Anthropogenic drivers of algal regime shifts in a Himalayan watershed and pathways for nature-based solutions.

Freshwater ecosystems worldwide are increasingly threatened by human activities, yet the ecological mechanisms linking anthropogenic pressures to algal community dynamics remain incompletely resolved in the Himalayan river systems. To bridge this gap, this study presents a rapid ecological assessment of algal diversity and water quality across three sites in the Kosi watershed, India, integrating comprehensive physicochemical analyses, Water Quality Index (WQI) evaluation, and multivariate statistics. We collected and analyzed 33 water and algal samples during the summer of 2023, identifying 11 algal genera spanning Cyanophyta, Chlorophyta, Xanthophyta, and Charophyta. Our results reveal a pronounced spatial gradient in water quality, with WQI values ranging from "excellent" (42.6) at the minimally impacted upstream site to "very poor" (266.3) at the most anthropogenically influenced downstream site. Canonical correspondence analysis demonstrates that turbidity and pH are the primary environmental drivers shaping algal assemblages, with nutrient-tolerant Spirulina and Oscillatoria dominating disturbed sites, while Oedogonium and Spirogyra prevail under more stable conditions. The rapid assessment approach enabled the timely detection of pollution hotspots and ecological responses, providing implementable steps for watershed management. Our findings underscore the urgent need for targeted conservation and pollution mitigation strategies to preserve riverine biodiversity and ecosystem services in Himalayan catchments. Nature-based solutions can be among the potential pathways to sustainability.