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

Wiener, Kenwinn Dane (2018-03)

Thesis (MScConEcol)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Oewersones van hoofwater riviere in bergopvangs gebiede van die Kaapse Floristiese Streek (KFS) is hewig getransformeer en degradeur deur houtagtige uitheemse indringer plante (UIPs). Houtagtige UIPs, veral stikstof fikserende Australiese Acacia spp. (Acacia mearnsii, A. longifolia en A. saligna) wat wydverspreid langs hierdie riviere voorkom, onderdruk en vervang inheemse fynbos plante, verminder stroomvloei en verander onderliggende grond se koolstof en stikstof siklusse. Die in-stroom effekte van die UIPs op sediment -en voedingstof ladings bly egter onbeslis. Die impak van die UIPs op water hulpbronne en biodiversiteit het die implimentering van grootskaalse opruimings insiatiewe soos die Werk vir Water (WfW) program regverdig. Hierdie program is hoofsaaklik gefokus op die verwydering van houtagtige UIPs met die doel om biodiversiteit te bewaar en water produksie te verhoog. Die verwydering van UIPs vanuit oewersones lei dikwels tot verdure versteurings en die gebiede word gereeld oorgelaat om op ‘n natuurlik wyse te herstel. Die doel van hierdie studie was om relatiewe sediment ladings en voedingstof konsentrasies in bergstrome wat geassosieer word met inheemse fynbos, uitheemse Acacia mearnsii en Acacia-verwyderde oewerwalsones te kwantifiseer. Veldwerk was uitgevoer in 'n kombinasie van bergstroom en bergstroom-oorgangsones van twee standhoudende rivier stelsels binne in die KFS. Gewilde tyd-geintegreede sediment monsterneming metodes was gebruik om tydelike en ruimtelike relatiewe sediment ladings op drie plekke op die onderskeie rivier stelsels te monitor. Elk van die geselekteerde rivier stelsels het bestaan uit; een inheemse fynbos, een uitheemse (hoofsaaklik A. mearnsii) en een skoongemaakte (skoongemaak van A. mearnsii meer as 10 jaar gelede) oewerwal-areas. Die sediment monsternemers is maandeliks vir 'n tydperk van een jaar geneem. Laboratorium ontledings was gefokus op 'n seleksie van fyn sediment (< 63 μm) eienskappe wat hoofsaaklik bestaan uit deeltjie grootte samestelling, totale koolstof, totale stikstof en totalefosfor inhoud. Dit was gekombineer met seisoenale rivier water monsters -en stadium metings. Dit is gevind dat relatiewe sediment ladings nie net afhangklik was van die bydraende opvanggebied nie, maar ook van geomorfologiese eienskappe (bv. kanaal morphologie, helling en topografie) en indringer status. Relatiewe sediment ladings verminder met 'n toename in bydraende opvanggebied op die Du Toit’s Rivier, wat 'n omgekeerde verhouding tot gevolg het. Daar is afgelei dat die oorheersende sediment bron op die Du Toit’s Rivier bestaan hoofsaaklik uit bank erosie en sediment deposisie in die rivier kanaal. Indringer -en skoongemaakte oewersones het geen effek gehad op sediment ladings in die Du Toit’s Rivier nie. Dit dui daarop dat die geomorfologiese eienskappe van die kanaal en rivier oewersones die onmiddellike kanaal-oewerwal konnektiwiteit verminder, en die effek van UIPs en verwydering daarvan op relatiewe sediment ladings beperk. In teenstelling hiermee was die toename in relatiewe sediment vragte in die Wit Rivier goed korreleer met die groote van die bydraende opvanggebied. Dit dui op 'n stelsel waar sediment lewering afgelei word uit 'n kombinasie van rivier oewersones en sediment deposisie in die rivier. Dus, die verhoogde relatiewe sediment lading in die uitheemse en skoongemaakte areas veroorsaak was deur verhoogde inset van sediment uit die uitheemse A. mearnsii en skoongemaakte oewersones. Alhoewel relatiewe sediment ladings in die skoongemaakte areas minder was as in die uitheemse A. mearnsii area in die Wit Rivier, was dit steeds ongeveer twee keer hoër as in die fynbos area na meer as 10 jaar van opruiming. Die ontleding van temporale en ruimtelike variasie het getoon dat voedingstof konsentrasies beduidend verskillend was tussen indringer status en oor seisoene. Die gemiddelde totale stikstof inhoud in sediment van die uitheemse A. mearnsii area op die Wit Rivier was tot 30% en die totale fosfor tot 43% hoër as in die fynbos-area. Hierdie resultate ondersteun die hipotese dat uitheemse A. mearnsii plante en verwydering daarvan 'n beduidende invloed op sediment verwante voedingstowwe in fynbos bergstrome van die KFS kan hê. Ten opsigte van die tydelike patrone, was dit duidelik dat voedingstof konsentrasies laag was gedurende die nat winters maande met beduidende hoër konsentrasies in die warm somer maande. Hierdie patroon dui daarop dat blaarval syfers, relatiewe sediment ladings en hidrometrologiese faktore (bv. Rivier vloei volume, stroom temperatuur en reënval) ‘n belangrike rol speel in die temporale dinamika. Geen beduidende tendense is opgemerk in die voedingstof konsentrasies in die rivier water ten opsigte van indringer status nie. Terwyl die konsentrasies van voedingstowwe in die skoongemaakte area merkwaardig laag was, bly fosfor vlakke aansienlik hoër as in die fynbos-area op die Wit Rivier en dui dit op 'n lang-durige effek wat voortduur na die verwydering van uitheemse A. mearnsii plante. Die beduidende korrelasie tussen yster en fosfor in hierdie stelsels dui daarop dat fosfor biologies onbeskikbaar is en opbou in hierdie stelsels. Verder is die impak van UIPs en die verwydering daarvan op sediment- en voedingstof ladings sterk afhanklik van die kanaaloewer konnektiwiteit en word hoofsaaklik beïnvloed deur kanaal morfologie. Hierdie studie dra by tot die kennis van sediment dinamika en in-stroomse biogeochemiese prosesse van fynbos bergstrome en die mate waartoe hierdie prosesse beinvloed word deur uitheemse A. mearnsii plante en opruimings aktiwiteite. Daarbenewens sal 'n beter begrip van hoe verskillende opvanggebiede en kanale sediment en geassosieerde voedingstowwe vervoer, sal waarde volle insigte lewer oor die gebruik van opvangsgebied – en kanaal eienskappe as 'n bestuurs instrument om die vervoer van sediment en voedingstowwe na riviere te verminder.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103941
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