Fish Community Differences Revealed by Water and Sediment eDNA in Thermal-Discharge-Impacted Waters of the Hongyanhe Nuclear Power Plant

Thermal discharge from coastal nuclear power plants forms persistent temperature gradients in waters adjacent to outfalls, and is one of the typical anthropogenic disturbances in nearshore waters. However, whether fish community patterns under thermal discharge are consistent between water and sediment, and whether the two media provide complementary information, remain poorly understood. In October 2024, we collected water (n = 16) and sediment (n = 15) samples near the Hongyanhe Nuclear Power Plant, Liaodong Bay, and applied environmental DNA (eDNA) metabarcoding to compare fish community composition and spatial patterns across media in near- and far-field zones. A total of 32 fish taxa were identified: 2 only in water, 6 only in sediment, and 24 in both media. α-Diversity calculated from OTU abundance data did not differ significantly among the four medium-zone groups, mean Shannon index was numerically higher in far-field water (1.64) than in near-field water (1.07), and Simpson index showed a similar non-significant trend. Water community similarity declined with both geographic and environmental distances (distance–decay pattern), and indicator taxon analysis identified Acanthopagrus schlegelii and Lateolabrax maculatus as near-field water indicators and Sardinella zunasi and Thryssa kammalensis as far-field water indicators. Sediment communities showed high within-group dispersion and no distance–decay pattern. PERMANOVA revealed a significant medium × zone interaction, indicating that near–far-field differences in fish community composition depended on sample medium. Water community structure was associated with seawater temperature, nitrate concentration, and salinity, whereas sediment community structure showed no significant associations with the measured environmental variables. Overall, water and sediment eDNA provided partially complementary information on fish detection; combined water–sediment sampling is recommended to broaden monitoring coverage. This single-survey study provides baseline data for multi-medium fish eDNA monitoring to support future management of thermal-discharge impacts.