Phytoplankton growth allometry and size- dependent C:N stoichiometry revealed by a variable quota model

Abstract

Size scaling of phytoplankton growth rates and size-dependent carbon to nitrogen (C:N) stoichiometry determine phytoplankton size structure and coupling of carbon and nitrogen cycling of marine ecosystems. They are critical in predicting the growth of phytoplankton spanning a wide range of sizes and their consequences for the biological pump in marine ecosystem models. The size scaling of phytoplankton growth and size-dependent C:N stoichiometry are modelled by embedding size-dependent light-harvesting, nutrient acquisition and storage into Droop's quota-dependent phytoplankton growth model. The size-scaling exponent of maximum growth rate of phytoplankton is -0.17 (which is higher than the universal size-scaling exponent of - 1 U4 predicted by the metabolic theory of ecology) under saturated light and NO3. The size-scaling exponent of growth rate (μ) decreases with increasing light under saturated NO3, and decreases with decreasing NO3 concentra- tion under saturated light. The allometry of equilibrium cellular C and N quota varies with light and NO3 concentrations. Under saturated light and NO3 concentration, C:N increases slightly with cell size. Under limiting light, but saturated NO3, C:N is close to the Redfield ratio and is not size depen- dent. Under limiting NO3 but saturated light, C:N is higher than the Redfield ratio and increases with cell size. We identified the uncertainty of the size-scaling exponent of μ associated with key parame- ters, for which more data need to be collected in the lab and field.