Investigating the potential of floating wetlands in small farm dams in the Western Cape, South Africa

[摘要] ENGLISH ABSTRACT: The rapid deterioration of surface water quality and habitat quality due to urban sprawl,intensive agricultural practices, land degradation and spread of invasive alien species is anunfortunate reality in many parts of the world. Urgent intervention is necessary to mitigatethe negative impacts on riparian and wetland ecosystems to preserve their functioning.Floating wetland studies across the globe have highlighted their potential to successfullypurify various wastewater types, attract biodiversity and promote environmentalawareness. Selection of appropriate plants to populate these wetlands is key for efficientnutrient removal, surviving herbivory by aquatic birds, excessive wind and fluctuatingnutrient levels. I investigated the suitability of floating wetlands to purify agricultural run-offand determined plant survival success on small farm dams in the Western Cape, SouthAfrica. The removal efficiency of nitrogen and phosphorus was investigated in a mesocosmexperiment planted with three endemic wetland plant species (Cyperus textilis, Juncuslomatophyllus and Prionium serratum) over one month. In terms of monitoring plantsurvival success on floating wetlands, it is important to understand the drivers of plantsurvival and growth as plants are exposed to various threats and pressures in open farmdam settings. Three field visits to existing floating wetlands were conducted over a period ofa year in order to better understand the survival success of certain plant species. Plantsurvival rate was determined by expressing the number of individuals that since theprevious field trip as a percentage, whilst growth rate was measured using plant height, andexpressed as a percentage of the size of the individual at the time of the first and third fieldvisit.Low nitrate, phosphate and ammonium uptake rates, yet high removal efficiencies wereobserved across treatments in the mesocosm experiment which suggests that mostnutrients that were added into the system were removed successfully. The lack of asignificant difference between planted floating wetlands and the control (unplanted) isattributed to insufficient nutrient enrichment in the experiment. Future studies should testmore eutrophic conditions. All plants survived and thrived in the simulated conditions andwere responsible for the uptake of some nutrients, however, non-significant differences innutrient storage (roots vs shoots) were observed. Visual observations on plants on floatingwetlands implemented on farm dams in South Africa suggest that herbivory by aquatic birdsappears to be a major threat to the successful establishment and survival of plants. Changesin water quality (pH, dissolved oxygen, run-off vs effluent storage, water temperature, totaldissolved solids and salinity) appear to be significant drivers of plant survival and plantgrowth. The following species are recommended for use on floating wetlands in theWestern Cape of South Africa due to high survival rates: Cyperus dives, Cyperus fastigiatus,C. textilis, Juncus effuses and Schoenoplectus scirpoides. Floating wetlands attractedbiodiversity such as aquatic birds, dragonflies and terrapins. Thus floating wetlands appearto be successful in attracting biodiversity to small farm dams. This study provides importantbaseline information on the potential use of floating wetlands for the dual purpose ofnutrient removal and attracting biodiversity.

[发布机构] Stellenbosch University