The unicellular NUM v.0.91: a trait-based plankton model evaluated in two contrasting biogeographic provinces

Abstract. Trait-based models founded on biophysical principles are becoming popular in planktonic ecological modeling, and justifiably so. They allow for slim, efficient models with a significant reduction in parameters that are well-suited to modeling past and future climate changes. In their idealized forms, trait-based models describe the ecosystem in one set of parameters defined by first principles and rooted in physics, chemistry, geometry, and evolution. The result is an emerging ecosystem defined by physical and chemical limitations at the cell level. At present, however, a significant part of these parameters is not fully constrained, which potentially introduces considerable uncertainty into the model results. Here, we investigate how these parameters influence the ecosystem structure of one of the simplest trait-based models, the Nutrient-Unicellular-Multicellular (NUM) model. We describe the unicellular module of the NUM model and, through an extensive parameter sensitivity analysis, we demonstrate that the model – with a large span in parameters – can capture the general features of the picoplanktonic, nanoplanktonic, and microplanktonic ecosystem in a high-productivity upwelling system. We demonstrate that it is possible to narrow the range of parameters to get a stable and acceptable solution. Finally, the model responds correctly in an oligotrophic downwelling system using parameters fitted to the upwelling system. Our analysis demonstrates that the unicellular module of the NUM model is broadly accessible without detailed knowledge of the parameter settings and that the first-principles approach is well-suited to modeling poorly resolved regions and ecosystem evolution during current and deep-time climate change.