Macroinvertebrates as ecological indicators of the wellbeing of the lower uMvoti and Thukela Rivers, KwaZulu-Natal, South Africa.

Abstract

The excessive use of water resources and climate change stressors is impacting the quality and quantity of surface aquatic ecosystems in South Africa, a semi-arid country. Although South Africa is considered to be a developing nation, riverine ecosystems have already been transformed and impacted on to meet human needs. This has altered the ecological characteristics of the rivers of which more than 70% are now threatened. The National Water Act (NWA) of South Africa and associated National Water Resource Strategy (NWRS) advocates the establishment of a suitable balance between the use and protection of water resources to ensure sustainability. The implementation of NWA and NWRS is limited in some South African rivers and the quality of these vulnerable ecosystems continues to deteriorate. Knowledge is needed to evaluate the response of the riverine ecosystems to changes in environmental variables so that we can understand the socio-ecological consequences of the continued deterioration of our resources and best manage them when resource demand exceeds supply. This study focusses primarily on lower uMvoti and Thukela Rivers along with their associated tributaries (Ntchaweni and Mandeni Streams). These rivers are among the highly threatened ecosystems and that can be attributed to water resource use stressors including overexploitation, invasion by exotic species, industrial pollution and effluents, extensive agricultural practices, mining activities, increased urbanization as well as social and economic development in peri-urban and urban centres. These stressors have been identified as determinants of the degradation of aquatic biodiversity and they result in the loss of key ecosystem services. Aquatic macroinvertebrates are good ecological indicators that have been used internationally to establish robust bio-monitoring lines of evidence or tools for the monitoring and management of river ecosystems. Today a suite of international and local lines of evidence incorporating macroinvertebrates are available to evaluate the wellbeing of macroinvertebrates communities, their response to environmental variable changes and the wellbeing of the rivers they occur in. To implement the use of macroinvertebrate communities as ecological indicators of the evaluation of the wellbeing of the uMvoti and Thukela Rivers, aquatic insects, mollusks, fresh water crustaceans, annelids, and other aquatic invertebrate communities were characterised. These use of these ecological indicators is well established due to: (1) the knowledge of the tolerances of taxa to different water quality, quantity and habitat stresses, (2) the high diversity of taxa that are representative of a wide range of river ecosystem types and (3) they are abundant, easy to collect (visible to the naked eye) and easy to identify. Two community metric measure tools namely the South African Scoring System (SASS, version 5) and the Macroinvertebrate Response Assessment Index (MIRAI) were used to evaluate the wellbeing of macroinvertebrate communities of the lowland uMvoti and Thukela Rivers in this study. The ecological integrity of both rivers were found to be adversely impacted and their integrity state ranged mostly from class C (moderately modified) to class E/F (seriously or extremely modified). Reduced habitat heterogeneity and altered water quality were found to be driving factors that cause the degradation in macroinvertebrate communities. Multivariate statistical analyses were used to evaluate the responses of macroinvertebrate communities to water resource use activities associated with the uMvoti and Thukela Rivers. In the early part of the study period many intolerant macroinvertebrate taxa contributed to the structure of communities. However, towards the latter part of the study, pollution tolerant taxa dominated communities. Both rivers also showed a decreasing trend in estimated macroinvertebrates estimated abundance and number of taxa. In the uMvoti River this can be attributed to the combined effect of the urban runoff, effluence discharge from the Gledhow sugar mill and Sappi Stanger Mill, informal settlements and agricultural activities. Results reported from the Thukela River can be ascribed to the synergistic effects of water quality stressors associated with the Isithebe Industrial complex, wastewater treatment works, effluent from the Sappi mill, sugarcane plantations as well as domestic use by local communities. The outcomes of this study showed that there is not sufficient protection and management measures afforded to the systems. The requirements of the National Water Act to establish a sustainable balance between the use and protection of the water resources in the system is not being achieved. No action is being taken to mitigate pollution from major sources in the study area. Thus, an appropriate management plan and its implementation is urgently needed, with monitoring activities, to mitigate these stressors and attain a balance between use and protection of these socio-ecologically important ecosystems. Failure to implement effective management plans may result in continued deterioration of the wellbeing of the ecosystem and potential loss of biodiversity, ecosystem services and functions that these rivers provide.