Partial Weir Opening Is Associated with Changes in Diatom Diversity and Ecological Recovery Trajectories in a Monsoonal River

Partial opening of three weirs (Sejong, Gongju, and Baekje) in the Geum River, South Korea, was implemented in 2020 to partially restore longitudinal flow connectivity. Using seasonal surveys conducted at eight stations from 2020 to 2024, we examined spatiotemporal variation in water quality and benthic diatom assemblages associated with this hydrological intervention. Annual basin-wide averages showed gradual interannual changes in water quality, with declines in total phosphorus, total nitrogen, chlorophyll-a, turbidity, and biochemical oxygen demand after 2021, accompanied by increased dissolved oxygen. Diatom community indices based on relative-abundance data exhibited corresponding temporal variation, with decreased dominance and increased Shannon diversity, evenness, and taxon richness, with significant shifts emerging approximately two years after weir opening. The Bray–Curtis-based Community Dissimilarity Index increased from 0.61 to 0.77 (p < 0.01), and β-diversity partitioning indicated that species turnover accounted for 72% of total dissimilarity, suggesting progressive compositional differentiation among sites. Ordination analyses indicated gradual differentiation between assemblages sampled before and after partial weir opening, while random forest models showed limited explanatory power and were treated as exploratory. Indicator species analysis identified period-specific characteristic taxa rather than deterministic species turnover. Overall, partial weir opening was associated with measurable changes in benthic diatom community structure and diversity in a monsoonal river, supporting the use of benthic diatoms as sensitive descriptors for tracking ecological change under flow-regulated conditions.