Ostracoda and Foraminifera as bioindicators of (aquatic) pollution in the protected area of uMlalazi estuary, South Africa

To mitigate ecological and health risks, implementing a comprehensive multidisciplinary monitoring strategy is imperative. This approach aims to effectively identify and record potential declines in water quality and ecological conditions. Utilizing cost-effective and efficient monitoring tools is crucial, especially for developing nations. Despite the previously reported uMlalazi River's pristine status within a protected natural reserve at South Africa's eastern coast, our findings challenge the assumption of its cleanliness, emphasizing the need for ongoing proactive monitoring. Here we reassess the pollution levels and ecological status of aquatic life of the river, and use this to enhance the indicator value of microfauna in South Africa. We analysed 25 surface sediment samples from the uMlalazi estuary, covering a salinity range from oligohaline to euhaline, with a focus on marginal marine Ostracoda and Foraminifera as potential indicators. All samples contained Ostracoda and Foraminifera, with the exception of two. Among the identified ostracod species, there were 17 species belonging to 14 genera. Typical taxa are the brackish water species Perissocytheridea estuaria, Sulcostocythere knysnaenis, and Australoloxoconcha favornamentata. We identified 19 Foraminifera species from 16 genera, with dominant taxa such as Ammonia sp., Quinqueloculina sp., and Miliolinella sp. Three distinct assemblages were observed: A) Ammonia sp. and Quinqueloculina sp., with very low diversity and abundances in general, located along the river course at stations exceeding Pollution Load Index (PLI), which indicates deterioration of sites quality; B) Ammonia sp., Quinqueloculina sp., and Sulcostocythere knysnaenis associated with higher salinity and lower PLI; C) Ammonia sp., Quinqueloculina agglutinans, and Cribroelphidium articulatum located in mudflats with minimal PLI. Our findings align with the commonly observed diversity trend, which indicates reduced species diversity corresponding to elevated pollution levels. Notably, the examined samples revealed a range of Foraminiferal Abnormality Index (FAI) up to 23%, exhibiting anomalies such as multiple tests, changes in coiling, and abnormal chamber shapes. Geochemical analysis indicates that the catchment is subjected to substantial anthropogenic pressure, as evidenced by elevated concentrations of heavy metals, sulphur, and microplastic. Sugarcane farming, urban sewage, titanium mining, and fish farming are the primary sources of pollution in the catchment area. Ongoing investigations in South African estuaries are expanding our dataset and will contribute to a better understanding of the species-specific responses of Ostracoda and Foraminifera to anthropogenic pressure.