It is the most abundant chemical element by mass
in the Earth's biosphere, air, sea and land. Oxygen is the third most abundant
chemical element in the universe, after hydrogen and helium.[4] About 0.9% of the Sun's mass is oxygen.[7] Oxygen constitutes 49.2% of the Earth's crust by mass[8] and is the major component of the
world's oceans (88.8% by mass).[7] Oxygen gas is the second most common
component of the Earth's atmosphere,
taking up 20.8% of its volume and 23.1% of its mass (some 1015
tons).[7][41
Oxygen is an important part of the atmosphere, and
is necessary to sustain most terrestrial life as it is used in respiration.
However, low-oxygen areas are
expanding in deep waters, killing some creatures outright and changing how and
where others live. It may get much worse.
UNSUAL TREND
This phenomenon
could transform the seas as much as global warming or ocean acidification will,
rearranging where and what creatures eat and altering which species live or die. It already is starting to scramble ocean food chains and threatens to compound almost every other problem
in the sea.
“I don’t think people realize this
is happening right now,” says Lisa Levin,
an oxygen expert with the Scripps Institution of Oceanography, in San Diego.
Marlin
and sailfish
are the oceans’ perfect athletes. A marlin can outweigh a polar bear, leap through
the air, and traverse the sea from Delaware to Madagascar. Sailfish can outrace
nearly every fish in the sea. Marlin can hunt in waters a half mile down, and
sailfish often head to deep waters too.
Yet in more and more places around
the world, these predators are sticking near the surface, rarely using their
formidable power to plunge into the depths to chase prey.
The discovery of this behavioral
quirk in fish built for diving offers some of the most tangible evidence of a
disturbing trend: Warming temperatures are sucking
oxygen out of waters even far out at sea, making
enormous stretches of deep ocean hostile to marine life.
“Two hundred meters down, there is a
freight train of low-oxygen water barreling toward the surface,” says William Gilly,
a marine biologist with Stanford University’s Hopkins Marine Station, in
Pacific Grove, California. Yet, “with all the ballyhoo about ocean issues, this
one hasn’t gotten much attention.”
These are not
coastal dead zones, like the one that sprawls across the Gulf of Mexico, but
great swaths of deep water that can reach thousands of miles offshore. Already
naturally low in oxygen, these regions keep growing, spreading horizontally and vertically. Included are
vast portions of the eastern Pacific, almost all of the Bay of Bengal, and an
area of the Atlantic off West Africa as broad as the United States.
Globally, these
low-oxygen areas have expanded by more than 1.7 million square miles (4.5 million square kilometers) in the past 50
years.
BAD
WATER RISING
Few understand marlin and sailfish
better than biologist Eric
Prince. He has studied them in Jamaica, Brazil, the Ivory Coast,
and Ghana. He has examined their ear bones in Bermuda, taken tissue samples in
Panama, and gathered their heads—with bayonet-like bills still attached—during
fishing contests in Puerto Rico.
One day a decade ago (2005), while
tracking satellite tags attached to these fish, Prince saw something bizarre:
Marlin off North Carolina fed in waters as deep as 2,600 feet (800 meters). But
marlin off Guatemala and Costa Rica hovered high in the water, almost never
descending beyond a few hundred feet. Sailfish followed a similar pattern.
These billfish have special tissues
in their heads that keep their brains warm in deep water. So why were they
bunching up at the ocean’s surface?
The culprit, it turned out, was a
gigantic pool of low-oxygen water deep off Central America. These fish were
staying up high, trying
to avoid suffocating below.
One day a decade ago (2005), while
tracking satellite tags attached to these fish, Prince saw something bizarre:
Marlin off North Carolina fed in waters as deep as 2,600 feet (800 meters). But
marlin off Guatemala and Costa Rica hovered high in the water, almost never
descending beyond a few hundred feet. Sailfish followed a similar pattern.
These billfish have special tissues
in their heads that keep their brains warm in deep water. So why were they
bunching up at the ocean’s surface?
The culprit, it turned out, was a
gigantic pool of low-oxygen water deep off Central America. These fish were
staying up high, trying
to avoid suffocating below.
One day a decade ago (2005), while
tracking satellite tags attached to these fish, Prince saw something bizarre:
Marlin off North Carolina fed in waters as deep as 2,600 feet (800 meters). But
marlin off Guatemala and Costa Rica hovered high in the water, almost never
descending beyond a few hundred feet. Sailfish followed a similar pattern.
These billfish have special tissues
in their heads that keep their brains warm in deep water. So why were they
bunching up at the ocean’s surface?
The culprit, it turned out, was a
gigantic pool of low-oxygen water deep off Central America. These fish were
staying up high, trying
to avoid suffocating below.
Prince’s discovery
came just as other scientists were figuring out that rising temperatures were
expanding natural low-oxygen zones in the deep ocean, pushing them skyward by as much as a meter (three feet) per year.
Over the next
decade, researchers figured out that this change already was driving marine
creatures—sailfish, sharks,
tuna, swordfish,
and Pacific cod,
as well as the smaller sardines, herring, shad, and mackerel they eat—into ever
narrower bands of oxygen-rich water near the surface.
“It leaves just a
very thin lens on the top of the ocean where most organisms can live,” says Sarah Moffitt,
of the Bodega Marine Laboratory at the University of California, Davis.
Oxygen is so central to life, even
in the marine world, that its loss is harming animals in countless other ways,
too.
ways, too.
WARMING
WATERS
Fish, squid, octopus, and crab all
draw dissolved oxygen from the water. And just as oxygen levels shift with
elevation, oxygen at sea varies with depth. But in the ocean, oxygen is also
dynamic, changing daily and seasonally with weather and tides or over years
with cycles of warming and cooling.
Oxygen gets into the sea in two ways:
through photosynthesis, which takes place only near the top where light
penetrates, or through the mixing of air and water at the surface by wind and
waves.
Deep ocean waters hold far less
oxygen than surface waters because they haven’t been in contact with air for
centuries. And in many places, decomposing organic matter raining down from the
surface uses up what little oxygen remains. These natural deep-water “oxygen
minimum zones” cover great swaths of ocean interior.
The expansion of deep-sea low-oxygen
zones is driven by temperature. Warm water carries less dissolved oxygen. It’s
also lighter than cold water. That leaves the ocean segregated in layers,
restricting delivery of fresh oxygen to the deep and making these oxygen-poor
zones much bigger.
“The
natural thing to expect is that as the ocean gets warmer, circulation will slow
down and get more sluggish and the waters going into the deep ocean will hang
around longer,” says Curtis Deutsch, a chemical oceanography professor
at the University of Washington, in Seattle. “And indeed, oxygen seems to be
declining.”
The zone off West Africa that’s as
big as the continental United States has grown by 15 percent since 1960—and by
10 percent just since 1995. At 650 feet (200 meters) deep in the Pacific off
southern California, oxygen has dropped 30 percent in some places in a quarter
century.
CONSEQUENCE
Most researchers
project that oxygen loss will keep driving many species toward the surface,
reducing habitat for some and concentrating prey for birds, turtles, and other surface predators.
Winds in some
regions will draw the oxygen-depleted water to the surface and push it onto
shallower continental shelves. When oxygen drops there, some sensitive species
that can’t move die. Even survivors experience stress, which can make them
vulnerable to predators, disease, or overfishing.
This has already
begun. The waters of the Pacific Northwest, starting in 2002, intermittently
have gotten so low in oxygen that at times they’ve smothered sea cucumbers, sea
stars, anemones, and Dungeness crabs. This biologically rich region—where winds
draw waters from the deep 50 miles (80 kilometers) offshore and push them to
the beach—is temporarily transformed into a lifeless
wasteland.
Since the 1950s, researchers every
year have dropped nets 1,000 feet (300 meters) down to catalog marine life many
miles off California. Most track commercially important species caught by the
fishing industry. But J.
Anthony Koslow tallies fish often credited with
keeping marine systems functioning soundly—tiny midwater bristlemouths, the
region’s most abundant marine species, as well as viperfish, hatchetfish,
razor-mouthed dragonfish, and even minnow-like lampfish.
All are significant parts of the
seafood buffet that supports life in the eastern Pacific, and all are
declining dramatically with the vertical rise of
low-oxygen water.
“If it was a 10 percent change, it
wouldn’t have been worth noting, but they’ve declined by 63 percent,” says
Koslow, of the Scripps Institution of Oceanography. And “what’s been amazing is
it’s across the board—eight major groups of deep-sea fishes declining
together—and it’s strongly correlated with declining oxygen.”
Koslow can’t say
precisely why these fish populations have collapsed. But he suspects they, too,
now spend more time closer to the surface seeking oxygen. That puts these fish
during the day in a region where light
penetrates, making them easier pickings for birds, marine mammals, rockfish,
and other sight-feeders.
If that’s the
case, Koslow says, “the ramifications would be huge.”
Such tiny fish are
a massive food source around the world. Globally, they account for far more
mass in the sea than the entire world’s catch of fish combined. But there isn’t
enough historical data in other parts of the world to determine if the trend is
unique to California.
“They are central
to the ecology of the world’s oceans,” Koslow says.
PROSPECT
FROM RETROPECT
Scientists recently examined
marine sediment cores from a period of glacial melt
17,000 to 11,000 years ago.
During that time, global average air
temperatures rose 3 to 4 degrees Celsius, the closest historical analog for the
projected future, says study co-author Tessa Hill, of the Bodega Marine Laboratory.
“The idea here is … let’s take an interval with somewhat analogous warming and
see how low-oxygen zones responded,” Hill says.
The results: Low-oxygen areas
exploded around the world.
“What we found is that their
expansion was just extremely large and abrupt,” says lead author Moffitt.
“Their footprint across ocean basins grew much more than we had anticipated.”
One low-oxygen region off Chile and Peru—combined, the two countries now have an anchovy fleet that makes up the world’s largest single-species fishery—was much larger then, thousands of years ago. It
stretched from 9,800 feet (3,000
meters) deep to within 490 feet (150 meters) of the surface. And off
California, low-oxygen waters came far closer to the surface than they do
today.
Their research showed that
“environments we might think of as stable, like the deep ocean, may not be so
stable at all,” Moffitt says.
In the blink of an eye, geologically
speaking, entire ocean basins changed. And many scientists suspect they are
doing so once again, at a cost they can’t yet quantify.
Source:
http://en.wikipedia.org/wiki/Oxygen
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