A non-classical biomanipulation experiment in Gonghu Bay of Lake Taihu: control of Microcystis blooms using silver and bighead carp

A non-classical biomanipulation experiment was carried out in Gonghu Bay of Lake Taihu in 2009. Silver and bighead carp were stocked in a large fish enclosure to control cyanobacterial blooms. Water quality, plankton abundance, and the intracellular and extracellular microcystins (MCs) in lake water were investigated monthly in 2009. The concentrations of nitrogen nutrients were significantly lower in the fish enclosure than in the surrounding lake, while phosphorus (especially total phosphorus) concentration was higher in fish enclosure. During the blooming period, Cyanophyta contributed to more than 90% of the phytoplankton in the surrounding lake, whereas it represented only 40-80% in the fish enclosure. The phytoplankton and crustacean zooplankton biomasses and the zooplankton/phytoplankton ratios were all significantly lower in the fish enclosure than in the lake. This result suggested that silver and bighead carp can effectively suppress the phytoplankton biomass with the initial stocking density of 7.5gm(-3) for silver carp and 1.1gm(-3) for bighead carp, despite a simultaneous decrease in the grazing pressure of the zooplankton on the phytoplankton. During the blooming period, the intracellular and extracellular MCs in the fish enclosure were reduced by 93.8% and 69.8% compared with the surrounding lake. MCs content varied from 0.34 to 18.8ng (mean 4.8ng) MC-LReqg(-1) wet weight in the muscle sample of silver and bighead carp in the experimental enclosure, which suggested that these fish were safe to consume for human. However, the long-term effects of MCs on aquatic ecosystem and on public health cannot be overlooked.