Photoacclimation contributes to Arctic primary production under sea ice and around the subsurface chlorophyll maximum

In the Arctic Ocean, a substantial portion of primary production occurs under sea ice and around the subsurface chlorophyll maximum. In these low-light environments, primary production will depend on photoacclimation, which increases phytoplankton’s chlorophyll content per unit of carbon biomass to enhance photon capture. Here, we incorporated a photoacclimation theory based on resource allocation optimization into a global ecosystem model, showing that remarkably high chlorophyll content responding to low-light environments is a primary driver of under-ice production. The distribution of primary production is more strongly correlated with phytoplankton carbon biomass than with chlorophyll concentration. We estimated that approximately half of the Arctic Ocean’s primary production occurs in regions where satellite observations are challenging (i.e., where ice concentration exceeds 10%). This mechanistic framework enhances our ability to interpret observed distributions of chlorophyll and primary production and predict ongoing changes in the Arctic Ocean primary production under shifting environmental conditions. Arctic chlorophyll content under ice and around the subsurface chlorophyll maximum is several times higher than surface open-water regions, resulting in a phytoplankton growth rate beneath the ice comparable to open-water areas, according to global ecosystem model simulations.