Masters Degrees (Conservation Ecology and Entomology)

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Browsing Masters Degrees (Conservation Ecology and Entomology) by Subject "Acacia"

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    Quantifying the impacts of invasive N2-fixing plants on relative sediment loads and nutrient concentrations in mountain streams of the Cape Floristic Region, South Africa
    (Stellenbosch : Stellenbosch University, 2018-03) Wiener, Kenwinn Dane; Simaika, John; Jacobs, Shayne Martin; Grenfell, Suzanne; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Riparian zones of headwater rivers in mountain catchments of the Cape Floristic Region (CFR) have been severely transformed and degraded by the invasion of woody trees species. Woody invading alien plants (IAPs), particularly N2-fixing invasive Acacia spp. (Acacia saligna, A. longifolia and A. mearnsii) that are widespread along these rivers, rapidly suppress and replace indigenous fynbos vegetation, reduce stream flow and alter underlying soil carbon (C) and nitrogen (N) cycles. However, the in-stream effects of invasion on sediment and nutrient loading remain unquantified. The impacts of IAPs on water resources and biodiversity have justified extensive IAP clearing initiatives such as the Working for Water (WfW) programme. This program has focussed mainly on the clearing of woody invasive plants with the aim to conserve biodiversity and increase water production. The clearing of IAPs often results in further disturbances and areas are often left to recover without intervention. The aim of this study was to quantify relative sediment loads and nutrient concentrations in mountain streams associated with natural, Acacia invaded and Acacia cleared riparian zones. Fieldwork was performed in a combination of mountain stream and mountain stream transitional zones of two perennial river systems within the CFR. Common time-integrating sediment sampling methods were used to monitor temporal and spatial relative sediment loads at three locations on the respective river systems. Each river consisted of one near pristine site (fynbos), one invaded site (predominantly A. mearnsii); and one cleared site (cleared from A. mearnsii more than 10 years ago). The suspended sediment (SS) samplers were emptied monthly for a period of one year. Laboratory analyses focused on a selection of fine sediment (< 63 μm) properties consisting mainly of particle size composition, total carbon, total nitrogen and total phosphorus content. This was combined with seasonal water samples and instantaneous river stage measurements. The relative sediment loads were found not to be entirely dependent on catchment contributing area, but also on geomorphic characteristics (i.e. channel morphology, slope and topography) and invasion statuses. Relative sediment loads decrease with an increase in contributing area on the Du Toit’s River, resulting in an inverse relationship. It was inferred that the dominant sediment source on the Du Toit’s River is from channel bank erosion and sediment deposits on the riverbed. Invasion and clearing appeared to have no effect on sediment loads at the Du Toit’s River. This suggests that the channel and riparian zone characteristics might reduce immediate sediment connectivity to the channel, thus limiting the effect of invasion and clearing on relative sediment loads. In contrast, the magnitude of increase in relative sediment loads on the Wit River scales well with catchment contributing area, which is indicative of a system where sediment supply is predominantly derived from a combination of in-channel and catchment sources. Thus, the elevated relative sediment load in the invaded and cleared sites could reflect enhanced input of sediment from the A. mearnsii invaded and cleared riparian ecotones. Although relative sediment loads were reduced to levels similar to natural fynbos sites, relative sediment loads at the cleared site on the Wit River remained around two times higher after more than 10 years of clearing. The analysis of temporal and spatial variation showed that nutrient concentrations differed significantly amongst invasion statuses and across seasons. Mean total nitrogen content in infiltrated sediments of the invaded site on the Wit River were up to 30% and total phosphorus up to 43% higher than in the fynbos site. These results support the hypothesis that A. mearnsii invasions and clearing may have a significant influence on sediment-associated nutrients in streams of the CFR. Nutrient concentrations in sediments were significantly lower during the wet winter months compared to warm summer months. The temporal results suggest that peak litterfall rates, relative suspended sediment loads and hydrometeological factors (i.e. discharge, stream temperature and rainfall) are important drivers of temporal dynamics. No significant spatial trends were noted in the concentrations of nutrient concentrations in surfaces water with regards to invasion statuses. While clearing may have reduced nutrient concentration to levels that resemble a natural state, P levels remained significantly higher than in the fynbos site on the Wit River and are suggestive of a long-lasting effect after the removal of invasive A. mearnsii. The significant relationship between iron and phosphorus concentrations in these systems suggests that P might accumulate in these systems and is biologically unavailable. Furthermore, the impacts of invasion and clearing on sediment and nutrient loads seemed to be strongly dependent on the level of channel-riparian connectivity and influenced by channel morphology. This study contributes to the knowledge of sediment dynamics and in-stream biogeochemistry of fynbos mountain streams and the extent to which these processes may be altered by A. mearnsii infestations and clearing activities. In addition, improved understanding of how different catchments and channels route sediments and associated nutrients can provide valuable insights into the use of site characteristics as a management tool to mitigate and reduce the downstream export of sediment and nutrients.