Seasonal variation of the phytoplankton communities and their environmental factors in the Bohai Sea

Phytoplankton play an important role in marine ecosystems, and the changes of phytoplankton community can reflect changes in marine environment. The Bohai Sea is severely affected by human activities, and the problems of eutrophication and harmful algal blooms in the Bohai Sea are becoming more and more serious. A comprehensive survey was carried out in the central Bohai Sea (37°70′－39°70′N, 119°00′－121°51′E) in February, May, September 2017 and December 2016. The taxonomic composition, abundance and diversity of phytoplankton communities and the distribution of dominant taxa were studied. A total of 131 taxa of 68 genera in 3 phyla were identified, including 90 diatom species in 46 genera, 37 dinoflagellate species in 19 genera and 4 chrysophyta species in 3 genera. The community structure and dominant species in the survey area showed obvious shifts with seasonal changes. The dominant species in winter, spring and summer all belonged to diatoms, while those in autumn belonged to diatoms and dinoflagellates. By comparing the results with historical data, we found that Paralia sulcata and Coscinodiscus were the common dominant taxa in this area. However, the dominance indices of Amphora lineolata, Cylindrotheca closterium and Thalassiosira rotula were higher than before. Nutrient concentration and structure are the important factors influencing phytoplankton community structure. This study is helpful to understand the biodiversity of the Bohai Sea, and can provide some reference for the protection and evaluation of the ecological environment of the Bohai Sea.