Studies on the changes of West Lake＇s zoobenthic communites after Qiantang River water was pumped into it

During Jan. 1995 to Dec. 1996, monthly investigations on the zoobenthic conummities of West Lake, samples were collected from six sampling stations. A total of 26 species of macmzoobenthes were identified. The seasonal changes in density and biomass of zoobenthos in this lake were analyzed. The annual mean densities were 980- 2751 ind/m^2 and mean biomass was 19.69 - 122.80 g/m^2. The densities in winter and early spring were higher than those in sumr~r and autunm. Comimrafive study of theprevious data ( 1982 to 1983) collected by the authors, showed that the succession of zoobenthic conammities, dominated by Procludius choreus in density and Bellamya purificata in biomass, had been occurred in Xiaonan sub-lake after Qiantang River water was drawn into it; and that the species and biomass of zoobenthes were then increased and the density was decreased. In other sub-lakes, the dominant species were Limnodrilus hoffmeisteri and Tokunag-ayusurika akamusi in density and Branchiura sowerbyi and Tokunagayusurika akamusi in biomass. The water quality was bad in these sub-lakes because these dominant species are indicators of eutrophication. According to the Margalef index and Goodnight index, West Lake is still an eutrophic lake. Only the water quality of Xiaonan sub-lake was improved after water drawn from the Qiantang River was introduced into it.