Land use and seasonality shape macroinvertebrate assemblages and water quality in an Afrotropical highland catchment, Kenya

Integrated assessments linking land use, seasonality, water quality, and macroinvertebrate assemblage structure remain scarce for highland stream catchments in East Africa. This study assessed the effects of land use (forest and agriculture) and season (dry and wet) on physicochemical water quality, habitat condition, and macroinvertebrate communities across the Molo River catchment, Kenya. Wet-season conditions were characterised by elevated soluble reactive phosphorus, total nitrogen, total soluble substances, and particulate organic matter, and lower electrical conductivity and riparian condition scores relative to the dry season. Agricultural sites showed significantly higher water temperature and conductivity but lower dissolved oxygen, substrate quality, instream cover, and total habitat scores than forest sites. Macroinvertebrate abundance and assemblage composition differed significantly between land-use types during the wet season (ANOSIM R = 0.24, p = 0.045), with Baetis, Hydropsyche, and Caenis as the primary contributors to between-site dissimilarity. Dry-season RDA ordination revealed that temperature, conductivity, and particulate organic matter were the dominant environmental drivers of assemblage structure, with agricultural sites clustering separately from forest sites along the first ordination axis. Land use exerted a stronger and more consistent influence on both physicochemical conditions and macroinvertebrate assemblages than seasonality alone. These findings confirm the utility of integrated biophysical monitoring for detecting degradation gradients in Afrotropical highland catchments and provide a basis for targeting catchment management and bioassessment in the Molo River basin.