Decadal Trends Reveal Recent Acceleration in the Rate of Recovery

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Decadal Trends Reveal Recent Acceleration in the Rate of Recovery from Acidification in the Northeastern U.S. Kristin E. Strock,*,†,⊥ Sarah J. Nelson,‡ Jeffrey S. Kahl,§ Jasmine E. Saros,† and William H. McDowell∥ †

School of Biology and Ecology and Climate Change Institute, University of Maine, 137 Sawyer Environmental Research Center, Orono, Maine 04469, United States ‡ Senator George J. Mitchell Center for Environmental and Watershed Research, 5710 Norman Smith Hall, University of Maine, Orono, Maine 04469, United States § James Sewall Company, 136 Center Street, Old Town, Maine 04468, United States ∥ Department of Natural Resources and the Environment, University of New Hampshire, Durham, New Hampshire 03824, United States ABSTRACT: Previous reports suggest variable trends in recovery from acidification in northeastern U.S. surface waters in response to the Clean Air Act Amendments. Here we analyze recent trends in emissions, wet deposition, and lake chemistry using long-term data from a variety of lakes in the Adirondack Mountains and New England. Sulfate concentration in wet deposition declined by more than 40% in the 2000s and sulfate concentration in lakes declined at a greater rate from 2002 to 2010 than during the 1980s or 1990s (−3.27 μeqL−1year−1 as compared to −1.26 μeqL−1year−1). During the 2000s, nitrate concentration in wet deposition declined by more than 50% and nitrate concentration in lakes, which had no linear trend prior to 2000, declined at a rate of −0.05 μeqL−1year−1. Base cation concentrations, which decreased during the 1990s (−1.5 μeq L−1 year−1), have stabilized in New England lakes. Although total aluminum concentrations increased since 1999 (2.57 μg L−1 year−1), there was a shift to nontoxic, organic aluminum. Despite this recent acceleration in recovery in multiple variables, both ANC and pH continue to have variable trends. This may be due in part to variable trajectories in the concentrations of base cations and dissolved organic carbon among our study lakes.

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INTRODUCTION Acidification was first documented in northeastern U.S. surface waters in 1972.1 Over the past 40 years, national and international policy led to widespread declines in S emissions, long considered to be the main driver of acidification. In the U.S., the Clean Air Act (CAA) was enacted in 1970 and has been modified several times since then including Title IV of the CAA Amendments, which was passed in 1990 and implemented in 1994. This amendment was the last major modification to the Act and regulated sulfur (S) and nitrogen (N) emissions, largely from coal-burning power plants. Since then, specific EPA rules such as the Clean Air Interstate Rule have resulted in further declines in emissions. As a result of these policies and similar measures in other countries, the rates of acidic deposition have decreased throughout large portions of North America and Europe.2 Studies by Stoddard et al.3 and Skjelkvale et al.4 suggested significant decreases in sulfate (SO42−) in surface waters throughout Europe and North America after the implementation of emission reductions. A lag in recovery time across North America and Europe was suggested in analyses by many authors, including Stoddard et al.3 and Skjelkvale et al.,4 because the rate of decline in S deposition was greater than the corresponding decline in surface water SO42− for most regions. © 2014 American Chemical Society

Several of the acid-sensitive regions included in these studies reported moderate increases in pH and acid neutralizing capacity (ANC), concurrent with SO42− declines.5 Other regions, including the northeastern U.S., reported variable trends in ANC, pH, and aluminum, which suggests more limited recovery.3−7 Lagged effects and limited recovery have been attributed to base cation depletion of soils, desorption of S accumulated in soils, and increased nitrate (NO3−) or organic anion delivery to surface waters.3,7−9 These mechanisms could slow the rate of recovery in surface waters until weathering rates exceed the leaching of base cations by acid anions.10 Modeling of soil recovery in both the Adirondack Mountains of New York and New England suggests that recovery will take decades and some watersheds may show no improvement despite continued reduction in acidic deposition.11 This raises questions about the time frame needed to measure and interpret trends in the recovery of freshwater ecosystems from acidic deposition. Received: Revised: Accepted: Published: 4681

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wet deposition since the 1980s.29 The large number of NADP/ NTN sites across the U.S. offered multiple sites within each region that would allow for adequate comparisons between regions. Nine sites in four states throughout New England, including Massachusetts, Maine, New Hampshire, and Vermont, were used (MA01, MA08, MA13, ME02, ME09, ME98, NH02, VT01, VT99). In the Adirondack Mountains, six sites were used (NY08, NY20, NH52, NY68, NY98, NY99). Wet deposition, as opposed to dry deposition, was used to more directly compare concentrations of ions in surface waters with the concentrations in deposition. The limited spatial and temporal coverage of dry deposition monitoring complicates any regional comparisons to surface water chemistry. Sulfur and N emissions data are from the U.S. Environmental Protection Agency (USEPA) monitoring of coal-fired plants subject to the Acid Rain Program or the Clean Air Interstate Rule. This does not include all sources of S and N emissions, but is representative of the emission sources most likely to see a reduction as a result of CAA Amendments. Correlations between annual emission and deposition were measured using Pearson’s correlation coefficient. Regional Response of Surface Water Chemistry. Lake and streamwater data from the USEPA long-term monitoring network have been used to evaluate recovery in response to the Clean Air Act Amendments.26 Surface water chemistry data used in this analysis are from an EPA acid rain aquatic effects monitoring program, the EPA Temporally Integrated Monitoring of Ecosystems (TIME) project.30 The TIME sampling sites were statistically chosen to target lakes likely to be responsive to changes in rates of acidic deposition (i.e., those with ANC < 100 μeq L−1) based on methods developed by the Environmental Monitoring and Assessment Program (EMAP).31,32 Lakes with low ANC (such as the TIME lakes) are also showing some of the greatest rates of recovery in North America and Europe.5,33 As a result, the lakes used in this study represent a subpopulation of lakes most affected by acidification and those expected to show the strongest signal of recovery over time. The regions included in this analysis are New England (31 sites located in Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and southern New York) and 43 sites in the Adirondack Mountains of New York. Many of the lakes in New England have significant human development around their shorelines whereas the lakes in the Adirondacks are largely undeveloped. Each lake was sampled annually in the summer index period (during late July or early August) from 1991 until present. All laboratory techniques follow standard EPA protocols outlined in Hillman et al.34 and summarized by Stoddard et al.26 To assess regional trends in recovery from acidic deposition, simple linear regression (SLR) is used to calculate a slope for each monitoring site from 2000 to 2010. The slopes calculated for multiple sites within a region are then analyzed for significant trends by calculating confidence limits surrounding the median value in the slope distribution and testing whether the confidence limits include zero. We used SLR instead of other techniques such as the Seasonal Kendall Tau test, because the sampling frequency (annual) precludes seasonal assessment. This simple linear regression approach was also used by Stoddard et al.26 and will allow us to compare our results to reported trends in recovery from 1991−2000. We used piecewise regression to explore nonlinear trends in data combined from both regions to infer overall change in acid

Recent studies also highlight the increasingly important effects of other environmental stressors, namely changes in land use and climate, on trends in surface water chemistry. Episodic acidification, which can temporarily delay recovery, has been observed throughout North America in response to climate events.12−15 Understanding these complex interactions is becoming increasingly important as temporal variation in precipitation is likely to increase with increased atmospheric warming.16 A long-term increase in atmospheric carbon dioxide concentrations can alter the retention of solutes in the watershed, which can influence long-term recovery from acidification.17 Long-term changes in air temperature can enhance organic matter mineralization, weathering, nitrification and S oxidation, and have been linked with increased alkalinity as well as increased SO42− and NO3− export from catchments.18,19 Many climate-related mechanisms associated with acidification are influenced by landscape characteristics and historical levels of acid deposition.20,21 Mechanisms that can modify the influx of dissolved organic carbon (DOC) are particularly important, because concentrations of this organic acid have been increasing in recent decades (in some cases more than doubling) in many regions of the Northern Hemisphere.22 Several mechanisms have been proposed to explain large-scale increases in DOC, including the widespread decline in S deposition as a consequence of reduced emissions, as well as enhanced organic matter production and/ or release as a consequence of increasing carbon dioxide and climate-mediated changes in temperature or precipitation.22−25 The variability in ANC and pH recovery described previously has been attributed to an increasing importance of organic acids in relation to mineral acids in several regions of the U.S. by Stoddard et al.26 as well as the United Kingdom by Chapman et al.27 and in Atlantic Canada by Clair et al.7 The objective of this study is to evaluate long-term trends in surface water recovery from acidic deposition a decade after previous assessments by Kahl et al.5 These analyses will focus on two regions of the northeastern U.S., the Adirondack Mountains of New York and New England. These regions are proximate to emission sources and naturally have a large proportion of lakes with low ANC.26 In previous assessments that analyzed trends in surface water chemistry from the 1980s to 2000, these two regions had differing trends in recovery. New England had limited recovery in ANC and pH as compared to the Adirondacks, where increases in ANC and pH as well as decreases in toxic aluminum were observed.4,26,28 By including another decade of monitoring and new trend analysis techniques, this study will address issues raised in the previous report concerning uncertainty in timeframes of existing data and possible lags in response as well as exploring trends that are not unidirectional. The goals of this study are to (1) analyze trends in emissions, deposition, and surface water chemistry from 2000−2010 for a set of lakes in the Adirondack Mountains and a set of lakes in New England and summarize these findings in the context of previous analyses; and (2) to evaluate an overall response in northeastern U.S. surface waters to declining acidic deposition using a nonlinear approach to identify breaks in linear trends and quantify rates of recovery during different time periods.

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MATERIALS AND METHODS Trends in Emissions and Deposition. Data from the U.S. National Atmospheric Deposition Program/National Trends Network (NADP/NTN 2012) were used to assess trends in 4682

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000 tons) ∼50% lower than those in 1980 (27 000 000 tons). A decrease in the concentration of SO42− and NO3− in wet deposition was observed at NADP monitoring stations throughout New England and the Adirondack Mountains from 2000−2010. The concentration of SO42− in wet deposition declined at a greater rate in the past decade in New England (−1.20 μeq L−1 year−1 from 2000−2010 as compared to −1.03 μeq L−1 year −1 from 1990 to 1999) and in the Adirondack Mountains (−1.94 μeq L−1 year−1 from 2000−2010 as compared to −1.88 μeq L−1 year−1 from 1990−1999). This resulted in a reduction in SO42− concentrations in wet deposition by 45% and 42% from 2000− 2010 at NADP sites in New England and the Adirondack Mountains, respectively (Figure 1). The SO42− concentration in wet deposition in 2010 (ranging from 11 to 17 μeq L−1) was over 70% lower than values measured in 1980 (ranging from 38 to 70 μeq L−1). Sulfur emission values were highly correlated with SO42− deposition in both New England (r = 0.86 from 1990−1999 and r = 0.88 from 2000−2010) and the Adirondack Mountains (r = 0.97 from 1990−1999 and r = 0.89 from 2000− 2010). The concentration of NO3− in wet deposition in New England and the Adirondack Mountains had a 55% and 59% reduction, respectively, from 2000−2010. There was no significant decline in NO3− deposition in the 1990s, but from 2000−2010, NO3− deposition declined at a rate of −0.91 μeq L −1 year −1 (New England) and −1.50 μeq L year −1 (Adirondack Mountains). The NO3− concentration in wet deposition in 2010 (ranging from 7 to 11 μeq L−1) was over 60% lower than values measured in 1980 (ranging from 19 to 34 μeq L−1). In the past decade, NO3− deposition is more highly correlated with N emissions than in the 1990s in New England (r = 0.63 from 1990−1999 and r = 0.92 from 2000− 2010) and the Adirondack Mountains (r = 0.74 from 1990− 1999 and r = 0.89 from 2000−2010). Correlations between nitrogen emissions and deposition are complicated by the importance of multiple emissions, such as mobile sources, that are not subject to Title IV of CAA amendments. Regional Response of Surface Water Chemistry. A greater decline in deposition led to continued chemical recovery, most notably for SO42− concentrations. Lake SO42− continued to decline from 2000−2010 and declined at a greater rate in both regions than recorded for the 1990s (Table 1, Figure 2). The rate of SO42− decline in lakes for 2000−2010 was greater than the rate of decline in wet deposition from 2000−2010 in both regions (−1.20 μeq L−1 year−1 and −1.94 μeq L−1 year−1 for New England and the Adirondack Mountains, respectively). As was observed in the 1990s, there was no significant directional trend in surface water NO3− concentration in either region, reflecting the importance of both terrestrial and aquatic processes in regulating nitrate loss. Concurrent with greater declines in SO42− was an increase in ANC in both regions. There was a greater increase in ANC from 2000−2010 (1.06 μeq L−1 year−1) in New England Lakes than in the 1990s (0.40 μeq L−1 year−1). The increase in ANC in the Adirondacks from 2000−2010 (0.51 μeq L−1 year−1) is slightly less than that observed in the 1990s (0.56 μeq L−1 year−1) (Table 1). Similar results were reported by Waller et al.37 for trends in Gran ANC in the TIME Adirondack lakes from 1991−2007 (0.76 μeq L−1 year−1). Although SO42− declined in the 1990s, there was limited evidence of recovery in ANC and pH, particularly in New England lakes.26 Increases in ANC in the past decade are most likely due to continued

sensitive lakes in the Northeast. In order to infer regional trends from this data, we assume that these sites are representative of the regional subpopulation of acid sensitive surface waters. Data were log-transformed to meet assumptions of normality and homogeneity of variance, which were tested with the Shapiro-Wilk’s test and Levene’s test, respectively. Zero, one, or two breaks were considered for all models. Piecewise models were considered significant if they were significantly different than the null linear model (ANOVA, p < 0.05). From the significant models, the best fit model was identified using Akaike Information Criteria (AIC), where the model with the lowest AIC score was chosen as the best fitting model.35 Linear regression was applied separately to data before and after the breakpoint year. All statistical analyses were conducted using R software (version 2.12.1, R Development Core Team (2011), Vienna, Austria).36

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RESULTS AND DISCUSSION Trends in Emissions and Deposition. Total emission of S and N compounds in the U.S. declined by ∼51% and ∼43%, respectively, from 2000−2010 (Figure 1). This is double the rate of decline for both S and N in the 1990s. A large reduction in emissions occurred between 2005 and 2010, putting 2010 S emission totals (8 000 000 tons) almost 70% lower than those in 1980 (26 000 000 tons) and 2010 N emission totals (13 000

Figure 1. Trends in emission and wet deposition concentrations of SO42− (top) and NO3− (bottom) from 1980 to 2010 for New England (gray) and the Adirondack Mountains (black). Solid lines are the mean values for all NADP monitoring sites within the region (nine in New England and six in the Adirondack Mountains), dashed lines are the standard error surrounding the mean. 4683

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Table 1. Regional Trend Results for 1990 to 2000 (from Stoddard et al.26 and Displayed Graphically in Figure 4 of Kahl et al.5) and 2000 to 2010a region

year

SO42−

NO3‑

base cations

ANC

H+

DOC

inorganic aluminum

New England

1990s 2000s 1990s 2000s

−1.88** −2.06** −2.10** −2.62 **

+0.02 −0.02 +0.01 −0.05

−1.57** −0.12 −1.22* −0.51*

+0.40* +1.06** +0.56* +0.51**

0.01 −0.05 −0.09 −0.04

+0.08* −0.05 +0.09* −0.03

−1.94 0.27 0.66 −0.80

Adirondack a

Values are the median slopes for each region, with significance determined by calculating confidence intervals around each regional median. All units are μeq L−1 year−1 except for Inorganic Aluminum which is μg L−1 year−1 and DOC which is in mg L−1 year−1. Significant trend **p < 0.01, *p < 0.05.

Figure 2. Summary of regional trends in surface water chemistry. Values are the median slopes for all lakes in each region. *Significant trend (p < 0.05).

SO42− decline and the simultaneous stabilization of base cation concentrations in surface waters. There was no significant trend in the concentration of hydrogen ions in either region, suggesting no evidence of further acidification but also no evidence of pH recovery in the region. Similar findings by Palmer et al.38 were documented at the Hubbard Brook Experimental Forest, where the pH of stream waters was unchanged or declined over time. These changes were attributed to the increasing role of organic acid deprotonation and hydrolysis of aluminum in buffering waters against changes in pH.38 Other long-term lake monitoring networks in the Adirondack region have identified long-term decreases in hydrogen ions (increase in pH); however, the rate of increase was highly variable.39 Base cation concentrations in New England lakes no longer have a significant decreasing trend and the rate of decline in lakes in the Adirondack Mountains is less than half that observed in the 1990s (−0.51 μeq L−1 year−1 from 2000−2010 as compared to −1.22 from 1990−2000). (Table 1). Mobilization of base cations from soil can occur with acidic deposition due to cation exchange buffering.10 Base cation availability in soils can be reduced further if base cation concentration in deposition declines. 40−42 Base cation depletion in all soil horizons was observed in the experimentally acidified Bear Brook Watershed and was comparable to excess base cation export in the stream.43

A survey of organic soils in the northeastern U.S. suggests that soils were continuing to acidify into the early 2000s with reduced calcium availability since 1984.44 The mobilization of cations from soil would be expected to decline as SO42− deposition decreases. A decrease in base cation concentration in surface waters was documented previously in lakes and streams in New England and the Adirondack Mountains; however, the strong decline in base cations in New England and Atlantic Canada exceeded the decreases in acid anions and was thought to be limiting the rate of recovery in these regions.4,26,28,41,43 Trends from 2000−2010 may suggest that the rate of base cation loss from soils is beginning to slow and that surface water base cation concentrations may increase in the future to return to an equilibrium with mineral weathering rates, as predicted in the model proposed by Fernandez et al.43 Aluminum has been a major focus of assessing biologically relevant chemical recovery in surface waters due to the toxicity of inorganic monomeric aluminum, which increased in concentration in surface waters in response to acidification in the Adirondack Mountains, New England, and parts of Europe.45−47 Less toxic forms of aluminum and lower aluminum concentrations occur when pH and DOC concentrations are high.48 As a result, reduced deposition should lead to lower aluminum concentrations and a shift to the less toxic organically bound and particulate forms of aluminum. These analyses suggest that total dissolved aluminum concentrations have increased from 2000−2010 in both regions (Figure 2). 4684

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units of the best model which suggests that this change in trend occurred throughout the first half of the decade.35 The decline in NO3− suggested in the simple linear regression results for 2000−2010 (Figure 2) is significant when looking across the Northeast (Figure 3). Concentrations of NO3−, which increased in the 1990s (0.53 μeq L−1 year−1), declined from 1999 to 2010 (−0.10 μeq L−1 year−1). Driscoll et al.28 documented declines in NO3 − concentrations in Adirondack Lakes during the 1990s. This was the opposite of trends observed in the 1980s and occurred during a period with little to no observable declines in N emissions or deposition. Both increasing N accumulation and retention due to increased atmospheric CO2 and a resulting fertilization effect or changing hydrologic patterns were suggested as possible mechanisms for increased N retention in the Adirondack Mountains.28,53,54 Goodale et al.55 documented dramatic decreases in stream NO3− concentration from the early-1970s to the mid-1990s and later identified an inverse relationship between stream NO3− and DOC concentrations.56 This relationship was also observed in response to DOC additions to streams.57 Forest regrowth in response to historic disturbances as well as climate variability could also be contributing to increased N retention in this region.54,58 While the mechanism is unclear, these results suggest increased watershed retention of N over time. If retention decreases in the future, increased NO3− delivery to surface waters could delay or reverse recovery from S deposition.59 Although there were significant increases in ANC in both regions using simple linear regression (Table 1), there is no significant increase in ANC since 1996 in the piecewise regression. This suggests that although individual lakes may have increasing trends in ANC, there is a large amount of variability across these acid-sensitive lakes that obscures a longterm trend. In New England, this variability could be due to the influence of human development on these lakes, including the influence of road salt on base cation concentrations.60 Piecewise regression results suggest that there is a significant decline in hydrogen ion concentration (increase in pH) since 1995 (−0.12 μeq L−1 year−1). The lack of regional response in ANC despite increasing pH is most likely due to the simultaneous decline in base cation concentration. The results of the piecewise regression suggest that the change in total aluminum from steady or declining concentrations to an increasing trend (2.57 μg L−1 year−1) began around 1999, and that marked increases in DOC (0.36 mg L−1 year−1) may be occurring since 2007 (as compared to 0.06 μeq L−1 year−1 from 1991−2006) (Table 2). This suggests that the increasing trends in DOC observed in some systems (i.e., SanClements et al.52) may be occurring more broadly in acid sensitive lakes only in recent years (Figure 3). Continued monitoring is needed to determine whether or not this recent change is an ongoing trend in the region. Over the past decade, recovery from acidic deposition has continued in the northeastern U.S., with a greater rate of decline in both SO42− and NO3− concentrations. Piecewise regression results suggest recovery in surface water concentrations has accelerated, with a greater rate of decline in SO42− and a broad decline in NO3− concentrations for the first time. The extent to which this resulted in a recovery in pH and ANC depended on the time frame in which trends were analyzed. Defining trends by decade suggested ANC recovery with limited response in pH, while piecewise regression (used to detect time periods with varying trends) suggested a limited

This increase in total aluminum concentrations appears to be largely driven by an increase in organic aluminum. In both regions, inorganic aluminum concentrations declined (p < 0.0001) and organic aluminum concentrations increased (p < 0.05) from 1990 until present (data not shown). A decline in inorganic monomeric aluminum was observed in the Adirondack Mountains following reduced S deposition in the 1990s.28 Recent work by Lawrence et al.49 documented significant increases in organic aluminum as DOC concentrations increased. Hydrolysis of aluminum in less acidic lakes may represent an important hydrogen ion source that could be contributing to a delayed recovery in lake pH (Table 1).50 This mechanism could be increasing in importance due to depleted soil pools of exchangeable base cations.51 These findings highlight the importance of including aluminum speciation in the USEPA long-term research and monitoring program. There are no significant trends in DOC in either region from 2000−2010 (Figure 2, Table 1). This is contrary to trends analyzed for the 1990s and to reports that suggested increasing trends in DOC throughout the northeastern U.S. and other regions of the Northern Hemisphere.22,26,28 In the lakes used in this analysis (n = 74), only seven had a significant increase in DOC over the past decade. Six of these lakes were in the Adirondack Mountains and one was in Maine. In an analysis of a subset of lakes that are also a part of the EPA long-term monitoring network, SanClements et al.52 found significant increases in DOC from 1993−2009 in five remote lakes located in the state of Maine. All of the lakes studied by SanClements et al.52 had significant decreasing trends in DOM fluorescence index, signifying an increase in terrestrially derived carbon. Although some lakes in the region have been shown to undergo linear changes in DOC concentration and composition, our statistically based regional sampling suggests that DOC concentrations have typically not increased consistently over time in acid-sensitive lakes. Exploring Nonlinear Trends Across the Northeast. The results of the two-segment linear piecewise regression indicate that there were significant changes in SO42−, DOC, and aluminum concentrations from 2000−2010 (Table 2, Figure 3). Table 2. Piecewise Regression Resultsa SO42− DOC NO3− ANC Base Cations H+ Aluminum

break point

pre break slope

post break slope

2002 2007 1999 1996

−1.35* 0.06* 0.53* −7.70*

−3.32* 0.36* −0.10* 0.60

1995 1999

2.02* 3.00

−0.12* 2.57*

a

Breakpoints are considered significant if different than null linear model (p < 0.05). Multiple breakpoint models were also tested and the model with the lowest AIC score was chosen. Slopes shown are untransformed rates of change (units L−1 Year −1), however significance testing is based on log transformed values, used to meet assumptions of normality. *Significant linear regression trend p < 0.05.

The piecewise regression indicates that the increased rate of decline in SO42− began around 2002 (where the rate of decline increased from −1.35 μeq L−1 year−1 to −3.32). Models that considered breakpoints at 2003, 2004, and 2005 were also significantly different than the null and had AIC scores within 2 4685

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Figure 3. Box plots of SO42−, NO3−, ANC, hydrogen ion, base cations, DOC, and total aluminum data for all TIME lakes in both New England and Adirondack regions. Simple linear regression lines are overlain for both pre and post break periods for each analyte (solid lines indicate significant trends, p < 0.05).

mediated mechanisms have been identified that can influence recovery from acid deposition including reoxidation of reduced sulfur during drought and dilution during rain events that lower ANC and pH, an effect which can be magnified when rain events increase the delivery of organic acids to surface waters.12,63,64 The relative importance of these different mechanisms may vary across different lake ecosystems depending on watershed size and landuse or water residence time. In a recent analysis in Hubbard Brook Experimental Forest by Mitchell and Likens,8 modern SO42− export exceeded deposition levels and was related to climate-mediated changes in soil moisture that altered the export of stored S from forest

response in ANC but recovery in pH from the mid-1990s. Despite this continued variability in ANC and pH, base cation concentrations stabilized and there was a shift from inorganic aluminum to nontoxic organic aluminum. These results suggest a recent acceleration in recovery, but continued monitoring is warranted to define ongoing effects on base cation loss from soils and surface water ANC, pH, and DOC concentrations. An additional consideration in this region is how variable weather may be influencing long-term trends in recovery. While emissions have been declining in the northeastern U.S., the frequency of extreme precipitation events (defined as >1 in. in 24 h) have increased, making it the region of the U.S. with the most substantial increase in these events.61,62 Several climate4686

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soils. This mechanism is particularly important in the Hubbard Brook watershed where historically high levels of atmospheric S inputs may have resulted in increased pools of S in soils.8 A change in the delivery of precipitation is particularly important for DOC concentration, where increased flows through the upper soil horizon can flush DOC-enriched interstitial water to streams.65,66 Continued monitoring is needed to ascertain the potentially antagonistic or additive interactions between reduced S emissions and changes in the frequency and intensity of extreme wet and dry years.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (717) 254- 8008; fax: (717) 254-1971; e-mail: [email protected]. Present Address ⊥

Environmental Studies Department, Dickinson College, 112 Kaufman Hall, Carlisle, Pennsylvania 17013, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The USEPA−USGS LTM/TIME project was funded by EPA ORD to J.S.K., W.H.M., S.J.N., K.E.W., and EPA CAMD to W.H.M., J.S.K., S.J.N. (IAG 06HQGR0143), processed through Grant/Cooperative Agreement G11AP20128 from the United States Geological Survey. Data from the Adirondack Mountain region were collected by the Adirondack Lakes Survey Corporation (ALSC). The work by the ALSC was performed with funding support from the New York State Energy Research and Development Authority, the New York State Department of Environmental Conservation, and the United States Protection Agency Long-Term Monitoring Network. The contents are solely the responsibility of the authors and do not necessarily represent the official views of the USGS. We thank four anonymous reviewers for their comments, which substantially improved this article.

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REFERENCES

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