Zandvlei Trust

Salinity Monitoring Community Programme

Since the 3rd March 2002,  there has been a co-operative monitoring initative on Zandvlei. This involves members of the community, who, on a voluntary basis, undertake weekly monitoring of salinity levels at a number of locations around Zandvlei for the City of Cape Town's Scientific Services department, which receives and collates the data into a database.

The information generated from this programme is valuable and is used to assist with management of this waterbody. Due to its location in an urban environment, management activities must cater for the needs of both the recreational users of Zandvlei and the ecological requirements of the ecosystem itself. thus maintaining ggod waterlevels for the human users(mouth closed) must be co-ordinated with the need to periodically open the mouth of the waterbody to ensure flushing and to allow pulses of saltwater in. Opening the mouth tends to be done mechanically and generally coincides with spring tides.

Health of the Estuary
It is vital that the salinity is kept at levels suitable for maintaining the health of this valuable estuary and the various esturine/brackish water species that inhabit it. This in turn is critical to support some of the ecological requirements of, for example the tubeworm (Ficopomatus enigaticus) optimal salinity range  9.5 30 parts per thousand (ppt), Southern Waters 2001, sand prawn (Callianassa kraussi) and resident estuarine and marine migrant fish populations.
Furthermore, careful salinity mangement assists with suppressing blooms of undesirable freshwater algae species while encouraging the establishment of aquatic plants such as the pondweed (Potamogeton pectinatus). The latter species can tolerate salinities between 5 14 ppt and flourishes in temporarily closed estuarine environments provided salinity levels do not remain elevated for long periods.

Some results of this monitoring programme are presented in the table below, 3 sample monitoring stations only, between March 2002 and March 2003. 
Handheld refractometers were used to measure the salinity.

Sample point Minium salinity (ppt) Maximum salinity (ppt) Average slinity (ppt)
Royal Road 3 35 13.1
Mid Vlei  0.5 16 9.1
Rutter Road 0 18 6.7

Candice Haskins      
City of Cape Town, Scietific Services.


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