Long-term changes of planktonic rotifers in a subtropical Chinese lake dominated by filter-feeding fishes

1. The long-term changes (1956-1998) in density and species composition of planktonic rotifers were studied at two sampling stations (I, II) of Lake D onghu, a shallow eutrophic Chinese Lake densely stocked with filter-feeding fishes. Annual average densities of rotifers increased with an increase in fish yield and eutrophication, whilst species number decreased from 82 in 1962-1963 to 62 in 1994-1998. 2. During 1962-98, some species such as Anuraeopsis fissa, Polyarthra spp. (including P. dolichoptera & P. vulgaris), Trichocerca pusilla and Synchaet a oblonga increased their percentage in abundance remarkably, whilst the pr oportion of Keratella cochlearis decreased at two relatively eutrophic stat ions from 19 to 4.2% at Station I and from 30 to 3.2% at Station IL 3. The high r(max) of A. fissa probably made it more successful than other rotifers under high predation pressure by planktivorous fish. The decrease in the K. cochlearis population might be attributed partly to predation by Cyclops vicinus. 4. Small rotifers were less vulnerable to fish predation than large-sized c ladocerans. Decreases in cladocerans coincided with increases in rotifers, suggesting that the indirect effect of fish predation on cladocerans might have partly contributed to the population development of rotifers in Lake D onghu during recent decades. 5. We also conducted surveys (1994-1998) of seasonal dynamics of rotifers a t four sampling stations (I-IV) which have varied in trophic status after f ragmentation of the lake in the 1960s. A total of 75 species were identifie d at the four stations. Both densities and biomass of rotifers were conside rably higher in the two more eutrophic stations than in the two less eutrop hic stations. This indicates that the population increase of rotifers at St ations I and II during recent decades might be partly attributed to eutroph ication of the lake water.