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Increases in Dissolved Organic Carbon Accelerate Loss of Toxic Al in Adirondack Lakes Recovering from Acidification Gregory B. Lawrence,† James E. Dukett,*,‡ Nathan Houck,‡ Phil Snyder,‡ and Sue Capone‡ †

United States Geological Survey, 425 Jordan Road, Troy, New York 12180, United States Adirondack Lakes Survey Corporation, 1115 New York State Route 86, Ray Brook, New York 12977, United States

‡

ABSTRACT: Increasing pH and decreasing Al in surface waters recovering from acidification have been accompanied by increasing concentrations of dissolved organic carbon (DOC) and associated organic acids that partially offset pH increases and complicate assessments of recovery from acidification. To better understand the processes of recovery, monthly chemistry from 42 lakes in the Adirondack region, NY, collected from 1994 to 2011, were used to (1) evaluate long-term changes in DOC and associated strongly acidic organic acids and (2) use the base-cation surplus (BCS) as a chemical index to assess the effects of increasing DOC concentrations on the Al chemistry of these lakes. Over the study period, the BCS increased (p < 0.01) and concentrations of toxic inorganic monomeric Al (IMAl) decreased (p < 0.01). The decreases in IMAl were greater than expected from the increases in the BCS. Higher DOC concentrations that increased organic complexation of Al resulted in a decrease in the IMAl fraction of total monomeric Al from 57% in 1994 to 23% in 2011. Increasing DOC concentrations have accelerated recovery in terms of decreasing toxic Al beyond that directly accomplished by reducing atmospheric deposition of strong mineral acids.

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INTRODUCTION

increases in ANCCalc were more than double the increases in ANCGran in Adirondack lakes from 1991 to 2007. Further complications of DOC arise when considering the effects of increasing pH on concentrations of inorganic Al, the form toxic to aquatic biota. Concentrations of inorganic monomeric Al (IMAl) above 2.0 μmol L−1 cause mortality in brook trout,8 and elevated IMAl concentrations have led to loss or reductions in fish populations in Adirondack lakes and elsewhere.9 In the absence of acidic deposition, dissolved Al is generally low in concentration and occurs in an organically complexed, non-toxic form.10,11 However, mineral acids formed from the atmospheric deposition of S and N can mobilize inorganic aluminum in the soil that can be hydrologically transported into surface waters.12 During recovery from acidification, increases in DOC may increase the solubility of Al by limiting increases in pH and increasing complexation of Al by organic anions.13 Increases in DOC also increase strongly acidic organic acids, which contribute directly the mobilization of IMAl and have been suggested as a possible factor that could suppress decreases in IMAl from decreased acidic deposition.11 Increases of both DOC and organic monomeric Al (OMAl) were previously shown in Adirondack lakes from 1992 to 2000,13 but the role of increasing DOC in Al speciation and concentration remains an uncertainty in the recovery process. The effects of variable DOC concentrations on the mobilization of Al can be addressed through the base-cation

Chemical recovery of surface waters, acidified from decades of acidic deposition, continues to concern researchers, policymakers, government agencies, and the public. Since the 1970s, air quality legislation has reduced anthropogenic emissions of SO2 and NOx. As a result, increases in pH and decreases in SO42− and NO3− concentrations have been measured in lakes in the Adirondack region of NY, which is one of the most impacted regions in the country.1 Nevertheless, lakes continue to experience acidification, particularly during periods of high precipitation and snowmelt,2 and neither the processes of recovery nor the chemical indices used to measure recovery are fully understood.1 The trend of increasing pH in surface waters recovering from acid deposition is being accompanied by an increasing trend in dissolved organic carbon (DOC), which has been observed in lakes and streams throughout the northeastern U.S., eastern Canada, and northern Europe.3,4 Increasing DOC has been attributed to several processes, including increased solubility as soils and waters become less acidic5 and altered carbon dynamics associated with climate warming.6 Increases in naturally derived organic acids associated with DOC partially offset the increases in pH achieved through lower emission levels. Organic acids can also lower acid-neutralizing capacity determined by Gran titration (ANCGran) because this measurement integrates overall solution chemistry.1,7 If ANC is determined by the sum of base cations (Ca2+, Mg2+, Na+, and K+) minus the sum of acid anions (SO42− + NO3− + Cl−), the effects of organic acids are not explicitly included. This effect was apparent in the results by Waller et al.,1 which showed that © XXXX American Chemical Society

Received: February 1, 2013 Revised: June 10, 2013 Accepted: June 10, 2013

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dx.doi.org/10.1021/es4004763 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Article

Table 1. Values of Minimum (min) and Maximum (max) IMAl Concentrations (μmol L−1) for 1994 and 2011 in Each Study Lake and the Rate of Change (μmol of C L−1) in DOC over the 17 Year Study Period IMAl

a

name

1994 min

1994 max

2011 min

2011 max

DOC trenda

Arbutus Lake Avalanche Lake Big Hope Pond Big Moose Lake Black Pond Stream Brooktrout Lake Bubb Lake Cascade Lake Clear Pond Constable Pond Dart Lake G Lake Grass Pond Heart Lake Indian Lake Jockeybush Lake Lake Colden Lake Rondaxe Limekiln Lake Little Hope Pond Long Pond Loon Hollow Pond Lost Pond Marcy Dam Pond Middle Branch Lake Middle Pond Middle Settlement Lake Moss Lake Nate Pond North Lake Otter Lake Owen Pond Queer Lake Raquette Lake Reservoir Sagamore Lake South Lake Squash Pond Squaw Lake West Pond Willis Lake Willys Lake Windfall Pond

0.00 7.00 0.00 0.93 0.00 1.19 0.00 0.00 0.00 3.63 0.59 0.19 0.59 0.00 2.00 1.19 5.15 0.00 0.00 0.00 3.60 12.79 0.00 0.00 0.00 0.00 0.59 0.00 0.00 1.04 0.82 0.00 0.48 0.26 0.00 1.07 4.23 0.04 0.56 0.00 10.27 0.00

0.96 17.72 1.26 8.52 1.07 7.82 4.86 4.56 0.96 18.01 9.30 15.49 18.64 0.70 11.34 15.01 11.16 5.71 5.26 4.15 8.04 21.87 17.94 8.78 8.12 1.07 10.97 4.56 2.11 11.49 11.04 0.44 2.82 11.45 8.82 18.31 16.31 1.96 8.64 1.48 16.27 1.04

0.00 0.76 0.00 0.00 0.00 0.10 0.00 0.00 0.00 1.10 0.14 0.00 0.08 0.00 0.25 0.10 0.74 0.00 0.00 0.00 1.60 1.69 0.00 0.02 0.00 0.00 0.06 0.00 0.00 0.00 0.07 0.00 0.00 0.13 0.00 0.00 1.64 0.00 0.00 0.00 0.99 0.00

0.55 5.07 0.24 2.24 0.33 1.28 0.59 2.66 0.28 4.31 2.07 3.48 10.95 0.27 2.00 2.10 3.44 1.51 0.61 0.88 3.90 5.67 2.81 1.71 2.18 0.23 3.24 1.02 0.50 3.38 2.35 0.63 2.19 2.80 1.84 4.50 6.89 0.20 2.79 0.34 3.54 0.29

c b d a d b d b a a a a d d c a a a b a c d d d d a c a b a d a b a b a a d a b a a

Significance at p < 0.01. bSignificance at p < 0.05. cSignificance at p < 0.1. dNon-significance (p ≥ 0.1).

monitored by the Adirondack Lakes Survey Corporation (ALSC) since the early 1990s as part of the Adirondack Long-Term Monitoring (ALTM) program. The objectives of this paper are to use the extensive ALSC data set to (1) evaluate long-term changes in DOC and associated strongly acidic organic acids and (2) use the BCS to assess the effects of increasing DOC concentrations on the Al chemistry of these lakes.

surplus (BCS), a chemical index that explicitly includes strongly acidic organic anions.10,11 Like ANCGran and ANCCalc, the BCS relates closely to IMAl below a threshold value, but unlike ANCGran and ANCCalc, the BCS threshold of zero does not shift with varying DOC concentrations.10,11 Values of BCS above zero provide a measure of base-cation availability and indicate how close a water is to the threshold for Al mobilization. The BCS therefore provides a useful reference point for evaluating acidification recovery in the presence of increasing DOC concentrations. Understanding the effect of increasing DOC on the Al concentration and speciation is a key step in determining the recovery potential of acidified surface waters. This includes lakes of the Adirondack region of New York, which have been

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MATERIALS AND METHODS The lakes included in this study are located throughout all but the easternmost portion of the Adirondack region, which continues to receive levels of acidic deposition that are among the highest in the northeastern U.S.A. (http://nadp.sws.uiuc. B



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remain unprotonated in this pH range contribute to the anion deficit. These anions can therefore be considered strongly acidic and can be used as an estimate of RCOOs− for the low pH samples. Values of pH ≤ 4.5 were measured in a total of 200 samples collected from 13 of the 42 study lakes over the 17 year study period. For the samples with pH values above 4.5 (most of the data set), the anion deficit could not be used to estimate RCOOs− because some of the weakly acidic functional groups would also be unprotonated (charged) and contribute to the anion deficit. However, the strong acid fraction of DOC is not likely to change with the pH of water. Acid−base properties of DOC have been considered to be similar within Adirondack lakes.16 Therefore, RCOOs− was estimated from DOC concentrations and the relationship shown in Figure 1.11

edu/; accessed January 2013). Bedrock and surficial geology are highly variable across the region; however, in many areas, particularly the west, parent material is ineffective at buffering inputs of acidic deposition.14 In the 1980s, the ALSC conducted a chemical and biological survey of 1469 Adirondack lakes.15 Following completion of this survey, monthly sampling and chemical analyses of 17 Adirondack lakes by Syracuse University was expanded to 52 lakes to form the current ALTM program. Data presented in this paper are from a sampling location either at the lake outlet or at 0.5 m below the surface over the deepest portion of the lake. Additional project information about the ALSC and the ALTM program, including current data, can be found at www. adirondacklakessurvey.org. The analysis in this paper used data from 42 ALTM drainage lakes (Table 1) collected each month from 1994 to 2011. Eight ALTM seepage lakes were excluded from our analysis because these lakes do not tend to reflect terrestrial effects of acidic deposition, such as soil acidification. These lakes have no inflowing streams or sustained surface outlets and, therefore, have poorly defined watersheds. Monthly surface water samples were analyzed in the ALSC laboratory for ANCGran, pH, NO3−, SO42‑, DOC, total monomeric Al (TMAl), OMAl, Ca2+, Mg2+, K+, Na+, NH4+, Cl−, and F−. Values with concentrations less than 1.3 μmol L−L are below the reporting limit for TMAl and OMAl. Concentrations below the reporting limit approximate zero. IMAl is calculated by the difference between TMAl and OMAl. The BCS was calculated from the following equation using chemical concentrations expressed in μeq L−1.

anion deficit = (Ca 2 + + Mg 2 + + K+ + Na + + IMAl3 + + OMAl2 + + NH4 + + H+] − (SO4 2 − + NO3− + Cl− + F−)

To calculate the anion deficit, the charge of IMAl was assumed to be 3 because below pH 4.5 other inorganic forms of Al are minimal.17 Because a reliable method has not been developed to estimate the charge of organically complexed Al species (OMAl), we assumed a charge of 2+ to calculate the charge balances.11 Had a charge of 1+ been assumed, the average annual estimate of strongly acidic organic anion concentrations would have been lowered by 2.1−3.3 μeq L−1. Trend analysis of the annual means of all lakes combined was performed by linear regression for BCS, RCOO−s, and concentrations of OMAl and IMAl. These data did not fail the test for normality. Concentrations of DOC for some of the individual lakes were non-normally distributed; therefore, the seasonal Kendall test was used for the trend analysis of individual lakes.18 The Kendall rank coefficient was used for trend analysis on the annual means of all lakes combined for DOC, percent OMAl, and percent IMAl, which were also non-normally distributed. Segmented regression was used to determine linear best-fit lines and break points in relationships between IMAl and BCS.19

BCS = (Ca 2 + + Mg 2 + + Na + + K+) − (SO4 2 − + NO3− + Cl− + RCOOs−)

(2)

(1)

where RCOO−s is the charge of strongly acidic organic anions. The term RCOO−s was estimated from the relationship between DOC and the anion deficit (net charge of all ions, except organic anions; eq 2) with samples that had pH values between 4.0 and 4.5 (Figure 1), as described in the study by Lawrence et al.11 In this pH range, weakly acidic anions are protonated and, therefore, uncharged. Organic anions that

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RESULTS AND DISCUSSION The acid−base properties of surface water DOC are generally considered to be similar within regions of similar climate and physiography.20 The strength of the relationship shown in Figure 1 indicates high similarity in strong acid anion properties over a large range in DOC in Adirondack lake samples within the pH range of 4.0−4.5. This was also shown previously by Lawrence et al.11 for Adirondack streams. The equations of the best fit lines for Adirondack lakes (RCOOs− = 0.060[DOC] − 3.2) and streams (RCOOs− = 0.071[DOC] − 2.1) were similar, although the relationship for streams yielded a value of strong acid anion concentration that was approximately 20% higher than that for the lakes over the DOC concentration range of 300−1200 μmol of C L−1. These results indicate the influence of the strong acid component of organic acidity associated with DOC. Furthermore, the increase in annual mean concentrations of DOC in Adirondack lakes (Figure 2) indicates that the influence of strong organic acids has increased. From 1994 to 2011, DOC concentrations increased in 31 of 42 study lakes at the p < 0.1 level and in 27 lakes at the p < 0.05 level. An additional 8 lakes exhibited non-significant increases in the DOC concentration

Figure 1. Anion deficits of charge balances as a function of DOC concentrations in Adirondack lake water samples with pH values of 4.0−4.5. C



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Figure 2. Annual means of DOC concentrations for the 42 lakes from 1994 to 2011.

for this period. For all 42 lakes, DOC showed an overall increase (p < 0.01) of 78 μmol of C L−1 over the 17 year study period (Figure 2). The increasing trend in DOC suggests that RCOOs− was also likely to increase over the study period. Because RCOOs− affects BCS negatively (eq 1), an increasing trend in RCOOs− would offset an increase in BCS. Nevertheless, the BCS showed a marked increase over the study period of 1.5 μeq L−1 year−1 (25.5 μeq L−1 for the study period), although year-to-year variability was relatively high (Figure 3). From the estimation of

Figure 4. IMAl as a function of the BCS for all lake data in 1994 and 2011. Data points with BCS values greater than 150 are not shown because they all plot closely to the x axis. Statistics refer to the sloped line to the left of the threshold.

concentrations that approached zero (0.5 and 0.9 μmol L−1 in 1994 and 2011, respectively) and similar BCS values of −0.5 μeq L−1 in 1994 and 10.0 μeq L−1 in 2011 (Figure 4). The relatively small increase in the BCS threshold value in 2011 was attributable to the method of Al analysis, which slightly overestimates IMAl concentrations at increased DOC concentrations10,21 or could be a result of imprecision in resolving the breakpoint with a larger number of values clustering near a BCS of zero. A pronounced decrease between 1994 and 2011 was also observed in the slope (p < 0.01) of the relationship between IMAl and the BCS to the left of the breakpoint (Figure 4). The decrease in the slope indicates that, for a given BCS value, the concentration of IMAl was considerably lower in 2011 than 1994. This change in slope, in combination with the increase in BCS, led to a substantial decrease (p < 0.01) in concentrations of IMAl (Figure 5a). However, there was not a significant change (p > 0.1) in OMAl over the study period (Figure 5a). The change in slope and lack of trend in OMAl, despite a strong increase in the BCS, were likely driven by the higher DOC concentrations that increased complexation of IMAl by weak organic acids. The shift in the distribution of IMAl and OMAl as a percentage of TMAl supports this interpretation (Figure 5b). From 1994 to 1997, IMAl and OMAl were each about 50% of TMAl; however, by 2011, approximately 80% of TMAl was in an organically complexed form, whereas IMAl had decreased to approximately 20%. The effect of increased organic complexation on Al can be illustrated relative to the effect of decreased strong acids from acidic deposition that has been occurring for several decades.22,23 Had the slope of the IMAl−BCS relationship not changed from 1994 to 2011, the IMAl in 2011 could be estimated by applying the BCS values from 2011 to the linear regression model developed from 1994 data. Without the effect of organic complexation, the 2011 values for individual lakes shown in Figure 6 would be equally distributed above and below the line representing the model for 1994 conditions. A majority of values below the line indicates that IMAl decreased substantially more than would have occurred had these

Figure 3. Annual mean concentrations of strongly acidic organic anions (RCOOs−) and the BCS, averaged for the 42 lakes.

RCOOs− from the relationship between DOC and the anion deficit of low-pH samples (Figure 1), the magnitude of the BCS trend can be compared to that of the RCOOs− trend. The average annual increase in RCOOs− of 0.2 μeq L−1 year−1 was only 13% of the average annual increase of BCS. If RCOOs− had remained unchanged throughout the study period, the BCS increase would have been 1.7 or 28.9 μeq L−1 year−1 for the study period. These estimates suggest that the increase in strongly acidic organic acids had a relatively minor effect on the recovery trajectory of the BCS. The increasing trend in BCS was reflected in a substantially higher number of samples with BCS values greater than 0 μeq L−1 and IMAl concentrations near zero in 2011 than in 1994 (Figure 4). Although these pronounced increases in BCS and decreases IMAl occurred, the BCS threshold for inorganic Al mobilization was relatively stable over the 17 years. Statistically significant thresholds in 1994 and 2011 were identified at similar BCS values through the segmented regression analysis. In both years, a strong linear relationship was observed between the BCS and IMAl to the left of the breakpoint, and the best-fit line to the right of the breakpoint was horizontal (p > 0.1). The thresholds for the BCS−IMAl relationship occurred at IMAl D



Article

The BCS−Al analysis presented in this paper excluded seepage lakes on the basis that they were not likely to reflect the effects of acidic deposition on soils, which is the primary source of Al reaching surface waters. To test this assumption, we evaluated the BCS−Al relationship for East Copperas Pond, which fits the criterion of no natural surface inlets or outlets. In this Adirondack pond, the lowest BCS values in 1994 and 2011 were −55 and −44 μeq L−1, respectively, which would suggest IMAl concentrations of 5−7 μmol L−1 or higher (panels a and b of Figure 5). However, maximum IMAl in both 1994 and 2011 was less than 1.0 μmol L−1. This pond also had high DOC concentrations (800−1000 μmol of C L−1) relative to most other lakes, although several lakes had similar DOC concentrations. Low BCS values, high DOC concentrations, and minimal Al concentrations suggested that these waters have had minimal contact with mineral soil, presumably as a result of peat deposits. Water that passes through the mineral soil tends to lose DOC from solution through adsorption to mineral surfaces, and its within the mineral soil that mobilization of IMAl occurs.12 A large influence from organic matter that occurs from peat deposits tends to lower pH (and BCS) and causes high concentrations of DOC. The overall effect of increased concentrations of DOC on the recovery of surface water chemistry is mixed. The accelerated decrease in the toxic, inorganic form of Al would be expected to benefit a wide range of aquatic biota.9,24 Brook trout survival in Adirondack streams experiencing episodic acidification was shown to vary from 100% when median IMAl was less than 1.7 μmol L−1 to 0% when median IMAl exceeded 7.5 μmol L−1.8 However, increased concentrations of strong organic acids lower pH, as do weak organic acids, which also tend to buffer increases in pH resulting from decreased acidic deposition. Nevertheless, the maximum IMAl concentration in 2011 was less than in 1994 in every lake except Owen Pond, where 1994 concentrations approached zero (Table 1). Furthermore, a comparison of BCS values in 1994 and 2011 shows that the lakes had become considerably less acidic over this period of increasing DOC (Figure 7). In 1994, 53% of the samples collected from these lakes had BCS values less than zero, whereas by 2011, this percentage had dropped to 28%, despite the negative effect of increased strong organic anions on BCS measurements. The increased concentrations of DOC and associated effects on water chemistry may represent a natural return to conditions that existed prior to acidic deposition.1

Figure 5. (a) Average annual concentrations of OMAl and IMAl and (b) OMAl and IMAl as a percentage of TMAl for the 42 study lakes, from 1994 to 2011.

Figure 6. Concentrations of IMAl in 2011 for the 42 study lakes as a function of the BCS. The sloped line represents the linear model developed from IMAl and BCS for 1994 lake data, and the individual values represent BCS values and IMAl measured in 2011.

decreases relied only on the increasing BCS values that directly resulted from decreased acidic deposition. Although increases in DOC and associated organic acidity have slowed recovery somewhat in terms of pH, BCS, and ANC,1 the effect of increasing concentrations of weak organic acids can be viewed as an acceleration of recovery in the context of reducing IMAl concentrations more than would have resulted from the BCS increase alone. This finding is significant because the presumed increase in RCOOs− associated with increased DOC could contribute to increased mobilization of inorganic Al.11 However, the influence of weak organic acids appeared to outweigh the effect of strong organic acids with regard to Al by increasing complexation of inorganic Al to a greater degree than RCOOs− increased the mobilization of inorganic Al. These results do not support the premise that increased DOC concentrations can increase IMAl concentrations.

Figure 7. Cumulative frequency of BCS values expressed as percentage for 1994 and 2011. E



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Chronic and episodic acidification of Adirondack streams from acid rain in 2003−2005. J. Environ. Qual. 2008, 37 (6), 2264−2274. (11) Lawrence, G. B.; Sutherland, J. W.; Boylen, C. W.; NierzwickiBauer, S. A.; Momen, B.; Baldigo, B. P.; Simonin, H. A. Acid rain effects on aluminum mobilization clarified by inclusion of strong organic acids. Environ. Sci. Technol. 2007, 41 (1), 93−98. (12) Driscoll, C. T.; Van Breemen, N.; Mulder, J. Aluminum chemistry in a forested spodosol. Soil Sci. Soc. Am. J. 1985, 49 (2), 437−444. (13) Driscoll, C. T.; Driscoll, K. M.; Roy, K. M.; Mitchell, M. J. Chemical response of lakes in the Adirondack region of New York to declines in acidic deposition. Environ. Sci. Technol. 2003, 37 (10), 2036−2042. (14) Driscoll, C. T.; Driscoll, K. M.; Roy, K. M.; Dukett, J. Changes in the chemistry of lakes in the Adirondack region of New York following declines in acidic deposition. Appl. Geochem. 2007, 22 (6), 1181−1188. (15) Baker, J. P.; Gherini, S. A.; Christiansen, S. W.; Munson, R. K.; Driscoll, C. T.; Newton, R. M.; Gallagher, J.; Reckhow, K. H.; Schofield, C. L. Adirondack Lakes Survey: An Interpretive Analysis of Fish Communities and Water Chemistry, 1984−1987; Adirondack Lakes Survey Cooperation: Ray Brook, NY, 1990. (16) Munson, R. K.; Gherini, S. A. Influence of organic acids on the pH and acid-neutralizing capacity of Adirondack Lakes. Water Resour. Res. 1993, 29 (4), 891−899. (17) Stumm, W.; Morgan, J. J. Aquatic Chemistry; Wiley and Sons: New York, 1981; p 780. (18) Hirsch, R. M.; Slack, J. R. A nonparametric trend test for seasonal data with serial dependence. Water Resour. Res. 1984, 20 (6), 727−732. (19) Oosterbaan, R. J. Frequency and regression analysis of hydrologic data. In Drainage Principles and Applications, ILRI Publication 16, 2nd ed.; Ritzema, H. P., Ed.; International Institute for Land Reclamation and Improvement: Wageningen, The Netherlands, 1994. (20) Hruska, J.; Kohler, S.; Laudon, H.; Bishop, K. Is a universal model of organic acidity possible: Comparison of the acid/base properties of dissolved organic carbon in the boreal and temperate zones. Environ. Sci. Technol. 2003, 37 (9), 1726−1730. (21) Driscoll, C. T. A procedure for the fractionation of aqueous aluminum in dilute acidic waters. Int. J. Environ. Anal. Chem. 1984, 16 (4), 267−283. (22) Dukett, J. E.; Aleksic, N.; Houck, N.; Snyder, P.; Casson, P.; Cantwell, M. Progress toward clean cloud water at Whiteface Mountain, New York. Atmos. Environ. 2011, 45, 6669−6673. (23) Lawrence, G. B.; Simonin, H. A.; Baldigo, B. P.; Roy, K. M.; Capone, S. B. Changes in the chemistry of acidifed Adirondack streams from the early 1980s to 2008. Environ. Pollut. 2011, 159 (10), 2750− 2758. (24) Gensemer, R. W.; Playle, R. C. The bioavailability and toxicity of aluminum in aquatic environments. Crit. Rev. Environ. Sci. Technol. 1999, 29 (4), 315−450.

Certainly the decrease in IMAl beyond that expected from increased BCS can be viewed as a positive recovery response. This analysis demonstrates that recovery processes are complex and need to be evaluated with a variety of chemical measures that include the BCS.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 518-897-1354. Fax: 518-897-1364. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work by the Adirondack Lakes Survey Corporation (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 Environmental Protection Agency (U.S. EPA) Long-Term Monitoring Network. The authors are thankful for support from ALSC and ALTM staff: Jeff Brown, Sara Burke, Mike Cantwell, Paul Casson, Pam Corey, Elizabeth Faucher, Matt Kelting, Karen Roy, Monica Schmidt, and Christopher Swamp. Tim Mihuc, SUNY Plattsburgh, provided a helpful review of the manuscript. The views expressed here are those of the authors and do not necessarily reflect those of the supporting entities or the individuals listed.

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REFERENCES

(1) Waller, K.; Driscoll, C. T.; Lynch, J.; Newcomb, D.; Roy, K. M. Long-term recovery of lakes in the Adirondack region of New York to decreases in acidic deposition. Atmos. Environ. 2012, 46, 56−64. (2) Civerolo, K. L.; Roy, K. M.; Stoddard, J. L.; Sistla, G. A comparison of the temporally integrated monitoring of ecosystems and Adirondack long-term monitoring programs in the Adirondack Mountain Region of New York. Water, Air, Soil Pollut. 2011, 222 (1−4), 285−296. (3) Monteith, D. T.; Stoddard, J. L.; Evans, C. D.; de Wit, H. A.; Forsius, M.; Hogasen, T.; Wilander, A.; Skjelkvale, B. L.; Jeffries, D. S.; Vuorenmaa, J.; Keller, B.; Kopacek, J.; Vesely, J. Dissolved organic carbon trends resulting from changes in atmospheric deposition chemistry. Nature 2007, 450 (7169), 537−541. (4) Clair, T. A.; Dennis, I. F.; Vet, R. Water chemistry and dissolved organic carbon trends in lakes from Canadaʼs Altlantic Provinces: No recovery from acidifcation measured after 25 years of lake monitoring. Can. J. Fish. Aquat. Sci. 2011, 68 (4), 663−674. (5) Evans, C. D.; Jones, T. G.; Burden, A.; Ostle, N.; Zielinski, P.; Cooper, M. D. A.; Peacock, M.; Clark, J. M.; Oulehle, F.; Cooper, D.; Freeman, C. Acidity controls on dissolved organic carbon mobility inorganic soils. Global Change Biol. 2012, 18, 3317−3331. (6) Fenner, N.; Freeman, C. Drought-induced carbon loss in peatlands. Nat. Geosci. 2011, 4, 895−900. (7) Driscoll, C. T.; Fuller, R. D.; Schecher, W. D. The role of organic acids in the acidification of surface waters in the eastern U.S. Water, Air, Soil Pollut. 1989, 43 (1), 21−40. (8) Baldigo, B. P.; Lawrence, G. B.; Simonin, H. A. Persistent mortality of brook trout in episodically acidified streams of the southwestern Adirondack Mountains, New York. Trans. Am. Fish. Soc. 2007, 136 (1), 121−134. (9) Driscoll, C. T.; Lawrence, G. B.; Bulger, A. J.; Butler, T. J.; Cronan, C. S.; Eagar, C.; Lambert, K. F.; Likens, G. E.; Stoddard, J. L.; Weathers, K. C. Acidic deposition in the northeastern United States: Sources and inputs, ecosystem effects, and management strategies. BioScience 2001, 51 (3), 180−198. (10) Lawrence, G. B.; Roy, K. M.; Baldigo, B. P.; Simonin, H. A.; Capone, S. B.; Sutherland, J. S.; Nierswicki-Bauer, S. A.; Boylen, C. W. F


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