Modular structure of plankton diversity along environmental gradients revealed by environmental DNA in Lake Shihwa, South Korea.

Aquatic biodiversity assessment using environmental DNA (eDNA) metabarcoding has been increasingly applied for biomonitoring and environmental assessment in coastal waters modified by humans. Lake Shihwa is a historically impacted and intensively managed semi-enclosed system, where seawater exchange by water gate and continuous freshwater inputs generate strong environmental gradients. In this study, surface water samples were collected from exterior to interior stations to investigate the community dynamics of eukaryotic plankton across distinct environmental regimes. To encompass broad biodiversity, eDNA metabarcoding targeting the cytochrome oxidase I (COI) and 18S rRNA genes was performed. Clustering of samples by salinity revealed significant biodiversity differences, which were further supported by salinity-dependent module differentiation identified using a simulated annealing algorithm. Notably, the optimal salinity ranges of taxa within modules corresponded closely with measured salinity conditions. These results demonstrate that distinct diversity patterns observed among stations are primarily structured by salinity along environmental gradients. Together, our findings support the growing reliability of eDNA approaches in capturing ecologically meaningful biodiversity patterns and underscore their strong potential for broader application in environmental monitoring and assessment.