Early warning on the potential harmful algal bloom species in Beibu Gulf of South China Sea under the background of climate change and human activity.

Human activity and global climate change increasingly affect marine environments, leading to increases in harmful algal blooms (HABs) caused by phytoplankton. These blooms pose significant threats to public health, tourism, fisheries, and ecosystems. As an important fishing ground and tourist destination, the Beibu Gulf faces growing environmental pressure. This study sought to assess the phytoplankton community structure and status of HABs, with a focus on potential HAB species. Using environmental DNA (eDNA) metabarcoding, summer and winter surveys at both coastal and offshore waters revealed 66 potential HAB species, 23 of which were newly recorded in the Beibu Gulf. The potential HAB species exhibited greater richness and relative abundance in summer than in winter. Offshore areas showed greater diversity, whereas coastal areas showed greater relative abundance. Temperature emerged as the most influential factor shaping phytoplankton composition, and pH was found to play an important role in coastal areas. Nutrients such as silicate and ammonium are critical for the distribution of potential HAB species. Among the potential HAB species, Cyclotella cryptica predominated in coastal areas during winter, whereas Chaetoceros tenuissimus predominated in summer. Some species that caused severe HAB events in other oceanic regions were first detected in this study, including Margalefidinium polykrikoides, Karlodinium veneficum, and Prorocentrum concavum. This study revealed the diversity and complexity of the phytoplankton community in the Beibu Gulf, emphasizing the critical importance of monitoring and early warning of potential HAB species, particularly those driven by human activities and climate change.