The Effect of Modelling Mechanistically Phytoplankton Photo-physiology on the Seasonal Cycle of Primary Production in Polar Regions

The ability of global models in simulating the seasonality of biogeochemical cycles constrains their reliability for projections of primary production and ocean carbon uptake. In particular, the phasing and amplitude of the seasonal cycle of primary production affect the net flux of carbon between the ocean and the atmosphere. Models’ characterization of the seasonal cycle of primary production in high latitudes generally shows an amplitude and/or phasing bias of the spring-summer bloom. The question that we tackle in this study is to which extent model simulations of the seasonal cycle of primary production would benefit from a more mechanistic description of the links between phytoplankton physiology and environmental drivers.

To explore that question we worked with the Regulated Ecosystem model version 2 (REcoM2) integrated within the Finite-Element Sea-Ice Ocean Model (FESOM). We included in the phytoplankton growth model a photodamage term that decreases the amount of active photosynthetic pigments when light becomes supersaturating. Eventually, the interplay between light-dependent photodamage and nutrient-dependent new synthesis of pigments determines the photosynthetic capacity of the cells. The immediate effect is that the model is able to simulate variations in the stoichiometry of phytoplankton with light, nutrients and temperature in better agreement with observations.

Regarding the seasonal variations of primary production in polar regions, model simulations show a less steep increase of biomass and net primary production during the growing season and lower biomass concentrations at the peak of the bloom. However, the start of the bloom happens relatively early when compared to satellite observations. We suggest to further evaluate the role of other environmental factors interacting with the physiology of primary producers and driving both bottom-up (e.g. vertical mixing) and top-down (e.g. grazing) control of the spring bloom in polar regions.