Many factors, such as dilution, temperature, adsorption, sedimentation and nutrient deficiencies influence self-purification of the sea. The marine environment is generally unfavourable to the survival of most pathogenic organisms. However, under special circumstances, particularly in temperate and warm coastal waters near large cities, pathogenic agents may be found in marine waters in the proximity of the coast-line and in estuaries.
The coastal marine environment is clearly being affected by the modification and destruction of habitats, over-fishing and pollution. Many of these impacts can be traced back to land-based human activities located far from the sea.
Many coastal waters carry excessive sediment and are contaminated by microbes and organic nutrients. Nitrogen, resulting from sewage discharges, agricultural and urban run-off, and atmospheric precipitation, is a particular problem. The destruction of wetlands and mangroves, which act as natural filters for sediment, excessive nitrogen and wastes, has also accelerated nutrient build-up. Additional pollution sources are oil leaks and accidental spills from shipping, discharge of bilge water, oil drilling and mineral extraction. Some persistent pollutants are even reaching deep ocean waters.
While coastal pollution is gradually being controlled in many industrialized countries, it is still rising rapidly as a result of population growth, urbanization and industrial development in developing regions.
In 1990 coastal cities and towns in Southern Africa discharged more than 850 million litres of industrial and human wastes into the sea daily through more than 80 pipelines, largely without any treatment (Cock and Koch 1991). In 1992, the lack of adequate infrastructure in Maputo caused significant coastal sewage and pollution problems, while in Angola untreated industrial waste pumped into the Bay of Luanda resulted in bacterial contamination (IUCN 1992). There are no immediate prospects of reducing the coastal pollution problems faced by many African countries.
Africa's coastal ecosystems are also threatened by industrial pollution, mining and oil exploration activities. Although the level of industrial development in Africa is still relatively low, the rate is accelerating along the coastal zone (World Bank 1995a). Most industries still discharge their untreated wastes directly into rivers and, ultimately, the oceans. The Mediterranean basin is now one of the most polluted, semi-enclosed basins in the world. But pollution also affects unenclosed seas. In 1993 industrial waste was found in the coastal waters near major centres along the entire coastline, stretching from Dar Es Salaam and Maputo on the east coast, to Durban and Cape Town in South Africa, and to Walvis Bay in Namibia and Boa do Cacuaco, 15 km north of Luanda in Angola (SARDC, IUCN AND SADC 1994).