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
- the warm Subtropical Province, which extends
from southern Mozambique down to about Port St Johns;
- the warm temperate South Coast Province,
extending from about Port St Johns to about Cape Point; and
- 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:
- the Pelagic zone
- the benthic zone [check]
Hence, the marine biodiversity of southern Africa is best
divided into the following main habitats:
- Rocky shores
- Sandy shores (beaches).
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.