Updated: Oct 22, 2019
Oil Oil spills cause huge damage to the marine environment - but in fact are responsible for only around 12% of the oil entering the seas each year. According to a study by the US National Research Council, 36% comes down drains and rivers as waste and runoff from cities and industry.
Over 80% of marine pollution comes from land-based activities.
Seas of garbage Solid garbage also makes its way to the ocean. Plastic bags, balloons, glass bottles, shoes, packaging material – if not disposed of correctly, almost everything we throw away can reach the sea.
Plastic garbage, which decomposes very slowly, is often mistaken for food by marine animals. High concentrations of plastic material, particularly plastic bags, have been found blocking the breathing passages and stomachs of many marine species, including whales, dolphins, seals, puffins, and turtles. Plastic six-pack rings for drink bottles can also choke marine animals.
This garbage can also come back to shore, where it pollutes beaches and other coastal habitats.
Sewage disposal In many parts of the world, sewage flows untreated, or under-treated, into the ocean. For example, 80% of urban sewage discharged into the Mediterranean Sea is untreated.
This sewage can also lead to eutrophication. In addition, it can cause human disease and lead to beach closures.
Toxic chemicals Almost every marine organism, from the tiniest plankton to whales and polar bears, is contaminated with man-made chemicals, such as pesticides and chemicals used in common consumer products.
Some of these chemicals enter the sea through deliberate dumping. For centuries, the oceans have been a convenient dumping ground for waste generated on land. This continued until the 1970s, with dumping at sea the accepted practise for disposal of nearly everything, including toxic material such as pesticides, chemical weapons, and radioactive waste. Dumping of the most toxic materials was banned by the London Dumping Convention in 1972, and an amended treaty in 1996 (the London Convention) further restricted what could be dumped at sea. However, there are still the problems of already-dumped toxic material, and even the disposal of permitted substances at sea can be a substantial environmental hazard.
Chemicals also enter the sea from land-based activities. Chemicals can escape into water, soil, and air during their manufacture, use, or disposal, as well as from accidental leaks or fires in products containing these chemicals. Once in the environment, they can travel for long distances in air and water, including ocean currents.
People once assumed that the ocean was so large that all pollutants would be diluted and dispersed to safe levels. But in reality, they have not disappeared - and some toxic man-made chemicals have even become more concentrated as they have entered the food chain.
Tiny animals at the bottom of the food chain, such as plankton in the oceans, absorb the chemicals as they feed. Because they do not break down easily, the chemicals accumulate in these organisms, becoming much more concentrated in their bodies than in the surrounding water or soil. These organisms are eaten by small animals, and the concentration rises again. These animals are in turn eaten by larger animals, which can travel large distances with their even further increased chemical load.
Animals higher up the food chain, such as seals, can have contamination levels millions of times higher than the water in which they live. And polar bears, which feed on seals, can have contamination levels up to 3 billion times higher than their environment.
People become contaminated either directly from household products or by eating contaminated seafood and animal fats.
Evidence is mounting that a number of man-made chemicals can cause serious health problems - including cancer, damage to the immune system, behavioural problems, and reduced fertility.