The main danger resulting from ocean contamination from runoff is the pervasive impact upon the food chain. Chemical contaminants do not just poison or contaminate the flesh of fish and large sea creatures: but also smaller animals eaten by fish, and the plankton eaten by the smallest sea creatures. The death of plankton by chemical contamination is devastating because it is the basis of the ocean food chain, and will ultimately lead to the extinction and decimation of many species.
River runoff is also damaging but at a more immediate human level. Rivers are ecosystems that many communities rely upon for food and drinking water. Contamination makes it dangerous to eat fish that contain PCBs, heavy metals, mercury or other chemicals. Ingestion of contaminated fish can lead to many illnesses and serious health problems, including thyroid disorders and various cancers.
Because runoff from industrialized countries such as ours carries with it pollution and waste which continually and consistently damage the ocean's ecosystems.
Just now are we starting to see serious environmental efforts at limiting polluted runoff from sewer systems, highways, waterfront developments, etc. In the meantime, huge volumes of waste and hazardous materials have already flowed into the oceans, damaging fisheries, coastlines and the ability of some species to survive.
The ocean covers over 75% of earth's surface. The rivers carry waste products towards the ocean. When rivers become contaminated with toxic waste the toxic materials travel down the rive into the ocean. Once the chemicals reach the ocean, they begin to disburse.
Some chemicals may result in the death's of certain organisms. Once other creatures feed on the organisms then they become contaminated. Humans may catch and consume the creatures, such as fish or crabs,and become contaminated as well. Side effects may include birth defects, higher level of toxins in the system, and could eventually include death. In addition, any animal life or plant life that ceases to exist would lead to long term damage to the water system and disturb the balance of the water cycle and food chain.