Abstract:
Steam ecosystem structure and function. Riffles and pools exhibit heterogeneity in physical, chemical, and biological characteristics, which in turn may affect stream function. However, information on the effects of land use and streambed topography on the functioning of tropical streams, is still limited. Leaf litter decomposition, a key ecosystem process that links trophic interactions, was used in this study as a functional indicator of stream ecosystem. This study was set up to assess the influence of land use and streambed topography on decomposition rates of Syzygium guineense and Eucalyptus saligna leaf litter in the Kamweti River, Kenya. The decomposition periment was conducted following the standard litter bag technique. A total of 400 coarse-mesh (0.5mm) litter bags were used to enclose 5 } 0.05 g of each plant species leaf litter, incubated in the selected sites and thereafter 5 replicates of litter bags were randomly
retrieved after an interval of 1, 3, 8, 14, 21, 28, 35, 42, 49 and 56 days of incubation. Also, the maximum leaching time and leaf nutrient content were evaluated in the laboratory. Triplicate benthic and drift macroinvertebrates samples were collected to help answer the question whether there was a similarity between leaf litter-associated, drifting and benthic macroinvertebrates. Benthic and drift macroinvertebrates were sampled using 0.5mm kick net and 0.1mm drift net sampler, respectively. Decomposition rates were estimated using the negative exponential decay model. Linear Mixed effect Models were used to evaluate the effects of land use and streambed topography on leaf litter domposition rates, macroinvertebrates and physico-chemical parameters. Syzygium guineense consistently had significantly higher concentrations of all the nutrient parameters than Eucalyptus saligna, except for total nitrogen. The maximum leaching time for both plant species was estimated to be 12 hours. Forested sites (Syzygium = 0.0269 } 0.004, Eucalyptus = 0.0408 } 0.004) had higher decomposition rates than agricultural sites (Syzygium=0.0205 } 0.004, Eucalyptus =0.0269 } 0.006), although not significantly different(p> 0.05). Riffle habitats had significantly higher decomposition rates (p<0.05) than pool habitats across the two land uses. Eucalyptus had significantly higher decomposition rate than Syzygium guineense (p=0.0007) across the land use. Macroinvertebrates colonizing the leaf litters were largely more similar to those from
benthic zone, but different to those from drift. Overall, Syzygium sp was found to be a better quality leaf litter nutritionally than Eucalyptus sp. Streambed topography had a significant effect on leaf litter decomposition than land use. Eucalyptus afforestation should be discouraged as it contributes leaf litter with a lower nutritional value to stream ecosystems. Conservation and management efforts should be directed to the local scale factor as opposed to only riparian and catchment factors.
