The influence of hydrology on the structure of invertebrate communities in two streams flowing into Lake Nakuru,Kenya

Stream fauna communities were studied in the Baharini Springbrook and the Njoro River, which flow into Lake Nakuru. The streams have significantly different flow regimes. Consequently, discharge-related factors influence their faunal composition and abundance. The Baharini Springbrook is perennial, slow-flowing with low steady discharge, a higher conductivity and a low conductivity gradient along its course. The Njoro River is intermittent, fast-flowing with a higher and variable discharge, lower conductivity and a steeper conductivity gradient. Due to its permanent and steady surface stream flow, the Baharini Springbrook has a higher number of invertebrate species and higher abundances than the Njoro River. However, in spite of the intermittent nature of the Njoro River, during periods of relatively steady discharge the composition of its fauna is similar to that of the Baharini Springbrook. Similarity in the substratum characteristics of the two streams and their close proximity is a possible explanation for the close relationship in their faunal composition. At the onset of flow after a drought, rapid colonisation of the lowland intermittent stretch of the Njoro River occurred probably by drift from upstream perennial sections and/or from the hyporheos. Initial small spates at the onset of surface flow seem not to affect the invertebrate abundance, but later bigger spates during and after much heavier rains reduced the faunal abundance significantly. The recovery of faunal abundance to pre-flood levels in the Njoro River was completed in 27 days. Taxon richness was, however, less affected by the spates.