Nitrogen uptake and nitrification in the subarctic North Atlantic Ocean

Nitrification, the oxidation of ammonium to nitrite and then nitrate, is often discounted as a source of nitrate in euphotic zone waters due to photoinhibition of nitrifying microorganisms and/or competition for ammonium with phytoplankton. However, there have also been counterarguments that nitrification represents a significant “regenerated” nitrate source to phytoplankton, augmenting the “new” nitrate supplied via physical processes. If nitrification is an appreciable nitrate source in the euphotic zone, then the assumption of a balance between nitrate uptake and organic matter export will overestimate export production. We investigated the relative importance of nitrification and nitrate uptake in the subarctic North Atlantic in late spring and late summer. The rates and vertical distributions of primary production, nitrogen uptake, and ammonium and nitrite oxidation were determined through isotope tracer experiments and the distributions of the nitrogen and oxygen isotopes of nitrate. In surface waters, ammonium and nitrite oxidation rates were low, representing an average of 5.2% and 2.5% of total euphotic zone nitrate uptake, respectively. The nitrogen and oxygen isotopes of nitrate confirmed that nitrification was not significant within the euphotic zone. Comparison of the rates of nitrogen uptake and primary production showed that while springtime phytoplankton growth could be fully supported by new nitrate and recycled ammonium, up to 50% of summertime productivity was likely fueled by dissolved organic nitrogen (DON). Uptake of DON implies that the fraction of primary production exported from surface waters in the late summer was significantly lower than the measured nitrate and ammonium uptake rates suggest.