Long-term changes in crustacean zooplankton and water quality in a shallow, eutrophic Chinese lake densely stocked with fish

From surveys made in 1962-1963, 1973-1974, 1979-1996 at two Stations in Lak e Donghu, a shallow eutrophic water body near Wuhan, P. R. China, the autho rs, derive long-term changes in species composition, standing crop and body -size of planktonic crustaceans. The species number decreased from the 1960 s to the 1990s. The cladocerans dropped from 46 (1960s) to 26 (1980s) to 13 (1990s); the copepods decreased from 14 (1960s) to 10 (1980s) to 7 (1990s) . From the mid-1980s on, the dominant crustaceans also changed: Daphnia hya lina and D. carinata ssp. were replaced by Moina micrura and Diaphanosoma b rachyurum at Stations 1 and 2, respectively; Cyclops vicinus replaced Mesoc yclops leuckarti. Densities and biomass of Cladocera decreased markedly after 1987. Annual av erage densities and biomass of cladocerans were statistically differences b etween 1962-1986 and 1987-1996 (P < 0.01). Annual average densities of Daph nia (Station 1 + Station 2) were negatively correlated with fish yield. Since the 1980s, annual average body length of Cladocera and Calanoida decr eased, while annual average body length of Cyclopoida increased. In the sam e years, average body length of copepods was lower during May-October than during January-April and November-December. A 12-yr data analysis showed annual average concentration of chlorophyll-a (Chl-a) to be negatively correlated with annual average density of Daphnia, whilst lake transparency was positively correlated with annual average den sities of Daphnia. The results imply that, since Daphnia feeds efficiently on phytoplankton, it could decrease concentration of Chl-a, and enhance wat er transparency.