By usan Moran of Nature magazine
Nitrate levels in the waters off China, Japan and the Korean Peninsula are soaring, according to a 30-year study published in Science today. Researchers attribute the nitrate spike to rises in nitrogen pollution, and warn that it could severely affect marine ecology, leading to algal blooms and 'dead zones' with low oxygen levels.
Researchers from South Korea and the United States have analysed measurements, taken from the 1980s to the 2000s, of the amount of nitrogen and phosphorous at various depths in the seas bordering China's east coast -- namely the Yellow Sea, the East China Sea and the Sea of Japan. They compared the nitrogen levels in the oceans to the amount of nitrogen deposited in the atmosphere, using measurements taken at four air-monitoring sites.
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Over that time span, rapid population growth and industrial activity in the region, particularly in China, have caused increases in nitrogen pollution, including nitrogen oxides from fossil-fuel burning and ammonia compounds from agriculture. This nitrogen reaches the sea from the air, and to some degree from rivers.The researchers found that as nitrogen pollution has increased, oceans in the region have become more enriched with nitrogen relative to phosphorus. They warn that this shift in nutrients could favour marine organisms that thrive in conditions of high nitrogen and low phosphorus.
Surprise shift
Kitack Lee of Pohang University of Science and Technology in South Korea, one of the authors of the study, says he was surprised to see that nitrogen levels were affected even in such large, deep bodies of water.
He does not single out any particular country as the primary source of the nitrogen pollution, but notes that the study areas are located downstream of China and thus the "contributions of anthropogenic nitrogen from China are inevitably large".
The study builds on previous research showing the growing impact of nitrogen pollution on the open ocean, and on isolated lakes, streams and estuaries, including Chesapeake and Tokyo bays and bodies of water in Norway and Sweden.
"The paper takes a large data set to prove the point that has been shown in other regions, but not here," says Hans Paerl, an oceanographer at the University of North Carolina in Chapel Hill, who has researched atmospheric nitrogen deposition and over-enriched coastal waters for decades. He cautions that increasing nitrate abundance in the ocean could spur a rise in the growth of algae, including toxic blooms that are harmful to marine life.
In their study, Lee and his colleagues found that nitrate levels rose most in the Yellow Sea east of China (and southwest of South Korea), where the Changjiang River reaches the sea.
In one of the blocks of water studied in that area, nitrate concentrations have surged from 2 micromoles per litre to 8 micromoles per litre over the past 25 years. The current concentration in that area "is 30 times higher than [in] the Gulf of Mexico," says Ken Johnson, an oceanographer at the Monterey Bay Aquarium Research Institute in Moss Landing, California. "If you saw 8 micromoles in the Gulf, the dead zones there would ultimately be a lot bigger."
James Galloway, an environmental scientist at the University of Virginia, Charlottesville, who co-authored an August 2011 report on nitrogen in the environment for the US Environmental Protection Agency, says that the Pacific study demonstrates the need for greater controls on combustion systems and on nitrogen discharges to waterways from agricultural runoff, sewage and other sources. "Ultimately, the challenge is how to prevent the environmental consequences without diminishing the ability to produce food and energy, for Asia and elsewhere," he says.Johnson is leading a group of scientists trying to launch a global network of sensors that would continuously measure the impact of climate on nutrient fluxes, including nitrogen, as well as monitoring ocean acidification and oxygen stocks. "It's embarrassing how little data we have on oceans worldwide," he says.
This article is reproduced with permission from the magazine Nature. The article was first published on September 22, 2011