Abstract:Liusha Bay is one of the main aquaculture areas of marine mollusks in Guangdong Province, characterized by the main producing areas of South Pearl. To assess the environmental effects of aquaculture activities, the phytoplankton composition, abundance, community structure, diversity, and water physicochemical factors were investigated in each season from August 2015 to May 2016. In total, 171 phytoplankton species were detected, with 122 species of diatom belonging to 43 genera, followed by dinoflagellates represented by 44 taxa (10 genera), blue-green algae by two taxa (two genera), chrysophyceae by two taxa (two genera), and euglena by one taxa (one genus). The range of phytoplankton density was 0.05×104~79.04×104 cells/L, which ranked as: summer > spring > autumn > winter. In general, the phytoplankton density in the outer bay was similar to that in the inner bay. The abundance of phytoplankton in summer was distinctly higher than that in the other three seasons. The most common dominant species were Chaetoceros crinitus, Trichodesmium erythraeum, and Coscinodiscus debilis in spring, Skeletonema costatum in summer, Chaetoceros pseudocurvisetus, Bacillaria paradoxa, and Chaetoceros lorenzianus in autumn, and Coscinodisus wailesii, Rhizosolenia delicatula, and Fragilaria islandica in winter. B. paradoxa was the dominant species in spring, autumn, and winter. T. erythraeum was the dominant species in all seasons except for autumn. Redundancy analysis suggested that the main variables affecting the dominant species were water temperature and nitrate (NO3-N) in spring, transparency in summer, and nitrite (NO2-N) in autumn and winter. The phytoplankton diversity and abundance of different cultured zones presented seasonal differences. Compared to 2012, the total coverage of the aquaculture zone in 2015 increased by approximately 50% in the outer bay. Furthermore, coverage of the pearl oyster farming zone decreased by about 90% in the inner bay. Both the distribution of the aquaculture zone and the scale of the mariculture species were significantly changed, and the characteristics of the phytoplankton community responded to these changes. In summer, cage fish farming could efficiently increase nutrient content in seawater and decrease phytoplankton diversity. Furthermore, phytoplankton abundance was decreased by large-scale scallop culture.
