Community Characteristics of Phytoplankton in Dongping Lake Revealed by eDNA and Implications for Water-Quality Assessment

Dongping Lake is a regulating lake where hydrodynamic alteration and heterogeneous inputs may reshape phytoplankton communities; this study aimed to characterize eukaryotic phytoplankton, assess water quality and identify key environmental drivers. In September 2025, eukaryotic phytoplankton were profiled using 18S rDNA V9 eDNA metabarcoding across 18 sites, and community–environment relationships were evaluated using diversity indices, principal coordinates analysis (PCoA), Spearman correlations and redundancy analysis (RDA). This study detected 101 eukaryotic phytoplankton species. Bacillariophyta dominated read abundance at 55.08%, followed by Cryptophyta at 22.20%, whereas species richness was highest in Chlorophyta with 40 species. Site richness ranged from 26 to 63, peaking at sampling sites D17 and D18 and reaching a minimum at sampling site D15; Cryptophyta dominated reads only at sampling site D6. Nine dominant species were identified. Mean diversity values were Shannon-Wiener diversity index (H) 3.45, Pielou evenness index (J) 0.92, Margalef richness index (D) 4.40 and Chao1 richness estimator 44.72, and overall water quality was assessed as slightly polluted, with sampling site D12 or D15 reaching moderate pollution under specific indices. Dominant-species responses were differentiated; for example, Stephanodiscus hantzschii was negatively correlated with NH4+ and TN, and Ceratium hirundinella was positively correlated with salinity but negatively correlated with NH4+. RDA ranked key drivers as salinity > NO2− > TN > NH4+ > TP > DO > temperature. Salinity and nitrogen-form gradients were closely associated with spatial community differentiation and dominant-species shifts, supporting targeted monitoring and management.