Inventory of nutrients in the Bohai

Biogeochemical observations were carried out to address the influence of major sources on nutrient composition and the ecosystem of the Bohai. Relatively high concentrations of nutrients off the Huanghe mouth and the shallow water areas were observed in the Bohai suggesting the effects of tidal and residual currents and anthropogenic perturbation. Sediment in the Bohai represents a source for ammonium, phosphate and dissolved silicate, while it is a sink for nitrite and nitrate. Benthic nutrient fluxes were 2–3 times higher than the riverine input with the regeneration rate of phosphate being slower relative to DIN and dissolved silicate. The release of dissolved silicate and phosphate from sediments may mitigate the decrease of dissolved silicate and phosphate due to the reduction of freshwater discharge. Compared with submarine groundwater discharge, nutrient regeneration in sediment provides similar DIN flux, 2–5 times phosphate and dissolved silicate fluxes. DIN/P molar ratios in the three mentioned sources were 155–845, indicating that phosphorus limitation for phytoplankton growth could be intensified, which likely results in changes of ecosystems of the Bohai.