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.