Water too cold to swim in

Few people will swim voluntary in the cold water of our West Coast. And even fewer will make reservations for holidays here just because they love these cold waters. To be honest, most people holidaying here during summertime will not set foot in these cold waters, unless of course, when dared to!

Ever wondered about these cold waters? I always thought it was because of cold waters flowing from Antarctica past the coast. This is only partially true, because those cold waters will actually never surface. No, the reason for these cold waters is more interesting and the cause much more amazing. It all has to do with “upwelling”.

Upwelling is the movement of relatively deep water to the surface. This happens when currents are deflected upwards because of obstacles like mid-ocean ridges. Or where the surface water at the equator is pushed away by Coriolis Force and the void is filled up from underneath. But this is not the case here with us. The surface water at our West Coast is blown away from the coast.

In spring and summer the south-easter is the prevailing wind.  And the "Cape doctor" will blow as if someone is paying him! This wind blows parallel to our western coastline and pushes the water in front of her in a northern direction. Because of Coriolis Force (the rotating of the earth) the water moves away from the coast and leaves a void. At the same time a warm katabatic (berg) wind blows at certain places of the coast north of Cape Columbine, because of the warm land and cold water. The dust blown up by these winds were seen and measured on satellite pictures up to 150km off shore! These strong wind from the inland, cause even more water to be displaced. And more cold, deep water upwells to replace it.  Because of all this the typical surface temperature of the Benguela ranges between 13-15°C, but will drop lower than 10°C at times.

Where do these cold waters come from?

The continental shelf running next to our western coast, is relatively narrow. Water displaced by winds are replaced by upwells from a depth of about 300m.  The result is that sea surface temperatures throughout the Benguela upwelling system are on average typically 5°C lower than those of the water farther offshore in the South Atlantic at similar latitudes.

Maximum upwelling takes place where the wind is strongest and the continental shelf is narrowest and deepest. These places are called “centres of upwelling” and are found in South Africa at Hondeklipbaai, Cape Columbine and the western seaboard of the Cape Peninsula.

Freshly upwelled water is a clear, turquoise colour because it contains no phytoplankton, the microscopic plant life of the sea. Phytoplankton cannot survive the darkness deep in the ocean, it needs light for photosynthesis to produce food.  The upwelled water is rich with nutrients, especially nitrogen in the form of nitrates, a by-product from decomposition of organic material that sank to the ocean bottom. These nutrient-rich waters cause the phytoplankton to grow and multiply at an amazing speed. This provides abundant food for zooplanktonic animals.  Both types of plankton are consumed by small pelagic fish such as anchovy and pilchard which in turn are a food source for predatory fish such as hake, as well as seabirds, seals and dolphins.  Because of this the West Coast is one of the richest fish areas on the planet and the headquarters of South African purse seine and trawl fisheries.

The nutrient rich water also feeds the giant kelp forests along the coast, which in turn is a hideout, habitat and feeding source for a horde of omnivore and herbivore like west coast rock lobster, abalone, limpets, sea urchin and a big variety of fish species.

Unfortunately the upwelling also causes what we call red tide. But this will be the subject for a new blog.