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Setting Limits: Using AirPollution Thresholds to Protect
and Restore U.S. EcosystemsMark E. Fenn, Kathleen F. Lambert,
Tamara F. Blett, Douglas A. Burns,
Linda H. Pardo, Gary M. Lovett, Richard A. Haeuber, David C.
Evers,
Charles T. Driscoll, and Dean S. Jeffries
Fall 2011 Report Number 14
Setting Limits: Using AirPollution Thresholds to Protect
and Restore U.S. Ecosystems
Issues in EcologyIssues in Ecology
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© The Ecological Society of America • [email protected] esa 1
ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
Setting Limits: Using Air Pollution Thresholdsto Protect and
Restore U.S. Ecosystems
SUMMARY
More than four decades of research provide unequivocal evidence
that sulfur, nitrogen, and mercury pollution havealtered, and will
continue to alter, our nation’s lands and waters. The emission and
deposition of air pollutants harmnative plants and animals, degrade
water quality, affect forest productivity, and are damaging to
human health. Many air qual-ity policies limit emissions at the
source but these control measures do not always consider ecosystem
impacts. Air pollutionthresholds at which ecological effects are
observed, such as critical loads, are effective tools for assessing
the impacts of air pol-lution on essential ecosystem services and
for informing public policy. U.S. ecosystems can be more
effectively protected andrestored by using a combination of
emissions-based approaches and science-based thresholds of
ecosystem damage.
Based on the results of a comprehensive review of air pollution
thresholds, we conclude:
l Ecosystem services such as air and water purification,
decomposition and detoxification of waste materials, climate
regu-lation, regeneration of soil fertility, production and
biodiversity maintenance, as well as crop, timber and fish
suppliesare impacted by deposition of nitrogen, sulfur, mercury and
other pollutants. The consequences of these changes maybe difficult
or impossible to reverse as impacts cascade throughout affected
ecosystems.
l The effects of too much nitrogen are common across the U.S.
and include altered plant and lichen communities,enhanced growth of
invasive species, eutrophication and acidification of lands and
waters, and habitat deteriora-tion for native species, including
endangered species.
l Lake, stream and soil acidification is widespread across the
eastern United States. Up to 65% of lakes within sensi-tive areas
receive acid deposition that exceeds critical loads.
l Mercury contamination adversely affects fish in many inland
and coastal waters. Fish consumption advisories formercury exist in
all 50 states and on many tribal lands. High concentrations of
mercury in wildlife are also wide-spread and have multiple adverse
effects.
l Air quality programs, such as those stemming from the 1990
Clean Air Act Amendments, have helped decrease airpollution even as
population and energy demand have increased. Yet, they do not
adequately protect ecosystemsfrom long-term damage. Moreover they
do not address ammonia emissions.
l A stronger ecosystem basis for air pollutant policies could be
established through adoption of science-based thresh-olds. Existing
monitoring programs track vital information needed to measure the
response to policies, and couldbe expanded to include appropriate
chemical and biological indicators for terrestrial and aquatic
ecosystems andestablishment of a national ecosystem monitoring
network for mercury.
The development and use of air pollution thresholds for
ecosystem protection and management is increasing in the United
States,yet threshold approaches remain underutilized. Ecological
thresholds for air pollution, such as critical loads for nitrogen
and sulfurdeposition, are not currently included in the formal
regulatory process for emissions controls in the United States,
although they arenow considered in local management decisions by
the National Park Service and U.S. Forest Service. Ecological
thresholds offer ascientifically sound approach to protecting and
restoring U.S. ecosystems and an important tool for natural
resource managementand policy.
Cover photo credit: Loch Vale in the Colorado Rocky Mountains.
Photo by SteveB in Denver
(http://www.flickr.com/people/darkdenver/) and used in
thispublication under a Creative Commons Attribution license.
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© The Ecological Society of America • [email protected] esa
Introduction
Natural ecosystems have been altered in vari-ous ways by
nitrogen, sulfur, and mercurydeposited in rain, snow, or as gases
and parti-cles in the atmosphere. Through decades ofscientific
research, scientists have documentedhow local, regional, and global
sources of airpollution can produce profound changes inecosystems.
These changes include acidifica-tion of soils and surface waters,
harmful algalblooms and low oxygen conditions in estuar-ies,
reduced diversity of native plants, highlevels of mercury in fish
and other wildlife,and decreased tolerance to other stresses,
suchas pests, disease, and climate change.Advancing our
understanding of the linkagesamong pollutant deposition rates or
concen-trations, ecosystem effects, and associated pol-icy
decisions is a priority in policy-relevantscience in the U.S.
Air pollutants that affect human health andecosystems are
primarily emitted from electricpower generation, industrial,
transportation,and agricultural activities. The benefits
andnecessities of these activities must be consid-ered in light of
the often detrimental effects ofatmospheric emissions on human
health, visi-bility, ecosystems, and on the services pro-vided to
society by these ecosystems (Table 1).The 1990 Clean Air Act
Amendments andother air quality regulations have led tomarked
declines in emissions of nitrogen, sul-fur and mercury. Some
emissions from powergeneration and other sources have decreasedby
over 50% since the 1970s, even as popula-tion and energy demand
have increased. Asthe emissions and deposition of most pollu-tants
have declined, some impacted ecosys-tems have started to recover.
In many parts ofthe country, however, ecological conditionsare
still declining due to the increase in otherforms of pollution such
as ammonia (NH3),the long term accumulation of sulfur and
nitrogen compounds in soils, and the ongoingbiomagnification of
mercury in food webs.
The purpose of this report is to distilladvances in the science
of air pollutionthresholds and to describe their use to
assess,protect and manage the nation’s ecosystemsand the vital
services they provide. We focushere on the environmental impacts of
nitro-gen, sulfur, and mercury and refer to connec-tions to climate
change. The discussion drawson the published research of hundreds
of sci-entists over the past several decades with afocus on U.S.
ecosystems and lessons fromCanada and Europe.
Air Pollution Thresholds
Thresholds of air pollution in the U.S. havebeen widely
discussed in the scientific litera-ture since the 1970s, when
research estab-lished that sulfur deposition was above levelsat
which damage occurs in many sensitiveecosystems in the eastern U.S.
More recently,nitrogen deposition has been shown to impactsensitive
ecosystem components and processesthroughout the United States.
Defining thespecific concentration or deposition input ofan air
pollutant that will cause adverse or sig-nificant ecosystem effects
has been the subjectof much scientific research. Pollutants
canaccumulate with little noticeable impact onplants or animals
until major changes occur asa tipping point is reached (Box 1).
Thesechanges are measured by scientifically deter-mined chemical or
biological indicators (Box2). Such environmental changes might
elimi-nate a single sensitive species, or a broad shiftmay occur in
biodiversity throughout anecosystem. Once a species or ecosystem
haspassed a tipping point, a return to the previousstate may not be
possible.
Air pollution thresholds can be definedbased strictly on
scientific research (ecologicalthresholds) or based on a balance of
policy con-
Setting Limits: Using Air Pollution Thresholdsto Protect and
Restore U.S. Ecosystems
Mark E. Fenn, Kathleen F. Lambert, Tamara F. Blett, Douglas A.
Burns, Linda H. Pardo, Gary M. Lovett,
Richard A. Haeuber, David C. Evers, Charles T. Driscoll, and
Dean S. Jeffries
ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
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© The Ecological Society of America • [email protected] esa 3
ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
siderations spanning law, economics, ecologi-cal effects, human
health and risk assessment(policy thresholds) (Box 1) (Figure 1).
One toolincreasingly used to integrate the science andpolicy of air
pollution thresholds for ecosystemprotection and management is
critical loads(Box 4).
Advances in the Science ofAir Pollution Thresholds
Based on research over the past decade, astrong scientific
foundation exists for definingair pollution thresholds using
critical loadsapproaches (Box 4). In the following sectionswe
synthesize the state of the science relatedto the ecological
effects, key indicators, andcritical loads approaches for
acidifying deposi-tion, nitrogen pollution and mercury
contami-nation.
1. ACIDIFYING DEPOSITIONA. Effects of Acidifying Deposition
Acidifying deposition (or “acid rain”) iscaused by emissions to
the atmosphere of sul-fur dioxide (SO2), nitrogen oxides (NOx),
andother acidifying compounds such as ammonia(NH3)(see Box 3 for
definition of chemicalnames and symbols). These pollutants returnto
Earth in rain, snow, fog, mist and gases informs such as nitric and
sulfuric acids andammonium (NH4
+) and can have long-termnegative impacts to terrestrial and
aquaticecosystems. Ecosystems in the western U.S.have not been
greatly affected by acidificationbecause acidifying deposition is
relatively lowin much of the region and because in manyarid or
semi-arid regions the soils are relativelyinsensitive to acid
inputs. Some high eleva-tion streams in the Colorado Rockies and
the
Box 1. DEFINITION OF TERMS
ACIDIFYING DEPOSITION. Deposition of substances from the
atmosphere as rain, snow, fog, or dry particles that have the
potentialto acidify the receptor medium, such as soil or surface
waters. Emissions of sulfur and nitrogen oxides and ammonia are the
most com-mon sources of acidifying air pollutants.
ACID NEUTRALIZING CAPACITY. A measure of the ability of a
solution to neutralize inputs of strong acids, commonly applied to
sur-face water or soil solution. The acronym ANC is widely used in
referring to acid neutralizing capacity.
ATMOSPHERIC DEPOSITION. The transfer of air pollutants from the
atmosphere to the Earth’s surface. Atmospheric depositionoccurs as
wet (e.g., rainfall, fog, or snow) and dry deposition (e.g.,
gaseous or particulate deposition).
BASE SATURATION. The fraction of exchangeable cations in soil
which are nonacid forming cations (Ca+2, Mg+2, K+ and Na+),
alsoreferred to as ‘base cations’. The higher the amount of
exchangeable base cations in soil, the more acidity can be
neutralized.
BIOACCUMULATION. The increase in concentration of a contaminant
in an individual organism relative to the surrounding environ-ment
or medium (e.g., water, sediment).
BIOMAGNIFICATION. The increase in concentration of a contaminant
from lower trophic levels to higher trophic levels in thefood
chain.
CRITICAL LOAD. The quantitative estimate of an exposure to one
or more pollutants below which significant harmful effects on
spec-ified sensitive elements of the environment do not occur
according to present knowledge.
ECOLOGICAL THRESHOLD. The dose of a pollutant at which a
measurable change occurs in the response of some component of
anecosystem (e.g., NO3
– leaching at nitrogen deposition of 8 kg/ha/yr).
ECOSYSTEM SERVICES. Benefits to society from a multitude of
resources and processes that are supplied by natural
ecosystems(e.g., clean drinking water).
ENDPOINT. The ultimate ecological, biological or human condition
or process to be protected from harm. Two examples of endpointsare
human health and forest sustainability.
INDICATOR. A measurable physical, chemical, or biological
characteristic of a resource that may be adversely affected by a
change inair quality (e.g., ANC).
NITROGEN SATURATION. Syndrome of effects occurring in an
ecosystem caused by an overload of nitrogen, usually from long
termatmospheric nitrogen deposition.
POLICY THRESHOLD. A quantitative value of desired ecological
condition established by policy and selected based on a balancingof
science and land management or policy goals.
SENSITIVE RECEPTOR. The indicator that is the most responsive
to, or the most easily affected by a type of air pollution.
TARGET LOAD. The acceptable pollution load that is agreed upon
by policy makers or land managers. The target load is set below
thecritical load to provide a reasonable margin of safety, but
could be higher than the critical load at least temporarily.
TIPPING POINT. The point at which an ecosystem shifts to a new
state or condition in a rapid, often irreversible,
transformation.
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Table 1. Linking air pollution impacts to ecosystem services,
indicators and thresholds. Ecological thresholdsgiven are typical
values that can vary depending on ecological and environmental
conditions.
Impact Ecosystem Ecological Response Ecosystem Services
Indicator Ecological ThresholdImpacted
Sulfur and Nitrogen Deposition
Acidification Terrestrial 1. Decreased forest 1. Timber
production Ca: Al+3 ratios in soil 10 – low risk
2. Increased 3. Biodiversitysusceptibility to 4. Resilience to
Soil percent base 30% - low risk
Foliar chemistry
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
Sierra Nevada Mountains do experience acidicepisodes when
pollutants retained in the snowpack over the winter are released
into soils andstreams during snowmelt. In the eastern UnitedStates,
depletion of available calcium and mag-nesium pools and
acidification of forest soils iswidespread and well documented in
theAppalachian Mountains, including the Cats-kills and the
Adirondacks, and in the Shenan-doah Mountain region of West
Virginia.
Mountain forests of the northeastern andsoutheastern United
States receive high ratesof acidifying deposition due to frequent
expo-sure to acidic clouds, fog, rain and snow.Changes associated
with acidifying depositionhave reduced the ability of some tree
speciesto cope with the cold temperatures commonto these mountain
environments. This effectcontributed to large-scale red spruce
deaths inthese regions in the 1980s and 90s, andremains a problem
today. In eastern U.S. hard-wood forests at lower elevations, many
sugarmaple, white ash, flowering dogwood, andother trees have high
calcium requirementsand therefore are also sensitive to
acidifica-tion. Tree declines have negative conse-quences for
forest productivity and ecosystemservices, including timber
production and cli-mate regulation (lower productivity means
lessremoval of carbon dioxide from the atmos-phere). Research has
attributed sugar mapledeclines in western Pennsylvania to
acidifica-tion acting in concert with insect outbreaks,and research
in New Hampshire has shownimproved growth and reproduction of
sugarmaple, and less frost damage to red spruce,when calcium was
added to an acidified forestfor experimental purposes.
Acidification of sensitive surface waters hasresulted in well
documented adverse effects onfish, zooplankton, aquatic insects,
microorgan-isms, and other aquatic biota. In many sensi-tive areas
receiving elevated acidifying deposi-tion, surface waters are too
acidic to supportany fish species. The reduction in the numberof
aquatic species and in the number of fishsupported diminishes
biodiversity and recre-ational fishing opportunities.
Long-termresearch on acidification impacts on forests,lakes and
streams has produced a wealth ofdata, from which are drawn the most
com-monly applied indicators for assessing acidifi-cation status
and effects (Table 1). Althoughterrestrial and aquatic indicators
are treatedseparately below, recognition should be givento the
connection of soil acidification toaquatic acidification.
B. Indicators - Acidifying Deposition
Indicators of Soil Acidificationand Forest Health
One way to assess the risk to acid sensitivetree species such as
red spruce and sugar mapleis by tracking chemical indicators in the
soiland in the leaves and needles of plants (i.e.,
Box 2. INDICATORS AND AIR POLLUTION THRESHOLDS
Just as physicians use a range of diagnostic measurements to
monitor humanhealth, scientists track chemical and biological
indicators to monitor ecosystemhealth. When many different studies
confirm an association between a pollutantamount and an ecosystem
response, threshold pollutant levels can often beidentified for
indicators that signal likely problems. Chemical indicators are
oftenused as surrogates for biological effects because chemical
indicators are typi-cally simpler and less expensive to measure.
Chemical indicators are imperfectsurrogates since accurate
prediction of just how plants and animals will respondto chemical
changes in their environment is not always possible.
Box 3. CHEMICAL NAMES AND SYMBOLS, AND UNITSOF MEASURE
Chemical Names and Symbols:Sulfur dioxide, SO2Nitrogen dioxide,
NO2Nitrogen oxides, NOxSulfur oxides, SOxAmmonia, NH3Ammonium,
NH4
+
Mercury, HgMethylmercury, MeHgSulfate, SO4
-2
Nitrate, NO3-
Dissolved organic carbon, DOCPhosphorus, PNitrogen, NCarbon,
CAluminum, Al+3
Calcium, Ca+2
Magnesium, Mg+2
Sodium, Na+
Potassium, K+
Calcium to aluminum ratio, Ca:Al pH, a measure of acidity or
hydrogenion concentrationNutrient ratios (e.g., N:P, N:Ca, C:N)
Units of Measure:Equivalents per hectare per
year,eq/ha/yrKilograms per hectare per year,kg/ha/yrParts per
million, ppmMicroequivalents per liter, µeq/LMilliequivalents per
square meter peryear meq/m2/yrMicrograms per liter, µg/L
Figure 1. Conceptualrepresentation of how ecologicaland policy
thresholds may bedeveloped. Both lines showestimates of
ecosystemdegradation as pollutantsincrease in ecosystems. Line“A”
represents a gradual declinein ecosystem condition, wheremanagers,
policy makers, andregulators can set policythresholds at any number
ofdifferent points depending ongoals (for example, A1, atbeginning
of decline or A2, atmidpoint of decline). Line “B”represents a
rapid decline inecosystem condition, with aclearly identified,
ecologicalthreshold at which a tippingpoint occurs (B1).
A
B
A1 A2 B1Deposition of Air PollutionS
upp
ly o
f E
cosy
stem
Ser
vice
s
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© The Ecological Society of America • [email protected] esa
foliage) (Table 1). Three elements naturallypresent in soils,
calcium (Ca+2), magnesium(Mg+2), and aluminum (Al+3), influence
theextent to which trees and other plants may beadversely affected
by acidifying deposition.Calcium and magnesium are nutrients
neededfor a variety of plant functions and their sup-ply helps
neutralize acid inputs to soils,whereas Al+3 can be harmful to
plants at highconcentrations when present in the readilyavailable
exchangeable form. Acid depositionslowly removes readily available
exchangeableCa+2 and Mg+2 from soils and replaces themwith
exchangeable Al+3 and hydrogen ion (oracidity), setting off a
cascade of adversechanges.
In general, greater availability of Ca+2 andMg+2 and low Al+3
provides favorable conditionsfor many acid-sensitive tree species
such as sugarmaple and red spruce. Calcium to aluminum ratio(Ca:Al)
in soils and soil solutions is one indica-tor used to assess the
health risk to acid sensitivetree species such as red spruce and
sugar maple.Soil percent base saturation is another useful
indi-cator for assessing sensitivity and extent of acid-ification.
Scientists generally concur that wheresoil percent base saturation
is low there is a highrisk of damage to the vitality of sensitive
treespecies due to nutritional deficits resulting
fromacidification. The risks to forest vegetation asso-ciated with
a range of Ca:Al ratios and soil per-cent base saturation values
are shown in Table 1.Other studies have focused on the
concentra-tion of exchangeable Ca+2 and Mg+2 as a usefulindicator
since soils can have widely varyingamounts of these nutrients that
are essential tothe health of forest vegetation. Concentrations
of
Ca+2 and Mg+2 in the leaves and needles ofplants (foliage) have
recently been identified asvaluable indicators for evaluating acid
deposi-tion impacts. For example, low concentrationsof these
nutrients have been identified as limit-ing the growth of sugar
maple (Table 1).
Indicators of Acidification inAquatic Ecosystems
Indicators of acidification in lakes and streamsare generally
based on changes in water chem-istry. Water chemistry strongly
affects thenumbers and types of aquatic organisms thatare present
in a water body. The indicatorsmost commonly used to track changes
in sur-face water acidification are ANC, pH, and/orconcentrations
of key elements.
Acid neutralizing capacity (ANC) is a com-monly used chemical
indicator of lake orstream sensitivity to acidification. ANC,
mea-sured in microequivalents per liter (µeq/L; SeeBox 3 for a list
of chemical units of measure),characterizes the ability of water to
neutralizestrong acids including those introduced byatmospheric
deposition. ANC is a good gen-eral indicator of acidity-related
water qualitybecause values are typically strongly correlatedwith
pH, Al+3 concentrations, and Ca+2 con-centrations. Specific concern
levels have beenidentified and are used to estimate criticalloads
(Table 2). The diversity of fish speciesdeclines precipitously with
decreases in ANCin Adirondack Lakes (Figure 2). In Shenan-doah
National Park (Virginia) streamsresearchers found that one fish
species, onaverage, is lost for every 21 µeq/L decline inANC.
Recent studies have demonstrated thatanother useful chemical
indicator is base cationsurplus. Low values indicate that the soil
hasbecome sufficiently acidified to enable toxicforms of aluminum
to be transported from thesoil into streams at concentrations of
concern.
The pH value of a water body is a funda-mental measure of
acidity or the hydrogen ionconcentration. A pH of 7 is neutral, and
pHvalues below 7 are increasingly acidic whilevalues above 7 are
increasingly basic or alka-line. Like ANC, decreases in pH are
associ-ated with decreases in the richness of aquaticspecies (Table
1). Studies have shown that inlakes of the Adirondack Mountains of
NewYork and the White Mountains of NewHampshire, one fish species
is lost for everypH decline of 0.8 units as values decrease from6
to 4. Few fish species can survive at pH val-ues of 4 or less
(Figure 3).
Figure 2. Number of fish speciesper lake as a function of
acidneutralizing capacity (ANC) inAdirondack lakes. The data
arepresented as the mean of speciesrichness for every 10 µeq/L
ANCclass. Lakes are also classifiedinto five descriptive
categoriesranging from low to acuteimpacts. (Adapted from:
Sullivan,T.J. and others 2006. Assessmentof the Extent to
WhichIntensively-Studied Lakes areRepresentative of the
AdirondackMountain Region. Final Report06-17. New York State
EnergyResearch and DevelopmentAuthority. Albany, NY).
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
Decreases in pH and ANC are often paral-leled by changes in
element concentrationsincluding increases in Al+3 concentrations
anddecreases in Ca+2. High dissolved Al+3 concen-trations can have
toxic effects on many types ofaquatic biota, and at extreme levels
few aquaticspecies can survive (Table 1). Organic forms ofAl+3 are
much less toxic than inorganic forms.Emerging research suggests
that Ca+2 concentra-tions in streamwater are also an important
bio-logical indicator. Acidifying deposition hasaccelerated the
leaching of Ca+2 from soils tosurface waters gradually decreasing
the avail-able pool of Ca+2 in soils and lowering Ca+2
concentrations in runoff. This soil depletiontogether with
decreases in leaching associatedwith declines in acidifying
deposition is con-tributing to decreases in surface water Ca+2.Many
lakes in the boreal forest of the CanadianShield now have Ca+2
concentrations that areconsidered sub-optimal for water fleas,
crayfishand other crustaceans and may be limiting thespecies
richness of lakes in this region.
C. Critical Loads – AcidifyingDeposition
Critical loads represent the deposition ratethat can occur
without surpassing tippingpoints for a given species or ecosystem
basedon established indicators and effect levels.The critical load
for a specific pollutant orgroup of pollutants will vary depending
on dif-ferences in landscape sensitivity and in theendpoints for
which the critical loads are cal-culated (e.g., forest soils, lake
chemistry).
Advances in understanding of chemical andbiological indicators
of acidification have sup-ported the development of critical loads
forsulfur and nitrogen in parts of the U.S. andCanada.
Table 2. Expected ecological effects and concern levels in
freshwater ecosystems at various levels of acid neu-tralizing
capacity (ANC). (Source: USEPA)a.
Category Label ANC level (µeq/L) Expected Ecological Effects
Low Concern >100 Fish species richness may be unaffected.
Reproducing brook trout populations are (No Effect) expected where
habitat is suitable. Zooplankton communities are unaffected and
exhibit expected diversity and distribution.
Moderate 50-100 Fish species richness begins to decline
(sensitive species are lost from lakes). BrookConcern trout
populations are sensitive and variable, with possible sub-lethal
effects. Diversity(Minimally and distribution of zooplankton
communities begin to decline as species that are sensi-Impacted)
tive to acid deposition are affected.
Elevated 0–50 Fish species richness is greatly reduced (more
than half of expected species are Concern missing). On average,
brook trout populations experience sub-lethal effects, including
(Episodically loss of health and reproduction (fitness). During
episodes of high acid deposition, brook Acidic) trout populations
may die. Diversity and distribution of zooplankton communities
declines.
Acute Concern
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Forests
U.S. researchers use models to develop criticalloads for forest
soil acidification (Box 4). Arecent study estimated the critical
acid loadsfor forest soils across the conterminous U.S.The critical
acid loads for S and N throughoutthe Appalachian Mountain Range and
Floridaare estimated to be less than 1,000 eq/ha/yr(critical loads
for combined sulfur and nitro-gen are expressed in terms of ionic
charge bal-ance as equivalents per hectare per year). Thisstudy
estimated that about 15% of U.S. forestsoils exceed their critical
acid load by at least25% including much of New England,
WestVirginia, and parts of North Carolina. Bycomparison, critical
load modeling in Canadaestimated that 30 to 40% of upland
forestareas in Canada are in exceedance of the criti-cal load for
acidification, while more than50% are in exceedance in eastern
Canada(Ontario, Quebec, New Brunswick, NovaScotia and
Newfoundland).
Surface Waters
Regional critical loads for surface waters havebeen developed
for acidifying deposition ofsulfur and nitrogen in sensitive
regions of theAdirondack Mountains of New York and inthe central
Appalachians of Virginia and WestVirginia. The median critical load
for a targetANC of 50 µeq/L is 129 milliequivalents persquare meter
per year (meq/m2/yr) in theAdirondacks and 45 meq/m2/yr in the
centralAppalachians with values ranging from less
than 0 to over 1,000 meq/m2/yr in relativelyinsensitive
ecosystems. The number of aquaticecosystems exceeding the critical
loads is stillquite high, but has declined with decreases inacid
deposition from the early 1990s to thelate 2000s (Figure 4).
Currently, 44% ofAdirondack lakes evaluated exceed the criti-cal
load and in these lakes recreationally valu-able fish species such
as trout are missing dueto acidification. In the Shenandoah area,
85%of streams evaluated exceed the critical loadresulting in losses
in fitness in fish species suchas the blacknose dace. The
persistence of criti-cal load exceedances despite
significantdecreases in SO2 emissions is related to con-tinued high
inputs of acidifying NOx, low ini-tial ANC conditions, and soil
depletion ofnutrient cations (Ca+2 and Mg+2) that haveleft many
watersheds more sensitive to aciddeposition over time.
A similar study of 2053 lakes in six north-eastern states and
four eastern Canadianprovinces estimated critical loads for
acidify-ing deposition of sulfur and nitrogen for a tar-get ANC of
40 µeq/L. Results show that 28%of the lakes studied have a critical
load in thecategories of ≤20 and 20–40 meq/m2/yr, sug-gesting
vulnerability to acidification with rela-tively moderate
atmospheric deposition. It isestimated that the critical load is
exceeded in12% of the study lakes, based on depositionlevels in
2002. These studies point to theimportance of long-term monitoring
andresearch for assessing the impact of emissionscontrol programs
on deposition and ecologicalrecovery (Box 5).
Box 4. UNDERSTANDING THE CRITICAL LOADS APPROACH
Critical loads, and other approaches that use models or
empirical observations to link deposition with effects, provide
tools that enableresource managers and policymakers to evaluate
tradeoffs between the costs of more stringent emissions controls
and the benefits ofecosystem services provided by healthy
ecosystems.
A critical loads approach can be used to synthesize scientific
knowledge about air pollution thresholds that cause adverse
impactsor ecosystem change. Describing air pollutant effects on
ecosystems in critical load terms quantifies estimates of “cause
and effect” ina way that allows researchers to communicate science
to air quality regulators and natural resource managers. Critical
loads are mostcommonly applied to evaluate the effects of nitrogen
and sulfur pollutants and their associated acidity or the
eutrophying effects ofnitrogen. When critical loads are exceeded
there is increased risk for a range of problems including ecosystem
acidification, excessnitrogen effects, declines in forest health,
and changes in biodiversity.
Critical loads are typically expressed as deposition loading
rates of one or more pollutants in amount per area per year (e.g.,
kilo-grams per hectare per year (kg/ha/yr)). Critical loads are
based on changes to specific biological or chemical indicators such
as speciescomposition of a given ecosystem (e.g., grassland) or
biotic community (e.g., understory plants or tree-dwelling lichens)
or acid neu-tralizing capacity (ANC) in soils, streams or lakes.
Because different sensitive receptors (e.g., forest soils, high
elevation lakes, speciesof lichen) or species may have varying
sensitivities to air pollutant loads, multiple critical loads can
be used to describe a continuum ofimpacts with increasing
deposition at a given location (See Figure 5).
In addition, even for the same organism, multiple critical loads
may be associated with biological thresholds for different
negativeeffects, such as stunted growth, reduced reproduction, and
increased mortality. Several different threshold levels may
therefore beincluded in a critical load assessment. The
policymaker, air regulator, or land manager can assess all the
critical loads (science-drivenecological thresholds) and select
target loads (policy thresholds) based on the level of ecosystem
protection desired, economic con-siderations, and stakeholder input
at a given location.
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
2. NITROGENA. Effects of Excess Nitrogen onEcosystems
The nitrogen gas that makes up most of theEarth's atmosphere is
inert, with little impacton ecosystems. Nitrogen converted to its
reac-tive forms such as NH3 and NOx, however,can cause profound
biological changes.Activities such as fertilizer
manufacturing,intensive livestock production and the burn-ing of
fossil fuels convert nitrogen to thesereactive forms which can then
enter andpotentially over-fertilize ecosystems. This canlead to
problems such as algal overgrowth inlakes, reduced water quality,
declines in foresthealth, and decreases in aquatic and
terrestrialbiodiversity by favoring “nitrogen loving”species at the
expense of other species withlow nitrogen preferences. For example,
mostestuaries and bays in the Northeast U.S. andMid-Atlantic
regions experience some degreeof eutrophication (where excess
nutrients pro-mote a proliferation of plant life, which candeplete
oxygen in the deeper waters), as aresult of nutrients from
atmospheric deposi-tion and agricultural, urban and
industrialrunoff. Excess nitrogen can also changespecies
composition. In Waquoit Bay,Massachusetts elevated nitrogen allows
tallcord grass to thrive but not eelgrass, whichdecreases critical
fish habitat.
Adding nitrogen to forests whose growth istypically limited by
its availability may appeardesirable, possibly increasing forest
growth and
timber production, but it can also haveadverse effects such as
increased soil acidifica-tion, biodiversity impacts, predisposition
toinsect infestations, and effects on beneficialroot fungi called
mycorrhizae. As atmosphericnitrogen deposition onto forests and
otherecosystems increases, the enhanced availabil-ity of nitrogen
can lead to chemical and bio-logical changes collectively called
“nitrogensaturation.” As nitrogen deposition from airpollution
accumulates in an ecosystem, a pro-gression of effects can occur as
levels of biolog-ically available nitrogen increase (Figure
5).Because of the multiple potential effects ofnitrogen deposition
in terrestrial and aquaticecosystems, the ecosystem services
affectedvary depending on the sensitive receptorsfound within a
given ecosystem and the levelof atmospheric deposition. Prominent
exam-ples of affected ecosystem services in forestsinclude timber
production, climate regulation,recreational use, and biodiversity
loss. In
Figure 4. Percentage of lakes inexceedance of the critical
loadfor sensitive eastern US surfacewaters in the Adirondacks
(169lakes in NY) and the centralAppalachians (92 streams in VAand
WV). The percent exceedingthe critical load has declined
asemissions and deposition havebeen decreasing (Source: JasonLynch-
USEPA).
Box 5. THE ROLE OF LONG-TERM MONITORING AND RESEARCH
Long-term studies measure baseline ecosystem conditions and
trends and can show how ecosystems respond when
atmosphericdeposition decreases below a threshold that was
previously exceeded. The trajectory of recovery is not always
consistent with modelsimulations, illustrating the importance of
long-term monitoring and research to improve the capabilities of
simulation models. A num-ber of regional- and national-scale air,
water, soil, and biota monitoring networks collect high-quality
data that are useful in assessingecosystem thresholds. However,
current efforts are not enough to provide continuous data at sites
across the country, and often lackthe coordination needed to
effectively combine datasets for maximum benefit. We recommend that
existing monitoring and researchprograms be continued, expanded and
better integrated. Some examples of federal monitoring programs
include:
• Federal agency air pollution monitoring programs such as the
Interagency Monitoring of Protected Visual Environments(IMPROVE)
http://vista.cira.colostate.edu/improve/ and the National
Atmospheric Deposition Monitoring Program
(NADP),http://nadp.sws.uiuc.edu/, and the Clean Air Status and
Trends Network (CASTNET) http://www.epa.gov/castnet/
• The U.S. Forest Service's Forest Inventory Analysis and Forest
Health Monitoring (FIA/FHM)
• The Environmental Protection Agency’s Temporally Integrated
Monitoring of Ecosystems/Long-term Monitoring (TIME/LTM)network
http://www.epa.gov/airmarkt/assessments/TIMELTM.html and National
Surface Water Surveys
• The U.S. Geological Survey's National Water-quality Assessment
Program (NAWQA) http://water.usgs.gov/nawqa/ andBiomonitoring of
Environmental Status and Trends (BEST) programs
• The U.S.Forest Service’s wilderness area surface
water-monitoring programs http://www.fs.fed.us/waterdata/
• The NSF-sponsored National Ecological Observation Network
(NEON) http://www.neoninc.org/ and Long-Term EcologicalResearch
(LTER) network - http://www.lternet.edu/
Appalachians
Adirondacks
% W
ater
Bo
die
s In
Exc
eed
ance
1989
-199
1
2006
-200
8
1989
-199
1
2006
-200
8
100
90
80
70
60
50
40
30
20
10
0
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
© The Ecological Society of America • [email protected] esa
freshwaters affected ecosystem servicesinclude recreational
fishing, other forms ofrecreation, and provision of high
qualitydrinking water (Table 1).
B. Indicators – Nitrogen Pollution
Nitrogen impacts on ecosystems can be identi-fied by examining
changes in biota, or by mea-suring chemical indicators. Ideally, a
chemicalindicator provides a warning suggesting thatsensitive biota
are at risk before biologicalharm occurs.
Nitrate Leaching
One of the most notable symptoms of nitrogensaturation is
increased leaching of nitrogenfrom soils into lakes, streams and
groundwater,primarily in the form of nitrate (NO3
–).Streamwater NO3
– concentration is a usefuland simple indicator of the nitrogen
status of acatchment because this measure integratesmany nitrogen
cycling processes that occurwithin the catchment, including the
process-ing of atmospheric nitrogen deposition. Innitrogen limited
ecosystems in the westernUnited States, U.S. Forest Service land
man-agers have set policy thresholds of 20µeq/L as a concern level
indicating potentiallyover-enriched systems. Studies in Europe
andthe northeastern U.S. show that nitrogenleaching begins to
increase in forests receiving
levels of atmospheric nitrogen depositiongreater than 8-12
kilograms of nitrogen perhectare per year (kg N/ha/yr), although
not allforests receiving those levels of depositionshow NO3
– leaching, due to land disturbancehistory, the presence of
wetlands and othercharacteristics.
Nutrient Ratios
Other commonly used chemical indicators ofnitrogen enrichment
include nutrient ratios infoliage such as nitrogen:phosphorus
(N:P),nitrogen:calcium (N:Ca+2), or carbon:nitrogen(C:N). C:N
ratios in organic or mineral hori-zons of the soil also indicate
ecosystem nitro-gen status or the predisposition to
nitrogensaturation. The C:N ratio of the soil and thegrowth rate of
the forest influence nitrogenleaching. Forests with soil C:N ratios
less than20-25 (indicating high N availability) aremore likely to
exhibit nitrogen leaching thanforests with higher C:N ratios.
Biological Indicators
Biological indicators of nitrogen over-enrich-ment or
eutrophication include shifts inspecies or biological communities,
enhancedestablishment of invasive species, or othermeasures of
biodiversity change (Box 6). Localextinction of sensitive species
or functionalgroups can also occur. The most sensitiveorganisms
exhibiting such changes in responseto nitrogen enrichment in
freshwater ecosys-tems such as lakes are small single-celled
algaeknown as diatoms. Even a small amount ofadditional nitrogen
deposition from air pollu-tion that is transported to waters can
inducemajor shifts in the species of diatoms. Eachdiatom species
has specific patterning in theshell, so by studying lake sediment
cores fromwestern lakes that extend back 100 years ormore,
researchers have been able to documentwhether recent diatom species
shifts corre-spond with changes in nitrogen
deposition.Tree-inhabiting lichens (epiphytes) are highlysensitive
indicators of nitrogen air pollution inforests and woodlands. The
lowest criticalloads for nitrogen effects are typically based
ondiatom or lichen community responses, mak-ing them good early
warning indicators forecosystem changes. Nitrogen also affects
thebiodiversity of herbaceous or grassland plantcommunities at
relatively low levels (Box 6).
Nitrogen effects on these biological indica-tors are linked to
changes in ecosystem func-
Figure 5. Continuum of nitrogendeposition impacts as
demonstrated from pastobservations and potential future
effects in Rocky MountainNational Park in Colorado. As
ecosystem nitrogenaccumulation continues,additional
acidification or
eutrophication impacts occur tovarious ecosystem receptors.Note
that the trajectory line is
conceptual even though theeffects below the current
nitrogen deposition level havebeen documented. Similar
trajectories of additionalecosystem effects as
nitrogenaccumulates in the ecosystem
likewise occur in other ecologicalregions. (Figure courtesy of
Ellen
Porter, National Park Service).
Synthesis: Continuum of Impacts in Rocky Mountain National
Park
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
tions including alteration of food webs,increased risk of fire,
and reduction in impor-tant nursery habitat for commercial
fisheries.For example, among the nitrogen sensitivelichen species
are those that are vital foragespecies for deer, elk, and the
northern flyingsquirrel; the latter is the major prey of the
fed-erally endangered spotted owl. Nitrogen depo-sition effects on
plant biodiversity can resultin major impacts on ecosystems,
includingenhanced invasion by exotic grasses, increasedfire danger,
vegetation type change, and disap-pearance of biological species
that depend ondeclining native plant species and communi-ties (Box
6). Finally, negative impacts to eel-grass beds from the
over-enrichment of coastalwaters can diminish the quality of
importantnursery, habitat, and feeding grounds for com-mercially
important fish and shellfish in theeastern U.S., such as
scallops.
C. Nitrogen Critical Loads
Air pollution thresholds at which excess nitro-gen effects on
ecosystems occur can be deter-mined using field studies or
estimated by mod-eling. Nitrogen critical loads can also beextended
over wide geographic areas or pre-dicted through time by use of
models. Critical
loads based on field observations across spatialgradients of
varying air pollution exposure orfrom field experiments are known
as empiricalcritical loads. Empirical critical loads for nitro-gen
deposition effects in selected ecoregions ofNorth America are
presented in Table 3.Recent studies demonstrate that exceedanceof
empirical critical loads for nitrogen is com-mon across the
U.S.
By comparing modeled and empirical criti-cal load values to
current and future deposi-tion data and estimates, policymakers
canassess current ecosystem condition, set goalsfor ecosystem
recovery, and track improve-ment. This information can also aid
decisionmaking processes for air pollution controls ormitigation
programs for damaged ecosystems.For example, the low end of the
critical loadrange in Mediterranean California mixedconifer forests
(3 kg N/ha/yr) describes thepoint where impacts begin in the most
sensi-tive parts of these ecosystems, specifically,changes in
lichen communities. Such a lownitrogen critical load provides a
‘canary in thecoal mine’ threshold that is indicative of ini-tial
ecosystem responses to added nitrogen.Ecoregions in the United
States where lichencommunities are likely affected by nitrogen
airpollution based on nitrogen deposition in
Box 6. BIODIVERSITY
Biological diversity (or “biodiversity”) may simply be defined
as the species richness of a geographic area. Biodiversity loss has
accel-erated in modern times due to land use change, the
introduction of invasive species and other disturbances. Climate
change and airpollution also contribute to changes in plant
community composition and biodiversity. In polluted regions, the
occurrence of sensitivespecies may decrease and lead to replacement
by pollution-tolerant species. When air pollution alters the
biodiversity or the composi-tion of biological communities,
detrimental effects on the provision of valued ecosystem services
can occur. The implications for biodi-versity shown by long-term
studies of acid deposition and nitrogen pollution are highlighted
in the case studies below.
Acid Rain: Diminishing Aquatic Diversity in the NortheastAquatic
organisms vary in sensitivity to acidity with sensitive species
showing limitations at pH 6.0 and many organisms declining
inabundance and richness at pH levels of 5.5 and lower. As acidity
increases, sensitive species or sensitive life history stages of
specieseither die or seek refuge in less-acidified habitats leaving
the original habitat less productive and diverse. The impacts are
most severein sensitive high elevation ecosystems that have
experienced chronic deposition. Of the 53 fish species recorded in
lake surveys in theAdirondack Mountains of New York, half are
absent from lakes with pH less than 6.0. Recreational fishes, such
as Atlantic salmon, tigertrout, bluegill, walleye and alewife, are
among those absent from low-pH lakes. These acidity effects can
extend further down the foodchain. In lakes of the Adirondacks and
the White Mountains (New Hampshire) an average of 2.4 zooplankton
species are lost with eachpH unit decrease. Long-term monitoring of
acidifying deposition and surface water chemistry confirm that
decreased emissions ofacidic pollutants have resulted in lower
deposition and some recovery in pH. The long and complex process of
biological recoveryincluding the restoration of soil base
saturation is only just beginning.
Nitrogen Pollution, Plant Communities and Biodiversity in
CaliforniaBiodiversity of plant communities is sensitive to N added
by air pollution. Nitrogen-loving species are often favored and
increase inprominence as ecosystem nitrogen availability increases.
Forests and woodlands in many regions of the world show large
changes inepiphytic lichen communities in response to chronic
atmospheric nitrogen deposition. These lichen community impacts
occur at nitro-gen pollution thresholds as low as 3-6 kg/ha/yr.
Ecologically important lichen species have been eliminated from
forests over largeareas of California. Similarly, in coastal
central California, native serpentine grassland plant communities
are exterminated when the Ndeposition load is 6 kg/ha/yr or greater
and are replaced by exotic grasses and other native plant species.
These changes result in theloss of the plant species needed for
reproduction and survival of the endangered Bay Checkerspot
butterfly, and has caused local pop-ulation extinctions of the
butterfly. Likewise, nitrogen from air pollution at levels around 8
kg/ha/yr in desert and scrub vegetation com-munities of California
favors the growth of exotic annual grasses, crowding out native
species and increasing fire risk in some areas.
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
© The Ecological Society of America • [email protected] esa
exceedance of the critical load for licheneffects are shown in
Figure 6.
Recent research has shown that by stimulat-ing increased growth
of non-native grasses,nitrogen deposition may increase the
fre-quency of wildfires in southwestern U.S. desertareas because
these grasses provide fuel to sus-tain the spread of fire in areas
with little or noprevious fire history. Simulation models
haveestimated that the lowest threshold level ofnitrogen that
initiates these changes inpinyon-juniper ecosystems is about 3
kgN/ha/yr of nitrogen deposition. Fire riskincreases exponentially
above this level toabout 5.7 kg N/ha/yr, at which level grasses
aregenerally fully established. This provides anexample of how
policymakers or stakeholderscould select a variety of policy
thresholds (3.0,5.7 kg/ha/yr, or levels in between) dependingon
risk tolerance and goals for protectingnative vegetation in
different types of ecosys-
tems. Nitrogen deposition currently exceedsthese ecological
thresholds in many southwesternU.S. ecosystems, indicating the
importance ofthis information in air pollution policy or
landmanagement decision making in this region.
Another example of nitrogen depositioneffects relative to
ecological thresholds occurs inthe Colorado Rocky Mountains. In
this region,alpine vegetation has begun to shift toward ahigher
proportion of grasses. This shift occursat a threshold of around 4
kg N/ha/yr, whichapproximates current nitrogen deposition lev-els
in areas of the Rockies most influenced byagricultural and urban
emissions. Theresponses of individual plant species to nitro-gen
(critical load of 4 kg N/ha/yr) is a muchmore sensitive indicator
of nitrogen effectsthan soil acidification (critical load of 10-15
kgN/ha/yr). Nitrogen deposition also has initi-ated shifts in
diatom species to those that favorhigher nitrogen levels in some
high-elevation
Table 3. Critical Loads (CL) of Nitrogen Deposition for Effects
on Selected NorthAmerican Ecosystems
Ecosystem Chemical or Biotic Response CL for N Deposition(kg
N/ha/yr)
Arctic Tundra Plant community change; grass growth 1-3
Arctic Tundra Changes in shrub, bryophyte, lichen cover 6-11
Boreal Shrublands Decrease in shrub cover; increase in grass
cover 6
Northern Forests Change in soil community structure 5-7
Northern Hardwood Increased surface water nitrate leaching 8and
Coniferous Forests; Eastern Temperate Hardwood Forests
Northwest Forested Lichen community change from oligotrophic
3-5Mountains and to eutrophic species dominanceMediterranean
California Mixed Conifer Forests
Alpine Changes in herb and grass species 4-10composition
Great Plains Tall Grass Change in biogeochemical N cycling,
plant and 5-15Prairie insect community shifts
Mediterranean California Increased surface water nitrate
leaching 17Mixed Conifer Forests
Mediterranean California Native herbs replaced by annual
grasses; 6Serpentine Grasslands loss of checkerspot butterfly
habitat
Rocky Mountain Western Freshwater eutrophication 2Lakes
From: Pardo, L.H., Robin-Abbott, M.J., and Driscoll, C.T., eds.
2011. Assessment of nitrogen deposition effects and
empiricalcritical loads of nitrogen for ecoregions of the United
States. Gen. Tech. Rep. NRS-80. Newtown Square, PA: U.S.
Departmentof Agriculture, Forest Service, Northern Research
Station.
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© The Ecological Society of America • [email protected] esa 13
ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
lakes at levels of 1.4 to 1.5 kg N/ha/yr as wetdeposition in
national parks and Class IWilderness areas in Colorado, the
greaterYellowstone Ecosystem, and the eastern SierraNevada
Mountains. This threshold is amongthe lowest identified for any
ecosystem changesresulting from nitrogen deposition, makingdiatoms
ideal early warning indicators ofdecline in aquatic ecosystem
condition fromnitrogen air pollution. Current nitrogen depo-sition
levels are higher than these thresholdsfor most of the western
U.S.
3. MERCURY POLLUTIONA. Effects
Mercury is a naturally occurring metal and alocal, regional and
global pollutant. Mercuryemissions from electric utilities,
incinerators andindustrial manufacturing are among the
largestsources of mercury to the environment in theU.S. Mercury in
the air and water are not directpublic health risks at levels
commonly found inthe U.S. The risk to human and ecologicalhealth
typically occurs through consumption ofmercury-contaminated fish
and other biota.Inorganic mercury (Hg) is deposited to
thelandscape, transported from soils to wetlandsand surface waters,
and converted by bacteria tomethylmercury (MeHg) – the organic form
ofmercury that is readily absorbed by fish andother organisms. Once
ingested, mercury canbioaccumulate in organisms and
biomagnifythrough the food web to elevated concentra-tions in fish
and other organisms that are con-sumed by people and wildlife. Fish
contamina-tion by MeHg poses a widespread problem infreshwater, and
in coastal and marine recre-ational and commercial fisheries.
As mercury sampling in lakes and rivers hasexpanded, the extent
of waters known to beimpaired by mercury pollution has increased.
In2008, all 50 states, one U.S. territory, and threeNative American
tribes issued mercury advi-sories for human fish consumption
covering16.8 million lake acres and 1.3 million rivermiles. That
was a 19% increase in lake areaunder advisory and a 42% rise for
rivers com-pared to 2006. The number of statewide mer-cury
advisories for coastal waters increased from12 in 2004 to 15 in
2008. These increases likelydo not reflect increases in Hg
deposition, butrather increases in measurements documentingthe
widespread nature of mercury contamina-tion. In addition to
contamination of fish,MeHg poses risks to fish-eating wildlife such
asloons, mink, eagles and otter. MeHg concentra-
tions can also be elevated in organisms thatfeed on aquatic
insects, such as songbirds andbats, and in organisms that dwell in
wetlandsand upland environments such as the Bicknell’sthrush – a
migratory songbird that breeds in theforested mountains of the
Northeast.
B. Indicators - MercuryIndicators for Human and
EcologicalHealth
Mercury concentrations in fish and other ani-mals routinely
exceed human and wildlifehealth levels. Human health indicators
formercury are based on the concentration ofMeHg in fish tissue
that is considered safe forthe average consumer (Figure 7). The
U.S.Environmental Protection Agency has recom-mended a human health
criterion of 0.3 partsper million (ppm) in fish tissue which
repre-sents the maximum advisable concentration ofMeHg in fish and
shellfish that protects theaverage consumer among the general
popula-tion. Particularly sensitive groups of peopleincluding women
in child-bearing years andchildren under 12 years of age are
advised tolimit consumption to fish low in mercury.Many states have
set even more stringent
Figure 6. Lichen Based CriticalLoad Exceedance Map. Areasshown
in red and orangereceived atmospheric nitrogendeposition at levels
deleteriousto communities of epiphytic(tree dwelling) lichens. This
mapshows that these effects occurin over half of the forested
landarea, including urban forests, ofthe continental U.S. Levels
ofcertainty in the critical loadexceedance estimates varyamong
ecoregions dependingon the amount of availablelichen community
data. (FromPardo, L.H. and others 2011.Effects of nitrogen
depositionand empirical nitrogen criticalloads for ecoregions of
theUnited States. EcologicalApplications,doi:
10/1890/10-234.1).
Exceedance of Critical Loads of N Uncertainty
Below CL Reliable
At CL Fairly Reliable
Above CL min Expert Judgement
Above CL max
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
© The Ecological Society of America • [email protected] esa
human health protection levels. Maine andMinnesota use 0.2 ppm
as the human healththreshold, as does Canada.
Ecological effects thresholds for mercury aregenerally based on
the concentrations of mer-cury in the tissue, blood, or diet (often
fish) ofan organism that are associated with adverseimpacts.
Adverse impacts to biota from mer-cury exposure include reduced
reproductivesuccess, decreased egg incubation time andother
behavioral changes, and neurologicalproblems such as the loss of
movement knownas ataxia. Several ecological effects thresholdshave
been defined in the literature and manyare lower than human health
effect thresh-olds. For example, reproductive effects in
fish-eating birds are reported at levels of 0.16 ppmin their prey
fish. Significant adverse repro-ductive impacts on loons are
commonly citedat the threshold of 3.0 ppm in loon blood.
These indicators of human and ecologicaleffects can be used to
assess and communicaterisk, to determine the presence of
biologicalmercury hotspots where concentrationsexceed established
thresholds, to establish tar-gets for critical loads estimates, and
to assessthe effectiveness of mercury emissions reduc-tions on
target species.
Chemical Indicators of FreshwaterSensitivity to Mercury
There is large variation in the degree to whichmercury deposited
onto the landscape will betransported to lakes and streams
throughdrainage waters and converted from inorganicmercury to MeHg
by bacteria in soils, wet-lands and lake and river sediments. The
rateof methylation by these bacteria is affected bypH, sulfur and
dissolved organic carbon(DOC) concentrations and, in Canadianshield
lakes, was found to increase with
increasing water temperatures. Once con-verted to the MeHg form,
mercury can bioac-cumulate in individual organisms and biomag-nify
in the food web. As a result, total mercuryand MeHg concentrations
in surface watersmay not correlate well with mercury
concen-trations in biota, such as fish. In areas wheremercury
deposition is low or moderate, levelsin fish and wildlife may be
disproportionatelyhigh if conditions are conducive to MeHg
pro-duction and bioaccumulation. This has beenobserved in some
Alaskan ecosystems, such asNoatak and Gates of the Arctic
NationalParks, in Kejimikujik National Park of NovaScotia, and
large areas of the Northeastincluding the Adirondacks.
In recent years scientists have turned theirattention to
understanding the specific condi-tions that make an ecosystem
sensitive to mer-cury. Sensitive systems may be more efficientat
converting inorganic mercury to MeHg ormore efficient at
bioaccumulating mercury ateach level in the food chain. In general,
acidicecosystems with low productivity and high sul-fate and DOC
tend to be sensitive to mercuryinputs and to exhibit higher fish
mercury con-centrations. Several of these chemical indica-tors are
influenced by inputs of acidifyingdeposition leading to interactive
effectsamong two or more atmospheric pollutants. Astudy of
freshwater ecosystems in the north-eastern U.S. used the following
chemical indi-cators of mercury sensitivity: total phosphorus,DOC,
ANC, and pH (Table 4). These waterchemistry indicators provide
managers with ameans for evaluating where MeHg concentra-tions in
fish are likely to be high and can helpprioritize monitoring and
assessment efforts.
Watersheds particularly sensitive to mercuryare more commonly
found in the southern andeastern U.S., Great Lakes, and isolated
areasin the western U.S. The sensitive regionsshown in Figure 8
were identified based on thephysical and chemical characteristics
of awatershed that cause it to convert inorganicmercury to MeHg at
a higher rate than otherwatersheds.
Recovery from mercury deposition has beenstudied in watersheds
where emission controlshave been implemented for large sources
suchas municipal waste incinerators. Studies fromsouthern New
Hampshire suggest that eventhough mercury is persistent and
bioaccumu-lates in the environment, decreased inputsfrom local
sources have been accompanied bydecreased concentrations in top
predatorsincluding fish and loons. In New Hampshire
Figure 7. Fish tissue mercuryconcentration ( ppm; same asµg/g)
across the U.S. All data
standardized to 14 inchlargemouth bass, skin-off fillets.(Figure
is derived from a modeland national dataset described
in: Wente, S.P., 2004. U.S.Geological Survey Scientific
Investigation Report2004-5199, 15 p.).
Fish-tissue MercuryConcentration (ppm)
states
1.0
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
mercury emissions upwind of a biological mer-cury hotspot
declined by 45% between 1997and 2002. Mercury concentrations in
yellowperch and loon blood in the region declined32 and 64%
respectively between 1999 and2002 – rates much greater than
observed else-where in the region. These results suggest
thatreduced emissions and deposition of mercuryfrom local and
regional sources are needed torestore healthy wildlife and safe
fisheries andthat return to levels consistent with humanhealth
criterion is likely to occur withindecades, not centuries. However
continuedmonitoring is essential in light of
increasingcontributions of mercury from global sourcesand the need
to better understand potentialinteractions with other pollutants
and withclimate change.
C. Mercury and Critical Loads
Science-based air pollution thresholds andcritical loads for
mercury are not as well estab-lished as those of sulfur, nitrogen
and acidity.Efforts are underway to develop and refinecritical
loads for mercury by investigating thelinkage between atmospheric
deposition lev-els, methylation processes and chemical
andbiological thresholds for human and ecologicalhealth. Critical
loads for mercury and othertrace metals have been estimated for
parts ofEurope using a set of very general assumptionsrelating
atmospheric Hg to Hg concentrationsin groundwater, food crops, and
aquaticorganisms. Available data for calculatingexceedances are
quite limited. However theresults suggest that a large part of the
land-scape (approximately 50%) exceeds mercury
critical loads for ecosystem effects. Current understanding of
the links between
mercury emissions and deposition and biologi-cal responses in
humans and ecosystems ishampered by a lack of consistently
collectedlong-term data on mercury levels in waters,soils, and
biota. Mercury monitoring effortsvary widely among states and are
difficult tointegrate and synthesize to establish responsepatterns.
In 1996 the Mercury DepositionNetwork (MDN) began measuring
mercurydeposition in precipitation (wet
deposition)(http://nadp.sws.uiuc.edu/mdn/) and currentlyincludes
115 sites in the United States andCanada as part of the National
AtmosphericDeposition Program (NADP). In 2009, theAtmospheric
Mercury Network (AMNet) alsojoined NADP and includes 21 sites that
trackthe concentration of different forms of mer-cury in
precipitation. There is further need fora comprehensive
environmental mercurymonitoring network.
USING AIR POLLUTIONTHRESHOLDS IN POLICY &MANAGEMENT
Ample evidence exists to advance the wideruse of air pollution
thresholds in policy, man-agement and regulatory issues. Policy and
reg-ulatory decisions in response to air pollutantemissions are
based on many economic, politi-cal, human health, environmental,
scientificand sociological considerations and tradeoffs.Air
pollution thresholds can be used to helpevaluate and monitor these
tradeoffs and sev-
Table 4. Indicators of Surface WaterSensitivity to Mercury. The
followingthresholds are associated with con-centrations of
methylmercury in yel-low perch greater than 0.3 ppm inlakes in the
northeastern U.S.
Indicator Threshold
Total phosphorus 4 mg/L
Surface water pH
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
cussed in the recent US-Canada ProgressReports, which detail
progress achieved inimplementing the US-Canada Air
QualityAgreement. These assessments rely on ecosys-tem element
cycling models from which criti-cal loads can be estimated and the
effects ofthe Clean Air Act and other emissions reduc-tions laws
and policies can be evaluated.Long-term measurements have
generallyshown improvements in some surface-waterindicators of
acidification, such as sulfate con-centrations and pH, in the
Northeast U.S.over the past 30 years. Most of these watershave not
recovered to pre-acidification condi-tions, and many remain in
excess of criticalload thresholds.
Dynamic ecosystem modeling can be usedto simulate likely
ecosystem responses in aspecific year to future emissions
reductions orincreases. Recent modeling work simulatesthat future
decreases in SO2 and NOx emis-sions of greater than 50% (relative
to clean airlaws implemented as of 2005) will be neces-sary to
decrease the number of chronicallyacidic lakes in the Adirondacks
by one-thirdto one-half by the year 2050. Despite
theseimprovements, model results indicate thatmany of the currently
chronically acidic lakeswill improve only to an episodically acidic
sta-tus, so the net change in the sum of chronicplus episodically
acidic lakes is likely to stayabout the same or improve slightly
dependingon the extent to which emissions decrease.This work shows
how models can simulatewhether emissions policy goals for
ecosystemrecovery are likely to be met within a specifictime
frame.
Finally, critical loads can inform the devel-opment of national
air quality standardsknown as the “secondary standards” that
areaimed at protecting environmental resourcesfrom air pollution.
The Clean Air Act requiresEPA to set national air quality standards
for sixcriteria pollutants (nitrogen oxides, sulfuroxides (SOx),
particulate matter, ozone, carbonmonoxide, and lead) based on
primary (health-based) and secondary (welfare-based)
consider-ations. “Welfare” includes consideration ofenvironmental
harm. The law also requiresEPA to periodically review the
scientific crite-ria upon which the standards are based.
While EPA generally reviews criteria andstandards for each of
the six criteria pollutantsindividually, EPA decided to jointly
examineNOx and SOx compounds in a recent review ofthe secondary
standards. In a policy assess-ment completed as a part of the
NOx/SOx sec-
eral examples of their effective applicationexist in the U.S.,
Europe and Canada.
1. U.S. Policy Use of Thresholds
Although the 1970 U.S. Clean Air Act man-dates protection of
human health and welfare(which includes ecological effects),
neitherthe Clean Air Act nor its 1990 amendmentsspecifically
mandates a critical loads approachfor addressing air pollution.
Class I areas aredesignated federal wilderness areas that weregiven
special protection from degradation byair pollution under the Clean
Air ActAmendments of 1977. It is becoming increas-ingly evident
that critical loads for effects onterrestrial and aquatic
ecosystems areexceeded in many Class I areas, even thoughthe human
health-based standards for NO2and SO2 are rarely exceeded in these
areas.State and federal environmental and regula-tory agencies and
multi-stakeholder organiza-tions are increasingly turning to
critical loadsas a type of threshold that can aid in thedevelopment
of air quality standards, theassessments of emissions regulations,
and otherpolicies aimed at protecting or improvingecosystem
condition. In Rocky MountainNational Park in Colorado, the critical
load fornitrogen deposition impacts on aquatic diatomcommunities
provides the basis for a nitrogendeposition goal to achieve
resource protection.The National Park Service, the State ofColorado
Department of Public Health andEnvironment, the U.S.
EnvironmentalProtection Agency, and interested
stakeholderscollaborate in the Rocky Mountain NationalPark
Initiative to develop strategies to reduceair pollutant emissions
that contribute tonitrogen deposition in the Park.
The EPA recently started using critical loadsto describe
threshold effects in its annual AcidRain Progress Reports. In the
2009 Acid RainProgress Report, critical loads for acid deposi-tion
were calculated for over 1,300 lakes andstreams in the Northeast
and Mid-AppalachianHighlands regions of the eastern United
States.By comparing critical loads to deposition databefore and
after implementation of the AcidRain Program, it was determined
that 37% oflakes and streams in those regions where atmos-pheric
deposition was in exceedance of the crit-ical load in the 1989-91
period were no longerreceiving sulfur and nitrogen deposition
loadsin 2007-2009 that threatened the health ofthese
ecosystems.
Critical loads are also presented and dis-
-
ondary standard review, EPA staff found thatalthough the current
secondary standardsserve to protect vegetation from direct
damageassociated with exposures to gaseous SO2 andNO2, “currently
available scientific evidenceand assessments clearly call into
question theadequacy of the current standards with regardto
deposition-related effects on sensitiveaquatic and terrestrial
ecosystems, includingacidification and nutrient enrichment”(USEPA
2011). They further conclude that“consideration should be given to
establishinga new ecologically relevant multi-pollutant,multimedia
standard to provide appropriateprotection from deposition-related
ecologicaleffects of oxides of nitrogen and sulfur on sen-sitive
ecosystems with a focus on protectingagainst adverse effects
associated with acidify-ing deposition in sensitive aquatic
ecosystems”(USEPA 2011). Finally, the policy assessmentrecommends
the use of a critical loadsapproach in establishing and monitoring
thissuggested ecologically relevant standard.
The conclusions of the EPA staff policy doc-ument were supported
in a review by the inde-pendent Clean Air Science AdvisoryCommittee
(CASAC), which advises EPA onscientific issues related to Clean Air
Actimplementation. Similar to the CASAC find-ings, the data and
information presented heresupport the advance of secondary air
qualitystandards to enhance recovery of sensitiveecosystems from
acidifying atmospheric nitro-gen and sulfur pollution. In
particular, sec-ondary standards for NOx and SOx could bedeveloped
using a critical loads approach tolink pollution concentrations in
the air withdeposition and ecological effects based onestablished
indicators for surface water chem-istry and biology. In addition,
the wealth ofresearch-scale data on thresholds and criticalloads
becoming available for U.S. ecosystemssupport the exploration of
other collaborativepolicy and management approaches to
strate-gically reduce pollution in areas where demon-strated
impacts exist. Such a process was usedin Colorado, as discussed
above.
2. European Policy Use of Thresholds
European scientists and policymakers haveused a critical loads
approach for addressingnitrogen and sulfur effects in ecosystems
since1994. The air pollution abatement strategiesunder the European
Union Convention onLong-range Transboundary Air Pollution andunder
the European Commission’s Thematic
Strategy on Air Pollution are linked to acidityand nitrogen
critical loads as the basis fornegotiating national emissions
maxima.Meanwhile, ecosystem monitoring has demon-strated that
pollution abatement efforts havedecreased acidification, and to a
lesser degree,nitrogen levels in ecosystems. Once
scheduledemissions controls are completely imple-mented, European
areas that exceed acidifica-tion critical loads will be reduced
from 93 mil-lion hectares in 1990 to an estimated 15million
hectares.
In Europe, application of critical loads hasconnected science to
policy by providingmethodologies for using scientific evidence
todefine pollution limits and to assist in settingemission control
targets within a broad multi-nation policy framework. As a result,
nitrogencritical loads have been developed for NO3
–
leaching from forests and for changes toEuropean plant species
diversity for mostmajor vegetation types found in Europe.Similarly,
acidification critical loads havebeen developed to protect
terrestrial ecosys-tems and thousands of lakes and streams.Critical
loads for mercury, lead and cadmiumhave also been developed in
Europe based onendpoints (e.g., human health and ecosystemfunction)
and indicators (metal concentrationsin soil, food crops, and
biota).
3. Canadian Policy Use of Thresholds
Soil nutrient declines caused by acidifyingdeposition have
resulted in extensive timber
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
Figure 9. Equipment usedto monitor air quality atGreat Smoky
MountainsNational Park.
-
productivity loss in Atlantic Canadian forests.Fish declines in
lakes and rivers of easternCanada have significantly impacted
theCanadian recreational fishing industry.Concern about
acidification impacts toecosystem services (Table 1)
motivatedCanadian policymakers to apply critical loadswhen
establishing regional and, later, nationalemissions reduction
policies. Sulfur-based crit-ical loads that initially served as the
basis foremission control policy have been loweredover time as
understanding of air pollutioneffects on forests and surface waters
expanded.Canada periodically reviews its critical loadsto ensure
that they remain consistent with thelatest scientific information
and policyrequirements. Since the 1990s both S and Nare considered
in critical load analyses,although airborne S pollutants continue
to bethe predominant, anthropogenic acidifyingagent in Canada.
The national emission control policy agreedto in 1998 by the
federal and all provincialand territorial governments is called
theCanada-Wide Acid Rain Strategy for Post2000. It seeks to meet
the environmentalthresholds of critical loads for acid
depositionacross Canada; decreasing SO2 emissionswhere needed to
meet critical loads, and mini-mizing growth in emissions of both
SO2 andNOx where acid deposition is currently belowcritical loads.
As of 2008, the application ofthis policy has decreased SO2
emissions inCanada 63% below1980 levels. Based on criti-cal loads
projections indicating that acid rainwill continue to damage
sensitive ecosystemseven after full implementation of
currentCanadian and U.S. control programs, furtheremission controls
will be needed.
4. Global Policy Use of Thresholds
Fifty-one European countries, Canada, andthe U.S. participate in
the United NationsConvention on the Long-Range Transport ofAir
Pollutants (CLRTAP). Under this treaty,the Gothenburg Protocol to
AbateAcidification, Eutrophication and Ground-level Ozone went into
effect in 2003, and estab-lished more stringent emissions targets
for SO2,NOx, NH3, and other pollutants. UnderCLRTAP, European
countries compare criticalloads of S and N deposition for
acidification andeutrophication to estimates of current deposi-tion
as a means of evaluating the effectivenessof emissions control
measures. Countries areencouraged to work together to reduce
harmful
impacts to surface waters, soils, and vegetation.In addition to
Europe, both Canada and theUnited States have ratified CLRTAP based
onan understanding of the global nature of air pol-lution transport
and the need to reduce air pol-lution impacts.
There is currently no mechanism in placefor decreasing global
sources of mercury.However, the February 2009 United
NationsEnvironment Program Governing Council,including the U.S.,
China, India and 138other countries established a process to
createa legally binding agreement to control globalmercury
pollution by 2013. Negotiationsstarted in June 2010, and will
address a broadset of issues such as decreasing mercury emis-sions
from human sources, improving manage-ment of mercury in the waste
stream and atstorage sites, curbing demand for mercury inproducts,
decreasing mercury mining, remedi-ating contaminated sites, and
enhancingglobal monitoring. Once the framework isestablished,
participating nations must eachratify the treaty in order for it to
go into effect.The impact of this framework will be assessedover
time based in part on the extent to whichfish tissue mercury
concentrations havedeclined to below human health thresholds.
ADVANCING THE USE OF AIRPOLLUTION THRESHOLDS INPOLICY
Nitrogen, sulfur, and mercury pollution arealtering the Earth’s
ecosystems. Coupled withother stressors including land use
changes,invasion of exotic species, and climate change,these
pollutants are threatening the provisionof ecosystem services such
as air and waterpurification, waste detoxification, climate
reg-ulation, soil regeneration, biodiversity mainte-nance, and
production of crops, timber, andfish. Ongoing and future projected
climatechange is likely to become an increasing envi-ronmental
stressor in coming decades.However, little research to date has
quantita-tively explored the interaction of climatechange with the
effects of nitrogen, sulfur, andmercury pollutants in
ecosystems.
In the face of large-scale global change, nat-ural resource
managers, air regulators, andpublic stakeholders need to know
whetheremissions controls are effective in producingthe ecosystem
benefits that were anticipated.While scientists can often determine
an airpollution threshold where ecological change islikely to occur
and define the nature and
© The Ecological Society of America • [email protected]
ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
18 esa
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degree of change, decision makers must weigha number of
scientific and societal considera-tions in deciding which ecosystem
changes areof concern and what level of protection isdesired to
address these concerns. Further-more, long-term monitoring programs
fortracking trends in air pollution, soil and waterchemistry, and
aquatic and terrestrial biota area critical component for
protecting naturalresources and for the development, refine-ment,
and application of ecological and policythresholds.
This interaction between science and deci-sion-making often
proceeds iteratively. Onceecosystem components that should be
pro-tected (sensitive receptors) have been identi-fied, scientists
can identify response thresholds(e.g., critical loads) for those
components, landmanagers and stakeholders can determinedesired
protection levels (policy thresholds suchas target loads), and
environmental regulatorscan evaluate tradeoffs to determine
whetheremissions controls are warranted to achievethese goals.
Field studies and modeling canhelp by linking potential threshold
responses(ecological thresholds) to stressors. Moni-toring is
essential to determine whether thedesired response is achieved.
The National Ambient Air Quality Standards(NAAQS) for air
pollutants such as NOx andSOx are based on concentrations of these
pollu-tants in ambient air rather than on depositionlevels
experienced by ecosystems. Scientificprogress has improved our
ability to relate ambi-ent air concentrations to atmospheric
deposi-tion inputs and effects through the estimation ofcritical
loads. The secondary standards provide aframework for addressing
these issues and ampleevidence exists for applying existing
researchand modeling to the case of acidifying deposi-tion impacts
on sensitive aquatic ecosystems.Similar applications of secondary
standardstoward protection of terrestrial ecosystems fromthe
effects of nitrogen and sulfur pollutionwould also be of great
benefit. Nitrogen asammonia and ammonium (NH3 and NH4
+), areincreasingly important sources of nitrogen airpollution,
but are not regulated by EPA as crite-ria pollutants in the
NAAQS.
Air pollution thresholds based on scienceprovide a mechanism for
evaluating the extentto which ecosystem services have been
com-promised and for restoring impaired ecosys-tems. Establishing
priorities such as the levelsat which various ecosystem services
should bemaintained will require the mutual engage-ment of public
stakeholders, policymakers,
and scientists. Use of ecological thresholds forassessing the
impacts of air pollution on essen-tial ecosystem services and for
informing pub-lic policy is gaining ground. These
ecologicalthresholds provide a strong basis for develop-ment of
policy thresholds and offer a scientifi-cally sound approach to
protecting and restor-ing U.S. ecosystems.
For Further Reading
Aber, J.D., C.L. Goodale, S.V. Ollinger, M.-L.Smith, A.H.
Magill, M.E. Martin, R.A.Hallett, and J.L. Stoddard. 2003. Is
nitrogendeposition altering the nitrogen status ofnortheastern
forests? BioScience. 53: 375-389.
Burns, D.A., T. Blett, R. Haeuber, and L.H.Pardo. 2008. Critical
loads as a policy toolfor protecting ecosystems from the effects
ofair pollutants. Frontiers in Ecology and theEnvironment. 6:
156-159.
Driscoll, C.T., G.B. Lawrence, A.J. Bulger, T.J.Butler, C.S.
Cronan, C. Eagar, K.F.Lambert, G.E. Likens, J.L. Stoddard, andK.C.
Weathers. 2001. Acidic deposition inthe northeastern United States:
Sourcesand inputs, ecosystem effects, and manage-ment strategies.
BioScience. 51: 180-198.
Driscoll, C.T., et al. 2007. Mercury Matters:Linking mercury
science with public policyin the northeastern United States.
HubbardBrook Research Foundation. Science LinksTM
Publication. Vol. 1, no. 3, Hanover, NH.Dupont, J., T.A. Clair,
C. Gagnon, D.S.
Jeffries, J.S. Kahl, S.J. Nelson, and J.M.Peckenham 2005.
Estimation of criticalloads of acidity for lakes in
northeasternUnited States and eastern Canada.Environmental
Monitoring and Assessment.109: 275–29.
Evers, D.C., Y.-J. Han, C.T. Driscoll, N.C.Kamman, W.M. Goodale,
K.F. Lambert,T.M. Holsen, C.Y. Chen, T.A. Clair, T.J.Butler. 2007.
Biological mercury hotspotsin the northeastern United States
andsoutheastern Canada. BioScience. 57: 1-15.
Fenn, M.E., J.S. Baron, E.B. Allen, H.M.Rueth, K.R. Nydick, L.
Geiser, W.D.Bowman, J.O. Sickman, T. Meixner, andD.W. Johnson.
2003. Ecological effects ofnitrogen deposition in the western
UnitedStates. BioScience. 53: 404-420.
The Heinz Center. 2010. Indicators of EcologicalEffects of Air
Quality. The H. John Heinz IIICenter for Science, Economics and
theEnvironment. Washington, D.C. Availableonline at:
http://heinzctr.org/Programs/Reporting/Air_Quality/index.shtml
Lovett, G.M., and T.H. Tear. 2008. Threatsfrom Above: Air
Pollution Impacts on
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ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011
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Ecosystems and Biological Diversity in theEastern United States.
The Nature Con-servancy and the Cary Institute of Eco-system
Studies.
Pardo, L.H., M.J. Robin-Abbott, and C.T.Driscoll (eds.). 2011.
Assessment of Nitro-gen Deposition Effects and EmpiricalCritical
Loads of Nitrogen for Ecoregions ofthe United States. Gen. Tech.
Rep. NRS-80. Newtown Square, PA: U.S. Departmentof Agriculture,
Forest Service, NorthernResearch Station.
USEPA. 2009. 2008 Biennial National Listingof Fish Advisories.
EPA
823-F-09-007.http://water.epa.gov/scitech/swguidance/fishshellfish/fishadvisories/upload/2009_09_16_fish_advisories_tech2008-2.pdf
USEPA. 2009. Acid Rain and Related Pro-grams: 2008 Environmental
Results.
http://www.epa.gov/airmarkt/progress/ARP_3/ARP_2008_Environmental_Results.pdf
USEPA. 2011. Policy Assessment for theReview of the Secondary
National AmbientAir Quality Standards for Oxides ofNitrogen and
Oxides of Sulfur. EPA-452/R-11-005a. U.S. Environmental
ProtectionAgency, Office of Air Quality Planning andStandards
Health and EnvironmentalImpacts Division, Research Triangle
Park,North Carolina. 364 p.
http://www.epa.gov/ttnnaaqs/standards/no2so2sec/data/20110204pamain.pdf
Acknowledgements
Funding for this project was provided byPurchase Order
EP09H001132 from the USEnvironmental Protection Agency to
theEcological Society of America.
About the Scientists
Mark E. Fenn, USDA Forest Service, PacificSouthwest Research
Station, Riverside, CA92507Kathleen F. Lambert, Harvard
University,Harvard Forest, Petersham, MA 01366Tamara Blett,
National Park Service, AirResources Division, Denver, CO 80225
Douglas A. Burns, U.S. Geological Survey,Troy, NY 12180Linda H.
Pardo, USDA Forest Service, North-ern Research Station, Burlington,
VT 05403Gary M. Lovett, Cary Institute of EcosystemStudies,
Millbrook, NY 12545 Richard Haeuber, U.S. EnvironmentalProtection
Agency, Clean Air Markets
Division, Washington, DC 20460David C. Evers, BioDiversity
ResearchInstitute, Gorham, ME 04038Charles T. Driscoll, Department
of Civil andEnvironmental Engineering, SyracuseUniversity,
Syracuse, NY 13244Dean S. Jeffries, Environment Canada,National
Water Research Institute,Burlington, ON L7R4A6
Science Writing and Layout
Mark Schrope, Science writerBernie Taylor, Design and layout
About Issues in Ecology
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Colorado StateUniversity, [email protected].
Advisory Board of Issues inEcology
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Negra, the H. John Heinz III Centerfor Science, Economics, and the
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Service, Jornada Experimental RangeDavid S. Wilcove, Princeton
University Kerry Woods, Bennington College
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