Climate-induced changes in high elevation stream nitrate dynamics

Product Type: Journal Article

Year: 2009

Author(s): Baron, J.S., T.S. Schmidt, and M.D. Hartman

Pages: 1777-1789

Suggested Citation: Baron, J.S., T.S. Schmidt, and M.D. Hartman. 2009. Climate-induced changes in high elevation stream nitrate dynamics. Global Change Biology 15: 1777-1789.

Abstract

Mountain terrestrial and aquatic ecosystems are responsive to external drivers of change, especially climate change and atmospheric deposition of nitrogen (N). we explored the consequences of a temperature-warming trend on stream nitrate in an alpine and subalpine watershed in the Colorado Front Range that has long been the recipient of elevated atmospheric N deposition. Mean annual stream nitrate concentrations since 2000 are higher by 50% than an earlier monitoring period of 1991-1999. Mean annual N export increased by 28% from 2.03 kg N ha -1yr-1 before 2000 to 2.84 kg N ha -1yr-1 in Loch Vale watershed since 2000. The substantial increase in N export comes as a surprise, since mean wet atmospheric N deposition from 1991 to 2006 (3.06 kg N ha -1yr-1) did not increase. There has been a period of below average precipitation from 2000 to 2006 and a steady increase in summer and fall temperatures of 0.12 degrees Cyr-1 in both seasons since 1991. Nitrate concentrations, as well as the weathering products calcium and sulfate, were higher from the period 2000-2006 in rock glacier meltwater at the top of the watershed above the influence of alpine and subalpine vegetation and soils. We conclude the observed recent N increases in Loch Val are the result of warmer summer and fall mean temperatures that are melting ice in glaciers and rock glaciers. This, in turn, has exposed sediments from which N produced by nitrification can be flushed. We suggest a water quality threshold may have been crossed around 2000. The phenomenon observed in Loch Vale may be indicative of N release from ice features such as rock glaciers worldwide as mountain glaciers retreat.