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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 open-ocean 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.

SEASONAL thermal fronts which occur in coastal waters are typified by sharp lateral gradients of temperature at the sea surface resulting from a surface outcropping of the thermocline. The frontal regions are areas of considerable physical and biological activity; in the case of the fronts in the Celtic and Irish Seas this increase in biological activity is reflected in marked increases of chlorophyll a at the sea surface1. Considerable data on the spatial distribution of physical and chemical characteristics of the water and chlorophyll a concentrations in the vicinity of these and other fronts are now available2–5. But, few observations have been made either on the temporal variability of these characteristics or on the more detailed distribution of phytoplankton productivity. During a survey of the Celtic Sea in September 1976 we made consecutive crossings of the frontal region in order to investigate the small-scale distributions of chlorophyll a and primary productivity and their interrelationships with selected physical and chemical variables.