Monitoring the health of the rivers of the Durban Metropolitan Area using fresh water invertebrates. A pilot study.
This document is aimed at all parties involved with conservation, planning and management of rivers within the Greater Durban Metropolitan Area, South Africa. The intentions of this study were to provide information on the health of the rivers using freshwater invertebrates and to identify areas where investigative efforts should be focused. In doing so, this document incorporates two data processing methods or techniques used in biomonitoring, the South African Scoring System (SASS) Rapid Bioassessment Technique (RBA) version 4 and the Integrated Habitat Assessment System (IHAS version 2c). The history, current status and future prospects and potential benefits of biological indicators both globally and in South Africa are reviewed and discussed. The RBA (SASS4) involved the standardised collection of samples using the "kick and sweep" technique from defined habitat types at representative sites on river stretches using a standardised net following defined methods. The total score per site is calculated for each family represented in the sample, which is then summed to give the SASS4 score. Lower scores are given to families with a high level of tolerance for polluted waters. The Average Score Per Taxon (ASPT) computation is obtained by dividing the SASS4 total by the number of taxa (families) in the sample. Both scores were considered when determining water quality impairment. The number and abundance of the different taxa and the number of biotopes present were considered as other measures of the river condition. The presence of numerous families of highly tolerant organisms (sludge worms and leeches) usually indicated poor water quality as represented in the Umlaas River, iSiphingo River, Tongati River, and the Ohlanga River. Several different types (or taxa) of stoneflies, mayflies and caddisflies (higher biodiversity) indicated a healthy site for example the iLovu River. Moderate river sites were characterized by declination in invertebrate diversity. Invertebrate diversity declines as the degradation of water quality increases. The manner in which SASS4 scores varied with "degree of anthropogenic impact" showed that the assessment does yield results, which follow water quality changes, provided that when SASS4 scores are less than 50, little attention is given to ASPT. At intermediate and high impacted sites ASPT did not follow habitat quality. However at low impacted water quality sites both SASS and ASPT tended to be greatest where habitat quality was the best. This study has identified that water quality "appears" to have a greater effect on macro invertebrate communities than the physical habitat and SASS distinguished sites with differing levels of water quality. This method was not designed to enable the exact nature of the disturbance to be determined, and it was intended that once an impairment of water quality had been established, it would be further assessed via intensive chemical and other studies. The use of more ecological indicators for example the inclusion of IHAS (version 2c) allowed a more comprehensive assessment of river health and more confidence could be linked to the outcome.