Phytoplankton community dynamics during Alexandrium blooms in 2019 off the Qinhuangdao coast, Bohai Sea, China

Alexandrium blooms in the northwest area of the Bohai Sea(Qinhuangdao coastal area),China, produce large amounts of toxins that could be enriched in shellfish and consequently harm human bodies. To understand the succession of the phytoplankton community structure during Alexandrium bloom events in the northwest area of the Bohai Sea off Qinhuangdao from April 2 to May 7, 2019, microscopy observations and high-performance chromatography(HPLC)-pigment analysis were performed.Sixty species of phytoplankton were identified, mainly diatoms and dinofl agellates. The abundance of Alexandrium reached the maximum on April 16(3.3×103cells/L). HPLC-pigment CHEMTAX analysis showed that the phytoplankton community was composed mainly of diatoms, dinofl agellates, prasinophytes,and cryptophytes. Diatoms were the main contributor to the total Chl-a pool. There was a downward trend for the proportion of diatom biomass to the total Chl-a pool, followed by an upward trend. The proportion of dinoflagellate biomass showed the opposite trend, whereas that of the prasinophyte biomass presented an obvious increasing trend. Temperature, nutrients, and nutrient structures were the main factors on the distribution of different phytoplankton groups in the study area as shown in the redundancy analysis. This work illustrates the succession of phytoplankton community structures during Alexandrium blooms and provided a theoretical basis for studies on the mechanism underlying the outbreak of harmful algal blooms in sea areas.