Links between N Deposition and Nitrate Export ... - ACS Publications

Subscriber access provided by Library, Special Collections and Museums, University of Aberdeen

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

Environmental Science & Technology

ACS Paragon Plus Environment


1

Links between N Deposition and Nitrate Export

2

from a High-Elevation Watershed in the Colorado

3

Front Range

4

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

5

†

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

6

‡

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

7

§

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

8

*

9

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

10

Page 2 of 32

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

Colorado Front Range

11


1

Page 3 of 32


12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

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

21

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

22

sulfur deposition.

23

INTRODUCTION

24

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

25

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

26

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

27

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

28

concentrations during the growing season suggesting atmospheric N deposition exceeds uptake

29

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

30

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

31

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

32

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

33

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

34

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


2


35

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

36

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

37

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

38

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

39

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

40

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

41

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

42

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

43

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

44

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

45

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

46

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

47

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

48

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

49

Page 4 of 32

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

50

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

51

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

52

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

53

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

54

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

55

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

56

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

57

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


3

Page 5 of 32


58

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

59

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

60

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

61

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

62

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

63

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

64

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

65

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

66

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

67

thawing of cryosphere features.

68

METHODS

69

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

70

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

71

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

72

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

73

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

74

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

75

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

76

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

77

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

78

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

79

Atmospheric Deposition Program (NADP) site (CO98).


4


80

Page 6 of 32

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

81

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

82

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

83

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

84

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

85

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

86

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

87

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

88

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

89

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

90

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

91

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

92

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

93

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

94

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

95

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

96

Supporting Information for details).

97

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

98

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

99

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

100

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

101

ROM406 CASTNET station (see Supporting Information for details).


5

Page 7 of 32


102

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

103

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

104

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

105

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

106

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

107

scale.

108

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

109

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

110

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

111

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

112

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

113

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

114

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

115

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

116

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

117

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

118

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

119

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

120

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

121

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

122

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

123

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

124

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


6


Page 8 of 32

125

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

126

evaluate multicollinearity among explanatory variables. Candidate variables included annual N

127

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

128

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

129

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

130

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

131

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

132

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

133

performed using the S-Plus statistical software.

134

RESULTS

135

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

136

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

137

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

138

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

139

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

140

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

141

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

142

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

143

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

144

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

145

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

146

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


7

Page 9 of 32


147

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

148

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

149

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

150

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

151

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

152

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

153

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

154

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

155

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

156

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

157

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

158

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

159

Atmospheric Deposition. Both nitrate and ammonium concentrations in precipitation

160

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

161

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

162

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

163

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

164

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

165

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

166

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

167

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

168

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

169

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


8


Page 10 of 32

170

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

171

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

172

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

173

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

174

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

175

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

176

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

177

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

178

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

179

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

180

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

181

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

182

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

183

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

184

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

185

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

186

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

187

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

188

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

189

2003-05.

190

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

191

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

192

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


9

Page 11 of 32


193

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

194

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

195

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

196

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

197

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

198

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

199

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

200

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

201

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

202

DISCUSSION

203

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

204

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

205

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

206

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

207

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

208

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

209

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

210

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

211

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

212

of N emissions to LV (34).

213

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

214

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


10


Page 12 of 32

215

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

216

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

217

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

218

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

219

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

220

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

221

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

222

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

223

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

224

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

225

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

226

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

227

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

228

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

229

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

230

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

231

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

232

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

233

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

234

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

235

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

236

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

237

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


11

Page 13 of 32


238

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

239

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

240

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

241

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

242

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

243

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

244

where surface-water nitrate concentrations are elevated.

245

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

246

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

247

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

248

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

249

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

250

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

251

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

252

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

253

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

254

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

255

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

256

in the early 2000s.

257

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

258

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

259

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

260

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


12


Page 14 of 32

261

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

262

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

263

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

264

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

265

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

266

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

267

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

268

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

269

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

270

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

271

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

272

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

273

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

274

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

275

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

276

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

277

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

278

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

279

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

280

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

281

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

282

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


13

Page 15 of 32


283

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

284

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

285

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

286

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

287

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

288

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

289

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

290

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

291

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

292

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

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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

301

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

302

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

303

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

304

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

305

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


14


306

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

307

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

308

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

309

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

310

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

311

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

312

evaluating the effectiveness of deposition reduction strategies.

Page 16 of 32

313

ACKNOWLEDGMENTS

314

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

315

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

316

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

317

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

318

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

319

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

320

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

321

comments on a previous version of the manuscript.

322

Supporting Information Available

323

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

REFERENCES

331 332

1. Baron, J. S.; Campbell, D. H. Nitrogen fluxes in a high elevation Colorado Rocky Mountain Basin. Hydrol. Process. 1997, 11, 783–799.

333

2. Burns, D. A. The effects of atmospheric nitrogen deposition in the Rocky Mountains of

334

Colorado and southern Wyoming USA—a critical review. Environ. Pollut. 2004, 127,

335

257–269.

336 337 338 339

3. Campbell, D. H.; Baron, J. S.; Tonnessen, K.; Brooks, P.; Schuster, P. Controls on nitrogen flux in alpine/subalpine watersheds. Water Resour. Res. 2000, 36, 37–48. 4. Williams, M. W.; Baron, J. S.; Caine, N.; Sommerfeld, R.; Sanford, R. L. Nitrogen saturation in the Rocky Mountains. Environ. Sci. Tech. 1996, I, 640–646.

340

5. Fenn, M. E.; Haeuber, R.; Tonnesen, G. S.; Baron, J. S.; Grossman-Clarke, S.; Hope,

341

D.; Jaffe, D.A.; Copeland, S.; Geiser, L.; Rueth, H. M.; Sickman, J. M. Nitrogen

342

emissions, deposition, and monitoring in the western United States. BioScience 2003,

343

53, 391-403.

344

6. Baron, J. S.; Rueth, H. M.; Wolfe, A. N.; Nydick, K. R.; Allstott, E. J.; Minear, J. T.;

345

Moraska, B. Ecosystem responses to nitrogen deposition in the Colorado Front Range.

346

Ecosystems 2000, 3, 352–368.

347 348

7. Williams, M. W.; Tonnessen, K. A. Critical loads for inorganic nitrogen deposition in the Colorado Front Range, USA. Ecol. Appl. 2000, 10, 1648–1665.


16


Page 18 of 32

349

8. Bergstrom, A. K.; Blomqvist, P.; Jansson, M. Effects of atmospheric nitrogen

350

deposition on nutrient limitation and phytoplankton biomass in unproductive Swedish

351

lakes. Limnol. Oceanogr. 2005, 50, 987–994.

352

9. Jassby, A. D.; Goldman, C. R.; Reuter, J. E. Long-term change in Lake Tahoe

353

(California-Nevada, USA) and its relation to atmospheric deposition of algal nutrients.

354

Arch. Hydrobiol. 1995, 139, 1–21.

355

10. Elser J. J.; Andersen T.; Baron, J. S.; Bergström, A. K.; Jansson, M.; Kyle, M.; Nydick,

356

K. R.; Steger, L.; Hessen, D. O. Shifts in lake N:P stoichiometry and nutrient limitation

357

driven by atmospheric nitrogen deposition. Science 2009, 326. 835–837.

358

11. Saros, J. E.; Interlandi, S. J.; Wolfe, A. P.; Engstrom, D. R. Recent changes in the

359

diatom community structure of lakes in the Beartooth Mountain Range, U.S.A. Arctic

360

Antarctic Alpine Res. 2003, 35, 18–23.

361

12. Wolfe, A. P.; Van Gorp, A. C.; Baron, J. S. Recent ecological and biogeochemical

362

changes in alpine lakes of Rocky Mountain National Park: a response to anthropogenic

363

nitrogen deposition. Geobiology 2003, 1, 153–168.

364

13. Bowman, W. D.; Gartner J. L.; Holland K.; Wiedermann M. Nitrogen critical loads for

365

alpine vegetation and terrestrial ecosystem response – Are we there yet? Ecol. Appl.

366

2006, 16, 1183–1193.

367

14. McDonnell, T. C.; Belyazid, S.; Sullivan, T. J.; Sverdrup H.; Bowman, W. D.; Porter,

368

E. M. Modeled subalpine plant community response to climate change and atmospheric


17

Page 19 of 32


369

nitrogen deposition in Rocky Mountain National Park, USA. Environ. Pollut. 2013,

370

187, 55–64.

371

15. Eshleman, K. N.; Sabo, R. D.; Kline, K. M. Surface water quality is improving due to

372

declining atmospheric N deposition. Environ. Sci. Technol. 2013, 47, 12193–12200.

373

16. Garmo, Ø. A.; Skjelkvåle, B. L.; de Wit, H. A.; Colombo, L.; Curtis, C.; Fölster, J.;

374

Hoffmann, A.; Hruška , J.; Høgåsen, T.; Jeffries, D. S.; Keller, W. B.; Krám, P.; Majer,

375

V.; Monteith, D. T.; Paterson, A. M.; Rogora, M.; Rzychon, D.; Steingruber, S.;

376

Stoddard, J. L.; Vuorenmaa, J.; Worsztynowicz, A. Trends in surface water chemistry

377

in acidified areas in Europe and North America from 1990 to 2008. Water Air Soil

378

Pollut. 2014, 225, 1880.

379

17. Strock, K. E.; Nelson, S. J.; Kahl, J. S.; Saros, J. E.; McDowell, W. H. Decadal trends

380

reveal recent acceleration in the rate of recovery from acidification in the northeastern

381

U.S. Environ. Sci. Technol. 2014, 48, 4681–4689; DOI: 10.1021/es404772n.

382

18. Rogora, M.; Arisci, S.; Marchetto, A. The role of nitrogen deposition in the recent

383

nitrate decline in lakes and rivers in Northern Italy. Sci. Total Environ. 2012, 417, 214–

384

223.

385

19. Bernal, S.; Hedin, L. O.; Likens, G. E.; Gerber, S.; Buso, D. C. Complex response of

386

the forest nitrogen cycle to climate change. Proc. Natl. Acad. Sci. U.S.A. 2012, 109,

387

3406–3411.

388 389

20. Goodale, C. L.; Aber, J .D.; Vitousek, P. M. An unexpected nitrate decline in New Hampshire streams. Ecosystems 2003, 6, 0075–0086.


18


Page 20 of 32

390

21. Argerich, A.; Johnson, S. L.; Sebestyen, S. D.; Rhoades, C. C.; Greathouse, E.;

391

Knoepp, J. D.; Adams, M. B.; Likens, G. E.; Campbell, J. L.; McDowell, W. H.;

392

Scatena, F. N.; Ice, G. G. Trends in stream nitrogen concentrations for forested

393

reference catchments across the USA. Environ. Res. Letters 2013, 8, 014039.

394

22. Park, J. H.; Mitchell, M. J.; McHale, P. J.; Christopher, S. F.; Meyers, T. P. Impacts of

395

changing climate and atmospheric deposition on N and S drainage losses from a

396

forested watershed of the Adirondack Mountains, New York State. Global Change

397

Biol. 2003, 9, 1602–1619.

398

23. Canham, C. D.; Pace, M. L.; Weathers, K. C.; McNeil, E. W.; Bedford, B. L.; Murphy,

399

L.; Quinn, S. Nitrogen deposition and lake nitrogen concentrations: a regional analysis

400

of terrestrial controls and aquatic linkages. Ecosphere 2012, 3, 1–16.

401

24. Brooks, P. D.; Campbell, D. H.; Tonnessen, K. A.; Heuer, K. Natural variability in N

402

export from headwater catchments: snowcover controls ecosystem retention. Hydrol.

403

Process. 1999, 13, 2191–2201.

404 405

25. Baron, J. S.; Schmidt, T. W.; Hartman, M. D. Climate-induced changes in high elevation stream nitrate dynamics. Global Change Biol. 2009, 15, 1777–1789.

406

26. Heath J.; Baron, J. S. Climate, not atmospheric deposition, drives the biogeochemical

407

mass-balance of a mountain watershed. Aquatic Geochem. 2014, 20, 157–181; DOI

408

10.1007/s10498-013-9199-2.


19

Page 21 of 32


409

27. Barnes, R.; Williams, M. W.; Caine, N.; Parman, J.; Hill, K. Thawing glacial and

410

permafrost features contribute to nitrogen export from Green Lakes Valley, Colorado

411

Front Range, USA. Biogeochemistry 2014, 117, 413–430.

412

28. Morris, K.; Mast, A.; Clow, D.; Wetherbee, G.; Baron, J.; Taipale, C.; Blett, T.; Gay,

413

D.; Heath, J. 2012 Monitoring and Tracking Wet Nitrogen Deposition at Rocky

414

Mountain National Park; Natural Resource Report NPS/NRSS/ARD/NRR 2014-757;

415

Department of Interior, National Park Service: Denver, CO, 2014.

416 417

29. Baron, J. Biogeochemistry of a subalpine ecosystem: Loch Vale Watershed; Ecological Study Series #90, Springer-Verlag: New York, 1992.

418

30. Clow, D. W.; Campbell, D. H.; Mast, M. A.; Striegl, R.G.; Wickland, K.P.; Ingersoll,

419

G.P. Loch Vale, Colorado: A Water, Energy, and Biogeochemical Budgets Program

420

Site; Fact Sheet 164–99; U.S. Geological Survey: Reston, VA, 2000.

421

31. Campbell, D. H.; Clow, D. W.; Ingersoll, G. P.; Mast, M. A.; Spahr, N. E.; Turk, J. T.

422

Processes controlling the chemistry of two snowmelt-dominated streams in the Rocky

423

Mountains. Water Resourc. Res. 1995, 31, 2811–2821.

424

32. Richer, E. E.; Baron, J. S. Loch Vale Watershed Long-Term Ecological Research and

425

Monitoring Program: Quality Assurance Report, 2003-2009; Open-File Report 2011-

426

1137; U.S. Geological Survey: Reston, VA, 2011.

427

33. Clow, D. W.; Nanus, L.; Verdin, K. L.; Schmidt, J. Evaluation of SNODAS snow depth

428

and snow water equivalent estimates for the Colorado Rocky Mountains, USA. Hydrol.

429

Process. 2012, 26, 2583–2591.


20


Page 22 of 32

430

34. Benedict, K. B.; Carrico, C. M.; Kreidenweis, S. M.; Schichtel, B.; Malm, W. C.;

431

Collett, J. L. A seasonal nitrogen deposition budget for Rocky Mountain National Park.

432

Ecological Applications 2013, 23, 1156–1169.

433

35. de Gouw, J. A.; Parrish, D. D.; Frost, G. J.; Trainer, M. Reduced emissions of CO2,

434

NOx, and SO2 from US power plants owing to switch from coal to natural gas with

435

combined cycle technology. Earth's Future 2014, 2, 75–82.

436

36. Watmough, S; Eimers, M. C.; Herne, J.; Dillon, P. J. Climate effects on stream nitrate

437

concentrations at 16 forested catchments in south central Ontario. Environ. Sci.

438

Technol. 2004, 38, 2383–2388.

439

37. Morecroft, M. D.; Burt, T. P.; Taylor, M. E.; Rowl, A. P. Effects of the 1995–1997

440

drought on nitrate leaching in lowland England. Soil Use and Management 2000, 16,

441

117–123.

442

38. Mast, M. A.; Ingersoll, G. P. Trends in lake chemistry in response to atmospheric

443

deposition and climate in selected Class I wilderness areas in Colorado, Idaho, Utah,

444

and Wyoming, 1993-2009; Scientific Investigations Report 2011-5123; U.S. Geological

445

Survey: Reston, VA, 2011.

446

39. Mast, M. A.; Turk, J. T.; Clow, D. W.; Campbell, D. H. Response of lake chemistry to

447

changes in atmospheric deposition and climate in three high-elevation wilderness areas

448

of Colorado. Biogeochemistry 2011, 103, 27–43.


21

Page 23 of 32


449

40. Mast, M. A.; Ely, D. Effect of power plant emission reductions on a nearby wilderness

450

area: a case study in northwestern Colorado. Environ. Monit. Assess. 2013, 185, 7081–

451

7095.

452

41. Eshleman, K. N.; Morgan, R. P. Temporal patterns of nitrogen leakage from mid-

453

Appalachian forested watersheds: role of insect defoliation. Water Resour. Res. 1998,

454

34, 2005–2116.

455 456

42. Swank, W. T.; Wade, J. B.; Crossley, D. A. Jr.; Todd, R. L. Insect defoliation enhances nitrate export from forest ecosystems. Oecologia 1981, 51, 297–299.

457

43. Rhoades, C. C.; McCutchan, J. H. Jr.; Cooper, L. A.; Clow, D.; Detmer, T. M.; Briggs,

458

J. S.; Stednick, J. D.; Veblen, T. T.; Ertz, R. M.; Likens, G. E.; Lewis, W. M.

459

Biogeochemistry of beetle-killed forests: Explaining a weak nitrate response. PNAS

460

2013, 110, 1756–1760.

461

44. Mitchell, M. J.; Driscoll, C. T.; Kahl, J. S.; Likens, G. E.; Murdoch, P. S.; Pardo, L. H.;

462

Climatic control of nitrate loss from forested watersheds in the northeast United States.

463

Environ. Sci. Technol. 1996, 30, 2609–2612.

464

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

465

T. J.; Fisk, M. C. Snow depth, soil freezing and nitrogen cycling in a northern

466

hardwood forest landscape. Biogeochemistry 2011, 102 223–238.

467

46. Judd, K. E.; Likens, G. E.; Buso, D.C.; Bailey, A. S. Minimal response in watershed

468

nitrate export to severe soil frost raises questions about nutrient dynamics in the

469

Hubbard Brook experimental forest. Biogeochemistry 2012, 106, 443–459.


22


470 471

Page 24 of 32

47. Brooks, P. D.; Williams M. W. Snowpack controls on nitrogen cycling. Hydrol. Process. 1999, 13, 2177–2190.

472

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

473

response to climate change in a high-elevation ecosystem in the Rocky Mountains.

474

Arctic Alpine Res. 1998, 30, 25–29.

475

49. Williams, M. W.; Knauf, M.; Cory, R.; Caine, N.; Liu, F. Nitrate content and potential

476

microbial signature of rock glacier outflow, Colorado Front Range. Earth Surf. Proc.

477

Landforms 2007, 32, 1032–1047.

478 479

50. Caine, N. Recent hydrologic change in a Colorado alpine basin: an indicator of permafrost thaw? Annals Glaciol. 2010, 51, 130–134.

480

51. Leopold, M.; Volkel, J.; Dethier, D.; Williams, M. W. Changing mountain permafrost

481

from the 1970s to today: comparing two examples from Niwot Ridge, Colorado Front

482

Range, USA. Zeitschrift fur Geomorphologie 2013, Supplementary Issue, available

483

online/fast track.

484

52. Clow, D. W.; Schrott, L.; Webb, R.; Campbell, D. H.; Torizzo, A.; Dornblaser, M.

485

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

486

Colorado Front Range. Ground Water 2003, 41, 937–950.

487 488


23

Page 25 of 32


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 ... - ACS Publications

Recommend Documents