U. WASHINGTON (US) — Nitrogen derived from human activities has polluted lakes for more than a century. The fingerprint is evident even in remote lakes thousands of miles from the nearest city.
The findings are based on historical changes in the chemical composition of bottom deposits in 36 lakes using an approach similar to aquatic archeology.
More than three quarters of the lakes, ranging from the U.S. Rocky Mountains to northern Europe, showed a distinctive signal of nitrogen released from human activities before the start of the 20th century, says Gordon Holtgrieve, a postdoctoral researcher at University of Washington and lead author of the report.
“When it comes to nitrogen associated with humans, most studies have focused on local and regional effects of pollution and have missed the planetary scale changes,” Holtgrieve says.
“Our study is the first large-scale synthesis to demonstrate that biologically-active nitrogen associated with human society is being transported in the atmosphere to the most remote ecosystems on the planet.”
Burning fossil fuel and using agricultural fertilizers are two key ways humans increase the amount of nitrogen entering the atmosphere. Once in the atmosphere, atmospheric currents distribute this nitrogen before it is deposited back on Earth in rain and snow, often thousands of miles from the source.
“Turns out the world, for nitrogen, is a much smaller place than we’d assumed,” says co-author Daniel Schindler, professor of aquatic and fishery sciences.
Although nitrogen is a vital nutrient for life—so much so that farmers apply fertilizers containing it to bolster food crops—too much nitrogen can be harmful.
It has been reported that humans already have doubled the rate of nitrogen released to the biosphere since 1950. Humans now contribute more nitrogen to the biosphere than all natural processes combined. When produced in developed areas, this excess nitrogen can lead to smog, acid rain, and water pollution.
The effects on remote forests, lands and lakes are largely unknown, Schindler says. An increasing body of evidence, however, shows that the biological composition of microscopic communities in Arctic lakes changed with the arrival of human-derived nitrogen.
This global nitrogen pollution may interact with climate change to produce a “double whammy” that could alter remote lakes in ways not seen in the past 10,000 years, Schindler says.
Using statistical models to analyze nitrogen characteristics of lake sediments, the authors show that the chemical fingerprint of nitrogen pollution started about 115 years ago, shortly after the Industrial Revolution, and that the rate of chemical changes increased during the last 60 years with industrial production of nitrogen for fertilizers.
The findings, published in the journal Science, conclude that climate, natural sources of nitrogen, and normal chemical processes on land and in water cannot account for the chemical signals they observe.
“Given the broad geographic distribution of our sites—and the range of temperate, alpine and arctic ecosystems—we believe the best explanation is that human-derived nitrogen was deposited from the atmosphere,” Holtgrieve says.
“The global change debate is dominated by discussions of carbon emissions, whether among scientists, politicians or the lay public,” says co-author Alexander Wolfe, professor of earth and atmospheric sciences at the University of Alberta.
“However, in a relative sense, the global nitrogen cycle has been far more perturbed by humanity than that of carbon.”
More than a dozen research universities contributed to the study.
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