Links between N Deposition and Nitrate Export from a High-Elevation

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Links between N Deposition and Nitrate Export from a High-Elevation Watershed in the Colorado Front Range M. Alisa Mast, David W. Clow, Jill S Baron, and Gregory Alan Wetherbee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es502461k • Publication Date (Web): 10 Nov 2014 Downloaded from http://pubs.acs.org on November 15, 2014

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Environmental Science & Technology

ACS Paragon Plus Environment


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Links between N Deposition and Nitrate Export

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from a High-Elevation Watershed in the Colorado

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Front Range

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M. Alisa Mast,*,† David W. Clow,†, Jill S. Baron,‡, and Gregory A. Wetherbee§

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†

U.S. Geological Survey, Colorado Water Science Center, Denver, CO 80225, USA

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‡

U.S. Geological Survey, Fort Collins Science Center, Fort Collins, CO 80526, USA

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§

U.S. Geological Survey, Branch of Quality Systems, Denver, CO 80225, USA

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*

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KEYWORDS: atmospheric deposition, nitrate fluxes, high-elevation streams, Loch Vale,

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corresponding author (email: [email protected], Tel: 303-236-6898, Fax: 303-236-4912)

Colorado Front Range

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ABSTRACT: Long-term patterns of stream nitrate export and atmospheric N deposition were

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evaluated over three decades in Loch Vale, a high-elevation watershed in the Colorado Front

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Range. Stream nitrate concentrations increased in the early 1990s, peaked in the mid-2000s, and

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have since declined by over 40%, coincident with trends in nitrogen oxide emissions over the

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past decade. Similarities in the timing and magnitude of N deposition provide evidence that

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stream chemistry is responding to changes in atmospheric deposition. The response to deposition

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was complicated by a drought in the early 2000s that enhanced N export for several years. Other

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possible explanations including forest disturbance, snow depth, or permafrost melting could not

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explain patterns in N export. Our results show that stream chemistry responds rapidly to changes

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in N deposition in high-elevation watersheds, similar to the response observed to changes in

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sulfur deposition.

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INTRODUCTION

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There is considerable interest on the effects of atmospheric nitrogen (N) deposition on high-

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elevation aquatic ecosystems in the Colorado Front Range (1-4). Atmospheric deposition ranges

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from 3-5 kg N ha-1 yr-1, among the highest in mountainous areas of the western US (2,5).

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Streams and lakes, particularly those on the east side of the Front Range, have elevated nitrate

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concentrations during the growing season suggesting atmospheric N deposition exceeds uptake

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by plant and microbial communities and that watersheds have become N saturated (4,6). This

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additional N can alter aquatic ecosystems through episodic acidification (7), shifts in the nutrient

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status and productivity of lakes (8,9), impaired food web performance (10), and changes in

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diatom community assemblages (11,12). Several studies in the Front Range have reported

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measureable changes in aquatic and terrestrial biota suggesting that N deposition has altered

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high-elevation ecosystems (6,12-14).


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High rates of mineralization and nitrification in soils particularly in N saturated watersheds of

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the northern hemisphere may mask the relationship between atmospheric deposition and stream

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N export, and some watersheds appear to respond to changes in deposition while others do not

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(15-17). Streams in the Appalachian Mountains have shown declines in nitrate concentrations

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and loads in response to declines in N deposition (15). In Italy, nitrate concentrations in

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subalpine and alpine streams increased during 1980-2000 but have decreased in more recent

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years (18). The decrease in nitrate was coincident with falling rates of N deposition after 2002

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due to reduced NOx emissions and lower precipitation amounts. Stream nitrate declined in

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mountains of New Hampshire during 1977-2007, despite relatively constant rates of N

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deposition (19). The reasons for the declines in stream nitrate are not entirely clear but may be

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associated with climate variability or land-disturbance history (19-21). In the Adirondack

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Mountains, little relation between stream N export and deposition was observed and climate

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variability, and processing and loss of N from forests appeared to be the best predictor of

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temporal patterns in N export by streams (22,23).

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In high-elevation watersheds in the Colorado Front Range, increasing stream nitrate during the

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growing season in the 1990s was interpreted to represent stages of N saturation (4). The trend

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appeared consistent with N deposition, which increased by more than 50% during 1985-1999 (2).

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Climate variability also has been postulated to be a major control on N export from alpine

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ecosystems in the Front Range. An earlier study suggested the depth of snowpack, which

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regulates soil microbial activity in winter, is the major control on interannual variability in

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stream export of N (24). More recently, several studies attributed notable increases in surface-

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water nitrate concentrations in the Front Range in the early 2000s to thawing of ice in glacial

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features and permafrost in response to increasing summer air temperatures (25-27). These studies


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suggest the roles of deposition and climate have yet to be clarified and a better understanding of

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controls on N export in high-elevation streams is needed, particularly in light of recent policy

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efforts aimed at reducing N deposition in the Front Range (28).

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In this paper, we reassess long-term patterns of stream N export in the Loch Vale watershed

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using data collected during 1984 through 2012. Stream nitrate concentrations in LV, which

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peaked in the mid-2000s, have since fallen rapidly to levels similar to those measured nearly two

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decades ago. We computed annual flow–normalized concentrations and yields at two stream

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gages in LV with over 20 years of chemistry and discharge data and evaluated possible causes,

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including changes in atmospheric N deposition, drought, forest disturbance, snow depth, and

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thawing of cryosphere features.

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METHODS

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Study site description. Loch Vale (LV) watershed is located in Rocky Mountain National

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Park in the Colorado Front Range (Figure S1 in Supporting Information) and has been a site of

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ecosystem research by the USGS since 1981 (29,30). A detailed description of the physical

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setting and biogeochemistry of LV is provided by Baron (29). The watershed drains 660 ha of

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mountainous terrain that ranges from 3050 to 4026 m. Subalpine spruce-fir forests cover the

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lower 6% of the watershed and the remainder is alpine tundra, bedrock and talus. Small snow

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glaciers, permanent snowfields, rock glaciers, and lakes occupy the two headwater cirques.

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Bedrock is granite and gneiss and soil cover is thin and discontinuous. Streamflow gaging

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stations are located at the outlet of The Loch (3050 m) and on the Andrews Creek tributary (3212

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m). A weather station is located at an elevation of 3159 m adjacent to the LV National

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Atmospheric Deposition Program (NADP) site (CO98).


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Data description and analysis. Long-term climate, hydrologic, and geochemical data

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collected by the USGS in LV were used in this analysis (http://co.water.usgs.gov/lochvale/).

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Mean, minimum, and maximum annual and monthly air temperatures at the weather station were

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computed from the daily means and daily minimum and maximum values. For comparison,

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annual precipitation also was computed for the C1 climate station on Niwot Ridge, which is

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located 25 km south of LV at an elevation of 3,010 m (http://culter.colorado.edu/NWT/).

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Annual precipitation amounts for 1984-2012 were obtained from the LV NADP station

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(CO98). Comparison of precipitation amount at CO98 with a nearby SNOTEL station indicated

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precipitation was underestimated in 2003 because a large spring snowstorm exceeded the

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capacity of the precipitation gage. Moreover, the comparison also revealed that precipitation at

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CO98 has been higher relative to nearby SNOTEL sites since 2008 (Figure S2 in Supporting

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Information), the year in which the mechanical rain gage (Belfort) was replaced with an

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electronic gage (EIT Noah-IV). Comparison of the two gages, which were collocated for three

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years, revealed that the Noah-IV collected on average 12% more precipitation than the Belfort

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(Figure S3 in Supporting Information). To account for these biases, precipitation amount at

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CO98 was increased by 14 cm in 2003 and decreased by 8-19% over the period 2008-2012 (see

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Supporting Information for details).

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Precipitation chemistry in wet deposition has been measured weekly at the CO98 site since

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1983. Annual mean concentrations of nitrate and ammonium and rates of inorganic N deposition

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were obtained from http://nadp.sws.uiuc.edu/ for water years 1984-2012. Total inorganic N

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deposition for LV was estimated as NADP wet deposition plus dry deposition from the nearby

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ROM406 CASTNET station (see Supporting Information for details).


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Precipitation chemistry data also were obtained for five other high-elevation NADP sites on

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the east side of the Front Range (CO02, CO19, CO94, WY00, WY95). Normalized values (z-

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scores) were computed as the annual concentrations minus the mean of the annual concentrations

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divided by the standard deviation for each site. Z-scores facilitate comparison of temporal

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patterns in concentrations among sites by putting the concentration magnitudes on a comparable

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scale.

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Continuous streamflow has been measured at The Loch outlet (USGS station

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401733105392404) since 1983 and at Andrews Creek (USGS station 401723105400000) since

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1992 (Figure S1). Water-quality samples were collected at the gaging sites weekly during

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spring/summer and monthly during winter. Details of sample collection and analytical methods

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are given in Campbell et al. (31) and Richer and Baron (32) and data are available at

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http://waterdata.usgs.gov/nwis. Daily stream nitrate flux for the two streams was computed by

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multiplying mean daily discharge by the daily nitrate concentration. Ammonium concentrations

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were at or below detection in both streams and were not included in the analysis. Linear

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interpolation was used to estimate nitrate concentrations between sampling dates. Daily fluxes

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were summed by water year and divided by watershed area to generate annual yields in kg ha-1

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yr-1. Water years were defined as October 1 of the previous year through September 30 of the

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indicated year. Annual yields were flow normalized to the average annual runoff (1984-2012) to

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reduce the influence of random variations in flow conditions. Annual flow-weighted nitrate

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concentrations were computed from the annual yield divided by annual runoff.

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Statistical analysis. Long-term trends in mean annual and monthly air temperature, and annual

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precipitation and runoff were evaluated using the nonparametric Mann-Kendall test. Relations

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between annual concentrations and yields with deposition and climate variables were tested


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using stepwise multiple-linear regression. The variance inflation factor (VIF) was used to

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evaluate multicollinearity among explanatory variables. Candidate variables included annual N

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concentrations in precipitation, annual N deposition, annual and winter precipitation amount,

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days of snow cover per year, annual runoff, annual air temperature, seasonal air temperatures,

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and the Palmer Drought Index (PDI). PDI is a meteorological drought index calculated from

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precipitation and temperature data over a broad area. Monthly PDI values were obtained for

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northern Colorado at http://www.ncdc.noaa.gov/temp-and-precip/drought/historical-palmers.php

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and were used to compute an annual average PDI by water year. All statistical analyses

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performed using the S-Plus statistical software.

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RESULTS

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Climate and hydrology. Mean annual air temperature at the weather station was 1.2 °C

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(Figure 1). The warmest years on record were 2000 and 2012 and the coldest were 1983 and

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1984. Statistically significant trends were not detected in annual mean, minimum, and maximum

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air temperatures during 1983-2012 (Figure 1). Few trends were detected in mean monthly air

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temperatures. July mean temperatures increased about 0.7 °C decade-1 (p = 0.010) during 1983-

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2012, the same rate of increase reported previously (25). Mean monthly temperatures in

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February showed a downward trend of -0.7° C decade-1 (p = 0.054). Air temperature trends also

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were examined at the nearby C1 station, which has a record extending back to 1953 (Figure S4 in

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Supporting Information). There was a small upward trend in annual air temperature (0.2° C

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decade-1, p = 0.007) for the entire period of record and a slightly larger trend (0.4° C decade-1, p

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= 0.015) during 1983-2012. The trend in the more recent period was largely driven by a cold

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period during 1983-86. Annual mean precipitation measured at CO98 was 105 cm for the period


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1984 to 2012. There were no long-term trends in precipitation although there was an extended

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wet period in the mid-1990s followed by a period of drought during the early 2000s (Figure 2A).

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Streamflow in LV is dominated by snowmelt with 55% of runoff occurring during June and

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July (31). Runoff at The Loch averaged 77 cm per year during 1993-2012 with a general pattern

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of increases in the first half of the record (through the late 1990s) and decreases in the second

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half of the record (Figure 2B). Runoff was only 58% of the long-term mean during the drought

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year of 2002. Notable high flow years occurred in 1997 during a strong El Nino event and 2011

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during a strong La Nina event (http://www.esrl.noaa.gov/psd/enso/mei/). Runoff at Andrews

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Creek averaged 98 cm per year, about 30% higher than at The Loch (Figure 2B). Greater runoff

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(per unit of watershed area) at Andrews largely reflects greater precipitation inputs, wind-loading

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of snow, and lower evapotranspiration rates due to its higher elevation (31, 33). Long-term

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trends were not detected in annual runoff at either gage over the period of record.

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Atmospheric Deposition. Both nitrate and ammonium concentrations in precipitation

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increased in the first half of the record and ammonium concentrations increased at a faster rate

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than those of nitrate (Figure 3A). Since the mid-2000s, concentrations of both N species have

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declined to the point where nitrate is similar to concentrations in the 1980s and ammonium is

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slightly higher. Concentration patterns in LV are similar to 5 other high-elevation NADP sites on

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the east slope of the Front Range as shown by z-scores of inorganic N concentrations (nitrate

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plus ammonium) (Figure 3B). At all sites, inorganic N concentrations peaked in the early 2000s

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followed by a decline through 2010. Dry deposition also peaked in the early 2000s and has since

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declined by over 40% (Figure S5 in Supporting Information). In contrast to recent declines in

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concentrations and dry deposition, wet deposition reported by NADP for CO98 has been fairly

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constant since the mid-1990s at approximately 3 kg ha-1 yr-1 (Figure 3C). Total N deposition in


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LV was estimated using the revised wet deposition at CO98 as described in the Supplemental

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Information plus dry deposition estimated from CASTNET. It is important to note that

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CASTNET does not measure gaseous ammonia, which may account for up to 20% of total N

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deposition in Rocky Mountain National Park (34). The estimated total deposition (revised wet

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plus dry) reveals a temporal pattern more similar to that observed in NADP concentrations, with

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a peak in the early 2000s and falling deposition over the past decade (Figure 3D).

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Stream chemistry. Annual mean nitrate concentration at The Loch was 18.8 µeq L-1, about

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30% lower than at Andrews Creek, which averaged 26.8 µeq L-1 (Figure 4). Lower

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concentrations at The Loch reflect N uptake in several lakes situated above the gage and in forest

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and meadows in the lower part of the drainage (31). Temporal patterns in annual nitrate

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concentrations were similar at The Loch and Andrews Creek gages with concentration rising

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after 1995, peaking in mid-2000s, then falling to near 1990 levels by 2012. Annual nitrate yields

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in kg N ha-1 yr-1 were 1.7 times higher at Andrews Creek compared to The Loch reflecting higher

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concentrations and runoff from the higher-elevation Andrews watershed. The temporal pattern in

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annual nitrate yield was noisier reflecting considerable variability in climate over the study

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period but the flow-normalized values showed a peak in the mid-2000s similar to annual

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concentrations (Figure 4). Net N export (stream nitrate yield minus wet N deposition) indicated

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about 30% of wet deposited N was retained in the watershed (Figure 5). Annual values were

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highly variable and showed no clear temporal trend except for a period of elevated export during

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2003-05.

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The best regression model for annual stream nitrate concentrations included 2 explanatory

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variables, N concentrations in precipitation (NPPT) and the drought index PDI (Table S1 in

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Supporting Information). NPPT is the sum of ammonium and nitrate at CO98 expressed in µeq


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L-1 and PDI is the annual Palmer Drought Index lagged by one year. These two factors explained

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83% of the variability at Andrews Creek and 73% at The Loch. The contribution of each factor is

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compared with the measured nitrate concentration for Andrews Creek in Figure 6. NPPT is

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generally a good predictor of stream concentrations with the exception of 2003-2005 when

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stream concentrations were 20% higher than predicted by NPPT (Figure 6). The PDI factor was

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positive during this period and appeared to explain the elevated stream nitrate in the years

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following the peak drought year in 2002. For stream nitrate yield, the best regression model

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included total N deposition (TDN) in kg ha-1 and PDI lagged by 1 year, which accounted for

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59% of the variability at Andrews Creek and 46% at The Loch (Table S1).

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DISCUSSION

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Similarities in the temporal pattern of N deposition and stream nitrate concentrations provides

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evidence that N export is responding to changing deposition in LV. N deposition and stream

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nitrate began increasing after 1995, peaked in the mid-2000s, and have since returned to levels

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observed in the early 1990s. Although nitrate yields were considerably more variable than

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concentrations due to interannual variability in runoff, the flow-normalized yields (Figure 5)

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were similar to deposition suggesting the pattern is not simply a response to variations in wet and

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dry years. Recent evidence shows a dramatic decline in NOx emissions in the US beginning

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about 2000 as power plants in the US switched from coal to natural gas (35). This transition is

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ongoing in Colorado, especially in the Front Range metropolitan area, which is the major source

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of N emissions to LV (34).

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The fairly direct watershed response to deposition seems plausible considering the paucity of

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soil and vegetation in alpine environments, the current high degree of N saturation in LV (1,3),


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and the fact that most of the N yield (up to 70%) occurs during spring snowmelt when soil

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microbial activity and plant uptake are lower than during the growing season. However, there

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were notable differences in the magnitude and timing of the stream response, which suggest

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watershed processing and transport of N also played a role, particularly in response to drought

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conditions in the early 2000s. For example, the peak in stream nitrate lagged the peak in

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deposition by 2 years and stream nitrate concentrations declined at a faster rate (45-50%) than

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deposition (35%) over the last decade. (Figure 6). A possible explanation is that atmospherically

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deposited N may be stored in watershed soils in pore water, as salts, or in organic matter during

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drought periods (the early 2000s in LV). As drought conditions subside and precipitation returns

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to normal (the mid-2000s in LV), some of the accumulated nitrogen may be released, providing

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the lagged response evident in LV. This is illustrated by the pattern in net N yield at The Loch,

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which showed higher watershed retention of N in low runoff years (2001-02) followed by an

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abrupt decrease in 2003 when runoff recovered (Figure 5). Mechanisms for release following the

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drought may include flushing of pore water and salts from soil and talus, and changes in rates of

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terrestrial N processing. Other studies in forested watersheds have reported elevated leaching of

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nitrate following summer dry periods, which was attributed to enhanced mineralization and

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nitrification rates when soils were rewetted (36,37).

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The temporal pattern in stream nitrate concentrations in LV was found to be similar to other

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high-elevation areas of the Front Range indicating that it is representative of a regional trend.

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Figure 7 shows trends in summer nitrate concentrations during 1992-2011 at 14 lakes in different

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areas of the Front Range (38). As observed in LV, some of the increase in the mid-2000s likely

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reflects the influence of the 2002 drought. Despite this complicating factor, lake concentrations

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patterns were very similar to total inorganic N concentrations at high-elevation NADP sites


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(Figure 3B) supporting the conclusion that deposition is an important influence on nitrate export.

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This result is consistent with recent studies in eastern North America and Europe where declines

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in watershed N export appear to be have occurred in response to reductions in N deposition over

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the last decade (15,16,18). We previously reported that stream-water sulfate in high-elevation

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watersheds have responded rapidly to reductions in sulfate deposition (39,40) and herein we

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show the response to N deposition may be similar at least in areas with high rates of deposition

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where surface-water nitrate concentrations are elevated.

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Although the combination of deposition and drought appeared to provide a reasonable

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explanation for patterns in N export from LV, we considered other possible drivers identified in

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the literature. Forest disturbance such as timber harvest and insect infestation has been linked to

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nitrate loss in forested watersheds in the eastern US (41,42), and attributed to reduced plant N

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uptake or increased nitrate production by soil microbes. In Colorado, the mountain pine beetle

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epidemic, which peaked in the late 2000, has caused extensive mortality of lodgepole pine

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forests in the Front Range. Despite the extent of the dieback, Rhoades et al. (43) found little

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evidence for enhanced stream N export from beetle-impacted watersheds. The forest canopy in

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LV is dominated by spruce and fir and these trees have been minimally affected by either

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mountain pine beetles or spruce beetles. Considering the forest composition as well as the results

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of Rhoades et al. (43), we ruled out forest disturbance as the cause of the nitrate increases in LV

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in the early 2000s.

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Climate-induced changes in watershed hydrology often are identified as important controls on

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N losses from seasonally snow-covered watersheds (44-46) and in LV we found that watershed

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N export was accelerated for several years following a period of drought. It also has been

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suggested that N export from high-elevation watersheds is controlled by spatial and temporal


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variability in snow cover because the snowpack insulates soil and regulates heterotrophic

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microbial activity during snowmelt (47,48). Brooks et al. (24) reported a strong positive

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correlation (r2 = 0.78) between watershed scale N retention (deposition minus stream export) and

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precipitation amount in LV for 1987-1996, which was attributed to greater soil microbial activity

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and retention of N in years with more snow. We repeated the analysis using nitrate yields at The

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Loch for 1984-2012 and wet N deposition similar to the method of the original study (24), but

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found little evidence for a relation between precipitation amount and net N export over the

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longer period of record (Figure S6 in Supporting Information).

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Increased nitrate concentrations in high-elevation streams and lakes in the Front Range during

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the last decade were noted in previous studies (25-27,38,49), and several investigators concluded

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the response indicates melting glacial and permafrost features caused by increasing air

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temperatures. At nearby Green Lakes Valley (GLV), discharge during September and October

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was reported to have increased nearly 50% since 1980, which was attributed to thawing of ice-

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rich permafrost on north facing slopes (50). In contrast to GLV, September discharge has

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declined at The Loch since the late 1990s (Figure S7 in Supporting Information). A slight

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downward trend in precipitation between 1993 and 2012 (Figure 2A) coupled with an increase in

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July air temperatures (Figure 1) seems to provide a plausible explanation for patterns in late-

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season flows in LV. The difference in flow response between the two study areas is not clear,

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although GLV is higher and smaller than LV so permafrost could underlie a greater area of the

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GLV watershed. However, a recent study on Niwot Ridge found evidence of permafrost only on

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some north facing slopes and did not find evidence that recent warming had resulted in

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significant degradation of permafrost since the 1970s (51).


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Increases in nitrate concentrations and fluxes in LV streams during 2000-2006 were attributed

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to glacier ice melt due to increased summer air temperatures in a previous study (25). Barnes et

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al. (27) also reported year-round increases in N export from GLV during 2000-2009, and

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attributed this to thawing glacial and permafrost features. Because stream nitrate concentrations

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at The Loch (but not at the headwaters directly below the glaciers) have declined to pre-2000

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levels, we suggest the biogeochemical consequences of glacial retreat in LV may not be as

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important at the watershed scale as previously thought. We attempted to place the contribution of

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cryosphere thawing into hydrologic perspective by estimating the magnitude of runoff generated

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by melting ice and comparing that to annual fluxes of water generated by snowmelt. Clow et al.

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(52) estimated an upper limit of 1.1x106 m3 of ice in non-rock glacier permafrost, 3x106 m3 of

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ice in rock glaciers, and 3x106 m3 in snow glaciers. If 1% of the total ice in the watershed melted

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each year it would generate about 1 cm of runoff, which is small compared to the annual average

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runoff of 77 cm at The Loch. Glacial meltwater would need to be 3-4 orders of magnitude more

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concentrated than stream water to have a measureable effect on annual solute yields. Although

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July air temperatures have increased significantly, which could enhance thawing, there were no

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long-term trends in annual air temperature or in annual discharge in LV. We suggest

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contributions from glaciers and permafrost to annual water and solute budgets in LV are minor

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even while they are responsible for high local concentrations adjacent to their source (27).

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These results are important in light of recent policy efforts to reduce N deposition in the Front

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Range. Published evidence of ecosystem changes in response to elevated N deposition in Rocky

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Mountain National Park (2,6,12) led to enactment of policy in 2007 titled the Nitrogen

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Deposition Reduction Plan (http://www.colorado.gov/cs/Satellite/CDPHE-

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AP/CBON/1251594862555). As part of the Nitrogen Deposition Reduction Plan, the Colorado


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Department of Public Health and Environment, the National Park Service, and the U.S.

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Environmental Protection Agency agreed on a resource management goal of reducing wet N

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deposition at CO98 in LV to 1.5 kg ha-1 yr-1 by 2035 (28). Our results suggest that reductions in N

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emissions and deposition on a regional scale should result in fairly immediate declines in stream nitrate

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concentrations in LV and in other high-elevation watersheds in the Colorado Front Range. However,

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climate variability may mask improvements in stream chemistry and needs to be considered when

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evaluating the effectiveness of deposition reduction strategies.

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ACKNOWLEDGMENTS

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This research was funded by the USGS Water, Energy, and Biogeochemical Budgets Program

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and the USGS Western Mountain Initiative Program. The individuals who assisted with sample

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collection, laboratory analysis, and data management over the nearly three decades of monitoring

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are too many to list, but we are thankful to all who helped build and maintain the extraordinary

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long-term record at Loch Vale. Data for the C1 climate station were provided by the NSF

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supported Niwot Ridge Long-Term Ecological Research project and the University of Colorado

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Mountain Research Station. Douglas Burns and three anonymous reviewers provided helpful

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comments on a previous version of the manuscript.

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Supporting Information Available

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Additional text provides details of adjustments to precipitation amount at CO98, estimation of

324

dry deposition, and computation of total deposition for Loch Vale. Table S1 lists results of

325

multiple regression analysis. Figures show topographic map of study area (S1), comparison of

326

annual precipitation measurements (S2-S3), mean annual temperature at a nearby climate station

327

(S4), dry deposition estimates for LV (S5), net N yield versus precipitation amount (S6) and


15

Page 17 of 32


328

September runoff at The Loch (S7). This information is available free of charge via the Internet

329

at http://pubs.acs.org/ .

330

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45. Groffman, P. M.; Hardy, J. P.; Fashu-Kanu, S.; Driscoll, C. T.; Cleavitt, N. L.; Fahey,

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48. Williams, M. W.; Brooks, P. D.; Seastedt, T. Nitrogen and carbon soil dynamics in

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485

Ground water occurrence and contributions to streamflow in an alpine catchment,

486

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487 488


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489

List of Figures

490

Figure 1. Annual and monthly air temperature trends at the Loch Vale weather station, 1984-

491

2012.

492

Figure 2. (A) Annual precipitation at the Loch Vale NADP site (CO98), and (B) annual

493

runoff at Andrews Creek and The Loch outlet.

494

Figure 3. (A) Annual nitrate and ammonium concentrations in wet deposition at the Loch

495

Vale NADP site (CO98), (B) z-scores of nitrate plus ammonium concentrations in wet

496

deposition at 6 Front Range NADP sites, (C) wet inorganic N deposition at CO98 reported by

497

NADP, and (D) total inorganic N deposition for Loch Vale computed using revised

498

precipitation amounts and dry deposition estimates from CASTNET.

499

Figure 4. Mean annual nitrate concentrations and annual nitrate yields at Andrews Creek and

500

The Loch outlet. Symbols are annual concentrations and yields and lines are flow-normalized

501

yields.

502

Figure 5. Net N export (stream nitrate yield minus wet N deposition at CO98) at The Loch

503

outlet.

504

Figure 6. Nitrate concentrations at Andrews Creek (Stream) compared to contribution of

505

each factor in the regression model where NPPT is the annual N concentrations in

506

precipitation and PDI is the Palmer Drought Index.

507

Figure 7. Summer nitrate concentrations at 14 high-elevation lakes (3140 to 3855 m) in the

508

Colorado Front Range, data from (38).


24


Annual

3 2 1 0 -1

Annual Minimum

Mean air temperature (°C)

-2 -3 -4 10 9 8 7 6 5

Annual Maximum

July

16 14 12 10 1980

1990

2000

2010

Year

Figure 1


Page 26 of 32


Annual precipitation (cm)

Page 27 of 32

150

A

125 100 75 50

Annual runoff (cm)

120

B

Andrews Creek

100 80 60

The Loch

40 1985

1990

1995

2000

2005

Year

Figure 2


2010


N concnetrations (µeq L-1)

20

N concentrations (z-score)

A

15

NO3

10 5 0 3

NH4 B

2 1 0 -1 -2 5

Wet N depostion (kg ha-1 yr-1)

Page 28 of 32

C

4 3 2

Total N depostion (kg ha-1 yr-1)

1 5

D

4 3 2 1

1985

1990

1995

2000

2005

Year

Figure 3 ACS Paragon Plus Environment

2010


Nitrate yield (kg ha-1 yr-1)

Nitrate concentration (µeq L-1 )

Page 29 of 32

40

Andrews Creek 30 20

The Loch

10

5 4 3 2 1 0

1980

1990

2000

2010

Year

. Figure 4



Net N export (kg ha-1 yr-1)

2 2003

1 0 -1 -2 -3 1980

1990

2000

2010

Year

Figure 5


Page 30 of 32


Nitrate concentration (µeq L-1)

Page 31 of 32

Stream

40 30 20 10

NPPT PDI

0 -10 1990

1995

2000

2005

Year

Figure 6


2010

Lake nitrate concentration (µeq L-1)


40

29 26 22

30 20

Page 32 of 32

24 29 28

33 N=13

26

27 12

34

29 21

30 36

28

29

10 0 1990

1995

2000

2005

Year

Figure 7


2010

Links between N Deposition and Nitrate Export from a High-Elevation

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