Short-term microbial community reassembly and sediment-driven water quality pulses following a managed weir opening in the Nakdong River

Abstract

The construction of large weirs in the Nakdong River, South Korea, has imposed quasi-lentic conditions that aggravate eutrophication and pollutant retention. To mitigate these impacts, four weirs were opened, yet the acute, short-term consequences for pollutant fate and microbial function remain unclear. We conducted an integrated short-term assessment combining hydrology and water quality with 16S rRNA amplicon sequencing, neutral community modeling, redundancy analysis, and co-occurrence networks. Weir openings induced streamflow and mobilized previously settled materials, evidenced by sharp increases in suspended solids, total phosphorus, and COD. Bacterial communities reorganized toward flow-adapted assemblages, with declines in eutrophic/lentic taxa (e.g., Limnohabitans, Flavobacterium) and concurrent rises in sediment-associated genera (Terrimicrobium, Luteolibacter) and cyanobacteria (Microcystis PCC-7914), indicating resuspension of benthic biomass and contaminants. Co-occurrence network analysis confirmed that altered hydrological conditions and water quality parameters were the primary drivers shaping the microbial community. Functional inference (FAPROTAX) predicted enhanced nitrogen cycling, consistent with the reactivation of natural biogeochemical mitigation pathways. Collectively, these results demonstrate that the reintroduction of flow produces a transient pollution pulse while initiating subsequent water quality improvement. Microbial indicators therefore provide rapid diagnostics for evaluating hydrological management actions and can inform adaptive management and policy decisions in highly regulated river systems.