Assemblage-based monitoring of the uMsunduzi River, using biotic and abiotic lines of evidence.

Abstract

Globally, urbanisation and industrialisation activities result in a multitude of impacts on freshwater ecosystems. Generally, frequent pollution of rivers from industrial effluents and sewage channels has caused the deterioration of the water quality of rivers. The availability of healthy, clean, and good-quality drinking water is a matter of concern in urban areas, particularly in developing countries such as South Africa. The instream physical barriers that lack fish ladders, development of industrial complexes adjacent to the river banks, extensive agricultural activities, abstraction of water for domestic and industrial use, and ageing sewage networks, as well as periodic industrial spills, have worsened the ecological well-being of the two socio-economically important rivers in the KwaZulu-Natal Province, South Africa, the uMsunduzi River a tributary of the uMngeni River. The present study evaluated the recovery of the uMsunduzi River, and efforts were made to improve fish population structures following a fish kill event using biotic and abiotic lines of evidence. It was hypothesised that water quality characteristics and biological communities would exhibit improvement since the occurrence of the spill. Abiotic lines of evidence included water quality and habitat characteristics, whereas biotic lines of evidence involved fish communities and fish population attributes of the indigenous KwaZulu-Natal yellowfish (Labeobarbus natalensis), macroinvertebrates, and benthic diatoms. The research was carried out quarterly from 2022 to 2023 at selected sections of the uMsunduzi system, with sites for fish collection upstream of the spill point and re-introduction sites downstream of the spill point. After the mixed product spill and fish kill in the Baynesspruit and uMsunduzi Rivers, nine sites were considered for fish community assessment and ecological monitoring. This included how assemblages respond to a wide range of environmental variables and for fish using the Fish Response Assessment Index (FRAI). The results showed that the uMsunduzi River is heavily polluted, and the stressors impact water quality, fisheries and habitat availability. The multivariate analyses indicated that the anthropogenic stressors that drive the ecological well-being and fish community structures of the uMsunduzi River could be related to changes in water quality and instream habitat modifications. The identified environmental modifications can be directly linked to human activities and flow modifications along the uMsunduzi system. Assessment of attributes of fish communities along the uMsunduzi catchment, with a focus on the KwaZulu-Natal yellowfish as an indicator species, showed a declining state of fish communities in terms of species richness and abundance. The poor fish communities include the fish collection sites upstream of the spill point selected for fish collection. The downstream site, further away from the city, showed some improvement in fish communities. The poor water quality and fish stock for collection sites highlight poor water quality and the impact of stressors on fish communities. The outcomes of this study can contribute to the holistic management, monitoring, and rehabilitation strategies for the uMsunduzi River after a fish kill event. The pre-spill fish data from (Dlamini et al. 2019) showed fish to be slowly recovering to attain the pre-spill abundances. Furthermore, biological stressors should be minimised, particularly inputs of toxic industrial effluents from industrial complexes, sewage from poorly maintained sewage systems and instream physical barriers that fragment fish populations and reduce environmental flows required by indigenous fishes. The development of evidence-based rehabilitation strategies is recommended to attain ecological health for the uMsunduzi River. Removing obsolete instream barriers and incorporating fishways on operational barriers is recommended to allow the movement of migratory fish between upstream and downstream niches. Moreover, inter-system translocation of the KwaZulu-Natal yellowfish from adjacent catchments with healthy breeding populations, such as the uMngeni River, should be collectively decided by the relevant stakeholders to improve population attributes of the yellowfish in the uMsunduzi River. The fisheries management could benefit local subsistence fishermen and improve river conditions. Before this, the genetics of the species need to be determined so as not to jeopardise the genetic fitness or resilience of the existing population. The existing legislative framework for environmental protection should be enforced for industrial and municipal operations to render minimal degradation to freshwater ecosystems. Conservancies should be established to strategically manage and protect freshwater resources and potential fisheries along the uMsunduzi system.