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Study Finds Nitrates up in lake Superior

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Study finds nitrates up in Lake Superior

John Myers Duluth News Tribune

Published Thursday, May 31, 2007

The level of nitrates in Lake Superior has increased five-fold over the past 100 years and continues to increase at a steady rate, a University of Minnesota study has found.

The study, published today in the journal Geophysical Research Letters, found that the nitrate isn’t necessarily coming from the expected culprits.

The increase in nitrates has been steady in the big lake, not reflecting the rapid increase in fertilizer and fossil fuel use and over the past 50 years.

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“The assumption has been for years that we’re getting atmospheric deposition from agricultural fertilizer and fossil fuel combustion. But the increase in the lake water doesn’t correlate with what is falling from the sky,’’ said Robert Sterner, chief researcher on the project for the University of Minnesota. “The nitrate in the lake can’t be accounted for by what’s falling from the sky.’’

Sterner said it’s unclear where the nitrates are coming from. They could be produced by a natural process in the lake, or from some other source of nitrogen. It’s not coming large amounts di-rectly from urban or agricultural runoff because most of the lake’s watershed is forested, Sterner said.

Most lakes use up their own nitrogen through plants. But with so few plants in Lake Superior, the nitrogen keeps building, Sterner said.

The level of nitrates in Lake Superior now sits at 25 micromolars, up from 5 micromolars in 1906. It would take levels above 700 micromolars of nitrate to be considered unsafe for human drinking water. A micromolar is a concentration of one-millionth molecular weight per liter.

While nowhere near a threat to people, it’s unclear what effects increased nitrates may have on other parts of the ecosystem or why they continue to increase. Sterner and other University of Minnesota scientists plan to look at the relationship of nitrate to Lake Superior’s unusual level of carbon use over carbon production.

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