Seasonal and spatial contrast in the surface layer nutrient content around the Kuroshio along 138°E, observed between 2002 and 2013

Seasonal and spatial contrasts of nitrate ? nitrite (N + N) concentrations in the slope area (the inshore area to the Kuroshio), Kuroshio and subtropical gyre areas were described by using data sets carried out along the O-line (138 degrees E) between 34 degrees 45/N and 30 degrees N over 11 years (2002-2013). The mean concentration of N + N in the surface mixed layer during winter was elevated to 4.8 +/- 2.7, 1.5 +/- 1.6 and 0.7 +/- 0.4 lM in the slope, Kuroshio and subtropical gyre areas, respectively, while it was depleted to <0.1 lM in all areas during summer. Applying regression analyses to the data, a significant negative correlation between N + N concentration and temperature was found at any depths above 1000 m in both summer and winter in these areas, except for the summer surface depletive layers. The analyses also showed that the diffusive upward flux and the depth difference between the surface mixed layer and the 15-mu M nitrate-isoconcentration depth were important factors to induce the seasonal contrast between winter and summer and horizontal contrast among the slope, Kuroshio and gyre areas in the N + N concentration in the surface mixed layer. Since the nitrate-isoconcentration depth is much shallower in the slope area than in the other two areas, the developing wintertime mixed layer reaches to the nitrate-rich water in the slope area, and that leads to a several micromolar increase in the surface N + N concentration there.