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Abstract

Dynamics in organic matter processing, ecosystem metabolism and trophic sources for consumers in the Mara River, Kenya.

Abstract

The overall objective of this dissertation was to better understand the functioning of the Mara River by assessing the spatio-temporal dynamics of organic matter sources and supply under different land-use and flow conditions and the influence of these dynamics on energy flow for consumers in the river. Benthic macroinvertebrates from open- and closed-canopy streams were collected and classified into functional feeding groups (FFGs) using gut content analysis. A total of 43 predators, 26 collectors, 19 scrapers and 19 shredders, were identified. Species richness was higher in closed-canopy forested streams where shredders were also the dominant group in terms of biomass. Seven shredder taxa occurred only in closed-canopy forested streams highlighting the importance of maintaining water and habitat quality, including the input of leaf litter of the right quality, in the studied streams. Subsequently, the composition of macroinvertebrate functional feeding groups (FFGs) and the ecosystem process of leaf breakdown was used as structural and functional indicators, respectively, of ecosystem health in upland Kenyan streams. Coarse- and fine-mesh litterbags were used to compare microbial (fine-mesh) with shredder + microbial (coarse-mesh) breakdown rates, and by extension, determine the role of shredders in litter processing of leaves of different tree species (native Croton macrostachys and Syzygium cordatum and the exotic Eucalyptus globulus). Breakdown rates were generally higher in coarse- compared with fine-mesh litterbags for the native leaf species and the relative differences in breakdown rates among leaf species remained unaltered in both agriculture and forest streams. Shredders were relatively more important in forest compared with agriculture streams where microbial breakdown was more important. Moreover, shredder mediated leaf litter breakdown was dependent on leaf species, and was highest for C. macrostachys and lowest for E. globulus, suggesting that replacement of indigenous riparian vegetation with poorer quality Eucalyptus species along streams has the potential to reduce nutrient cycling in streams. To study organic matter dynamics in these streams, the influence of land use change on the composition and concentration of dissolved organic matter (DOM) was assessed and its links with whole-stream ecosystem metabolism was investigated. Optical properties of DOM indicated notable shifts in composition along a land use gradient. Forest streams were associated with higher molecular weight and terrestrially derived DOM whereas agriculture streams were associated with autochthonously produced and low molecular weight DOM and photodegradation due to the open canopy. However, aromaticity was high at all sites irrespective of catchment land use. In agricultural areas high aromaticity likely originated from farmlands where soils are mobilized during tillage and carried into streams and rivers by runoff. Gross primary production (GPP) and ecosystem respiration (ER) were generally higher in agriculture streams, because of slightly open canopy and higher nutrient concentrations. Lastly, natural abundances of stable carbon (γ13C) and nitrogen (γ15N) isotopes was used to quantify spatial and temporal patterns of carbon flow in food webs in the longitudinal gradient of the Mara River. River reaches were selected that were under different levels of human and mammalian herbivore (livestock and wildlife) influences. Potential primary producers (terrestrial C3 and C4 producers and periphyton) and consumers (invertebrates and fish) were collected during the dry and wet seasons to represent a range of contrasting flow conditions. Overall periphyton dominated contributions to consumers during the dry season. This study highlights the importance of large mammalian herbivores on the functioning of riverine ecosystems and the implications of their loss from savanna landscapes that currently harbour remnant populations.