Occurrence and persistence of water level/salinity states and the ecological impacts for St Lucia estuarine lake, South Africa

Abstract

The St Lucia estuarine lake in South Africa forms part of a World Heritage Site and is an important local source of biodiversity. Like many estuarine systems worldwide, St Lucia has experienced significant anthropogenic impacts over the past century. Abstractions have decreased fresh water inflows from the lake catchments by about 20%. Furthermore the Mfolozi river, which previously shared a common inlet with St Lucia and contributed additional fresh water during droughts, was diverted from the system in 1952 because of its high silt loads. The separated St Lucia mouth was subsequently kept artificially open until the onset of a dry period in 2002 when the mouth was left to close naturally. These changes and the current drought have placed the system under severe stress with unprecedented hypersaline conditions coupled with desiccation of large portions of the lake. Long-term simulations of the water and salt balance were used to estimate the occurrence and persistence of water levels and salinities for different management scenarios. The risks of desiccation and hyper-salinity were assessed for each case. The results show that the configuration of the Mfolozi/St Lucia inlets plays a key role in the physicochemical environment of the system. Without the Mfolozi link desiccation (of about 50% of the lake area) would occur for 32% of the time for an average duration of 15 months. Artificially maintaining an open mouth would decrease the chance of desiccation but salinities would exceed 65 about 17% of the time. Restoring the Mfolozi link would reduce the occurrence of both desiccation and hypersaline conditions and a mostly open mouth state would occur naturally. Integrating these modeled scenarios with observed biological responses due to changes in salinity and water depth suggests that large long-term changes in the biological structure can be expected in the different management scenarios.