Evaluating multiple stressors on aquatic ecosystems in an urban environment.

Abstract

The issue of compromised aquatic ecosystems is a global dilemma; with existing and new stressors emerging, freshwater ecosystems continue to degrade tremendously. Multiple stressors exist in aquatic ecosystems, from invasive species to overexploitation of aquatic resources, habitat degradation, flow modifications and pollution. The uMsunduzi catchment in Pietermaritzburg, KwaZulu-Natal Province, South Africa, is one such stressed freshwater body. The present study was undertaken to determine the major threats to aquatic ecosystems and the possible strategies to mitigate the factors that compromise freshwater biodiversity, health, and ecological status. The first problem was to investigate the impact of anthropogenic barriers along the longitudinal pathway of the rivers. Hence, an extensive literature review on managing river barriers was conducted. This systematic review showed a need for river connectivity restoration projects in Africa, particularly in South Africa. The Northern Hemisphere countries are more advanced in restoring river connectivity by removing barriers or retrofitting fishways. A prevalence of physical barriers in the uMsunduzi catchment was found, and most barriers did not have fishways or fish passage structures. The fish communities, together with their associated habitat features, within the uMsunduzi mainstem and tributaries, were assessed to deduce which environmental factors influence the fish communities' structures in the system. There was a clear indication of a decline in species diversity and deterioration of the ecological health of the uMsunduzi catchment. Of the 18 expected fish species, according to the Freshwater Biodiversity Information System (2023), only 50% of these were caught. This is concerning, especially as the “near threatened” (IUCN status) Enteromius gurneyii (redtail barb) was not caught, and Amphilius natalensis (Natal mountain catfish) was caught once in low abundance. The uMsunduzi River had a highly deteriorated ecological integrity per the Fish Response Assessment Index (FRAI) scores, especially close and downstream of the city centre where the industrial areas are concentrated. Similarly, there is a great deal of microbial contamination, putting the uMsunduzi River in a matter of public health. There was also a significant presence of magnesium, calcium and fatty acids. In conclusion, the connectivity, ecological health and water quality of the uMsunduzi catchment were compromised, giving a clarion call for mitigation and management actions on the systems.