HUIJUAN TANG, PING XIE 1 , MIN LU, LIQIANG XIE and JIAN WANG Donghu Experimental Station of Lake Ecosystems, State Key Laboratory for Freshwater Ecology and Biotechnology of China, Institute of Hydrobiology, The Chinese Academy of Sciences, Wuhan 430072, P. R. China Studies on the Effects of Silver Carp (Hypophthalmichthys molitrix) on the Phytoplankton in a Shallow Hypereutrophic Lake through an Enclosure Experiment key words: enclosure, silver carp, phytoplankton, crustacean, biomass Abstract The responses of nutrients, water transparency, zooplankton and phytoplankton to a gradient of sil- ver carp biomass were assessed using enclosure methods. The gradient of four silver carp biomass levels was set as follows: 0, 116, 176 and 316 g m –2 . Nutrients did not show any statistically significant dif- ferences among the treatments. An outburst of Daphnia only occurred in fishless enclosures where phytoplankton biomass was the lowest and water clarity significantly increased. While among fish enclosures, the small-sized Moina micrura dominated throughout the experiment and both zooplankton and phytoplankton biomasses decreased with increased fish biomass. No large colonial cyanobacterial blooms occurred in the fishless enclosures as predicted. This might be due to low water temperature, short experiment time and the occurrence of large bodied Daphnia in our experiment. Cryptophyta was the most dominant group in most of the enclosures and the lake water throughout the experiment. The fishless enclosure had much lower proportion of Cyanophyta but higher proportion of Trachelo- monas sp. 1. Introduction The Chinese carp or silver carp is one of the most intensively cultured fish species, com- prising much of the production of Chinese aquaculture (LIANG et al., 1981; TANG, 1981). Sil- ver carp is an obligate filter-feeding planktivore, consuming mainly phytoplankton, zoo- plankton, and/or suspended small particles (LAZZARO, 1987; SPATARU and GOPHEN, 1985). However, the impact of silver carp on phytoplankton communities is in controversy due to the variability in fish feeding response according to body size, stocking density, food avai- lability, and environmental conditions (SPATARU and GOPHEN, 1985). On one hand, silver carp was reported to successfully control Cyanophyta blooms, thus improve water quality (KAJAK et al., 1975; STARLING, 1993; Xie, 1996; DATTA and JANA, 1998). On the other hand, due to size selectivity, small phytoplankton can not be controlled efficiently (SMITH, 1989) or are even increased by reducing herbivore grazing from zooplankton (DRENNER et al., 1987). In addition, silver carp may increase phytoplankton biomass because of ichthyoeu- trophication (OPUSZYNSKI, 1979). It was demonstrated that the effect of planktivores on phytoplankton biomass is a function of fish biomass (JANUSZKO, 1974 and STARLING, 1993). Lake Donghu is heavily stocked by silver carp, which exert strong influence on plankton community of the lake ecosystem. Although many studies were made to clarify the zoo- plankton-fish interaction in the lake (XIE et al., 2000; YANG et al., 1999 and WU et al., Internat. Rev. Hydrobiol. 87 2002 1 107–119 © WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2002 1434-2944/02/0101-0107 $ 17.50+.50/0 1 Corresponding author: PING XIE, Xieping@public.wh.hb.cn