Long-term changes in the changjiang estuary plankton community related to anthropogenic eutrophication

The Changjiang River is well-known to channel significant anthropogenic influences to the estuary. However, long-term changes in plankton community related to anthropogenic eutrophication in the estuarine water are not well documented. In this paper, data from a nine year study (1996, 1998-2005) on suspended matter, chemical oxygen demand, nutrients and plankton community were collected and analyzed by principal component analysis. These data reveal that N/P ratio was always much higher than 40 and Si/N ratio decreased from 1.35 to 0.94 between 1996 and 2005. As an ecological consequence to such nutrient balance changes, phytoplankton species diversity and the dominance of diatoms decreased, dinoflagellates increasingly dominated the phytoplankton population, and the frequency of harmful algal blooms caused by dinoflagellates also increased. Furthermore, due to the reduced diatom availability, the dominance of copepods decreased, zooplankton diversity increased, and medusae increasingly dominated the zooplankton population. Water column nutrient ratios, therefore, are also important factors affecting anthropogenic eutrophication, as are the contamination loadings from land.