Zooplankton are a secondary producer in the world's oceans. Changes in the species assemblage or abundance of zooplankton has a direct impact on both primary productivity and the biomass of fish and other marine animal resources. Because, zooplankton are a key member of marine food webs, they have been the focus of a number of studies in the Yellow Sea. However, the majority of these studies have evaluated zooplankton population dynamics at a large scale (e.g., the entire Yellow Sea). To date, there has been little effort to understand the effect of local anthropogenic activity, such as the discharge of nuclear cooling water, on zooplankton ecology. We evaluated the species composition, quantitative distribution, dominant species, and community structural characteristics of zooplankton at 14 inshore sites near the Tianwan nuclear power station (34.62°-34.76°N,119.46°-119.66°E) in August 2009. We identified 43 species belonging to 10 groups, including 5 species of Protozoa, 17 species of Hydromedusae, 2 Ctenophora, 2 Cladocera, 11 species of Copepoda, 2 Mysidacea, 1 Sergestidae, 1 Euphausiacea, 1 Chaetognath, and 1 Urochordata. The abundance of zooplankton ranged from 99 to 2546 ind/m³ (mean 834 ind/m³). The most abundant group were the Hydromedusae followed by the Copepoda. Twelve species had a dominance index ranking of >0.02 within the study area. Among these, Obelia spp., Acartia pacifica, Centropages dorsispinatus, Penilia avirostris, Oikopleura dioica, Calanopia thompsoni, and Sagitta crassa were the primary dominant species (dominance index: 0.804, 0.586, 0.569, 0.485, 0.197, 0.140, and 0.116, respectively). The amplitude of the index varied significantly (species: 9 to 17, richness: 0.913-1.770, diversity index: 1.170-3.212, evenness index: 0.369-0.803) suggesting the community structure was instable. The mean diversity index of zooplankton was 2.188, the mean richness index was 1.336, and the mean evenness index was 0.587.
Water temperature and the abundance and the number of species were significantly correlated (P<0.05) (correlation coefficients: -0.615 and -0.574, respectively; P<0.05). We observed a significant relationship between the number of species and the diversity, richness, and evenness indices (correlation coefficients: 0.730, 0.759 (P<0.01), and 0.552 (P<0.05), respectively. There was a reduction in the number of zooplankton species as the temperature increased. We found a significant negative correlation between ammonia nitrogen content and water temperature and between ammonia nitrogen content and the number of species. Increases in temperature lead to increased ammonia, resulting in a decline in the number of zooplankton species. Thus, we hypothesize that increases in water temperature due to discharge of nuclear power cooling water have caused a decrease in the zooplankton community diversity, richness, and evenness indices in the nearshore region around the Tianwan nuclear power station.
The zooplankton diversity index has since been adopted to assess water quality around the Tianwan nuclear power plant. Monitoring results suggest a low level of water pollution.