Abstract
For the past 20 years management of the Bot/Kleinmond estuarine system in the south-western Cape has been based on the premise that, barring intervention, the estuary was naturally evolving into a freshwater coastal lake. This paper presents evidence, based on a 20-year series of water-level data, updated runoff estimates from the catchment and dimensional data, that, in the absence of anthropogenic influences, the system is not progressing naturally, but artificially, towards becoming a freshwater system. It is concluded that the increasingly closed state of the Bot Estuary in recent years is most likely due to reduction in runoff from its tributaries and premature artificial breaching of the Kleinmond arm of the system. These findings, coupled with the high conservation importance of the Bot River Estuary, suggest that the current management plan needs urgent revaluation and that the two estuaries cannot be managed separately.
Highlights
Introduction preservation of estuarine biodiversity inSouth Africa (Turpie et al., 2002; Maree et al, 2003), bringing some new urgency to the needEstuarine management is a complex task because it deals with the for a sound management plan.use and care of the interface between land, river and sea where aThis paper reviews past management practices and the data on combination of terrestrial, freshwater and coastal management is which they were based
Estuarine dynamics are governed by natural conditions and modified through human interference
The main conclusion is that anthropogenic interventions are responsible for the Bot Estuary’s perceived shift from an estuarine system towards a freshwater coastal lake, and that it is not a natural phenomenon as was previously claimed
Summary
Estuarine dynamics are governed by natural conditions and modified through human interference. Should sufficient rain continue to fall (State 2b) - and this is conditional, since in some dry years (1980, 1982, 1999 in Fig. 6 offer examples) this state does not fully materialise - the estuary water level increases to about 1.7 m MSL when water overflows via the channel and the salinity in the Bot Estuary continues to drop further This cyclical process is strongly influenced by the state of the Kleinmond mouth. Bot Estuary mouth can breach (naturally or artificially) to drain estuary water into the sea In these cases, the water level drops from +3 m to about 0.4 m or even 0 m MSL within a few hours, after which the mouth stays open (State 4) for about four months on average and the salinities in the system can equate that of seawater. This process may be emulated through a managed artificial breaching policy to produce more natural ecological conditions
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