Addition of Pico-phytoplankton with Variable Nutrient Stoichiometry to the Biogeochemical Elemental Cycling Component of the Community Earth System Model

Common to nearly all ocean biogeochemistry-circulation models is the representation of phytoplankton cellular nutrient quotas that are fixed at Redfield stoichiometry, resulting in a biological carbon pump that varies primarily as a function of nutrient delivery to the ocean surface. Nearly all Earth System Models (ESMs) predict increasing ocean stratification from 21^st century planetary warming 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 model represented phytoplankton types. The updated model simulates Prochlorococcus and Synechococcus populations that dominate the productivity of the tropical and subtropical ocean and pico-eukaryote populations that contribute significantly to productivity within the subtropical to subpolar transition zone. Pico-phytoplankton cellular stoichiometry patterns inversely track the distribution of surface phosphate, with low phosphate regions supporting the most P-poor phytoplankton stoichiometries. Collectively, pico-phytoplankton contribute ~40% of global NPP and ~25% of global carbon export below 100 meters. Surface western boundary currents are identified as regional hotspots of carbon-rich/P-poor ecosystem stoichiometry and thus enhanced carbon export.