Sea areas starved of oxygen will soon damage fish stocks even more than unsustainable catches, the United Nations believes.
Oxygen depletion: Set to have a big impact in the 21st century
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.
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.
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.
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."
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.
Unep launched its Geo Year Book, highlighting emerging issues, at the meeting here of its governing council from 29 to 31 March.
Delegations from more than 150 countries are expected to take part.