ELEVATED CONSUMPTION OF CARBON RELATIVE TO NITROGEN IN THE SURFACE OCEAN

UPTAKE of atmospheric CO2 by the ocean's 'biological pump' is driven b y export of carbon from the euphotic zone to deeper waters1,2. As nitr ate is a limiting nutrient in large regions of the ocean, measurements of nitrate uptake are often used to estimate the amount of carbon exp orted in this way3-6. This presupposes knowledge of the molar C:N rati o in the organic material exported from the upper waters, which is usu ally taken to be 6.6 (the Redfield ratio7,8). Recent studies have sugg ested, however, that the consumption ratio of C:N may deviate from thi s value in coastal waters9-11. Here we present time-series from both c oastal waters and open-ocean sites which demonstrate that net organic carbon production greatly exceeded that predicted from nitrate consump tion and the Redfield C:N ratio. We found a similar discrepancy in sec tions across broad regions of the North Atlantic during eutrophic peri ods. Our results suggest that extrapolating from nitrate consumption u sing the Redfield ratio leads to significant underestimates of organic carbon export from the euphotic zone.