^w^ Anything added to the ocean that does not belong there is pollution. Humans are the major source of pollution.
^w^ Instances of widespread disease may be caused by pollution. Because the ocean is so large, it is difficult to know the exact effects of pollution.
^w^ Many fish that live in heavy industrial areas show high levels of chemicals, particularly mercury.
^w^ Oil spills clog the surface of the water and prevent sunlight from reaching the oxygen-producing phytoplankton, thus depriving all successive layers of the food chain of nourishment. Oil poisons many ocean dwellers outright.
