ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 Sierra nevada mountains do e e acidic episodeswhen pollutants retained in the snow Box 2.INDICATORS AND AIR POLLUTION THRESHOLDS pack ove r the winter are re nesium pools and acidification of forest soils is widespread and well documented n the al indic e oof just hov doah Mountain region of West Virginia. to chemical changes in their environment is not always possible. Montatnfs5fthenorttS and A oth lines to these mountain environments.This effect tafegTrd80cedcathsim remains problem today n.S.hard can set A1 A2 B1 nt d ing on gwo Deposition of Air Pollution and therefore are also sensitive to acidifica B.Indicators.Acidifying Deposition dition with quence and cli Indicators of Soil Acidification and forest Heatth eclines in w pl ncert with insect outbreaks, and in the leaves and needles of plants (i.e., of suga Box 3.CHEMICAL NAMES AND SYMBOLS,AND UNITS OF MEASURE Chemical Names and Symbols: Sodium,Na' dioxide.NO Potassium.K ie evated Nitrogen oxides.NO tio.Ca:Al Sulfur oxides,SO. on concentration any fish species.The reduction in then Nutrient ratios(e.g..N:P.N:Ca.C:N) of aquatic he number ry.MeHg Units of Measu omfe的eo Sulfate,SO Equivalents per hectare per year, trate,NO eq/ha/yr data srams a monly applied indicators for a Nitrogen,N Parts per million,ppm terrest aquatic ind Calcium,Ca Milliequivalents per square meter per Magnesium.Ma liter,ug/L aquatic acidification. The Ecological Society of America esahq@esa.org esa 5
© 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 4 Ca"and Mg'in the leaves and needles of 12 Acute Low 10 pleng acid ing the growth of sugar maple (Table 1). 4 2 0 200 .100 100200 300 400 50 ANC(ueq/L) foliage)(Table 1).Three elen nents naturallv stream sensitivity to acidification.ANC,mea in soils,calcium edata are M+2 per er( ttowhich trees and other plants may be characterizes the ability of water to neutraliz 10L Calcium strongacidsincludingthg agnes five des are nu raliz acid inputs to soils. Adaoted values are typically strongly correlated with pH,Al concentrations,and Ca' con- cie readil ific conce eve been d Lake s are wly reme Ca and Mg readily available exchangeable laces them n ion (or enan researchers found that on n ical indic io .Low values indi cate that the soil has ons is one indica d to enab e toxi soil into st mm The pH value of a water body is a funda extent or ure tio A or th nt hase ration is low there is a high singly acidic while alues above asic or alka ne decre es in pH are assoc ciated with a range of CaAl ratios and hown in Table 1. cs hav ns of New indicator since soils can have widely varying pH decline of.alues 6 esa The Ecological Society of America esahq@esa.org
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)
ISSUES IN ECOLOGY NUMBER FOURTEEN FALL 2011 Critical pH Ranges of Fish rce Central mudminnov Adiro tions in streamwater are also an important bio White sucke bass NY) surface wate Arctic char acofin sols and lowering his soil depletion tributing to decreases in surface water Ca Many lakes in the considered sub-optimal for water fleas,crayfish d dac Fathead mi nose da C.Critical Loads-Acidifying Deposition Critical loads re nt the deposition Safe range,no acid-related efects occ Critca acd-reeefec ely sed rs a ific pollu Advances in understanding of chemical and biological indicators of acidification have sup ois lake che Table 2.Expected ecological effects and concern levels in freshwater ecosystems at various levels of acid neu- tralizing capacity (ANC).(Source:USEPA). Category Label ANC level (ueq/L) Expected Ecological Effects NoE9ec6em >100 exhibit expected diversity and distribution. Moderate 50-100 Fish speci ies richness beains to decline (sensitive s pe cies are lost from lakes).Brook trout tions in to e st e as spe impacted) tive to acid affected. Elevated 0-50 Fish sp esrichness is greatyreduced (more than half of loss of he ealth and re tion (f Acidic) <0 t are greati and from s The Ecological Society of America.esahg@esa.ord esa 7
© 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)