Vertical profiles of nitrate uptake obtained from in situ15N incubation experiments in the western North Pacific

Vertical profiles of nitrate uptake were obtained in pelagic waters of the western North Pacific. Uptake rates were determined by a conventional, particulate-based 15N tracer method with in situ incubations for 24 h. Among the five profiles reported in this paper, three of them are obtained at one station in the subarctic Pacific (45°N, 165°E) on different occasions, and two of them are obtained at stations in the subtropical Pacific (25°N, 165°E and 25°N, 170°E). Nitrate uptake sharply decreased with depth at the subarctic station. At the subtropical stations where nitrate concentrations in surface waters were low, 15N-nitrate was added both at a trace level (ca. 4 nM–8 nM) and at a saturated level (ca. 8 μM). Uptake of the trace 15N varied less pronouncedly with depth, and the rates were slightly higher in the upper 70 m. Uptake of the saturated 15N was substantially higher in the upper 50 m and decreased with depth. No uptake near the depths of nitracline or deep chlorophyll maximum was larger than those at other depths in the euphotic zone. The estimated depth-integrated uptake rates were 0.97–1.22 mmol N m− 2 d− 1 at the subarctic station and 0.36–0.54 mmol N m− 2 d− 1 at the subtropical stations.