Upper Water Column Nitrous Oxide Distributions in the Northeast Subarctic Pacific Ocean

This is the first study to investigate the magnitude and distribution of N2O concentrations along the Line P oceanographic transect in the Northeast (NE) subarctic Pacific Ocean. Concentrations of N2O were measured from the surface to 600 m depth at five stations between 126 W and 145 W. Although nitrification within the mixed layer may produce some N2O, we conclude that mixing and diffusion processes, which vertically transport N2O upwards from below the mixed layer, are the primary sources of N2O to the surface waters of the NE subarctic Pacific Ocean. Below the mixed layer, nitrification appears to be the dominant source of N2O, and based on correlations of excess N2O (Delta N2O) versus apparent oxygen utilization and NO3- concentrations, we estimated that the N2O yield from nitrification was approximately 0.028 to 0.040%. The longitudinal distributions of N2O concentrations below the mixed layer were variable and we consider the potential role that different transiting water masses may play in contributing to this variability. Finally, we estimated that the regional average sea-to-air N2O flux was 4.37 mol of N2O km(-2) d(-1), a value which is approximately four times that of the global average seawater-to-air flux rate. Our N2O yield estimates are within the range of those expected under oxic conditions, leading us to conclude that decreasing dissolved O-2 concentrations in the NE subarctic Pacific Ocean, and the water masses that influence this region, over the past 50 years have yet to produce a substantial increase in N2O production. Given the expectation that dissolved O-2 concentrations in the subarctic Pacific Ocean will continue to decrease during this century, this study has provided an important baseline from which future studies will be able to track changes in seawater N2O concentrations and fluxes to the atmosphere.