[Mike]

http://news.bbc.co.uk/2/hi/science/nature/3577711.stm

By Alex Kirby
BBC News Online environment correspondent in Jeju, Korea

Oxygen depletion: Set to have a big impact in the 21st century
Sea areas starved of oxygen will soon damage fish stocks even more
than unsustainable catches, the United Nations believes.
The UN Environment Programme says excessive nutrients, mainly
nitrogen from human activities, are causing these “dead zones” by
stimulating huge growths of algae.

Since the 1960s the number of oxygen-starved areas has doubled every
decade, as human nitrogen production has outstripped natural sources.

Unep made its remarks as it launched its Global Environment Outlook
Year Book 2003.

Human disturbance

About 75% of the world’s fish stocks are already being overexploited,
but Unep says the dead zones, which now number nearly 150 worldwide,
will probably prove a greater menace.

Unless urgent action is taken to tackle the sources of the problem,
it is likely to escalate rapidly

Dr Klaus Toepfer
Unep executive director
It quotes research by a team of scientists at the Virginia Institute
of Marine Science in the US.

They concluded: “The history and pattern of human disturbance in
terrestrial, aquatic, coastal and oceanic ecosystems have brought us
to a point at which oxygen depletion is likely to become the keystone
impact for the 21st Century, replacing the 20th Century keystone of
overfishing.”

Ironically, Unep says, nitrogen is desperately needed in parts of the
world, including much of Africa, where the lack of it is reducing
farmers’ yields.

Washed away

The amount of nitrogen used as fertiliser globally is 120 million
tonnes a year, more than the 90 million tonnes produced naturally.

Yet only 20 million tonnes of that is retained in the food we eat,
with the rest washed away into rivers and out to sea.

The burning of fossil fuels in vehicles and power plants, and of
forests and grasslands, and the draining of wetlands all contribute
more nitrogen to the cycle.

This leads to the explosive blooms of algae, tiny marine plants,
which sink to the seabed and decompose, using up all the oxygen, and
suffocating other marine life.

Unep’s executive director, Dr Klaus Toepfer, said: “Humankind is
engaged in a gigantic global experiment as a result of the
inefficient and often excessive use of fertilisers, the discharge of
untreated sewage, and the ever-rising emissions from vehicles and
factories.

“Hundreds of millions of people depend on the marine environment for
food, for their livelihoods and for their cultural fulfilment. Unless
urgent action is taken to tackle the sources of the problem, it is
likely to escalate rapidly.”

Remedies available

Some of the dead zones are less than a square km in size, while
others are up to 70,000 sq km. Examples include Chesapeake Bay in the
US, the Baltic and Black Seas and parts of the Adriatic.

One of the best-known is in the Gulf of Mexico, affected by nutrients
washed down the Mississippi river.

Other zones have appeared off South America, Japan, China, Australia
and New Zealand.

Not all are permanent: some appear annually or only intermittently.

Unep says reducing nitrogen discharges can restore the seas to
health: an agreement by states along the River Rhine has cut the
amount of nitrogen entering the North Sea by 37%.

Other remedies include wasting less fertiliser, cleaning vehicle
exhausts, and using forests to soak up excess nitrogen.