Spatio-temporal variations in physicochemical factors, and stream fauna communities were studied in the transition zones between Baharini Springbrook, Njoro River and Lake Nakuru. The two streams have significantly different flow regimes and consequently discharge-related factors influence their faunal composition and abundance. Baharini is perennial, slow-flowing with low steady discharge, a higher conductivity and a low conductivity gradient along its course. Njoro is intermittent, fast-flowing with a higher and variable discharge, lower conductivity and a steeper conductivity gradient.
Ostracods, copepods, corixids and chironomids dominate the fauna of the two streams. Due to permanence and steady surface stream flow, Baharini has a higher number of species and higher abundances than Njoro River. However, in spite of the intermittent nature of Njoro River, the composition of its fauna is similar to that of Baharini during periods of relatively steady discharge. Similarity in the substrate 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, fast colonisation of the lowland intermittent stretch of 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 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 Njoro was completed in 27 days. Taxon richness was, however, less affected by the spates.
The substrata of Baharini and Njoro are predominantly fine-grained and hence the relative proportions of the larger grain size classes are important determinants of the faunal distribution in the streams. High faunal abundances, taxon richness and most species of the benthic fauna (dominated by chironomids) are associated with large grain size substrates with low organic carbon concentrations.
Faunal abundance and number of species decline with increasing conductivity, though this relationship is not significant over the intermediate conductivity ranges and a number of species show a wide distribution with respect to conductivity. Thus an appreciation of scale and consideration of other intervening factors is fundamental to any interpretation of the relationship between conductivity and the fauna in the two streams.