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 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."