eflows19
Oral
eflows
The St. Lucia estuarine lake system – a review of drought induced changes and management options.
The St. Lucia estuarine lake system comprises 80% of the estuarine area of KwaZulu-Natal, and about half of the entire South African estuarine area. It has been declared not only a World Heritage Site, but also a Ramsar Site, and is the oldest protected estuarine system world-wide. Since most estuaries along the KwaZulu-Natal coast are temporarily open/closed estuaries (TOCE) and are relatively small in size, this large estuarine system serves an important role in terms of estuarine habitat and productivity. Its value as a fish and crustacean nursery is estimated to be R 600 million per annum when it is functioning properly. The system has been altered through freshwater abstractions in the catchment, and the separation from the Mfolozi estuary with which it historically shared an opening to the sea. This connection served to stabilise the mouth and to provide freshwater to the St. Lucia system, which was especially important during drought years. During the last drought period, the mouth remained closed for
7 years (2002-present, except a short opening in 2007 induced by high ocean swell), the lakes frequently turned saline (up to 200 ppt) and water levels dropped dramatically leaving wide areas of formerly inundated areas dry (up to 70%). A hydrodynamic model has been developed, drawing attention to the importance of the connection between the Mfolozi and the St. Lucia system. As a consequence of the drought, fish recruitment is hindered which is apparent from a low number of recruits in the estuary and the nearby ocean. In addition, the estuarine resident populations of zooplankton, macrobenthos and meiofauna have declined in biomass. This information is currently being incorporated into ecosystem models, following a time-series of seasonal variations over several years of empirical data, thereby characterising ecosystem behaviour during a severe drought period. Possible management solutions for the near future are in concordance with the history of the S. Lucia-Mfolozi system: (1) Slow down and reverse catchment degradation (2) part or all of the Mfolozi floodplain is rehabilitated to natural land restoring its sediment trapping capacity and (3) restore natural functioning of the combined Mfolozi - St. Lucia Mouth.
2009-10-20
St. Lucia, Mfolozi, drought, water levels, salinity, biomass, hydrodynamic model, ecosystem model
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