Flow regime variability and wastewater pollution in estuarine systems: a longitudinal assessment of ecological and societal impacts in South Africa’s Eastern Cape

Estuaries are vital socio-ecological systems that support biodiversity, provide nursery habitats, and deliver essential ecosystem services for human well-being. This study focuses on highly impacted estuarine systems affected by chronic wastewater inputs at selected Eastern Cape estuaries in South Africa. The study assesses the temporal and spatial variation patterns of pollutant loads on societal impacts and the influence of inter- and intra-annual flow variability in the estuaries. A mixed-method approach integrated laboratory analyses, field surveys, and a rare seven-year longitudinal dataset (2017–2021) of quarterly monitored physico-chemical and Potentially Toxic Element (PTEs) parameters from six estuaries: Swartkops, Qholora, Mtimubu, Mnyameni, Kowie, and Buffalo. Spatiotemporal analyses and Bayesian Hierarchical Models (BHMs) were applied to investigate ecological–socioeconomic interactions and support adaptive management. Dominant variables influencing estuarine dynamics included salinity (0.80), temperature (0.74), chromium (0.72), nickel (0.72), lead (–0.70), and oxidation–reduction potential (–0.63), reflecting the link between salinity and metal contamination, which is influenced by mixing processes and redox conditions. Increasing temperature and declining nutrient loads were especially evident in the Kowie and Buffalo estuaries. The BHMs achieved strong predictive performance (r = 0.75). Findings underscore the importance of integrated spatial planning and adaptive management strategies to strengthen estuarine resilience. The study provides a critical baseline for future monitoring and restoration, with implications for water governance, environmental policy, and climate adaptation in water-stressed regions such as South Africa and the broader Global South.