Suffocating dead zones spread across world's oceans
Critically low oxygen levels now pose as great a threat to life
in the world's oceans as overfishing and habitat loss, say experts
* David Adam, environment correspondent
* Friday August 15 2008 00:01 BST
With more than 400 oxygen-starved dead zones in global coastal waters,
scientists are calling for such dead zones to be recognised as one of
the world's great environmental problems
Man-made pollution is spreading a growing number of suffocating dead
zones across the world's seas with disastrous consequences for marine
life, scientists have warned.
The experts say the hundreds of regions of critically low oxygen now
affect a combined area the size of New Zealand, and that they pose as
great a threat to life in the world's oceans as overfishing and
The number of such seabed zones - caused when massive algal blooms
feeding off pollutants such as fertiliser die and decay - has boomed
in the last decade. There were some 405 recorded in coastal waters
worldwide in 2007, up from 305 in 1995 and 162 in the 1980s.
Robert Diaz, an oceans expert at the US Virginia Institute of Marine
Science, College of William and Mary, at Gloucester Point, said:
"Dead zones were once rare. Now they're commonplace. There are
more of them in more places."
Marine bacteria feed on the algae in the blooms after it has died and
sunk to the bottom, and in doing so they use up all of the oxygen
dissolved in the water. The resulting 'hypoxic' seabed zones can
asphyxiate swathes of bottom dwelling organisms such as clams and
worms, and disrupt fish populations.
Diaz and his colleague, Rutger Rosenberg of the department of marine
ecology at the University of Gothenburg, call for more careful use of
fertilisers to address the problem.
Writing in the journal Science, the researchers say the dead zones
must be viewed as one of the "major global environmental
problems". They say: "There is no other variable of such
ecological importance to coastal marine ecosystems that has changed so
drastically over such a short time."
The key solution, they say, is to "keep fertilisers on the land
and out of the sea". Changes in the way fertilisers and other
pollutants are managed on land have already "virtually
eliminated" dead zones from the Mersey and Thames estuaries, they
Diaz says his concern is shared by farmers who are worried about the
high cost of fertilisers. "They certainly don't want to see their
dollars flowing off their fields. Scientists and farmers need to
continue working together to minimise the transfer of nutrients from
land to sea."
The number of dead zones reported has doubled each decade since the
1960s, but the scientists say they are often ignored until they
provoke problems among populations of larger creatures such as fish or
lobsters. By killing or stunting the growth of bottom-dwelling
organisms, the lack of oxygen denies food to creatures higher up the
The Baltic Sea, site of the world's largest dead zone, has lost about
30% of its available food energy, which has led to a significant
decline in its fisheries.
The lack of oxygen can also force fish into warmer waters closer to
the surface, perhaps making them more susceptible to disease.
The size of marine dead zones often fluctuates with the seasons. A
massive dead zone, some 8,000 square miles across, forms each summer
in the Gulf of Mexico as floodwater flushes nitrogen-rich fertiliser
into the Mississippi River.
Experts said it was slightly smaller than expected this year because
Hurricane Dolly stirred up the water. Dead zones require the water to
be separated into layers, with little or no mixing between.
As well as fertilisers rich in nitrates and phosphates, sewage
discharges also contribute to the problem because they help the algal
blooms to flourish.
Diaz and Rosenberg say: "We believe it would be unrealistic to
return to pre-industrial levels of nutrient input [to oceans], but an
appropriate management goal would be to reduce nutrient inputs to
levels that occurred in the middle of the past century," before
the rise in added nutrients began to spread dead zones globally.
Climate change could be adding to the problem. Many regions are
expected to experience more severe periods of heavy rain, which could
wash more nutrients from farmland into rivers.
In May, scientists reported that oxygen-depleted zones in tropical
oceans are expanding. They analysed oxygen levels in samples of
seawater and found the effect was largest in the central and eastern
tropical Atlantic and the equatorial Pacific. The increase could push
oxygen-starved zones closer to the surface and give marine life such
as fish less room to live and look for food.
The scientists, led by Lothar Stramma from the Leibniz Institute of
Marine Sciences in Kiel, Germany, say the change could be linked to
warming seas. At 0C, a litre of seawater can hold about 10ml of
dissolved oxygen; at 25C this falls to 4ml. Stramma said:
"Whether or not these observed changes in oxygen can be
attributed to global warming alone is still unresolved." The
reduction could also be down to natural processes working on shorter
timescales, he said.