A review of the importance of freshwater inflow to the future conservation of Lake St Lucia

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Abstract

  • 1.Lake St Lucia, the oldest formally protected estuary in the world is under threat from historic and present manipulation of freshwater supplies to the system.
  • 2.Constraints to the functioning of the ecosystem began in 1914 with the commencement of draining and canalization of the Mfolozi Swamp to open up the swamp and river floodplain for sugar cane cultivation. Warner's Drain was completed in 1936 and the sediment filtering capability of the swamp on river floodwaters was effectively removed. This resulted in exceptionally high sediment loads from the Mfolozi River entering directly into the St Lucia system and the river was therefore diverted southwards and provided with a separate estuary mouth. This deprived St Lucia of its single largest freshwater supply.
  • 3.During the past 50–60 years, increasing freshwater abstractions from the Mkhuze, Mzinene, Hluhluwe and Nyalazi rivers have contributed to the increasingly severe salinity extremes experienced by Lake St Lucia. In the past decade, desiccation of large areas of False Bay, North and South Lake has occurred, due primarily to natural estuary mouth closure combined with a prolonged drought and unnaturally low freshwater inflows during the closed phase. These events have pushed the system into an extreme state that has not been recorded previously and would not have occurred if Mfolozi River water had been available to the St Lucia system over this drought period.
  • 4.Forestry plantations have further exacerbated the freshwater supply situation, although recent removal of pine plantations on the eastern and western shores of the lake are likely to help restore groundwater flows to the system.
  • 5.This review explores the possibility of relinking the Mfolozi River, once the Mfolozi floodplain swamp has been re-established, to the St Lucia system, thereby bringing urgently needed fresh water to this threatened World Heritage Site. Copyright © 2009 John Wiley & Sons, Ltd.

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