Net-phytoplankton community structure and its environmental correlations in central Bohai Sea and the Bohai Strait

Phytoplankton is an important indicator of, and responder to, environmental changes. This study aims to reveal the response of a phytoplankton community to environmental changes, especially the level of nutrients in the Bohai Sea. The distribution pattern of net-phytoplankton communities in the central Bohai Sea and the Bohai Strait in winter and summer were studied, and the relationship between the phytoplankton communities and environment was explored. The results showed that diatoms (e.g. A. octonarius, Paralia sulcata and Detonula pumila) dominated the phytoplankton communities in winter, while diatoms (e.g. Chaetoceros sp. and Thalassiothrix frauenfeldii) and dinoflagellates (e.g. Ceratium fusus and Ceratium tripos) were both dominant in summer. By cluster analysis, the phytoplankton communities were divided into three clusters in winter and two in summer. The community that inhabited the waters around Qinhuangdao had higher abundance and distinct taxa composition in winter, related to the higher level of DIN and phosphate. Influenced by different water masses (the Yellow Sea Warm Current and the Bohai Sea Coastal Current), the phytoplankton community composition in the northern and southern parts of the Bohai Strait were also significantly different in winter. In summer, the difference in abundance and dominant species between the two phytoplankton communities was more closely linked to the N:P ratio in the environment. These results reflect that both nutrient level and hydrodynamic condition greatly influence the phytoplankton communities in the central Bohai Sea and the Bohai Strait. Our study will provide basic data for the eutrophication and environmental changes in the Bohai Sea.