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

摘要

Nitrogen fixation in the subtropical East China Sea (ECS) and the southern Yellow-Sea (YS) were measured using ~(15)N_2 tracer assay during June and July 2006. Depth-integrated nitrogen fixation (2 to 221 μmol 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 = [PO_4] - [NO_3]/16) near the Changjiang (Yangtze River) mouth. In the mesohaline waters, N_2 fixation was positively correlated with the vertical density (σ_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 N2 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.