 15.0 ug C c o p e p o d - 1 d a y 1 ( H a r r i s et a l . 1980) or 19.8 ug C copepod" 1 d a y 1 ( s i m u l a t i o n ) . Observed g r a z i n g r a t e s by c i l i a t e s are u n a v a i l a b l e , but i t was suggested e a r l i e r t h a t they, too, were overestimated. If 50 ug C l " 1 d a y 1 i s a more reasonable g r a z i n g l o s s from phytoplankton, then 183 (net p r o d u c t i o n ) - 50 (grazing) - 9 ( s i n k i n g ) - 2 (mixing) - 3 (observed i n c r e a s e i n phytoplankton standing stock) = 119 ug C 1 _ 1 d a y 1 (= 65% of 183 ug C l " 1 d a y 1 ) would be l e f t over as a p o t e n t i a l source of organic carbon f o r b a c t e r i a l uptake. Summmary of n i t r o g e n c y c l i n g — T h e two d i f f e r e n t n i t r o g e n and carbon budgets i l l u s t r a t e two c o n t r a r y i n t e r p r e t a t i o n s of the events i n CEE2. Ne i t h e r i n t e r p r e t a t i o n i s c o n s i s t e n t with a l l a v a i l a b l e o b s e r v a t i o n s . The f i r s t i n t e r p r e t a t i o n r e j e c t e d W i l l i a m s ' (1980) and Fuhrman and Azam's (1980) measurements of 157 F i g u r e 28. P r e d i c t e d and observed i n g e s t i o n r a t e of a d u l t female Pseudocalanus. S o l i d l i n e - simulated i n g e s t i o n i n su r f a c e 8 m with phytoplankton c o n s t r a i n e d to f o l l o w observed c o n c e n t r a t i o n . Gaps are times when no a d u l t females were present i n CEE2 c i r c l e s - observed i n g e s t i o n by specimens removed from o u t s i d e CEE2 and p l a c e d i n water from 4-8 m of CEE2 ( H a r r i s et a l . 1980) 159 m i c r o h e t e r o t r o p h i c a c t i v i t y , and accepted the simulated n i t r o g e n r e g e n e r a t i o n r a t e . Here, net primary p r o d u c t i o n i n the s u r f a c e l a y e r was estimated as 136 ug C 1 _ 1 d a y 1 , c l o s e to the average observed 1 *C p r o d u c t i v i t y of 97 ug C l " 1 d a y 1 . The s u r f a c e pool of d i s s o l v e d i n o r g a n i c n i t r o g e n was turned over every 3.4 days, 6 and new/total p r o d u c t i o n (0-20 m) was n70%. Conversely, the second i n t e r p r e t a t i o n r e j e c t e d both the n i t r o g e n r e g e n e r a t i o n r a t e p r e d i c t e d by the r e f e r e n c e run and the observed 1*C p r o d u c t i v i t y . Instead i t accepted the much f a s t e r 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 r a t e measured by W i l l i a m s . Net primary p r o d u c t i o n was estimated as 183 ug C l " 1 d a y 1 (0-8 m); d i s s o l v e d i n o r g a n i c n i t r o g e n was turned over every 2.5 days (0-8 m); new/total p r o d u c t i o n was n50% (0-20 m). The r a p i d 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 r a t e of 0.76 ug-atom N l " 1 d a y 1 (based on W i l l i a m s ' measured r e s p i r a t i o n r a t e of 214 ug O r2 l - 1 d a y 1 , assuming an RQ of 0.8, C/N mass r a t i o of 6, and no b a c t e r i a l uptake of i n o r g a n i c nitrogen) i s s i m i l a r to the r e m i n e r a l i z a t i o n r a t e s of 0.3 to >1.4 ug-atom N l " 1 d a y 1 observed i n other l a r g e - s c a l e CEPEX enc l o s u r e s ( H a r r i s o n 1978). H a r r i s o n a l s o reported that the median time r e q u i r e d f o r plankton to consume an amount of NH r4 + e q u i v a l e n t to the ambient c o n c e n t r a t i o n was n2.4 days. The separate e x i s t e n c e of these two i n t e r p r e t a t i o n s hinges 'Turnover time was c a l c u l a t e d as mean c o n c e n t r a t i o n of d i s s o l v e d i n o r g a n i c n i t r o g e n d i v i d e d by estimated net phytoplankton uptake of n i t r o g e n = 5.46 ug-atom N l " 1 • (94.1 ug-atom N l " l / 5 8 d a y s ) " 1 . 160 upon the i n c o m p a t i b i l i t y of measured 1 4 C p r o d u c t i v i t y and measured m i c r o h e t e r o t r o p h i c a c t i v i t y . T h i s i n a b i l i t y to r e c o n c i l e primary p r o d u c t i o n as determined by 1 4 C b o t t l e experiments with the high a c t i v i t y of b a c t e r i o p l a n k t o n o b t a i n e d by other means c u r r e n t l y plagues marine r e s e a r c h ( S i e b u r t h 1977). 5.1.6 S i l i c o n The r e f e r e n c e run p r e d i c t e d that l a r g e diatoms r a p i d l y s t r i p p e d s i l i c o n (as o r t h o - s i l i c i c a c i d ) out of the s u r f a c e l a y e r a f t e r each n u t r i e n t a d d i t i o n ( F i g . 29). S i l i c o n was removed more slowly a f t e r the f i n a l s i l i c o n a d d i t i o n on day 62 because l a r g e diatom growth was r e s t r a i n e d by l a r g e f l a g e l l a t e shading. A f t e r diatoms exhausted s i l i c o n i n the s u r f a c e l a y e r , S i c o n c e n t r a t i o n s were low (<0.3 ug-atom l " 1 ) but not zero due to mixing inputs from the deep l a y e r . Deep-layer S i was p r e d i c t e d to d e c l i n e from 24.6 ug-atom l " 1 at the s t a r t of the run (day 4) to 8.2 ug-atom l " 1 on day 62, then i n c r e a s e s l i g h t l y due to mixing from the t e m p o r a r i l y e n r i c h e d s u r f a c e l a y e r . Twenty-seven per cent of the p r e d i c t e d net l o s s of s i l i c o n from the deep l a y e r was due to mixing i n t o the s u r f a c e , with the remaining 73% due to i_n s i t u uptake. In s t a r k c o n t r a s t to the r e f e r e n c e run's p r e d i c t i o n s , observed s i l i c o n accumulated in both the s u r f a c e ( F i g s . 4b, 29) and deep l a y e r s of CEE2. Deep S i i n c r e a s e d from 24.6 ug-atom l ~ l on day 4 to >30 ug-atom l " 1 a f t e r day 50. The d i s p a r i t y between observed and p r e d i c t e d S i c o n c e n t r a t i o n s must 161 F i g u r e 29. C o n c e n t r a t i o n of s i l i c i c a c i d i n the s u r f a c e 8 m p r e d i c t e d by the r e f e r e n c e run. Octagons are c o n c e n t r a t i o n s observed in CEE2. Silicon ( u g - a t S i I"') 0 3 0 [ ] j 1 1 1 =~l 1 I I I I I C D 163 have been at l e a s t p a r t l y due to the absence of any S i r e g e n e r a t i o n i n the r e f e r e n c e run. T h i s i s c o n v i n c i n g l y demonstrated f o r the deep l a y e r of CEE2. S i was observed to i n c r e a s e from 24.6 ug-atom l " 1 on day 4 to 34.6 ug-atom l " 1 on day 79 even though S i was not added to the deep l a y e r . Moreover, a s i m u l a t i o n run i n which S i was c o n s t r a i n e d t o f o l l o w i t s observed c o n c e n t r a t i o n s i n the top and bottom l a y e r s p r e d i c t e d a mixing l o s s of 8.3 ug-atom S i l " 1 from the bottom l a y e r . Thus nl8.3 ug-atom S i l " 1 must have been regenerated over 75 days i n the deep l a y e r , not counting f u r t h e r s i l i c o n needed to s a t i s f y diatom uptake. In the s u r f a c e l a y e r the s i t u a t i o n was complicated by an unknown s i l i c o n uptake. Consider the p e r i o d from day 13 to day 25, the p e r i o d when l a r g e diatoms bloomed i n the s u r f a c e l a y e r . Observed S i c o n c e n t r a t i o n s f e l l by 10.5 ug-atom l " 1 ; 17.4 ug-atom l " 1 was added; a mixing i n f l u x of 2.2 ug-atom l * 1 was simulated with S i l e v e l s c o n s t r a i n e d to t h e i r observed c o n c e n t r a t i o n s . Hence a t o t a l uptake of 10.5 + 17.4 + 2.2 = 30.1 ug-atom S i 1'1/12 days c o u l d have been supported without having to invoke S i r e g e n e r a t i o n . U n f o r t u n a t e l y the amount of S i taken up by diatoms duri n g t h i s 12-day p e r i o d i s unknown. In t h i s p e r i o d , p o s i t i v e increments of diatom biomass t o t a l l e d 735 ug C l " 1 ; assuming a low C/Si mass r a t i o of 1.25, t h i s biomass i n c r e a s e would remove only 20.9 ug-atom S i l " 1 . Between days 13 and 25 the observed 1 4 C p r o d u c t i v i t y was 37.8 ± 34.3 ug C l - 1 / 4 hours (mean ± s.dev., n=12) i n the s u r f a c e 8 m of CEE2. Diatom carbon comprised 60 - 89% of t o t a l p h o t o s y n t h e t i c 164 carbon. Even assuming that diatoms were r e s p o n s i b l e f o r a l l pho t o s y n t h e s i s and that C/Si = 1.25, the mean d a i l y p r o d u c t i v i t y (37.8 • 2.4 =91 ug C l " 1 , see footnote 5 , p. 151) would remove 2.6 ug-atom S i l " 1 day" 1 = 31 ug-atom S i 1" 1/12 days, m a r g i n a l l y g r e a t e r than the 30.1 ug-atom S i l " 1 l i m i t . The a v a i l a b l e o b s e r v a t i o n s are c l e a r l y of no use i n e s t a b l i s h i n g the e x i s t e n c e of S i r e g e n e r a t i o n i n the s u r f a c e 8 m of CEE2. Of course, the r a p i d d e p l e t i o n of s i l i c o n p r e d i c t e d i n the su r f a c e l a y e r c o u l d have simply been due to the o v e r e s t i m a t i o n of diatom uptake. T h i s might have been a consequence of o v e r e s t i m a t i n g gross p h o t o s y n t h e s i s , underestimating the C/Si r a t i o of newly s y n t h e s i z e d biomass, or assuming t h a t S i uptake was t i e d to gross p h o t o s y n t h e s i s r a t h e r than some other measure of diatom growth. Because the d i s c r e p a n c y between p r e d i c t e d and observed c o n c e n t r a t i o n s of s i l i c o n i n CEE2 was so g l a r i n g , and because measurements with which to c o n s t r u c t u s e f u l s i l i c o n budgets were l a c k i n g , the r e f e r e n c e run was abandoned and f u r t h e r s i m u l a t i o n s were run to expl o r e s i l i c o n dynamics. The r e s u l t s of these runs, to be presented i n s e c t i o n 5.-2, suggest that S i was s t r i p p e d from the s u r f a c e l a y e r i n the r e f e r e n c e run p r i m a r i l y because the p h o t o s y n t h e t i c r a t e of l a r g e diatoms was never s e r i o u s l y l i m i t e d , except f o r b r i e f p e r i o d s f o l l o w i n g S i exhaustion. 165 5.2 S i 1 icon S i m u l a t i o n s The i n t e r a c t i o n among s i l i c o n uptake, s i l i c o n r e g e n e r a t i o n and diatom growth was e x p l o r e d by f o r c i n g l a r g e and small f l a g e l l a t e biomass and d i s s o l v e d i n o r g a n i c n i t r o g e n to t r a c k t h e i r observed l e v e l s over time i n the s u r f a c e and deep l a y e r s of CEE2. Only the dynamics of l a r g e and small diatoms and s i l i c i c a c i d were e x p l i c i t l y s i mulated, thereby e l i m i n a t i n g any ambiguity due to u n r e a l i s t i c p r e d i c t i o n s of n i t r o g e n or f l a g e l l a t e c o n c e n t r a t i o n s . The r e f e r e n c e run was m o d i f i e d i n a v a r i e t y of ways to determine i f S i r e g e n e r a t i o n s t r o n g l y i n f l u e n c e d S i dynamics in Foodweb I. 5.2:.l S i m u l a t i o n S i - 1 . Comparison with r e f e r e n c e run The parameters of the r e f e r e n c e run were unchanged i n S i - 1 , except f o r the d e a c t i v a t i o n of n i t r o g e n , f l a g e l l a t e and b a c t e r i a l dynamics. T h i s run was a " c o n t r o l " to which l a t e r runs were compared. The p r e d i c t i o n s of Si-1 confirmed those of the r e f e r e n c e run ( F i g . 30; c f . F i g s . 20 and 29). T h i s was expected because diatoms were S i - l i m i t e d i n the r e f e r e n c e run u n t i l a f t e r the f i n a l S i a d d i t i o n . S i l i c o n i n the deep l a y e r f e l l even f a s t e r i n S i - 1 than i n the r e f e r e n c e run. By day 62 i t had d e c l i n e d to 1.9 • 10" 4 ug-atom S i l " 1 , whereas the r e f e r e n c e run p r e d i c t e d a minimum of 8.2 ug-atom S i l " 1 on day 62. T h i s d i f f e r e n c e was a consequence of g r e a t e r diatom growth in the deep l a y e r i n 166 F i g u r e 30. S i m u l a t i o n S i - 1 . P r e d i c t e d l a r g e diatom biomass ( s o l i d l i n e ) and s i l i c i c a c i d c o n c e n t r a t i o n (dashed l i n e ) i n the s u r f a c e 8 m. Only S i and diatom dynamics are enabled. Parameters as i n r e f e r e n c e run. Si l i con (ug-at S i 1"') 0 30 1 i ' • ' ' • ' • i i 167 168 S i - 1 , i n turn a r e s u l t of l e s s e r shading by the small biomass of f l a g e l l a t e s observed in the s u r f a c e l a y e r before the Ceratium bloom. Small diatoms were q u i c k l y e l i m i n a t e d from the water column as was the case in the r e f e r e n c e run and a l l subsequent runs. T h e i r impact on simulated S i dynamics was t h e r e f o r e i n s i g n i f i c a n t . In Foodweb I, however, small diatoms were probably r e s p o n s i b l e f o r the f a l l i n s u r f a c e S i c o n c e n t r a t i o n s between days 50 and 60 ( F i g . 4b) d u r i n g a bloom of Chaetoceros  danicus and C. s o c i a l i s ( F i g . 2b). 5.2.2 S i m u l a t i o n Si-2 and S i - 3 . L i m i t a t i o n of diatom growth To see i f the r a p i d removal of S i c o u l d have been due to o v e r e s t i m a t i o n of gross p h o t o s y n t h e s i s by l a r g e diatoms, the h a l f - s a t u r a t i o n constant f o r S i - l i m i t a t i o n of gross p h o t o s y n t h e s i s , K r S i , was i n c r e a s e d i n runs Si-2 and S i - 3 . In s i m u l a t i o n S i - 2 , K r S i of l a r g e diatoms was i n c r e a s e d to 4.00 ug-atom S i l " 1 from the value of 2.06 used i n the r e f e r e n c e run and S i - 1 . There was v i r t u a l l y no d i f f e r e n c e between the p r e d i c t i o n s of Si-1 and Si-2 ( c f . F i g . 30 and 31): the peak biomass of l a r g e diatoms i n blooms p r e d i c t e d by Si-2 was never l e s s than 89% of the analogous peak biomass in S i - 1 , and s i l i c o n was d e p l e t e d only s l i g h l y more slowly i n S i - 2 . Doubling the K r S i of diatoms t h e r e f o r e had no s i g n i f i c a n t e f f e c t on modelled dynamics. To f u r t h e r l i m i t ' d i a t o m growth, K r S i was i n c r e a s e d to 30 ug-atom S i l " 1 in s i m u l a t i o n S i - 3 . Although the value of 30 169 F i g u r e 3 1 S i m u l a t i o n S i - 2 . P r e d i c t e d l a r g e diatom biomass ( s o l i d l i n e ) and s i l i c i c a c i d c o n c e n t r a t i o n (dashed l i n e ) i n the s u r f a c e 8 m. K r S i = 4 ug-atom S i 1" 1. Si l i con (ug-at S i 1_1) p 3 0 i i ' ' i i i i i i i 171 ug-atom S i l " 1 f o r a h a l f - s a t u r a t i o n constant i s a b s u r d l y h i g h , the i n t e n t was merely to c u r t a i l gross p h o t o s y n t h e s i s : t h i s c o u l d have been achieved i n a v a r i e t y of other ways with e s s e n t i a l l y the same r e s u l t . S i l i c o n accumulated i n the s u r f a c e l a y e r i n S i - 3 , reaching a peak of 29.2 ug-atom l " 1 a f t e r the n u t r i e n t a d d i t i o n on day 20 ( F i g . 32). In the deep l a y e r , S i f e l l to a minimum of 2.4 ug-atom l " 1 on day 62. Over the e n t i r e s i m u l a t i o n the average gross p h o t o s y n t h e t i c r a t e of l a r g e diatoms i n the s u r f a c e 8 m was 43.1 ug C l " 1 day" 1, only s l i g h t l y l e s s than the 45.6 ug C l " 1 day" 1 p r e d i c t e d by S i - 1 . Most of the su r f a c e S i accumulated before day 20 i n S i - 3 ; p r i o r to t=19.5, gross p h o t o s y n t h e s i s of l a r g e diatoms averaged 49.6 ug C l " 1 day" 1 i n S i - 3 , 37% l e s s than that p r e d i c t e d i n S i - 1 . 5.2.3 S i m u l a t i o n Si-4 and S i - 5 . V a r i a b l e C/Si r a t i o In these s i m u l a t i o n s K r S i was r e s e t to the o r i g i n a l values used i n S i - 1 . In the hope of reducing S i demand, the C/Si r a t i o of newly s y n t h e s i z e d diatom biomass was made a f u n c t i o n of the degree of S i - l i m i t a t i o n as d e s c r i b e d i n s e c t i o n 3.4.2 (p. 57). That i s , the C/Si mass r a t i o i n c r e a s e d l i n e a r l y from 1.25 with no S i - l i m i t a t i o n to 6.25 at complete l i m i t a t i o n . Si-4 p r e d i c t e d a l a r g e r s t anding crop of diatoms i n the s u r f a c e l a y e r than d i d S i - 1 , and s i l i c o n f a i l e d to accumulate i n e i t h e r the top or bottom l a y e r s ( F i g . 33). Large diatom biomass i n Si-4 g r a d u a l l y d i v e r g e d from that p r e d i c t e d i n S i - 1 when s i l i c o n was s t r o n g l y l i m i t i n g . S i uptake per u n i t of diatom gross p h o t o s y n t h e s i s i n run Si-4 was l e s s than h a l f that 172 F i g u r e 32. S i m u l a t i o n S i - 3 . P r e d i c t e d l a r g e diatom biomass ( s o l i d l i n e ) and s i l i c i c a c i d c o n c e n t r a t i o n (dashed l i n e ) i n the s u r f a c e 8 m. K r S i = 30 ug-atom S i 1" 1. 174 F i g u r e 33. S i m u l a t i o n S i - 4 . P r e d i c t e d l a r g e diatom biomass ( s o l i d l i n e ) and s i l i c i c a c i d c o n c e n t r a t i o n (dashed l i n e ) i n the s u r f a c e 8 m. 1.25 < C/Si < 6.25 S i l i c on (ug-at S i I - 1) p 3 0 i i i i i i i i i i i 176 i n S i - 1 whenever the ambient c o n c e n t r a t i o n of S i f e l l below 3.8 ug-atom I'1 ( K r S i =2.06 ug-atom S i l ' 1 ) . T h e r e f o r e , at low S i c o n c e n t r a t i o n s the reduced uptake of S i per u n i t of gross p h o t o s y n t h e s i s more than compensated f o r the higher p h o t o s y n t h e s i z i n g biomass i n S i - 4 . As a r e s u l t , S i was not as s e v e r e l y d e p l e t e d between n u t r i e n t a d d i t i o n s i n Si-4 as i t was i n S i - 1 . Consequently, gross p h o t o s y n t h e s i s was not as s t r o n g l y l i m i t e d i n Si-4 and the net r a t e of biomass decrease was slower i n Si-4 a f t e r d e p l e t i o n of ambient S i . T h i s i s why diatom biomass was higher i n Si-4 than i n S i - 1 . The r a p i d d e p l e t i o n of s u r f a c e - l a y e r S i p r e d i c t e d i n Si-4 c l e a r l y demonstrates that a v a r i a b l e C/Si r a t i o t i e d to the degree of S i - l i m i t a t i o n c o u l d not have r e s t r a i n e d S i uptake at the high ambient c o n c e n t r a t i o n s observed i n CEE2. Is i t p o s s i b l e that the C/Si r a t i o of S i - r e p l e t e diatoms was underestimated in the model? P. J . H a r r i s o n et a l . (1977) re p o r t a C/Si mass r a t i o of 4.0 f o r Skeletonema costatum grown under no n u t r i e n t l i m i t a t i o n , and a r a t i o of 15.6 f o r S i - s t a r v e d c e l l s . When these l i m i t s were employed i n s i m u l a t i o n S i - 5 , l a r g e diatoms bloomed to huge c o n c e n t r a t i o n s , and again S i was s t r i p p e d from the s u r f a c e l a y e r ( F i g . 34). By i t s e l f , a v a r i a b l e C/Si r a t i o c o u l d not account f o r the observed behavior of S i i n CEE2. 177 F i g u r e 34. S i m u l a t i o n S i - 5 . P r e d i c t e d l a r g e diatom biomass ( s o l i d l i n e ) and s i l i c i c a c i d c o n c e n t r a t i o n (dashed l i n e ) i n the s u r f a c e 8 m. 4 < C/Si < 15.6 Note change of s c a l e f o r diatoms. S i l i c o n (ug-at S i p 3 0 i i i i i i i i ' i i 178 L a r g e Diatoms (ug C 1"') o 1000 (—i i ' i i i i i i i i i CD 179 5.2.4 S i m u l a t i o n S i - 6 . S i l i c o n uptake l i n k e d to net  pho t o s y n t h e s i s In a l l previous s i m u l a t i o n s the uptake of s i l i c o n was c a l c u l a t e d as a f r a c t i o n of gross p h o t o s y n t h e s i s by diatoms. T h i s i s d e c i d e d l y u n r e a l i s t i c : s e v e r a l s t u d i e s have found S i uptake to be coupled to growth r a t h e r than gross p h o t o s y n t h e s i s . The i n c o r p o r a t i o n of s i l i c o n from a m e t a b o l i c a l l y a c t i v e i n t e r n a l pool i n t o the diatom's s i l i c e o u s v a l v e s occurs during c e l l d i v i s i o n (Chisholm et a l . 1978; Lewin et a l . 1966). According to the model of Davis et a l . (1978), uptake of S i from the e x t e r n a l medium equals the r a t e of i n c o r p o r a t i o n i n t o the f r u s t u l e when the small i n t e r n a l pool i s f u l l . Except f o r b r i e f p e r i o d s when the i n t e r n a l pool i s f i l l i n g , the maximal uptake r a t e i s r e g u l a t e d by the growth ra t e ( i . e . r a t e of f r u s t u l e formation = r a t e of c e l l d i v i s i o n ) of the p o p u l a t i o n . In s i m u l a t i o n s S i -1 through S i - 5 , s i l i c o n was taken up even i f growth ( i . e . net ph o t o s y n t h e s i s ) was negative provided that some gross p h o t o s y n t h e s i s o c c u r r e d . To remidy t h i s a b e r r a t i o n , S i uptake i n s i m u l a t i o n S i - 6 was c a l c u l a t e d as a f r a c t i o n of instantaneous net photosynthesis whenever net photosynthesis exceeded z e r o . 7 The f r a c t i o n was determined by the same v a r i a b l e C/Si r a t i o used i n run S i - 4 . The c o n c e n t r a t i o n of diatom carbon i n the surface l a y e r 7 I t might be more d e s i r e a b l e to l i n k S i uptake with d a i l y r a t h e r than instantaneous net p h o t o s y n t h e s i s , but such a fo r m u l a t i o n would have been much more d i f f i c u l t and not worth the e x t r a e f f o r t . 180 p r e d i c t e d by Si-6 exceeded t h a t i n S i - 4 , and once again the e l e v a t e d biomass n u l l i f i e d the e f f e c t of lower S i uptake per u n i t of net p h o t o s y n t h e s i s . S i d i d not accumulate in the s u r f a c e or bottom l a y e r s ( F i g . 35). 5.2.5 S i m u l a t i o n S i - 7 . D i s s o l u t i o n of s i l i c a The parameters of Si-7 were i n d e n t i c a l to that of S i - 6 , except that d i s s o l u t i o n of s i l i c a i n diatom f r u s t u l e s was a c t i v a t e d (see s e c t i o n 3.7.3, p. 104). S i uptake was l i n k e d to net p h o t o s y n t h e s i s and the C/Si mass r a t i o of diatoms v a r i e d between 1.25 and 6.25. Again, because of the very l a r g e diatom biomass p r e d i c t e d by S i - 7 , s i l i c o n was q u i c k l y removed from the s u r f a c e l a y e r a f t e r each n u t r i e n t a d d i t i o n ( F i g . 36). D i s s o l u t i o n of s i l i c a w i t h i n the s u r f a c e l a y e r was p r e d i c t e d to c o n t r i b u t e 132 ug-atom S i l " 1 between t=3.5 and 80 days, compared to the 111 ug-atom S i l " 1 added a r t i f i c i a l l y . 25 ug-atom S i l " 1 was mixed i n t o the s u r f a c e , but t h i s was c e r t a i n l y overestimated because the c o n c e n t r a t i o n g r a d i e n t between the s u r f a c e and deep l a y e r s was u n r e a l i s t i c a l l y l a r g e ; a run i n which S i was c o n s t r a i n e d to f o l l o w i t s observed c o n c e n t r a t i o n i n the s u r f a c e and deep l a y e r s p r e d i c t e d a mixing i n f l u x of only 13 ug-atom S i l ~ l . U n l i k e a l l p r e v i o u s s i m u l a t i o n runs, d i s s o l v e d S i was not d e p l e t e d i n the deep l a y e r i n S i - 7 ( F i g . 37). I n t e r p o l a t e d to t=80 days, the observed S i c o n c e n t r a t i o n was 34.3 ug-atom l ~ l , whereas Si-7 p r e d i c t e d a f i n a l c o n c e n t r a t i o n of 26.2 ug-atom 1 _ 1 . I f s i l i c o n had been mixed i n t o the s u r f a c e l a y e r 181 F i g u r e 35. S i m u l a t i o n S i - 6 . P r e d i c t e d l a r g e diatom biomass ( s o l i d l i n e ) and s i l i c i c a c i d c o n c e n t r a t i o n (dashed l i n e ) i n the s u r f a c e 8 m. S i uptake l i n k e d to net p h o t o s y n t h e s i s . 1.25 < C/Si < 6.25 Si l i con (ug-at S i 1~') Q 30 i i i ' i i i — i — i — i — i 182 183 F i g u r e 36. S i m u l a t i o n S i - 7 . P r e d i c t e d l a r g e diatom biomass ( s o l i d l i n e ) and s i l i c i c a c i d c o n c e n t r a t i o n (dashed l i n e ) i n the s u r f a c e 8 m. S i l i c a d i s s o l u t i o n a c t i v a t e d . 1.25 < C/Si < 6.25 S i uptake l i n k e d to net p h o t o s y n t h e s i s . Note change of s c a l e f o r diatoms. S i l i c o n (ug-at S i 1"') a 30 i ' ' ' i ' i i i i i 185 F i g u r e 37. P r e d i c t e d c o n c e n t r a t i o n of s i l i c i c a c i d i n the 8-20 m l a y e r . S i l i c a d i s s o l u t i o n a c t i v a t e d . 1.25 < C/Si < 6.25 S i uptake l i n k e d to net p h o t o s y n t h e s i s . Octagons are c o n c e n t r a t i o n s observed i n CEE2. Deep S i l i c on (ug-at S i 1"'; 8-20 m) CD-Q 50 ro CO 4^ CD CD C D G G G G G 3 G G (3 G G 3 !G G IG G G G G G G G G G G G G CO C D G 187 at the r a t e expected from the observed c o n c e n t r a t i o n d i f f e r e n c e s between the two l a y e r s , S i - 7 would have p r e d i c t e d a f i n a l c o n c e n t r a t i o n of n34 ug-atom S i 1'1 i n the deep l a y e r . 5.2.6 C o n c l u s i o n s r e g a r d i n g S i dynamics Except f o r s i m u l a t i o n S i - 3 , none of the manipulations of S i demand or r e g e n e r a t i o n s u c c e s s f u l l y allowed S i to accumulate in the s u r f a c e 8 m. They f a i l e d because the o v e r a l l p h o t o s y n t h e t i c r a t e of l a r g e diatoms was s u s t a i n e d at high l e v e l s by the high p r e d i c t e d c o n c e n t r a t i o n of diatoms. The exception was Si-3 with K r S i set to 30 ug-atom S i l " 1 . What i s remarkable about Si-3 i s that " S i accumulated to high c o n c e n t r a t i o n s i n the s u r f a c e l a y e r d e s p i t e a large-diatom gross p h o t o s y n t h e t i c r a t e which averaged only 5% l e s s than that p r e d i c t e d i n S i - 1 . The c r u c i a l d i f f e r e n c e i n Si-3 was that p h o t o s y n t h e s i s was not s u s t a i n e d at an approximately constant r a t e but r a t h e r there was a p e r i o d of low p h o t o s y n t h e s i s when Si accumulated, f o l l o w e d by a spurt i n growth. The main reason why S i was s t r i p p e d from the s u r f a c e l a y e r i n the r e f e r e n c e run was not because there was no s i l i c a d i s s o l u t i o n or C/Si was constant or S i uptake was l i n k e d to gross p h o t o s y n t h e s i s , but because the p h o t o s y n t h e s i s of l a r g e diatoms was never s e v e r e l y reduced (except f o r b r i e f p e r i o d s f o l l o w i n g S i e x h a u s t i o n ) . The importance of s i l i c a d i s s o l u t i o n as a source of s i l i c i c a c i d f o r diatom growth remains u n c e r t a i n . T h i s u n c e r t a i n t y i s based on three unanswered q u e s t i o n s : 1) what was the o v e r a l l r a t e of net p h o t o s y n t h e s i s i n CEE2?, 2) what 188 p r o p o r t i o n of t o t a l p h o t o s y n t h e s i s was c o n t r i b u t e d by diatoms?, and 3) what was the bulk C/Si r a t i o of diatom biomass? Consider the p e r i o d from t=3.5 to 61.5 days examined i n s e c t i o n 5.1.5. Net p r o d u c t i o n averaged roughly 136 to 183 ug C l " 1 day" 1 i n the s u r f a c e 8 m. Assume f o r the sake of argument that diatoms were r e s p o n s i b l e f o r a l l primary p r o d u c t i o n . If we take a C/Si mass r a t i o of 1.25, the higher net production would r e q u i r e 302 ug-atom S i l _ 1 / 5 8 days. If i n s t e a d we assume that C/Si=4 and take the lower net p r o d u c t i o n , only 70 ug-atom S i l _ 1 / 5 8 days would be r e q u i r e d . The maximum q u a n t i t y of S i that c o u l d have been removed from the s u r f a c e l a y e r without r e q u i r i n g s i l i c o n r e g e n e r a t i o n was 95 ( a r t i f i c i a l a d d i t i o n s ) + 10 ( i n f e r r e d mixing i n f l u x ) - 8 (observed accumulation of s u r f a c e S i ) = 97 ug-atom S i l _ 1 / 5 8 days. T h i s i l l u s t r a t e s the i m p o s s i b i l i t y of a s s e s s i n g the s i g n i f i c a n c e of S i r e g e n e r a t i o n f o r diatom growth i n Foodweb I. 5.3 Phytoplankton Growth and Loss N u t r i e n t dynamics have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n i n e a r l i e r s e c t i o n s . In two. key a r e a s — r a t e of n i t r o g e n r e g e n e r a t i o n and r a t e of s i l i c o n r e g e n e r a t i o n — i n t e r p r e t a t i o n of experimental evidence and model p r e d i c t i o n s hinged upon hypothesized r a t e s of a l g a l growth. The purpose of t h i s s e c t i o n i s to come to g r i p s with two fundamental q u e s t i o n s concerning phytoplankton growth and l o s s i n Foodweb I. 189 5.3.1 Have maximum r a t e s of primary p r o d u c t i o n been  underestimated? — Y e s , at l e a s t f o r l a r g e - c e l l e d f l a g e l l a t e s . The maximum gross p h o t o s y n t h e t i c r a t e s of diatoms and f l a g e l l a t e s i n the model were d e r i v e d from Chan's (1978b) r e s u l t s f o r f i v e diatom and four d i n o f l a g e l l a t e s p e c i e s as d e s c r i b e d on p. 65 ' et, seq. Chan found that diatoms have much higher d i v i s i o n r a t e s than d i n o f l a g e l l a t e s of equal c e l l p r o t e i n . Although l a r g e - c e l l e d f l a g e l l a t e s had a mean c e l l diameter 29% smal l e r than l a r g e - c e l l e d diatoms ( s e c t i o n 3.4.1, p. 51), the maximum s p e c i f i c gross p h o t o s y n t h e t i c r a t e of l a r g e f l a g e l l a t e s was n e v e r t h e l e s s only 36% of that of l a r g e diatoms i n the model (Eq. 18, p. 69). Because of t h i s small p h o t o s y n t h e t i c r a t e , l a r g e f l a g e l l a t e s i n c r e a s e d slowly throughout the ref e r e n c e run ( F i g . 23). Is such a low maximum ph o t o s y n t h e t i c r a t e c r e d i b l e ? Two arguments suggest not. F i r s t , the model's p r e d i c t e d net r a t e of change of l a r g e f l a g e l l a t e biomass was compared t o observed net r a t e s of change in the s u r f a c e 8 m of CEE2. A s i m u l a t i o n was run i n which a l l model dynamics were d i s a b l e d so that phytoplankton biomass and n u t r i e n t c o n c e n t r a t i o n s f o l l o w e d t h e i r observed l e v e l s " over time (parameter values were those of the r e f e r e n c e r u n ) . The model c a l c u l a t e d the net r a t e of change of l a r g e - f l a g e l l a t e carbon expected at each time step; the average net ra t e of change over s u c c e s s i v e 24-hour i n t e r v a l s was then p l o t t e d to e l i m i n a t e day-night o s c i l l a t i o n s ( F i g . 38). Al s o p l o t t e d i n F i g . 38 are observed net r a t e s of change, c a l c u l a t e d as 190 F i g u r e 38. S p e c i f i c net growth r a t e of l a r g e f l a g e l l a t e carbon in the s u r f a c e 8 m of CEE2. Continuous l i n e i s d a i l y avarage r a t e p r e d i c t e d by a s i m u l a t i o n i n which a l l s t a t e v a r i a b l e s t r a c e d t h e i r observed c o n c e n t r a t i o n s . Unconnected l i n e s represent average r a t e of change between o b s e r v a t i o n s of f l a g e l l a t e carbon. - 2 S p e c i f i c Growth R a t e (day-') C D -r o C D ro CL CO CO C D CD. C D 2 -J 1 1 L -J I I L. 4 CO C D 192 [ l n ( B r i + l / B r i ) ] / ( t r i + l - t r i ) , (40) where B r i and B ri+1 are l a r g e f l a g e l l a t e carbon observed at the s u c c e s s i v e sampling times t r i and t r i + l . P r e d i c t e d net r a t e s of change were much smal l e r than observed r a t e s . I t i s u n l i k e l y that s p e c i f i c l o s s r a t e s from l a r g e f l a g e l l a t e s have been g r e a t l y overestimated i n the model, or that growth l i m i t a t i o n was e x c e s s i v e , and thus the l a r g e disagreement between p r e d i c t e d and observed net growth would seem to be due to underestimation of P rmax f o r l a r g e f l a g e l l a t e s . The second argument i s based on observed r a t e s of 1 4 C f i x a t i o n normalized to phytoplankton carbon ( F i g . 15). The f i r s t t h i n g to note i s that there appears to be no systematic d i f f e r e n c e between the s p e c i f i c 1*C f i x a t i o n r a t e s of diatom-and f l a g e l l a t e - d o m i n a t e d communities i n CEE 2 or 3. Yet the p r e d i c t e d net rate of growth ( a l l dynamics d i s a b l e d ) of l a r g e f l a g e l l a t e s was s u b s t a n t i a l l y s m a l l e r than t h a t of l a r g e diatoms ( F i g . 39). T h i s was a l s o t r u e f o r p r e d i c t e d gross or net p h o t o s y n t h e s i s . One might o b j e c t that f l a g e l l a t e s r e s p i r e more of t h e i r f i x e d carbon than diatoms, t h e r e f o r e y i e l d i n g a lower growth r a t e f o r f l a g e l l a t e s i n s p i t e of equal 1 4 C f i x a t i o n r a t e s . T h i s o b j e c t i o n i s g e n e r a l l y not supported by the l i t e r a t u r e (see s e c t i o n 3.4.4, p. 71). The second i n t e r e s t i n g f e a t u r e of F i g . .15 i s the extremely high s p e c i f i c f i x a t i o n r a t e s o c c a s i o n a l l y observed. Some are much l a r g e r than the v a l u e s of 0.03 - 0.11 h " 1 used f o r maximum s p e c i f i c gross p h o t o s y n t h e s i s i n the model. Eppley (1972, h i s 193 F i g u r e 39. S p e c i f i c net growth r a t e s of l a r g e diatoms ( s o l i d l i n e ) and l a r g e f l a g e l l a t e s (dashed l i n e ) i n the s u r f a c e 8 m p r e d i c t e d by a s i m u l a t i o n i n which a l l s t a t e v a r i a b l e s were i n t e r p o l a t e d between t h e i r observed c o n c e n t r a t i o n s . Parameters as i n r e f e r e n c e run. D a i l y averages are p l o t t e d . 195 Eq. (1)) p r e d i c t s a maximum growth r a t e of 0.058 h r 1 at 13.5 °C (the mean water temperature i n CEE2), which, a f t e r s c a l i n g upwards by a f a c t o r of 2.4 to roughly account f o r r e s p i r a t o r y and e x c r e t o r y l o s s e s as d e s c r i b e d on p. 68, s t i l l f a l l s short of the h i g h e s t observed f i x a t i o n r a t e s . The h i g h e s t growth r a t e s t a b u l a t e d by Goldman et a l . (1979) f o r n a t u r a l marine waters are those of Sheldon and S u t c l i f f e (1978), who found d o u b l i n g times of 3.35 h (22 °C) and 2.91 h (27 °C) f o r Sargasso Sea microplankton. Normalized to 13.5 °C ( f o l l o w i n g Eppley 1972) and s c a l e d by a f a c t o r of 2.4, these r a t e s equal gross p h o t o s y n t h e t i c r a t e s of 0.29 and 0.24 h ' 1 , e c l i p s i n g a l l but the highest f i x a t i o n r a t e s i n F i g . 15. Thus the s p e c i f i c r a t e s of 1 4 C f i x a t i o n determined f o r CEE 2 and 3 are not r i d i c u l o u s l y high. Were they n e v e r t h e l e s s overestimated? If they were, then e i t h e r 1 4 C f i x a t i o n was overestimated or phytoplankton carbon was underestimated. Measurements of carbon and c h l o r o p h y l l i n the CEEs suggest that phytoplankton carbon was not underestimated because 1) phytoplankton carbon commonly made up >50% of p a r t i c u l a t e organic carbon, thus was not s y s t e m a t i c a l l y low, and 2) the mean C / C h l o r o p h y l l - a r a t i o of phytoplankton assemblages with >0.1 h _ 1 s p e c i f i c 1*C f i x a t i o n was 32.2 (range 12.2 - 88.2), which i s q u i t e reasonable f o r v i g o r o u s l y growing phytoplankton (A n t i a et a l . 1963). 8 1 4 C However, carbon or c h l o r o p h y l l i n the CEEs may have been l e s s than that i n the i n c u b a t i o n b o t t l e s under circumstances f a v o r a b l e to production when s i g n i f i c a n t s y n t h e s i s of m a t e r i a l o c c u r r e d during, the 4-hour i n c u b a t i o n . The importance of t h i s source of e r r o r i s unknown. 196 f i x a t i o n s p e c i f i c to c h l o r o p h y l l - a (and c o r r e c t e d to 13.5 °C) was <10 mg C (mg Chl-a • h ) _ 1 which i s high but w i t h i n the range r e p o r t e d by H a r r i s o n and P i a t t (1980). To conclude, the s p e c i f i c 1 4 C f i x a t i o n r a t e s shown i n F i g . 15 are high but not absurd. Since many of the observed r a t e s of 1 4 C f i x a t i o n were much higher than modelled r a t e s , c o n s i d e r a b l e l a t i t u d e e x i s t s f o r upward adjustment of modelled maximum gross p h o t o s y n t h e t i c r a t e s of diatoms and e s p e c i a l l y f l a g e l l a t e s . Some upward adjustment was a l s o suggested on p. 154 as being necessary to account f o r the high 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 r a t e s r e p o r t e d by P. J . l e B . W i l l i a m s . 5.3.2 Have l o s s e s or growth l i m i t a t i o n been underestimated? — Y e s , i n t e r m i t t e n t l y . The most .serious f a u l t of the model i s i t s p r e d i c t i o n of s u s t a i n e d p o s i t i v e net growth of l a r g e diatoms and l a r g e f l a g e l l a t e s i n the s u r f a c e 8 m. T h i s i s v i v i d l y demonstrated by the s i m u l a t i o n i n which a l l phytoplankton and n u t r i e n t dynamics were d i s a b l e d i n order to p r e d i c t the net change of phytoplankton biomass as a f u n c t i o n of observed n u t r i e n t and phytoplankton c o n c e n t r a t i o n s at each time st e p . L a r g e - c e l l e d phytoplankton growth (averaged over 24-hour p e r i o d s to remove day-night o s c i l l a t i o n s ) was p r e d i c t e d to be negative f o r l e s s than 9% of the simulated 76.5 days ( F i g . 39). T h i s i s i n marked c o n t r a s t to the e x t e n s i v e p e r i o d s of d e c l i n e c h a r a c t e r i s t i c of the observed phytoplankton p o p u l a t i o n s ( F i g s . 2a, 2c, 38). 197 Net phytoplankton growth c o u l d be s u s t a i n e d at high l e v e l s only i f the maximum p h o t o s y n t h e t i c r a t e was c o n s i s t e n t l y high and growth l i m i t a t i o n and l o s s e s ( g r a z i n g , exudation, s i n k i n g , r e s p i r a t i o n ) were c o n s i s t e n t l y low. These c o n d i t i o n s were s a t i s f i e d i n the s i m u l a t i o n . Prmax was constant, l i g h t s e v e r e l y l i m i t e d growth on only a few s c a t t e r e d o c c a s i o n s ( F i g . 40), and l o s s e s from l a r g e - c e l l e d phytoplankton were not enough to reduce standing c r o p i n the absence of s t r o n g growth l i m i t a t i o n . The acute disagreement between the model's p r e d i c t i o n s of s u s t a i n e d growth and o b s e r v a t i o n s of dramatic d e c l i n e s of phytoplankton biomass suggests that phytoplankton growth and l o s s were e p i s o d i c i n Foodweb I. T h i s i s i n v e s t i g a t e d more f u l l y i n s e c t i o n 6. Two other p i e c e s of c i r c u m s t a n t i a l evidence support the n o t i o n that there were d i s j u n c t episodes of pronounced a l g a l a c t i v i t y and l o s s i n CEE2. F i r s t , s e c t i o n 5.2.6 concluded that s i l i c i c a c i d c o u l d have accumulated i n the s u r f a c e l a y e r of CEE2 only when the p r o d u c t i o n of new diatom biomass was sharply c u r t a i l e d . Indeed, S i was observed to accumulate most r a p i d l y d u r i n g the f i n a l h a l f of the c o l l a p s e of l a r g e - c e l l e d diatoms. Second, s e c t i o n 5.1.5 noted that the i n f l u x of carbon to b a c t e r i a i n the s u r f a c e l a y e r c o u l d have r e q u i r e d as much as 74% of net primary p r o d u c t i o n . Although t h i s estimate i s h i g h l y u n c e r t a i n , i f i t were even approximately c o r r e c t then most of t h i s i n f l u x to b a c t e r i a must have o c c u r r e d through the degradation of i n a c t i v e c e l l s f o l l o w i n g s p u r t s of a l g a l growth i f the observed p e r i o d s of r a p i d net a l g a l growth are to be 198 F i g u r e 40. P r e d i c t e d l i m i t a t i o n of l a r g e diatom growth i n the s u r f a c e 8 m due to l i g h t , d i s s o l v e d n i t r o g e n and s i l i c o n . Values p r e d i c t e d by a s i m u l a t i o n i n which a l l s t a t e v a r i a b l e s t r a c e d t h e i r observed c o n c e n t r a t i o n s . Gross p h o t o s y n t h e s i s i s most l i m i t e d f o r p l o t t e d v a l u e s near zero, n o n - l i m i t e d f o r values near u n i t y . Values of l i g h t - l i m i t a t i o n have been averaged over s u c c e s s i v e 24-hour i n t e r v a l s to remove day-night o s c i l l a t i o n s . P r e d i c t e d l i m i t a t i o n f o r other a l g a l groups i s s i m i l a r , except f l a g e l l a t e s were not S i - l i m i t e d . 200 accounted f o r . B a c t e r i a i n the s u r f a c e l a y e r d i d i n f a c t i n c r e a s e to t h e i r highest c o n c e n t r a t i o n d u r i n g the c o l l a p s e of l a r g e - c e l l e d diatoms; P. P a r s l e y (pers. comm.) observed clumps of Stephanopyxis c e l l s h e a v i l y c o l o n i z e d by b a c t e r i a d u r i n g the c o l l a p s e . 201 6. WHAT WENT WRONG? It i s abundantly c l e a r that the model developed i n s e c t i o n 3 c o u l d not account f o r even the major f e a t u r e s of phytoplankton and n u t r i e n t dynamics i n CEE2. The model's most s p e c t a c u l a r f a i l u r e was i t s p r e d i c t i o n of s u s t a i n e d p o s i t i v e net growth of l a r g e - c e l l e d diatoms and f l a g e l l a t e s i n response to observed time s e r i e s of n u t r i e n t s , l i g h t , and zooplankton. .The p r e d i c t e d biomass of l a r g e - c e l l e d diatoms u s i n g the parameter values of Table IV e x e m p l i f i e s t h i s f a i l u r e ( F i g . 41). What went wrong? The reason f o r the f a i l u r e was a l l u d e d to i n s e c t i o n 5.3.2: phytoplankton growth and l o s s was e p i s o d i c i n CEE2 whereas the model assumed constant maximum p h o t o s y n t h e t i c r a t e s , constant s i n k i n g and r e s p i r a t i o n r a t e s , a constant f r a c t i o n of pho t o s y n t h e s i s l o s t v i a exudation. I t had been o r i g i n a l l y thought that f l u c t u a t i o n s i n g r a z i n g p r e s s u r e , 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 l e v e l s were d i r e c t l y r e s p o n s i b l e f o r the observed changes i n phytoplankton biomass, r a t h e r than f l u c t u a t i o n s i n the i n t e r n a l p h y s i o l o g y of the phytoplankton ( i . e . changes i n P rmax, K rN, K r S i , s i n k i n g r a t e , e t c . ) . The model's f a i l u r e prompted a reexamination of t h i s premise. To l i m i t the scope of i n v e s t i g a t i o n to a manageable s i z e , a t e s t case was c o n s i d e r e d : c o u l d the model be a d j u s t e d to q u a n t i t a t i v e l y account f o r the observed bloom and c o l l a p s e of l a r g e - c e l l e d diatoms i n the bubbled l a y e r .of CEE2? I t i s shown below that the modelled p h y s i o l o g y of l a r g e diatoms c o u l d be a d j u s t e d to account f o r t h e i r bloom and c o l l a p s e , but the 202 F i g u r e 41. Biomass of l a r g e - c e l l e d diatoms p r e d i c t e d by a s i m u l a t i o n i n which the c o n c e n t r a t i o n s of n u t r i e n t s , small diatoms, and l a r g e and small f l a g e l l a t e s were i n t e r p o l a t e d between o b s e r v a t i o n s . Parameter values as i n Table IV. a.) observed (octagons) and p r e d i c t e d ( s o l i d l i n e ) biomass of l a r g e diatoms i n the s u r f a c e 8 m of CEE.2 b) r e s i d u a l s 203 0 20 40 60 80 Time (days) residuals ® • • A * # in +-• I ro —I • • • cu tin h -T~WI—,—r~ "i—i—i—i—|—l—i—i—r 40 Time (days) -1—i—I—i—|—r—i—i—i—|—i—i—i—i—| 60 ' 80 20 204 reasons u n d e r l y i n g the changes i n physiology remain hidden. 6.1 The Bloom and C o l l a p s e of L a r g e - C e l l e d Diatoms Between days 13 and 51, l a r g e - c e l l e d diatoms i n CEE2 (mainly Stephanopyxis t u r r i s ) bloomed to huge c o n c e n t r a t i o n s , formed r e s t i n g spores, and c o l l a p s e d ( F i g . 42). Davis et a l . (1980) d e s c r i b e a s i m i l a r sequence of events which occurred i n the 1977 CEPEX experiment. As L e p t o c y l i n d r u s danicus bloomed, n i t r a t e + ammonium c o n c e n t r a t i o n f e l l below 0.5 ug-atom N l " 1 , and the m a j o r i t y of c e l l s formed r e s t i n g spores. The spores then q u i c k l y sank out of the water column. The approach taken here was not to e x p l a i n what caused spore p r o d u c t i o n i n the 1978 experiment, nor to c r e a t e a general model of phytoplankton bloom and c o l l a p s e . I nstead, the p e r i o d from days 13 to 51 was s p l i t i n t o a bloom phase (days 13 to 25) and c o l l a p s e phase (days 25 to 51); model parameters were s e p a r a t e l y a d j u s t e d to f i t p r e d i c t e d l a r g e - d i a t o m biomass to observed biomass i n each phase. 6.2 Parameter E s t i m a t i o n Technique The model used to p r e d i c t diatom growth was that developed i n s e c t i o n 3; only the dynamics of l a r g e - c e l l e d diatoms i n the s u r f a c e 8 m were a c t i v a t e d ; l a r g e - c e l l e d diatoms below 8 m, as w e l l as s m a l l - c e l l e d diatoms and l a r g e and sma l l f l a g e l l a t e s , n i t r o g e n and s i l i c o n t r a c k e d t h e i r observed c o n c e n t r a t i o n s over 205 F i g u r e 42. Observed c o n c e n t r a t i o n of v e g e t a t i v e c e l l s (0-8 m depth; s o l i d l i n e ) and r e s t i n g spores (0-20 m depth; dashed l i n e ) of l a r g e diatoms i n CEE2. S p o r e s (ug C 1"') 0 2 5 i i i i i i i i i i i 207 t i m e . 1 Although spore dynamics per se were not modelled, spore p r o d u c t i o n was i n d i r e c t l y simulated by i n c r e a s i n g , f o r example, the s i n k i n g r a t e of v e g e t a t i v e c e l l s and reducing t h e i r p h o t o s y n t h e s i s . The o b j e c t i v e was to f i n d values f o r model parameters which allowed simulated l a r g e diatom biomass t o most c l o s e l y f i t the observed bloom and c o l l a p s e . The " c l o s e s t f i t " was d e f i n e d as the minimum sum of squared d e v i a t i o n s (minimum SSQ) between p r e d i c t i o n s and o b s e r v a t i o n s over the i n t e r v a l of i n t e r e s t . T h i s minimum was found by the technique of system i d e n t i f i c a t i o n (Bard 1974; e.g. Parslow et a l . 1979), whereby a simulated time s e r i e s of l a r g e diatom biomass i n the su r f a c e l a y e r was generated from i n i t i a l parameter guesses, then the parameters were s y s t e m a t i c a l l y a l t e r e d to minimize the sum of squared d e v i a t i o n s between the simulated and observed time s e r i e s . In p a r t i c u l a r , only the va l u e s of three parameters were so a l t e r e d : 1) the maximum gross p h o t o s y n t h e t i c r a t e of l a r g e - c e l l e d diatoms, Prmax, 2) the h a l f - s a t u r a t i o n constant of ni t r o g e n l i m i t a t i o n of gross p h o t o s y n t h e s i s , K rN, and 3) the U n l i k e s e c t i o n 5, n u t r i e n t c o n c e n t r a t i o n s i n s e c t i o n 6.3 were l i n e a r l y i n t e r p o l a t e d between o b s e r v a t i o n s without accounting f o r expected i n c r e a s e s a f t e r n u t r i e n t a d d i t i o n s . The e r r o r i n t r o d u c e d by t h i s simpler method of i n t e r p o l a t i o n does not a f f e c t the c o n c l u s i o n s of s e c t i o n 6. 2S i s the e f f e c t i v e s i n k i n g r a t e i n the su r f a c e bubbled l a y e r , which i s l e s s than the true s i n k i n g r a t e measured i n un d i s t u r b e d water. S can be more g e n e r a l l y viewed as an u n s p e c i f i e d l o s s of carbon from l a r g e diatoms: t h i s u n s p e c i f i e d mode of l o s s can be expressed i n u n i t s of d a y 1 by d i v i d i n g S by the s u r f a c e l a y e r depth of 8 m. 208 s i n k i n g r a t e , S. J Zooplankton g r a z i n g r a t e s were not a l t e r e d because there was no evidence to suggest nor any reason to expect that the f u n c t i o n a l response of grazers changed between the bloom and c o l l a p s e phases. The c h o i c e of which parameters of diatom p h y s i o l o g y to a l t e r was l a r g e l y a r b i t r a r y . C l o s e r f i t s of p r e d i c t i o n s to o b s e r v a t i o n s might have been achieved by v a r y i n g parameters other the three mentioned above. However, as the number of parameters to be v a r i e d i n c r e a s e s , computational time r i s e s e x p o n e n t i a l l y , and worse, the l i k e l i h o o d of f i n d i n g the g l o b a l minimum SSQ can become very s m a l l . C o n s i d e r i n g that the whole p o i n t of the e x e r c i s e was to demonstrate the inadequacy of the o r i g i n a l model by showing that at l e a s t some p h y s i o l o g i c a l parameters formerly thought to be constant a c t u a l l y v a r i e d over time, i t was unnecessary to simultaneously vary more than three parameters to achieve c l o s e f i t of p r e d i c t i o n s to o b s e r v a t i o n s . Two methods were used to f i n d minimal SSQs. The simplest method, p r a c t i c a l when v a r y i n g only one or two parameters, was to search the SSQ s u r f a c e over a r e g u l a r g r i d of parameter v a l u e s . T h i s was used to c o n s t r u c t contour p l o t s of SSQ over two-dimensional parameter space. The second, more s o p h i s t i c a t e d method was Powell's d i r e c t search using conjugate d i r e c t i o n s (see pp. 75-78 of F l e t c h e r 1972). Prmax, K rN and S were bounded below by 0 d a y - 1 , 0 ug-atom N l " 1 and 0 m day" 1, r e s p e c t i v e l y , and were bounded above by l i m i t s imposed before each search. To s i m p l i f y the search, these parameters were transformed by the i n v e r s e s i n e f u n c t i o n to y i e l d a problem of unconstrained 209 o p t i m i z a t i o n (see pp. 42-43 of Powell 1972). "Convergence" of the search was assumed to occur when the transformed v a r i a b l e s changed by l e s s than 10"* i n magnitude on s u c c e s s i v e i t e r a t i o n s . In p r a c t i c e one may be unable to f i n d the g l o b a l minimum SSQ, i . e . the c l o s e s t p o s s i b l e f i t , because of the complexity of the SSQ s u r f a c e . T h i s i s what happened here. With s u f f i c i e n t l y d i f f e r e n t i n i t i a l guesses, Powell's method converged to d i f f e r e n t f i n a l estimates of "opt i m a l " parameter v a l u e s . These l o c a l minima had s i m i l a r SSQs and were l o c a t e d a t the bottom of the same v a l l e y of low SSQ, meaning that s e v e r a l combinations of parameter values y i e l d e d p r e d i c t i o n s which f i t the o b s e r v a t i o n s e q u a l l y w e l l and that the d i f f e r e n t parameters were h i g h l y c o r r e l a t e d . 6.3 R e s u l t s and D i s c u s s i o n 6.3.1 The diatom bloom, days 13 to 25 Two searches using Powell's method converged to neighb o r i n g p o i n t s i n parameter space with very s i m i l a r SSQs (Table V ) . E f f e c t i v e s i n k i n g r a t e was low, nO.l m day" 1 (= l o s s r a t e of 0.0125 day" 1 from l a r g e diatoms), K rN <0.07 ug-atom N l " 1 , and Prmax was moderate, nl.7 day" 1. F i g . 43, a contour graph of the SSQ su r f a c e s l i c e d at S = 0.11 m day" 1, shows that f o r Prmax near nl.6 day" 1, K rN values <0.4 ug-atom N l " 1 had v i r t u a l l y no e f f e c t on c l o s e n e s s of f i t . T h i s i m p l i e s that diatom growth i n CEE2 was not l i m i t e d by n i t r o g e n i f K rN <0.4 V \ o CM Table V. Sum of squared deviations between predicted and observed large-diatom biomass i n the surface 8 m of CEE2 f o r d i f f e r e n t parameter values. The maximum gross photosynthetic rate ( P m a x ) > t n e h a l f - s a t u r a t i o n constant of N - l i m i t a t i o n (K^) , and the e f f e c t i v e sinking rate (S) i n the surface bubbled layer were simultaneously varied from i n i t i a l guesses using Powell's method (see pp. 75-78 of Fletcher 1972). This method converged to f i n a l parameter values which yielded close f i t s to observed large-diatom biomass. Two p e r i o d s — t h e large diatom bloom (days 13 to 25) and collapse (days 25 to 51)—were separately analyzed. Pmax ( d a y - 1 ) ^ ( u s - a t N s ( m d a y - 1 ) S SQ upper upper upper ^ 2 - 2 i n i t i a l bound f i n a l i n i t i a l bound f i n a l i n i t i a l bound f i n a l (10 (ug C) 1 ) Bloom 1.74 5 1.63 0.00 5 0.24 0.32 40 0.11 7.04 2.09 5 1.80 0.77 5 0.67 0.32 40 0.00 7.36 lollapse 0.50 2 2.00 0.77 5 0.58 15.00 40 4.26 5.37 0.50 2 2.00 4.50 5 0.78 3.00 4 3.97. 5.23 0.00 5 4.76 0.77 5 0.76 2.00 40 8.42 4.22 211 F i g u r e 43. SSQ s u r f a c e f o r p e r i o d of l a r g e diatom bloom, days 13 to 25. S = 0.106 m d a y 1 . Contour i n t e r v a l i s 10 s (ug C l ' 1 ) 2 . S t a r marks l o c a l minimum of 7.045 • 10* (ug C l " 1 ) 2 at Prmax=1.63 day" 1, KrN=0.242 ug-atom N l " 1 . Contoured g r i d was 10 by 10 p o i n t s . 213 ug-atom N l ' 1 . When the SSQ s u r f a c e i s s l i c e d at K rN = 0.24 ug-atom N l " 1 , a narrow v a l l e y of low SSQ i s seen t o run from Prmax «1.5 day" 1, S n0 m day" 1 to Prmax a5 day" 1, S n6.5 m day" 1 ( F i g . 44). Since Prmax and S are opposing terms of gain and l o s s from l a r g e diatom biomass, the p r e d i c t i o n of such a v a l l e y was expected: as long as S was l a r g e enough t o coun t e r a c t an e x p l o s i v e l y high Prmax, growth of diatoms c l o s e l y mimicked o b s e r v a t i o n s d u r i n g the bloom ( F i g . 45). 6.3.2_ The diatom c o l l a p s e , days 25 to 51 When the same parameter v a l u e s used i n the p e r i o d between days 13 and 25 were a p p l i e d to the p e r i o d a f t e r day 25, the simulated biomass of l a r g e diatoms skyrocketed from a s t a r t i n g c o n c e n t r a t i o n of 687 ug C l " 1 on day 25, i n v i o l e n t disagreement with the observed c o l l a p s e ( F i g . 46). When parameter val u e s were s y s t e m a t i c a l l y searched f o r c l o s e f i t s to the observed c o l l a p s e , a s u p r i s i n g r e s u l t was obtained (Table V ) : Powell's method c o n s i s t e n t l y converged t o P rmax values at or near the upper bound p l a c e d on Prmax, with S a l s o being very high to counteract the l a r g e maximum p h o t o s y n t h e t i c r a t e . The f i t s were good (e.g. F i g . 47), but d i f f i c u l t to r e c o n c i l e with the observed accumulation of s i l i c i c a c i d i n the su r f a c e l a y e r d u r i n g the l a r g e diatom c o l l a p s e . With such high Prmax v a l u e s , the primary p r o d u c t i o n of diatom biomass would r a p i d l y s t r i p S i from the s u r f a c e l a y e r unless S i was very q u i c k l y regenerated. T h i s seems u n l i k e l y , although Nelson and Goering (1978) speculate that s i l i c i c a c i d may be more r a p i d l y 214 F i g u r e 44. SSQ s u r f a c e f o r p e r i o d of l a r g e diatom bloom, days 13 to 25. K rN = 0.242 ug-atom N l " 1 . Contour i n t e r v a l i s 10 s (ug C l " 1 ) 2 . Contours above 10 • 10 s (ug C l - 1 ) 2 not p l o t t e d . Contoured g r i d was 11 by 11 p o i n t s . 215 S i n k i n g Rate (m day - 1 ) 216 F i g u r e 45. Co n c e n t r a t i o n of l a r g e diatoms i n surface 8 m p r e d i c t e d with Prmax = 1.63 d a y 1 , KrN=0.242 ug-atom N S=0.106 m d a y 1 ; days 13 to 25. Octagons are observed c o n c e n t r a t i o n s . SSQ = 7.045 • 10* (ug C 1" 1) 2 . 218 F i g u r e 46. Co n c e n t r a t i o n of l a r g e diatoms i n s u r f a c e 8 m p r e d i c t e d with Prmax=1.63 day" 1, KrN=0.242 ug-atom N S=0.106 m day" 1; days 25 to 51. Octagons are observed c o n c e n t r a t i o n s . SSQ = 2.131 • 10 7 (ug C 1" 1 ) 2 . Large Diatoms (ug C i"1) 1 0 0 0 i—I_J i i i i i i i i i i r o C D r o CO C D G G Q G Q Q Q G G G 19 C D CD C O O 220 F i g u r e 47. C o n c e n t r a t i o n of l a r g e diatoms i n s u r f a c e 8 m p r e d i c t e d with Prmax=2.00 d a y 1 , RrN=0.783 ug-atom N l " 1 , S=3.97 m d a y 1 ; days 25 to 51. Octagons are observed c o n c e n t r a t i o n s . SSQ = 5.229 • 10* (ug C 1" 1) 2 . 222 regenerated than combined n i t r o g e n i n some marine 'ecosystems. I n t e r e s t i n g l y , observed 1 4 C f i x a t i o n r a t e s (as ug C l " 1 h~ 1 or ug C (ug Chl-a • h ) " 1 ) were s i m i l a r d u r i n g the p e r i o d s of bloom and c o l l a p s e . Thus, Prmax value s i n the c o l l a p s e phase which are comparable to bloom P rmax's cannot be dis m i s s e d out of hand, but a high c o l l a p s e Prmax of, f o r example, 4.76 day" 1 (Table V) i s b i o l o g i c a l l y n o n s e n s i c a l . 6.4 Co n c l u s i o n s Regarding P h y s i o l o g i c a l V a r i a b i l i t y I t i s ev i d e n t that the model c o u l d be tuned t o f i t the separate p e r i o d s of l a r g e diatom bloom and c o l l a p s e i n CEE2. The important p o i n t i s that s i g n i f i c a n t a l t e r a t i o n s of at l e a s t some p h y s i o l o g i c a l parameters were necessary to achieve t h i s f i t . Although g r a z i n g pressure c o u l d have been s i m i l a r l y a l t e r e d to achieve a c l o s e f i t to the bloom and c o l l a p s e of l a r g e diatoms, there was no evidence to support such an ad hoc man i p u l a t i o n . One must t h e r e f o r e conclude that fundamental s h i f t s i n the ph y s i o l o g y of l a r g e diatoms were r e s p o n s i b l e f o r t h e i r c o l l a p s e , r a t h e r than a prolonged i n c r e a s e i n g r a z i n g p r e s s u r e or prolonged shortage of n i t r o g e n , s i l i c o n or l i g h t . What t r i g g e r e d the diatom c o l l a p s e , and what was r e s p o n s i b l e f o r t h e i r d e c l i n e ? I t seems f a i r l y c e r t a i n that a r a p i d l y growing p o p u l a t i o n of Stephanopyxis t u r r i s , which comprised 99% of large-diatom carbon at the height of the bloom, removed a l l d i s s o l v e d i n o r g a n i c n i t r o g e n from the su r f a c e l a y e r s h o r t l y before day 25. The p o p u l a t i o n experienced a sudden n u t r i e n t shock which induced a p o r t i o n of the 223 S. t u r r i s p o p u l a t i o n to form spores. On day 25, peak numbers of both v e g e t a t i v e c e l l s (263 per l i t r e , 0-8 m) and spores (15 per l i t r e , 0-8 m) were pres e n t . T h i s d e s c r i p t i o n i s s i m i l a r to that proposed by Davis et a l . (1980) f o r the i n i t i a t i o n of a c o l l a p s e of L e p t o c y l i n d r u s danicus i n an e n c l o s e d ecosystem i n 1977. Davis et a l . (1980) note that the number of v e g e t a t i v e c e l l s + spores of L. danicus was g r e a t l y reduced f o l l o w i n g spore formation, and they imply that the d i f f e r e n c e c o u l d be accounted f o r i f "spore formation [ i n L. danicus] i s a sexual process r e q u i r i n g s e v e r a l spermatogonial c e l l s and perhaps s e v e r a l o o g o n i a l c e l l s f o r each s u c c e s s f u l [ r e s t i n g - s p o r e forming] auxospore." T h i s same argument cannot e x p l a i n the marked d e c l i n e of v e g e t a t i v e biomass of l a r g e diatoms i n CEE2 because spore formation i n Stephanopyxis t u r r i s i s asexual with each v e g e t a t i v e c e l l capable of forming one r e s t i n g spore (von Stosch and Drebes 1964; Drebes 1966). Only a small p r o p o r t i o n of large-diatom c e l l s formed r e s t i n g spores immediately a f t e r n i t r o g e n exhaustion, as i n d i c a t e d by the s i g n i f i c a n t biomass of v e g e t a t i v e c e l l s d u r i n g the c o l l a p s e ( F i g . 42). Therefore, some f a c t o r s other than spore formation must have been p a r t l y r e s p o n s i b l e f o r the observed c o l l a p s e of Stephanopyxis. To assess p o t e n t i a l g r a z i n g , s i n k i n g , and mixing l o s s e s of v e g e t a t i v e carbon, a s i m u l a t i o n was run using the parameter v a l u e s of Table IV (except e f f e c t i v e s i n k i n g r a t e i n the bubbled s u r f a c e l a y e r was set at 1/4 of the 1.58 m day' 1 r a t e observed by Bienfang (1980) f o r the >20 um f r a c t i o n s e t t l i n g i n 224 unbubbled water on day 32) and c o n s t r a i n i n g phytoplankton to t h e i r observed c o n c e n t r a t i o n s . The p r e d i c t e d l o s s of larg e - d i a t o m carbon from the s u r f a c e l a y e r between days 25 and 51 was 194 ( s i n k i n g ) + 413 (grazing) + 13 (mixing) = 620 ug C 1-V26 days. (Grazing may have been overestimated by a f a c t o r of n2 and s i n k i n g underestimated by a f a c t o r of n2.) A l o s s of 620 ug C l - 1 almost equals the observed d e c l i n e i n l a r g e diatom biomass of 682 ug C l " 1 , and the d i f f e r e n c e c o u l d e a s i l y be made up by r e s t i n g spore formation. U n f o r t u n a t e l y t h i s balance n e g l e c t s any primary p r o d u c t i o n which may have o c c u r r e d . The 1*C p r o d u c t i v i t y i n the su r f a c e 8 m between days 25 and 51 was 30.3 ± 27.5 ug C l _ 1 / 4 hours (mean ± s.dev., n=23) • 2.4 (see footnote 5 on p. 151) • 26 1.9 mg C l~1/26 days. I f t h i s average i s roughly r e p r e s e n t a t i v e of the net primary p r o d u c t i o n of l a r g e diatoms i n the su r f a c e l a y e r ( l a r g e diatom carbon averaged 79% of t o t a l p h o t o s y n t h e t i c carbon between days 25 and 51), then a huge l o s s of 1.9 + 0.7 -0.6 = 2.0 mg C 1-V26 days i s unaccounted f o r . Some of the carbon was undoubtedly l o s t through spore formation, but other avenues of l o s s such as exudation or c e l l l y s i s must have been important. In any event, the l a r g e diatoms c o l l a p s e d i n CEE2 because of a dramatic change i n p h y s i o l o g y : e i t h e r t h e i r c e l l growth was a r r e s t e d or c e l l l y s i s or exudation was g r e a t l y a c c e l e r a t e d . None of t h i s e x p l a i n s , however, why l a r g e diatoms continued to d e c l i n e a f t e r n u t r i e n t s had regained t h e i r former high c o n c e n t r a t i o n s . F i g . 40 demonstrates that with 225 h a l f - s a t u r a t i o n constants of 0.77 ug-atom N l " 1 and 2.06 ug-atom S i l " 1 , growth would have been l i m i t e d to no l e s s than 60% of maximum a f t e r day 27. With a s a t u r a t i o n i r r a d i a n c e ( I r k ) of nlOO - 200 uEin n r 2 s _ 1 as modelled, the s e v e r a l cloudy days a f t e r August 9 (day 32) would have i n t e r m i t t e n t l y l i m i t e d growth ( i n the s u r f a c e l a y e r at l o c a l apparent noon) to no l e s s than 42% of maximum. Why d i d n ' t the v e g e t a t i v e c e l l s resume r a p i d growth? One c o u l d , of course, vacuously p o s t u l a t e that some r e q u i r e d growth f a c t o r was absent d u r i n g the l a r g e - d i a t o m c o l l a p s e . On the b a s i s of a v a i l a b l e evidence, however, i t appears t h a t l a r g e - c e l l e d diatom growth i n CEE2 was " e p i s o d i c . " In t h i s context e p i s o d i c growth means that d i s t i n c t changes or " f l i p s " i n c e l l u l a r p h y s i o l o g y occur, and that a f t e r a f l i p the p o p u l a t i o n may be locked i n t o a new p h y s i o l o g i c a l s t a t e f o r an extended l e n g t h of time even i f the environment changes and f a v o r s the e a r l i e r p h y s i o l o g i c a l s t a t e . An example of such a f l i p i s when an e n t i r e p o p u l a t i o n forms r e s t i n g spores. Other examples are d r a m a t i c a l l y i n c r e a s e d growth r a t e s of diatom c e l l s r e c e n t l y d e r i v e d from auxospores (Davis et a l . 1973; C o s t e l l o and Chisholm, unpublished) or abrupt shift-down of b a c t e r i a l metabolism i n c u l t u r e s which have exhausted a carbon or n i t r o g e n source (Schaechter 1973). The no t i o n of v e g e t a t i v e c e l l s " l o c k e d i n t o " c e r t a i n p h y s i o l o g i c a l s t a t e s i s unconventional and runs counter to the common wisdom of "adaptive p l a s t i c i t y ; " the author i s unaware of p u b l i s h e d work su p p o r t i n g t h i s n o t i o n , except f o r p o p u l a t i o n s which have been s e v e r e l y g r o w t h - l i m i t e d f o r one or more months (Davis et a l . 226 1973). The v a r i o u s unanswered questions r e g a r d i n g the p r o t r a c t e d d e c l i n e of l a r g e - c e l l e d diatoms in the su r f a c e of CEE2 suggest that blanket terms used to d e s c r i b e c o l l a p s i n g a l g a l p o p u l a t i o n s , l i k e "senescent" or "moribund," conceal more than they e x p l a i n . What i s completely l a c k i n g at present i s a q u a n t i t a t i v e understanding of the c o l l a p s e of a l g a l blooms. 227 7. FINAL CONCLUSIONS One might wonder why the attempt to model p h y t o p l a n k t o n - n u t r i e n t dynamics i n Foodweb I was such a f a i l u r e when models of other marine systems, using s i m i l a r methods, have been reasonably s u c c e s s f u l . How have these other models "gotten away with" assuming smoothly changing p h y s i o l o g i c a l s t a t e s when i t was necessary to invoke an abrupt change i n diatom p h y s i o l o g y to even roughly account f o r some events of Foodweb I? T h i s q u e s t i o n can perhaps best be answered by c o n s i d e r i n g two widely c i t e d s i m u l a t i o n models of c o a s t a l marine ecosystems. Consider f i r s t the Narragansett Bay ecosystem model of Kremer and Nixon (1978). Kremer and Nixon simulated phytoplankton, zooplankton and n u t r i e n t dynamics over a 1-year p e r i o d ( s t a r t i n g January 1) i n e i g h t s p a t i a l elements l i n k e d by t i d a l f l u s h i n g and exchange of water between elements. Most b i o l o g i c a l parameters were e x p o n e n t i a l f u n c t i o n s of water temperature: maximum growth r a t e of phytoplankton, maximum r a t i o n per zooplankter, zooplankton r e s p i r a t i o n , r a t e of j u v e n i l e development, n u t r i e n t r e g e n e r a t i o n . Temperature changed s e a s o n a l l y between l i m i t s of 3 °C on February 9 and 20 °C on August 9. The standard model run r e a l i s t i c a l l y p r e d i c t e d both the magnitude and t i m i n g of the s p r i n g bloom of phytoplankton i n Narragansett Bay and roughly accounted f o r the ge n e r a l f e a t u r e s of the summer b u i l d u p of zooplankton and winter d e c l i n e of both phytoplankton and zooplankton. When water temperature was h e l d constant the seasonal c y c l e was 2 2 8 d e s t r o y e d . Phytoplankton q u i c k l y bloomed and a f t e r a s e r i e s of damped o s c i l l a t i o n s reached approximate e q u i l i b r i u m by J u l y ; phytoplankton biomass d e c l i n e d only s l i g h t l y i n November and December due to reduced l i g h t . When water exchange was e l i m i n a t e d and the e i g h t s p a t i a l elements were simulated as i s o l a t e d systems, phytoplankton and zooplankton o s c i l l a t e d w i l d l y i n each element and the model f a i l e d to p r e d i c t the d e c l i n i n g s t anding crop of algae towards the mouth of the bay. A l t e r a t i o n of b i o l o g i c a l parameters d i d not s t r o n g l y i n f l u e n c e the p r e d i c t e d seasonal c y c l e s or s p a t i a l t r e n d s . These r e s u l t s imply that s e v e r a l major b i o l o g i c a l f e a t u r e s of Narragansett B a y — t h e seasonal c y c l e of phytoplankton and zooplankton biomass, reduced plankton biomass towards the bay's mouth, a l a r g e measure of the ecosystem's s t a b i l i t y — w e r e c o n t r o l l e d by the p h y s i c a l f a c t o r s of temperature, water movement, and to a l e s s e r degree, l i g h t . Kremer and Nixon's model was l e a s t s u c c e s s f u l i n i t s p r e d i c t i o n of plankton and n u t r i e n t dynamics in summer when temperature and l i g h t were not s t r o n g l y l i m i t i n g . I t was p r e c i s e l y i n t h i s p e r i o d that manipulation of b i o l o g i c a l parameters most a f f e c t e d model p r e d i c t i o n s . The l e s s o n to be learned from the Narragansett Bay model i s t h i s : The t r a d i t i o n a l assumption of smoothly changing phytoplankton p h y s i o l o g y i s most s u c c e s s f u l when p r e d i c t i n g gross seasonal changes f o r c e d by p h y s i c a l f a c t o r s , but i s w o e f u l l y inadequate whenever b i o l o g i c a l i n t e r a c t i o n s dominate. Winter, Banse and Anderson (1975) d i d not examine seasonal changes, but i n s t e a d simulated phytoplankton growth w i t h i n the 229 c e n t r a l b a s i n of Puget Sound over 75 and 35 days i n the s p r i n g of 1966 and 1967, r e s p e c t i v e l y . Puget Sound i s g e n e r a l l y c h a r a c t e r i z e d by a seaward flow of l o w - s a l i n i t y water at the s u r f a c e complemented by subsurface movement of more s a l i n e water i n t o the Sound from the S t r a i t of Georgia. Winter et a l . (1975) modelled a l g a l growth i n the s u r f a c e 30 m i n response to computed v e r t i c a l mixing and a d v e c t i o n and d a i l y o b s e r v a t i o n s of i n s o l a t i o n , n i t r a t e d i s t r i b u t i o n and h e r b i v o r e c o n c e n t r a t i o n . Phytoplankton p h y s i o l o g y was simulated i n a c o n v e n t i o n a l manner. Winter et a l . (1975) s u c c e s s f u l l y p r e d i c t e d not only the general c h l o r o p h y l l c o n c e n t r a t i o n w i t h i n the c e n t r a l basin but a l s o the r e c u r r i n g blooms seen in both y e a r s . They concluded that " a l g a l growth i n the c e n t r a l b a s i n i s l i m i t e d by a combination of hydrodynamic f a c t o r s ... and the modulation of the underwater l i g h t i n t e n s i t y by s e l f - s h a d i n g and by i n o r g a n i c p a r t i c u l a t e s . " As with Kremer and Nixon's model, Winter et a l . ' s model was s u c c e s s f u l i n so f a r as p h y s i c a l f a c t o r s ( e s t u a r i n e c i r c u l a t i o n , t i d a l mixing, l i g h t a t t e n u a t i o n ) dominated ecosystem dynamics. T h e i r success emboldened Winter et a l . (1975) to s t a t e "We conclude that the f u n c t i o n s and parameters t r a d i t i o n a l l y employed to d e s c r i b e phytoplankton metabolism are m a r g i n a l l y adequate f o r use in a s hort-time s c a l e model, such as the one developed here." The d i f f i c u l t i e s encountered i n t h i s t h e s i s -demand that Winter et a l . ' s statement be q u a l i f i e d : ' T r a d i t i o n a l d e s c r i p t i o n s of phytoplankton metabolism are m a r g i n a l l y adequate p r o v i d e d that p h y s i c a l f a c t o r s l a r g e l y f o r c e b i o l o g i c a l dynamics. They are 230 e n t i r e l y inadequate otherwise.' So we have come f u l l c i r c l e . I t was o r i g i n a l l y supposed that phytoplankton c o u l d be modelled as p a s s i v e l y changing t h e i r p h y s i o l o g i c a l s t a t e i n c o n c e r t with changes i n i r r a d i a n c e and n u t r i e n t c o n c e n t r a t i o n . The enclosed water columns of Foodweb I were thought to be i d e a l systems i n which to c o n f i r m that phytoplankton biomass indeed v a r i e d with on-going changes i n l i g h t , n u t r i e n t s and g r a z i n g p r e s s u r e . M o d e l l i n g of events was f a c i l i t a t e d by an e x t e n s i v e and d e t a i l e d body of ob s e r v a t i o n and the absence of l a r g e - s c a l e a d v e c t i o n . However, the removal of ad v e c t i o n and the short d u r a t i o n of the experiment allowed b i o l o g i c a l i n t e r a c t i o n s to dominate the enclosed water columns, thereby d e f e a t i n g the o r i g i n a l m o d e l l i n g attempt. 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H e t e r o t r o p h i c u t i l i z a t i o n of d i s s o l v e d o r g a n i c compounds i n the sea. I I . Observations on the responses of h e t e r o t r o p h i c marine p o p u l a t i o n s to abrupt i n c r e a s e s i n amino a c i d c o n c e n t r a t i o n . J . mar. b i o l . Ass. U.K. 50:871-881. W i l l i a m s , R. B., and J . P. B a p t i s t . 1966. Physiology of Mnemiopsis i n r e l a t i o n to i t s r o l e as a pred a t o r . B u l l . Ass. South East B i o l o g i s t s 13:48. Wilson, J . H. 1979. Observations on the g r a z i n g r a t e s and growth of Ostrea e d u l i s L. l a r v a e when fed a l g a l c u l t u r e s of d i f f e r e n t ages. J . exp. mar. B i o l . E c o l . 38:187-199. Winter, D. F., K. Banse, and G. C. Anderson. 1975. The dynamics of phytoplankton blooms i n Puget Sound, a f j o r d i n the northwestern United S t a t e s . Mar. B i o l . 29:139-176. Wright, R. T., and J . E. Hobbie. 1965. The uptake of organic s o l u t e s i n lake water. Limnol. Oceanogr. 10:22-28. Wright, R. T., and J . E. Hobbie. 1966. Use of glucose and a c e t a t e by b a c t e r i a and algae i n a q u a t i c ecosystems. Ecology 47:447-464. Yonge, C. M. 1926. S t r u c t u r e and p h y s i o l o g y of the organs of fe e d i n g and d i g e s t i o n i n Ostrea e d u l i s . J . mar. b i o l . Ass. U.K. 14:295-386. APPENDIX 1 Appendix 1. Species composition of the four phytoplankton groups d i s t i n g u i s h e d i n the s i m u l a t i o n model. Included are c e l l s i z e ( e q u i v a l e n t s p h e r i c a l diam-e t e r ) , c e l l carbon, and average c o n c e n t r a t i o n of each s p e c i e s observed i n CEE2 between days 2 and 79. taxon c e l l diam. c e l l carbon c o n c e n t r a t i o n (um) (pg C) (ug C / l ) Group 1. L a r g e - c e l l e d diatoms. 23.0 15.4 66.4 30.4 Act i n o p t y c h u s  undulatus A s t e r i o n e l l a  -japonica B i d d u l p h i a  l o n q i c r u r i s C e r a t a u l i n a berqoni i Chaetoceros  c o n s t r i c t u s  convolutus  dec i p i e n s  didymus  l a c i n i o s u s  s i m i l i s C o s c i n o d i s c u s  excentr i c u s D i t y l u m b r i g h t w e l l i i Eucampia  zoodiacus Grammatophora  marina L e p t o c y l i n d r u s  danicus A Licmorpha a b b r e v i a t a 15.7 28.4 25.0 21.0 20.0 15.4 52.1 47.4 21.8 18.1 18.9 30.8 290. 116. 3225. 545. 122. 466. 350. 235. 210. 115. 1859. 1500. 258. 168. 186. 562. .00999 .635 1.08 1.07 4.06 .263 .585 .200 .442 .193 .568 .0227 .721 .295 .410 .130 257 Appendix 1. (continued) taxon c e l l diam. c e l l carbon c o n c e n t r a t i o n (um) (pg C) (ug C / l ) Group 1. (continued) N a v i c u l a sp. A 48.3 N i t z s c h i a sp. A 15.8 Pleurosigma f a s i c o l a 38 .9 R h i z o s o l e n i a d e l i c a t u l a 21.4 f r a g i l i s s i m a 32.8 set i g e r a 28.2 s t o l t e r f o t h i i 26.1 s t y l i f o r m i s 59.2 S c h r o d e r e l l a d e l i c a t u l a 26.6 Skeletonema costatum A 15.9 Stephanopyxis t u r r i s 60.2 S t r i a t e l l a u n i p u n c t a t a 57.9 Thalassionema n i t z s c h i o d e s 18.0 T h a l a s s i o s i ra dec i p i e n s 29.8 n o r d e n s k i o l d i i 23.0 polychorda 37.4 r o t u l a 41.7 T r o p i d o n e i s a n a r c t i c a 131.9 1563. 123. 955. 246. 650. 460. 387. 2485. 402. 125. 2583. 2364. 165. 522. 290. 874. 1121. 15350. 1.65 .225 .0427 .0724 .253 1.19 .0282 .00861 1.85 .112 77.2 .108 5.40 1.84 2.28 7.04 5.18 1.51 Appendix 1. (continued) taxon c e l l diam. (um) c e l l carbon (pg C) c o n c e n t r a t i o n (ug C / l ) Group 2. S m a l l - c e l l e d diatoms. Amphiprora sp. Chaetoceros sp. A sp. B compressus danicus d e b i l i s densus g r a c i l i s r a d i c a n s s e p t e n t r i o n a l e soc i a l i s s u b t i l i s C y l i n d r o t h e c a  c l o s t e r i u m A c l o s t e r i u m B F r a g i l a r i a  c r o t o n e n s i s L e p t o c y l i n d r u s  dan i c u s B N a v i c u l a sp. B N i t z s c h i a d e l i c a t i s s i m a palea pungens Skeletonema  costatum B T h a l a s s i o s i r a s u b t i l i s 12.7 11, 4, 14, 9, 14, 14, 8, 12, 8, 11, 7, 9.5 4.9 11.2 10.8 10.9 8.8 8.0 12.1 11.3 10.5 74.4 55.2 6.32 92.8 38.4 94.1 97.7 30.7 65.8 30.3 55.2 20.0 38.9 8.61 56.8 52.3 52.7 32.4 26.1 67.3 57.1 48.5 .148 3.27 .0473 .143 1.81 .228 .454 .0392 .0267 .145 5.71 .00623 .971 .209 .0396 .0250 .418 1.16 1.42 .691 1.60 .145 Group 3. L a r g e - c e l l e d f l a g e l l a t e s . Cerat ium f u r c a 36.3 2236. 1.23 fusus 36.1 2210. 82.8 259 Appendix 1. (continued) taxon c e l l diam. c e l l carbon c o n c e n t r a t i o n (um) (pg C) (ug C / l ) Group 3. (continued) D i n o f l a g e l l a t e s , u n i d e n t i f i e d A u n i d e n t i f i e d B u n i d e n t i f i e d C u n i d e n t i f i e d D Dinophysis  acuminata Distephanus  speculum Glenodinium  danicum Gonyaulax  l o n g i s p i n a  s p i n i f e r a S c r i p p s i e l l a trochoideum 25. 35. 45. 55. 34.1 16.4 21.0 37.1 30.9 19.9 848. 2033. 3906. 6578. 1905. 281. 536. 2370. 1469. 470, .835 .362 .594 1.93 .726 .187 .252 .884 .0794 .422 Group 4. S m a l l - c e l l e d f l a g e l l a t e s . A p e d i n e l l a  r a d i a n s Chroomonas  amphioxeia  b a l t i c a  minuta Chrysochromulina sp. kappa Chrysophyte, u n i d e n t i f i e d Cryptophyte, u n i d e n t i f i e d Dinobryon sp. 7.0 7.3 10.4 4.7 4.0 6.5 6.6 10.9 4.6 31.6 34.3 86.1 11.1 7.08 26.0 26.7 98.3 10.5 .218 1.37 .101 .401 .978 3.33 .180 .0205 .0139 260 Appendix 1. taxon (continued) c e l l diam. (um) c e l l carbon (pg C) co n c e n t r a t ion (ug C / l ) Group 4 (continued) D i n o f l a g e l l a t e , u n i d e n t i f i e d E Green a l g a , unident i f i e d Gymnodinium  punctatum F l a g e l l a t e s , u n i d e n t i f i e d A u n i d e n t i f i e d B u n i d e n t i f i e d C Katodinium  rotundatum Leucocryptos sp. Ochromonas sp. Pachysphaera sp. Prorocentrum  micans Pyramimonas  micron Prymnesium  parvum 15. 14.8 14.4 3.5 7.5 15.0 12.3 7.7 8.7 7.6 14.5 6.4 7.1 225. 217. 201. 5.13 34.4 225. 133. 39.5 54.5 38.1 206. 24.5 32.4 .754 .00785 2.78 2.85 .457 .107 .0418 .0704 2.83 .0219 .00150 .385 .0286 APPENDIX 2 Appendix 2. Zooplankton taxa t r a c k e d by the main model. Included are estimated or measured body carbon (see notes at end of appendix) and f e e d i n g s e l e c t -i v i t i e s (see s e c t i o n 3.5.2 f o r d e t a i l s ) on phyto-plankton and b a c t e r i a . No s e l e c t i v i t i e s are shown fo r non-feeding taxa or taxa assumed to be s t r i c t l y c a r n i v o r o u s . "-" i n d i c a t e s a s e l e c t i v i t y of zero . taxon body weight s e l e c t i v i t y 1 (ug C) LD SD LF SF B Copepods: Acart i a 2 Nl .0220 N2 .0374 N3 .0719 .019 .433 .010 .671 -N4 .124 .032 .577 .021 .748 -N5 .193 .047 .696 .037 .792 -N6 .276 .063 .786 .055 .812 -CI .350 .076 .840 .072 .817 -C2 .538 .104 .919 .111 .810 -C3 .827 .138 .966 .166 .781 -C4 1.44 .194 .972 .261 .717 -C5 2.52 .265 .916 .387 .628 -C6 male 3.70 .322 .846 .487 .558 -C6 female 6.04 .404 .730 .623 .465 -Calanus p a c i f i c u s 3 CI 2.72 .276 .904 .406 .615 -C2 5.44 .385 .757 .594 .485 -C3 14.3 .566 .497 .847 .308 -C4 27.2 .688 .336 .959 .209 -C5 47.6 .780 .222 .992 .141 -C6 male 35.0 .732 .281 .982 .176 -C6 female 68.0 .824 .165 .978 .106 -Centropaqes 4 CI 1.14 .169 .977 .217 .747 -C2 1.75 .217 .960 .302 .688 -C3 2.70 .275 .905 .404 .616 -C4 4.68 .360 .793 .552 .514 -C5 8.23 .460 .648 .709 .406 -C6 male 12.0 .533 .543 .807 .337 -C6 female 19.7 .628 .414 .911 .256 -Appendix 2. (continued) taxon body weight (ug C) LD s e l e c t i v i t y SD LF SF B Corycaeus 5 Nl .0105 N2 .0178 N3 .0343 .008 .265 .003 .540 -N4 .0590 .015 .384 .008 .639 -N5 .0922 .024 .497 .014 .709 -N6 .132 .033 .594 .023 .756 -copepodid .680 .122 .949 .139 .797 -C6 male 2.08 .239 .942 .341 .661 -C6 female 2.88 .284 .894 .421 .604 -E p i l a b i d o c e r a 4 CI .998 .156 .975 .195 .763 -C2 1.53 .201 .969 .274 .708 -C3 2.36 .256 .926 .371 .639 -C4 4.09 .338 .824 .514 .539 -C5 7.19 C6 male 10.5 C6 female 17.2 O i t h o n a 5 Nl .00438 N2 .00743 N3 .0143 .003 .129 .001 .375 -N4 .0246 .006 .205 .002 .477 -N5 .0384 .009 .288 .004 .561 -N6 .0548 .014 .366 .007 .626 -copepodid .320 .071 .820 .065 .816 -C6 male 1.12 .167 .977 .214 .750 -C6 female 1.20 .174 .977 .227 .741 -Oncaea 7 copepodid .360 .077 .846 .074 .817 -C6 male .880 .144 .970 .175 .775 -C6 female 1.08 .164 .977 .208 .754 -Appendix 2. (continued) taxon body weight s e l e c t i v i t y (ug C) LD SD LF SF B P a r a c a l a n u s 6 T o r t a n u s 8 Nl .00760 N2 .0114 N3 .0152 .003 .136 .001 .386 -N4 .0228 .005 .193 .002 .462 -N5 .0456 .011 .325 .005 .593 -N6 .0760 .020 .447 .011 .680 -CI .152 .038 .632 .027 .771 -C2 .304 .068 .809 .062 .815 -C3 .798 .135 .963 .161 .784 -C4 1.52 .200 .970 .272 .709 -C5 2.66 .272 .908 .400 .619 -C6 male 2.76 .278 .902 .410 .612 -C6 female 3.80 .326 .840 .494 .553 -anus 6 Nl .0216 N2 .0324 N3 .0432 .011 .313 .005 .583 -N4 .0648 .017 .407 .009 .654 -N5 .130 .033 .590 .023 .754 -N6 .216 .051 .725 .042 .800 -CI .432 .088 .882 .089 .816 -C2 .864 .142 .969 .172 .777 -C3 2.27 .251 .931 .361 .646 -C4 4.32 .346 .812 .529 .529 -C5 7.56 .444 .671 .686 .422 -C6 male 7.90 .452 .659 .698 .414 -C6 female 10.8 .511 .573 .781 .356 -CI .858 C2 1.3.2 C3 2.03 C4 3.52 C5 6.19 C6 male 9.06 C6 female 14.8 H a r p a c t i c o i d a 9 5. Appendix 2. (continued) taxon body weight (ug C) LD s e l e c t i v i t y SD LF SF B C i l i a t a : 1 0 1 7 f e e d i n g p r e f e r e n c e <3 um .00015 - - - -3-15 um .0003 - 1 - 1 >15 um .0005 .134 - .019 -unassigned .0004 .134 1 .019 1 Metazoan l a r v a e : Polychaete l a r v a e 1 1 5.4 .178 1 .019 1 T r o c h o p h o r e s 1 2 .14 .178 1 .019 1 Pelecypod l a r v a e 1 2 .14 .178 1 .019 1 Gastropod v e l i g e r s 1 3 .14 .178 1 .019 1 Cyphonautes 1 3 .14 .178 1 .019 1 00 00 .01 .01 .01 .01 .01 L a r v a c e a n s : 1 4 trunk l e n g t h -125 um .0243 - .014 - .250 1 175 .0581 • - .014 - .250 1 225 .112 - .014 - .575 .1 275 .189 - .098 - .608 1 325 .292 - .310 - .775 1 375 .425 ' - .342 - .781 1 425 .590 - .911 - .782 1 475 .790 - .956 - .785 1 525 1.03 - 1 - .949 1 575 1.30 .007 1 - 1 1 625 1.62 .045 1 .002 1 1 675 1.99 .045 1 .002 1 1 725 2.40 .097 1 .002 1 1 775 2.86 .101 1 .007 1 1 825 3.37 .103 1 .010 1 1 875 3.93 .109 1 .010 1 1 925 4.54 .129 1 .010 1 1 950 + 6.34 .134 1 .019 1 1 spent 9.00 .134 1 .019 1 1 Appendix 2. (continued) taxon body weight s e l e c t i v i t y (ug C) LD SD LF SF B C o l o u r l e s s f l a g e l l a t e s 1 5 .00928 C t e n o p h o r e s : 1 4 P l e u r o b r a c h i a 0- 1 mm .1 1- 2 2. 2- 4 16. 4-8 80. 8-16 550. B o l i n o p s i s 0-2 mm .71 2- 4 7.4 4-8 34. 8- 16 156. 16+ 1136. C h a e t o g n a t h s : 1 6  S a g i t t a 0-3 mm 1.0 3- 6 9.7 6-9 39. 9- 12 101. 12-15 202. 15-21 454. 266 Appendix 2. (continued) Note: 1. LD = l a r g e diatoms, SD = small diatoms, LF = l a r g e f l a g e l l a t e s , SD = small f l a g e l l a t e s , B = b a c t e r i a . Notes on d e r i v a t i o n of body weights: 2. From Durbin and Durbin (1978) f o r Acart i a c l a u s i . 3. Adult male and female weights are from M u l l i n and Brooks (1970). Weights of other stages i n p r o p o r t i o n to a d u l t female weight are given -by G. G r i c e ( l e t t e r of June 13, 1979). 4. Adult male weight from CEPEX Data Report 3, March 1979. P r o p o r t i o n a l weights of other stages from Durbin and Durbin (1978). 5. Adult male, a d u l t female, and copepodid weights from CEPEX Data Report 3, assuming C/dry wt. =0.4. P r o p o r t i o n a l weights of other stages from Durbin and Durbin (1978). 6. Adult male, a d u l t female weights from CEPEX Data Report 3. P r o p o r t i o n a l weights of other stages given by G. G r i c e ( l e t t e r of June 13, 1979). 7. Adult male, a d u l t female, copepodid weights from CEPEX Data Report 3. 8. Adult female weight from Amber and F r o s t (1974), assuming C/dry wt. = 0.4. P r o p o r t i o n a l weights of other stages from Durbin and Durbin (1978). 9. Very approximate estimate based on F e l l e r (1977). 10. Very approximate estimates based on Beers and Stewart (1970, 1971) and Beers et a l . (1971). 11. From Ikeda (1974), assuming C/dry wt. = 0.4. 12. Assumed to equal body weight of gastropod v e l i g e r s . 13. From CEPEX Data Report 3. 14. Based on data of K. King, p e r s o n a l communication. 15. Average c e l l carbon of s p e c i e s observed i n CEE2; d i f f e r e n t f l a g e l l a t e s p e c i e s s t a t i s t i c a l l y weighted a c c o r d i n g to mean abundance in CEE2 between days 1 to 80. 267 Appendix 2. (continued) 16. M. Reeve, pe r s o n a l communication. Other notes: 17. A water c l e a r a n c e r a t e of 4 u l c i l i a t e " 1 h " 1 i s assumed when c a l c u l a t i n g the fe e d i n g s e l e c t i v i t y on b a c t e r i a (see s e c t i o n 3.5.2 f o r d e t a i l s ) . 'As I w r i t e t h i s I happen to be i n an a i r p l a n e at 30,000 f e e t , f l y i n g over Wyoming en route home from San F r a n c i s c o to Boston. Below, the e a r t h looks very s o f t and c o m f o r t a b l e — f l u f f y c l o u d s here and t h e r e , snow t u r n i n g pink as the sun s e t s , roads s t r e t c h i n g s t r a i g h t a c r o s s the country from one town to another. I t i s very hard to r e a l i z e t h a t t h i s i s a l l j u s t a t i n y p a r t of an overwhelmingly h o s t i l e u n i v e r s e . I t i s even harder to r e a l i z e t h a t t h i s present u n i v e r s e has evolved from an unspeakably u n f a m i l i a r e a r l y c o n d i t i o n , and f a c e s a f u t u r e e x t i n c t i o n of endless c o l d or i n t o l e r a b l e heat. The more the u n i v e r s e seems comprehensible, the more i t a l s o seems p o i n t l e s s . 'But i f there i s no s o l a c e i n the f r u i t s of our r e s e a r c h , there i s at l e a s t some c o n s o l a t i o n i n the r e s e a r c h i t s e l f . Men and women are not content to comfort themselves with t a l e s of gods and g i a n t s , or to c o n f i n e t h e i r thoughts t o the d a i l y a f f a i r s of l i f e ; they a l s o b u i l d t e l e s c o p e s and s a t e l l i t e s and a c c e l e r a t o r s , and s i t at t h e i r desks f o r endless hours working out the meaning of the data they gather. The e f f o r t to understand the u n i v e r s e i s one of the very few t h i n g s that l i f t s human l i f e a l i t t l e above the l e v e l of f a r c e , and g i v e s i t some of the grace of tragedy.' — S t e v e n Weinburg, The F i r s t Three Minutes, 1977. 

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