Alister Doyle, Reuters 1 May 08;
OSLO (Reuters) - Global warming could gradually starve parts of the tropical oceans of oxygen, damaging fisheries and coastal economies, a study showed on Thursday.
Areas of the eastern Atlantic and Pacific Oceans with low amounts of dissolved oxygen have expanded in the past 50 years, apparently in line with rising temperatures, according to the scientists based in Germany and the United States.
And models of global warming indicate the trend will continue because oxygen in the air mixes less readily with warmer water. Large fish such as tuna or swordfish avoid, or are unable to survive, in regions starved of oxygen.
"Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies," according to the scientists writing in the journal Science.
The north of the Indian Ocean, along with the Arabian Sea and the Bay of Bengal, is also oxygen-low but the available data showed no substantial change in the size of the oxygen-minimum zone in recent decades.
Lothar Stramma, lead author at IFM-GEOMAR in Kiel, Germany, said there were signs the oxygen-low bands between 300 and 700 meters depths were getting wider and moving into shallower coastal waters.
"The expansion of the oxygen-minimum zones is reaching more to the continental shelf areas," he told Reuters. "It's not just the open ocean." That could disrupt ever more fisheries.
Problems of lower oxygen supply add to woes for the oceans led by over-fishing as the world struggles to feed an expanding population. A U.N. conference in 2002 set a goal of trying to reverse declines in fish stocks by 2015.
The scientists said levels of dissolved oxygen in the oceans had varied widely in the past and more study was needed. "We are far from knowing exactly what will happen," Stramma said.
In the most extreme case, at the end of the Permian period about 250 million years ago, there were mass extinctions on land and at sea linked to high levels of carbon dioxide and extremely low oxygen levels in the waters.
The U.N. Climate Panel said last year that global warming, stoked by human use of fossil fuels, would push up temperatures and bring more droughts, floods, heatwaves and rising sea levels. More and more species would be at risk of extinction.
Thursday's study showed that a swathe of the eastern Pacific from Chile to the United States and a smaller part of the eastern Atlantic, centered off Angola, were low in oxygen.
Stramma said the oxygen-poor regions were away from major ocean currents that help absorb oxygen from the air. And warmer water is less dense and so floats more easily -- that makes it less prone to mix with the deeper levels of the oceans.
(Editing by Matthew Jones)
Ocean Dead Zones Growing; May Be Linked to Warming
James Owen, National Geographic News 1 May 08;
The world's hypoxic 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."
Underwater Deserts
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.
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.
Past Episodes
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.
"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]."
Study: Warmer ocean water means less oxygen
Randolph E. Schmid, Associated Press Yahoo News 1 May 08;
Low-oxygen zones where sea life is threatened or cannot survive are growing as the oceans are heated by global warming, researchers warn.
Oxygen-depleted zones in the central and eastern equatorial Atlantic and equatorial Pacific oceans appear to have expanded over the last 50 years, researchers report in Friday's edition of the journal Science.
Low-oxygen zones in the Gulf of Mexico and other areas also have been studied in recent years, raising concerns about the threat to sea life.
Continued expansion of these zones could have dramatic consequences for both sea life and coastal economies, said the team led by Lothar Stramma of the University of Kiel in Germany.
The finding was not surprising, Stramma said, because computer climate models had predicted a decline in dissolved oxygen in the oceans under warmer conditions.
Warmer water simply cannot absorb as much oxygen as colder water, explained co-author Gregory C. Johnson of the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle.
There are complex biological and chemical interactions in these low-oxygen regions, Stramma said, adding that this needs to be more closely studied.
Frank A. Whitney of Canada's Institute of Ocean Sciences said, "As oceans lose oxygen, this will reduce habitat for many organisms."
"Many species will lose their deep habitat, meaning competition will become stronger in the remaining favorable habitat, and increased vulnerability to predation will likely occur," said Whitney, who was not part of Stramma's team.
He said the most rapid oxygen declines he has seen have occurred in the subarctic Pacific Ocean, and fish and crab kills have been reported in the last few years off Oregon.
Steven J. Bograd, a research oceanographer at the National Oceanic and Atmospheric Administration's Environmental Research Division in Pacific Grove, Calif., called the finding "compelling" but not surprising.
Bograd, who also was not part of Stramma's team, has studied trends in dissolved oxygen in the ocean off California, finding an expansion of the area of the continental shelf there that is exposed to low-oxygen conditions.
"So, why should we care?" Bograd said. "Most marine species have minimum oxygen thresholds that they need for survival. As oxygen decreases, these animals will suffer and/or be compelled to move to other areas. Over time, the optimal area for various species will be compressed," he explained.
Bograd's findings are reported in a paper scheduled for publication in Geophysical Research Letters.
"We are not able to say definitively what has caused the oxygen declines off California. But we do know that waters from the eastern tropical Pacific" — a reduced-oxygen area studied by Stramma — flow into this region, Bograd said.
"So, their results and ours are consistent," he said, adding that there could also be other processes at work off California.
The general pattern is for colder ocean waters in the north and south to absorb oxygen, cool and sink below the surface to then flow toward the equator, Johnson explained.
Along the way, organic matter drifts down into the deeper water and its decay uses up some of this oxygen.
The oxygen balance depends on this movement and the amount of oxygen reaching the warmer waters, Johnson said, and this can be reduced if less is absorbed and moved in the deep currents.
"That means that eventually, at the end of the line, there will be less oxygen," he said.
In cold surface water, oxygen levels can reach as high as 300 to 400 micromols per kilogram, Johnson said. A mol of a gas such as oxygen occupies a volume of just under six gallons and a micromol is one-thousandth of that. A kilogram of water is the amount that would weigh 2.2 pounds.
Dissolved oxygen varies widely in the oceans, and sea life becomes stressed when it reaches between 60 and 120 micromols per kilogram.
The researchers found concentrations as low as 10 in parts of the eastern Pacific and the northern Indian Ocean and larger areas in the Atlantic and Pacific were below 150.
Stramma's team noted declines in affected areas ranging from 0.09 to 0.34 per year over the last half century.
The research was funded by the German Research Council, U.S. National Oceanic and Atmospheric Administration and U.S. National Science Foundation.
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