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Environment: Forest stream study traces nitrates

Even some ‘pristine’ streams show signs of human impacts

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New research helps shed light on long-term nutrient level changes. Bob Berwyn photo.

By Summit Voice

SUMMIT COUNTY — Nitrates are increasing even in some pristine forest streams in the mountain West and the South, while declining in the Pacific Northwest, in the Northeast, and in Puerto Rico, according to a new study led by Oregon State University researchers.

The long-term data from the Forest Service Experimental Forest and Range network, a system of 80 locations across the country. Many of the sites have long-term monitoring programs and data sets spanning decades and so provide unique opportunities to evaluate long-term trends.

The research leaders said it’s one of the first studies to compare water quality trends in forested streams across the country that are largely undisturbed by land use or land cover changes.

“Much of what we know about changes in stream water quality comes from studies where basins have been impacted by human activity,” said Alba Argerich, a postdoctoral research associate with Oregon State University and the study’s lead author. “Our work intentionally focused on relatively undisturbed streams, the very reference sites that serve as benchmarks for evaluating water quality trends.”

Argerich and colleagues analyzed concentrations of stream nitrogen, which, despite regulations, have been on the rise across the country as energy and food production release reactive forms of the compound into waterways. Once there, reactive nitrogen—nitrate and ammonium—can alter stream function and cause substantial changes in stream communities.

“These long-term water quality data from experimental forests are a treasure,” said Sherri Johnson, a research ecologist with the Pacific Northwest Research Station and a co-author of the study. “Some sites have over 40 years of weekly data.”

The researchers analyzed 559 years of stream nitrate and 523 years of stream ammonium data from 22 streams in 7 experimental forests across the country.

They observed that, within a forest, trends were not always in sync — at some sites, two streams within an experimental forest had opposing trends for the same type of nitrogen for the same period of time, suggesting that the controls on stream nitrogen concentrations may vary among and within sites.

“Understanding how nutrient concentrations are changing over time in reference streams is vital for informing best management practices that are aimed at protecting water resources,” Argerich said.

The study is published in the current issue of the journal Environmental Research Letters.

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