Linking weir imprints with riverine water chemistry, microhabitat alterations, fish assemblages, chlorophyll-nutrient dynamics, and ecological health assessments

Abstract

Abstract The weir construction in larger rivers has displayed harmful impacts due to unpredicted modifications in hydrology and microhabitats. This study illustrated the impacts of three massive weirs installed in the mainstream Geum River, South Korea. We used the modified multimetric index of biotic integrity (mmIBI) and water pollution index (mmWPI) to understand the imapcts of weir installations on riverine water chemistry and fish assemblages. The results divulged seasonal and spatial heterogeneities by indicating the gradual degradation of water quality. Phosphorus and sediments displayed an excessive influence of seasonal rainfall patterns. The nutrients (N, P) showed week relationships at weir (GW: R2 = 0.21, SW: R2 = 0.12, BW: R2 = 0.25), but stronger links in the upstream river (R2 = 0.74) sites. The mmWPI revealed the weir sites and riverine zones into ‘fair to very poor’ water quality status, while mmIBI specified the sites in ‘fair to poor’ ecological health. Overall, 60 fish species sampled as 64,637 individuals were observed, indicating 81.24% of individuals at the weir sites. Squalidus japonicus coreanus (18.47% RA) and Hemibarbus labeo (9.25% RA) were relatively the most abundant fish species in this investigation. The total number of fish species and individuals gradually declined along the river gradient. The principal component analysis (PCA) grouped the dominant factors by distinct riverine zones, with a percent cumulative variance of 81.80%. The tolerant and omnivorous fish species were established in aggregations in downstream, with increased insectivorous species. In conclusion, the impacts of weir installation were assiduously manifested and indicted an exceedingly deteriorating water chemistry that could have caused a decline in sensitive species, favored the tolerant species, and overall reduced the habitat quality suitable as natural breeding and feeding grounds. Furthermore, the declining fish assemblages and their distribution, perturbed microhabitat conditions, and inclusively deteriorating ecological health status were linked to the weir imprints.