Algal community response to water quality gradients across the wetlands in the Koshi Tappu Ramsar site, Nepal

Wetlands support diverse aquatic communities with algal assemblages serving as sensitive indicators of ecosystem health and environmental change. Algal abundance and diversity are shaped by complex interactions among physical, chemical, and biological factors within aquatic ecosystems. Monitoring these parameters is essential for evaluating ecosystem health, water quality, and plankton dynamics. This study investigated seasonal variation in algal diversity in relation to physicochemical parameters of water at the Koshi Tappu Ramsar site, Nepal. Samples were collected during the pre- and post-monsoon periods, and algal richness, diversity, and evenness were analysed alongside key physicochemical water variables. A total of 194 algal taxa were recorded, with significantly higher species richness and Shannon-Wiener diversity index (H′) observed during the post-monsoon season (p < 0.001), indicating improved ecological conditions. Multivariate analyses revealed that phosphate, electrical conductivity, sulphate and calcium hardness were primary drivers of algal distribution. Bacillariophyceae was the most dominant class during both seasons. Bacillariophyceae such as Gomphonema and Pinnularia were associated with eutrophic, ion-rich waters, whereas Pinnularia angusta, Melosira and Cosmarium granatum reflect oxygenated water conditions. These findings underscore the influence of seasonal hydrology and nutrient dynamics on phytoplankton communities, providing a scientific basis for wetland monitoring and conservation in Nepal’s lowland ecosystems.