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
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.
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
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
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