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MEDIA RELEASE: Public warned of presence of Blue-Green Algae in Zandvlei Marina canals

posted May 14, 2012, 12:18 AM by Damian Gibbs



11 MAY 2012




Public warned of presence of Blue-Green Algae in Zandvlei Marina canals


The City of Cape Town would like to alert residents to the presence of a Blue-Green algal bloom in the Marina canals at the Zandvlei Estuary Nature Reserve.


The Zandvlei Estuary Nature Reserve experienced various problems over the past few weeks which included, amongst others, a bloom of Golden Algae and fish deaths. The most recent analytical results from the City’s Scientific Services laboratory (25 April 2012) showed a decline in the extent of the Golden Algae bloom. Although the algae is still present, it does not appear to have caused further fish to die off.


The Biodiversity Management Branch of the City’s Environmental Resource Management Department is undertaking regular inspections several times a week at a number of locations in both the Marina Da Gama and main vlei area. These inspections have shown improvement in water clarity. The sand prawns are active indicating that the golden algae bloom did not impact these organisms.


However, in the last day or so there has been a visible green colour in some of the blind-ending Marina Da Gama canals. It has been established that this is as a result of various species of Blue-Green Algae (Cyanophyceae). The occurrence of these species is of concern, because they can produce toxins. Eight samples were collected from various locations around the estuary (mainly the Marina canals and the outlet channel) on 10 May 2012, and analysed to test for the presence of toxins. No toxins were detected in the samples.


Although the algal bloom is not toxic, people and their pets should not drink from or swim in the Marina canals as an extra precautionary measure. Canoeing, sailing and other non-contact recreation sports are fine and the estuary therefore remains safe for these recreational activities.


Blue-Green Algae bloom in response to nutrients and warm, still conditions. Residents are asked to assist the City by reporting any increases in the extent of the bloom or any other unusual conditions that they observe. The succession of blooms from the Golden Algae to Blue-Green Algae not surprising, as nutrients are being recycled from one species to the next as the blooms wax and wane.


The City will continue to monitor the situation very carefully.


Although the Marina Da Gama canals are a concern, the rest of the vlei and the mouth is recovering from the recent Golden Algae bloom. At this stage the mouth will probably remain open for the duration of winter, in accordance with the Mouth Management Protocol.


The pondweed harvesting was halted to assist the vlei in its recovery. However, loose weed broken during winter storms and from previously cut pondweed, is at present being removed. The Zandvlei Estuary Management Forum’s pondweed task team will discuss the way forward regarding harvesting within the week.


Zandvlei is Cape Town’s last effectively functioning estuary and supports at least 23 species of indigenous fish. Estuaries are particularly important as nurseries for many marine fish species and, as such, Zandvlei not only supports recreational fishing but acts as a haven for fish stocks targeted by small-scale commercial fishers in the adjacent marine environment. Zandvlei is one of the City’s 16 official nature reserves.





Issued by: Communication Department, City of Cape Town


Media enquiries:


For operational enquiries: Julia Wood, Manager: Biodiversity Management, Environmental Resource Management Department, City of Cape Town, Tel: 021 514 4155 or Cell: 084 464 9153


Dalton Gibbs, Manager: Area South, Environmental Resource Management Department, City of Cape Town, Tel: 021 713 0510


For any other enquiries: Alderman Belinda Walker, Mayoral Committee Member for Economic, Environmental and Spatial Planning, City of Cape Town, Tel: 021 400 1299 or Cell: 083 629 8031,