Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

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Southern Waters Ecological Research and Consulting 4.4 Water level regulation and drawdowns 4.4.1 Response of Zeekoevlei to the drawdown procedure The use of drawdowns as a management technique was instituted at Zeekoevlei during 1997, and at Rondevlei the following year. The original purpose of lowering the water levels was to enable physical access to the shoreline areas of the vlei for the purposes of litter and reed removal. The 1997 event was subjected to detailed scientific evaluation of changes to the biota of the vlei, and in particular the phyto- and zooplankton populations. The findings were profound. The receding water levels isolating the reed beds, and the absence of deep water refugia, resulted in a high level of fish predation by piscivorous birds. This reduction in pressure on the zooplankton enhanced phytoplankton grazing pressure, and with a sustained clear-water phase being the result. This characteristic was reconfirmed during the second (1998) drawdown. The improved water clarity also resulted in the reappearance of pondweed (Potamogeton pectinatus) in the shallow northern and north-western bays of the vlei. Subsequent drawdowns (post-1998) have not been subject to detailed investigation, and no comparative studies have been conducted for Rondevlei. Notwithstanding this, aggregate measurement of the level of algal pigments (chlorophyll-a) in Zeekoevlei has revealed that, despite no change in the ambient level of nutrient availability, phytoplankton populations have remained at approximately 50% of the level measured prior to implementation of the drawdown procedure. 4.4.2 Why the drawdowns benefit Zeekoevlei The drawdowns have resulted in improved ecological functioning for the following reasons: 1. At a gross level, the procedure served to relieve top-down pressure on the vlei food chain by reducing grazing by fish on the organisms that feed on algae. This cause and effect response is both predictable, and relatively easy to measure. 2. At a more subtle but no less important level, the procedure has released pressure on a mechanism common to shallow lakes, viz. that water level regulation forces the system towards dominance by phytoplankton rather than by more desirable rooted plant species such as pondweeds – i.e. plant communities that support a wide range of valued ecosystem services. Shallow lakes are known to exist in one of two 20 Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan
Southern Waters Ecological Research and Consulting predominant conditions, namely a clear water condition dominated by rooted plants, or a turbid state in which algae are the major form of aquatic plant life. The implication here is that the response will be directly proportional to the release on water level regulation. This benefit will, however, be limited by other prevailing factors such as the presence of sediment in the vlei (see below). 4.4.3 Negative impacts of the drawdowns - midges A negative consequence of the drawdown procedure is a disturbance of the fish – aquatic insect (midge) dynamic. The absence of deep water refugia during the drawdowns resulted in a high level of fish predation by birds, as the fish had no means of escaping this impact. Annual re-implementation of the drawdowns under the same conditions has allowed the impact to be sustained. The predominant fish species in Zeekoevlei is the common carp (Cyprinus carpio), a species known to be an excellent biological control for the development of chironomid midge larvae in shallow lakes. Immediately following the 1997 winter drawdown of Zeekoevlei, as well as during the subsequent years, midge emergence reached plague proportions. This is now considered to be a direct consequence of the reduction in the fish population, and the consequent reduction in control on the midge population. However, as midge emergence on the Cape Flats varies widely on inter-annual basis, and no emergence trapping trials have been conducted after the drawdowns, other natural forcing functions may also be in effect. 4.4.4 The need for deep water areas and sediment removal Apart from generating an enormous internal nutrient load (see above), the sediments accumulated in Zeekoevlei occupy in toto the deep water areas (> 1.9 m) of the vlei. During the drawdowns, average water levels are reduced to 0.5 and < 1m, a time when, should the sediment basins that cover an area of some 60 ha (= 40% of the vlei area), be open water, they would provide an additional water depth of up to 3 m. The sediments currently present in the vlei have accumulated over a period of approximately 70 years, i.e., from the time of establishment of the first wastewater treatment facility on the south-eastern shoreline. Rehabilitation of the vlei, as is envisaged and recommended in this report, would reduce algal sedimentation rates to very low levels. Thus the benefits for the Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan 21
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Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

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