Abstract: Based on the survey data of two cruises in June and September 2007, the species composition and biodiversity of zooplankton in Oujiang River estuary were studied, with the effects of water mass on them analyzed. A total of 76 species (excluding larvae and juvenile zooplankton, fish eggs, and larval fish) werecollected, belonging to 41 families, 20 orders, 8 classes, and 7 phyla. Copepoda was most dominant. In this estuary, most of zooplankton species were subtropical species. The species number of offshore species was more than that of neritic species, but neritic species were numerically predominant. The species number was significantly greater in autumn than in summer, which was responsible for the significant difference of ShannonWiener index (H′)  between autumn (2.89)
 and summer (2.23). In summer, the area with the highest biodiversity was located at the waters off Damen Island, where several water masses mixed. The H′ increased gradually from the estuary to the offshore.

Key words: Megranate field, Aphis gossypii, Natural enemies