Nitrogen fixation in the East China Sea and southern Yellow Sea during summer 2006

Nitrogen fixation in the subtropical East China Sea (ECS) and the southern Yellow Sea (YS) were measured using N-15(2) tracer assay during June and July 2006. Depth-integrated nitrogen fixation (2 to 221 mu mol N m(-2) d(-1)) was highest in the oceanic main path of the Kuroshio Current in the northeastern ECS, and in the mesohaline (surface salinity 30 to 34) stations. Very little N-2 fixation was encountered in the low-salinity (surface salinity <30) area, and we suggest that N-2 fixation was hindered by the nutrient conditions (lack of 'excess' phosphate relative to nitrate, xsPO(4) = [PO4] -[NO3]/16) near the Changjiang (Yangtze River) mouth. In the mesohaline waters, N-2 fixation was positively correlated with the vertical density (sigma(t)) gradient in the upper water column (30 m), indicating that N-2 fixation can also be controlled by physical regime, and enhanced water column stratification may promote N-2 fixation during summer. N-2 fixation met 0.01 to 4.6% of nitrogen demand by primary production, suggesting that N-2 fixation was not a major contributor to primary production in the study area. Estimated new N flux by N-2 fixation to the ECS continental shelf (13 Gg N) during summer is at the same order of magnitude as atmospheric deposition, but much lower than either the Kuroshio subsurface water upwelling or the Changjiang riverine input.