Linking nitrogen pollution in estuaries to rocky shores: A stable isotope approach

Shivanee Pillay1, Deborah Robertson-Andersson1; AJ Smit1,2

1UKZN 2UWC

Many coastal regions in KwaZulu-Natal (KZN) are impacted by sewage effluent, but the knowledge and extent of this impact remains poor. Additionally, previous impact studies were restricted to community dynamics and nutrient trends in estuaries and coastal habitats. However, this information can be supplemented by tracing the fate of nitrogen and its sources to coastal systems. An accepted methodology is the use of stable isotopes of Nitrogen (N) to determine anthropogenic origin to both terrestrial and marine systems. This study aimed to determine anthropogenic sources and impacts of N-pollution to three estuaries in southern KZN. To achieve this, stable isotope analysis together with nutrient and community composition data, were used in the selected temporarily open/closed estuaries. Source differentiation between the open (summer) and closed (winter) phase of the estuaries and the importance of anthropogenic nitrogen (via estuarine input), was made possible using two species of rocky shore macroalgae, Hypnea spicifera and Jania intermedia. The δ15N in algae and mussels` provided evidence that estuarine inputs were more important during the open phase than closed phase and were from anthropogenic sources in two sites (Mhlugwa and Kongweni). Additionally, stable isotope analysis, physicochemical water properties and determination of community composition, when integrated, were found to be useful tools in nutrient pollution studies. The condition of sites chosen for this study indicated a need for mitigation of nutrient pollution in estuaries and a greater need for development of Wastewater Treatment Works along the coast of KZN.