Environmental DNA analysis of phytoplankton biodiversity and seasonal succession in a marine ranching area around Nan'ao Island, southern China.

The coastal waters surrounding Nan'ao Island serve as important areas for marine ranching in southern China. In this study, we examined the structure and seasonal succession of the phytoplankton community in this region using an environmental DNA (eDNA) metabarcoding approach. Across four seasons, we identified 6154 eukaryotic phytoplankton amplicon sequence variants (ASVs), with 510 known species annotated. The summer community was dominated by diatoms, primarily warm-water species such as Chaetoceros tenuissimus and Leptocylindrus sp. By contrast, dinoflagellates prevailed in other seasons, with Dinophyceae being the main group in spring and winter and Syndiniales in autumn. The biodiversity and stability of the communities were found to be the highest in spring and lowest in summer. Summer upwelling and winter Zhe-Min coastal currents led to significant seasonal changes in environmental conditions across habitats. NMDS analysis revealed that temperature, salinity, and ammonium nitrogen were the main factors driving community succession, while RDA analysis showed that pH and nutrient concentrations were the primary factors contributing to spatial differences in community distribution. In addition, we identified 52 harmful algal bloom species, among which, toxic species such as Karlodinium veneficum and Dinophysis acuminata pose the greatest threat in spring, potentially causing toxin accumulation or even mortality in farmed fish and shellfish. The seasonal eDNA data obtained in this study will contribute to gaining a better understanding of the successional dynamics of phytoplankton communities and their biodiversity in response to the influence of multiple stressors in coastal waters.