Phytoplankton variable elemental composition modifies the marine biological pump and largely determines the global patterns of nutrient limitation

Phytoplankton acclimate to increased nutrient stress by decreasing their cellular quotas (nutrient:carbon ratios). Reducing cellular quotas reduces the export efficiency of the limiting nutrient, helping sustain biological productivity. Here we present a version of the Community Earth System Model with phytoplankton group specific, fully variable C:N:P:Fe:Si ratios constrained by field observations of particulate organic matter stoichiometry and individual cell spectroscopy. We compare the results of a steady-state fully fixed stoichiometry model to the fully variable model and find that using a fixed Redfield stoichiometry leads to a decrease of 1PgC/yr carbon export, increase of 18 ppm atmospheric CO2, decrease of 55 TgN/yr nitrogen fixation, and decrease of 27/yr TgN nitrogen fixation. We also investigate the impacts of variable nutrient acquisition on global patterns of nutrient limitation and find that the weaker ability of phytoplankton to acclimate to N stress by lowering their cellular quotas relative to other nutrients pushes marine ecosystems towards nitrogen limitation. Only when the nutrient supply ratios are highly skewed, exceeding the ability of the phytoplankton to acclimate, do other nutrients become growth-limiting, as with iron in the High Nitrate, Low Chlorophyll (HNLC) regions. We show that in the oligotrophic gyres, variable plankton stoichiometry, given sufficient time, pushes the marine ecosystems towards co-limitation, as non-limiting nutrients are more efficiently drawn down and exported (higher cellular quotas), relative to the growth-limiting nutrient (lower cellular quotas).