The Effect of Urbanization on Freshwater Quality and Biodiversity Loss in Urban Watersheds

Urbanization is a major anthropogenic impact that is reshaping freshwater ecosystems worldwide; however, the cumulative impact of urbanization on water quality and aquatic biodiversity degradation has not been accurately evaluated at the watershed level in rapidly developed tropical cities. This study aims to evaluate how urban land cover affects physicochemical parameters of freshwater and the biodiversity of macroinvertebrates and fish in four watersheds within the Noyyal River Basin, Coimbatore, India, from the highly urbanized to the rural reference sites across a gradient of urban land cover. The physicochemical parameters of water quality, including biochemical oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen (DO), total nitrogen, total phosphorus, pH, and turbidity, were sampled seasonally over two years (2022-2024) at 48 monitoring stations. The biological assessment of macroinvertebrate and fish diversity followed the Shannon-Wiener diversity index as well as BMWP scoring and the Index of Biotic Integrity (IBI). Data showed the relationship between urban land cover and diversity indices was negative and very strong (r =- 0.93, p< 0.001), while 17 times greater increase of BOD occurred from the rural reference site (1.8 mg l -1) to the urbanized watershed (28.4 mg l -1). Richness of the EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa decreased by 89% along the urbanization gradient. Additionally, impervious surfaces and stormwater runoff routes were the main drivers of nutrient loading and habitat homogenization. These results emphasize the need for integrated urban watershed management policies that include ecological flow standards, green infrastructure, and real-time water quality monitoring to protect freshwater biodiversity from urban growth in rapidly developing countries in the Global South.