Global-scale variations of the ratios of carbon to phosphorus in exported marine organic matter

The ratio of carbon (C) to phosphorus (P) in marine phytoplankton is thought to be constant throughout the worlds'oceans. Known as the Redfield ratio, this relationship describes the links between carbon and phosphorus cycling and marine ecosystems. However, variations in the stoichiometry of phytoplankton have recently been identified, in particular strong latitudinal variability. Here we assess the impact of this variability in the C:P ratio of biomass on the C:P ratio of organic matter that is exported to the deep ocean using a biogeochemical inverse-model based on a data-constrained ocean circulation model and a global database of dissolved inorganic carbon and phosphate measurements. We identify global patterns of variability in the C:P ratios of exported organic matter, with higher values in the nutrient-depleted subtropical gyres, where organic matter export is relatively low, and lower ratios in nutrient-rich upwelling zones and high-latitude regions, where organic matter export is high. This suggests that total carbon export is relatively constant throughout the oceans, in agreement with recent estimates of carbon fluxes. We conclude that the latitudinal patterns of C:P in exported organic matter are consistent with the large-scale stoichiometric variations in phytoplankton C:P. We suggest that a future expansion of nutrient-depleted waters could result in a shift to more efficient C export that compensates for the expected decline in productivity