The flux of organic material sinking to depth is a major control on the inventory of carbon in the ocean. To first order, the oceanic system is at equilibrium such that what goes down must come up. Because the export flux is difficult to measure directly, it is routinely estimated indirectly by quantifying the amount of phytoplankton growth, or primary production, fuelled by the upward flux of nitrate. To do so it is necessary to take into account other sources of biologically available nitrogen. However, the generation of nitrate by nitrification in surface waters has only recently received attention. Here we perform the first synthesis of open-ocean measurements of the specific rate of surface nitrification and use these to configure a global biogeochemical model to quantify the global role of nitrification. We show that for much of the world ocean a substantial fraction of the nitrate taken up is generated through recent nitrification near the surface. At the global scale, nitrification accounts for about half of the nitrate consumed by growing phytoplankton. A consequence is that many previous attempts to quantify marine carbon export, particularly those based on inappropriate use of the f-ratio (a measure of the efficiency of the 'biological pump'), are significant overestimates.

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N regeneration was measured on a transect of the North and South Atlantic, from the United Kingdom to the Falkland Islands, that included extreme oligotrophic conditions. NH z 4 and NO { 2 oxidation rates were measured from the surface and base of the photic zone at 16 stations, using an isotope dilution technique in conjunction with gas chromatography/mass spectrometry analysis. NH

A range of marine phytoplankton was grown in closed systems in order to investigate the kinetics of dissolved inorganic carbon (DIC) use and the in£uence of the nitrogen source under conditions of constant pH. The kinetics of DIC use could be described by a rectangular hyperbolic curve, yielding estimations of K G(DIC) (the half saturation constant for carbon-speci¢c growth, i.e. C·) and · max (the theoretical maximum C·). All species attained a K G(DIC) within the range of 30^750 mM DIC. For most species, NH 4 ‡ use enabled growth with a lower K G(DIC) and/or, for two species, an increase in · max . At DIC concentrations of 41.6 mM, C· was 4 90% saturated for all species relative to the rate at the natural seawater DIC concentration of 2.0 mM. The results suggest that neither the rate nor the extent of primary productivity will be signi¢cantly limited by the DIC in the quasi-steady-state conditions associated with oligotrophic oceans. The method needs to be applied in the conditions associated with dynamic coastal (eutrophic) systems for clari¢cation of a potential DIC rate limitation where cells may grow to higher densities and under variable pH and nitrogen supply.