Nutrient dynamics and cross shelf transport in the East China Sea

To understand the temporal and spatial variations in nutrient dynamics, as well as the potential cross-shelf transport of nutrients between the East China Sea (ECS) shelf and the northwestern Pacific Ocean, six field observations covering the ECS were conducted in spring, summer, and autumn in 2011 and 2013. Nutrient dynamics in the ECS and nutrient exchange between shelf water and the open ocean were examined. High concentrations of dissolved inorganic nutrients were detected in the nearshore surface layer and offshore bottom layer in different seasons, and the concentrations of dissolved inorganic nutrients in surface seawater were lower in summer and autumn than in spring. The concentrations of dissolved organic nutrients in Kuroshio surface water were slightly lower in summer than in spring, but the concentrations in Kuroshio subsurface water were slightly higher in summer than in spring. There were abundant nutrient reservoirs in the euphotic zone of the ECS, which explained the high primary productivity. The evaluation of cross-shelf transport indicated that nutrients from shelf water were transported out across the 200 m isobath through the surface layer with the density (σ) less than 23.0 kg/m³ in spring. The flux of dissolved inorganic nitrogen transported from the ECS shelf to the Northwest Pacific Ocean in spring was equivalent to 21% of the atmospheric nitrogen deposition in the Northwest Pacific Ocean. In summer, the onshore flux in the surface and bottom layers accounted for 80% of the total flux, and the transportation of nutrients along the surface layer to the continental shelf contributed to the nutrient storage and primary productivity of the euphotic zone in the ECS shelf in summer.