Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

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Southern Waters Ecological Research and Consulting The findings of this report clearly indicate that, despite the high levels of eutrophication present in both vleis (see Table below), meaningful rehabilitation is possible, although at a relatively high initial cost. The cost implications should, however, be weighed against the period of time that the vleis have been abused. Also, the eco-recreational value of having the wetlands in a significantly better condition than at present, is likely to render the cost of rehabilitation relatively small when viewed long-term. Comparison Of Zeekoevlei And Rondevlei With The Trophic State Classification System For African Lakes And Lacustrine Wetlands Indicator Upper limit of Zeekoevlei Rondevlei mesotrophy (median) (median) Phosphorus concentration 0.05-0.06 0.70 0.24 mg l -1 Chlorophyll-a concentration 10-15 200 125 mg m -3 Nitrogen concentration mg l -1 0.2-1.0 2.0 2.8 Primary production level g C m -2 d -1 2-3 1.2 – 4.4 Not determined Dominant algal family Cyanophyta Cyanophytes Cyanophytes Chlorophytes 12 Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan
Southern Waters Ecological Research and Consulting 4.2.1 Sources of phosphorus loading in Zeekoevlei On average an estimated 34 000 kg of phosphorus enters Zeekoevlei per year. This exceeds the guideline annual loading for a vlei of this area and depth by two orders of magnitude (100x). The largest single contribution, 35%, originates from seepage from the adjacent WWTP; the second-largest contribution, from the catchment of the Big Lotus River (28%); and the third-largest contribution as internal loading generated from the considerable volume of organically-rich sediments (25%). The various phosphorus loadings for Zeekoevlei are summarized in the following diagram: Groundwater 3.8% Lake surface 0.1% Septic tanks 1.5% Big Lotus catchment 28.4% Seepage 34.6% Little Lotus catchment 4.7% Reed beds 1.1% Internal loading 25.1% Local catchment 0.6% Diagrammatic representation of the identified annual phosphorus loads, by source and percentage of the total, for Zeekoevlei. This study concluded that reducing the total phosphorus loading to an in-lake growing season median concentration of between 0.1 and 0.2 mg l -1 , in conjunction with continued drawdowns and other management options (see elsewhere in this summary) would: • reduce algal growth and sedimentation; • restore clear water lake conditions; • allow sufficient light penetration to support the re-establishment of desired rooted aquatic plants; • lead to increased levels of biodiversity; • reduce the rate of reed encroachment. Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan 13
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Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

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