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

• Published in: Deep-sea research. Part I, Oceanographic research papers Vol. 164; p. 103304
• Main Authors: Shiozaki, Takuhei, Tada, Yuya, Fukuda, Hideki, Furuya, Ken, Nagata, Toshi
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2020
• Subjects: F-ratio; N2 fixation; New production; N2 fixation; New production; F-ratio
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/j.dsr.2020.103304

We examined primary production (PP) and the assimilation rates of nitrate (ρNO3-), ammonium (ρNH4+) and dinitrogen (ρN2) using 13C and 15N 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), ρNO3- (9.45–650 nmol N L−1 d−1) and ρNH4+ (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 ρNO3- and ρNH4+, ρN2 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. N2 fixation largely contributed to total new production (up to 29%) in mid-summer. The mean f-ratio, estimated as the ratio of (ρNO3- + ρN2) to (ρNO3- + ρN2 + ρNH4+) 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 (ρNO3- + ρN2 + ρNH4+) 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 fC-ratio was estimated from the ratio of (ρNO3- + ρN2) 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 fC-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. •N2 fixation contributed max. 29% to new production in this interfrontal region.•The maximum new production and f-ratio were at the high end of values in the Pacific.•Nitrogen limitation was stronger in the Sanriku bays than offshore.