The response of phytoplankton in a subtropical lake to the cessation of aquaculture: a case study of Lake Dianshan, China

To assess the effectiveness of aquaculture cessation for improving water quality, we analysed the response of the phytoplankton community to the cessation of aquaculture in Lake Dianshan, Shanghai, China. Quantitative analyses of phytoplankton and nutrient content were undertaken monthly from 2004 through 2008. After the cessation of aquaculture, no significant changes in nutrient concentration were detected. Nevertheless, the phytoplankton community changed greatly after the cessation. Firstly, the phytoplankton species richness as well as the Shannon-Wiener diversity index increased. Secondly, the chlorophylla concentration and the phytoplankton density declined sharply. This decrease was caused primarily by a decrease in Cyanophyta. Additionally, the Chlorophyta, Bacillariophyta and Cryptophyta increased, leading to a decrease in the relative biomass of Cyanophyta. Thirdly, the dominant species shifted from the toxic cyanophyte Microcystis sp. to less harmful Cyanophyta such as Merismopedia sp. and Chroococcus sp., along with Chlorophyta, Bacillariophyta and Cryptophyta. The phytoplankton community composition improved, indicating that the overall water quality was improved after the cessation. However, the changes in the phytoplankton community were more likely to be associated with the physical and biotic changes caused by the removal of aquaculture than with the variation in nutrient levels. Our study indicated that phytoplankton monitoring, especially in summer, is essential for assessing the water quality because an overall improvement in water quality may also be achieved without significant changes in nutrient concentrations. The rebounding of Cyanophyta in 2008 highlighted the importance of controlling nutrient loading by inflow for a substantial improvement in water quality. These results are relevant for managing subtropical eutrophic lakes.