Land use shapes benthic algal assemblage structure and biomass metrics in Afrotropical headwater streams

Benthic algae integrate catchment- and reach-scale pressures and serve as reliable indicators of ecological conditions in streams. This study assessed whether benthic algal assemblage structure, chlorophyll-a, ash-free dry mass, and Autotrophic Index are sensitive indicators of land-use and seasonal gradients in headwater streams of the Sondu–Miriu River Basin, Kenya. We sampled 24 headwater streams across four land-use categories: natural forest (NF), tea and tree plantations (TTP), smallholder agriculture (SHA), and smallholder tea (SHT). Synoptic sampling was conducted once during the wet season in August 2025, while monthly monitoring was conducted from November 2024 to May 2025 at representative NF and SHA streams covering dry- and wet-season months. NF streams had higher dissolved oxygen and lower nutrients, SHA streams showed elevated electrical conductivity and total dissolved solids, and SHT streams recorded higher turbidity and temperature; nutrient concentrations were largely similar across land uses. Shannon and Simpson diversity indices were highest in NF streams and lowest in SHT streams. Chlorophyll-a and AFDM were highest in SHA streams; chlorophyll-a was lowest in SHT streams, while AFDM and Autotrophic Index were lowest in NF streams. Relative abundance showed that Lyngbya and Arthrospira dominated SHA streams, Stigeoclonium dominated SHT streams, and NF streams maintained diatom-rich assemblages (Diatoma, Cyclotella, Eunotia) that persisted across seasons. NMDS and RDA ordinations confirmed significant separation of assemblages by land use and season, with DO, turbidity, EC, and temperature as the primary drivers of compositional variation. Benthic algal assemblage composition and biomass metrics, particularly the Autotrophic Index, sensitively tracked land-use gradients and seasonal shifts, confirming the value of periphyton-based bioassessment for monitoring anthropogenic disturbance in Afrotropical headwater streams.