Ocean: Dead zones increase climate change

10, 000-fold amount of nitrous oxide from oxygen-depleted zones in coastal waters

Zones of oxygen depletion on the American coast. It often occurs as a result of summer plankton flowers, here red. © NASA / SVS / GSFC
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Flat coastal waters, when deficient in oxygen, emit 10, 000 times more of the strong greenhouse gas nitrous oxide than is normally the case with seawater. Scientists report this in Science. As these "dead zones" spread through warming and over-fertilization, they could significantly fuel climate change.

Nitrous oxide, chemically nitrous oxide (N2O), is an extremely powerful greenhouse gas and a key factor in stratospheric ozone depletion. However, its content in the atmosphere is still negligible. But that could change. Because the release of the gas, which is mainly produced in nutrient-rich, oxygen-poor soils and waters of bacteria, increases. Now American researchers have discovered that even the so-called "dead zones" in the ocean play a key role in nitrous oxide release.

The "dead zones" are areas of the seabed and deeper water layers where the oxygen content of the water is extremely low or even oxygen-free - anoxic. Such areas have been exploited in recent years in some coastal areas of the oceans, as a result of over-fertilization and warming of seawater. Lou Codispoti of the University of Maryland's Center for Environmental Science and his colleagues have now studied the mechanisms of nitrous oxide production in these waters.

Coastal areas with 10, 000-fold laughing gas emissions

They found that N2O production is particularly high when the dead zones lie in shallow coastal waters less than 90 meters deep. Because here, in sun-flooded, but still low-oxygen water, the production rates of nitrogen bacteria are particularly high. Driven by abundant nutrients, phytoplankton provides a great deal of organic material, from which the nitrogen bacteria deplete. Up to 10, 000x the average open ocean price, N2O production rates can climb in these areas. In view of the strong greenhouse effect of the gas not a good prospect for the climate.

Strong greenhouse gas increase possible

"As the volume of hypoxic waters expands along our shores and moves toward the ocean surface, so does their production of nitrogen oxide greenhouse gases, " explains Codispoti. "The oxygen-depleted marine areas currently produce about half of the total nitrogen oxide release from the oceans. As these zones continue to expand, we will see a significant increase in atmospheric concentrations. Display

The future of marine nitrous oxide production, as well as of the climate and the ozone layer, could therefore crucially depend on how the so far around ten percent of the water volume develops, which are low in oxygen or free of oxygen. "Low-level gas data from many coastal areas with low-oxygen water have so far been dormant, " according to Codispoti. We should therefore further intensify our observations of the relationship between low oxygen concentrations and nitrous oxide in coastal waters.

(University of Maryland Center for Environmental Science, 15.03.2010 - NPO)