Zeekoevlei in the 1920s. See how much the shoreline has changed.
Before agricultural and then urban development, wetlands on the Cape Flats were mainly seasonal wetlands, and dried up in the summer. Even the larger vleis in what is now the Ramsar Site of Zeekoevlei / Rondevlei retreated into smaller deepwater sections, leaving expanses of salt-crusted sand. These cycles of drought and flooding are crucial for natural wetland biodiversity. Summer drought creates heat stress for roots, helping to limit invasion by plants such as bulrush. It also controls aquatic weeds, which die out in the summer.
However, development brought with it increased nutrients (from fertilisers, livestock and then treated sewage effluent) as well as an increased water table and permanent stormwater inflows into wetlands such as Zeekoevlei. These changes allowed bulrushes and other reeds that need permanent water and nutrients to expand into wetlands.
Rondevlei, Zeekoevlei, Princess Vlei, the Philippi Horticultural Area, the Khayelitsha Wetlands Park and similar sites remain seasonal wetlands in the Cape Flats, and are areas of high biodiversity importance. Many of them support plant species that are now endangered, largely because so many seasonal wetlands have been infilled or have changed to permanent systems.
Hydrodictyon africanum. Credit: Alex Lansdowne
Aponogeton angustifolius (Witches’ Pool, Rondevlei). Credit: Theo Stock
For example, this little waterblommetjie Aponogeton angustifolius requires periods of drought followed by a period of steady rain for its sustained presence in wetlands and slow-flowing rivers. It still occurs in the Van Blommestein seasonal wetlands and elsewhere in remnant seasonally inundated (flooded) wetlands in the south western Cape.
Another increasingly rare plant that occurs in seasonal wetlands only is the beautiful “water net” algae (Hydrodictyon africanum). It is known from only a few seasonal wetlands in the City today.
Adult Daphnia (bottom left), eggs (right) and massed eggs released as the system dries out.
Many of our naturally seasonal wetlands also support interesting macroinvertebrate communities – that is, communities of small crustaceans and insects whose life cycles require periods of seasonal drought, when their wetland soils dry out, followed by a few months of inundation. Some of these species are endemic to the Western Cape (that is, they occur here and nowhere else in the world). During the dry season, aquatic fauna living in seasonal wetlands need a strategy to survive the dry period. They can migrate elsewhere, which many winged insects can do; they can enter a state of diapause of hibernation; or they can survive genetically as drought-resistant fertilised eggs.
Water fleas (Daphnia pulex) are charismatic microcrustaceans that illustrate the importance of seasonal drought very well. During the dry season they survive as eggs (or ephippia). Sustained inundation over a week or two (signalling the onset of the wet season rather than an out of season rainfall event) is the cue for hatching. The eggs hatch as females, and go into a phase of rapid asexual reproduction, producing large numbers of females. This allows them to increase their numbers quickly to take advantage of a short wet season. As the wetland starts to dry out, they are cued to produce some males. Towards the end of the season, they mate sexually, producing multiple fertilised eggs, some of which will survive into the next season.
When wetlands no longer dry out, these important lifecycle cues are lost. Some species disappear altogether, often being out-competed by other insect and animal species which do not need to leave the wetland in summer. Instead they remain year-round, affecting predator-prey relationships, as does the introduction of (often alien) fish to these now perennial species. These new species can affect the value of wetlands habitats to other threatened fauna such as Western Leopard Toads, as the fish prey on eggs and tadpoles.
The same wetland in winter – supporting diverse, often threatened invertebrates and wetland plants including watersterretjie Spiloxene aquatica.
The photos above show that many somewhat scruffy-looking open space areas in the Cape Flats transform miraculously in winter, forming seasonal wetlands with high biodiversity value. It’s well worth stopping and taking a better look at them, and certainly worth efforts to conserve them where we can. Citizens can do a lot to help. We can: