Molecular diversity and ecological characteristics of the eukaryotic phytoplankton community in the coastal waters of the Bohai Sea, China

Brown tides caused by the picophytoplankton Aureococcus anophagefferens have been observed in the Bohai Sea, China on a regular basis, which should be relative with the change in phytoplankton community structure. To better understand this change in the coastal waters of the Bohai Sea, we sequenced 18S rDNA genes extracted from field samples collected from April through October 2013 and evaluated the composition and relative abundance of the eukaryotic phytoplankton community. Most of the operational taxonomy units (OTUs) belonged to Dinophyceae and their sequences were relatively abundant. In late spring and early summer, Mamiellophyceae (chlorophytes), Cryptophyceae (cryptophytes) and Mediophyceae (diatoms) comprised a high proportion of total algal abundance. Small phytoplankton (pico- or nano-sized) were highly abundant in our work, which might dominate the phytoplankton community and provide a 'background' for the formation of A. anophagefferens brown tides and other small-celled algal blooms. A total of 71 algal species were identified, among which 34 were new records. Further analysis revealed that half of the identified species were known bloom-forming or toxic algae, including 11 harmful algal species that were previously undetected or unreported in the Bohai Sea. Most of these harmful species are mixotrophs, which provides a competitive advantage during the formation of harmful algal blooms (HABs). In conclusion, our work provides a detailed evaluation of eukaryotic phytoplankton diversity and dynamics in the Bohai Sea, suggesting the need for long-term monitoring to further study and guard against the potential risk of HABs.