Molecular diversity and potential ecological risks of toxic HAB species in the coastal waters off Qinhuangdao, China

Harmful algal blooms (HABs) have occurred frequently in the coastal waters off Qinhuangdao (CWQ) of the Bohai Sea during the past two decades, with paralytic shellfish toxins frequently exceeding safe levels in recent years. However, the biodiversity of toxic HAB species remain poorly understood. Cruise observations in the CWQ of the Bohai Sea from August to November 2021 were used, to investigate the biodiversity, geographical and temporal distributions of toxic HAB species, and associated environmental factors. Through amplicon sequence variants (ASVs)-based metabarcoding analysis, 4261 ASVs of five microalgae phyla were identified in this study, of which Dinoflagellata was the most dominant phylum in most sampling sites. Consequently, 257 microalgae species were annotated in this study, in which 70 were identified as HAB species, including 33 toxic HAB species have been reported to produce toxins or potentially toxic substances. Notably, most HAB species were widely distributed in the CWQ in August to November, especially the toxic species Karlodinium veneficum. Moreover, some toxic HAB species may be distributed in the CWQ all year round, with a high risk of toxic HAB outbreak. Eight environmental factors were evaluated, and the temperature was found to be the key environmental factor influencing the distribution and seasonal variation of dominant HAB species. This research highlights the necessity for monitoring toxic HAB species for accurate prevention and mitigation of HABs.