Ecological Health Assessments, based on Chemical Pollution and the Multimetric Fish Model, in a Temperate Monsoon River and their Spatiotemporal Variability

Longitudinal and seasonal physiochemical gradients were evaluated in a temperate monsoon river along with functional traits and their links to biological health indicators and status. A 10-year dataset of water quality and fish monitoring data from five sampling sites was used to assess the river health. The multimetric water pollution index (mWPI) and fish-based index of biotic integrity (mIBI-F) were applied to the monsoon river, along with an ordination analysis of fish communities. Significant increases in nutrients, organic matter, ionic contents, and chlorophyll-a were evident at the sites near pollution sources. The summer monsoon exacerbated these conditions, leading to very poor chemical health at middle reach and downstream sites. Although the chemical health improved slightly in autumn, conditions remained poor at downstream sites. Fish trait indicators showed down river declines in species richness, increases in tolerant species, and a proportional shift towards omnivorous species, reflecting ecological stress and deterioration. The relative abundance of invasive alien species increased post-monsoon, particularly at downstream sites, reflecting heightened biological stress. The mIBI-F assessment revealed a decline in biological health at downstream sites, that was strongly correlated with chemical health (R² = 0.61, p < 0.001). Ordination and regression analyses revealed significant relations between guild-based fish composition and chemical health, influenced by both pollution gradients and seasonal changes. These findings highlight the risks to river health due to agricultural and urban runoff, along with discharge from wastewater treatment plants linked to large industries and metropolitan sewage.