Lake St Lucia, Africa’s largest estuarine lake in crisis: Combined effects of mouth closure, low levels and hypersalinity

Digby Cyrus,Hendrik Jerling,Leon Vivier,Fiona Mackay

doi:10.4102/sajs.v107i3/4.291

Abstract

The Lake St Lucia estuarine system is the most important nursery ground for juvenile marine fish and prawns along the KwaZulu-Natal coast. The estuary mouth closed in June 2002 because of drought and remained so for 4 years and 9 months. A study to determine the impacts of extended mouth closure, hypersalinity and low lake levels on the mesozooplankton, macrobenthic invertebrates and fish fauna was initiated in 2004. Zooplankton and benthic invertebrate diversity declined, benthic invertebrate community composition changed and the diversity and abundance of fish decreased between 2004 and 2007. In the case of fish, the declines were related to die-offs in the lake and the failed recruitment of post-larvae and juveniles from the marine environment as a result of the mouth having been closed. Options for management intervention under closed-mouth conditions are limited at this time, particularly in the short term, to breaching the mouth and facilitating the inflow of sea water. In the medium term, as was the historical situation, the reconnection of the Mfolozi system to St Lucia should be viewed as a major priority.

Highlights

Drought conditions that started in 2002 resulted in the closure, in June, of the mouth of the Lake St Lucia estuarine system, on the south-east coast of Africa, which lasted 4 years and 9 months
We reviewed the impacts of hypersaline and low lake level conditions resulting from the mouth closure in June 2002 until the breaching in March 2007, on the mesozooplankton, macrobenthos and fish and the functioning of the St Lucia system as a nursery for marine fish and prawns
It is evident that the zooplankton community species richness of St Lucia has been reduced substantially during the drought period, compared to that reported by Grindley[15]

Summary

Introduction

Drought conditions that started in 2002 resulted in the closure, in June, of the mouth of the Lake St Lucia estuarine system, on the south-east coast of Africa, which lasted 4 years and 9 months. Lake St Lucia is the most important estuarine system on the south-east coast of southern Africa, comprising approximately 80% of the estuarine area of KwaZulu-Natal, South Africa.[1] The system forms part of the iSimangaliso Wetland Park, a designated World Heritage Site. Reductions in freshwater flow have resulted in periods of mouth closure[5] with salinities that have reached as high as 102 parts per thousand (ppt)[6]. These reductions have been seriously exacerbated by human interventions such as the separation of the Mfolozi from St Lucia. Several ecological ’states’, from fresh through estuarine and marine to hypersaline, may occur in the lake system at different times, with the marine-estuarine being the dominant state.[7]

Methods

Results

Conclusion