"Much of what we know about changes in stream water quality comes from studies of basins that have been affected by human activity," explained Alba Argerich, a postdoctoral research associate at Oregon State University and the study's lead author. "Our work, on the other hand, focused on relatively undisturbed streams – the very reference sites that serve as benchmarks for evaluating water-quality trends for many other studies."

The researchers analysed 559 years of nitrate data and 523 years of ammonium records from 22 streams in seven Experimental Forests across the US. "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 more than 40 years of weekly data and the land-use histories are also well documented."

The study found that even these near-pristine forested streams are subject to change, as stream nitrate has declined in the Pacific Northwest, in the Northeast, and in Puerto Rico, but has increased in the Mountain West and the South. The researchers also observed that, within a forest, trends were not always in sync – at some sites, two nearby streams behaved differently for the same type of nitrogen over an equivalent period of time. This suggests that stream nitrogen concentrations may vary among and within the same sites.

Such trends also appear to vary with data-record length at some catchments, a finding that confirms how important long-term records are, and how important it is to be cautious when extrapolating trends from short time periods to longer ones. "Understanding how nutrient concentrations vary over time in reference streams is vital for developing better management practices aimed at protecting water resources," said Argerich.

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