Estimating Three-Dimensional Carbon-To-Phosphorus Stoichiometry of Exported Marine Organic Matter

Abstract

© 2022. The Authors.The stoichiometric carbon to phosphorus ratios (rC:P) in suspended particulate organic matter (POM) are generally inversely correlated with surface phosphate (PO4) concentration. However, it is uncertain if previously suggested relationships between rC:P and PO4 are appropriate for the vertical export flux of organic matter. Using a global steady-state inverse ocean biogeochemistry model and annual-mean observed tracers, we estimate optimal parameters for both linear and power law representations of rP:C (= 1/rC:P), and find rP:C = (0.0066 ± 0.0018) × [PO4] + (0.0053 ± 0.0010) and rP:C = (0.0112 ± 0.0018) × [PO4](0.36±0.07), respectively, where [PO4] is in μM. Both parameterizations allow us to fit global tracer observations equally well, but the power law model implies an up to 50% larger uptake rC:P in oligotrophic gyres. For both formulations, the POM export rC:P from the euphotic zone is nearly equal to the uptake rC:P, while the dissolved organic matter export rC:P is up to two times larger than the uptake rC:P. Although weakly constrained, our model suggests that in eutrophic regions the vertical organic P fluxes are attenuated faster with depth than the organic C fluxes. By contrast, in oligotrophic regions there are no discernible differences between the organic P and C flux-attenuation profiles. As a result, the large spatial range of rC:P spanning 50–200 at the base of the euphotic zone is diminished to 110–160 at 2000 m depth. In oligotrophic regions at 150–500 m depths, our estimated export rC:P values are significantly lower than those measured with sediment traps, implying a potentially large modulation of export rC:P by migrating zooplankton within the twilight zone.