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Posts Tagged ‘suboxic waters’

Ocean Deserts Source of GHG

Posted by feww on March 12, 2010

Dead zones contribute to climate change

Hypoxic Waters Elevate Greenhouse Gasses in the Atmosphere

A University of Maryland Center for Environmental Science oceanographer says  that the increased amount of nitrous oxide (N2O) produced in aquatic dead zones, low-oxygen (hypoxic) waters, increases concentrations of the potent GHG in the atmosphere, worsening the impacts of global warming and contributing to the widening of ozone “holes” that allow harmful UV radiation through.

Eutrophication in the Sea of AzovEutrophication is caused by human activity. (Source: NASA).

“As the volume of hypoxic waters move towards the sea surface and expands along our coasts, their ability to produce the greenhouse gas nitrous oxide increases,” explains Dr. Codispoti of the UMCES Horn Point Laboratory. “With low-oxygen waters currently producing about half of the ocean’s net nitrous oxide, we could see an additional significant atmospheric increase if these ‘dead zones’ continue to expand.”

N2O, a highly potent greenhouse gas, is present in minute concentrations in Earth’s atmosphere, and is now a major factor in the destruction of the stratospheric ozone layer. “For the past 400,000 years, changes in atmospheric N2O appear to have roughly paralleled changes in carbon dioxide CO2 and have had modest impacts on climate, but this may change. Just as human activities may be causing an unprecedented rise in the terrestrial N2O sources, marine N2O production may also rise substantially as a result of nutrient pollution, warming waters and ocean acidification. Because the marine environment is a net producer of N2O, much of this production will be lost to the atmosphere, thus further intensifying its climatic impact,” a UMC news release said.

As dissolved oxygen levels decline in coastal waters, the N2O production increases. “Under well-oxygenated conditions, microbes produce N2O at low rates. But at oxygen concentrations decrease to hypoxic levels, these waters can increase their production of N2O.”

Shallow suboxic and hypoxic waters produce high rates of N2O “because respiration and biological turnover rates are higher near the sunlit waters where phytoplankton produce the fuel for respiration.”

“When suboxic waters (oxygen essentially absent) occur at depths of less than 300 feet, the combination of high respiration rates, and the peculiarities of a process called denitrification can cause N2O production rates to be 10,000 times higher than the average for the open ocean. The future of marine N2O production depends critically on what will happen to the roughly ten percent of the ocean volume that is hypoxic and suboxic.

“Nitrous oxide data from many coastal zones that contain low oxygen waters are sparse, including Chesapeake Bay,” said Dr. Codispoti. “We should intensify our observations of the relationship between low oxygen concentrations and nitrous oxide in coastal waters.”

The article “Interesting Times for Nitrous Oxide” appears in the March 12, 2010 edition of the journal Science.

University of Maryland Center for Environmental Science

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