Phosphorus (P) is an essential nutrient element of phytoplankton, as well as a limiting factor for phytoplankton growth. It controls the succession of blooms from diatoms to dinoflagellates, especially in the East China Sea (ECS), where the eutrophication is serious. Most studies have only considered a single aspect of dissolved inorganic phosphorus (DIP) or dissolved organic phosphorus (DOP). In contrast, we investigated the growth interactions among the major bloom-forming marine diatom Skeletonema costatum and dinoflagellates Prorocentrum donghaiense and Karenia mikimotoi by using bi-algal cultures combined with DIP and DOP. Our results revealed that (1) P. donghaiense and K. mikimotoi have survival strategies that are superior to those of S. costatum in a bi-algal culture, whether under P-sufficient or P-deficient conditions, and (2) P. donghaiense has a slight competitive advantage over K. mikimotoi when P is sufficient, but the reverse is true when P is deficient. The difference in interspecific competition results at different P concentrations with DIP and DOP mainly arises from the variation in the utilization abilities of different species as regards different P sources, a finding which can also provide strong evidence for revealing the succession of diatoms and dinoflagellates blooms in the ECS.

The Western Subarctic Gyre (WSG) is a cyclonic upwelling gyre in the northwest subarctic Pacific, which is a region with a high concentration of nutrients but low chlorophyll. We investigated the community structure and spatial distribution of phytoplankton in this area by using HPLC-pigment CHEMTAX (a chemotaxonomy program) and metabarcoding sequencing during the summer of 2021. The phytoplankton community showed significant differences between the two methods. The CHEMTAX analyses identified eight major marine phytoplankton assemblages. Cryptophytes were the major contributors (24.96%) to the total Chl a, followed by pelagophytes, prymnesiophytes, diatoms, and chlorophytes. The eukaryotic phytoplankton OTUs obtained by metabarcoding were categorized into 149 species in 96 genera of 6 major groups (diatoms, prymnesiophytes, pelagophytes, chlorophytes, cryptophytes, and dinoflagellates). Dinoflagellates were the most abundant group, accounting for 44.74% of the total OTUs obtained, followed by cryptophytes and pelagophytes. Sixteen out of the 97 identified species were annotated as harmful algal species, and Heterocapsa rotundata, Karlodinium veneficum, and Aureococcus anophagefferens were assigned to the abundant group (i.e., at least 0.1% of the total reads). Nutrients were more important in shaping the phytoplankton community than temperature and salinity. The 24 stations were divided into southern and northern regions along 44°N according to the k-means method, with the former being dominated by high Chl a and low nutrients. Although different phytoplankton assemblages analyzed by the two methods showed various relationships with environmental factors, a common feature was that the dinoflagellate proportion showed a significantly negative correlation with low nutrients and a positive correlation with Chl a.

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