Chemical Recovery of Surface Waters Across the Northeastern United

Jul 23, 2005 - Changes in lake water chemistry between 1984 and 2001 at 130 stratified ... deposition across the entire northeastern United States. Al...
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Environ. Sci. Technol. 2005, 39, 6548-6554

Chemical Recovery of Surface Waters Across the Northeastern United States from Reduced Inputs of Acidic Deposition: 1984-2001 RICHARD A. F. WARBY,* CHRIS E. JOHNSON, AND CHARLES T. DRISCOLL Department of Civil and Environmental Engineering, 151 Link Hall, Syracuse University, Syracuse, New York 13244

Changes in lake water chemistry between 1984 and 2001 at 130 stratified random sites across the northeastern United States were studied to evaluate the population-level effects of decreases in acidic deposition. Surface-water SO42- concentrations decreased across the region at a median rate of -1.53 µequiv L-1 year-1. Calcium concentrations also decreased, with a median rate of -1.73 µequiv L-1 year-1. This decrease in Ca2+ retarded the recovery of surface water acid neutralizing capacity (Gran ANC), which increased at a median rate of 0.66 µequiv L-1 year-1. There were small increases in pH in all subregions except central New England and Maine, where the changes were not statistically significant. Median NO3- trends were not significant except in the Adirondacks, where NO3concentrations increased at a rate of 0.53 µequiv L-1 year-1. A regionwide decrease in the concentration of total Al, especially in ponds with low ANC values (ANC < 25 µequiv L-1), was observed in the Adirondack subregion. These changes in Al were consistent with the general pattern of increasing pH and ANC. Despite the general pattern of chemical recovery, many ponds remain chronically acidic or are susceptible to episodic acidification. The continued chemical and biological recovery at sites in the northeastern United States will depend on further controls on S and N emissions.

Introduction Over the past 40 years, the effects of acidic deposition on aquatic and terrestrial ecosystems have been the focus of much research (1, 2). Since the passage of the 1970 and 1990 Amendments of the Clean Air Act (CAAA), wet deposition of SO42- and H+ have decreased across the northeastern United States, while changes in wet deposition of NO3- have been small (3, 4). As acidic deposition declines, research in the United States and Europe has shifted to understanding the recovery of these complex ecosystems, both at intensive study sites and on a regional scale (3, 5-14). This overall decrease in acidic deposition has resulted in some chemical recovery of surface waters, with increases in pH and ANC and decreases in strong acidic anions and inorganic monomeric Al (2, 8, 10). In the northeastern United States the decreases in strong acid anion concentrations have been accompanied by nearly * Corresponding author: email: [email protected]; telephone: (315) 559-7680, fax: (315) 443-1243. 6548

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stoichiometric decreases in base cation concentrations, resulting in lower rates of ANC recovery of surface waters than expected on the basis of the decreases in strong acid anions alone. During the 1980s a similar pattern was observed across Europe. However, during the 1990s, decreases in strong acid anions in Europe were not accompanied by corresponding decreases in base cations, resulting in a greater rate of ANC increase (7, 9-11). The rate of ANC recovery of surface waters appears to vary markedly with different geologic and geographic settings (3). It is therefore convenient to examine the recovery of ANC based on ANC class. Three ANC classes were considered in this paper: chronically acidic (ANC < 0 µequiv L-1), low ANC (0 < ANC < 25 µequiv L-1), and moderate ANC (ANC > 25 µequiv L-1). Typically, ponds in the chronically acidic ANC class are more susceptible to acidification and should tend to show a greater rate of ANC recovery. Although this study covers the entire northeastern United States (Figure 1), other studies have previously examined subregions within the region. The Adirondacks is probably the most intensively studied subregion, with 52 Adirondack Long-Term Monitoring (ALTM) sites (e.g., ref 3). The Catskills and New England have also been studied (e.g., refs 4 and 7). The general findings reported in the literature for these subregions are that surface waters have shown some chemical recovery following reductions in SO2 emissions mandated in the 1970 and 1990 CAAAs. These papers report decreases in SO42- and Ca2+ concentrations and increases in pH and ANC for some surface waters. Driscoll et al. (3) reported decreasing NO3- concentrations at some ALTM sites for the period 1992-2000. They also found decreasing concentrations of monomeric Al at the ALTM sites in the 1990s and a shift in the speciation of Al from the more toxic inorganic forms to the less toxic organic species. To date, no other study has examined the recovery of populations of surface waters following decreases in acidic deposition across the entire northeastern United States. Although regional patterns have been inferred from different studies across the northeastern United States conducted at different times, we report regional trends between 1984 and 2001 based on 130 study sites spanning the region. Using a probability-based approach (15, 16), we extrapolated the results from 130 lakes sampled in 2001 to a target population of 3666 lakes across the region. By investigating patterns across five subregions of the Northeaststhe Adirondacks, the Catskills/Poconos, southern New England, central New England, and Maine (Figure 1)swe examined the differing responses of individual subregions to reduced inputs of acidic deposition in the context of the entire northeastern United States. We compare and contrast our results with existing literature to provide additional understanding of the chemical recovery of surface waters in response to decreases in acidic deposition. We also report trends for areas in the northeastern United States for which there is little or no existing literature. This synoptic study complements previous time series studies (e.g., refs 3, 7, 8, and 10) by placing the results in a regional context.

Methods The Direct/Delayed Response Program (DDRP) was initiated in 1984 at the request of the Administrator of the U.S. Environmental Protection Agency (EPA) (15). The DDRP was conducted under the National Acid Precipitation Program (NAPAP) and was designed to assess the then-current and future effects of acidic deposition on surface waters in three 10.1021/es048553n CCC: $30.25

 2005 American Chemical Society Published on Web 07/23/2005

FIGURE 1. Map of the 130 Direct Delayed Response Project (DDRP) study lakes sampled in 2001. Shown are sample sites for each subregion. regions of the eastern United States (17). The central question of the DDRP was the following (16): How many surface waters would become acidic due to the then-current or altered levels of acidic sulfur deposition, and on what time scales would these changes occur? The DDRP watershed selection process was designed to allow results to be extrapolated to the population of lakes studied in the earlier Eastern Lakes Survey (ELS). Using preliminary results from the ELS, lakes were divided into three ANC classes. A random sample of 50 lakes was selected from each ANC class (15). All lakes were >4 ha in surface area. Refusal of access and other factors ultimately reduced the total to 145 lake watersheds in the northeastern U.S. region. During the summer of 2001 (May 28-Aug 4) we collected water from 130 of the original 145 DDRP lakes and watersheds across the northeastern United States, from Pennsylvania to Maine (Figure 1). The remaining 15 sites were not sampled due to refusal of access and/or inaccessibility. Because of the stratified sampling approach used in the DDRP, the inclusion probabilities of lakes ranged from 0.02445 to 0.08296. The reciprocal of a lake’s inclusion probability is the number of lakes in the target population “represented” by that lake. Thus, each lake in the DDRP survey represented between 12 and 41 lakes in the ELS population. We used this approach to compute populationlevel statistics from our results. However, because we collected samples from 130 sites, the inclusion probabilities in the subregions with unsampled sites were adjusted so that the total number of lakes represented by the populationlevel estimates (3666 lakes) was unchanged between 1984 and 2001. Thus, in our study each lake represented between 12 and 53 lakes in the ELS population. Samples were collected at the water surface from the center of the ponds using an inflatable raft. The pH of the

water samples was determined within 8 h of collection using a combination glass electrode and portable pH-meter. These samples were neither acidified nor filtered in the field. The water samples were stored at 4 °C before analysis of the samples at Syracuse University. The samples were then filtered through 0.45 µm nitrocellulose filters. The ANC of the samples was determined using a strong acid titration with a Gran plot analysis. Sulfate, NO3and Cl- were determined using ion chromatography, whereas Ca, Mg, Na, K, and total Al were determined by inductively coupled plasma mass spectrometry (ICPMS). The aliquots used for the determination of Ca, Mg, Na, K, and total Al were acidified with HNO3 (0.15 M). The measured concentrations of Ca, Mg, Na, and K were assumed to equal the ionic concentrations (Ca2+, Mg2+, Na+, and K+). For the original DDRP, the pH was determined in the field in an unfiltered aliquot using a combination glass electrode. Gran ANC was measured in an unfiltered aliquot using a strong acid titration with a modified Gran plot analysis. Sulfate, NO3-, and Cl- were determined using ion chromatography, whereas Ca, Mg, Na, and K were determined using flame AAS. Total Al was determined by flame AAS and graphite furnace AAS in an unfiltered aliquot. The aliquots for the determination of Ca, Mg, Na, K, and total Al were acidified with HNO3 (18). In this paper we discuss the changes in pH, ANC, SO42-, NO3-, Ca2+, Mg2+, and total Al in the DDRP ponds. For each site, the rate of change in chemical concentration was computed as the difference between the 2001 and 1984 values, divided by the 17-year interval between sampling. Because neither the 1984 data nor the 2001 data were normally distributed, nonparametric statistics were used. Median rates of change rather than mean values are reported. In the calculation of the median trends the 130 common sites between 1984 and 2001 were used. The significance of the VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Regional Trends in the Chemistry of the 130 DDRP Lakes Resampled in 2001a region

no. of ponds

pH

Gran ANC

SO42-

NO3-

AlTOT

Ca2+

Mg2+

whole ADR CATPOC CNE SNE Maine

130 25 21 30 22 32

0.002* 0.007ns 0.008* -0.002ns 0.007* -0.001ns

0.66** 0.23ns 0.81ns 0.65ns 0.84* 0.94**

-1.53** -1.82** -1.12** -1.08** -2.14** -1.73**

0.002** 0.53* 0.005ns 0.05); *, p < 0.05; **, p < 0.01.

changes was determined using the Wilcoxon matched-pairs test. The only assumption required by this test is that the underlying distribution of the variable of interest is continuous; no assumptions about the nature or shape of the underlying distribution are required. The test simply computes the number of times that the value of the first variable (A) is larger than that of the second variable (B). The null hypothesis for the test (that the two variables are not different from each other) predicts that A > B 50% of the time. The Wilcoxon matched-pairs test uses the binomial distribution to compute a z value for the observed number of cases where A > B and computes the associated tail probability for that z value (19). Hydrologic conditions are important in controlling the acid-base chemistry of surface waters (20) and for assessing the recovery of surface waters from reduced inputs of acidic deposition. Because discharge is not monitored in most of the DDRP watersheds, precipitation amount was used as a surrogate. To determine if precipitation amount, and therefore hydrologic flow, was similar during the 12 months preceding each sampling period in 1984 and 2001, precipitation amounts from 18 NADP/NTN sites across the region were analyzed. Monthly precipitation amounts from August 1983 to July 1984 and from June 2000 to May 2001 were summed to estimate the annual precipitation at each site. The Wilcoxon matched-pairs test was then used to determine if the precipitation amounts were statistically significantly different for the two periods. The median value of the 18 NADP/NTN sites was used as a regional precipitation amount.

Results and Discussion Trends in Atmospheric Deposition and Precipitation Amount. From the analysis of the 18 NADP/NTN sites across the study region, we found that the 12 months preceding the 1984 sampling period were wetter than the same period before the 2001 sampling period. The median precipitation amount for the region for the 1984 sampling period was 132 cm, whereas 94 cm of precipitation occurred for the 2001 sampling period. These precipitation amounts were statistically significantly different (p < 0.001). A similar analysis showed that 3 months prior to sampling in 2001 the region received 29% less precipitation and 6 months prior to sampling received 39% less precipitation than during the same periods in 1984. In the 17 years between samplings, the median precipitation amount for the region ranged between 83 and 138 cm, indicating that conditions prior to the 1984 sampling period were considerably wetter than average, whereas in 2001 conditions were drier than average. The median precipitation over the period was 107 cm. A regional decrease in the concentration of SO42- in precipitation is well documented, with trends of -1.47 and -0.96 µequiv L-1 year-1 for the Adirondacks and New England, respectively, between 1990 and 2000 (8). Decreases in precipitation NO3- and H+ concentrations in these subregions have also been observed (8). 6550

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FIGURE 2. Chemistry of Constable Pond in the Adirondack subregion. Time series data were collected monthly by the ALTM. All units are µequiv L-1. Detailed time series data (O) are compared with the DDRP and 2001 synoptic surveys (2). Trends in Lake Sulfate and Nitrate. Sulfate concentrations in surface waters reflect the reduced S deposition resulting from stricter controls on SO2 emissions mandated by the 1970 and 1990 CAAAs. In our study, subregional trends in lake-water SO42- concentrations ranged from -1.08 µequiv L-1 year-1 in central New England to -2.14 µequiv L-1 year-1 in southern New England. Concentrations of SO42- in lakes decreased across the whole study region, with the median rate of change of -1.53 µequiv L-1 year-1 (Table 1; Figure 3). Stoddard et al. (21) observed decreases in SO42- in surface waters in long-term monitoring (LTM) lakes throughout the northeastern United States, with small differences in significance or magnitude of the trends. They reported trends in the range of -1.6 to - 2.1 µequiv L-1 year-1. The magnitudes of the trends for surface-water SO42- reported by Driscoll et al. (3) (-2.06 µequiv L-1 year-1) in the Adirondacks and by Murdoch and Stoddard (22) (ranging from -0.9 to -3.3 µequiv L-1 year-1) for the Catskills region are comparable to our results. Time series studies, such as those cited above, provide detailed patterns of environmental change. However, the amount of sampling and analysis required for time series studies makes it difficult to include large numbers of sites. Our synoptic survey, with the population-based estimates of water chemistry changes, complements time series studies by providing a better understanding of the extent of the recovery of surface waters in the northeastern United States following decreases in acidic deposition. Our results are generally comparable with time series measurements, as illustrated in Figure 2 for Constable Pond in the Adirondacks,

trend in the Adirondacks (0.53 µequiv L-1 year-1; Table 1) between 1984 and 2001. Nitrate concentrations tend to be higher in late spring, when we sampled the Adirondack sites, than in late summer and autumn, when the DDRP sampling was conducted (24). Therefore, the significant increase in NO3- concentrations of Adirondack lakes that we observed may be at least partly due to differences in sampling dates. The study region as a whole showed a small, but statistically significant, increase in NO3-, but this was driven by the increase in the Adirondacks, as the concentrations (and trends) of NO3- in the other regions were negligible. Trends in Lake Basic Cations. An almost ubiquitous decreasing trend in base cations has been reported for surface waters in the northeastern United States and Europe since the 1980s (3, 4, 7-11). In our study trends in Ca2+ concentrations ranged from -0.51 µequiv L-1 year-1 for southern New England to -2.79 µequiv L-1 year-1 for the Catskills and Poconos, with a median rate of change for all sites of -1.73 µequiv L-1 year-1 (Table 1; Figure 3). Trends for Mg2+ concentrations were smaller, ranging from -0.17 µequiv L-1 year-1 (Adirondacks) to 0.72 µequiv L-1 year-1 (southern New England), with a median rate of change for all sites of 0.2 µequiv L-1 year-1. In this paper we focused on divalent cations. Any changes in Na+ were almost exactly balanced by changes in Cl-, whereas K+ showed little or no significant trends in either the subregions or the region as a whole.

FIGURE 3. Trends in sulfate, calcium, and ANC for the whole region and the subregions for the interval 1984-2001. Boxes indicate the interquartile ranges; caps indicate the 10th and 90th percentiles; median trends are indicated by the line in the box; dots indicate the 10th and 90th percentiles of the outliers. an Adirondack Long-Term Monitoring (ALTM) site that is also a DDRP site. Over a similar time period of study, Evans et al. (11) reported a median trend of surface-water SO42- of -1.9 µequiv L-1 year-1 across five subregions in Europe, ranging from -1 µequiv L-1 year-1 in Italy to -4.17 µequiv L-1 year-1 in Slovakia. However, more recent studies in Europe looking only at trends in the 1990s have generally shown greater rates of SO42- decrease than when both the 1980s and 1990s were considered (9, 10). Stoddard et al. (7) found a pattern similar to that observed by Evans et al. (11) between the two decades for rates of SO42- decrease in southern/central Ontario and areas in Europe. However, an opposite trend was observed in the Adirondacks and Catskills, where SO42decreased at a slower rate in the 1990s than in the 1980s (7). The effects of decreasing NO3- deposition on lake-water chemistry are less clear than for SO42-. For example, an increasing trend in NO3- was observed in a number of Adirondack lakes between 1982 and 1991 (23). Over the longer period of 1982-2001 a significant decrease in NO3- was observed in half of the same lakes (3). We found an increasing

The decrease in Ca2+ can be attributed to decreases in acidic deposition coupled with decreases in atmospheric Ca2+ deposition. Acid inputs are partially neutralized through weathering and exchange reactions in soils, which release Ca2+ and other base cations to soil solutions. Thus, reductions in acid inputs may result in lower Ca2+ concentrations in drainage waters. The magnitude of the decreases in lake Ca2+ concentrations across the region generally follows the decreasing acid deposition gradient from the southwest to the northeast (25). Areas showing the greatest declines in acidic deposition have also shown the greatest declines in Ca2+. In southern New England and Maine, areas that receive lower inputs of acidic deposition, there have been relatively small decreases in Ca2+, whereas Mg2+ has shown small, but statistically significant, increasing trends. This pattern has resulted in the largest increases in Gran ANC in these subregions (see below). Less is known about regional patterns in Ca2+ deposition. Recent studies at the Hubbard Brook Experimental Forest in New Hampshire have suggested that decreases in deposition of base cations have contributed to declines in stream concentrations, although declines in atmospheric deposition of SO42- appear to be the major controlling factor (26, 27). The decreases we observed in Ca2+ in the Adirondacks (-2.67 µequiv L-1 year-1) were greater than those reported for 1982-2000 by Driscoll et al. (-1.30 µequiv L-1 year-1) (3). Stoddard et al. (7) reported an increasing trend in base cation concentrations in the Adirondacks and the Catskills in the 1980s (0.8 µequiv L-1 year-1) and then a large decrease in the 1990s (-4.5 µequiv L-1 year-1). In another paper Stoddard et al. (8) reported a -2.29 µequiv L-1 year-1 trend in base cations in the Adirondacks during the 1990s. Despite the variation in the magnitudes reported in the literature, it is clear that the sum of divalent base cations, Ca2+ in particular, has decreased in the Catskills and Adirondacks over the past 10-20 years. Finally, our data suggest that the welldocumented decline in base cation concentrations in the Adirondack and Catskills surface waters extends regionwide, with the possible exception of the coastal areas of southern New England. Similar trends of decreasing base cations were found in northern/central Europe and Scandinavia in the 1980s (7). However, base cations in European surface waters during the 1990s decreased at a slower rate than in the 1980s VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Number of Lakes Sampled (n) and Population-Level Estimates for the Region (N) in 1984 and 2001 in Chronically Acidic, Low-ANC, and Moderate-ANC Classes, and Rates of ANC Changes over the Intervala 1984

2001

ANC class

ANC range

n

N

n

N

∆Gran ANC (µequiv L-1 year-1)

chronically acidic low ANC moderate ANC

ANC < 0 0 < ANC < 25 ANC > 25

14 29 87

223 473 2970

10 24 96

166 404 3096

0.80** 0.62** 0.65ns

a Values are median rates of ANC change for each ANC class. Significance indicators: ns, ANC class trend not significant (p > 0.05); **, p < 0.01.

(9-11), in contrast with patterns observed at North American sites. Trends in Total Al. In regions where the soils are rich in available base cations, there will be an approximately stoichiometric relationship between changes in acidic deposition and cation export from the watershed, resulting in little change in ANC or pH (21). However, in base-poor areas, cation export due to increased acidic deposition is accompanied by increases in Aln+ and H+ export to maintain charge neutrality. We found a decrease in total Al across the entire region, with a median trend of -1.02 µg L-1 year-1 (Table 1). This pattern suggests a reversal of acidification, which is supported by increases in pH and ANC across the region. The low ANC waters of the Adirondacks showed the greatest decrease in total Al (-3.39 µg L-1 year-1), although the change was not significant. This pattern is consistent with Adirondacks soils exhibiting the lowest concentrations of exchangeable base cations in the study region. Trends in Lake pH and ANC. The ANC, pH, and Al concentrations of surface waters are probably the most important indicators of chemical recovery of aquatic ecosystems from acidic deposition. Decreases in the pH and increases in Al of ponds result in the loss of diversity and abundance of plankton, invertebrates, and fish in acidsensitive waters (2). The pH of the surface waters across the northeastern United States exhibited an increasing trend of ∼0.002 pH unit year-1 (Table 1). Trends in central New England and Maine were not statistically significant. Clair et al. (14) reported similar trends for the Atlantic Canada region. The magnitudes of trends in ANC ranged from 0.23 µequiv L-1 year-1 for the Adirondacks to 0.94 µequiv L-1 year-1 for Maine (Table 1; Figure 3). The ANC in lakes increased across the whole study region, with the median rate of change of 0.66 µequiv L-1 year-1. Stoddard et al. (7) and Skjelkvåle et al. (10) found similar trends in southern/central Ontario, the Adirondacks and Catskills, and midwestern North America. Over a similar time period to our study, Evans et al. (11) reported changes in ANC ranging from 0.7 µequiv L-1 year-1 in Italy to 4.5 µequiv L-1 year-1 in the Czech Republic. In Europe and Scandinavia, however, ANC showed greater recovery in the 1990s than in the 1980s, with surface-water ANC in northern/central Europe recovering at 2.5 µequiv L-1 year-1 in the 1980s and at 7.0 µequiv L-1 year-1 in the 1990s (7). Lower ANC lakes are highly sensitive to acidic deposition and are therefore expected to be the first to recover following reductions in acidic deposition, due to their low buffering capacity (21). Our results are consistent with this finding. In chronically acidic ponds, ANC increased 0.80 µequiv L-1 year-1, whereas lakes in the low-ANC class recovered at 0.62 µequiv L-1 year-1 (Table 2; Figure 4). Although lakes in the moderate-ANC class showed a recovery similar to those in the low-ANC class, this change was not statistically significant. In both the chronically acidic and low-ANC classes, pH showed similar trends, increasing at rates of 0.015 and 0.016 pH unit year-1, respectively. Lakes in the moderate-ANC class 6552

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FIGURE 4. Estimated cumulative frequency diagram for ANC in 1984 (gray symbols) and 2001 (black symbols) for 3666 lakes (as determined by population-level extrapolations) in the northeastern United States. showed a very small decreasing trend; however, these results were also not significant. The trends in ANC largely reflect the trends in Ca2+ relative to changes in SO42-. In the chronically acidic and low-ANC classes, SO42- decreased more and Ca2+ decreased less than when all of the sites were considered (Tables 1 and 3). Similarly, the rapid increases in ANC observed in European waters in the 1990s were due to much greater decreases in SO42- concentrations than base cations (7). Perched seepage lakes are highly sensitive and responsive to changes in acidic deposition (3). Seven of our 130 sites are seepage lakes. However, we found a slower median rate of ANC recovery in seepage lakes (0.39 µequiv L-1 year-1) than in drainage lakes (0.66 µequiv L-1 year-1). Of the seven seepage lakes, five were in the low-ANC class in 1984 and two were in the chronically ANC class. Although the combined number of lakes in the two ANC classes did not change throughout the study period, only one site was chronically acidic in 2001. Furthermore, the two seepage lakes in the moderate-ANC class both exhibited significant decreases in Ca2+ while SO42remained essentially unchanged, resulting in significant decreases in ANC between 1984 and 2001. Because seepage lakes receive most of their water from direct precipitation and groundwater inflows, the wet year preceding the 1984 sampling period might have contributed to the observed pattern. Population-Level Extrapolations. In 1984, 29 lakes sampled, representing 473 lakes across the region, were in the low-ANC class (Table 2). Fourteen lakes, representing 223 lakes, were chronically acidic. By 2001 only 24 of the sampled lakes, representing 404 lakes, were in the low-ANC class. Ten lakes, representing 166 in the region, remained chronically acidic. Thus, as a result of reductions in acidic deposition over the past 20 years, more than 50 lakes in the northeast region are no longer chronically acidic. More than

TABLE 3. Regional and Subregional Trends in Lake Chemistry for the 43 Chronically Acidic and Low-ANC Lakes Sampled in 2001 (ANC < 25 µequiv L-1 in 1984)a region

no. of ponds

pH

Gran ANC

SO42-

NO3-

AlTOT

Ca2+

Mg2+

whole ADR CATPOC CNE SNE Maine

43 13 5 7 13 5

0.02** 0.009* 0.001ns 0.04* 0.02** 0.005ns

0.66** 0.33* 0.35ns 0.91ns 1.05** 0.62*

-1.87** -1.92** -1.29* -1.24* -2.37** -2.01*

-0.006ns 0.53** -0.004ns -0.006ns -0.02ns -0.01ns

-0.90** -3.39ns -0.26ns -0.84ns -0.81* -1.58ns

-1.24** -2.46** -2.39* -0.68** -0.23ns -0.78*

0.02ns -0.17** -0.20ns 0.01ns 0.68** 0.07ns

a Values are median rates of change for the region and each subregion. Units for Gran ANC, sulfate, nitrate, calcium, and magnesium are µequiv L-1 year-1. Units for pH are pH units/year, and Al units are µg L-1 year-1. Significance indicators: ns, regional trend not significant (p > 0.05); *, p < 0.05; **, p < 0.01.

100 lakes have moved out of the low-ANC class into the moderate-ANC class. Kahl et al. (4) reported that approximately one-third of the sites in the Adirondacks that were chronically acidic in the early 1990s were no longer so in 2000. However, over the longer period of our study, the number of chronically acidic lakes in the Adirondacks in our study remained unchanged (3 lakes, representing 48 lakes). However, the number of lakes in the low-ANC class decreased from 10 lakes (representing 188 lakes) in 1984 to 7 lakes (representing 112 lakes) in 2001. Of all the subregions in this study, rates of ANC increase were lowest in the Adirondacks (0.23 µequiv L-1 year-1) (Table 1; Figure 3) and not statistically significant. Our population estimates suggest a systematic increase in ANC throughout the region, especially at the low end of the ANC scale (Figure 4). For example, in 1984 ∼28% of the sampled lakes in the entire region had an ANC < 50 µequiv L-1. The number of lakes with ANC < 50 µequiv L-1 decreased to ∼24% by 2001 (Figure 4). Despite regionwide increases in ANC in recent years, waters with ANC < 50 µequiv L-1 remain sensitive to acidic deposition and may experience adverse biological response associated with episodic acidification (3, 8). Our results demonstrate that widespread recovery from acidic deposition is occurring for lakes across the northeastern United States. However, there are several factors that should be considered when these results are interpreted. The DDRP survey was conducted using lakes >4 ha. Smaller lakes, especially those occurring at higher elevations, are more sensitive to acidic deposition than larger lakes. As a result, estimates of the numbers of acidic and low-ANC lakes would increase if smaller lakes were considered. The surveys were conducted during the summer and fall under low flow conditions when ANC values are typically maximum (24). Sampling during winter and spring would undoubtedly yield larger numbers of acidic and low-ANC lakes. Finally, the 2001 survey was conducted following a drier period than the original 1984 survey. This difference between the two surveys undoubtedly served to increase the ANC change observed. Higher flow rates result in less contact time between water flowing through the watersheds and the soils, resulting in less neutralization of acidity by base cations (20). Therefore, the “true” rate recovery of ANC may be somewhat lower than the values reported here, considering the yearto-year variations in hydrology. Management Considerations. Despite decreases in S emissions and decreases in SO42- concentrations in surface waters across the northeastern United States, significant decreases in divalent base cations have retarded the recovery of ANC. Consequently, many of the surface waters in our study region remain chronically acidic or subject to episodic acidification, limiting the anticipated improvement in the diversity and population of fish and other aquatic biota. However, pH has shown a small but statistically significant

increase across the region, whereas Al concentrations have decreased, especially in the subregions (Adirondacks and Catskills, and Poconos) most affected by acidic deposition. It appears that further recovery of ANC in these sensitive ecosystems will require additional controls on the emissions of S and N.

Acknowledgments We thank the W. M. Keck Foundation for its financial support, without which this project would not have been possible. We thank Mario Montesdeoca and the laboratory staff at Syracuse University for their help in analysis of the samples. We also thank the 10 members of the field crew who helped with the collection of the samples in 2001. We are grateful to the Adirondack Lakes Survey Corporation for the use of Constable Pond data.

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Received for review September 15, 2004. Revised manuscript received May 12, 2005. Accepted June 9, 2005. ES048553N