Scientists gave warning yesterday that a rapid escalation in nitrogen and phosphorus levels in water, caused by fertiliser use and vehicle and factory emissions, had created almost 150 oxygen-starved dead zones around the world.

One of the worst places is the Gulf of Mexico, where an area of nearly 50,000 square miles — the size of Scotland — has been classified as deoxygenated and unable to sustain life.

Experts said that the soaring rate of coastal pollution posed a major threat to marine creatures and their habitats, as well as communities that depended on the sea.

Known by marine biologists as “suffocating waters”, dead zones have had dramatic repercussions in some areas, causing fish to flee in their hundreds of thousands while slow-moving creatures on the seabed, such as clams, lobsters and snails, have been wiped out. Klaus Toepfer, executive director of the UN’s Environment Programme, said that immediate action was needed to cut coastal pollution and stem the proliferation of such zones.

“Humankind is engaged in a gigantic, global, experiment as a result of the inefficient and often overuse of fertilisers, the discharge of untreated sewage and the ever-rising emissions from vehicles and factories,” he said.

“The nitrogen and phosphorus from these sources are being discharged into rivers and the coastal environment or being deposited from the atmosphere, triggering these alarming and sometimes irreversible effects.” Mr Toepfer said that while some dead zones could be as small as a square mile, others were far larger and growing at frightening rates.

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

Dead zones were first identified in the 1960s and have grown steadily in size and number over the past 30 years. Some of the earliest recorded zones were in Chesapeake Bay in the United States, the Baltic Sea, the Kattegat, the Black Sea and the northern Adriatic. Others have been reported in Scandinavian fiords.

The problem in the Gulf of Mexico, the most well-known area of depleted oxygen, is directly linked to nutrients or fertilisers brought to the Gulf by the Mississippi.

Others have been appearing off South America, China, Japan, southeast Australia and New Zealand.

The UN report, published yesterday at the Global Ministerial Environment Forum in Jeju, South Korea, says that the impact of agriculture, human wastes and air pollution must be reduced to avert a humanitarian and ecological crisis.

In some parts of the world action has been taken to reduce the amount of fertiliser and sewage running off the land, as well as the planting of coastal forests and grasslands to soak up excess nitrogen.

An agreement for the River Rhine in Europe, in which countries agreed to reduce by half the levels of nitrogen being discharged, has cut by 37 per cent the quantities of nitrogen entering the North Sea.

However, there is concern that more oxygen-starved areas will emerge in coastal waters off parts of Asia, Latin America and Africa as industrialisation and more intensive agriculture increase the discharge of nutrients.

Marion Cheatle, head of the UN environment pro- gramme’s early warning section, said that although nitrogen was essential in the form of fertiliser for farming and was scarce in many parts of the world, its overuse in other areas was creating major maritime problems.

In what experts call a “nitrogen cascade”, the chemical flows untreated into oceans and triggers the proliferation of plankton, which in turn depletes oxygen in the water.

Ms Cheatle said that the consequences of environmental problems fell most heavily on the poor people of the globe, and the economic costs, which were as yet unknown, would be extremely significant.

Experts believe that global warming, with increases in rainfall and temperatures, will further aggravate the problem.

Times Recommends

• Time-worn Freddie gives up Test matches
• Brighton company 'involved in cyber attack'
• The art of survival: it's all in the mind