Spatiotemporal Variation in Phytoplankton and Physiochemical Factors during Phaeocystis globosa Red-Tide Blooms in the Northern Beibu Gulf of China

Phaeocystis globosa blooms frequently in the Beibu Gulf of China. This species has a distinct life cycle that includes colonies and solitary cells. Colonies are formed during a bloom, while solitary cells are produced between blooms. Information about the abundance of solitary cells and other picophytoplankton in the northern Beibu Gulf is limited. To elucidate phytoplankton variation trends during periods of frequent P. globosa blooms and to determine the main physiochemical factors affecting phytoplankton distribution, four cruises were conducted between November 2018 and April 2019. Seawater was collected, and water temperature, salinity, and nutrient concentrations were simultaneously determined. Redundancy analysis was performed to understand the relationship between environmental factors and phytoplankton assemblages. Seven phytoplankton clusters were present during the cruises. Picophytoplankton abundance (including Synechococcus and Picoeukaryote groups I and II) dominated during the four cruises. Synechococcus abundance was restricted by the low temperatures in winter, decreasing from November to February and increasing in April. Picoeukaryote I abundance was almost unaffected by low temperatures and was mainly affected by nutrient concentration. P. globosa solitary cell abundance increased from November to January and decreased in February and April, and phosphorus was the key factor affecting P. globosa blooms. This is the first study to reveal the abundance and distribution of P. globosa solitary cells in this area.