Freshwater acidification research in Atlantic Canada: a review of results and predictions for the future

Freshwater acidification research in AtlanticCanada: a review of results and predictions forthe future

Thomas A. Clair, Ian F. Dennis, David A. Scruton, and Mallory Gilliss

Abstract: Atlantic Canada receives the lowest acid deposition amounts in eastern North America, but has some of themost acidic surface waters on the continent, due to the low buffering provided by regional bedrock and wetlands that pro-duce natural organic acids. Southwestern and eastern parts of Nova Scotia combine poor buffering, high organic acidity,and higher acid deposition, to produce extremely low surface water pH and acid neutralization capacity (ANC) values.Although sulfate deposition is decreasing, concurrent reductions in dissolved base cations, as well as the acid-base charac-teristics of natural organic acids, are not allowing the recovery of ANC or surface water pH. Spring-time acid pulses occurin Atlantic Canada, though these have been reduced in severity with decreases in winter acid deposition, while autumnallow pH pulses caused by organic acids are a regular occurence in Nova Scotia and must be separated from mineral aciditypulses. Geochemical modeling using both critical load and dynamic approaches, nevertheless predict improvements in thewater chemistry of Nova Scotia lakes within the next 20 years. However, re-establishment of pre-acidification water chem-istry in most of its lakes will require greater reductions in S emissions than are currently planned in Canada and the UnitedStates.

Key words: Atlantic Canada, acidification, freshwater, total organic carbon, organic acidity.

Resume : La cote Atlantique du Canada recoit les plus faibles depositions de pluies acides du nord-est Americain, maispossede les eaux de surface les plus acides du continent, dues au faible pouvoir tampon de la roche mere et aux terres hu-mides qui generent naturellement des substances organiques acides. Le sud-ouest et l’est de la Nouvelle-Ecosse combinentune haute acidite organique et la plus importante deposition acide, ce qui engendre des eaux de surface qui engendre despH de l’eau de surface et des capacites de neutralisation de l’acide (ANC) extremement faibles. Bien que la deposition dessulfates soit a la baisse, une reduction simultanee des cations basiques, ainsi que les caracteristiques acide-base des acidesorganiques naturels, ne permettent pas la recuperation de l’ANC ou du pH des eaux de surface. Des poussees printanieresd’acidite surviennent sur la cote Atlantique du Canada, bien que la severite de celles-ci ait ete reduite avec une diminutiondes precipitations acides hivernales, alors que les poussees de forte acidite automnales causees par les acides organiquesreviennent regulierement en Nouvelle-Ecosse, et doivent etre distinguees des poussees d’acidite minerale. Le modelagegeochimique, a l’aide des approches dynamique et de charge critique, predisent neanmoins des ameliorations de la chimiede l’eau des lacs en Nouvelle-Ecosse, au cours des 20 prochaines annees. Cependant, le retour a la chimie de l’eau d’avantl’acidification necessitera des reductions plus elevees des emissions de S que celles couramment envisagees, au Canada etaux Etats-Unis.

Mots-cles : cote Atlantique du Canada, acidification, eau douce, carbone organique total, acidite organique.

[Traduit par la Redaction]

Introduction

The first major studies of water chemistry in Canada’s At-lantic Provinces were made in the late 1950s. Gorham(1957) studied lakes in Halifax County, Nova Scotia (NS),while Hayes and Anthony (1958) assessed a wide variety oflakes throughout that province. Both studies identified themain themes that would affect freshwater acidification. Gor-ham (1957) measured very low specific conductance and

dissolved ions at his study sites and showed them to besome of the most dilute reported in the literature of thetime. He also showed that lakes near the Atlantic Oceancoast had unusually high Na+ and Cl– values, caused by sea-spray deposition. Hayes and Anthony (1958) also showedseasalt effects and found that many lakes in this regionwere low in phosphorus (P) and thus had low biological pro-ductivity. They also found that organic matter, which wasresponsible for natural acidity, had a more important influ-

Received 9 November 2006. Accepted 6 June 2007. Published on the NRC Research Press Web site at er.nrc.ca on 28 September 2007.

T.A. Clair1 and I.F. Dennis. Aquatic Ecosystem Impact Research Division, Environment Canada, PO Box 6227, Sackville, NB E4L1G6, Canada.D.A. Scruton. Fisheries and Oceans Canada, PO Box 5667, St John’s, NL A1C 5X1, Canada.M. Gilliss. NB Dept. of Environment, P.O. Box 6000, Fredericton, NB E3B 5H1, Canada.

1Corresponding author (e-mail: [email protected]).

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ence on the acidity of lakes in NS than in other regions ofEurope and North America then reported.

From the 1950s to early 1970s, water chemistry samplingwas done in Atlantic Canada by federal and provincial gov-ernmental organizations. Both the Departments of Fisheriesand the precursors to Environment Canada conducted large-scale water chemistry surveys to assess fish habitat and toevaluate potable and industrial usages in the region (Thomas1960). Acidification was not a concern at that time, so thatsampling and analytical protocols were not adequate for theevaluation of data for spatial or temporal acidification trendanalyses.

By the mid-to-late 1970s, acidification was identified as aserious concern in Canada and Scandinavia. Shaw (1979)showed that Atlantic Canada, the eastern-most part of theNorth American continent, received low, but measurableacid deposition originating mostly in central Canada, theOhio Valley and the eastern seaboard of the United States.Later studies by Watt et al. (1979, 1983) and Kerekes et al.(1982) showed that some surface waters were very acidic inthe region, and that deleterious effects to aquatic ecosystemsfrom acid rain could be measured. The main objectives ofaquatic chemistry research in the early part of the 1980swere to (a) follow up on the earlier work of Gorham (1957)and Hayes and Anthony (1958) to better understand howwater chemistry in the region differed from that of other re-gions in North America and Europe, (b) delineate the spatialextent of acidification effects in Atlantic Canada, and (c) as-sess long-term and seasonal trends in water chemistry to seeif changes attributable to acid rain could be detected. Asbetter information became available, research emphasischanged to the use of geochemical modeling to determine ifand how reductions in acid deposition might produce im-provements in water chemistry.

This paper reviews the freshwater acidification chemistrywork that has been conducted in Atlantic Canada over thelast 25 years. We analyzed the main data sets collected byvarious agencies. We also evaluated published scientific re-ports to assess spatial and temporal trends in the acidifica-tion-related variables measured. We then interpret the studyresults based on our improved current knowledge.

How is water chemistry different in AtlanticCanada?

Natural buffering capacityThe granite and shale bedrock found in large parts of the

Atlantic region contain little buffering material (Shilts1981). Soils that are formed from this parent material andthe waters draining them thus contain low base cation (Cb)concentrations and are thus vulnerable to acidification, evenunder low acid deposition conditions (Stumm et al. 1987).Kerekes et al. (1982) also identified a number of sites in Ke-jimkujik National Park (KNP), Nova Scotia, which had ex-tremely low Cb values.

Clair et al. (1982) assessed the Atlantic Region’s sensitiv-ity to acidification using alkalinity values from EnvironmentCanada databanks. Unfortunately, early alkalinity valueswere determined by regional laboratories using the two end-point acid titration method (APHA 1989) that has a detec-tion limit of 0.5 mg L–1 as CaCO3. The method works well

in waters with high carbonate (CO3–2) and bicarbonate

(HCO3–), but which are less sensitive to acidification. How-

ever, the method’s detection limit is too high to be useful inmany of the dilute waters of the region. In waters with highcarbonate content, acid neutralization capacity (ANC) is ap-proximately equal to carbonate alkalinity. In low carbonatewaters, organic acids can be an important component ofacid buffering. In this case, another analytical method,which will be described below, must be used to estimateANC.

In the absence of dependable ANC values in early data,Clair et al. (1982) constructed contour lines of the calcitesaturation index (CSI) from 575 data points from federalgovernment databases collected before 1982. The CSI, anindex of carbonate mineral precipitation (and thus relativebuffering capacity), was calculated as:

½1� CSI ¼ pH

� ðpðCaþ2Þ þ pðANCÞ � pK2þ pKs þ 5:5pfmÞ

where Ca+2 is the calcium ion concentration in moles (un-corrected for seasalts), ANC is the acid neutralization capa-city expressed as moles of CaCO3, K2 is the seconddissociation constant for carbonic acid, Ks is the solubilityconstant for CaCO3 and fm is the activity coefficient formonovalent species at laboratory temperature (APHA1989). If ANC was below the detection limit of 0.5 mg L–1

as CaCO3, that value was set = 0. Though not ideal, themethod at least allowed a view of relative acid buffering inthe region.

Much of southwestern NS, the Eastern Shore of NS, theCape Breton Highlands, southwestern New Brunswick(NB), and southern part of the Island of Newfoundland(NF) showed low CSI values (Clair et al. 1982). Labrador(LAB) also had low CSI’s, suggesting sensitivity to acidifi-cation. Prince Edward Island (PEI) and northern NB had lowsensitivity because of the well buffered bedrock and soils.Measured pH values correlated well with CSI.

A number of new sampling programs were conducted inthe Maritime Provinces (NB, NS, and PEI) post 1985 by En-vironment Canada (EC), Fisheries and Oceans Canada(DFO), and the Province of New Brunswick (Peterson1980; Esterby et al. 1989; Gorham et al. 1998; Pilgrim etal. 2003; Dennis et al. 2005a). On the Island of Newfound-land (NF) and in LAB, survey work was conducted by DFO(Ryan et al. 1990; Scruton 1983, 1984) and EC (Clair 1992)as well as the provincial government to provide more de-tailed information on that region’s susceptibility and re-sponse to acid deposition. Analysis of the newer sampleswere done using the more accurate Gran titration ANCmethod (Gran 1952), which is able to measure the sum ofcarbonate buffering (alkalinity), plus organic buffering. Italso allows the calculation of negative ANC values, whichin effect, indicates excess acidity.

These new data allowed us to map the distribution ofacidification-related surface water chemistry parameters(ANC, Ca+2, Mg+2, total and seasalt corrected SO4

–2, DOC,and pH) and to provide good coverage of most of the region.To accurately describe the extant of surface water chemistryconcentrations, we used a rigorous approach to calculatingspatial distribution of variables. This method extended the

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area of influence of a data point for only a 20 km radius(Dennis et al. 2005b) and each point used in the spatial anal-ysis was an average of the values found at a particular sam-pling site. In some cases, this value came from only onesample, while at other sites which were part of intensivesampling networks, several hundred values collected over anumber of years were averaged.

The lower the ANC, the more likely acidification will bea problem. Waters with low acid buffering capacity werefound in the southwestern and eastern shores and Cape Bre-ton Highlands (the extreme eastern part) of NS (Fig. 1).Southwestern NB also had an area of low ANC, as did SEand NW NF and southern LAB. The remainder of NB,northern and eastern NS and PEI showed waters well buf-fered against acidity. The dissolved base cations Ca+2 andMg+2 released by the production of ANC when soils andsurficial bedrock are weathered, very closely matched theANC regional distribution (Figs. 2a, 2b).

Base cation values from NS and NL were the lowest re-ported in Canadian regions, while NB values were moresimilar to those from Ontario and Quebec (Jeffries et al.2004). Though NS ANC results were also significantly

lower than those from the rest of Canada, NF and LAB,they did not vary much from the other regions.

Organic acidityHayes and Anthony (1958) reported that a number of the

NS sites had relatively high colour due to dissolved organicmatter (DOM) leached from wetland soils. Water from wet-land drainage can contain more than 15 mg L–1 of organiccarbon. Organic C in water is usually referred to as dis-solved organic carbon (DOC) or total organic carbon(TOC). The difference in nomenclature is based on themethod of collection and analysis. Total organic carbon isreported when the analysis is done on unfiltered samplesand thus can contain small particulate matter. For DOCanalysis, water is pre-filtered through a 0.45 �m filter beforeanalysis. Based on in-house laboratory studies, particulateorganic matter is not often a problem in Atlantic Canadaand pre-filtering is rarely done in regional studies, so thatmost of the values reported are TOC.

Automated colourimetric, wet oxidation methods used byEnvironment Canada and other laboratories in the 1980’s,under-estimated TOC by up to 25%, compared to high tem-

Fig. 1. Acid neutralization capacity (ANC) in Atlantic Canada as measured by Gran alkalinity titration method. Values are in mg L–1. Theinset shows the region’s location in North America as well as its provinces.

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perature combustion approaches (HTC) (Koprivnjak et al.1995). Environment Canada and other laboratories eventu-ally changed to HTC methods in 1994, which means thatthe earlier values are not comparable to the better post-1990results. At high concentrations, TOC has an important rolein the acid-base status of freshwaters. It contains natural or-ganic acids (NOAs) that increase water acidity, and thuscomplicates our understanding of anthropogenic acidifica-tion effects (Kerekes et al. 1986a). Clair et al. (1992),Kramer et al. (1990), and Oliver et al. (1983) all showedthat NOAs in Atlantic Canada waters can also act as buffersin freshwaters, depending on initial water pH conditions.

The behaviour of NOAs in the anthropogenic acidificationprocess thus depends on the concentration of TOC, the ac-tivity coefficients of NOA, pH based on background ANCand the magnitude of acid deposition. Though a number ofattempts have been made to quantify NOA activity, there isstill no routine method available for directly estimating or-ganic acidity of these complex solutions. Natural organicacid concentrations can be estimated using TOC and pH assurrogates (Oliver et al. 1983), using complex titration tech-niques (Kramer et al. 1990) or by difference using the elec-tro-neutrality concept (Driscoll et al. 1989; Clair et al. 1992;Lawrence et al. 2007). Natural organic acids are also effi-cient complexers of metals (Altmann and Buffle 1988). Inhighly acidified areas, soil ion exchanges cause the desorp-tion and export of ionic aluminum into stream and lakewaters (Cronan et al. 1986). At low pH levels, ionic Al

binds to fish gills, which, at sufficiently high concentrations,leads to the animal’s suffocation and death (Baker and Scho-field 1982). In waters with significant NOA concentrations,such as are found in Nova Scotia, there is little Al toxicityas this metal is complexed with organic acids, thus render-ing them unavailable to fish gills (Peterson et al. 1989; La-croix and Townsend 1987).

With the available post-1994 TOC data, we produced adistribution map for Atlantic Canada (Fig. 3). Large parts ofNB, NS, and NF have high TOC concentrations, whereasvalues were lower in PEI and southern LAB. High TOCconcentrations were shown to be due to the presence of wet-lands, which in turn were determined by topography andprecipitation (Clair and Ehrman 1996). These conditions ex-ist in east-central NB, much of NS and in eastern NF, andare reflected in the water chemistry.

Seasalt influence on freshwater chemistryMarine aerosols generated by ocean waves are deposited

over terrestrial catchments in Atlantic Canada and elsewhereand contribute ions to drainage waters (Watt et al. 1979; Far-rell et al. 1998). The ionic composition of the marine aero-sols has been measured, and is consistent with that inseawater (Junge 1972), so that in assessing marine influence,Cl– measured in freshwaters can be generally used as a quan-titative indicator of marine ionic contribution. As an exampleof its presence in the region, the Kejimkujik National Park(KNP), Canadian Air Precipitation Monitoring (CAPMoN)

Fig. 2. Calcium (a) and magnesium (b) distribution in Atlantic Canada. Values are in mg L–1.


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site, located 60 km from the Bay of Fundy and 80 from theAtlantic Ocean, reported an average of 15 kg ha–1 year–1 ofCl– deposited in rain and measurable dry deposition between1985 and 2000. The true marine influence at KNP is prob-ably greater than measured by CAPMoN, as Yanni et al.(2000a) showed that more than twice the amount of Cl–measured by the CAPMoN site was exported from localcatchments. They suggested that this unmeasured Cl– deposi-tion originated in fog intercepted by forest canopies and wasnot measured by conventional atmospheric precipitation sam-pling equipment. Because other ions are transported by theaerosols, their contribution to dilute freshwaters can measur-ably increase SO4

–2, NO3–, Ca+2, Mg+2, and Na+. However,

because the aerosol inputs are electronegatively balancedwith bicarbonate, they were thought to have no long-termpermanent influence on the acidity of surface waters (Wattet al. 1979). The assumption of no seasalt effect will be fur-ther discussed below.

Regional Cl– values were highest in coastal regions(Fig. 4), though there were clear instances where higher val-ues could be ascribed to the use of road salt in winter or tobedrock lithology. In central LAB, sites in and aroundGoose Bay may be contaminated by road salt, as they areanomalous for the region. We suspect that a major highwaybetween Halifax and Amherst in central NS may be respon-sible for the higher values measured there as well. There are

evaporite beds between Saint John and Moncton in southernNB and to the east of the Annapolis Valley in NS, whichmay be responsible for the higher Cl– values in these re-gions.

Generally, Fig. 4 shows that much of NS, PEI, and coastalNF receive relatively high amounts of Cl–. Not surprisingly,most of western and central NB and LAB do not containelevated concentrations, as they are far removed fromoceans. The southern part of NF shows surprisingly low lev-els that may be due to the regional topography as this coastis characterized by a high plateau (>300 m). Though the At-lantic Ocean off NF is well known for active marineweather systems, these apparently do not carry the aerosolsto the plateau lakes and streams.

Sulfate and nitrogen in Atlantic Canada watersThe two anions associated with acid precipitation are

SO4–2 and nitrate (NO3

–), with NO3– deposition currently

being 30% of SO4–2 in Atlantic Canada (Vet et al. 2005).

Nevertheless, NO3– is rarely found above analytical detec-

tion levels in non-agricultural or non-urban waters, so thatit usually has no influence in acidifying waters though itcan occasionally be measured during snowmelt events inthe winter and spring. However, receiving forest catchmentsseem to absorb and process most of the deposited N(Freedman and Clair 1987; Yanni et al. 2000a) and because

Fig. 3. Total organic carbon (TOC) distribution in Atlantic Canadafreshwaters. Values are in mg L–1.

Fig. 4. Chloride distribution in Atlantic Canada freshwaters. Valuesare in mg L–1.

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of this, we were unable to develop an NO3– distribution map

for the region.Sulfate measurements in surface waters initially provided

another problem caused by TOC and it’s complexation capa-bilities. Sulfate measurements using the methyl thymol blue(MTB) colourimetric method were overestimated by 10–25%, compared to those analyzed using ion chromatography(IC) in waters with TOC levels > 5 mg L–1 (Kerekes et al.1984). The main reactant in the colourimetric method re-acted with NOA’s, as well as SO4

–2 to produce falsely highvalues so that the MTB method was replaced by the moreaccurate ion chromatographic approach in the mid 1980’s.Figure 5 which was generated by using all IC data andMTB data for samples with less than 5 mg L–1 TOC showsthe regional distribution of SO4

–2.We expected that SO4

–2 concentrations would be greatestin the western parts of the Atlantic Region, nearest the mainCanadian and American pollution sources due to long-rangetransport (Shaw 1979; Vet et al. 2005), decreasing towardsthe north and east. We also suspected that we could detectlocal emission hotspots, around smelters and urban areas, ashad Watt et al. (1979) and Keizer et al. (1993). To detect ifthe seasalt effect might mask SO4

–2 distribution, we alsoplotted both total and seasalt corrected SO4

–2. The results(Figs. 5a, 5b) showed that though SO4

–2 originating in sea-salt caused higher concentratrions in parts of NS and NB, itdid not affect regional distribution patterns. Our hypothesis

of decreasing SO4–2 going from west to northeast was

clearly upheld as SO4–2 concentrations in western NB and

NS were visibly greater than in LAB and NF.The map nevertheless shows the importance of local SO4

–2

sources to surface waters in the region. SW and NE NBshow regions of high deposition which may be related to acoal burning power plant in Chipman; the city of Saint Johnand its industries; paper mills in the Miramichi region; andindustries near Belledune and Bathurst. There are higherSO4

–2 concentrations in NS downwind of the Halifax region;the industrial Pictou-Stellarton region; and eastern Cape Bre-ton which is affected by a coal-fired generating plant at Lin-gan. The coincidence of high SO4

–2 concentrations withpotential emission sources should be confirmed by deposi-tion modeling, but these results can be taken as startingpoints for further study.

Geological sources are the most likely causes of higherSO4

–2 in SE NB, in the Annapolis Valley of NS and in west-ern Cape Breton (Figs. 5a, 5b) as these regions contain gyp-sum-rich (CaSO4) deposits. Newfoundland and LAB hot-spots can only be explained by geology, as no industriescan account for the higher than average values in these iso-lated regions.

In Atlantic Canada, contributions from seasalt may con-tribute more than 20% of sulfate in coastal regions to 0%further inland. Nevertheless, the presence of seasalt SO4

–2

does not mask the general patterns of relative abundance.

Fig. 5. Total (a) and seasalt corrected (b) sulfate distribution in Atlantic Canada freshwaters. Values are in mg L–1.


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Acidity of Atlantic Canada freshwatersThe interactions between ANC, DOC, and SO4

–2 are whatdetermine pH of surface waters. When pH was mapped(Fig. 6), we produced a figure that somewhat resembled theANC distribution (Fig. 1). The areas shaded in deep pink tored have values below 5.5 and thus are of most interest.Prince Edward Island has no areas of concern because ofwell buffered soils. New Brunswick has four regions withelevated acidity. The largest area is in the SW that is af-fected by sensitive geology and high long-range deposition,a second is located in the NE of the Province, near Bathurstand is likely due to local industrial influences, and twosmaller regions are in waters with high DOC levels (Fig. 3).

The most acidic part of Atlantic Canada is SW and east-ern NS. The combination of regionally higher SO4

–2 deposi-tion and surface water concentration, poor soil bufferingcapacity as well as NOA activity, combine to produce thelowest surface water pH values in Canada outside pointsource areas such as Sudbury, Ontario. The low pH areasare clearly limited in their extent and defined by the re-gional geology.

All NF and LAB low pH regions are associated with lowSO4

–2, but are also in areas with low ANC and high DOCwaters (Fig. 3). In the south and east of the Island of New-

foundland, there is a very large area where resistant bedrockhas produced very thin soils since the last glaciation whichoffers no buffering against acidification. Similarly, the gran-itic plateau of the Long Range Mountains in the northwest ofthe Island offers little buffering to any acidification pressure,however low it may be. Finally, much of LAB has pH val-ues > 6.0 despite low ANC (Fig. 1), because of low SO4

–2

deposition in this region.The above data show that surface waters in large parts of

the Atlantic Region are highly acidic. The chemistry ofthese regions is clearly controlled by low ANC values af-fected by geology, upon which is superimposed sulfate dep-osition. Organic acids have a role to play in low pH, but acomparison of DOC levels in SW NS and eastern NF, showsthat despite somewhat higher DOC in NF, surface water pHvalues in NS are considerably lower because of the greaterSO4

–2 deposition.Our data suggest that local point sources affect surface

water acidity in NE NB. The other local point sources thatare identified with the SO4

–2 data do not cause increasedacidity because of high ANC waters due to local geology.Surface water acidity in the greater Halifax region does notseem to be inordinately affected by local SO2 sources, aswater acidity levels are somewhat lower than those from theSW and eastern shore regions. However, the influence ofhuman development (fertilizers, road salting, urban runoff)in this region may be counteracting the area’s acid sensitiv-ity (Keizer et al. 1993) and causing increased pH values.Much of NB and northern NS are resistant to acid depositionbecause of relatively high ANC waters. Labrador, thoughcontaining very low ANC values, has acceptable pH valuesdue to the low acid deposition in that part of the continent.

The results of the above spatial analysis must be takeninto a larger context. Despite the fact that much of the re-gion has water pH values > 6.0, surface water acidity hasnevertheless been increased by the long range transport ofacidifying substances. Ryan et al. (1990) and Clair et al.(2003) both show that on the Island of Newfoundland, thepH and ANC of lakes in a region receiving low amounts ofacid deposition have decreased measurably over recent time.What the above figures show is that acid deposition has rel-ative effects in the region. The only region that seems not tohave been affected is LAB, which has low ANC values andreceives little acid deposition, as shown by low SO4

–2 con-centrations. Newfoundland, which receives more acid depo-sition with equally sensitive terrain, has large numbers oflow pH lakes and rivers, though DOC plays a role in this.Clearly, the area most affected is SW NS, where a combina-tion of low ANC and relatively high acid deposition createthe highest acidity conditions.

Temporal trends of acidificationThe assessment of long-term water chemistry changes

caused by acidification was done two ways in Atlantic Can-ada. The first approach was used when groups of lakes orrivers were sampled at sporadic, irregular intervals andwater chemistry values were compared from one samplingtime to the next using analysis of variance or paired t testapproaches. These sampling programs were opportunistic innature, but nevertheless provided useful data. The secondtype of approach was the long-term sampling network that

Fig. 6. pH distribution in Atlantic Canada surface waters.

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involved regularly scheduled sample collections. In thiscase, the data were analyzed using non-parametric statisticaltrend detection methods such as those developed by Hirschand Slack (1984) and Cluis et al. (1989).

Sporadic studies assessing acidification trendsThe first of the acid rain sporadic sampling studies in At-

lantic Canada was done by Watt et al. (1979) who revisitedthe Halifax (NS) area lakes first sampled by Gorham (1957).They found that water acidity increased at all 16 undisturbedlakes initially sampled in 1955. They correlated SO4

–2 levelsto the proximity of local power plants in the Halifax area,but also felt that a long-distance component of air pollutionwas perhaps also responsible for part of the problem. Theselakes were resampled in 1991 by Keizer et al. (1993) whofound no changes in sulfate or acidity levels from Watt etal. (1979), suggesting that the acidification situation had sta-bilized in that region. Keizer et al. (1993) also found thaturbanization had modified the chemistry of most of the orig-inal Gorham (1957) lakes, causing increases in Na+ and Cl–from road salting and nitrates and phosphates from lawn andgarden fertilizers. Generally, the Keizer et al. (1993) studyshowed that whatever important changes might be happen-ing to these lakes since the late 1970’s seemed to be con-trolled by land-use changes, and not local or long-rangeacidification sources.

Another sporadic sampling study was conducted in south-western NB and reported by Pilgrim et al. (2003). They ana-lyzed data from 39 lakes sampled in 1986, 1993, and 2001,and showed that with reductions in atmospheric S deposi-tion, regional lake waters showed a general improvement inpH and ANC, especially between 1986 and 1993, when thegreatest reductions in S deposition occurred.

Long-term continuous monitoringThe first study that reported trends based on continuous

water sampling in the Atlantic Region was by Thompson etal. (1980). Using data from Environment Canada databases,they showed that pH decreased continually from the mid-1960’s to the mid 1970’s in three NS rivers, the Tusket,Medway and St Mary’s. Longer data sets from these threerivers, plus a further 8 scattered throughout NB, NF andPEI, were reanalyzed later using Box-Jenkins time-seriesanalysis (Clair and Whitfield 1983). It was found that of theeight rivers which were considered sensitive to acidification,six showed increasing acidification and the other two eitherhad too short data sets (Lepreau R. in southern NB), or re-ceived too low an acid load (Piper’s Hole, NF) to showtrends.

Further monthly sampling was done at 18 rivers in Atlan-tic Canada (including the ones analyzed by Clair and Whit-field (1983)) from 1983 to 1992. Analysis of that data seriesshowed that SO4

–2 concentrations were still increasing from1983 to 1992 at 7 of the 18 sites sampled monthly, eventhough acid emissions and deposition were then rapidly de-creasing in North America (Clair and Ehrman 1995). De-spite the increasing SO4

–2 trend, however, H+ and Ca+2

decreased while ANC increased at three sites. This counter-intuitive result was partly explained by analyzing ion exporttrends. In terms of total amounts exported from catchments,SO4

–2 decreased in five rivers from 1983 to 1992, H+ ex-

ports did not change, while ANC increased at six of thesites. Watt et al. (2000) studied water chemistry from an-other nine rivers in southern NS from 1982 to 1996 andshowed decreases in SO4

–2, Ca+2 and Mg+2, but again nochanges were detected in H+. The main reason for the lackof acidity response probably lay in the organic acidity effect,as the decrease in SO4

–2 measured at some of the sites wasmatched by an increase in ANC. However, organic anionsreleased more H+ to compensate, thus keeping the pH fromincreasing.

The data from river studies show that the movement ofions from catchments to drainage waters is a complex phe-nomenon. Though ion concentrations and export must becontrolled by ion balance considerations, variations in pre-cipitation, discharge, the selective release of ions fromcatchment soils (e.g., Heath et al. 1992), as well as analyti-cal and measurement variability, made it difficult to deter-mine consistent trend patterns. What the river studies allshowed, however, was that SO4

–2 decreased at a large num-ber of sites, but that acidity did not improve as was ex-pected. This was likely due to concurrent decreases in basecations and the amphoteric effect of NOAs.

Another long-term, regular monitoring network in Atlan-tic Canada was run by Environment Canada. Over 80 lakesextending from southwestern NB, all through NS, and at twoNational Parks in NF, one located on the western margin(Gros Morne NP) and the other on the eastern margin (TerraNova NP) of the island lakes were sampled twice yearlyduring spring and fall overturns (Clair et al. 1995, 2001b;Dennis et al. 2005a). The trend results from this networkmatched those of the river networks and showed that SO4

–2

and base cation concentrations have decreased throughoutthe region, but that these have not been joined by the ex-pected increases of ANC and pH. Further analysis of datacollected from 1990 to 2001 by Jeffries et al. (2004) againshowed similar patterns, with 34% of lakes showing de-creasing ANC values. We suspect that the decreasing ANCreflects the fact that decreasing SO4

–2 deposition is leadingto a lesser export of base cations from soils that have beendepleted of buffering capacity.

A recent study by Skjelkvale et al. (2005) compared theAtlantic Canada results with those from elsewhere in Europeand North America, and showed that the pattern of continu-ing acidification in this region is unique. Out of 12 regionaldata sets from Europe and North America, only the 18 lakesin their Maine and Atlantic Canada data set had a negativemedian ANC slope denoting a continued reduction in thatparameter. They felt that the cause of the continuing acidifi-cation probably was due to the response of mitigating varia-bles such as low base cations, organic acidity, or perhaps theslow release of sulphur stored in the soils.

The above reports show the unique nature of surfacewaters trends in Atlantic Canada. The main reason for thelack of recovery in ANC and thus in acidity, is the generallylow buffering capacity of regional soils and the depletion ofbase cations caused by the long-term acid titration of soilsby acid rain. As will be discussed below, a reduction ofacid titration pressure on soils already severely depleted ofbase cations, will result in a reduction of base cation exportsequal to the reduction in acid deposition. Acid neutralizationcapacity will therefore only improve when acid deposition


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will be lower than the catchment parent material’s ability toreplace soil base cations through weathering.

Short-term episodesThe lake and river monitoring networks described above

only offer indications of long-term chronic effects on catch-ments. In Atlantic Canada, as elsewhere, there are short-term acid pulses driven by seasonal hydrological phenom-ena. Three kinds of acid pulses have been identified in theliterature (Wigington et al. 1992), and all three have beenreported in Atlantic Canada. These are caused by snowmelt,organic acids, and seasalts.

The acid pulses most associated with acid rain are snow-melt-driven mineral acid pulses that can occur during winterand early spring and can produce significant short-term in-creases in acidity (Wigington et al. 1992). During winter,when soils are frozen, deposited acids built up in wintersnowpacks can be released suddenly to streams and rivers.This acidic snow melt is usually not filtered through soils,and thus SO4

–2 and NO3– and the accompanying H+ travel

directly to streams and lakes with few chemical changes ex-cept for dilution. Clair et al. (2001a), using daily and weeklysampling data from three streams in southwestern NS,showed that several low pH episodes lasting from days insmall streams to weeks in larger rivers, can occur in regionalwater courses. Wigington et al. (1992) showed that thesharpest pH episode ever reported in the literature was fromthe Mersey River in Nova Scotia (Freedman and Clair 1987)with a 3.2 pH unit decline in the early 1980s. This low valuehas not been remeasured, though occasional values of 4.0 aremeasured during snowmelt episodes in this region.

Though reductions in SO4–2 deposition have not improved

ANC and pH under annual conditions, winter conditionshave been shown to improve. Laudon et al. (2002) analyzedthe intensity of snowmelt winter acidic episodes at MoosePit Brook and the Mersey River and found that, in step withreductions in continental acid deposition, winter mineral

acid pulses in the KNP area have been reduced in severityin the last 20 years, as shown by an improvement of winterANC values.

Mineral acid pulses are not the only types that occur inregional streams. Because of the prevalence of wetlands inlarge parts of the region, high flow events can wash NOAsfrom soils and cause acidity increases unrelated to mineralacids (Wigington et al. 1992). Organic acid-driven pulseshave been reported in the Kejimkujik NP region by Clair(1995). In streams that drain wetlands, acid pulses often oc-cur in the fall after the onset of heavy rainfalls. In Moose PitBrook, for example, monthly mean precipitation, pH, SO4

–2,and NOA values (calculated using the Oliver et al. 1983method) were averaged from 5 years of data (Fig. 7) andshow a rapid increase in the importance of NOAs to thecontribution of protons, compared to SO4

–2. The general pat-tern noted at this 17 km2 catchment, and in other streams ofsouthwestern Nova Scotia (Lacroix and Townsend 1987), isthat as precipitation increases from September to November,NOAs increase and SO4

–2 decreases so that the main acidifi-cation source becomes NOA. This type of NOA-driven in-crease in acidity has also been reported in Sweden (Laudonand Bishop 2002) and throughout North America and Eu-rope (Wigington et al. 1992). Despite their frequent occur-rences, organic acid pulses have not received a great deal ofstudy and their effect on aquatic ecosystems is not wellunderstood.

Acid pulses in streams can also be caused by seasalt dep-osition episodes in coastal regions. These happen when Na+

and Mg+2 from seawater exchange with H+ and Al in poorlybuffered soils, causing low pH values in runoff waters(Wright et al. 1988). Sullivan et al. (1988) suggested that,because the acid anion Cl– is balanced by base cations, sea-salts produce no long-term acidification effects to catch-ments. However, they can have temporary effects onweathering processes. During ocean storms, marine base cat-ions, principally Na+ and Mg+2 are retained by catchment

Fig. 7. Monthly changes in H+, natural organic acids (NOA), sulfate, and precipitation at Moose Pit Brook, near Kejimkujik National Park,NS. Values are 5 year averages.

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soils even as Cl– flows through the catchments. To maintainsolution charge balance, the exported Cl– ions are accompa-nied by H+ and Al in runoff, causing low pH episodes. Laterin the year, the base cations are released into water creatinghigher pH conditions. The low pH events can obviously pro-duce short-term acidification problems in acid susceptiblesoils.

Wright et al. (1988) simulated seaspray deposition in asmall catchment in western Norway and found that outflowstream pH and ANC could be temporarily significantly de-creased. Heath et al. (1992) showed that seasalt acidificationoccurred during sea storms in Maine, USA, which is locatedadjacent to Atlantic Canada. Seasalt generated acid pulseshave not been reported in Atlantic Canada, but are probablyoccurring, especially in catchments on windward coasts.

A related phenomenon was noted in NF as Thompson(1982) showed that Na+ originating in seaspray exchangedwith weatherizing horizon material Ca+2 during high seasaltdeposition episodes, resulting in Ca+2 being released duringhigh runoff periods. Catchment soils were then impover-ished of a biologically important ion. No other work hasbeen done to address this phenomenon in Atlantic Canada,and this remains a potential problem that should be betterstudied.

Catchment ion budgets and weatheringThe Kejimkujik National Park (KNP) area of south-cen-

tral Nova Scotia is the only region in Atlantic Canada wherea consistent intensive effort has been made to produce quan-titative, long-term data with which to assess catchment acid-ification processes. One of the first studies that looked atacidification effects on water was by Kerekes et al. (1982)who had first described water chemistry of the newlyformed Kejimkujik National Park (Kerekes and Schwing-hamer 1973). Kerekes and co-workers confirmed the earlierwork of Gorham (1957) and Hayes and Anthony (1958), andreported that base cation concentrations from KNP were stillsome of the lowest reported in the scientific literature, andshowed little bicarbonate influence in the waters, and thus avery high susceptibility to deposited acids. They also re-ported on the importance of natural organic acids in addinganother dimension to the acidity of lake and stream waters(Kerekes and Freedman 1989; Kerekes et al. 1982, 1986b).

In 1980, a long-term intensive weekly sampling programwas begun on the Mersey River at Mills Falls which drainsa 297 km2 catchment. A further three sites were added in1983, Moose Pit Brook (17 km2), as well as Whitebourneand Roger’s Brook which were each about 9 km2 in size(Clair and Freedman 1986). Sampling at Whitebourne andRoger’s was discontinued in 1987, and replaced in 1990 bya smaller site, Pine Marten Brook with a 1.30 km2 area(Allen et al. 1992). Discharge at the Mersey River andMoose Pit Brook was measured by hydrometric gaugeswhile runoff was estimated using the ForHyMod model de-veloped at the University of New Brunswick at the othersites (Yanni et al. 2000b).

Freedman and Clair (1987) had access to between 3 to5 years of data from the KNP watersheds (1980 to 1983 or1985, depending on the catchment). When they comparedion inputs (measured wet and estimated dry) and outputs

from the catchments, they measured more SO4–2 and Cl–

being exported from all catchments than was measured asdeposition. Yanni et al. (2000a), using a longer data seriesfrom these catchments (1980 to 1989), and only wet deposi-tion, measured net SO4

–2 adsorption in the catchments, andan excess of exported stream Na+ and Cl– over what wasmeasured in wet deposition. They suggested that unmeas-ured fog deposition was probably responsible for the excessCl– export. The change in SO4

–2 from being in excess in thefirst study to being retained in the second was due to thechange in analytical methods which was described above,as IC SO4

–2 which was used in the second part of the studyproduced values roughly 20% lower than the old MTBmethod which was all Freedman and Clair had available tothem initially. Not surprisingly, both papers measured morebase cation exports than deposited, as expected from weath-ering and soil exchange effects.

A further analysis of 15 years worth of sampling by Clair etal. (2005), combined with improved dry deposition estimatesfrom the CAPMoN network, showed that S inputs and outputswere approximately equal at the two sites over the study pe-riod, suggesting that S is not stored in catchment soils overtime. However, the new analysis also showed that there isonly a slight decreasing trend in S exports from Moose PitBrook, and none in the Mersey River, compared to a strongdecrease in deposition. This suggests that there must be someS storage in the catchments, but the tools that we have meas-uring inputs and outputs may be too blunt to allow us to moreaccurately measure the changes that are occurring.

Clair et al. (2005) also showed that half of the Cl– ex-ported from the Mersey catchment and 25% from MoosePit Brook could not be explained by the CAPMoN deposi-tion results. They showed that the probable fog contributionwas compatible with what is known about fog ion concen-trations and potential entrapment by vegetation (e.g., Weath-ers 1999). Little is known of what repercussions this loadinghas on catchment acidification processes.

Nitrogen export from Atlantic Canada catchments aremostly in the dissolved organic form (DON), as NO3

– wasrarely measurable above the method detection limit of0.02 mg L–1 (Freedman and Clair 1987; Clair and Ehrman1995; Yanni et al. 2000a; Clair et al. 2005). Approximately70% of deposited N was retained or denitrified by the KNPstudy catchments, with the remaining 30% exported asDON. These data also fall within the range of catchmentNO3

– retention values collected by Campbell et al. (2004)and Van Breemen et al. (2002) for the northeastern USA.Wright et al. (2001) show that, in Europe, areas receivingless than 10 kg N ha–1 year–1 have low NO3

– concentrationsin stream waters and are thus not acidified by this nutrient.As N deposition at KNP averaged 4.6 kg N ha–1 year–1 overthe study period, the Wright et al. (2001) hypothesis alsoheld for this part of North America.

The data collected in Moose Pit Brook between 1983 and2003 were used in a large-scale comparison of base cationexports in catchments from Canada, the United States, andEurope by Watmough et al. (2005). They showed thatMoose Pit Brook base cation losses are typical of thosefound in other acidified areas studied. Currently, catchmentsin southwestern Nova Scotia are showing a net loss of soilbase cations, which must inevitably lead to a reduction in


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ANC and the impoverishment of plant communities. The re-sults from these catchment studies show that, despite largereductions in S emissions and deposition, surface waters inwestern and eastern NS and most likely from SW NB arenot yet recovering and that further S reductions will beneeded before acid inputs into the sensitive terrains are com-pensated by weathering processes that will replenish soilbase cations.

Modeling and predictionsThe previous sections show that, despite S deposition re-

ductions in Atlantic Canada, there have been no measurableimprovements in water chemistry, and thus we could expectno improvements in ecosystem function. It was therefore im-portant to determine what deposition conditions will causewater chemistry to begin improving, so that policy makerscan understand how the region’s ecosystems would be af-fected by future reductions in acid deposition.

A number of predictive approaches have been used to seehow and when freshwater chemistry will recover in AtlanticCanada. Jeffries and Ouimet (2005) have assessed and re-viewed the concept of critical acid loads for eastern Canada.Critical loads are defined as ‘‘a quantitative assessment of anexposure to one or more pollutants below which significantharmful effects on specified sensitive elements of the envi-ronment do not occur according to present knowledge’’. Inpractice, the critical loads are the amount of acid depositionthat an ecosystem can tolerate without significant ecologicaldamage. Jeffries and Ouimet (2005) used a series of steady-state water chemistry models which allowed the calculationof critical loads within an expert model called the IntegratedAcidification Model (IAM) (Lam et al. 1998). They showedthat critical acid loads were currently exceeded for SW NB,most of NS, and western NF. They also show that AtlanticCanada is the only region where critical loads are currentlyexceeded in eastern Canada.

Dupont et al. (2005), using a dataset contributed by east-ern North American provincial, state, and federal govern-ments also calculated critical loads for 2053 lakes, of which10% were located in Atlantic Canada. Their results alsoshowed lakes in SW NS, SW NB, and western NF to havecritical loads exceeded and that these values were similar tothose found in the nearby State of Maine. Interestingly, boththe Jeffries and Ouimet (2005) and Dupont et al. (2005)analyses showed that, despite the low ANC and pH valuesin eastern NF, critical loads were not exceeded in that re-gion because of the low acid deposition it receives. Whit-field et al. (2006a) calculated critical loads to 20 lakes ineastern and southwestern Nova Scotia using two approachesand found that 13 of the sites exceeded loads and pointedout that further, significant reductions in S deposition wereneeded to assure the recovery of lake acidity to levels suit-able for healthy aquatic ecosystems. Further work done us-ing different weathering estimation approaches confirmedthis finding (Whitfield et al. 2006b).

Dynamic modeling is another geochemical approach usedin Atlantic Canada. Dynamic models are time dependentmodels which assume that the concentrations of major ionsin soils and drainage waters are governed by simultaneousreactions involving sulfate adsorption, cation exchange, alu-

minum speciation and speciation of inorganic carbon. Theyuse a mass balance approach where the flux of major ionsto and from the soil is assumed to be controlled by atmos-pheric inputs, chemical weathering, and losses from soils torunoff. At the heart of dynamic models is the size of thepool of exchangeable base cation (Cb) in the soil. As thefluxes to and from this pool change over time because ofchanges in atmospheric deposition, the chemical equilibriabetween soil and soil solution shift to change surface waterchemistry. The degree and rate of change of surface wateracidity thus depend both on flux factors and the inherentcharacteristics of the affected soils (Cosby et al. 1985,2001).

Clair et al. (2003, 2004) used the dynamic ‘‘Model ofAcidification of Groundwaters’’ (MAGIC) (Cosby et al.1985) to determine how changes in acid deposition from theonset of acidification to the present, affected water chemis-try of the region’s lakes. They also used this approach toforecast if and how predicted reductions in acid emissionswould further improve the chemistry of sensitive lakes. Theadvantage of MAGIC over steady-state models is that it canprovide a range of outcomes based on changeable inputs.The disadvantage of the method is that its use requires largeamounts of data for calibration and requires a large numberof estimated variables as model inputs (Dennis et al. 2005b).

Clair et al. (2003) studied the lakes in the Atlantic Canadatemporal monitoring network using MAGIC and assumedfuture deposition values based on existing or proposedCanadian and American emission reduction scenarios de-scribed by Kaminsky (2002). They found that lakes in NBand NF should return to pre-acidification levels much morequickly than those from NS because of better buffered ter-rain in NB and much lower deposition in NF. Clair et al.(2004) then estimated that because of soils with low buffer-ing capacity, many Atlantic salmon rivers in NS will takemore than 60 years to begin showing signs of recovery.They showed that, although pH should recover within thenext decade if proposed Canadian and American emissionreductions are in place, ANC and base cations will take upto 40 years to return to levels suitable for supporting sus-tainable populations and another four decades to return topre-acidification levels.

The Atlantic Canada predictions were compared toMAGIC outputs for the rest of Canada. New Brunswick andNF lakes reacted to acid deposition reductions similarly tothose from more central parts of the country. In contrast,NS lakes were significantly more acidic than lakes fromother parts of Canada, even before acid rain began, mostlybecause of low Cb and high NOA conditions (Clair et al.2007). Acidity in NS increased significantly from the 1950sto the 1970s, and have not recovered with decreasing aciddeposition. While recovery of water chemistry is expectedto be rapid (though not complete) in most of Canada undernew proposed emission control scenarios, the same will notbe the case in NS, where lake chemistry will recover muchmore slowly (Clair et al. 2007).

ConclusionsThe picture that emerges from this analysis is that large

parts of Atlantic Canada are very sensitive to and have been

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affected by acid deposition. Though the region receives rel-atively low acid deposition compared to the rest of NorthAmerica (Vet et al. 2005), the combination of low soil andparent material buffering with moderate acid deposition,combine to produce surface waters with the lowest pH andANC values in Canada (Jeffries et al. 2005). Critical loadsare also exceeded in southwest NB as the region receivessimilar acid deposition to SW NS. The original analysis ofNS waters by Gorham (1957) and Hayes and Anthony(1958) showing the most dilute waters known at the time,has not been modified by the addition of further samplingin the last ~50 years. The Island of Newfoundland also haslow ANC waters, but the reduced acid deposition loads tothat area have not caused the widespread low pH valuesthat are found in NS, nor are critical loads currently ex-ceeded in the eastern part of the Island.

Though we have a generally good understanding of thecurrent impact of acidification to Atlantic Canada fresh-waters, there are a number of issues that need to be addressedto better predict how reductions in acidification will affectimprovements in water chemistry. The first is the role ofNOAs in acidifying and buffering waters. Dennis et al.(2005c) have adapted the MAGIC dynamic model to betterreflect the influence of NOAs but the method could bearwith further refinements to improve predictions. Another out-standing issue is the importance of seasalts in coastal regions.Though they may not be acidifying in the long-term, theystill may be involved in long-term base cation depletion, andhave not been properly understood in Atlantic Canada.

There are two further research needs that must be ad-dressed to ensure that our predictive models provide reliableinformation. The first is the issue of cation denudation rates.As acid rain exchanges H+ for Cb in soils, continuous titra-tion of soils poor in buffering material will lead to a reduc-tion in base cations (Watmough and Dillon 2003). As Cb isreplenished from bedrock and till, the rate of the recoverywill depend on the mineral type as well as weathering rateswhich are poorly understood in nature (Drever 1994). A crit-ical part of dynamic and other modeling approaches is there-fore subject to uncertainty and needs to be better studied.

The role of nitrogen in acidification processes is stillpoorly understood. Nitrogen deposition currently seems topose no problem to the acidification of Atlantic Canadawaters, but little is known of long-term effects. Van Bree-men et al. (2002) estimated that slightly over 50% of depos-ited N in eastern US forests was denitrified and lost fromthe ecosystem with no impact on acidification levels. If thisholds for the coniferous forests of Atlantic Canada, thencombined with the low levels the region receives, nitrogendeposition may prove not to be of great concern. However,a concerted effort should be made to arrive at an accurateassessment of the problem.

Finally, acid rain is not a problem which operates in avacuum. An especially relevant issue is that acid waters inAtlantic Canada often support biota with high mercury lev-els (Evers et al. 1998). Mercury transport is mediated byNOAs in water and its uptake in biota is enhanced by acidicconditions (Dennis et al. 2005b). We currently have a poorunderstanding of the Hg transfers between ecosystem com-ponents and thus have a poor ability to quantitatively predict

if and how reductions in acidification might affect the up-take of Hg in biota.

Research on the acidification of freshwaters in AtlanticCanada has shown the extent of the problem, and has pro-vided a preliminary estimate of potential recovery rates.There are a number of issues that require further research toallow us to better predict recovery rates and thus the needfor mitigation to protect sensitive ecosystems. It is clear,however, that this part of eastern North America will mostlikely be the last to recover from acidification on the conti-nent, and that the problem will need to be monitored andstudied for a long time into the future.

AcknowledgementsWe thank Dr. Ali Khan of the Government of Newfound-

land for providing data for this analysis as well as comment-ing on the manuscript. The paper greatly benefited bycomments from Dr Peter Dillon and an anonymous re-viewer.

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