Marine Life Gasping for Breath?

ScienceNOW Daily News, May 1, 2008

Chalk up another environmental problem that could be stemming from global warming: New research shows that oxygen is vanishing from ever-larger swaths of the oceans. If the trend continues, it could disrupt marine ecosystems.

Warm water can't hold as much oxygen as cold water. So ever since scientists began to worry about the impact of rising ocean temperatures, they have been gathering data on oceanic oxygen levels. Up to now, however, most of the research has focused on colder waters, where much of the world's commercial fishing occurs. Researchers knew very little about the issue in the tropics, which contain some of the world's most diverse marine ecosystems.

To fill that gap, an international team cobbled together all available data on oxygen content of tropical waters collected since 1960, concentrating on six areas for which the records were the most complete. The figures came from sources such as research vessels, buoys, and satellites. The team also added some of its own measurements for the same six areas.

The results were alarming: Levels of dissolved O₂ have dropped in some cases by more than 15% during the past 5 decades. Areas such as the tropical Atlantic off the African coast suffered even more dramatic changes, says physical oceanographer and co-author Janet Sprintall of the Scripps Institution of Oceanography in San Diego, California. The top-to-bottom thickness of the low-oxygen zone in that region has expanded by 85% during that time, the team reports tomorrow in Science. "The results definitely exceeded my expectations," Sprintall says.

The team doesn't yet know why some areas are worse off than others, adds physical oceanographer and lead author Lothar Stramma of the University of Kiel in Germany. Although models predict oxygen reduction due to warming, he says, ocean circulation may also play a role, perhaps by transporting oxygen away.

A real worry is the vertical expansion of what scientists call open-ocean oxygen-minimum zones, or OMZs, says biological oceanographer Lisa Levin, who is also at the Scripps Institution but was not involved in the research. When regions of low O₂ extend vertically, she says, it can alter patterns of vertical migration by fish and plankton. It can also disrupt predator-prey interactions and food webs.

Marine biogeographer John Guinotte of the Marine Conservation Biology Institute in Bellevue, Washington, notes that nonmobile organisms such as cold-water corals could also be at risk if dissolved oxygen levels continue to decline. Such creatures "don't have the luxury of being able to swim away from increasingly inhospitable conditions."

http://sciencenow.sciencemag.org/cgi/content/full/2008/501/1

'Ocean deserts' are growing

Low-oxygen regions have expanded over the past half-century.

Nature.com, May 1, 2008

Low-oxygen 'underwater deserts' in the tropical oceans have expanded over the past 50 years, according to new measurements.

The most likely cause of the change is global warming, and climate models predict that the trend will continue, potentially threatening marine ecosystems.

The discovery concerns a layer of the ocean called the 'oxygen-minimum zone', where concentrations of dissolved oxygen are particularly low. The new study shows that this zone has been expanding both upwards and downwards into the adjacent layers in tropical waters.

Climate models predict that warming of the sea's surface as a result of human activity will hamper the mixing of oceanic waters, preventing dissolved oxygen from mixing evenly through the water column. The new results suggest that this process has already begun.

Researchers led by Lothar Stramma of the University of Kiel, Germany, measured the oxygenation of the oceans at depths of between 300 and 700 metres during a series of observation cruises in tropical regions of the world's three main oceans. They added their new data to previous oxygen measurements to build up a picture of the trend over the past 50 years.

Overall levels of oxygen have dropped in these zones, Stramma and his colleagues report in Science ¹. Regions of the eastern tropical Pacific Ocean and the northern reaches of the Indian Ocean are now classed as 'suboxic', meaning that the amount of oxygen has dropped sufficiently to harm the functioning of ecosystems.

Starving waters

In suboxic waters, nitrogen cannot react with oxygen to form biologically available nitrate. This means that organisms at the base of food chains, such as plankton, do not get enough nutrients to survive, Stramma explains.

The ultimate effect on commercially important ecosystems such as fisheries are difficult to predict, Stramma adds. "There are many complicated mechanisms involved that we need to understand better to predict changes for the future," he says. "I see our results as a starting point to be able some day to tell what changes in biogeochemistry, biology and fisheries we have to expect."

Any effect on fisheries is likely to be indirect, because these low-oxygen zones are far from the coastal waters that host most commercial fishing, suggests Andrew Solow, director of the Marine Policy Center at Woods Hole Oceanographic Institution in Massachusetts. "I don't know many fisheries that take place between 300 and 700 metres in the tropical ocean," he says.

These 'underwater deserts' should not be confused with the 'dead zones' created in coastal waters, most famously in the Gulf of Mexico, by runoff of nitrogen-rich fertilizer, Solow adds. Coastal waters lose their oxygen as a result of booms in phytoplankton growth; when these organisms die, they provide food for microbes that suck up all of the oxygen.

Going down

"It's a worrying trend," comments Laurence Mee, director of the Marine Institute at the University of Plymouth, UK. "This is one more piece in the argument that we need to do something about climate change."

Mee agrees that it's quite difficult to say whether, or how much, the decline in oxygen levels will affect ecosystems, including economically important ones. If the low-oxygen zones extend closer to the surface, they may reach the shallow, sunlit waters where many valuable fish species live. "When you start to mess around with the food chain, it has all kinds of knock-on effects that we don't know about yet," Mee says.

Team member Gregory Johnson of the National Oceanic and Atmospheric Administration in Seattle says that he and his colleagues now plan to take more measurements, to see whether the low-oxygen zones are spreading across the oceans, or spreading upwards and downwards within the water column.

References

1. Stramma L., Johnson., G.C., Sprintall, J. & Mohrholz, V., Science 320 655-658

http://www.nature.com/news/2008/080501/full/news.2008.795.html