Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

Southern Waters Ecological Research and Consulting
3.3 Ecology
The hydraulic regimes of both vleis are attenuated by weirs that were constructed during the
1940s. Both vleis are hypertrophic (see below), but with the degree of anthropogenic nutrient
enrichment in Zeekoevlei equivalent to 3-fold that for Rondevlei (see below). Both vleis
exhibit the microphyte (phytoplankton) dominance typical of shallow, regulated lakes, but
with this dominance considerably more sustained and ingrained in Zeekoevlei. Prior to the
recent implementation of water level drawdowns, Zeekoevlei was characterized by nearpermanent
algal blooms, an impoverished population of small-bodied cladoceran
zooplankton, and a large population of coarse fish. The drawdowns served to release the topdown
pressure on the zooplankton community, resulting in a considerable reduction in the
algal population. Algal dominance in Rondevlei followed a more distinct periodicity prior to
drawdowns being applied. Given a preceding greater level of ecological balance and stability
in this smaller vlei, the benefits that may have accrued from the drawdown procedure have
not been visibly apparent, and have not been subject to scientific investigation.
Macro-hydrophyte dominance in both vleis is limited to dense stands of emergent reeds. In
Zeekoevlei Typha capensis predominates, with a lesser contribution by Scirpus nodosus in the
deeper water. Rondevlei reflects an apposite situation with S. nododus as the dominant genus,
but with Typha encroachment. Rondevlei supports some 18 species of endangered or
vulnerable flora, and with one endemic species.
Avifaunal composition is much more diverse in Rondevlei than in Zeekoevlei, but the recent
drawdown-facilitated improvement in water clarity has allowed a return of greater numbers of
piscivorous birds to Zeekoevlei. The low human disturbance levels of the Rondevlei Nature
Reserve environment, together with a greater seasonal availability of shallow wading habitat
(that Zeekoevlei lacks), makes the smaller vlei more attractive to bird life. Rondevlei
provides habitat for 10 species of birds that are of rare or indeterminate importance.
3.4 Wetland importance
Both vleis have been provisionally assessed in terms of their present condition, and their
ecological importance and sensitivity. Ecological importance implies the value of a wetland
environment to maintain ecological diversity and functioning on local and wider scales, while
sensitivity relates to the capacity of the wetland to resist disturbance, and its ability to recover
once an impact has been endured (resilience). The level of variation between the present
Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan
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