Abstract: Based on three cruise surveys in 2009 and 2010, an investigation was carried out on the community structure and its seasonal variation of phytoplankton in the adjacent waters of the nuclear plant being constructed at the second stage in Yangjiang City of Guangdong Province. The water range affected by the thermal effluent and the effects of the nuclear plant on the phytoplankton were predicted. In 78 samples, a total of 125 species (including 2 varieties) were identified, belonging 50 genera of 6 classes, among which, diatoms were predominant in the species composition. Dinoflagellates contributed the second major group. Chromophyta abundance increased sharply in November but not reached the algae bloom threshold of cell abundance. The dominant species were Leptocylindrus danicus, Guinaridia striata, Skeletonema costatum, Pseudonitzchia pungens, Melosira moniliformis, Phaeocystis globosa, Ceratium furca, and Chaetoceros densus. The species composition and phytoplankton cell abundance showed significant seasonal changes, with different high abundance regions. The seasonal average cell abundance of phytoplankton was (111.8±120.6)SymboltB@103
cells·L-1. The overall horizontal distribution of phytoplankton cell abundance had a trend of decreased from nearshore in east to offshore in southwest. The Shannon diversity index was coincident with Pielou evenness index. The low values of the two indices in November suggested that in the survey areas, phytoplankton community was unstable, and water quality was unhealthy. The potential water range affected by the thermal effluent was within 10 km. The seasonal variation of phytoplankton species composition indicated its flexibility to temperature. The occurrence of high abundance of Phaeocystis globosa should be paid attention. The high velocity of flow and the widen water area around Yangjiang Nuclear Power Plant benefited water exchange, and thus, the second-stage project of the Yangjiang Nuclear Power Plant would not make a notable impact on the phytoplankton community structure in the adjacent waters of the Plant.

Key words: abundant bacterial groups, Proteobacteria, Firmicutes.