There are several factors which cause ocean pollution.
Pollution of the ocean by sewage - Many cities of the word dump garbage and sewage into the oceans. This consists of waste like human excrement, kitchen waste, bath water etc. Ships plying on the ocean also often release untreated swage into the water. These biological substances contains high levels of nutrients like nitrogen, phosphorus etc. This makes the environment perfect for the growth of algae. This is called eutrophicaton. Overgrowth of algae results in a depletion of the waters oxygen levels subsequent destruction of aquatic flora and fauna.

Pollution by oil- Oil spills are a main source of ocean pollution. Runoff, oil spills and offshore drilling contribute to this kind of ocean pollution. The layer of oil formed on the surface of the ocean damages the ecosystem in the ocean. The oil forms a layer over the wings and bodies of aquatic birds and thus reduces their ability to insulate the bird. As a result, birds die of hyperthermia. Oil pollution also results in the death of several kinds of fish.

Pollution by garbage- Garbage from coastal communities and garbage dumped from ships contribute to ocean pollution in a big way. Large pieces of garage can trap and kill animals in the ocean. Small pieces of garbage can be ingested by the fish and marine animals. This in turn can kill them.

Pollution by toxins- Untreated industrial waste, heavy metals like cadmium, mercury etc, persistent organic matter etc can cause severe impact, not only on marine life but on humans too. This is because these toxins enter the food chain through the body of fishes and as a result top predators like large fishes contain high levels of toxin in their bodies. When this is consumed by man, it enters their body and causes adverse effects like mercury and lead poisoning.

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