Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

Southern Waters Ecological Research and Consulting
2. Background to the Zeekoevlei / Rondevlei Rehabilitation
Study
2.1 The Problems – History And Causes
Zeekoevlei and Rondevlei exist as two adjacent shallow vleis (lacustrine wetlands) that are
physically separate in terms of their surface characteristics, but linked via groundwater not
only to each other, but to the pattern of sub-surface water movement across the Cape Flats in
the general direction of False Bay. Despite their geographic proximity, the vleis are managed
separately for the purposes of recreation (Zeekoevlei) and nature conservation (Rondevlei).
For several decades (almost 80 years for Zeekoevlei), both vleis have been subject to water
level controls and/or excessive loads of catchment-derived nutrient enrichment or deliberate
waste disposal, with the effects thereof being evident particularly in Zeekoevlei as sustained
blooms of blue-green algae and accelerated accumulation of organically-rich sediment. The
non-limiting availability of phosphorus, in particular, has for many years been recognized as
the cause of high levels of both plants and algae. The consequence of this sustained impact
has been a loss of biodiversity and reduced ecosystem health. Both vleis exhibit the effects of
eutrophication common in shallow, well-mixed lakes where natural cycles of flushing and
water level variation have been tampered with.
Eutrophication (from the Greek meaning well-nourished), refers to excessive enrichment of
a waterbody by the plant nutrients nitrogen and phosphorus, usually the consequence of
deliberate (“point source”) discharges, or the combined impact of polluted runoff (“diffuse
pollution”) emanating from the catchment(s) draining into a river, lake or wetland. The effect
may be sudden, i.e., the after effects of a single spill or discharge, or alternatively slow and
insidious, with the receiving water changing progressively through increasing levels of
enrichment from oligo- (poor) to hypereutophic (grossly enriched). The typical consequences
of eutrophication are imbalanced algal and/or plant growth, reduced water clarity, reduced
oxygen availability, blooms of toxic algae, fish kills and increased levels of (organic)
sedimentation. The process is often accompanied by an increasing level of resistance to
restoration or rehabilitation, i.e., the longer the impact is allowed to continue, the more
difficult it is to remedy. While eutrophication is relatively easy to diagnose, it is exceedingly
difficult to reverse.
Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan
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