Long-term environmental changes and the responses of the ecosystems in the Bohai Sea during 1960–1996

The Bohai Sea (BHS), located at the western boundary of the NW Pacific, is a shallow semienclosed sea with an area of about 77×103 km2 and average depth of 18 m, surrounded by fast-developing economic zones and populous lands. Through the Bohai Strait, the BHS connects to the Yellow Sea, one of the 50 large marine ecosystems in the world ocean. The hydrographic conditions there are substantially influenced by river discharges, wind–tide–thermohaline circulation, stratification in summer, and mixing in winter. During the period of 1960–1996, temperature, salinity, dissolved inorganic nitrogen, and the N:P ratio increased from 0.005 to 0.013 °C year−1, 0.04 to 0.13 year−1, 0.45 to 0.61 μmol L−1 year−1, and 1.27 to 1.40 year−1, respectively; while dissolved oxygen, phosphorus, silicon, and the Si:N ratio decreased from −1.59 to −2.30 μmol L−1 year−1, −0.007 to −0.011 μmol L−1 year−1, −0.385 to −0.602 μmol L−1 year−1, and −0.064 to −0.324 year−1, respectively. These changes were primarily caused by a reduced freshwater inflow. Since 1985, the concentrations of P and Si, and the Si:N ratio, have dropped to near-critical levels for diatom growth, while the N:P ratio has been below the Redfield ratio. These changes not only have had an influence on phytoplankton production, but also can decrease recruitment of the Penaeid prawn (Penaeus chinensis) and change fish community structure and diversity.