Multi-scale land use effects on stream health in the Walawe Basin: integrating habitat, water quality, and biological indicators

Abstract

Understanding the relationships between spatial scales of catchment land use and indicators of stream ecosystem health is crucial for the management of tropical river basins that are undergoing rapid environmental changes. This study was designed to assess stream health in Sri Lanka’s Walawe River Basin by integrating the changes in water quality, stream habitat quality, and macroinvertebrate-based indicators in response to different types of catchment land cover at multiple spatial scales. Twenty-nine sites (12 in the River Walawe and 17 in its tributaries) were sampled to evaluate the spatial and seasonal variations of water quality, habitat quality, and macroinvertebrate-based indicators. Seventeen tributaries (sub-catchments) were specifically selected to investigate the effects of types of land cover on stream health. Bivariate regression analysis and canonical correspondence analysis were conducted to evaluate the impact of spatial scales of land cover in the catchment on stream health. The regression analysis revealed that forest cover within the 200-m and 500-m stream buffers strongly predicted indices of stream habitat quality [coefficient of determination (r²) = 0.69, p < 0.05] and macroinvertebrates [percent Ephemeroptera, Plecoptera, and Trichoptera) (r² = 0.49, p < 0.05)], and Shannon diversity (r² = 0.25, p < 0.05). Sub-catchment-scale forest cover was the strongest predictor of overall water quality (r² = 0.35, p < 0.05). Stream habitat quality served as a strong predictor of species richness, percentage Ephemeroptera, Plecoptera, and Trichoptera, and Shannon diversity, suggesting that habitat diversity is important for securing stream ecological health. The findings underscore the importance of catchment-wide forest conservation for the health of stream ecosystems, as it is crucial for maintaining, and enhancing, water quality, while stream buffers are vital for the preservation of the biological integrity and complexity of stream habitats.