A Physical Perspective of Recurrent Water Quality Degradation: A Case Study in the Jiangsu Coastal Waters, China

Estuarine-coastal systems receive pollutants from various sources at an alarming rate. Some systems are extremely susceptible to terrestrial input, as a relatively small amount of pollutant loading induces lasting water quality degradation. The Jiangsu coastal waters (JCWs), located in the southwestern Yellow Sea, are such areas that lack large river discharge but suffer from recurrent eutrophication recently. We hypothesize that the low flushing efficiency in the JCWs largely accounted for the susceptibility to coastal pollution. To test it, transit time (TT) and pollutant flushing rates in the JCWs were estimated under various scenarios using a validated three-dimensional hydrodynamic model. Results show that TT in many inshore waters of the JCWs and the northern radial sand ridges is > 240 days, which are consistent with the observed frequently eutrophic areas. The JCWs are better flushed and thus less eutrophic in summer, as a consequence of the stronger tidal residual coastal circulation. The baroclinic pressure gradient, modulated by the formation and destruction of thermal stratification in offshore waters, poses a dominant control on the seasonal variations of the coastal circulation and flushing rates, while wind effects are secondary. River-borne pollutants from the Yangtze and Jiangsu local rivers are shown to primarily affect regions south and north of 32.5 degrees N, respectively. Our study interprets the vulnerability of the JCWs to pollutant enrichment from a physical perspective and finds the flushing efficiency to be a good indicator of eutrophication, providing references for environmental managers of local and global estuarine-coastal systems with recurrent pollution.