Leaf litter transport and retention in a tropical Rift Valley stream: an experimental approach

The dynamics of leaf litter transport and retention in two contrasting reaches in the Njoro River, Kenya, is reported, analysed and discussed. The study focussed on two aspects: experimental leaf litter transport and retention in the Maryjoy and Turkana reaches which differ in structural heterogeneity and natural leaf litter transport in the two reaches. Leaves of Dombeya goetzenii, Syzygium cordatum and Grewia forbesii were dyed differently with 'Herbol(R)' dyes and used to elucidate the dynamics of leaf transport and retention. Experimentally, about 40% of each of the three leaf species released into the water column travelled beyond the experimental 50-m mark in the Maryjoy reach (devoid of major retention structures) and about 95% of the leaves were retained within the Turkana reach (with major retention structures) and never reached the 50-m mark. Based on the calculated average leaf travel distance (1/k), the leaves of D. goetzenii could have drifted 10-113 m in the Turkana reach and between 60 and 127 m in the Maryjoy reach. The transport trends of the three leaf species were reach- but not leaf species-specific. The efficiency of trapping the drifting leaves differed from one retention structure to another in both reaches. Rock outcrops and debris dams were the main descriptors of the stream heterogeneity in the Turkana reach and retained about 55% and 20% of the experimental leaves, respectively. The leaves of S. cordatum dominated the leaf litter transported naturally within the two reaches, contributing 214.4 +/- 18.7 g d(-1) to 274.1 +/- 36.8 g d(-1) (+/- standard error) of the leaf litter biomass in the Turkana reach. It was estimated that 640.9 kg of leaf litter are exported to Lake Nakuru by the Njoro River annually.