| Summary: | Primary Production by phytoplankton is the base of the marine food web. Phytoplankton are often limited by nitrogen availability, which is usually supplied in the form of nitrate in the open ocean. Nitrate can either be supplied by mixing or upwelling from subsurface waters or from the process of nitrification within the surface layer. This work investigated the possibility that nitrification occurs in the epipelagic North Atlantic Ocean at rates appreciable enough to have an impact on primary production there. To quantify the importance of nitrification relative to nitrogen uptake by phytoplankton, experiments were performed on two cruises ¿ EN532 (Fall, 2013) and EN538 (Spring 2014) ¿ at two process stations per cruise. Experiments included measuring: 1) ammonium, nitrate, and bicarbonate assimilation rates, 2) ammonium oxidation rates, and 3) nitrite oxidation rates. Experiments were performed at multiple depths spanning the surface water column at each process station, so that integrated biological rates over the relevant depth interval (upper mixed layer or euphotic zone) could be calculated and compared between experiments at each station. While nitrification rates were on average higher during the spring, nitrate assimilation rates were higher as well, such that nitrification met less of the nitrate-uptake demand in the spring than in the fall. In general, however, nitrification was only able to provide between 0.7% and 0.8% of the nitrate demand in the spring and between 3% and 100% of the nitrate demand in the autumn, with the latter percentage likely being an anomaly due to relatively high concentrations of ammonium at this process station. These results indicate that the principal influx of nitrate to the epipelagic North Atlantic Ocean is not derivative of nitrification occurring within the epipelagic layer itself.
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