Buffering of Ocean Export Production by Flexible Elemental Stoichiometry of Particulate Organic Matter

One of the most important factors that determine the ocean-atmosphere carbon partitioning is the sinking of particulate organic matter (POM) from the surface ocean to the deep ocean. The amount of carbon (C) removed from the surface ocean by this POM export production depends critically on the elemental ratio in POM of C to nitrogen (N) and phosphorus (P), two essential elements that limit productivity. Recent observations indicate that P:N:C in marine POM varies both spatially and temporally due to chemical, physical, and ecological dynamics. In a new approach to predicting a flexible P:C ratio, we developed a power law model with a stoichiometry sensitivity factor, which is able to relate P:C of POM to ambient phosphate concentration. The new factor is robust, measurable, and biogeochemically meaningful. Using the new stoichiometry sensitivity factor, we present a first-order estimate that P:C plasticity could buffer against a generally expected future reduction in global carbon export production by up to 5% under a future warming scenario compared to a fixed, Redfield P:C. Further, we demonstrate that our new stoichiometry model can be implemented successfully and easily in a global model to reproduce the large-scale P:N:C variability in the ocean. Plain Language Summary Sinking of particulate organic carbon (C) from the surface to the deep ocean acts to draw down atmospheric CO2. Essential nutrients such as nitrogen (N) and phosphorus (P) both limit C export production and control its efficiency. The efficiency of C export for a given amount of N or P is expressed by their relative abundances in terms of their elemental ratios in organic matter. This P:N:C ratio is assumed to be fixed (Redfield ratio) in the most global biogeochemical models. Here we present a new method for allowing P:C to vary. Using this newly developed model, we quantify how flexible P: C can buffer changes in export production under a future warming scenario.