MACROINVERTEBRATE ASSEMBLAGES AND BODY SIZE DISTRIBUTION IN RESPONSE TO LAND-USE CHANGES IN HEADWATER STREAMS OF THE SONDU-MIRIU RIVER BASIN, KENYA

Abstract

Assemblages of aquatic macroinvertebrates have spatial and temporal variations in structure in response to environmental changes of their habitats, such as streams and rivers. Taxonomic approaches for monitoring the ecological status of aquatic ecosystems using macroinvertebrate assemblages face several limitations, creating a need for ataxonomic methods that are valid, disturbance-sensitive, and cost-effective for freshwater monitoring. This study evaluated the influence of seasonality and land-use changes on macroinvertebrate assemblage structure and water physico-chemical parameters of headwater streams of the Sondu-Miriu River basin, Kenya. Additionally, the study evaluated the utility of macroinvertebrate size-spectrum metrics and abundance-biomass comparison (ABC) curves as ataxonomic methods of evaluating land-use influence on the river’s ecological condition. Macroinvertebrate taxonomic abundance (individuals/m2), wet weight (mg), and physico-chemical variables were measured during the wet and dry seasons in March and September 2024, respectively, from 24 headwater streams distributed across four land-use types: natural forest (NF), tea and tree plantations (TTP), smallholder tea (SHT), and smallholder agriculture (SHA). Results based on water quality showed turbidity, total suspended solids (TSS), total dissolved solids (TDS), electrical conductivity (EC), particulate organic matter (POM), and nitrate (NO3 - -N) identified streams in NF as least disturbed and in SHA as most disturbed; TTP and SHT streams were intermediate in water quality. Taxon richness, diversity, and Ephemeroptera, Plecoptera, and Trichoptera (EPT) indices indicated land-use specific influences on assemblage structure. Redundancy analysis (RDA) showed thatsmall-bodied taxa (<8 mg wet weight) were associated with high disturbance indicators, while large-bodied taxa (>32 mg) were associated with low disturbance indicators and sites. Slopes (λ) of normalized abundance- and biomass-based size-spectrum (a measure of trophic transfer efficiency) deviated from theoretical steady-state conditions (λ=-2.0 and -1.0, respectively), indicating that the sites were disturbed, but the slopes did not vary significantly between land uses or seasons, suggesting size-spectrum slopes had low sensitivity to land-use-based changes in water quality. In contrast, size-spectrum midpoint heights (a measure of ecosystem production) differed significantly between sites, highest at SHT and NF and lowest at SHA and TTP streams. Spectrum midpoint heights were, therefore, more responsive to disturbance than slopes, highlighting their potential as indicators of land-use influence on the Afrotropical streams. Although the ABC curves indicated undisturbed conditions for all sites, Warwick’s (W) statistics revealed subtle differences among streams, suggesting variation in local-scale ecological conditions. This study demonstrated that catchment land use significantly influences water quality and macroinvertebrate assemblages in headwater streams of the Sondu-Miriu River basin. Traditional metrics (%EPT and diversity indices) are useful indicators of land use-based disturbance, while size-spectrum midpoint heights are potentially useful ataxonomic indicators of disturbance in the studied streams. It is recommended to integrate both community indices and functional metrics (especially midpoint height) into stream biomonitoring as complementary method for evaluating the ecological status of headwater streams. Management efforts should prioritize riparian buffers and nutrient/sediment control in SHA areas of the basin, while conserving less-impacted NF sites as reference areas. Long-term and broader spatial-scale studies are needed to validate the stability of size-spectrum metrics and ABC curves as rapid tools for monitoring the ecological status of headwater streams in response to anthropogenic influences.