The seas have been used as a dumping ground for debris during many centuries. With human civilization consuming more raw materials than any time in history, the volume and variety of waste is greater than ever. Wastes that end up in the sea include plastic, metals, pesticides, persistent toxic and even radioactive chemicals that derive from agriculture and industrial sources.
Nevertheless, not all pollutions involve man-made chemicals. Any substance put into the oceans in appropriate quantities can be harmful. Slit, for example, is a fine sediment that occurs naturally in rivers and is ultimately washed out to sea. But in areas where deforestation leads to excess erosion, the slit load can be immense, choking life closer to the river estuary.
Oil is another natural substance, but released into the sea it can cause severe damage not only to the environment, but to all the species that rely on the water. Oil spills mainly come from oil tankers that run aground. Smaller and more frequent spills that come from ships that illegally wash out their tanks can be just as destructive to marine life. Spills may take weeks, months or even years to clean up.
Find out more about ocean pollution at library.thinkquest.org