Physiological flexibility of phytoplankton impacts modeled chlorophyll and primary production across the North Pacific

Phytoplankton of various taxa are known to adjust their elemental compositions and pigment ratios in response to the changing light and nutrient conditions in the near-surface ocean. Although a wide variety of physiological photo-acclimation models have been developed and tested against laboratory results, their application and testing against oceanic observations remains limited. Hence the biogeochemical implications of photo-acclimation in combination with ocean circulation have yet to be fully explored. We compare modeled biomass and production from a recently developed phytoplankton model (FlexPFT), which incorporates photo-acclimation and variable C:N:chl ratios, to that obtained with an inflexible control model, which is a typical NPZD-type model based on Monod kinetics for growth assuming fixed C:N:chl ratios. We conduct simulations using a 3-D physical model (OFES) coupled to these two biological models (FlexPFT and NPZD, respectively), to evaluate each model’s performance. We compare model results to observed seasonal cycles of primary production, chlorophyll, and nutrients in the surface layer at two contrasting time-series sites, stn. K2 (subarctic gyre) and stn. S1 (subtropical gyre) in the western North Pacific and investigate the seasonal and spatial patterns of primary productivity, chlorophyll, and nutrients in the surface layer across the North Pacific. These two models produce different vertical distributions and seasonal cycles of primary production at the two observation stations. Compared to the NPZD model, the FlexPFT has more intense peak production, which occurs closer to the surface at both stations, and a more even seasonality with a more intense sub-surface chlorophyll maximum at stn. S1. The models also produce different patterns of primary production at the basin scale, in particular for the Kuroshio, Kuroshio Extension, and California Coastal regions.