It is no secret that our beautiful vlei is facing serious challenges which appear to have increased rather drastically over the past few years. Sewage leaks (mainly) have caused an overload of nutrients into the system, resulting in murky water, a shift in aquatic plant growth and often leaving homes on the Marina enveloped by rotting, floating plant matter.
This leaves us all concerned as we face both the environmental and human health risks that come with this form of pollution. So, what makes pondweed – Stuckenia pectinata – so important?
Pondweed grows seasonally, with dense beds forming in summer, which then begins to die-off towards winter. During winter the pondweed hibernates, and its growth is triggered in early summer by changes in light and water temperature. It is a food source for many aquatic birds (e.g. coots) and provides refuge for marine fish and aquatic invertebrate species. The pondweed absorbs nutrients, including nitrogen and phosphorus, through its roots and its leaves.
It is increasing concentrations of nitrogen and phosphorus in the water that drives the algal blooms we have witnessed with increasing regularity in recent years. Algal and phytoplankton blooms prevent sunlight from entering the water. This results in the death of aquatic plant life which are decomposed by bacteria that use up all the oxygen in the water and put the ecosystem at risk of becoming uninhabitable for many organisms. Therefore, a healthy population size of S pectinata is probably central to helping prevent this from happening.
There are multiple sources of nutrient pollution into Zandvlei Estuary, including sewage pollution, fertilizer and stormwater runoff. The rubble weir at the estuary mouth was lowered a few years ago with the aim to increase salinity and flushing the system. The subsequent increase in salinity has benefitted the fish in the estuary, but it also may be the driver behind the pondweed decline.
I have been a Marina resident all my life and I am currently a research student at UCT. My current research project aims to improve the understanding of current environmental factors operating in Zandvlei including nutrient levels, water temperature, salinity, water clarity, dissolved oxygen and phytoplankton abundance. It will also address the nutrient removal capacity of the pondweed that used to be prevalent in the estuary, but which has recently declined significantly.
I will use an experiment to determine whether, or to what extent, this decline is due to the increase in salinity over the past few years. This should give a deeper understanding of the potential role that S pectinata pondweed plays as a natural nutrient sink (absorber) and determine whether the current regime of salinity levels needs to be reassessed in the management of the Zandvlei Estuary.
I hope my research will contribute to theoretical and practical solutions in the management of the Zandvlei Estuary, and show the way for the balanced co-existence of humans and the waterways we have all come to love and appreciate.
Kelly is currently an MSc student in Marine Biology at University of Cape Town. Her research is supported by the Zandvlei Trust as well as Orms Photographic www.ormsdirect.co.za.