Evolution of community structure of aquatic macrophytes in East Taihu Lake and its wetlands

Two groups of aquatic plants dominated in East Taihu Lake during the field survey in 2002. The submersed macrophytes covered 73.6% of the lake surface, while the floating-leaved macrophytes covered 18.3%, respectively. After detailed classification, five mainly macrophyte asssemblages such as Elodea nuttalli (non-native species) association (30.7% of the total vegetation area of the waterbody), Ceratophyllum demersum association(17.2%), Elodea nuttalli-Potamogeton macckianus association (16.7%), Nymphoides peltata-Elodea nuttalli-Potamogeton macckianus association (15.8%) and Vallisneria spiralis-Potamogeton malaianus-Hydrilla verticillata association (9.3%) were set up to describe the macrophytes community structure in the lake. Previous studies showed that significant changes in the macrophyte community structure started in the 1960s after the introduction of Zizania caduciflora; Phragmites australis in the littoral zone disappeared at the beginning of the 1980s; Potamogeton malaianus was replaced by Potamogeton macckianus, which then occupied 40% of the water area, and became the dominant species of submersed macrophytes in the lake. In the past 10 years, enclosure aquaculture of crabs developed rapidly in East Taihu Lake. As a result, the marsh plant Zizania caduciflora (which was covering 25.6% before) and Potamogeton macckianus (which was covering 41% before) disappeared. They were replaced by the introduced species Elodea nuttalli and the rootless plant Ceratophyllum demersum, which covered over 90% of the lake in 2002. The macrophyte communities in the lake have changed markedly since the primary succession status in the 1950s towards the secondary succession status observed in this study. Despite the substantial economic benefits from the crab culture, the aquatic environment in East Taihu Lake is getting worse, especially resulting in water pollution and the acceleration of its swampiness.