《森林生态学》课程教学资源（生态学热点）Issues in ecology - 14 大气污染临界 Setting Limits - Using Air Pollution Thresholds to Protect and Restore U.S. Ecosystems.pdf_大学文库

© The Ecological Society of America • esahq@esa.org esa 1 ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 Setting Limits: Using Air Pollution Thresholds to Protect and Restore U.S. Ecosystems SUMMARY More than four decades of research provide unequivocal evidence that sulfur, nitrogen, and mercury pollution have altered, and will continue to alter, our nation’s lands and waters. The emission and deposition of air pollutants harm native plants and animals, degrade water quality, affect forest productivity, and are damaging to human health. Many air quality policies limit emissions at the source but these control measures do not always consider ecosystem impacts. Air pollution thresholds at which ecological effects are observed, such as critical loads, are effective tools for assessing the impacts of air pollution on essential ecosystem services and for informing public policy. U.S. ecosystems can be more effectively protected and restored 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 regulation, regeneration of soil fertility, production and biodiversity maintenance, as well as crop, timber and fish supplies are impacted by deposition of nitrogen, sulfur, mercury and other pollutants. The consequences of these changes may be 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 deterioration 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 sensitive areas receive acid deposition that exceeds critical loads. l Mercury contamination adversely affects fish in many inland and coastal waters. Fish consumption advisories for mercury exist in all 50 states and on many tribal lands. High concentrations of mercury in wildlife are also widespread and have multiple adverse effects. l Air quality programs, such as those stemming from the 1990 Clean Air Act Amendments, have helped decrease air pollution even as population and energy demand have increased. Yet, they do not adequately protect ecosystems from 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 thresholds. Existing monitoring programs track vital information needed to measure the response to policies, and could be expanded to include appropriate chemical and biological indicators for terrestrial and aquatic ecosystems and establishment 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 sulfur deposition, are not currently included in the formal regulatory process for emissions controls in the United States, although they are now considered in local management decisions by the National Park Service and U.S. Forest Service. Ecological thresholds offer a scientifically sound approach to protecting and restoring U.S. ecosystems and an important tool for natural resource management and 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 this publication under a Creative Commons Attribution license

2 esa © The Ecological Society of America • esahq@esa.org Introduction Natural ecosystems have been altered in various ways by nitrogen, sulfur, and mercury deposited in rain, snow, or as gases and particles in the atmosphere. Through decades of scientific research, scientists have documented how local, regional, and global sources of air pollution can produce profound changes in ecosystems. These changes include acidification of soils and surface waters, harmful algal blooms and low oxygen conditions in estuaries, reduced diversity of native plants, high levels of mercury in fish and other wildlife, and decreased tolerance to other stresses, such as pests, disease, and climate change. Advancing our understanding of the linkages among pollutant deposition rates or concentrations, ecosystem effects, and associated policy decisions is a priority in policy-relevant science in the U.S. Air pollutants that affect human health and ecosystems are primarily emitted from electric power generation, industrial, transportation, and agricultural activities. The benefits and necessities of these activities must be considered in light of the often detrimental effects of atmospheric emissions on human health, visibility, ecosystems, and on the services provided to society by these ecosystems (Table 1). The 1990 Clean Air Act Amendments and other air quality regulations have led to marked declines in emissions of nitrogen, sulfur and mercury. Some emissions from power generation and other sources have decreased by over 50% since the 1970s, even as population and energy demand have increased. As the emissions and deposition of most pollutants have declined, some impacted ecosystems have started to recover. In many parts of the country, however, ecological conditions are still declining due to the increase in other forms of pollution such as ammonia (NH3), the long term accumulation of sulfur and nitrogen compounds in soils, and the ongoing biomagnification of mercury in food webs. The purpose of this report is to distill advances in the science of air pollution thresholds and to describe their use to assess, protect and manage the nation’s ecosystems and the vital services they provide. We focus here on the environmental impacts of nitrogen, sulfur, and mercury and refer to connections to climate change. The discussion draws on the published research of hundreds of scientists over the past several decades with a focus on U.S. ecosystems and lessons from Canada and Europe. Air Pollution Thresholds Thresholds of air pollution in the U.S. have been widely discussed in the scientific literature since the 1970s, when research established that sulfur deposition was above levels at which damage occurs in many sensitive ecosystems in the eastern U.S. More recently, nitrogen deposition has been shown to impact sensitive ecosystem components and processes throughout the United States. Defining the specific concentration or deposition input of an air pollutant that will cause adverse or significant ecosystem effects has been the subject of much scientific research. Pollutants can accumulate with little noticeable impact on plants or animals until major changes occur as a tipping point is reached (Box 1). These changes are measured by scientifically determined chemical or biological indicators (Box 2). Such environmental changes might eliminate a single sensitive species, or a broad shift may occur in biodiversity throughout an ecosystem. Once a species or ecosystem has passed a tipping point, a return to the previous state may not be possible. Air pollution thresholds can be defined based strictly on scientific research (ecological thresholds) or based on a balance of policy conSetting Limits: Using Air Pollution Thresholds to 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

© The Ecological Society of America • esahq@esa.org esa 3 ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 siderations spanning law, economics, ecological effects, human health and risk assessment (policy thresholds) (Box 1) (Figure 1). One tool increasingly used to integrate the science and policy of air pollution thresholds for ecosystem protection and management is critical loads (Box 4). Advances in the Science of Air Pollution Thresholds Based on research over the past decade, a strong scientific foundation exists for defining air pollution thresholds using critical loads approaches (Box 4). In the following sections we synthesize the state of the science related to the ecological effects, key indicators, and critical loads approaches for acidifying deposition, nitrogen pollution and mercury contamination. 1. ACIDIFYING DEPOSITION A. Effects of Acidifying Deposition Acidifying deposition (or “acid rain”) is caused by emissions to the atmosphere of sulfur dioxide (SO2), nitrogen oxides (NOx), and other acidifying compounds such as ammonia (NH3)(see Box 3 for definition of chemical names and symbols). These pollutants return to Earth in rain, snow, fog, mist and gases in forms such as nitric and sulfuric acids and ammonium (NH4 + ) and can have long-term negative impacts to terrestrial and aquatic ecosystems. Ecosystems in the western U.S. have not been greatly affected by acidification because acidifying deposition is relatively low in much of the region and because in many arid or semi-arid regions the soils are relatively insensitive to acid inputs. Some high elevation 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 potential to acidify the receptor medium, such as soil or surface waters. Emissions of sulfur and nitrogen oxides and ammonia are the most common 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 surface 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 deposition occurs 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+ ), also referred 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 environment or medium (e.g., water, sediment). BIOMAGNIFICATION. The increase in concentration of a contaminant from lower trophic levels to higher trophic levels in the food chain. CRITICAL LOAD. The quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified 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 an ecosystem (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 endpoints are human health and forest sustainability. INDICATOR. A measurable physical, chemical, or biological characteristic of a resource that may be adversely affected by a change in air quality (e.g., ANC). NITROGEN SATURATION. Syndrome of effects occurring in an ecosystem caused by an overload of nitrogen, usually from long term atmospheric nitrogen deposition. POLICY THRESHOLD. A quantitative value of desired ecological condition established by policy and selected based on a balancing of 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 the critical 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

© The Ecological Society of America • esahq@esa.org esa 5 ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 Sierra Nevada Mountains do experience acidic episodes when pollutants retained in the snow pack over the winter are released into soils and streams during snowmelt. In the eastern United States, depletion of available calcium and magnesium pools and acidification of forest soils is widespread and well documented in the Appalachian Mountains, including the Catskills and the Adirondacks, and in the Shenandoah Mountain region of West Virginia. Mountain forests of the northeastern and southeastern United States receive high rates of acidifying deposition due to frequent exposure to acidic clouds, fog, rain and snow. Changes associated with acidifying deposition have reduced the ability of some tree species to cope with the cold temperatures common to these mountain environments. This effect contributed to large-scale red spruce deaths in these regions in the 1980s and 90s, and remains a problem today. In eastern U.S. hardwood forests at lower elevations, many sugar maple, white ash, flowering dogwood, and other trees have high calcium requirements and therefore are also sensitive to acidification. Tree declines have negative consequences for forest productivity and ecosystem services, including timber production and climate regulation (lower productivity means less removal of carbon dioxide from the atmosphere). Research has attributed sugar maple declines in western Pennsylvania to acidification acting in concert with insect outbreaks, and research in New Hampshire has shown improved growth and reproduction of sugar maple, and less frost damage to red spruce, when calcium was added to an acidified forest for experimental purposes. Acidification of sensitive surface waters has resulted in well documented adverse effects on fish, zooplankton, aquatic insects, microorganisms, and other aquatic biota. In many sensitive areas receiving elevated acidifying deposition, surface waters are too acidic to support any fish species. The reduction in the number of aquatic species and in the number of fish supported diminishes biodiversity and recreational fishing opportunities. Long-term research on acidification impacts on forests, lakes and streams has produced a wealth of data, from which are drawn the most commonly applied indicators for assessing acidification status and effects (Table 1). Although terrestrial and aquatic indicators are treated separately below, recognition should be given to the connection of soil acidification to aquatic acidification. B. Indicators - Acidifying Deposition Indicators of Soil Acidification and Forest Health One way to assess the risk to acid sensitive tree species such as red spruce and sugar maple is by tracking chemical indicators in the soil and 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 human health, scientists track chemical and biological indicators to monitor ecosystem health. When many different studies confirm an association between a pollutant amount and an ecosystem response, threshold pollutant levels can often be identified for indicators that signal likely problems. Chemical indicators are often used as surrogates for biological effects because chemical indicators are typically simpler and less expensive to measure. Chemical indicators are imperfect surrogates since accurate prediction of just how plants and animals will respond to chemical changes in their environment is not always possible. Box 3. CHEMICAL NAMES AND SYMBOLS, AND UNITS OF MEASURE Chemical Names and Symbols: Sulfur dioxide, SO2 Nitrogen dioxide, NO2 Nitrogen oxides, NOx Sulfur oxides, SOx Ammonia, NH3 Ammonium, NH4 + Mercury, Hg Methylmercury, MeHg Sulfate, SO4 -2 Nitrate, NO3 - Dissolved organic carbon, DOC Phosphorus, P Nitrogen, N Carbon, C Aluminum, Al+3 Calcium, Ca+2 Magnesium, Mg+2 Sodium, Na+ Potassium, K+ Calcium to aluminum ratio, Ca:Al pH, a measure of acidity or hydrogen ion concentration Nutrient ratios (e.g., N:P, N:Ca, C:N) Units of Measure: Equivalents per hectare per year, eq/ha/yr Kilograms per hectare per year, kg/ha/yr Parts per million, ppm Microequivalents per liter, µeq/L Milliequivalents per square meter per year meq/m2 /yr Micrograms per liter, µg/L Figure 1. Conceptual representation of how ecological and policy thresholds may be developed. Both lines show estimates of ecosystem degradation as pollutants increase in ecosystems. Line “A” represents a gradual decline in ecosystem condition, where managers, policy makers, and regulators can set policy thresholds at any number of different points depending on goals (for example, A1, at beginning of decline or A2, at midpoint of decline). Line “B” represents a rapid decline in ecosystem condition, with a clearly identified, ecological threshold at which a tipping point occurs (B1). A B A1 A2 B1 Supply of Ecosystem Services Deposition of Air Pollution

ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 6 esa © The Ecological Society of America • esahq@esa.org foliage) (Table 1). Three elements naturally present in soils, calcium (Ca+2), magnesium (Mg+2), and aluminum (Al+3), influence the extent to which trees and other plants may be adversely affected by acidifying deposition. Calcium and magnesium are nutrients needed for a variety of plant functions and their supply helps neutralize acid inputs to soils, whereas Al+3 can be harmful to plants at high concentrations when present in the readily available exchangeable form. Acid deposition slowly removes readily available exchangeable Ca+2 and Mg+2 from soils and replaces them with exchangeable Al+3 and hydrogen ion (or acidity), setting off a cascade of adverse changes. In general, greater availability of Ca+2 and Mg+2 and low Al+3 provides favorable conditions for many acid-sensitive tree species such as sugar maple and red spruce. Calcium to aluminum ratio (Ca:Al) in soils and soil solutions is one indicator used to assess the health risk to acid sensitive tree species such as red spruce and sugar maple. Soil percent base saturation is another useful indicator for assessing sensitivity and extent of acidification. Scientists generally concur that where soil percent base saturation is low there is a high risk of damage to the vitality of sensitive tree species due to nutritional deficits resulting from acidification. The risks to forest vegetation associated with a range of Ca:Al ratios and soil percent base saturation values are shown in Table 1. Other studies have focused on the concentration of exchangeable Ca+2 and Mg+2 as a useful indicator since soils can have widely varying amounts of these nutrients that are essential to the health of forest vegetation. Concentrations of Ca+2 and Mg+2 in the leaves and needles of plants (foliage) have recently been identified as valuable indicators for evaluating acid deposition impacts. For example, low concentrations of these nutrients have been identified as limiting the growth of sugar maple (Table 1). Indicators of Acidification in Aquatic Ecosystems Indicators of acidification in lakes and streams are generally based on changes in water chemistry. Water chemistry strongly affects the numbers and types of aquatic organisms that are present in a water body. The indicators most commonly used to track changes in surface water acidification are ANC, pH, and/or concentrations of key elements. Acid neutralizing capacity (ANC) is a commonly used chemical indicator of lake or stream sensitivity to acidification. ANC, measured in microequivalents per liter (µeq/L; See Box 3 for a list of chemical units of measure), characterizes the ability of water to neutralize strong acids including those introduced by atmospheric deposition. ANC is a good general indicator of acidity-related water quality because values are typically strongly correlated with pH, Al+3 concentrations, and Ca+2 concentrations. Specific concern levels have been identified and are used to estimate critical loads (Table 2). The diversity of fish species declines precipitously with decreases in ANC in Adirondack Lakes (Figure 2). In Shenandoah National Park (Virginia) streams researchers found that one fish species, on average, is lost for every 21 µeq/L decline in ANC. Recent studies have demonstrated that another useful chemical indicator is base cation surplus. Low values indicate that the soil has become sufficiently acidified to enable toxic forms of aluminum to be transported from the soil into streams at concentrations of concern. The pH value of a water body is a fundamental measure of acidity or the hydrogen ion concentration. A pH of 7 is neutral, and pH values below 7 are increasingly acidic while values above 7 are increasingly basic or alkaline. Like ANC, decreases in pH are associated with decreases in the richness of aquatic species (Table 1). Studies have shown that in lakes of the Adirondack Mountains of New York and the White Mountains of New Hampshire, one fish species is lost for every pH decline of 0.8 units as values decrease from 6 to 4. Few fish species can survive at pH values of 4 or less (Figure 3). Figure 2. Number of fish species per lake as a function of acid neutralizing capacity (ANC) in Adirondack lakes. The data are presented as the mean of species richness for every 10 µeq/L ANC class. Lakes are also classified into five descriptive categories ranging from low to acute impacts. (Adapted from: Sullivan, T.J. and others 2006. Assessment of the Extent to Which Intensively-Studied Lakes are Representative of the Adirondack Mountain Region. Final Report 06-17. New York State Energy Research and Development Authority. Albany, NY)

© The Ecological Society of America • esahq@esa.org esa 7 ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 Decreases in pH and ANC are often paralleled by changes in element concentrations including increases in Al+3 concentrations and decreases in Ca+2. High dissolved Al+3 concentrations can have toxic effects on many types of aquatic biota, and at extreme levels few aquatic species can survive (Table 1). Organic forms of Al+3 are much less toxic than inorganic forms. Emerging research suggests that Ca+2 concentrations in streamwater are also an important biological indicator. Acidifying deposition has accelerated the leaching of Ca+2 from soils to surface waters gradually decreasing the available pool of Ca+2 in soils and lowering Ca+2 concentrations in runoff. This soil depletion together with decreases in leaching associated with declines in acidifying deposition is contributing to decreases in surface water Ca+2. Many lakes in the boreal forest of the Canadian Shield now have Ca+2 concentrations that are considered sub-optimal for water fleas, crayfish and other crustaceans and may be limiting the species richness of lakes in this region. C. Critical Loads – Acidifying Deposition Critical loads represent the deposition rate that can occur without surpassing tipping points for a given species or ecosystem based on established indicators and effect levels. The critical load for a specific pollutant or group of pollutants will vary depending on differences in landscape sensitivity and in the endpoints for which the critical loads are calculated (e.g., forest soils, lake chemistry). Advances in understanding of chemical and biological indicators of acidification have supported the development of critical loads for sulfur and nitrogen in parts of the U.S. and Canada. Table 2. Expected ecological effects and concern levels in freshwater ecosystems at various levels of acid neutralizing 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). Brook Concern 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 sensiImpacted) 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 <0 Near complete loss of fish populations is expected. Planktonic communities have (Chronically extremely low diversity and are dominated by acid-tolerant forms. The numbers of Acidic) individuals in plankton species that are present are greatly reduced. a Based on data from Southern Appalachian streams and from Shenandoah National Park. Figure 3. Critical aquatic pH ranges for fish species. (Source: Baker, J.P. and Christensen, S.W. 1991. pp. 83-106, In: Acidic Deposition and Aquatic Ecosystems: Regional Case Studies. Charles, D.F. (ed). Springer-Verlag, New York. Figure redrawn in Jenkins, J. and others 2005. Acid Rain and the Adirondacks: A Research Summary. October, 2005. Adirondack Lakes Survey Corporation, Ray Brook, NY)

ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 8 esa © The Ecological Society of America • esahq@esa.org Forests U.S. researchers use models to develop critical loads for forest soil acidification (Box 4). A recent study estimated the critical acid loads for forest soils across the conterminous U.S. The critical acid loads for S and N throughout the Appalachian Mountain Range and Florida are estimated to be less than 1,000 eq/ha/yr (critical loads for combined sulfur and nitrogen are expressed in terms of ionic charge balance as equivalents per hectare per year). This study estimated that about 15% of U.S. forest soils exceed their critical acid load by at least 25% including much of New England, West Virginia, and parts of North Carolina. By comparison, critical load modeling in Canada estimated that 30 to 40% of upland forest areas in Canada are in exceedance of the critical load for acidification, while more than 50% are in exceedance in eastern Canada (Ontario, Quebec, New Brunswick, Nova Scotia and Newfoundland). Surface Waters Regional critical loads for surface waters have been developed for acidifying deposition of sulfur and nitrogen in sensitive regions of the Adirondack Mountains of New York and in the central Appalachians of Virginia and West Virginia. The median critical load for a target ANC of 50 µeq/L is 129 milliequivalents per square meter per year (meq/m2 /yr) in the Adirondacks and 45 meq/m2 /yr in the central Appalachians with values ranging from less than 0 to over 1,000 meq/m2 /yr in relatively insensitive ecosystems. The number of aquatic ecosystems exceeding the critical loads is still quite high, but has declined with decreases in acid deposition from the early 1990s to the late 2000s (Figure 4). Currently, 44% of Adirondack lakes evaluated exceed the critical load and in these lakes recreationally valuable fish species such as trout are missing due to acidification. In the Shenandoah area, 85% of streams evaluated exceed the critical load resulting in losses in fitness in fish species such as the blacknose dace. The persistence of critical load exceedances despite significant decreases in SO2 emissions is related to continued high inputs of acidifying NOx, low initial ANC conditions, and soil depletion of nutrient cations (Ca+2 and Mg+2) that have left many watersheds more sensitive to acid deposition over time. A similar study of 2053 lakes in six northeastern states and four eastern Canadian provinces estimated critical loads for acidifying deposition of sulfur and nitrogen for a target ANC of 40 µeq/L. Results show that 28% of the lakes studied have a critical load in the categories of ≤20 and 20–40 meq/m2 /yr, suggesting vulnerability to acidification with relatively moderate atmospheric deposition. It is estimated that the critical load is exceeded in 12% of the study lakes, based on deposition levels in 2002. These studies point to the importance of long-term monitoring and research for assessing the impact of emissions control programs on deposition and ecological recovery (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 enable resource managers and policymakers to evaluate tradeoffs between the costs of more stringent emissions controls and the benefits of ecosystem services provided by healthy ecosystems. A critical loads approach can be used to synthesize scientific knowledge about air pollution thresholds that cause adverse impacts or ecosystem change. Describing air pollutant effects on ecosystems in critical load terms quantifies estimates of “cause and effect” in a way that allows researchers to communicate science to air quality regulators and natural resource managers. Critical loads are most commonly applied to evaluate the effects of nitrogen and sulfur pollutants and their associated acidity or the eutrophying effects of nitrogen. When critical loads are exceeded there is increased risk for a range of problems including ecosystem acidification, excess nitrogen 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., kilograms per hectare per year (kg/ha/yr)). Critical loads are based on changes to specific biological or chemical indicators such as species composition of a given ecosystem (e.g., grassland) or biotic community (e.g., understory plants or tree-dwelling lichens) or acid neutralizing capacity (ANC) in soils, streams or lakes. Because different sensitive receptors (e.g., forest soils, high elevation lakes, species of lichen) or species may have varying sensitivities to air pollutant loads, multiple critical loads can be used to describe a continuum of impacts 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 negative effects, such as stunted growth, reduced reproduction, and increased mortality. Several different threshold levels may therefore be included in a critical load assessment. The policymaker, air regulator, or land manager can assess all the critical loads (science-driven ecological thresholds) and select target loads (policy thresholds) based on the level of ecosystem protection desired, economic considerations, and stakeholder input at a given location

© The Ecological Society of America • esahq@esa.org esa 9 ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 2. NITROGEN A. Effects of Excess Nitrogen on Ecosystems The nitrogen gas that makes up most of the Earth's atmosphere is inert, with little impact on ecosystems. Nitrogen converted to its reactive forms such as NH3 and NOx, however, can cause profound biological changes. Activities such as fertilizer manufacturing, intensive livestock production and the burning of fossil fuels convert nitrogen to these reactive forms which can then enter and potentially over-fertilize ecosystems. This can lead to problems such as algal overgrowth in lakes, reduced water quality, declines in forest health, and decreases in aquatic and terrestrial biodiversity by favoring “nitrogen loving” species at the expense of other species with low nitrogen preferences. For example, most estuaries and bays in the Northeast U.S. and Mid-Atlantic regions experience some degree of eutrophication (where excess nutrients promote a proliferation of plant life, which can deplete oxygen in the deeper waters), as a result of nutrients from atmospheric deposition and agricultural, urban and industrial runoff. Excess nitrogen can also change species composition. In Waquoit Bay, Massachusetts elevated nitrogen allows tall cord grass to thrive but not eelgrass, which decreases critical fish habitat. Adding nitrogen to forests whose growth is typically limited by its availability may appear desirable, possibly increasing forest growth and timber production, but it can also have adverse effects such as increased soil acidification, biodiversity impacts, predisposition to insect infestations, and effects on beneficial root fungi called mycorrhizae. As atmospheric nitrogen deposition onto forests and other ecosystems increases, the enhanced availability of nitrogen can lead to chemical and biological changes collectively called “nitrogen saturation.” As nitrogen deposition from air pollution accumulates in an ecosystem, a progression of effects can occur as levels of biologically available nitrogen increase (Figure 5). Because of the multiple potential effects of nitrogen deposition in terrestrial and aquatic ecosystems, the ecosystem services affected vary depending on the sensitive receptors found within a given ecosystem and the level of atmospheric deposition. Prominent examples of affected ecosystem services in forests include timber production, climate regulation, recreational use, and biodiversity loss. In Figure 4. Percentage of lakes in exceedance of the critical load for sensitive eastern US surface waters in the Adirondacks (169 lakes in NY) and the central Appalachians (92 streams in VA and WV). The percent exceeding the critical load has declined as emissions and deposition have been decreasing (Source: Jason Lynch- 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 atmospheric deposition decreases below a threshold that was previously exceeded. The trajectory of recovery is not always consistent with model simulations, illustrating the importance of long-term monitoring and research to improve the capabilities of simulation models. A number of regional- and national-scale air, water, soil, and biota monitoring networks collect high-quality data that are useful in assessing ecosystem thresholds. However, current efforts are not enough to provide continuous data at sites across the country, and often lack the coordination needed to effectively combine datasets for maximum benefit. We recommend that existing monitoring and research programs 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/ and Biomonitoring 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 Ecological Research (LTER) network - http://www.lternet.edu/ Appalachians Adirondacks % Water Bodies In Exceedance 1989-1991 2006-2008 1989-1991 2006-2008 100 90 80 70 60 50 40 30 20 10 0