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

Abstract. Phytoplankton growth, and hence biomass responds to changing light and nutrient conditions in the near-surface ocean. Although a wide variety of physiological photoacclimation models have been developed and tested against laboratory results, their application and testing against oceanic observations remain limited. Hence the biogeochemical implications of photoacclimation in combination with ocean circulation have yet to be fully explored. We compare modeled phytoplankton biomass and primary production from a recently developed flexible phytoplankton functional type model (FlexPFT), which incorporates photoacclimation and variable carbon (C) : nitrogen (N) : chlorophyll (Chl) ratios, to that obtained with an inflexible control model (InFlexPFT), which assumes fixed C : N : Chl ratios. We couple each plankton model with a 3-D eddy-resolving ocean circulation model of the North Pacific and evaluate their respective performance versus observations of Chl, nutrients, and primary production. These two models yield different horizontal and vertical distributions of Chl and primary production. The FlexPFT reproduces observed subsurface Chl maxima, although it overestimates Chl concentrations. In the subtropical gyre, where light is sufficient, even at low nutrient concentrations, the FlexPFT yields faster growth rates, as well as high Chl concentration and primary production in the subsurface layer. Compared to the FlexPFT, the InFlexPFT yields slower growth rates, and lower Chl and primary production. In the subpolar gyre, the FlexPFT also predicts faster growth near the surface, where light and nutrient conditions are most favorable. Compared to the InFlexPFT, the key differences that allow the FlexPFT to better reproduce the observed patterns are its assumption of variable, rather than fixed, C : N : Chl ratios and inter-dependent, rather than strictly multiplicative, effects of light- and nutrient-limitation.