The Zandvlei estuary is situated at the bottom of the catchment, which makes it a receptacle for whatever happens higher up. As such it invariably receives nutrients and other contaminants. These nutrients can lead to algal blooms and eutrophication. Managing Zandvlei Estuary includes managing these nutrients as far as possible. Having plants grow in the estuary is a better way to absorb the nutrients than having algae growth, because the ‘weeds’ can be harvested and hence removed from the system, effectively removing the nutrients with them, while algae has the potential to be toxic or lead to rotting (called eutrophication). Zandvlei is a recreational body of water and the weeds also need to be removed to improve the experience of people using the estuary for recreational activities like sailing, canoeing and windsurfing.

Stuckenia pectinata

The current dominant and preferred weed is Stuckenia pectinata. It has many benefits:

  • Valuable nursery habitat
  • Takes up and stores nutrients
  • Creates shade, restricts algae

But…

  • Overgrowth due to high nutrient load
  • Conflict with user groups
  • Interruption of flow

The weeds are currently being

Stuckenia pectinata close up

harvested by a mechanical harvester from the 1980’s. It is old and breaks down often, but a new one will cost in the region of R4.2 million. In the current drought and faced with competing needs, it is unlikely that this cost can be covered by the City. ZPAAC is currently looking at other alternatives, for example manual harvesting or more creative approaches like rigging up bicycles to create an urban gym while removing weed. We are also considering ways to find uses for the harvested weed.

Reports that give more information about weed harvesting:

The weed harvester

2012_ZEMF_Final_Recommendations_from_Zandvlei_Pondweed_TWG_201208

 

Also look at Invasive Species and Nutrient Management.