Study: Warmer ocean water means less oxygen
Pastor Dale Morgan
Study: Warmer ocean water means less oxygen*
By RANDOLPH E. SCHMID,
AP Science Writer
WASHINGTON - 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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