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A review of studies on the Mfolozi estuary and associated flood plain, with emphasis on information required by management for future reconnection of the river to the st Lucia system
Expanded Title:This report is structured around 14 contributions from various scientific disciplines, together with a synthesis section (see pages 256-260), all of which assist in understanding how the Mfolozi/Msunduzi rivers and floodplain link with the functioning of the St Lucia ecosystem. The end result is an endorsement for the relinkage of the Mfolozi and St Lucia estuaries and the implementation of measures that will reduce any excessive input of sediment from the former into the latter system. Historical evidence from early maps and anecdotal evidence at our disposal indicate that changes in the Mfolozi/Msunduzi floodplain have had profound impacts on the Mfolozi Estuary and indeed on the whole St Lucia system. The separation of the Mfolozi from St Lucia in the early 1950s resulted in a major change in the way that St Lucia functioned. Only now are we beginning to see and experience the full implications of that separation for the well-being of the ecosystem, with the lake virtually drying out completely for the first time in living memory. However, it is not simply the loss of this riverine input to the salinity and water levels of the lake compartments that has caused the ecological ‘pendulum’ to swing off the scale and endanger St Lucia’s World Heritage status. There are many other impacts, some of which are well documented scientifically, while others are based primarily on observation or the behaviour of models, including:  The St Lucia mouth closes more frequently than in the past and, once closed, remains closed for much longer. Modeling of these impacts has shown that the mouth, instead of staying closed for less than 30% of the time after closure, could now remain closed for as much as 80% of the time before breaching naturally.  River sediment dynamics are affected. Mouth closure means that, with full connection, more river sediment during the high-flow periods enters St Lucia than if the mouth was functioning naturally. Under natural mouth regimes, when the mouth is more likely to be open during wet periods, much of the river sediment would be flushed into the sea. Under closed mouth conditions these sediments may accumulate within the system.  Marine sediment dynamics are affected. These sediments tend to accumulate inside the estuary mouth, carried in by inflowing tidal water. In the former configuration, the combined flow from the Mfolozi and St Lucia through a single mouth would have been stronger than is the case for either of the separated mouths. Thus marine sediment dynamics are likely to be very different from what they were under natural conditions.  Sediment flushing on a large scale would have occurred whenever the combined mouth breached, especially when there was a substantial head of water at the time of breaching (estimated to have been approximately 4.6 m above MSL in one case). Separation of the Mfolozi from the St Lucia system and artificial breaching has effectively resulted in a loss of this massive scouring process in the ‘bay’ area.  Biotic connectivity between the Mfolozi and St Lucia has shown that freshwater prawns can breed prolifically in the Mfolozi wetlands. Their larvae are carried into St Lucia by small floods and there are likely to be several other similar cases for other species when there is a connection between the two systems.  Prolonged mouth closure affects recruitment and breeding of many species of fish and crustaceans. This should be seen in the context of the life histories of key taxa (e.g. Fish and penaeid prawns). For estuary-associated marine fish species in particular, once the populations in St Lucia have been reduced or extirpated locally, it takes several years after larval recruitment for the fish populations to recover and be able to contribute to the South African marine spawner stock.
Date Published:01/02/2011
Document Type:Research Report
Document Subjects:Ecosystem - Biodiversity protection, Ecosystem - Invertebrates
Document Keywords:Guidelines, Environment
Document Format:Report
Document File Type:pdf
Research Report Type:Consultant
WRC Report No:KV 255/10
ISBN No:9781431200658
Authors:Bate GC; Whitfield AK; Forbes AT
Project Leader:Bate GC
Project No:K8/930
Organizations:Nelson Mandela Metropolitan University; South African Institute of Aquatic Biodiversity; Marine & Estuarine Research, Durban
Document Size:12 764 KB
Attachments:EXECUTIVE SUMMARY KV 255.pdf
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