The coastal waters of Qinhuangdao in the Bohai Sea represent one of the few marine areas in China that have successively experienced outbreaks of red tides, brown tides, and green tides, posing a severe challenge for the prevention and control of harmful algal blooms (HABs). Meanwhile, microalgal toxins, particularly paralytic shellfish toxins (PSTs), frequently exceed regulatory limits in routine market monitoring. However, research on the diversity and seasonal succession patterns of toxigenic bloom-forming species in this region remains notably insufficient, which critically hampers the early warning and mitigation of HAB events. From March 2022 to February 2023, our team conducted monthly cruises for systematic sampling in the coastal waters off Qinhuangdao. Employing metabarcoding technology, we comprehensively investigated the molecular diversity, spatiotemporal distribution characteristics, and key environmental factors of phytoplankton communities. A total of nine phyla were identified, with Dinoflagellata and Chlorophyta being the dominant groups. Furthermore, 251 phytoplankton species were detected, among which 51 were recognized as HAB species, including 18 HAB species with confirmed or potentially toxigenic effects. It is noteworthy that from August to December 2022, multiple toxigenic HAB species belonging to the genera Alexandrium (producing PSTs), Pseudo-nitzschia (producing domoic acid, DA), and Karenia (producing brevetoxins, BTX) were widely distributed in the Qinhuangdao coastal waters. Results from redundancy analysis (RDA) and Spearman correlation analysis indicated that temperature and salinity were the key environmental factors governing the seasonal succession of dominant phytoplankton phyla and HAB species in this region. During the period from 2014 to 2023, based on morphological microscopy and metabarcoding techniques, our team identified at least 129 potential HAB species in the Qinhuangdao coastal area, of which 43 HAB species were confirmed or suspected to produce toxins. Noctiluca scintillans and Pseudo-nitzschia pungens were among the commonly encountered HAB species in annual cruise surveys, and have repeatedly formed HAB events in the region, particularly during summer and autumn, thus warranting close attention. Although only 26 species have been recorded as causative microalgae species of HAB events in the region from 2006 to 2024, this does not imply that future monitoring and management efforts should be confined solely to these known causative species. The 129 potential HAB species identified in this study represent a substantial seed bank for HAB initiation in this region. Under suitable environmental conditions, novel HAB events are highly likely to occur, underscoring the urgent need for sustained surveillance and preventive measures.

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