Macroinvertebrate distributions in relation to human and animal-induced physical disturbance of the sediment surface in two Kenyan tropical Rift Valley streams

Abstract

A study of macroinvertebrate distribution in relation to different levels of human and animal physical disturbance of the sediments of the Njoro (highly disturbed) and Ellegirini (less disturbed) rivers was conducted from October 2001 to June 2002, and again in 2012 only in the Njoro River. The rivers have similar climate and land-use characteristics but differ in the frequency and intensity of human and animal disturbance of their sediment surfaces. Benthic macroinvertebrates were sampled during both low discharge, when the disturbances were high, and high discharge, when the disturbances were low. Macroinvertebrate abundance and diversity were higher in the Ellegirini than in the Njoro River. Sites characterised by high disturbance had low macroinvertebrate abundance and diversity, and were dominated by one taxon that tolerated disturbance irrespective of flow regimes. A consistent decline in macroinvertebrate abundance and diversity in the Njoro River as a result of increased disturbance was evident in 2001–2002 and 2012. Macroinvertebrates are most probably redistributed via migration and dispersal, due to these physical disturbances.

Keywords:

• abundance
• benthic macroinvertebrate
• discharge regime
• dispersal
• diversity
• perturbation