| dc.description.abstract | South Africa’s water resources are limited and, in global terms, scarce. The demand for water is, however, growing continuously due to population growth, a developing economy, global warming, the spreading of alien plant species and the urgent need to supply water services to millions of people. In the past wetlands were perceived as having no value; they were dammed, overgrazed, mined for their soils, polluted with chemicals and litter, or drained for agriculture/housing development. Wetlands have also been converted for other land-use purposes, with more than 50% of the country's wetlands already lost, thus monitoring of wetland water quality is needed in order to make sound management decisions. Wetlands are very important ecosystems; they prevent flooding, they filter and purify surface water, and are important in providing habitats for various animals and plants. Diatoms, which form part of the group of protists known as algae, are good environmental indicators and are one of the key groups of organisms that can reflect ecological status.
Diatoms form an important part of the ecosystem as they, along with other algal groups, are primary producers and form the base of aquatic food webs. Diatoms are also excellent water quality indicators, they respond to climatic and chemical influences and their siliceous frustules preserve well over long periods of time in sediments. For all of these reasons, diatom analysis of wetlands can reveal much about environmental change at various timescales and can also be used to determine historical environmental conditions. Moreover, they can be used to monitor ongoing water quality in a given wetland and act as an early warning signal should human impacts intensify. Although diatoms are used regularly as indicators in rivers both internationally and in South Africa, less is known about diatom communities in wetland systems in South Africa. Several diatom indices have been successfully applied in South African rivers and in addition the same indices in a preliminary study on wetlands in the Western Cape. The study indicated that diatoms can be used for inferring aspects of the water quality of wetlands since changes in physico-chemical quality correlate well with the diatom index scores.
The main aim of the present project was to contribute information about the distribution and environmental preferences of diatoms in wetlands in the Western Cape. During the 1980s wetlands were investigated in the Cape Floristic Region and re-examined in the present study. A part of this study was to determine whether diatoms can be used to monitor the water quality of the wetlands in the Western Cape. A total number of 28 wetlands with different types of hydrogeographical (HGM) characteristics were investigated during the study. A total of 324 diatom species belonging to 60 genera were found in samples collected for the present study. Species sensitive towards pollution were observed during the study namely; Eunotia genuflexa, Eunotia flexuosa, Eunotia sp., Rhopalodia gibberula, Navicula reichardtiana, Achnanthidium crassum, Brachysira brebissonii and Tabellaria flocculosa.
Information pertaining to ecological tolerances of the species was obtained using statistical analysis, in particular, Canonical Correspondence Analysis (CCA). Species typically considered to be from Europe were identified in this and responded in a similar way to water quality variables found in Europe, suggesting that they are sub-cosmopolitan.
In particular, significant correlations existed between the Specific Pollution sensitivity Index (SPI) and certain water quality variables, such as dissolved oxygen, electrical conductivity, pH, temperature, orthophosphate and turbidity, but no correlations existed between nitrate, nitrite and ammonium. Despite the once-off sampling regime and the little studied diatom flora of the south-western Cape wetlands, these wetlands could be separated into water quality classes based on an analysis using diatom indices. Bio-assessment using diatoms, therefore represents a potential tool for assessing the ecological condition of wetlands of the south-western Cape of South Africa. The feasibility of using diatoms as a national tool for monitoring wetland water quality needs to be investigated objectively in conjunction with other potential biotic indicators. The SPI showed potential for describing water quality in freshwater wetlands but caution should be used in interpreting results for naturally occurring saline pans. A new indicator system may need to be developed for these systems. While diatom samples from different substrata gave similar results, further investigations are needed into the effect of substratum on the ‘ecological condition’ or water quality classes identified from diatom analyses. The preliminary data set herein illustrated the potential of the use of these benthic diatoms as bio-indicators in wetlands | en_US |
