Application of the relative risk model for evaluation of ecological risk in selected river dominated estuaries in KwaZulu-Natal, South Africa

Effective environmental management and restoration of impacted estuaries in South Africa necessitates a holistic understanding of the contribution of various stressor-related impacts throughout the catchment. Ecological risk assessment for aquatic ecosystems is an important tool for water resource management. In this study, we describe results of a preliminary assessment that was conducted to evaluate the relative risks of multiple anthropogenic stressors currently acting within the catchments of uMvoti, Thukela and aMatikulu/Nyoni estuaries using Bayesian Network Relative Risk Model (BN-RRM) framework. Four socio-ecological endpoints selected for the present study included biodiversity habitat, safe environment, fisheries and productivity. We constructed a conceptual model which depicted potential and effect pathways from the source, to the stressor, to the habitat and to the endpoint. We also developed five scenarios (including historical and future scenarios) to predict the potential risk distributions in different proposed scenarios. Results revealed that productivity was the endpoint at the lower risk in all the estuaries and all scenarios except for scenario 5. Results also showed that scenario 3 which is a scenario before major resource development had the lowest risk scores for all the endpoints. Scenario 4 (year 2025 if no laws and management measures are implemented) had the highest risk scores for all the endpoints. Overall endpoints generally displayed low to medium risk throughout all scenarios (except scenario 3) and different flows. All endpoints generally displayed zero risk in scenario 3. All endpoints were at a highest risk in the uMvoti Estuary followed by aMatikulu/Nyoni and then Thukela Estuary. Results highlighted that in the uMvoti and Thukela estuaries, people were at a higher risk when compared with the ecological components of these systems as social endpoints displayed higher risk scores than the ecological endpoints, however the opposite was observed in the aMatikulu/Nyoni Estuary. This study provided the foundation for evaluating the risks of multiple stressors in the catchments of these estuaries to a variety of endpoints. Management options and research should focus on collecting necessary data and information to refine the developed RRM. By establishing such framework, we believe that stakeholders within the catchments of these systems together with government organisations will be able to make more informed and risk-based management decisions pertaining restoration and rehabilitation options for these three estuaries.