Phase-transfer of nitrogen species within the water column of the Bohai Sea

Biogeochemical constituents, especially the different phases of nitrogen, were investigated in the central and southern Bohai Sea, China, during fall 1998 and spring 1999 by repeatedly sampling a regular station grid. Apart from at the area off the Huanghe River, dissolved organic nitrogen dominated the nitrogen fraction, indicating the seasonally maturated stage of the ecosystem. Inputs of dissolved inorganic nitrogen from the Yellow Sea (by the “Yellow Sea Warm Current”) as well as by the Huanghe River fuelled primary production and led to high primary production at the entrance of the Bohai Sea during both seasons and in the shallow areas, especially at the Huanghe River mouth, during spring. Beside the nitrogen import, resuspension and remineralisation processes of nitrogen contributed to the ongoing phytoplankton blooms and, thus, to the conversion of nitrogen. In spite of the high variability indicated by strong changes within a fortnight, seasonal differences could be detected, such as specific contribution of phytoplankton to suspended matter. Correlations of particulate nitrogen with total suspended matter as well as changes in concentrations of the different nitrogen species as well as in primary production rates helped in distinguishing between conversion and transport/diffusion processes. Overall, the Bohai Sea was revealed as a huge “bioreactor” for the conversion of nitrogen, and—due to permanent nitrogen input and extensive water–sediment interaction processes—as a possible important sink on a global scale for nitrogen via denitrification processes.