Xinghu Lake, a shallow tropical lake consisting of five basins was examined for its metazoan zooplankton in 2002 and 2003. The size structure of the metazoan zooplankton was analyzed by taking body mass as the size variable. Body length varied from 50μm and 1300μm, with all dominant species less than 1mm long. The smallest dominant species were the Rotifera Brachionus forficula, Brachionus angularis and Polyarthra vulgaris, with an average length less than 100μm. Asplanchna priodonta was the largest rotifer. The main cladocerans, Diaphanosoma brachyurum, Moina micrura, Bosminopsis deitersi and Bosmina longirostris, had an average size between 220 and 430μm. In copepod, Thermocyclops taihokuensis and Mesocyclops thermocyclopoides were dominant, and their average lengths were 730μm and 680μm, respectively. Nauplii and cyclopoid copepodites also had high abundance and biomass.
The body mass of the metazoan zooplankton ranged from 0.034μg to 70.24μg, and included more than two size classes for almost every species examined. Asplanchna priodonta had the largest range of body mass, 0.548～70.24μg. Both species number and abundance of the metazoan zooplankton were dominated by the small-sized species, and decreased with body mass, while biomass distribution had a peak in the medium size class, 4.39μg to 8.78μg. In two basins, Bohai and Qinglian, Rotifera contributed most to biomass in almost each size class, followed by copepod. Both the size range of the zooplankton and the body size at which peak biomass occurred were lower than in temperature lakes. Although Xinghu Lake is located far from the Equator, the size structure of its zooplankton is that of a typical equatorial lake. Besides the eutrophic status，predation by planktivorous fish and temperature appear to be the main factors controlling this size structure. The fact that the zooplankton community was dominated by small-sized animals suggests that predation effects need to taken into account in the application of biomanipulation to the lake.