Marine biodiversity

Sandy shore on Robberg Peninsula, Plettenberg Bay, Western Cape. [photo H. Robertson, Iziko ©]

Rocky shore at Kleinmond, Western Cape. [photo H. Robertson, Iziko ©]

There are over 10 000 species of marine plants and animals around the coast of southern Africa, constituting about 15% of the coast marine species known worldwide. Part of the reason for this high diversity is that the west, south and east coasts of southern Africa experience different currents and water temperatures, resulting in three main biodiversity 'provinces'. In addition, off shore there are the pelagic species living in the surface waters where there is light penetration, and the deep benthic species living in subdued to dark conditions on the ocean floor and relying mainly on organic matter coming down from above. 

Southern Africa is in the special position of being at the junction of two of the world's major oceans: the Atlantic with its cold Benguela Current flowing up the west coast and the Indian with its warm Agulhas current moving down the east coast. In addition, the Southern Ocean is not far off our coast and influences the biodiversity we encounter. For instance, the Southern Right Whales spend part of their time in the Southern Ocean where they feed on krill and the other part along our coasts where they bear their young.

The Benguela Current is cold partly because it is derived from waters coming from down south and partly because of the upwelling of water from deep down, which is the result of the combination of changing water depth and the effect of the South East wind. The upwelling results in rich nutrients from the depths being brought to the surface where they are utilised by phytoplankton. The phytoplankton are in turn eaten by the zooplankton and the zooplankton by fish. The fish are eaten by predators such as seals, dolphins, birds and people. This high productivity caused by the upwhelling has benefitted commercial fisheries, spoilt somewhat by senseless overfishing that has resulted in the collapse of certain fisheries such as sardine.  

The Agulhas Current is warm because it its tropical origins and flows strongly down the east coast at a rate of up to 2.6 m per second. The continental shelf off the south coast deflects the Agulhas Current southwards, resulting in the south coast receiving colder water than the main Agulhas. There is a strange phenomenon where a countercurrent forms and flows up the east coast in the opposite direction to the main Agulhas current. The famous annual sardine run off southern KwaZulu-Natal and northern Eastern Cape is the result of sardines following this current up the coast and being gradually squeezed inshore by the Agulhas coming from the opposite direction. Towards the south, the Agulhas peals off eastward as the return Agulhas current. 

Hence, there are three main biodiversity 'provinces' along our coastline: 

1. the warm Subtropical Province, which extends from southern Mozambique down to about Port St Johns; 
2. the warm temperate South Coast Province, extending from about Port St Johns to about Cape Point; and 
3. the cold temperate West Coast Province, extending from about Cape Point to about Walvis Bay. 

Irrespective of which 'province' you are in, biodiversity is entirely different on rocky shores versus sandy shores. There are also the estuaries where salt water meets freshwater and which create special conditions to which certain animals are specially adapted. 

Off the coast, biodiversity is influenced mainly be depth and can be divided into the following main zones:

1. the Pelagic zone 
2. the benthic zone [check]

Hence, the marine biodiversity of southern Africa is best divided into the following main habitats:

1. Rocky shores
2. Sandy shores (beaches).
3. Estuaries
4. Pelagic
5. Benthic.

• Branch, G. and Branch, M. 1981. The living shores of southern Africa. C. Struik Publishers, Cape Town. 

• Branch, G.M., Griffiths, C.L., Branch, M.L. and Beckley, L.E. 1994. Two oceans. A guide to the marine life of southern Africa. David Philip, Cape Town.