Lake Ulungur is one of the ten largest freshwater lakes in China. This large inland endorheic lake of Northwestern China (Xinjiang) supports a significant fishery production of the region. It plays an important role in maintaining ecosystem balance and provides various ecosystem services to the sustainable social development of the region. Information regarding the variation or succession of crustacean zooplankton will be critical for understanding how regional climate changes or anthropogenic activities (or both) impacts the evolution and resilience of the lake ecosystem. In this study, we investigated the community structure and the seasonal succession pattern of crustacean zooplankton and their correlation to various environmental factors in Lake Ulungur between November 2006 and August 2008. The samples were collected from three layers: 0.5 m under the water surface, the middle layer, and 0.5 m above bottom. For each sample, 15L of lake water was strained using a plankton net with a mesh size of 64μm. The samples were fixed with Lugol's solution. The biomass (in wet weight) of Cladocera and Copepoda was estimated by the equations of weight vis-a-vis length. Results showed that a total of 25 zooplankton species were identified, including 6 Cladocera species and 19 Copepoda species. Cladocera had the highest value of Shannon-wiener index in the summer of 2007, 2008 (1.662, 1.402) and the lowest value in the winter of 2007 (0), while the value of Pielou's index was high in the winter of 2006 (0.990). For Copepoda, values of Shannon-wiener indices and Pielou's indices were higher in the spring, summer and autumn (3.032、3.130、3.102; 0.876、0.942、0.934). Analysis of Variance (ANOVA) showed significant seasonal variations in density and abundance of cladocerans and nauplius, respectively. The peak density (2.116ind./L) and biomass (0.0619mg/L) of cladoceran appeared in Summer of 2007, and nauplius showed the maximal density (5.140ind./L) and biomass (0.0154mg/L) in the Spring of 2007.
The species whose abundance is more than 5% of the total crustacean density were considered 'dominant’. Bosmina longirostris, Onychocamptus mohammed were the dominant species in all seasons. In addition, there were seasonal dominant species of cladocera, Calanoida and Cyclopoidea, likely as a result of combined effects of the predation of pond smelt (Hypomesus nipponensis), the dominant planktivorous fish species in the lake, and the seasonal variation of phytoplankton abundance. Canonical correspondence analysis (CCA) was applied to explore the relationship between variation of crustacean zooplankton species and environmental factors. The results showed that certain species, such as Daphnia longispina, were positively associated with pH and Secchi disk transparency as well as negatively associated with phytoplankton biomass. Among other species, Bosmina longirostris were correlated positively with water depth while Nauplius were showed positive correlation with water temperature.