Computer Simulation of Phytoplankton and Nutrient Dynamics
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COMPUTER SIMULATION OF PHYTOPLANKTON AND NUTRIENT DYNAMICS IN AN ENCLOSED MARINE ECOSYSTEM by ALAN BOYD CARRUTHERS B.Sc. University 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 this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA 19 May 1981 © Alan Boyd Carruthers, 1981 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of ^—00/0 The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date tn+r n, /rrt DE-6 (2/79). i i ABSTRACT This thesis presents a quantitative model of interactions among phytoplankton, nutrients, bacteria and grazers in an enclosed marine ecosystem. The enclosed system was a 23 m deep, 9.6 m diameter column of surface water in Saanich Inlet, British Columbia. Dynamics of large- and small-celled diatoms and flagellates in response to observed irradiance and zooplankton numbers and observed or simulated nitrogen and silicon concentrations were modelled over a simulated 76-day period between July 12 and September 26. The model's predictions poorly matched the observed events in Controlled Experimental Ecosystem 2 (CEE2), but nevertheless provided some important insights into system behavior. Ciliate grazing probably prevented small-celled phytoplankton from increasing to large concentrations in CEE2. By virtue of their tremendous numbers, colourless flagellates were potentially the most important grazers on bacteria, much more important than larvaceans or metazoan larvae. Whereas small-celled phytoplankton were limited by grazers, large phytoplankton dynamics were not markedly affected by grazing. The average observed rate of 14C fixation in the surface 8 m was roughly consistent with an interpretation in which artificial additions of nitrogen contributed 62% of inferred net uptake of nitrogen by phytoplankton, mixing from subsurface water contributed 18%, bacterial remineralization 12%, and zooplankton excretion 9%. However, independent observations of rapid activity by microheterotrophs (presumably bacteria) suggested that 1*C fixation considerably underestimated net primary production. This yielded an alternative interpretation in which nutrient additions contributed 46% of inferred net uptake of nitrogen in the surface layer, mixing 13%, bacteria 35%, and zooplankton 7%. Dissolution of silica was responsible for the observed accumulation of silicic acid below 8 m depth in CEE2, but the importance of silica dissolution as a source of Si for diatom growth in the surface 8 m is uncertain. The model's major failing was its assumption of unchanging maximum growth rates of phytoplankton, and unchanging rates of exudation, sinking, and respiration. Physiological parameter values which accounted for the huge bloom of Stephanopyxis in CEE2 could not account for the ensuing collapse. Traditional modelling assumptions of slowly changing internal physiology, although adequate for marine systems dominated by physical factors such as seasonality or water movement, cannot capture the behavior of biologically dominated systems like the enclosed system considered here. iv TABLE OF CONTENTS ABSTRACT ii LIST OF TABLES vii LIST OF FIGURES viii ACKNOWLEDGEMENTS .xiii 1. INTRODUCTION .. 1 2. THE FOOD WEB I EXPERIMENT 5 2.1 Experimental Design 5 2.2 Sampling Protocol and Accuracy 6 2.3 Data Sources 8 2.4 Observed Events in CEE2 9 3. THE COMPONENTS OF THE SYSTEM 39 . 3.1 Overview of the Model 39 3.2 Light 41 3.2.1 Surface irradiance 41 3.2.2 Extinction of light within the water column .. 43 3.3 Mixing 47 3.4 Phytoplankton 50 3.4.1 Species and cell sizes 50 3.4.2 Chemical composition of phytoplankton 51 3.4.3 Photosynthesis 57 3.4.4 Phytoplankton respiration 71 V 3.4.5 Phytoplankton exudation 72 3.4.6 Nutrient limitation 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 particulate silica 104 3.8 Final Comments 108 4. COMPUTER IMPLEMENTATION 112 4.1 Approach 112 4.2 Nutrient Interpolation 113 4.3 Accuracy of the Finite Difference Approximation ...115 5. RESULTS AND DISCUSSION 116 5.1 The Reference Simulation 117 5.1.1 Large diatoms 117 5.1.2 Large flagellates 127 5.1.3 Small diatoms and small flagellates 134 5.1.4 Bacteria 137 5.1.5 Nitrogen cycling 142 vi 5.1.6 Silicon 160 5.2 Silicon Simulations ..165 5.2.1 Simulation Si-1. Comparison with reference run .165 5.2.2 Simulation Si-2 and Si~3. Limitation of diatom growth 168 5.2.3 Simulation Si-4 and Si-5. Variable C/Si ratio 171 5.2.4 Simulation Si-6. Silicon uptake linked to net photosynthesis 179 5.2.5 Simulation Si-7. Dissolution of silica ....'...18 0 5.2.6 Conclusions regarding Si dynamics 187 5.3 Phytoplankton Growth and Loss ....188 5.3.1 Have maximum rates of primary production been underestimated? 189 5.3.2 Have losses or growth limitation been underestimated? 196 6. WHAT WENT WRONG? 201 6.1 The Bloom and Collapse of Large-Celled Diatoms ....204 6.2 Parameter Estimation Technique 204 6.3 Results and Discussion 209 6.3.1 The diatom bloom, days 13 to 25 209 6.3.2 The diatom collapse, days 25 to 51 213 6.4 Conclusions Regarding Physiological Variability ...222 7. FINAL CONCLUSIONS 227 REFERENCES 231 APPENDIX 1 256 APPENDIX 2 261 LIST OF TABLES Table I. Literature estimates of volume of water swept clear by ciliates grazing bacteria or other small particles 88 Table II. Literature estimates of the volume of water cleared by gastropod and pelecypod larvae grazing on small phytoplankton 90 Table III. Literature estimates of specific dissolution rate of silica in living and dead phytoplankton .106 Table IV. Summary of parameter values used in the main simulation model. 109 Table V. Sum of squared deviations between predicted and observed large-diatom biomass in the surface of CEE2 for different parameter values 210 vi i i LIST OF FIGURES Figure 1. Photosynthetically active quanta immediately below the surface of CEE2 10 Figure 2. Separate biomass of four groups of phytoplankton observed in the surface 8 m of CEE2 13 Figure 3. Accumulative biomass of four groups of phytoplankton observed in the surface 8 m of CEE2 18 Figure 4. Observed and interpolated concentrations of dissolved nitrogen and silicon in the surface 8m of CEE2 20 Figure 5. Biomass of copepods observed in the surface 8 m of CEE2. 24 Figure 6. Biomass of ctenophores and chaetognaths observed in the surface 20 m of CEE2 26 Figure 7. Biomass of ciliates observed in the surface 8 m of CEE2 28 Figure 8. Biomass of metazoan larvae observed in the surface 8 m of CEE2. " 30 Figure 9. Biomass of larvaceans observed in the surface 20 m of CEE2 32 Figure 10. Biomass of colourless flagellates observed in the surface 8 m of CEE2 34 Figure 11. Biomass of bacteria observed in the surface 8 m of CEE2 37 Figure 12. Attenuation coefficient of photosynthetically active quanta vs. phytoplankton carbon in CEE2 45 Figure 13. Observed phytoplankton carbon vs. particulate organic carbon in CEE2 53 Figure 14. Particulate organic carbon and nitrogen in CEE2 55 Figure 15. Specific carbon fixation rate vs. average photosynthetically active irradiance during 4-hour midday incubations 60 Figure 16. Hypothetical depth profile of Irk in CEE2 63 Figure 17. Modelled excretion rates of ciliates and colourless flagellates, ctenophores, and other zooplankton 99 Figure 18. Biomass of large-celled diatoms in the surface 8 m predicted by the reference run 118 Figure 19. Modelled gain and loss rates specific to predicted large diatom biomass; reference run; surface 8 m 121 Figure 20. Predicted specific grazing loss of large diatoms; reference run; surface 8 m 124 Figure 21. Predicted biomass of large diatoms in the surface and deep layers; reference run 128 X Figure 22. Biomass of large-celled flagellates in the surface 8 m predicted by the reference run 130 Figure 23. Modelled gain and loss rates specific to predicted large flagellate biomass; reference run; surface 8 m .132 Figure 24. Predicted specific grazing loss of small diatoms; reference run; surface 8 m 135 Figure 25. Biomass of bacteria in the surface 8 m predicted by the reference run 138 Figure 26. Concentration of total dissolved inorganic nitrogen (nitrate + nitrite + ammonium) in the surface 8 m predicted by the reference run 143 Figure 27. Predicted excretion of ammonium-N by zooplankton; reference run; surface 8 m 146 Figure 28. Predicted and observed ingestion rate by adult female Pseudocalanus 157 Figure 29. Concentration of dissolved silicon in the surface 8 m predicted by the reference run 161 Figure 30. Simulation Si-1. Predicted large diatom biomass and silicic acid concentration in the surface 8 m 166 Figure 31. Simulation Si-2. Predicted large diatom biomass and silicic acid concentration in the surface 8 m 169 Figure 32. Simulation Si-3. Predicted large diatom biomass and silicic acid concentration in the surface 8 m 172 Figure 33.