South African marine coastal fauna and its habitat

South African has a coastal coastline of some 3000km and stretches from Ponta do Ouro on the Mozambique border on the sub-tropical east coast to the cold temperate west coast of Alexander Bay on the Namibian border. This includes a number of large number of sandy beaches and rocky shores interrupted by estuaries and river mouths some of which are permanently open to the sea while others are closed periodically.

Currents and tides

Gravitational forces of the sun and the moon govern the rise and fall of the tides. Spring tides occur twice a month, during full and new moon, when the sun and the moon's forces work together to increase their influence on the earth water bodies. The tides rise and fall twice a day. High tide is about six and a quarter hours after low tide. Spring low tide falls around 10am along the southern African coast and the maximum tidal range around the southern African coast is around two meters. The events between spring tides are called neap tides and occurred when the moon is in its first or last quarter.

The distribution of fauna and flora around the southern African coast is a direct result of the influence of the different water masses that flow on the west and east of the southern tip of Africa. The powerful Agulhas current (one of the most powerful in the world) flows from the Mozambique channel down the east coast of Africa bringing warm water from the sub-tropics. North of East London the continental shelf is narrow and the warm water of the Agulhas current flows close inshore. The current is steadily pushed away from the coast as the continental self widens and the coastal waters become slightly cooler from East London to Port Elizabeth. The Agulhas current then swings back some 300km offshore south of Cape Agulhas (Branch et al., 1994; Lubke et al., 1988) where the continental shelf is at is widest. The west coast of South Africa is influenced by north drifting cold water. Events of up-welling take place when surface waters is blown offshore and cold deep water moves to the surface near the coast This water is rich in nutrients and enables microscopic algae (phytoplankton) and macroscopic algae (seaweed) to grow and flourish. The west coast is thus characterised by its productivity and sustains large fisheries. This is different from the less productive but more diverse east coast.

Species richness and distribution

It is estimated that area known as southern Africa (north Namibia to southern Mozambique), have over 10 000 marine fauna and flora species, of which about 12% is endemic (Branch et al., 1994). More is know about the organisms and their habitat on intertidal rocky shores and sandy beaches than is the case for their subtidal counterparts. Processes on the intertidal zone are easier to study as they are more accessible than the subtidal habitats.

The tides, currents, water temperature, effects of sun and wind exposure as well as a number of terrestrial predators are but a few factors that play important roles in adaptation and survival of animals, plants and algae inhabiting the shore and intertidal environments. An example of this is the increase in physical stress as one moves up shore due to the increase in heat stress and desiccation. Other factors that influence the fauna and flora of our coastal areas both inter and sub-tidally are the presence of bays and protected areas e.g. some species are prominent within protected bays where the water is calmer and local currents less severe than in exposed areas of the coast. Other factors are the input of fresh water from local estuaries and rivers and their sediment and organic matter load, reef type, depth and sediment movement. Estuaries and river mouths are important breeding and nursery grounds for a number of crustacean and fish species.