An integrated modelling approach to the management of freshwater inflow to South African estuaries.
Estuaries are recognised for their biological diversity and productivity, as well as the vital role they play in providing habitat for organisms which utilise them as nurseries and feeding grounds. In many parts of the world concern has been expressed that the important functions and values of estuaries are being increasingly impacted upon by human activity. In South Africa diminishing freshwater inflow is a particular concern as this has led to an increase in the frequency and duration of mouth closure, which together with other factors has resulted in a marked deterioration in the condition of many estuaries. Global environmental imperatives require an approach to ecosystem management that is defensible and sustainable in the long term. Current approaches to estuary management in South Africa do not meet these criteria, and consequently, this study set out to develop methodologies to address these shortcomings. Three modelling approaches are presented, which can be used independently, or conjunctively, in defining the freshwater requirements of estuaries. The models assess the consequences of change in freshwater inflow for (i) juvenile fish which utilise estuaries as nurseries, (ii) the availability of intertidal and species specific habitats, and (iii) the population structure and production of a common estuarine invertebrate (Upogebia africana), endemic to the region. These techniques are applied in a case study of the Great Brak estuary (Western Cape, South Africa). The results indicate the utility of the approach and are supported, in part, by the findings of a long-term monitoring programme. The study also recognises the need for resource management to occur in the context of an integrated framework, which includes the explicit definition of ecological goals. Such a framework is presented, and is consistent with the Ecological Society of America's guidelines on sustainable ecosystem management. As this approach has been devised to be applicable to South African estuaries, characterised by poor data availability, it is anticipated that methodologies will be equally applicable to estuaries in other developing countries with a similar lack of data. The methodologies also extend current international approaches to the management of estuary freshwater inflow, and would therefore be of value to estuaries in the United States of America, Australia and other regions where diminishing freshwater inflow has been raised as a concern.