Modeling photosynthesis and the exudation of DOM in the subtropical oceans

Many parameterizations of algal photosynthesis commonly employed in global biogeochemical simulations fail to capture the observed vertical structure of both Chlorophyll-a and primary production due to multiplicative representation of nutrient and light limitation. We examine the consequences of removing this explicit limitation, decoupling photosynthesis and biosynthesis. In our parameterization, the flow of carbon and other nutrients allows for explicit exudation of dissolved organic carbon (DOC) when the light (energy) supply is in excess. Doing so improved simulations of the profile of primary production, increased modeled global primary production by ~30% and modeled surface labile DOC by a similar amount. Examining the contributions by phytoplankton functional group suggests that nanoplankton make the largest contribution on the global scale, but the specific exudation rate of picoplankton is predicted to be higher than that of nanoplankton.