Stormwater: Urban runoff, or stormwater, is more difficult to control: Cape Town alone has more than one hundred stormwater outlets discharging over beaches and rocks. Stormwater drains collect and channel the water from all non-porous surfaces, such as pavements and tarred roads, within a catchment. Stormwater flow depends on rainfall, and the first flows following a dry spell often contain high levels of pollutants, such as heavy metals (particularly lead), oil residues, nutrients (nitrogen and phosphorus), and pathogenic (disease causing) micro-organisms, such as viruses, bacteria and protozoa, from faecal material.

Rivers: Rivers carry to the sea water containing contaminants such as nutrients and pesticides from agricultural runoff, faecal pollution resulting from poor sanitation, and industrial discharges. Soil erosion, partly a result of poor catchment management, can cause increased water turbidity and siltation of estuaries (see Enviro Facts "River catchments", "Wetlands" and "Estuaries").

At sea: Pollution sources at sea include accidental and deliberate discharges of oil, operational dumping of garbage (particularly plastics), and controlled dumping. Unlike many countries in Europe and the USA, South Africa does not dump industrial wastes or sewage sludges. Its dumping is limited to dredge spoils (mostly routine harbour dredging) and unserviceable equipment. Dredge spoils are often rich in heavy metals (e.g. lead, copper, zinc, mercury, and cadmium), and are dumped at designated sites.

* Decomposition of organic matter causes a drop in dissolved oxygen, particularly in calm weather and sheltered bays. This can cause the death of marine plants and animals, and may lead to changes in biodiversity (see Enviro Facts "Biodiversity").

* Effluent, rich in nitrogen and phosphorus, results in 'eutrophication' (overfertilization), which may cause algal blooms. These blooms can discolour the water, clog fish gills, or even be toxic, e.g. red tides. Microbial breakdown of dead algae can cause oxygen deficiencies.

* Pathogenic microorganisms cause gastric and ear-nose- throat infections, hepatitis, and even cholera and typhoid. Filter feeding animals (e.g. mussels, clams, oysters) concentrate pathogens in their gut, so eating shellfish from polluted waters is a health risk.

* Effects from industrial discharges in South Africa are generally limited to the area next to the discharge (the `mixing zone'). Water quality guidelines specify maximum levels of pollutants allowed in the receiving water.

2) Oil spills smother plants and animals, preventing respiration. In seabirds and mammals it can cause a breakdown in their thermal insulation. Chemical toxicity can cause behavioral changes, physiological damage, or impair reproduction. Oil pollution is an eyesore, and cleanup and subsequent disposal of oily wastes is difficult.

3) Pesticides, such as DDT, and other persistent chemicals e.g. PCBs, accumulate in the fatty tissue of animals. These chemicals can cause reproductive failure in marine mammals and birds.

4) Ships often paint their hulls with anti-fouling substances, e.g. tributyl-tin or TBT, to prevent growth of marine organisms. These substances leach into water and, in high traffic areas such as harbours and marinas, can affect animal life. There is a world wide trend towards limiting the use of TBT containing paints.

5) Plastics kill many marine animals. Turtles, for example, often swallow floating plastic bags, mistaking them for jelly- fish. Animals are often strangled when they become entangled with plastic debris.

Shipping and oil: South Africa lies on one of the world's busiest shipping routes. Rough seas, an ageing world tanker fleet, human error, and deliberate discharge, make oil pollution a real threat around our coasts. South Africa has developed contingency plans allowing equipment and manpower to be mobilised at short notice to protect beaches, estuaries, bird colonies and other sensitive areas in the event of a major spill. The `Kuswag' fleet of four antipollution vessels, and a patrol aircraft, plus an unsympathetic approach to illegal oil discharges at sea, has contributed to a decline in the number of oil slicks off our coast: Sightings of oil slicks by the patrol aircraft has dropped more than five-fold since 1985.

* Careful planning of all future developments through Integrated Environmental Management (see Enviro Facts "Integrated Environmental Management"), which allows any interested group to have a voice, should ensure that developments, such as marinas or harbours, are managed in such a way that they have little adverse effect on the environment.

* Pick up litter as you walk along the beach.

* Be alert and report any sign of marine pollution to the Department of Environment Affairs and Tourism, address below.

SECRETS OF THE SEAS. Illustrated guide to marine life off southern Africa. A. Payne and R. Crawford (eds). Vlaeberg Publishers, Cape Town, 1992.

THE LIVING SHORES OF SOUTHERN AFRICA. M. and G. Branch. Struik, Cape Town, 1981.

THE GAIA ATLAS OF PLANET MANAGEMENT. N. Meyers (ed). Pan Books, London, 1985.

MARINE POLLUTION. R. Johnston. Academic Press Inc., 1976.

All books are available from Russel Friedman Books, PO Box 73 Halfway House 1685. Tel. 011-7022300/1.

Oceanographic Research Institute (ORI). P O Box 10712, Marine Parade, Durban 4056. Tel. 031-373536.

South African National Foundation for the Conservation of Coastal Birds (SANCCOB). Cape Town, PO Box 11116, Bloubergrant, 7443. Tel. 021-5576155.

Dolphin Action and Protection Group. P O Box 22227, Fish Hoek 7975. Tel. 021-782 5845.