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Regional trends in aquatic recovery from acidification in North America and Europe

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Rates of acidic deposition from the atmosphere (`acid rain') have decreased throughout the 1980s and 1990s across large portions of North America and Europe. Many recent studies have attributed observed reversals in surface-water acidification at national and regional scales to the declining deposition. To test whether emissions regulations have led to widespread recovery in surface-water chemistry, we analysed regional trends between 1980 and 1995 in indicators of acidification (sulphate, nitrate and base-cation concentrations, and measured (Gran) alkalinity) for 205 lakes and streams in eight regions of North America and Europe. Dramatic differences in trend direction and strength for the two decades are apparent. In concordance with general temporal trends in acidic deposition, lake and stream sulphate concentrations decreased in all regions with the exception of Great Britain; all but one of these regions exhibited stronger downward trends in the 1990s than in the 1980s. In contrast, regional declines in lake and stream nitrate concentrations were rare and, when detected, were very small. Recovery in alkalinity, expected wherever strong regional declines in sulphate concentrations have occurred, was observed in all regions of Europe, especially in the 1990s, but in only one region (of five) in North America. We attribute the lack of recovery in three regions (south/central Ontario, the Adirondack/Catskill mountains and midwestern North America) to strong regional declines in base-cation concentrations that exceed the decreases in sulphate concentrations.
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NATURE
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VOL 401
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letters to nature
.................................................................
Regional trends in aquatic recovery
from acidification in
North America and Europe
J. L. Stoddard
1
, D. S. Jeffries
2
,A.Lu
¨kewille
3
, T. A. Clair
4
, P. J. Dillon
5
,
C. T. Driscoll
6
, M. Forsius
7
, M. Johannessen
8
. J. S. Kahl
9
, J. H. Kellogg
10
,
A. Kemp
11
, J. Mannio
7
, D. T. Monteith
12
, P. S. Murdoch
13
, S. Patrick
12
,
A. Rebsdorf
14
, B. L. Skjelkva
˚le
8
, M. P. Stainton
15
, T. Traaen
8
,
H. van Dam
16
, K. E. Webster
17
, J. Wieting
18
& A. Wilander
19
1
Environmental Protection Agency, 200 SW 35th Street, Corvallis, Oregon 97333,
USA
2
Environment Canada, PO Box 5050, Burlington, Ontario, Canada L7R 4A6
3
Norwegian Institute for Air Research, PO Box 100, 2027 Kjeller, Norway
4
Environment Canada, PO Box 6227, Sackville, New Brunswick, Canada E4L 1G6
5
Ontario Ministry of the Environment, PO Box 39, Dorset, Ontario, Canada
P0A 1E0
6
Syracuse University, 220 Hinds Hall, Syracuse, New York 13244, USA
7
Finnish Environment Institute, Box 140, 00251 Helsinki, Finland
8
Norwegian Institute for Water Research, PO Box 173, 0411 Oslo, Norway
9
University of Maine, Sawyer Research Center, Orono, Maine 04469, USA
10
Vermont Department of Environmental Conservation, 103 S. Main Street,
Waterbury, Vermont 05676, USA
11
Environment Canada, 105 McGill, Montreal, Quebec, Canada H2Y 2E7
12
Environmental Change Research Centre, University College, 26 Bedford Way,
London WC1H 0AP, UK
13
US Geological Survey, 425 Jordan Road, Troy, New York 12180, USA
14
National Environmental Research Institute, PO Box 314, 8600 Silkeborg,
Denmark
15
Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba,
Canada R3T 2N6
16
Aquasence TEC, PO Box 95125, 1090 Amsterdam, The Netherlands
17
Wisconsin Department of Natural Resources, 1350 Femrite Drive, Monona,
Wisconsin 53716, USA
18
Umweltbundesamt, Seelstrasse 6– 10, 13581 Berlin, Germany
19
University of Agricultural Sciences, PO Box 7050, 750 07 Uppsala, Sweden
.................................. ......................... ......................... ......................... ......................... ........
Rates of acidic deposition from the atmosphere (‘acid rain’) have
decreased throughout the 1980s and 1990s across large portions of
North America and Europe1,2. Many recent studies have attributed
observed reversals in surface-water acidification at national3and
regional4scales to the declining deposition. To test whether
emissions regulations have led to widespread recovery in
surface-water chemistry, we analysed regional trends between
1980 and 1995 in indicators of acidification (sulphate, nitrate
and base-cation concentrations, and measured (Gran) alkalinity)
for 205 lakes and streams in eight regions of North America and
Europe. Dramatic differences in trend direction and strength for
the two decades are apparent. In concordance with general
temporal trends in acidic deposition, lake and stream sulphate
concentrations decreased in all regions with the exception of
Great Britain; all but one of these regions exhibited stronger
downward trends in the 1990s than in the 1980s. In contrast,
regional declines in lake and stream nitrate concentrations were
rare and, when detected, were very small. Recovery in alkalinity,
expected wherever strong regional declines in sulphate con-
centrations have occurred, was observed in all regions of
Europe, especially in the 1990s, but in only one region (of five)
in North America. We attribute the lack of recovery in three
regions (south/central Ontario, the Adirondack/Catskill moun-
tains and midwestern North America) to strong regional declines
in base-cation concentrations that exceed the decreases in
sulphate concentrations.
In the past two decades, the passage of national (for example, the
Clean Air Act in the United States and the Eastern Canada Acid Rain
Program in Canada) and international (for example, the United
Nations Economic Commission for Europe’s Convention on Long-
Range Transboundary Air Pollution (UN/ECE LRTAP) Sulphur
Protocols) environmental regulations and agreements has led to
widespread declines in the rates of acidic deposition, especially of
sulphur, across large regions of North America and Europe. In
north/central Europe, SO
2
concentrations in air decreased by 63%,
and SO
2
4
in precipitation by 40%, between 1985 and 1996 (ref. 2). In
the United States and Canada, SO
2
emissions declined 28% between
1980 and 1995 (ref. 1); sulphur deposition in the same period
decreased by 29% in the northeastern United States and by 35% in
the upper midwestern United States1. In eastern Canada, decreases
in SO
2
4
in precipitation ranged from 31% in Atlantic Canada to
36% in south/central Ontario5. In Great Britain, both emissions
(32%) and wet deposition (43%) of sulphur declined between 1979
and 1993, with most of the decrease observable in the early 1980s
(ref. 6). Few of these regions have recorded changes in the rate of
nitrogen deposition during this time period. These pervasive
declines in rates of acidic deposition create an expectation of
widespread recovery in acidified surface waters. Reasoning that
consistent recovery across a regional population of streams or lakes
would provide the strongest evidence for the success of controls on
acidic deposition, we report here on surface water trends at the
regional scale.
The primary source of data for this analysis is the International
Cooperative Programme on Assessment and Monitoring of Acidi-
fication of Rivers and Lakes (ICP-Waters), an international body
Midwestern
North America
South/Central
Ontario
Quebec/Vermont
Maine/
Atlantic
Canada
Adirondacks/
Catskills
Great Britain
Nordic Countries
North/Central
Europe
Figure 1 Lake and stream monitoring sites. Data from 205 long-term monitoring sites,
located in eight regions of Europe and North America, were used in this analysis.
© 1999 Macmillan Magazines Ltd
organized under the UN/ECE LRTAP. Additional data from
national, provincial and international monitoring programmes
(for example, in the United Kingdom7, Canada8, the United
States9and the International Cooperative Programme on Integrated
Monitoring in Europe10) were added in regions with inadequate
sample sizes. The sampling frequencies used by these programmes
varied substantially, from weekly in some cases, to quarterly or
biannually in others. In order to standardize the data used in trend
analyses, we chose quarterly sampling as a reasonable goal (achiev-
able for most sites) and calculated seasonal mean values for the
more temporally intensive data sets. We excluded sites with mean
Gran alkalinity greater than 200 microequivalents per litre (200
mequiv. l
1
) in order to focus on those sites most likely to show
recovery11, as well as sites with fewer than three quarterly samples
(on average), less than 5 years of data per decade, or where climate-
driven trends have been documented. A total of 168 sites had
sufficient data for the 1980s, and 205 sites had sufficient data for
the 1990s. Sites were grouped into regions based on similarities in
their geochemistry, in deposition trends in the 1980s and 1990s, and
on their geographical proximity (Fig. 1).
Preliminary analyses indicated that temporal patterns in chemi-
cal variables were not monotonic; they tended to be curvilinear,
with an inflection point around 1990. For this reason, and also to
permit a comparison of trends between decades, we performed site-
specific trend tests separately on data from the 1980s and 1990s
using the non-parametric Seasonal Kendall test modified to account
for serial dependence12. In order to infer regional trends, we applied
a meta-analytical technique first described by van Belle and
Hughes13 that allows, with some restrictions, trend results from
multiple sites to be combined into a single estimate of trend.
In performing the regional trend tests, we first tested the Z-
statistics from individual Seasonal Kendall tests for homogeneity by
attributing the variance among them to each of the possible sources
of interest (sites, seasons and interactions) through an analysis of
variance, and tested the resulting sums of squares against a x
2
distribution14. There is no accepted threshold for deciding when
heterogeneity among sites is sufficient to invalidate combining the
trend results regionally. We followed the recommendation of van
Belle and Hughes13, set the threshold for tests of homogeneity
conservatively (for example, p,0:01), and examined the trend
statistics more critically if they failed this test. In some cases
(particularly for SO
2
4
), Z-statistics failed the homogeneity test
despite having the same sign (for example, all Z,0). In these
cases, we conclude that the evidence for reporting a downward trend
is sufficient to regard the results as reliable.
At least two criteria should be fulfilled before extrapolation of
site-specific trends to the regional level11: (1) the sites must be
representative of the region or the regional subpopulation (for
example, acid-sensitive surface waters) of interest; and (2) the sites
should exhibit consistency of trend behaviour. We can test for the
latter condition through the homogeneity test described above, but
we have no current test for the former condition. Here we assume
that monitoring programmes have made good judgments in choos-
ing sites representative of acid-sensitive surface waters in each
region. All of the ICP-Waters sites were selected using published
criteria (for example, sites in headwater regions, free from local
disturbance, and located on sensitive geology)15. The national and
provincial programmes presented in this analysis used similar
criteria.
The strongest evidence for a regional surface-water response to
decreasing deposition comes from SO
2
4
trends (Table 1). Sulphate
(we used non-marine SO
2
4
in regions within 100 km of oceans) has
declined across most of Europe and North America, generally with
more rapid declines during the 1990s. Only Great Britain failed to
show a significant decline in SO
2
4
in the 1990s. Further, SO
2
4
trends
were generally homogeneous within a region, suggesting strongly
that SO
2
4
trends are driven by changes in atmospheric deposition.
Only two subsets of data failed the homogeneity test. For south/
central Ontario during the 1980s, all of the Z-statistics were
negative, indicating decreasing SO
2
4
, and we consider the regional
trend to be valid despite high x
2
values. Vermont/Quebec in the
1980s was the only region that exhibited heterogeneity strong
enough to invalidate a significant trend test; all of the sites with
increasing SO
2
4
in this region and decade were located in Quebec,
although none of the positive site-specific trends were significant.
Nitrate increases were nearly universal, but largely restricted to
the 1980s. The largest increases, and probably the only ecologically
significant ones, occurred in the Adirondack/Catskill mountains
and north/central Europe. The nitrate increases in these regions
during the 1980s have received considerable attention, generating
concern that these high-deposition regions are experiencing nitro-
gen saturation16. Interestingly, these two regions also show the
largest reversals of NO
3
trends in the 1990s.
Both conceptual17 and mechanistic18 models of acidification
suggest that decreases in acid anion concentrations (S½SO22
4þ
NO2
3ÿ) should be balanced by smaller decreases (relative to acid
anions) in base cations (C
B
) and increases in alkalinity (which we
define as recovery). Large-scale patterns in recovery were evident
across all the European regions (north/central Europe, the Nordic
countries and Great Britain) during the 1990s (Table 1, Fig. 2). The
lack of recovery in the Nordic countries in the 1980s has been
reported elsewhere19, and declines in C
B
identified as the likely
mechanism preventing alkalinity from recovery despite large
decreases in sulphur deposition. Our analysis (Fig. 2) reinforces
this interpretation, but highlights how this pattern changed in the
1990s. Nordic trends in C
B
in the 1990s are relatively flat (Table 1),
and continued strong decreases in SO
2
4
in this region appear to be
leading to a recovery in alkalinity. It should be noted that the
‘‘recovery’’ observed in Great Britain does not appear to be driven
by changes in acid anions (non-marine SO
2
4
is unchanged in the
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Figure 2 Regional acidification trend results. Histograms exhibit slopes of regional trends
in a, acid anions (SO22
4þNO2
3), b, measured (Gran) alkalinity, and c, base cation (C
B
)
concentrations for eight regions of Europe and North America for the 1980s and 1990s.
All slopes are in mequiv. l
1
yr
1
.
© 1999 Macmillan Magazines Ltd
1990s, while NO
3
has increased slightly), but instead results from
increasing C
B
concentrations (this is the only region to exhibit this
phenomenon). The pattern in Great Britain appears to be driven by
climate variations, particularly as they affect the transport of seasalt
aerosols across the region, rather than by changes in anthropogenic
emissions20.
Despite large decreases in acid anions in south/central Ontario,
the Adirondack/Catskill mountains and midwestern North America
in the 1990s, there was either no regional recovery in alkalinity or
continued acidification (Fig. 2); we attribute this pattern to com-
pensating negative trends in C
B
. Although some decrease in C
B
is
expected as SO
2
4
concentrations decline, C
B
declines in excess of
SO
2
4
declines are unexpected, and will preclude any recovery in
alkalinity21. Among the mechanisms that have been hypothesized to
produce excessive C
B
declines are (1) decreasing C
B
deposition22,
and (2) cation depletion of watershed soils23,24. While the former
mechanism has been discounted in at least some regions14,itis
possible that long-term high rates of acidic deposition have leached
enough cations from sensitive soils that adsorbed cation pools are
severely depleted. If this depletion continues even at diminished
levels of acidic deposition, it could produce the steep C
B
trends we
observe in these regions. The possibility that this mechanism will
prevent or delay recovery deserves critical attention.
It is important to consider the effects that climate variation could
have on the trends we report, especially for the relatively short 1990s
time series we use in our analysis. The effects of climate on non-
marine SO
2
4
trends and C
B
behaviour in Great Britain have already
been mentioned. Droughts in both south/central Ontario and
midwestern North America have been shown to cause re-oxidation
of reduced sulphur in wetlands and lake littoral regions25,26. The
resulting increase in sulphate export can be relatively long-term,
temporarily delaying the recovery response in lakes with plentiful
wetlands in their watersheds; this mechanism undoubtedly con-
tributes to the lack of homogeneity in SO
2
4
trends observed in
south/central Ontario. In midwestern North America, however,
where long water residence times create a great potential for climatic
effects27, declines in SO
2
4
in the 1990s far exceed what is attributable
to a recovery from drought in the 1980s28; these trends doubtless
reflect declining rates of deposition. Climate may also play a role in
producing strongly decreasing C
B
trends. In lakes with long resi-
dence times, prolonged periods of drought produce increased C
B
concentrations and decreased rates of C
B
export29; as lakes recover
from drought, C
B
concentrations may decline rapidly (for example,
in drainage lakes, where increases result primarily from evapotran-
spiration) or slowly (for example, in seepage lakes, where increases
result from changes in groundwater flowpaths)27. Although data
from the current study are not sufficient to identify mechanisms
incontrovertibly, the widespread declines in SO
2
4
we report are
completely consistent with observed changes in rates of sulphur
deposition in Europe and North America. Lack of lake and stream
recovery, and particularly the strong decreases in C
B
concentrations
observed in some regions, may be due to a number of mechanisms,
including climate fluctuations.
The three North American regions where recovery might be
expected but was not observed (south/central Ontario, Adiron-
dack/Catskill mountains and midwestern North America) exhibit
the largest North American declines in acid anions and largest
decreases in C
B
in the 1990s, and are closest to significant emissions
sources in the midwestern United States. Analysing whether the
proximity of these regions to significant sources (and therefore
higher, and possibly longer-term, inputs of acidic deposition) is
related to their C
B
behaviour is beyond the scope of this Letter.
These trend patterns are very similar to those observed in the 1980s
in the Nordic countries, where recovery is now occurring. Increas-
ing trends in alkalinity and unchanging concentrations of C
B
in the
Nordic countries during the 1990s may be the result of higher rates
of SO
2
4
decline (Fig. 2a), or may reflect the cumulative effects of
longer-term declines in sulphur deposition (beginning in the late
1960s) in this region30. Long-term decreases in sulphur deposition
could be expected to result in the recovery of soil cation pools (as
rates of primary weathering begin to exceed loss rates due to
leaching by acid anions), and eventual recovery after a sufficient
time lag17. The recovery pattern in the Nordic countries suggests
that larger decreases in sulphur deposition and/or a longer response
time may be required before similar recovery is widely observed in
North America. This suggestion of lagged recovery, coupled with
evidence of the deleterious effects of climate variability in detecting
recovery, highlights the importance of continued coordinated
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Table 1 Regional trend
Region Decade N*SO
2
4
trend NO
3
trend Alkalinity trend C
B
trend
pSlope pSlope pSlope pSlope
(mequiv. l
1
yr
1
)(mequiv. l
1
yr
1
)(mequiv. l
1
yr
1
)(mequiv. l
1
yr
1
)
...................................................................................................................................................................................................................................................................................................................................................................
North/central Europe 1980s 12 ,0.001 5.8 0.011 +1.3 0.089 +2.5 0.921 2.0
1990s 23 p0.001 5.9 p0.001 2.4 ,0.001 +7.0 0.909 0.3
...................................................................................................................................................................................................................................................................................................................................................................
Nordic countries 1980s 27 p0.001 0.8 0.002 +0.1 ,0.001 1.6p0.001 2.6
1990s 33 p0.001 3.1 0.418 0.0 p0.001 +2.1 0.778 0.0
...................................................................................................................................................................................................................................................................................................................................................................
Great Britain 1990s 18 0.510 +0.0 0.024 +0.1 p0.001 +1.5 p0.001 +2.4
...................................................................................................................................................................................................................................................................................................................................................................
Maine/Atlantic Canada 1980s 10 0.264 0.0 0.829 0.0 ,0.001 +1.2 0.180 0.9
1990s 10 p0.001 1.3 0.883 0.0 0.111 0.7 0.685 0.1
...................................................................................................................................................................................................................................................................................................................................................................
Vermont/Quebec 1980s 47 p0.001 1.1† ,0.001 +0.0 0.515 +0.1 ,0.001 0.6
1990s 49 p0.001 3.5 0.133 0.0 0.028 +0.9 0.007 1.1
...................................................................................................................................................................................................................................................................................................................................................................
South/central Ontario 1980s 13 p0.001 1.2‡ 0.002 +0.0§ p0.001 +1.0† 0.030 +0.4
1990s 13 p0.001 5.8 0.591 0.0 0.817 0.5 p0.001 5.9
...................................................................................................................................................................................................................................................................................................................................................................
Adirondack/Catskill mountains 1980s 25 p0.001 1.7 p0.001 +0.7k,0.001 1.3 0.775 +0.8
1990s 25 0.012 0.9 p0.001 2.4 0.426 +0.1 ,0.001 4.5
...................................................................................................................................................................................................................................................................................................................................................................
Midwestern North America 1980s 34 0.023 1.2 ,0.001 +0.1§ 0.123 0.9 ,0.001 +1.5
1990s 34 p0.001 2.6 0.782 0.0 p0.001 1.1 p0.001 2.8
...................................................................................................................................................................................................................................................................................................................................................................
Results of meta-analysis of trends in SO
2
4
,NO
3
, alkalinity and base cation (C
B
) concentration for 1980s and 1990s in various regions. Statistics (pvalues based on x
2
tests) are from an analysis of variance of
site trend Zscores, and constitute a test of trend homogeneity within the regions. x
2
values were considered significant at p,0:01 for site and season effects. Statistics for a site– season interaction term are
not shown, as they were never significant. Significant trends ( p,0:05) are shown in bold.
* Sample sizes vary among variables. Nvalues listed are for SO
2
4
.
† Site heterogeneity is significant; trend statistics are questionable.
‡ Site heterogeneity is significant, but all slopes are negative; trend statistics considered valid.
§ Seasonal heterogeneity significant; trend statistics questionable.
kSeasonal heterogeneity significant, but all slopes are positive; trend statistics considered valid.
© 1999 Macmillan Magazines Ltd
international monitoring to assess the success of acidic deposition
control measures. M
Received 20 April; accepted 19 August 1999.
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Acknowledgements
We thank the LRTAP Working Group on Effects for their support; this Working Group
supports the production of international, quality-controlled, comparable data. We also
acknowledge the work of the ICP Programme Centre at the Norwegian Institute of Water
Research (NIVA), where the data are collated, verified and archived, and thank T. J.
Sullivan and M. R. Church for comments and suggestions. This work was supported by the
US Environmental Protection Agency.
Correspondence and requests for materials should be addressed to J.L.S.
(e-mail: stoddard@mail.cor.epa.gov).
letters to nature
578 NATURE
|
VOL 401
|
7 OCTOBER 1999
|
www.nature.com
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Sterilization and canopy modification
of a swollen thorn acacia tree
by a plant-ant
Maureen L. Stanton*, Todd M. Palmer†‡, Truman P. Young*†‡,
Amanda Evans§& Monica L. Turner
*Center for Population Biology, University of California, Davis, California 95616,
USA
Mpala Research Centre, PO Box 555, Nanyuki, Kenya
Department of Environmental Horticulture, University of California, Davis,
California 95695, USA
§Department of Biology, University of Oregon, Eugene, Oregon 97403, USA
.......................................... ......................... ......................... ......................... .........................
Obligate symbioses between specialized arboreal ants and plants
have evolved independently in many lineages1,2. Ant-plants (myr-
mecophytes) typically provide hollow nest cavities and nutrition
to the occupying ant colony1,3–6. In turn, resident plant-ants
often protect their hosts from herbivory7–11 and/or overgrowth
by surrounding vegetation12,13. As individual plants are rarely
occupied by more than one ant colony14–17, co-occurring plant-
ant species compete intensely for hosts13,14,18,19. In such multi-
species systems, ecological interactions among potential partners
may lead to the evolution of cheating20,21. Previous studies have
revealed that some specialized plant-ants are effectively parasites
of their host-plants8,18,22,23, but the selection pressures favouring
such behaviours are poorly understood. Here we describe host
parasitism in an east African plant-ant that prunes and sterilizes
its host-tree canopies, apparently to minimize contact with
competitively dominant ants occupying neighbouring trees. We
propose that the high density of ant-trees and low diversity of
tree species in this savanna habitat have selected for induced,
parasitic pruning of host trees by this competitively subordinate
ant species.
Whistling Thorn (Acacia drepanolobium) trees dominate vast
areas of savanna on black cotton soils in upland east Africa24,25.At
each branch node, A. drepanolobium trees produce either a pair of
slender thorns or a swollen thorn pair joined by a bulbous, hollow
base 1.5–6 cm in diameter (Fig. 1). When living on a tree, ants chew
Figure 1 Close-up of
A. drepanolobium
branches showing axillary leaves, paired slender
thorns and a node occupied by a fused swollen thorn pair. To show architectural features
clearly, this photograph was taken at the beginning of the rainy season, when relatively
few axillary leaves were persistent on older growth. Resident ants had not yet chewed
entry holes into the recently formed swollen thorn (centre). Swollen thorns are produced
even in the absence of ants26.
... Eventually, acid rain became widely recognized as the greatest environmental threat of modern times, stimulating the development of less polluting technologies and their implementation. With the establishment of environmental regulations, measures, and policies in Europe and North America in the 1980s and 1990s, acidic precipitation was significantly reduced [18][19][20]. Today, there is far less deposition of sulfuric compounds, as Europe and Northern America have reduced their SO 2 emissions by 70-80% since 1990. ...
... Today, there is far less deposition of sulfuric compounds, as Europe and Northern America have reduced their SO 2 emissions by 70-80% since 1990. [19,21]. Europe has reduced its SO 2 emissions from 55 Tg in 1980 to 15 Tg in 2004 [22]. ...
... Changes in these conditions are very likely to cause biodiversity loss. These forest landscapes are also still recovering from the effects of acidic precipitation in the 20th century [18,19]. Future challenges such as climate change-induced droughts as well as forest loss due to bark beetle outbreaks only further increase the vulnerability of these sensitive systems to biodiversity loss. ...
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Over the course of industrialization in the 20th century, vast emissions of air pollutants have occurred. The exhaust gasses contain sulfur and nitrogen oxides, which are converted to sulfuric acid and nitric acid in the atmosphere. This causes acid rain to enter aquatic and terrestrial ecosystems, the most serious consequence of which is large-scale forest dieback across Europe and North America. However, through various political measures, the exhaust gasses have been reduced and, thus, acid rain and forest dieback were stopped. Nevertheless, the lingering effects of this pollution are still present today and are reflected in hydrochemistry. More recently, fluctuating precipitation regimes are causing additional stress to ecosystems in Central Europe. Climatic extremes are becoming more pronounced with climate change. Substantial differences between drought years and years with regular precipitation are directly altering the discharge of springs. Now, two overlapping and interacting syndromes of environmental pressures can be studied in these small catchments at a landscape scale: (1) acidification and (2) climate change. In this long-term study, the waters of 102 forest springs, located in two neighboring forest landscapes in north-eastern Bavaria, Germany (Frankenwald and Fichtelgebirge), were investigated over 24 years (1996 to 2020). By linking changes in pH values with changes in precipitation and spring discharge, we found that pH increases with decreasing discharge and decreasing precipitation. This effect was strongest in the Frankenwald compared to the Fichtelgebirge. We hypothesize that this temporal pattern reflects the longer residence time and, in consequence, the increased buffering of acidic interflow in small catchments during periods of drought. However, this should not be misinterpreted as rapid recovery from acidification because this effect fades in times of enhanced precipitation. We recommend that fluctuations in weather regimes be considered when investigating biogeochemical patterns throughout forest landscapes.
... Following reductions in anthropogenic sulfur emissions in North America and Europe, the acidification problem was widely considered solved. Many studies observed steady improvements in stream chemistry (Evans et al., 2001;Monteith et al., 2014;Skjelkvåle et al., 2005;Stoddard et al., 1999;Warby et al., 2005), including reduced concentrations of Al in the United States (Baldigo & Lawrence, 2000;Buchanan et al., 2017;Burns et al., 2006) and Europe (Beneš et al., 2017;Davies et al., 2005;Monteith et al., 2014). However, recent evidence highlights delayed recovery from acidification in certain regions with slow-weathering geology (Houle et al., 2006;Warby et al., 2009;Watmough et al., 2016), including Nova Scotia, Canada (Clair et al., 2011;Sterling et al., 2020). ...
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Elevated concentrations of toxic cationic aluminum (Al i ) are symptomatic of terrestrial and freshwater acidification and are particularly toxic to salmonid fish species such as Atlantic salmon ( Salmo salar ). Speciated metal samples are rarely included in standard water monitoring protocols, and therefore the processes affecting Al i dynamics in freshwater remain poorly understood. Previous analysis of Al i concentrations in Nova Scotia (Canada) rivers found that the majority of study rivers had concentrations exceeding the threshold for aquatic health, but a wide‐scale survey of Al i in Nova Scotia has not taken place since 2006 (Dennis, I. F., & Clair, T. A., 2012, Canadian Journal of Fisheries and Aquatic Sciences, 69 (7), 1174–1183). The observed levels of dissolved aluminum in Atlantic salmon ( Salmo salar ) rivers of Atlantic Canada have potential serious and harmful effects for aquatic populations. We present the findings of the first large‐scale assessment of the Al i status of Nova Scotia rivers in 17 years; we measured Al i concentrations and other water chemistry parameters at 150 sites throughout the Southern Uplands region of Nova Scotia from 2015 to 2022. We found that Al i concentrations exceeded toxic thresholds at least once during the study period at 80% of the study sites and that Al i concentrations increased during the study period at all four large‐sample study sites. Modeling of relationships between Al i concentrations and other water chemistry parameters showed that the most important predictors of Al i are concentrations of the dissolved fractions of Al, iron, titanium, and calcium, as well as dissolved organic carbon and fluoride. We developed a fully Bayesian linear mixed model to predict Al i concentrations from a test data set within 15 μg/L. This model may be a valuable tool to predict Al i concentrations in rivers and to prioritize areas where Al i should be monitored. Environ Toxicol Chem 2024;00:1–12. © 2024 The Author(s). Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.
... In particular, the Oslo and Geneva protocols have achieved an 80 % decrease in both North American and European SO 2 emissions since 1980 (Grennfelt et al., 2020). Despite these successes, the reduction in acid deposition in ecosystems has been unexpectedly slow, leaving them sensitized and necessitating continued deposition monitoring (Stoddard et al., 1999;Kuylenstierna et al., 2001). ...
Article
Full-text available
An important transport process for particles and gases from the atmosphere to aquatic and terrestrial environments is through dry and wet deposition. An open-source, modular, off-grid, and affordable instrument that can automatically collect wet-deposition samples allows for more extensive deployment of deposition samplers in fieldwork and would enable more comprehensive monitoring of remote locations. Precipitation events selectively sampled using a conductivity sensor powered by a battery-based supply are central to off-grid capabilities. The prevalence of conductive precipitation – that which initially contains high solute levels and progresses through trace-level concentrations to ultrapure water in full atmospheric washout – depends on the sampling location but is ubiquitous. This property is exploited here to trigger an electric motor (via limit switches) to open and close a lid resting over a funnel opening. The motors are operated via a custom-built and modular digital logic control board, which has a low energy demand. All components, their design and rationale, and their assembly are provided for community use. The modularity of the control board allows the operation of up to six independent wet-deposition units, such that replicate measurements (e.g., canopy throughfall) or different collection materials for various targeted pollutants can be implemented as necessary. We demonstrate that these platforms are capable of continuous operation off-grid for integrated monthly and bimonthly collections performed across the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (47 to 53° N) during the growing seasons of 2015 and 2016. System performance was assessed through the measured power consumption from 115 V of alternating current (VAC; grid power) or 12 V of direct current from battery supplies during operation under both standby (40 or 230 mA, respectively) and in-use (78 or 300 mA, respectively) conditions. In the field, one set of triplicate samplers was deployed in the open to collect incident precipitation (open fall), while another set was deployed under the experimental forest canopy (throughfall). The proof-of-concept systems were validated with basic measurements of rainwater chemistry, which found (i) pH values ranging from 4.14 to 5.71 in incident open fall rainwater, (ii) conductivities ranging from 21 to 166 µS cm-1, and (iii) dissolved organic carbon concentrations in open fall and canopy throughfall of 16±10 and 22±12 mg L-1, respectively, with incident fluxes spanning 600 to 4200 mg C m-2 a-1 across the transect. Ultimately, this demonstrates that the customized precipitation sampling design of this new platform enables more universal accessibility of deposition samples for the atmospheric observation community – for example, those who have made community calls for targeting biogeochemical budgets and/or contaminants of emerging concern in sensitive and remote regions.
... Surface water acidification was recognized as a major threat in Europe and North America (Stoddard et al., 1999). Its adverse impacts, including fish mortality, the inhibition of microbial activity , disruption of organic matter decomposition, and perturbations of the acid-base balance of phytoplankton (Cornut et al., 2012;Flynn et al., 2012), have attracted public and political attention. ...
Article
Full-text available
For many decades, acid deposition used to pose a significant regional air pollution challenge in China. After substantial emission control of anthropogenically derived sulfur and nitrogen containing gasses, both sulfur and nitrogen deposition, as well as the acid rain-affected area, have significantly decreased compared to their peak levels. Forests, particularly in the humid subtropics, are sensitive to acid deposition, as evidenced by soil acidification, sulfate and nitrate leaching in stream water, and elevated soil nitrous oxide emission. Reduction in the total deposition of sulfur and nitrogen, caused a significant decline in sulfate and nitrate leaching from subtropical forest and subsequently in sulfate and nitrate concentrations in stream water, although there was about a 5-year delay. This delay may be attributed to the desorption of accumulated sulfate and continued elevated mineralization of accumulated nitrogen pools. Emissions of nitrous oxide, a potent greenhouse gas, also declined in nitrogen-saturated subtropical forest soils, as soil water nitrate concentration decreased. Therefore, subtropical forests in China suffering from elevated acid deposition have begun to recover. Yet, the current levels of sulfur and nitrogen deposition continue to exceed the critical loads, i.e., the assigned threshold levels in accordance with emission control policies, in more than 10% of the country’s land area, respectively, indicating remaining risks of acidification and eutrophication. Thus, further emission reductions are urgently needed, also because they will help achieving goals related to air quality and nitrous oxide emissions.
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Despite a decreasing trend in acidic deposition rates over the past two to three decades, acidified surface waters in the northeastern United States have shown minimal changes. Depletion of soil Ca pools has been suggested as a cause, although changes in soil Ca pools have not been directly related to long-term records of stream chemistry. To investigate this problem, a comprehensive watershed study was conducted in the Neversink River Basin, in the Catskill Mountains of New York, during 1991-1996. Spatial variations of atmospheric deposition, soil chemistry, and stream chemistry were evaluated over an elevation range of 817-1234 m to determine whether these factors exhibited elevational patterns. An increase in atmospheric deposition of SO4 with increasing elevation corresponded with upslope decreases of exchangeable soil base concentrations and acid-neutralizing capacity of stream water. Exchangeable base concentrations in homogeneous soil incubated within the soil profile for one year also decreased with increasing elevation. An elevational gradient in precipitation was not observed, and effects of a temperature gradient on soil properties were not detected. Laboratory leaching experiments with soils from this watershed showed that (1) concentrations of Ca in leachate increased as the concentrations of acid anions in added solution increased, and (2) the slope of this relationship was positively correlated with base saturation. Field and laboratory soil analyses are consistent with the interpretation that decreasing trends in acid-neutralizing capacity in stream water in the Neversink Basin, dating back to 1984, are the result of decreases in soil base saturation caused by acidic deposition.
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In the absence of true regional data on changes in the acid/base status of lakes in the northeastern United States, we explore the possibility of using site-specific trends information from a judgment sample of lakes to assess the efficacy of the Clean Air Act Amendments. A meta-analytical technique is used to combine trends results from 44 Long-Term Monitoring (LTM) lakes in the Northeast for the period 1982-1994, with the goal of producing estimates of overall trends in the region. The lakes are subdivided into subpopulations (High ANC, Intermediate Till Drainage, Thin Till Drainage and Perched Seepage lakes) on the basis of their expected response to changes in acidic deposition, and they appear to represent the most acid-sensitive of these lake classes well. While the overall tendencies in the trends are as expected (e.g., most of the recovery is observed in the most sensitive subpopulations), there is significant trend heterogeneity among the lakes within most of the subpopulations; this heterogeneity prohibits the summarizing of trends at the regional level (i.e., for all of the Northeast). This heterogeneity is explained by differences in the responses of lakes in two subregions (Adirondacks vs. New England), and we present trends results separately for each subpopulation within these two subregions. All subpopulations in both subregions showed decreasing trends in sulfate concentrations, probably a reflection of decreasing trends in sulfur deposition in the region. Few trends in nitrate concentrations were observed. Recovery (as evidenced by increasing trends in acid-neutralizing capacity) was evident in Thin Till and Intermediate Till Drainage lakes in New England, but not in the Adirondacks. Most groups of lakes exhibited downward trends in base cations (Σ[Ca2+ + Mg2+ + Na+ + K+]); the magnitudes of these trends were always greater in Adirondack lakes than in similar New England lakes. This suggests that the depletion of soil cation pools in the Adirondacks may be responsible for some of the differences in recovery between Adirondack and New England lakes. While export of base cations may be the key difference producing different trends results in the two subregions, the site-specific nature of the trends, and their possible lack of regional representation, should be considered in interpreting the overall results.
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Nitrogen saturation occurs when the supply of nitrogenous compounds from the atmosphere exceeds the demand for these compounds on the part of watershed plants and soil microbes. Several factors predispose forested watersheds to N saturation, including chronically high rates of N deposition, advanced stand age, and large pools of soil N. Many watersheds in the eastern United States exhibit symptoms of N saturation. A sequence of recognizable stages produces characteristic long-term and seasonal patterns of lake-water and stream-water NO3- concentrations that reflect the changes in rates and relative importance of N transformations as these watersheds become more N sufficient. The early stages of N saturation are marked by increases in the severity and frequency of NO3- episodes. The later stages of N saturation are marked by elevated baseflow concentrations of NO3- from groundwater. The most advanced symptoms of N saturation usually occur in regions with the most elevated rates of N deposition. Long-term increases in surface-water NO3- have important implications for surface-water acidification, but probably will not lead to freshwater eutrophication.
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In the absence of true regional data on changes in the acid/base status of lakes in the northeastern United States, we explore the possibility of using site-specific trends information from a judgment sample of lakes to assess the efficacy of the Clean Air Act Amendments. A meta-analytical technique is used to combine trends results from 44 Long-Term Monitoring (LTM) lakes in the Northeast for the period 1982–1994, with the goal of producing estimates of overall trends in the region. The lakes are subdivided into subpopulations (High ANC, Intermediate Till Drainage, Thin Till Drainage and Perched Seepage lakes) on the basis of their expected response to changes in acidic deposition, and they appear to represent the most acid-sensitive of these lake classes well. While the overall tendencies in the trends are as expected (e.g., most of the recovery is observed in the most sensitive subpopulations), there is significant trend heterogeneity among the lakes within most of the subpopulations; this heterogeneit...
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Long-term data from the Hubbard Brook Experimental Forest, New Hampshire, suggest that although changes in stream pH have been relatively small, large quantities of calcium and magnesium have been lost from the soil complex and exported by drainage water because of inputs of acid rain and declines in atmospheric deposition of base cations. As a result, the recovery of soil and streamwater chemistry in response to any decreases in acid deposition will be delayed significantly.
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Regional trends of seasonal and annual wet deposition and precipitation-weighted concentrations (PWCs) of sulfate in the United States over the period 1980–1995 were developed from monitoring data and scaled to a mean of unity. To reduce some effects of year to year climatological variability, the unitless regional deposition and PWC trends were averaged (hereafter termed CONCDEP). The SO2 emissions data over the same period from the United States, Canada, and northern Mexico, aggregated by state and province, were weighted appropriately for each deposition region in turn to produce scaled trends of the emissions affecting each region. The emission-weighting factors, which were held constant year to year, were estimated by exercise of a regional transport model. The sulfate CONCDEP regional trends are generally similar to those of regionally weighted SO2 emissions, although the latter trends are less steep and the former trends have more year to year variability. In eastern regions, sulfate CONCDEPs and SO2 emissions patterns both generally show an initial decrease, an essentially trendless middle period, and a final decrease as reductions mandated by the Acid Rain Provisions of the 1990 Clean Air Act Amendments began. Linear regressions of regional sulfate CONCDEPs on corresponding regionally weighted SO2 emissions produced statistically significant relationships in all regions. The analysis indicated that although regional sulfate CONCDEPs decreased relatively faster than did SO2 emissions during the period in all regions except the Great Plains, in general the slopes were not significantly different from unity.
Article
The Acidifying Potential (AP) of wet deposition has been previously defined by the expression AP = SO4--[Ca++ + Mg++]. The time series over the period 1981 to 1993 for sulphate, calcium plus magnesium and AP concentrations are examined at five ecological monitoring research sites in the eastern half of Canada. At all sites there was a reduction in the concentrations of all 3 parameters over the whole period. As a result the reduction in AP was between 70 and 80% of the sulphate reduction at the 4 sites where precipitation acidity is highest. The annual wet deposition of AP showed a substantial decline in the first half of the period, but has levelled off in the second half, in spite of continuing sulphate decline. In assessing the impact of acidic deposition on surface water systems it is, thus, important to account for the counteraction of sulphate decreases by base cation decreases. This is particularly important for simulation modelling, either diagnostic using past data, or predictive using future sulphur dioxide emission scenarios, where in general cations have been assumed to remain constant.