In order to better understand the seasonal variations in the phytoplankton community structure in the Bohai Sea (BS) and the North Yellow Sea (NYS), we carried out three cruises during 12 to 24 April 2019, 8 to 18 June 2019, and 12 to 22 October 2019. A total of 212 taxa (75 genera and three phyla) were identified, among which 83 taxa in 40 genera, 96 taxa in 43 genera, and 151 taxa in 62 genera were found in spring, summer, and autumn, respectively. Diatoms including Paralia sulcata and Coscinodiscus granii were the most dominant phytoplankton group during the three seasons, while several species of dinoflagellates, e.g., Scrippsiella troichoidea, Tripos massiliensis f. armatus, Gyrodinium spirale, and Prorocentrum minimum were found in warmer, saltier, and nutrient-poor waters. The diversity index of phytoplankton community was highest in autumn and lowest in summer. Based on cluster and multidimensional scaling analyses, the phytoplankton community of the BS and the NYS was divided into three ecological provinces: the BS, the coastal area, and the NYS. These three ecological provinces differed in physicochemical properties induced by the complicated water masses and circulations. Due to the influence of nutrient concentration, the phytoplankton diversity had the highest value in autumn, followed by spring, and the smallest in summer.

Marine phytoplankton play crucial roles in the ocean’s biological pump and have great impacts on global biogeochemical cycles, yet the knowledge of environmental variables controlling their seasonal dynamics needs to be improved further, especially in the coastal ecosystems. In order to explore the determinants affecting the seasonal variation of phytoplankton, here we conducted three surveys during spring, summer and autumn along the coastal Yellow Sea. Among the phytoplankton community, 49 species of diatoms and 9 species of dinoflagellates were observed in spring, 63 species of diatoms and 10 species of dinoflagellates in summer, and 62 species of diatoms and 11 species of dinoflagellates in autumn. These results thus suggested that there were obvious differences in the number of species across the three seasons, of which diatoms were the most diverse group, followed by dinoflagellates. Additionally, diatoms were the most dominant species of the phytoplankton community and varied largely during different seasons. According to the redundancy analysis, the abundance of phytoplankton community was mainly related to water temperature and dissolved inorganic nitrogen (DIN) during the three seasons, indicating that water temperature and DIN could be the key factors controlling the seasonal variability of phytoplankton community along the coastal Yellow Sea. Also, significant correlations were observed between phytoplankton abundance and heavy metals Zn, As, and Hg during the three seasons, suggesting that these metals also had potential influences on the seasonal dynamics of phytoplankton community in the coastal Yellow Sea.

The phytoplankton sinking rate in the eastern Indian Ocean was measured during spring 2017 based on the SETCOL method. The range of phytoplankton sinking rates was −0.291 to 2.188 md⁻¹, with an average of 0.420 ± 0.646 md⁻¹. The phytoplankton sinking rate in the Equator (EQ) and the eastern boundary of the Indian Ocean near Sumatra (EB) was lower than that in the Bay of Bengal (BOB). The sinking rate above 100 m was low and increased rapidly below 100 m in all the three regions. The phytoplankton community composition had an important impact on the phytoplankton sinking rate in the east Indian Ocean. The strong stratification in BOB resulted in Trichodesmium spp. bloom and a lower phytoplankton diversity and evenness in BOB, while the phytoplankton in the deep layer are senescent cells that sink down from the upper layer and cannot actively regulate the state of the cells, resulting in a higher sinking rate. Depth and temperature have a great impact on the physiological state of phytoplankton. The sinking rate of phytoplankton depend on the dominant groups composing the phytoplankton community. For the eastern Indian Ocean, seawater stratification caused by temperature changes the distribution of nutrients in the upper layer, and phytoplankton are affected by temperature and nutrients, resulting in changes in community structure, and finally showing different subsidence characteristics.