The Modeling Study On The Effect Of Nutrient On The Evolutionary Dynamics Of Phytoplankton

Phytoplankton are the basis of material circulation and energy flow in marine ecosystems,which are sensitive to environmental changes.The great changes in the marine environment caused by climate change and human activities seriously affect the growth and reproduction of phytoplankton,resulting in a significant decline in phytoplankton biomass and a shift in community structure.Among the multiple abiotic factors,nutrient is the key limiting factor for phytoplankton growth.Therefore,it is of great significance to study the response mech-anisms and evolutionary behavior of phytoplankton to nutrient changes.By using dynamical models,this dissertation is devoted to investigate the effect of nutrient content and structural transformation on phytoplankton cell size,carbon concentrating mechanisms,and intracellular carbon-to-nitrogen ratio.To study the effect of limiting nutrient input on the biomass and cell size of marine phytoplankton,a nitrogen-phytoplankton model is formulated,taking the size-dependent growth rate,sinking rate,nutrient quota,and competition intensity into account.The ecological reproductive indexes are defined to analyse the threshold dynamical behavior of the system.Based on the theory of adaptive evolution,an evolutionary model of phytoplankton cell size is developed.Conditions for the continuously stable strategy or evolutionary branching point are obtained.Numerical results suggest that the reduction of nitrogen input can drive the phytoplankton to evolve toward smaller cell size.Small phytoplankton can survive when the vertical mixing rate is too fast or too slow,while large phytoplankton dominate in the environment when the vertical mixing rate is moderate.Thus,changes in limiting nutrient input not only alter phytoplankton density but also affect the evolution of phytoplankton cell size.To study the effect of rising dissolved inorganic carbon concentration on the carbon con-centrating mechanisms of phytoplankton,a carbon-phytoplankton model is established based on the different transport modes of dissolved CO₂and carbonate.An evolutionary model of inorganic carbon uptake strategies is developed by considering the allocation rate of inorganic carbon absorbed by phytoplankton.The effect of increasing CO₂concentration on the carbonate system,phytoplankton density,and inorganic carbon uptake strategies in the water column are investigated.The results show that phytoplankton preferentially absorb dissolve CO₂and re-duce the active transport of HCO₃^-and CO₃^2-in adaptation to the increase of dissolved inorganic carbon.It reveals that the increase of CO₂concentration leads to the down-regulation of the carbon concentrating mechanisms of phytoplankton.The results of this study shed new light on the effect of elevated atmospheric CO₂on phytoplankton carbon concentrating mechanisms.To study the effect of the changes in inorganic carbon content,inorganic nitrogen content,and their ratio on the intracellular carbon-to-nitrogen ratio of phytoplankton,a carbon-nitrogen-phytoplankton model is developed.The dynamical behaviors of the nitrogen-limited system and carbon-limited system are investigated separately,and the existence and stability of the equilibria is proved.An evolutionary dynamical model of nutrient uptake allocation rate is established,and the dynamics of the evolutionarily singular strategy are obtained.The results show that the intracellular carbon-to-nitrogen ratio under the co-limitation of two nutrients is the optimal carbon-to-nitrogen ratio in phytoplankton cells,and the nutrient uptake allocation rate that enables phytoplankton cells to reach the optimal carbon-to-nitrogen ratio is a continuously stable strategy.Numerical results suggest that either a decrease in inorganic nitrogen input or an increase in inorganic carbon input leads to an increase in the intracellular carbon-to-nitrogen ratio in phytoplankton cells.The results of this study provide a theoretical basis for studying the response mechanisms of phytoplankton ecological stoichiometry characteristics under the shift of nutrient structure.