Biodiversity and Stoichiometric Plasticity Increase Pico‐Phytoplankton Contributions to Marine Net Primary Productivity and the Biological Pump.

Earth System Models generally predict increasing upper ocean stratification from 21st century warming, which will cause a decrease in the vertical nutrient flux forcing declines in marine net primary productivity (NPP) and carbon export. 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. The climate forced changes in nutrient flux stoichiometry and phytoplankton community composition are 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, with the western subtropical regions of each basin supporting the most P‐poor stoichiometries. 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. Collectively, pico‐phytoplankton contribute ∼58% of global NPP and ∼46% of global particulate organic carbon export below 100 m through direct and ecosystem processing pathways. Biodiversity and cellular nutrient plasticity in marine pico‐phytoplankton combine to increase their contributions to ocean productivity and the biological carbon pump. Key Points: Simulated Prochlorococcus, Synechococcus, and pico‐eukaryotes contribute ∼60% of marine net primary productivity (NPP)Pico‐phytoplankton cycling contributes half of the marine export production, approaching parity with their contribution to NPPPico‐eukaryotes and diatoms with elevated C:P stoichiometry enhance carbon export at poleward flanks of western boundary currents [ABSTRACT FROM AUTHOR]