Aquatic dead zones, stretches of water where little or nothing can survive, have increased by a third in little over a decade. More than 400 dead zones were identified last year, covering a total area of 95,000 square miles, about the size of New Zealand.

The dead zones suffer from hypoxia, a lack of oxygen, which scientists believe is caused by fertilisers washing off the land. When hypoxia sets in, it can drive away tens of thousands of marine animals and, in severe cases, kill them.

Scientists believe that hypoxia ranks with overfishing and habitat destruction as one of the most damaging problems facing sealife.

Since the Sixties, when there were 49 dead zones, the number has increased rapidly and from 1995 to 2007 it rose from 305 to 405. Among the most alarming outbreaks of hypoxia were those in major fishing areas of the Baltic, the Black Sea, the Gulf of Mexico and the East China Sea. One of the largest was identified at the mouth of the Mississippi River and was 8,500 square miles.

“Dead zones were once rare. Now they’re commonplace. There are more of them in more places,” said Professor Robert Diaz, of the Virginia Institute of Marine Science, College of William and Mary, in the United States. He said that dead zones were rarely “a naturally recurring event”.

In a paper published in the journal Science, Professor Diaz and his colleague, Rutger Rosenberg, of the University of Gothenburg, in Sweden, said that dead zones “now rank with overfishing, habitat loss and harmful algal blooms as major global environmental problems”. They wrote: “There is no other variable of such ecological importance to coastal marine eco-systems that has changed so drastically over such a short time.”

According to the scientists, the dead zones occur when nutrients used to enhance farmland, such as nitrogen and phosphorus, wash into the sea and fertilise huge blooms of algae. When dead, the algae are eaten by bacteria, which absorb oxygen from the water as the algae decompose.

The scientists said that keeping fertilisers out of the sea was the best way to reduce the number of dead zones.

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