The Kuroshio nutrient stream and its temporal variation in the East China Sea

Using in situ data from 88 cruises from 1987 to 2009 in the East China Sea, downstream nutrient flux (the product of velocity and nutrient concentration) and nutrient transport (integration of flux over a section) by Kuroshio were examined. The presence of a maximum nutrient flux core in the middle layer was confirmed. Seasonal variation in the nutrient flux was not significant and was much smaller than interannual variations. The change in the Kuroshio speed and current structure were major causes for interannual variations in the nutrient flux. The downstream nitrate transport by the Kuroshio in the East China Sea had a mean value of 170.8 kmol s−1 and a standard deviation of 41.6 kmol s−1. The mean seasonal nitrate transport ranged between about 161 and 177 kmol s−1 and the absolute interannual variation from about 100 to 280 kmol s−1. The phosphate flux and phosphate transport can be approximately estimated by the ratio (13.64) of nitrate concentration to phosphate concentration. The nitrate concentration in the middle and bottom layers across the Kuroshio in the East China Sea was found to increase significantly over the 23 year period, and especially after 2004 but not at ratios with oxygen that suggest increased remineralization of organic matter. The nutrient transport, however, did not increase significantly because increases in the surface layer were offset by decreases in the middle and bottom layers caused by reduction in velocity in the density ranges of 26.0 to 27.2 σθ below the Kuroshio. Key Points Confirm presence of nutrient stream in the East China Sea Clarify mean downstream nutrient flux and transport by the Kuroshio Reveal role of variability of current field in transporting nutrients