SPATIAL VARIABILITY IN WATER QUALITY AND MACROINVERTEBRATE ASSEMBLAGES ACROSS A DISTURBANCE GRADIENT IN THE MARA RIVER BASIN, KENYA

Abstract

Across the Afrotropical ecosystems, human activities are increasingly modifying natural flow regimes, nutrient and organic matter loading and processing in streams and rivers, with implications on ecosystem structure and functioning. Macroinvertebrates functional composition data is important in assessing the effects of anthropogenic activities on ecological conditions of rivers and streams. The Mara River Basin has undergone extensive land-use change, but the influences of these changes on water quality and aquatic communities are still not well understood. This study, which was conducted in the months of August 2013 to February 2014, investigated changes in water quality and macroinvertebrate assemblages across a disturbance gradient arising from rural human activities in nineteen sites; grouped into three human-impact categories (reference undisturbed sites (n = 7), moderately disturbed sites (n = 6) and disturbed sites (n = 6). Temperature, dissolved oxygen, total dissolved solids, pH and electrical conductivity were measured in situ at each sampling site, and samples were collected for analysis of nutrients and total suspended solids. Sampled macroinvertebrates were identified to the lowest-possible taxonomic level, mostly family level, for analysis of structure (richness and diversity indices) and functional composition. There were significant spatial variations in water quality variables across the disturbance gradient (p < 0.05). The highest mean temperature and suspended solids values were recorded at the highly disturbed sites while the lowest values were recorded at undisturbed sites. Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa dominated (70% in abundance) the macroinvertebrate taxa in undisturbed sites with a few taxa (notably Lepidostoma, Colembolla and Leptophlebia sp.) being restricted to these sites, while Diptera dominated (48% in abundance) the macroinvertebrate taxa at the disturbed sites. Additionally, higher macroinvertebrate diversity and richness indices were recorded at the undisturbed sites. In regard to functional feeding groups, collectors were the numerically dominant taxa at all the site categories with the abundance of shredders being highest at the undisturbed sites. This study adds further evidence that land-use change from forestry to agriculture has a strong influence on the structural and functional composition of macroinvertebrates in the Mara River Basin. Attention should also be given to riparian management and monitoring of in stream activities by people and their livestock. Future studies should focus on restoration of degraded ecosystems and monitoring of restoration processes.