Ocean Dead Zones Growing; May Be Linked to WarmingBy 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