Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

Southern Waters Ecological Research and Consulting
4.2 Eutrophication and Phosphorus Loading
Nutrient concentrations in waterbodies can range from very low to very high. Such
concentrations are normally characterized in terms of the trophic state or condition of the
system, and ranked as oligotrophic (= poorly nutrient enriched) through meso- (= moderately
enriched) to eutrophic or even hypertrophic (highly to grossly-high in-lake levels of
phosphorus availability). For both Zeekoevlei and Rondevlei, restriction of the natural cycles
of filling, flushing and drying (through the construction of weirs), and their use as detention
ponds for effluent and urban runoff, has resulted in their being classified as highly eutrophic,
with impaired ecosystem functioning and biodiversity. While some relief of water level
regulation has been possible through the implementation of drawdowns, the greater problem
of excessive nutrient enrichment continues unabated.
Total phosphorus
(mg/l)
1
0.8
0.6
0.4
0.2
0
400
350
300
250
200
150
100
50
0
90/91
91/92
92/93
93/94
Chlorophyll-a (ug/l)
94/95
95/96
96/97
97/98
98/99
Growing season (GS)
Total phosphorus
Chlorophyll-a
Graph showing the change in growing season phosphorus concentration and chlorophyll-a in
Zeekoevlei during the 1990s. Note, despite the continuing year-to-year increase in the
ambient concentration of phosphorus, the marked decrease in chlorophyll-a as a consequence
of the applied drawdowns.
Appropriate management philosophy for eutrophication encompasses two broad options,
either: (a) minimizing the causes or (b) learning to live with the symptoms. The process, once
ingrained in an aquatic ecosystem, often adopts a high level of resilience to rehabilitation, and
the “live with it” option is a common choice, unwillingly made by many, in the face of the
extremely high costs associated with reversing the process.
Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan
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