Vertical Nitrate Fluxes in the Oligotrophic Ocean

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Abstract
The vertical flux of nitrate across the thermocline in the upper ocean imposes a rigorous constraint on the rate of export of organic carbon from the surface layer of the sea. This export is the primary means by which the oceans can serve as a sink for atmospheric carbon dioxide. For the oligotrophic open ocean regions, which make up more than 75% of the world's ocean, the rate of export is currently uncertain by an order of magnitude. For most of the year, the vertical flux of nitrate is that due to vertical turbulent transport of deep water rich in nitrate into the relatively impoverished surface layer. Direct measurements of rates of turbulent kinetic energy dissipation, coupled with highly resolved vertical profiles of nitrate and density in the oligotrophic eastern Atlantic showed that the rate of transport, averaged over 2 weeks, was 0.14 (0.002 to 0.89, 95% confidence interval) millimole of nitrate per square meter per day and was statistically no different from the integrated rate of nitrate uptake as measured by incorporation of 15N-labeled nitrate. The stoichiometrically equivalent loss of carbon from the upper ocean, which is the relevant quantity for the carbon dioxide and climate question, is then fixed at 0.90 (0.01 to 5.70) millimole of carbon per square meter per day. These rates are much lower than recent estimates based on in situ changes in oxygen over annual scales; they are consistent with a biologically unproductive oligotrophic ocean.