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Coastal agriculture poses a bigger threat to oceans than previously thought because nitrogen-based fertilisers reach and pollute the sea soon after being applied — a problem predicted to increase off the coasts of developing countries — says research published in Nature last week.

J. Michael Beman and colleagues at Stanford University, United States, showed that large 'algal blooms' of tiny marine plants called phytoplankton appeared in the Gulf of California within one week of nitrogen fertilisers being applied to fields in Mexico's Yaqui Valley.

Such sudden increases in phytoplankton can block light from penetrating the ocean and can cause the water's oxygen levels to decline rapidly. This makes the water inhospitable to fish and other marine life, which can die in large numbers in a short period of time.

In addition, some algal blooms, known as red or brown tides, release toxins that can poison fish and shellfish.

Beman analysed satellite pictures of the gulf taken between 1998 and 2002 to see if algal blooms were linked to patterns of fertilisation and irrigation in the Yaqui river valley.

"There were roughly four irrigation events per year and right after each one, you would see a bloom appear within a matter of days," explains Beman.

Although the findings confirm researchers' expectations, the close relationship between irrigation and blooms in the Gulf of California took the study's authors by surprise.

Kevin Arrigo, one of Beman's co-authors, says this is the first time that researchers have documented a "one-to-one correspondence between an irrigation event and a massive algal bloom".

The Gulf of California is one of many ocean regions in the tropics and subtropics with naturally low nitrogen concentrations, which the authors suggest contributes to the phytoplankton's sensitivity to the chemical.

"These marine regions are adjacent to rapidly developing agricultural areas in the tropical Americas, West Africa, South Asia, and South-East Asia that will experience large increases in fertiliser use over the next few decades," warn the authors.

They predict that by 2050, at least 27 per cent of nitrogen-rich fertilisers will be applied upstream of nitrogen-deficient coastal waters. The problem, they say, will be greatest in China.

Link to full paper by Beman and colleagues in Nature

Reference: Nature 434, 211 (2005)

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