Habitat scale and seasonality influence macroinvertebrate functional feeding groups in a tropical Kenyan montane stream

Habitat quality and diversity strongly influence the composition of macroinvertebrate communities in lotic ecosystems. We evaluated the functional organization of macroinvertebrates in response to changes in habitat type and seasonality in the Afromontane Moiben River in western Kenya. In-stream substrate characteristics were evaluated at the reach, channel unit/habitat and microhabitat scales at nine sites in different land-use areas. Water and habitat quality were assessed during the dry (January–March) and wet (April–June) seasons. A total of 81 macroinvertebrate samples were collected every month from three channel units (pools, riffles and runs) defined by different substrate types and classified into five functional feeding groups (FFGs): collector-filterers, collector-gatherers, predators, scrapers and shredders. Numerical abundance differed significantly between FFGs, with gatherers > filterers > predators > scrapers > shredders. Coarse substrate (bedrock, boulders and cobbles) in riffles had the highest richness and abundance of filterers and scrapers while sand, mud and detritus in pools recorded the lowest richness and abundance. Shredders and scrapers occurred predominantly in upstream sites with minimal human disturbance, while gatherers were most abundant at disturbed sites. The greatest variability in FFGs was at the smallest microhabitat scale, and not at the reach scale. This study shows that attention should be given to both reach-scale and in-stream disturbances when assessing the ecological condition of streams and rivers.