Diversity and community differentiation of phytoplankton in the coastal waters around Hainan Island: An eDNA Metabarcoding approach.

To examine survey-period spatial variation and environmental associations of phytoplankton communities around Hainan Island, we conducted a single survey of 19 surface-water stations from 5 to 14 August 2025 using environmental DNA (eDNA) metabarcoding targeting the 18S rRNA V9 region together with hydrographic, water-quality, and nutrient measurements. The mean relative sequence abundances of Bacillariophyta, dinoflagellates, and Chlorophyta were 40.23%, 29.27%, and 23.91%, respectively. Taxa with high sequence representation included Chaetoceros, Thalassiosira, Pseudo-nitzschia, Skeletonema, Gyrodinium, Micromonas, and Ostreococcus, indicating a mixed taxonomic composition of diatoms, dinoflagellates, and Chlorophyta-related taxa. Alpha-diversity varied among stations, with mean values of 1407.58, 4.61, 0.94, and 0.63 for observed species, Shannon, Simpson, and Pielou's evenness, respectively. Beta-diversity analysis showed pronounced community turnover among coastal sectors, with greater similarity among adjacent stations or stations with similar environmental backgrounds. After Benjamini-Hochberg correction, four taxon-environment associations remained significant. The variance inflation factor (VIF)-screened RDA model explained 55.2% of the total community variation (adjusted R2 = 0.193; global permutation P = 0.0115), and Mantel analysis showed a significant positive relationship between Bray-Curtis community dissimilarity and environmental distance (r = 0.357, P = 0.0052). Overall, the August 2025 survey revealed a mixed eDNA-based taxonomic composition, station-scale α-diversity variation, pronounced β-diversity turnover, and statistical associations between community differentiation and environmental heterogeneity, providing a survey-period baseline for future repeated monitoring of fishery-relevant coastal ecosystems.