Changes in nitrogen and phosphorus and their effects on phytoplankton in the Bohai Sea

Systematic studies of the changes in dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) and their effects on phytoplankton over the last 30 years in the Bohai Sea are presented. The amount of sewage disposal, use of fertilizer and the Huanghe River runoff were found to have a significant influence on the DIN or DIP concentrations in the Bohai Sea over the last 30 years. Moreover, the changes in DIN and DIP resulted in changes in the limiting nutrients of phytoplankton in the Bohai Sea from nitrogen in the early 1980s to nitrogen-phosphorus in the late 1980s, and then to phosphorus after the 1990s. In addition, changes in nitrogen and phosphorus had a significant effect on the phytoplankton community structure. The half saturation constant (Ks) was used to evaluate the effect of nutrients on the phytoplankton community structure in the Bohai Sea over the last 30 years. Cell abundance percentages of dominant phytoplankton species with high Ks values for phosphorus and low Ks values for nitrogen have decreased since the 1980s, while those of dominant phytoplankton species with low Ks values for phosphorus and high Ks values for nitrogen increased during this period.