Phytoplankton variable stoichiometry modifies key biogeochemical fluxes and the functioning of the ocean biological pump

Ocean biota take up carbon in surface waters and export some of it to the ocean interior (the biological pump), modifying surface carbon concentrations, air-sea CO ₂ exchange, and thus, Earth's climate. The growth of marine phytoplankton is often limited by one of several key nutrients (nitrogen, phosphorus, iron, silicon), and the efficiency of carbon export is constrained by nutrient availability, and the nutrient/carbon ratios in the biota (stoichiometry). Recent field observations suggest widespread variability in phytoplankton stoichiometry (C/N/P/Fe/Si). We show that accounting for phytoplankton dynamic stoichiometry dramatically shifts the magnitude and spatial patterns of carbon export by the biological pump, relative to a model with fixed ratios. Not accounting for dynamic stoichiometry also leads to increases in atmospheric CO ₂ , thereby underestimating the ocean carbon inventory. Thus, Earth System Models (ESMs) must account for dynamic plankton stoichiometry to make accurate projections of the carbon cycle and climate. Further research is needed to better constrain environmental controls on the stoichiometry of exported organic matter, particularly ecosystem-level processing of organic matter initially produced by the phytoplankton.