Climate‐induced changes in high elevation stream nitrate dynamics

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 À1 yr À1 before 2000 to 2.84 kg N ha À1 yr À1 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 À1 yr À1 ) 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 1C yr À1 in both seasons since 1991. Nitrate concentrations, as well as the weathering products calcium and sulfate, were higher for 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 Vale 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.

Melting Alpine Glaciers Enrich High-Elevation Lakes with Reactive Nitrogen

“…Samples that we collected from the Sundance Glacier revealed NO 3 -concentrations of 11 µg N L -1 in the ice, 28 µg N L -1 in a meltwater stream on the surface of the glacier, and 158 µg N L -1 in a pond situated on the surface of the glacier, receiving the surface meltwater stream. Similar glacier ice NO 3 -concentrations occur at Taylor Glacier in the Colorado Front Range (7). Processes including the alpine distillery (36) as well as relatively high sublimation and evaporation rates on glaciers in alpine regions appear to concentrate NO 3 -in these environments, prior to delivering high NO 3 -meltwaters to downstream aquatic ecosystems.…”

“…Isotopic studies of aqueous NO 3 -in an Arctic glacier system on Svalbard revealed that this mechanism explained more than 80% of the increase in NO 3 -concentrations observed between supra-and subglacial streams, whereas contributions from rock-derived NH 4 + appeared minimal (38). Additional hypotheses, including those involving rock glaciers and landscape development, are discussed elsewhere (7,13,39,40), although we note that our analysis of watershed characteristics did not reveal significantly different NO 3 -contributions associated with the extent of talus or vegetation types across GSF versus SF watersheds. Determining the age of talus (Pleistocene versus Holocene neoglacial) across these watersheds may provide additional insight; studies in a high N deposition region of the southern Rocky Mountains suggest that talus age contributes, in part, to differences in watershed retention of N (13, 41).…”

Thawing glacial and permafrost features contribute to nitrogen export from Green Lakes Valley, Colorado Front Range, USA

“…concentrations were always below 4 lM at all other sites, with little spatial variation. In order to assess how water chemistry changes in response to drought, we followed the protocol of Baron et al (2009), separating data into two periods corresponding to wet (1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999) …”

“…In the northern Rockies, Saros et al (2010) report significantly greater nitrate (NO 3 -) concentrations in glacial and snowmelt fed lakes compared to those collecting snowmelt alone. Similarly, Baron et al (2009) report that recent NO 3 -concentration increases in Loch Vale watershed result from melting ice in permafrost and rock glaciers. In Green Lakes Valley (GLV), 45 years of glacial mass balance and climate data show that recent increases in summer air temperature have resulted in the Arikaree Glacier crossing a threshold that will likely result in its disappearance in the next several decades (Hoffman et al 2007).…”

The Disappearing Cryosphere: Impacts and Ecosystem Responses to Rapid Cryosphere Loss

“…These conditions are transient and slowly change as higher plants occupy the landscape over time scale of decades to centuries. High alpine waters are typically oligotrophic and are therefore susceptible to ecological changes that result from increases in nitrogen export from the land (Baron et al 2009). Williams and colleagues (2007) characterized the nutrient content in the outflow of the Green Lake 5 rock glacier, located in the Green Lakes Valley of the Colorado Front Range.…”