Uptake of atmospheric CO2 by the ocean's 'biological pump' is driven by export of carbon from the euphotic zone to deeper waters1,2. As nitrate is a limiting nutrient in large regions of the ocean, measurements of nitrate uptake are often used to estimate the amount of carbon exported in this way3-6. This presupposes knowledge of the molar C:N ratio in the organic material exported from the upper waters, which is usually taken to be 6.6 (the Redfield ratio7,8). Recent studies have suggested, however, that the consumption ratio of C:N may deviate from this value in coastal waters9-11. Here we present time-series from both coastal waters and openocean sites which demonstrate that net organic carbon production greatly exceeded that predicted from nitrate consumption and the Redfield C:N ratio. We found a similar discrepancy in sections across broad regions of the North Atlantic during eutrophic periods. Our results suggest that extrapolating from nitrate consumption using the Redfield ratio leads to significant underestimates of organic carbon export from the euphotic zone.
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