A conceptual model to represent phytoplankton size in oceans

Size is a fundamental trait in marine phytoplankton, linking physiological responses to ecosystem functioning and services. Modelling size diversity presents the challenge of striking a balance between realism and parsimony. Here, we introduce a simple model of marine phytoplankton dynamics that accounts for the optimisation of phytoplankton size with external environment. In this optimal size model, the size of the phytoplankton community is an evolving trait and depends on nutrient availability and internal trade-offs between growth and resource storage capacity. We compare this model against a more complex size-structured food web model in terms of their ability to reproduce observations at two canonical time-series sites: BATS and OSP. These sites represent contrasting physical and biogeochemical conditions. We find that the optimal size model outperforms the size-structured model at both locations, demonstrating an efficient approach to represent the diversity within phytoplankton functional types in Earth System Models.