Ocean Dead Zones Growing; May Be Linked to Warming
By James Owen, National Geographic News
May 1, 2008
HYypoxic zones—swaths of ocean too oxygen-deprived to support fish and other marine organisms—are rapidly expanding as sea temperatures rise, a new study suggests.
Researchers have tracked a decline in dissolved oxygen levels since 1960 in the tropical Pacific and Atlantic Oceans, which has extended the size of these undersea deserts and intensified their effects.
The oxygen level in these zones "is below the critical oxygen level for fish and other large marine animals," said team leader Lothar Stramma, of the Leibniz Institute of Marine Sciences at the University of Kiel in Germany.
The team constructed a time line of oxygen concentrations at depths of between 985 and 2,295 feet (300 and 700 meters) using oxygen data records going back 50 years. The results fit predictions of the effects of global warming.
The oxygen declines were found to be most marked in tropical Atlantic regions, the study team reports in the latest issue of the journal Science.
In the east Atlantic, for example, the low-oxygen layer was found to have increased in height by 85 percent, growing from 1,215 to 2,265 feet (370 to 690 meters).
"The vertical area covered by some of these layers has almost doubled in the Atlantic," Stramma said.
Conditions have also become more suffocating for life within these hypoxic waters, he said.
"In general this low-oxygen zone had widened, and in some areas the oxygen value also got lower."
The study team suspects these underwater deserts are also spreading horizontally to cover wider areas of ocean, though more research is needed.
"We think there are areas that are extending, but we don't have the maps to show that right now," Stramma said.
The study team notes that seas have warmed substantially over the past 50 years and that climate models predict falling levels of oceanic oxygen in response to global warming.
Rising temperatures can prevent oxygen-rich surface waters from circulating to lower depths, since water becomes less dense as it warms, Stramma explained.
"Then you have less ventilated water for deep and middle layers, which means you have less oxygen supply from the surface," he said. "That is a problem for the larger fish, which need a lot of energy."
Commercial fisheries, particularly in the tropical east Atlantic, could suffer, he added.
"This is an area where there are a lot of tunas," Stramma said. To feed, "bigeye tuna go quite deep to 250 meters [850 feet], so for these guys the area where they live reduces.
"We see these low-oxygen zones rising [in depth]," Stramma added. "This means they may move on to the [continental] shelf and influence areas of intense fishing."
Hypoxic zones also severely reduce overall biological productivity, which reverberates throughout the food chain, he said. (Related: "Porpoises Starving in Europe Due to Ocean Warming" [January 10, 2007].)
Other recent studies have also noted a long-term trend of falling oxygen levels in non-tropical seas. In the sub-Arctic Pacific, for instance, reduced oxygen concentrations have been reported at depths of 330 to 1,310 feet (100 to 400 meters) between 1956 and 2006.
While the new study suggests human-caused global warming as a possible cause, the team notes that oxygen-deprived seas aren't a new phenomenon.
Paleontological records show that Earth has experienced episodes of much more severe ocean suffocation in its past.
For instance, drastically reduced oxygen levels linked to an era of global warming marked the planet's worst ever extinction event: the end of the Permian period 251 million years ago, when more than 90 percent of all marine species were wiped out.
Today, however, other factors may be amplifying the effects of global warming in spurring the growth of tropical hypoxic zones, said Thomas Wagner, professor of earth systems science at Newcastle University, U.K.
Local human impacts due to land use changes may contribute to the effects highlighted in the new study, he said.
"If you warm up the atmosphere, at the same time you increase the activity of water in the atmosphere," which in turn leads to increased rainfall and river flows into tropical coastal waters, Wagner said.
These flows contain organic runoff such as agricultural fertilizers and sewage, fueling algal blooms that consume the oxygen in seas, he said. (Related: "Gulf of Mexico 'Dead Zone' Is Size of New Jersey" [May 25, 2005].)
"This is a second mechanism which may also have an impact on [oceanic] oxygenation in certain areas," he said.
"Many mechanisms can play together, and it's very difficult to identify which is the leading one," he added. "I think we are still far away from being really confident of how an ocean system will respond [to global warming]."