Distribution of the Net-Phytoplankton Community and Chlorophyll-a in the Bohai Sea in Summer and Its Impacts on Fishery Resources

Phytoplankton are the major primary producers in the ocean and play a crucial role in the marine food chain. Understanding the dynamics of the marine phytoplankton community can provide insights into the succession process and current status of marine fishery resources. Based on the net-phytoplankton samples collected from the Bohai Sea in the summer (June and August) of 2015, we studied the phytoplankton community composition, diversity, abundance, and dominant taxa in the Bohai Sea. Compared with historical data, the phytoplankton community structure in the Bohai Sea was changed markedly. The chlorophyll-a concentration had changed slightly during recent years, but was much higher than that in the 1980s and 1990s, and the regions with lower values (<1.0 g/L) were much smaller than those 20 years ago. In terms of the dominant taxa, the dominance of the genera Chaetoceros and Coscίnodiscus was found to be slightly decreased, whereas the species Skeletonema costatum, which had been an important dominant species in the Bohai Sea, was not observed in the two months of summer in 2015. In contrast, Paralia sulcata and Pyrrophyta (Noctiluca scientillans in June and Ceratium tripos in August) were still dominant in the Bohai Sea, and the abundance and dominance of Achnanthes brevipes and Pseudo-nitzschia pungens was higher than those in the years before 2015. The diversity indices (Shannon diversity and species richness and evenness) of phytoplankton were at a moderate level. Such a trend was mainly caused by the significant change of the nutrient structure in the Bohai Sea. This may change the structure of the marine food chain and influence the growth and reproduction of commercial fishes and shrimps (e.g., Konosirus punctatus, Liza haematocheila, and Acetes chinensis) that feed on phytoplankton. Consequently, this will have potential impacts on fishery production and the structure and function of the ecosystem in the Bohai Sea.