Pico-phytoplankton contribute half of global marine carbon export

Earth System Models generally predict increasing upper ocean stratification from 21st century planetary warming, which will cause a decrease in the vertical nutrient flux resulting in declining marine net primary productivity (NPP) and carbon export fluxes. Recent advances in quantifying marine ecosystem carbon to nutrient stoichiometry have identified large latitudinal and biome variability, with low-latitude oligotrophic systems harboring pico-sized phytoplankton exhibiting large phosphorus to carbon cellular plasticity. Climate forced changes in nutrient flux stoichiometry and phytoplankton community composition is thus likely to alter the ocean’s biogeochemical response and feedback with the carbon-climate system. We have added three pico-phytoplankton functional types within the Biogeochemical Elemental Cycling component of the Community Earth System Model while incorporating variable cellular phosphorus to carbon stoichiometry for all represented phytoplankton types. The model simulates Prochlorococcus and Synechococcus populations that dominate the productivity and sinking carbon export of the tropical and subtropical ocean, and pico-eukaryote populations that contribute significantly to productivity and export within the subtropical to mid-latitude transition zone, contributing a combined 50 – 70% of these fluxes. Pico-phytoplankton cellular stoichiometry and resulting sinking export patterns inversely track the distribution of surface phosphate, with the western subtropical regions of each basin supporting the most P-poor stoichiometries. Collectively, pico-phytoplankton contribute ~58% of global NPP and ~46% of global particulate organic carbon export below 100 meters. Subtropical gyre recirculation regions along the poleward flanks of surface western boundary currents are identified as regional hotspots of enhanced carbon export exhibiting C-rich/P-poor stoichiometry, preferentially inhabited by pico-eukaryotes and diatoms.