| Lugu Lake is the deep plateau lake on the Yunnan-Guizhou Plateau with the highest latitude and altitude. Study of the zooplankton community in Lugu Lake was first reported in 1983 and focused on Rotifera, Copepoda and Cladocera, but no published research has been found on the zooplankton of Lugu Lake. To characterize zooplankton community structure, a seasonal investigation of the zooplankton in Lugu Lake was carried out in January, April, August and November of 2010 at nine sample sites. Characteristics of the zooplankton community including structure, species composition, distribution and seasonal variation were described and the relationship of zooplankton with water quality and macrophytes were analyzed to reveal the primary factors affecting zooplankton biomass. Zooplankton samples for qualitative analysis were collected with a No.25 plankton net and fixed with Lugol’s solution after filtration. For quantitative analysis of protozoa and rotifers, 1L water samples were collected 0.5m below surface. For quantitative analysis of copepods and cladocerans, 10L water sample were filtered with a No.25 plankton net. Counting and species identification in all samples were carried out under a microscope. A total of 80 species of zooplankton, belong to 33 families and 58 genera were observed in Lugu Lake, including Protozoa (10 families, 13 genera and 19 species), Rotifera (10 families, 22 genera and 32 species), Cladocera (4 families, 9 genera and 13 species), Copepoda (3 families, 8 genera and 10 species), and other taxa (6 families, 6 genera and 6 species). The range of plankton densities with average number and percent contribution to total zooplankton density in parentheses were as follows: total zooplankton, 219.4-2 200.3ind/L (813.2ind/L, 100%); Protozoan, 30.0-2 400.0ind/L (660.7ind/L, 81.25%,); Rotifera; 3.0-780.0ind/L, (145.4ind/L, 17.88%), Cladocera, 0-12.0ind/L (4.1ind/L, 0.50%); Copepoda, 0-13.0ind/L (2.7ind/L, 0.33%); other taxa, 0-4.0ind/L, (0.3ind/L, 0.04%). The seasonal distributions of zooplankton density in spring, summer autumn and winter were, respectively, 693.2ind/L, 903.4ind/L, 933.0ind/L and 722.9ind/L. Spatially, zooplankton density was higher in the north area than in the south, no obvious difference in density was observed between east and west, but the density was significantly lower in the central area of the lake. The species number in spring, summer, autumn and winter were, respectively, 55, 48, 71 and 63, with the highest species richness in the Niaodao area and the area near Caohai. Species richness was higher in the east than the west, higher in south than the north, and higher near shore than in the central part of the lake. The zooplankton community was composed primarily of species with large ecological amplitude, including eurythermal species, southern species and cold water species, along with many planktonic species and a few benthic-periphyton species. Among the four dominant groups of zooplankton, no species were particularly dominant, but commonly observed species included Arcella vulgaris, Conochilus unicornis, Keratella. valga, Polyarthra trigla, Daphnia hyaline, Bosmina longirostris, Neutrodiaptomus mariadvigae, and Tropocyclops prasinu. Correlation analysis between zooplankton populations and environmental factors indicates that Protozoan densities are primarily influenced by food availability; Rotifera by water temperature, water depth and total alkalinity and food availability; Cladocera by water temperature, dissolved oxygen, degree of mineralization and ammonia nitrogen; Copepoda by water temperature, transparence, ammonia nitrogen and degree of mineralization.
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