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 
habitat loss.

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 say.

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 
food chain.

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.