Biogenic fluxes of carbon and oxygen in the ocean

Rates of oxygen utilization (OUR) at depth in the ocean have been interpreted as showing that rates of carbon fixation by phytoplankton, as estimated by 14CO2 assimilation in vitro, must be in error1. The oxygen is consumed in the decomposition of organic matter sinking from the photic zone: there is a stoichiometrically equivalent flux of nitrate from the deep water towards the surface2. For comparison with the 14C data, it is conventional to extrapolate OUR to total equivalent phytoplankton production through a constant factor f, the ratio of nitrate based production (Pnew) to total production (Pt) as defined by Dugdale and Goering3. The alternative hypothesis, that scaling up OUR by a constant factor f is inappropriate, has not been examined in detail. We show here that f is variable in space and time for most provinces of the ocean. Furthermore, we show that in nitrogen-limited systems, such as the pelagic of the open ocean, Pt and f should be positively correlated. Applying these results to data from the Sargasso Sea, we find that the carbon fluxes estimated by 14C assimilation are consistent with the oxygen fluxes estimated by OUR. The conclusion is of profound importance for understanding the major biogeochemical cycles of the ocean.