HE REGIONAL SEAS, hugging the coasts and largely surrounded by land, have been the cradles of many ancient civilizations and lent their names to many of the world’s regions, for example the Mediterranean, Caribbean and Baltic. With major trading cities and states huddling around their shores they have been routes for the exchange of goods, information and culture, and have supplied large populations with food, raw materials and, increasingly, entertainment and leisure.

Mostly occupying shallow continental shelves and closely tied to coastal ecosystems such as mangroves and coral reefs, regional seas are naturally rich in marine life. They have been viewed as an endless supply of fish, as well as a bottomless pit for garbage. But their capacity to absorb the impacts of human exploitation is in many cases being stretched to the limit.

Urbanization and rising population densities along their shores are turning many of these seas into reservoirs of undispersed pollution. Their sediment flows are becoming impoverished, their ecosystems are overfished and invaded by alien species, and sometimes even their circulation patterns are disrupted.

The Mediterranean, home to the Egyptian, Phoenecian, Greek and Roman empires, now has 160 million residents on its shores and a similar number of annual visitors. More than 500 million tons of sewage are poured into the sea each year, along with 120 000 tons of mineral oils, 60 000 tons of detergents, 100 tons of mercury, 3 800 tons of lead and 3 600 tons of phosphates¹. One fifth of all the world’s oil spills have occurred in its waters². It was the first sea to have its own treaty and an action plan to reduce pollution and protect coastal ecosystems – so far with only moderate success.

About 75 percent of marine pollution worldwide originates on land, reaching the sea either directly, down rivers or via the fallout of atmospheric pollution. Nutrients from agricultural run-off and sewage discharge are causing algal blooms that starve the waters of oxygen and drive away sea life – a process known as eutrophication. Inputs of nitrates to the North Sea in Northern Europe have risen fourfold and phosphate inputs eightfold since the 1970s³, causing eutrophication on its eastern shores and tides of toxic algae that have killed stocks in offshore fish farms.

One consequence of eutrophication can be the formation of “dead zones” on the seabed. As excessive amounts of algae die and decay, the water’s oxygen levels drop, depriving other species of the oxygen they need to survive. The collapse of the Baltic Sea cod fishery in the early 1990s is blamed on oxygen loss in deep waters, which interfered with cod reproduction. One of the largest dead zones has formed along the United States shoreline of the Gulf of Mexico⁴, where increasing volumes of fertilizer wash into the Gulf from the Mississippi river system. The river water is rich in nutrients from both pollution and natural sources which, along with the algae that feed on it, consume all the available oxygen. This hypoxic zone was first documented in 1972.

Offshore mining and the extraction of oil and gas reserves from the continental shelf are a further pollution threat to regional seas. The North Sea hydrocarbon industry, for instance, has left hundreds of piles of drill cuttings on the seabed. Contaminated with metals such as boron and cadmium as well as diesel used to lubricate drilling, there is an estimated 2 million tons of this debris spread across hundreds of square kilometers.

Enclosed seas, cut off from the wider ocean, have seen some of the worst ecological damage, including the loss of water itself. Forty years ago the Aral Sea in Central Asia was the fourth largest inland sea in the world. Its fisheries were sufficiently plentiful to feed the Soviet empire. But since then more than 90 percent of the water from the Amu Darya and Syr Darya rivers that feed it has been diverted to irrigate cotton fields in Uzbekistan and Turkmenistan. The sea has lost two thirds of its volume and almost all native organisms, including its 24 known fish species, have died out⁵. This decline has triggered further environmental damage as winds whip up sand on 3 million hectares of exposed salt and pesticide-impregnated seabed, contaminating ecosystems, water supplies and food over a wide area. Local doctors blame it for epidemics of anemia, kidney disease, cancer and other health problems⁶.

Increasingly, countries are banding together in an effort to save their seas. The environmental action plan for the Mediterranean has been followed by others for the Black, Aral, Baltic and North Seas. The North Sea programs have had conspicuous success in reducing the discharge of many pollutants and in lowering fish catch limits, though others have fared less well.