Anthropogenic nutrient discharge and microbial community shifts associated with typhoon-driven runoff in an artificial coastal lake.

This study investigated changes in nutrient sources and microbial community composition across a typhoon-associated runoff period in Lake Sihwa, an anthropogenically influenced artificial coastal lake. Stable isotope analyses (δ13CPOC, δ15NPN, δ15NNO₃, and δ18ONO₃) and water-column environmental DNA (eDNA) metabarcoding were combined to characterize nutrient sources and compare ecological conditions between the pre- and post-typhoon surveys. Before the typhoon-associated rainfall period, nitrate inputs were primarily associated with urban runoff, whereas particulate organic matter (POM) reflected a substantial autochthonous plankton contribution. In the post-typhoon survey, nitrate-source contributions were dominated by industrial (39 ± 23%) and wetland (39 ± 6%) catchments. POM isotopic signatures (δ13CPOC, -22.6 ± 3.3 ‰; δ15NPN, 8.0 ± 1.9 ‰) indicated an increased influence of marine-plankton-like organic matter in the lake. eDNA metabarcoding revealed a pronounced taxonomic reorganization, with community composition shifting from heterotrophic groups (e.g., Cercozoa and Bacteroidota) toward phototrophic and nutrient-responsive taxa (e.g., Ciliophora and Proteobacteria), while several nitrogen-cycle-associated groups within Actinobacteriota, Nitrospirota, Verrucomicrobiota, and Proteobacteria showed lower relative representation. The spatial correspondence between these community shifts and land-use, salinity, and nutrient gradients suggests that runoff-related nutrient redistribution and freshwater-seawater mixing were important factors associated with the observed ecological changes. These findings demonstrate the value of integrating stable isotope tracing and eDNA metabarcoding as complementary approaches for evaluating nutrient-source redistribution and ecological reorganization across episodic storm-associated runoff periods in artificial coastal ecosystems.