After receiving complaints about the colour of the sea water at Muizenberg, the City would like to emphasise that this phenomenon is caused by a naturally occurring sea water algae and not because of ‘Khayelitsha sewerage’ or ‘effluent from the waste water treatment works on the Cape Flats’ as is often speculated.
Although the colour of sea water is often a cause for concern by bathers, it is generally caused by a variety of reasons – all natural.
The brown colour of the water seen currently from Muizenberg for several kilometres to the east along the False Bay Coastline is caused by the presence of a type of marine diatom (a microscopic type of algae) known as Annaulus. This is a natural phenomenon that occurs at certain times of the year and under certain seasonal and climatic conditions, not only in this part of South Africa, but also in parts of Australia, New Zealand, Hawaii and California. Onshore winds may play a role in bringing this brown water closer to the shore. There is no danger to swimmers, beach users or surfers from being in contact with this water, as it is not toxic. The diatoms are a source of food for prawns, fish and filter feeding molluscs.
On other occasions, another cause of discoloured sea water is a phenomenon called Red Tide, which is often seen off the False Bay coastline. During a Red Tide, the water may be tinged with orange or red due to the presence of another type of marine microscopic diatom. These Red Tide blooms often result in shellfish becoming toxic, and people should avoid eating shellfish if a bloom is present along the coastline.
Sea water may be discoloured by the outflow of river water into the sea. Rivers in the Western Cape are often stained dark brown due to the presence of naturally occurring humic acids/tannins in fynbos soils. The sea tends to be stained brown in the region where these rivers have their mouths. This can be observed along Cape Town’s shoreline at, amongst other places, Strand (Lourens River mouth), Macassar (Eerste River mouth) and Clovelly (Silvermine River mouth).
Monitoring of bathing water quality at Cape Town’s beaches takes place every fortnight and the quality of the water is generally good. Bathers should always be aware, however, that water quality may decline during and immediately after rainfalls, when land-based pollutants are washed down from the catchments towards the sea. Beach-goers should also avoid swimming or paddling in areas where stormwater outlets are located.
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