Primary production and nitrogen assimilation rates from bay to offshore waters in the oyashio-kuroshio-Tsugaru Warm Current interfrontal region of the northwestern north Pacific Ocean

We examined primary production (PP) and the assimilation rates of nitrate (rho NO3-), ammonium (rho NH4+) and dinitrogen (rho N-2) using C-13 and N-15 tracer techniques along bay to offshore transects in the Oyashio-Kuroshio-Tsugaru Warm Current interfrontal region in the northwestern North Pacific Ocean, during five research cruises covering a full seasonal cycle. Surface inorganic N was depleted in the bays and the offshore region from summer to fall, but the higher particulate organic carbon (POC)/particulate nitrogen (PN) ratio and higher maximum N assimilation rate observed kinetics experiments in the bays suggest that N limitation was stronger in bays than offshore. PP (295-19,200 nmol C L-1 d(-1)), rho NO3- (9.45-650 nmol N L-1 d(-1)) and rho NH4+ (11.6-494 nmol N L-1 d(-1)) in the surface waters were generally low in summer and high in spring. Different from the cases of rho NO3- and rho NH4+, rho N-2 was high (up to 12 nmol N L-1 d(-1)) in mid-summer, especially in offshore regions, and moderate or low (<= 2.26 nmol N L-1 d(-1)) during other seasons. N-2 fixation largely contributed to total new production (up to 29%) in mid-summer. The mean f-ratio, estimated as the ratio of (rho NO3- + rho N-2) to (rho NO3- + rho N-2 + rho NH4+) at two offshore stations varied within the range of 0.29-0.83 (mean 0.54 +/- 0.20). Comparison of the PP and total N assimilation (rho NO3- + rho N-2 + rho NH4+) indicated that the ratio of C to N assimilation rates (max. 95) far exceeded the Redfield ratio (6.6). This finding suggests that other sources of N (e.g., dissolved organic nitrogen) contributed significantly to total N assimilation; if this is correct, the estimated f-ratio may be too high. Alternatively, the f(C)-ratio was estimated from the ratio of (rho NO3- + rho N-2) to PP by assuming the Redfieldian assimilation ratio, and it ranged from 0.06 to 1.10 (mean 0.45 +/- 0.31). Despite uncertainties in both the f- and f c -ratio estimation, our data provide evidence that a large fraction of PP is potentially available for export to deep waters and to higher trophic levels in the interfrontal region of the western North Pacific.