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Sources of Methane - The Oceans

seaGlobally, oceans are thought to add around 15 million tonnes of methane to the atmosphere each year. Parts of the surface of the world's oceans have relatively high concentrations of dissolved methane.

The methane is largely produced by methanogenic microbes called Archaea which, because they need anaerobic conditions, are to be found producing methane within sinking particles in the surface waters.

The methane produced in this way then diffuses into the surrounding water and is emitted to the atmosphere from the ocean surface.

Large amounts of oceanic methane also arise from methanogenesis in marine sediments, particularly in nutrient rich areas such as those of estuaries.

Human Impact

Man's impact on oceanic methane emission lies primarily in our effect on oceanic nutrient inputs through rivers and estuaries. The nutrient rich waters in many rivers, produced by sewage input and agricultural run-off, lead to eutrophic conditions in estuaries and coastal waters.

Such nutrient rich waters and sediments are ideal for methanogenesis, with oxygen levels in the water often being very low and with plenty of organic carbon on which the methanogenic bacteria are able to grow.

Potential for control

Eutrophication of estuaries and coastal waters through sewage and agricultural run-off has many damaging effects on the environment, including toxic algal blooms aswell as increased nitrous oxide and methane emissions.

Steps have been taken in some countries to reduce key pollutants, such as nitrate, through better land management practices. Similarly, efficient sewage treatment has also led to reductions in levels of coastal eutrophication in some areas. However, in many parts of the world such marine pollution continues to increase in line with a growing human population and the need for ever more intensive agriculture.

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