Occurrence of aquatic macrophytes in a eutrophic subtropical lake in relation to toxic wastewater and fish overstocking

We studied nutrient level and macrophytes in three different naturally separated basins of Lake Liangzi, a large subtropical shallow lake subjected to different loadings of nutrients, toxic wastewater, and fish/crab farming. The biomass of macrophytes decreased rapidly two years after the toxic wastewater entered the lake. Values of total phosphorus (TP), total nitrogen (TN) and chlorophyll a were significantly higher, while Secchi depth, pH, and dissolved oxygen were significantly lower in the toxic area than in the overstocked area and the area subjected to nutrient loading only. This study shows that lake management changes can induce a shift from a vegetation-dominated to a phytoplankton-dominated system. Our analysis suggests that TN rather than TP may be the limiting nutrient in this and other lakes in the middle to lower reaches of the Yangtze River.