Acid Deposition: The Ecological Response
A WORKSHOP REPORT
TABLE OF CONTENTS
Section ...................................................Page Number
Executive Summary ...................................................v
Policy Context ...................................................2
Current Scientific Understanding ...................................................3
Sulfur Deposition ...................................................7
Nitrogen Deposition ...................................................8
ANC and Base Cation Cycling ...................................................11
Response and Recovery of Ecological Systems ...................................................12
Chemical Formulas ...................................................19
Appendix 1: Workshop Agenda ...................................................20
Appendix 2: Workshop Planning Group and Participants...................................................22
The workshop was sponsored by the Ecological Society of America (ESA), Environmental Protection Agency (EPA), United States Forest Service (USFS), United States Geological Survey (USGS), and the National Acid Precipitation Assessment Program (NAPAP). The authors would like to thank the workshop participants, especially those who provided valuable insight and editorial comments to the report. The development of the workshop was also the result of the hard work and planning of the following members of the steering committee: Stephanie Benkovic, Rona Birnbaum, Greg Lawrence, Mark Nilles, Richard Pouyat, Paul Ringold, Doug Ryan, Michael Uhart, and Kathie Weathers.
Over the past two decades, our understanding of atmospheric pollution and its effects on the environment has gradually developed. This increased understanding is in large part the product of long-term ecological and atmospheric studies. Results of these studies suggest that current emission levels of sulfur dioxide and nitrogen oxides continue to affect terrestrial and aquatic ecosystems, often in ways that were unexpected. As Congress determines whether to reauthorize the Clean Air Act (CAA), the scientific community came together to evaluate their current knowledge on this topic. This summary highlights the goals, results, conclusions, and recommendations from the March 1999 workshop, "Atmospheric Deposition*: The Ecological Response."
- Evaluate the status and trends of various types of ecosystems in response to acid deposition;
- Determine whether and how the extent of ecological damage from this disturbance has evolved since observed, projected, and reported in the 1990 National Acid Precipitation Assessment Program (NAPAP) Report; and,
- Ascertain whether some properties of certain ecosystems are showing signs of recovery.
Findings from the workshop are synthesized into four categories:
- Sulfur Deposition;
- Nitrogen Deposition;
- Acid Neutralizing Capacity and Base Cation Cycling; and
- Response and Recovery of Ecological Systems.
- In some regions of the United States, such as the Midwest and Northeast, emission reductions of sulfur dioxide correspond to decreases in the concentrations and fluxes of sulfate in both precipitation and surface waters;
- In some regions and ecosystems, current reductions of sulfur emissions may be insufficient for ecological recovery to occur;
- Factors limiting the recovery of ecosystems from acidification include nitrogen deposition, depletion of exchangeable base cations, unaccounted for inputs of sulfur (which might result from underestimates of total deposition or internal sources of sulfur such as weathering of minerals or mineralization of organic sulfur in soil);
- Given current knowledge of nitrogen deposition and its impact on ecological systems, the importance of nitrogen must be fully considered in formulating future air quality policies;
- Acidic episodes can have significant biological effects (e.g., loss of biodiversity; changes in community structure), and these effects are more widespread geographically than chronic acidification;
- Monitoring the chemistry of atmospheric inputs and surface waters should be continued and enhanced, and include efforts to improve measurements of total (wet and dry) deposition; and
- Current monitoring efforts should be maintained and expanded through inclusion of biological indicators of ecosystem response to acidifying deposition.
In order to evaluate the effectiveness of environmental policies and programs, a firm commitment is needed to long-term monitoring programs that help in assessing the status and trends of ecological systems and ecosystem response to historic and future changes in stress (i.e., deposition), as well as discerning when, where and why recovery is occurring.
Much has been learned over the last two decades as a result of research regarding the sources, mechanisms, and effects of acid deposition on ecological systems. Our understanding of the impacts of sulfur, nitrogen, base cation depletion, and the response and recovery of ecological systems has been refined. Still, many questions must be answered to fully assess whether the environmental goals of the 1990 CAAA are being met as a result of sulfur and nitrous oxide emission reductions. Future research should focus on these particular areas: nitrogen deposition surface water chemistry, watershed/soil chemistry, and forest ecosystems.
*Bolded words indicate terms defined in glossary
This document reports the results of a workshop attended by over forty leading academic and Federal agency researchers from the United States and Canada (Appendix 2). The workshop was held on March 1-3, 1999 in Washington, D.C. and was sponsored by the Ecological Society of America and several Federal agencies, including the Environmental Protection Agency (EPA), U.S.D.A Forest Service (USFS), U.S. Geological Survey (USGS), and the National Acid Precipitation Assessment Program (NAPAP). The workshop goals were to: 1) evaluate the status and trends of various types of ecosystems in response to acid deposition; and 2) determine whether and how the extent of ecological damage from this disturbance has evolved since observed, projected, and reported in the 1990 NAPAP Report.
Scientific evaluation of these patterns is timely due to the availability of long-term data sets developed by Federal agency monitoring programs, investigations initiated by EPA and the National Science Foundation's Long-Term Ecological Research Network, especially the Hubbard Brook data base that was collected and assembled by scientists, and monitoring and research programs sponsored by a variety of other agencies and organizations. Moreover, this is a timely opportunity to assess the response of ecological systems to Title IV emissions reductions called for in the 1990 amendments to the Clean Air Act. It is important to note that the workshop was not intended as a rigorous review of the science and data presented. Instead, the workshop accepted, at face value, peer-reviewed data and information in an effort to assess the current state of the science regarding acid deposition.
1. What have been the ecological responses to chronic deposition?
3. What will be the ecological response to projected future deposition rates?
In 1980, Congress passed the Acid Precipitation Act (PL 96-294, Title VII) to develop information regarding the formation and environmental effects of acid deposition. The Act mandated creation of a Federal interagency group referred to as the National Acid Precipitation Assessment Program (NAPAP). Through NAPAP, federal agencies gained a forum to define common research interests, develop coordinated research programs, submit coordinated research budgets to Congress, and conduct assessments. Beginning in 1980, NAPAP examined the relationships among fossil fuel combustion, pollutants resulting from emissions and the effects of these pollutants (particularly sulfur dioxide and nitrogen oxides) on the environment, economy, and human health. Results of that research were published as the NAPAP 1990 Integrated Assessment Report.
In 1990, the Clean Air Act Amendments (CAAA) (PL 101-549, Title IV) were passed to reduce the adverse effects of acid deposition through phased reductions of annual emissions of its precursors, sulfur dioxide (SO2) and nitrogen oxides (NOX), in the forty-eight contiguous states and the District of Columbia. The Acid Deposition Control Program created by Title IV is being implemented in two phases, with two major goals, to reduce total SO2 and NOX emissions.
When Congress enacted acid rain provisions in the 1990 CAAA, it developed an approach specifically designed to address long-range air pollution problems having regional environmental and human health effects. Title IV also introduced market-based mechanisms to implement emissions reductions at lower cost while capping the overall level of SO2 emissions. The depth of research on cause and effects, the existence of baselines established through environmental monitoring, and the use of innovative policy approaches all led to the acid rain control program being referred to as the "big experiment" by the scientific and policy communities. Evaluating the ecological response to changes in emissions of SO2 and NOx is an important link between scientific research and environmental policy in the context of implementing the 1990 Clean Air Act Amendments.
Understanding the current quantities and trends in emissions and deposition of sulfur (S) and nitrogen (N), and the extent and rate of ecological responses to these changes, is important for other reasons as well. Through the Government Performance and Results Act (GPRA) and the National Performance Review (NPR), both Congress and the White House have directed all Federal agencies to initiate performance-based management programs. Performance-based management entails establishment of justifiable goals or objectives, and a periodic assessment of progress toward those goals. In the case of environmental policy, a performance-based system requires a common metric of environmental or biological relevance that is measurable over time. Taken together, GPRA requirements, NPR mandates, and the Congressional directive to prepare a NAPAP 2000 Integrated Assessment Report create the need for a scientific evaluation of current issues and data related to acid precipitation and patterns of ecosystem response.
SO2 emissions--Reduce by 10 million tons below 1980 levels by 2010 (roughly a 50% reduction below 1980 levels); Phase I initiated in January 1995 and Phase II began in 2000; utility SO2 emissions are limited, or "capped," at 8.95 million tons per year; non-utility (i.e., industrial) emissions of SO2 capped at 5.6 million tons per year beginning in 1995.
NOx emissions-- Reduce by 2 million tons below projected levels from coal-fired boilers by 2000 (roughly a 10% reduction off of levels without Title IV); Phase I began in January 1996, and Phase II became effective in 2000; no cap on NOx emissions.
Current Emission Levels
Current Scientific Understanding
The 1980s was a decade in which knowledge of acid deposition increased dramatically. Scientists developed greater understanding regarding patterns of acid deposition, the mechanisms through which it affects aquatic and terrestrial ecosystems, and the ecological response to acid deposition. These advances in scientific knowledge and understanding built on research, observations and hypotheses developed in the 1970s, and reflected the substantial policy attention and funding that the issue of acid deposition received during the 1980s. The understanding developed in the 1980s was summarized in the 1990 State of the Science and Technology and 1990 Integrated Assessment reports published by NAPAP in 1991. Research has continued into the 1990s, although at reduced levels of funding.
The following broad messages developed by workshop participants summarize the current state of our knowledge of acid deposition and patterns of ecosystem response:
In the late1980s, scientists hypothesized that sustained atmospheric inputs of nitrogen would exceed (or "saturate") the biological demand for nitrogen in forest ecosystems where growth is normally limited by availability of nitrogen (see box on nitrogen saturation). They expected saturation to result in diminished forest health, degraded water quality, and greater leaching of nitrogen from ecosystems that generally were characterized by strong retention of atmospheric nitrogen deposition, such as the Catskill Mountains in New York and the Great Smoky Mountains in Tennessee. Evidence of the extent of nitrogen saturation is fragmentary and varies from one region to another in the United States. In the eastern United States, increased leaching of nitrate from the soil and large shifts in the nutrient ratios in tree leaves have been observed only in forests at intermediate to high elevations that receive large amounts of nitrogen deposition. In the western United States, the early stages of nitrogen saturation have been observed in high elevation ecosystems of the Colorado Rockies Front Range. Elsewhere in the western United States, however, nitrogen saturation is much more advanced. In mixed conifer forests and chaparral stands surrounding the Los Angeles Basin, CA, for example, nitrogen deposition is so high and has been going on for so long that these systems have progressed to the later stages of nitrogen saturation. Similarly, nitrogen saturation in the more exposed regions of the San Gabriel and San Bernardino Mountains, CA is severe, with the highest streamwater nitrate concentrations reported for wildland systems in North America (concentrations greater than 400 umol/L in otherwise undisturbed watersheds). Observations from these areas, as well as long-term experimental additions of nitrogen on various forest types, have provided a greater understanding of the mechanisms associated with nitrogen retention and loss from forest ecosystems, and have resulted in focused concerns associated with nitrogen deposition onto forested watersheds.
Nitrogen is commonly known to be a limiting nutrient in many ecosystems.
For example, plant growth in temperate forests is frequently limited by
nitrogen availability. So, as more nitrogen enters an ecosystem, plants
utilize the extra nitrogen as a nutrient for increased growth. When atmospheric
deposition of nitrogen results in nitrogen availability in excess of the
demand in some forest ecosystems, the condition is termed nitrogen saturation.
This condition occurs over several decades of deposition. After levels
of nitrogen entering an ecosystem exceed the nutrient requirements of organisms,
nitrogen begins to have adverse effects, including, decreased plant function
due to leached nutrients (e.g., calcium) from the soil; loss of fine root
biomass; decreases in symbiotic mycorrhizal fungi; and leaching into surface
waters and ground waters, which increases acidification.
Ecosystem responses to acid deposition: non-linear, case specific, and sub-lethal
Scientists have greater understanding of the role that episodic events play in ecosystem responses to acid deposition as well as the spatial extent of areas experiencing or at significant risk of experiencing episodic acidification. Episodic acidification refers to short-term decreases in acid neutralizing capacity (ANC) that often occur during high streamflow associated with rainstorms and snowmelt. Episodic acidification occurs in many regions and may be the result of natural processes, or a response to the combination of natural processes and acid deposition. Winter and spring acid deposition is of particular concern due to the role of snowmelt and heavy spring rains in episodic acidification. Surface waters affected by acid deposition experience acidification episodes that are of greater magnitude and longer duration than would have resulted from natural processes alone. However, chemical changes involving increasing concentrations of inorganic aluminum in surface waters are the primary characteristic distinguishing acid deposition driven episodic events from those caused by natural processes.
Aluminum (Al) mobilization
Concentrations of aluminum are naturally very low in most surface waters because aluminum is only slightly soluble at neutral pH values. In regions that are not impacted by acid deposition, aluminum in soil water and surface waters is largely associated with naturally occurring processes. There is a direct relationship between increasing aluminum and increasingly acidic water. Inputs of strong acid anions, such as sulfate or nitrate, to acidic soil mobilizes inorganic forms of aluminum within the soil that can be transported to surface waters. Inorganic forms of aluminum can be taken up by plants and animals more readily than organic forms of aluminum, and are toxic to vegetation and aquatic biota.
Base cation depletion and soil acidification
In 1990, it was unclear whether acid deposition had contributed to base cation depletion of forest soils. Some scientists hypothesized that acid deposition would not cause significant base cation depletion because of the large amounts of base cations found in most forest soils. Recent research in North America, however, has shown that acid deposition has resulted in both acidification (decreased pH) and depletion of nutrient cations (e.g., calcium and magnesium) essential for tree growth in soils with relatively low available pools of those cations
(i.e., soils of relatively low fertility). Soils with a base cation
saturation of less than 20% have low buffering capacity. These types
of watersheds tend to have surface waters that can be acidified by inputs
of sulfuric and nitric acids (see box on base cation depletion).
Of the four base cations (calcium, magnesium, potassium, and sodium),
calcium is generally the most abundant in soils and surface waters, and
is therefore most important for neutralization of acid inputs.
Calcium (as well as many of the other base cations) is supplied to the
soil by weathering, the physical and chemical breakdown of rocks and minerals.
Once released by weathering, the calcium can become loosely attached to
particle surfaces through the process of adsorption. Adsorbed
calcium can exchange with hydrogen supplied by acid rain, which results
in the deacidification of soil water and the leaching of calcium out of
the soil. If the rate of leaching exceeds the rate at which
calcium is added through weathering, the adsorbed calcium can become depleted
in soils, therefore leaving the soil vulnerable to current and future acidification.
Forest landscapes are highly variable in their sensitivity to acid deposition. High elevation regions are particularly susceptible to acid deposition because they receive greater amounts of total deposition including cloud water, and often have shallow soils with low buffering capacity. At lower elevations, ecosystem sensitivity to acid deposition often decreases because there is less atmospheric deposition and soils are deeper, though this generalization may not hold true for areas that are close to a significant source of pollution (e.g., areas near Los Angeles). Land use history also plays an important role in sensitivity to acid deposition. For example, harvesting may deplete soil pools of nutrient cations (e.g., calcium, magnesium, and potassium), making harvested sites more susceptible to the effects of acid deposition. Forests that have been highly disturbed (e.g., through fire or cultivation) may have low soil pools of nitrogen and consequently retain atmospheric nitrogen deposition for extended periods of time.
This section of the Report summarizes workshop discussions on current scientific understanding of the effects of sulfur and nitrogen emissions and deposition on surface water chemistry and biology, watershed/soil chemistry and biology, and forest ecosystems. In addition, the Report describes issues involved in recovery of these systems as a result of decreases in deposition of sulfur and potential future decreases in deposition of nitrogen. The statements reported below represent the conclusions reached during workshop plenary and breakout sessions, and reflect the general sense of workshop participants.
The majority of the workshop conclusions reflect a high degree of agreement. The high degree of agreement by no means indicates that we have a complete understanding of acid deposition, rather the conclusion section of this Report calls for continued and new research. Scientific participants at the workshop used existing resources to generate understanding and certainty regarding scientific conclusions reached through earlier research. As mentioned, this Report identifies research and monitoring needs that should be considered in order to continue to achieve the purpose and goals of the Clean Air Act Amendments of 1990.
The ranking system reflects the amount of agreement among workshop participants. Rankings are as follows: high, moderate, little, and no agreement. "No agreement" statements are included as they were important issues raised by participants, or were added to the Report after the workshop during the peer review process.
The current human impact on the sulfur cycle is unprecedented in the geologic record. The use of fossil fuels to meet energy demands for electricity, industrial combustion, transportation, home heating, and other needs has added a large flux of gaseous sulfur to the atmosphere, more than doubling the rate at which sulfur was mobilized from the Earth's crust 200 years ago. The net effect of these activities is to increase the pool of "oxidized" sulfur (sulfate) in the global cycle, while decreasing the storage of "reduced" sulfur in the Earth's crust. Although these activities have resulted only in a small change in the global pool of sulfur in the lithosphere, they have caused enormous changes in the annual flux of sulfur through the atmosphere and, thus, the amount and rate of atmospheric sulfur deposition to aquatic and forest ecosystems (see box on workshop findings for sulfur deposition).
Sulfur in the atmosphere reaches the Earth's surface and ecological systems through wet deposition (e.g., rain, snow), dry deposition (e.g., particles and gases), and in some ecosystems through fog or cloud water deposition. When SO2 is emitted from human activities, it may form sulfuric acid (H2SO4) through reactions in the atmosphere. Sulfuric acid falls to Earth as precipitation, or is deposited as cloud or fog water in some high-elevation or coastal areas. Dry deposition involves deposition of sulfate particles and gases to the land and vegetation surfaces (e.g., leaves) during periods without precipitation. Another mechanism of dry sulfur deposition is the absorption of sulfur-containing gases directly from the atmosphere by vegetation or other surfaces, a process that is particularly important in humid regions.
Atmospheric sulfur enters ecosystems as sulfur dioxide and sulfate. Both forms can generate acidity directly. The leaching of sulfate from soil may result in the depletion of nutrient cations (e.g., calcium, magnesium, and potassium) or the release of acid cations to surface waters (e.g., hydrogen, aluminum). The depletion of nutrient cations from soils may decrease the productivity of terrestrial ecosystems. Furthermore, high concentrations of acid cations in the soil may be toxic to terrestrial biota and be linked to forest decline under some conditions. The transport of acid cations to surface waters contributes to surface water acidification and may be toxic to aquatic biota, such as fish.
Thus, reducing SO2 emissions and sulfur deposition should
enhance the health of both terrestrial and aquatic ecosystems. Phase I
of the Acid Deposition Control Program has been successful in effecting
a decrease in SO2 emissions in some regions. Emission reductions
began on time in 1995 and SO2 reductions were greater than projected
by the Phase I implementation schedule. Although sulfur deposition has
been reduced, it is unclear whether reductions in SO2 emissions
have been, or will be, sufficient for recovery of terrestrial and aquatic
ecosystems to occur. Moreover, the level of recovery depends on the fate
of sulfur that has accumulated in soils during periods of high sulfur deposition,
as well as on the amount delivered directly to trees, surface waters, and
other ecosystem components. The leaching of accumulated sulfur will slow
the recovery of both terrestrial and aquatic ecosystems due to the deleterious
effects of sulfate mobilization on acidification and the release of cations
to soil water and, ultimately, surface waters.
Patterns in surface water chemistry
Nitrogen is one of the four most common chemical elements in living tissue, and is an essential component of key organic molecules. In short, all organisms require nitrogen in order to live. However, most plants and animals cannot use nitrogen directly from the air, which is composed of 78% nitrogen gas (N2). Instead, they require nitrogen to be bonded to hydrogen molecules through a process called "fixation," creating biologically usable compounds. Generally, this short supply of bio-available nitrogen plays an important role in controlling productivity, biodiversity, and nutrient cycling in terrestrial and aquatic ecosystems. For example, estuarine ecosystems normally are characterized by low nitrogen availability. In estuarine ecosystems along the Atlantic and Gulf of Mexico coasts of the United States, greater amounts of available nitrogen have led to changes in aquatic communities and losses in biological diversity.
The 1990 NAPAP Report and Title IV of the CAAA concentrated principally on atmospheric sulfur deposition. Concerns over nitrogen emissions focused specifically on nitrate deposition under Title IV. Research on nitrogen as a component of acid deposition is less mature than our understanding of the role and effects of sulfur deposition. Moreover, the role of nitrogen in acid deposition is much more complicated because of biological mediation. Consequently, the effects of nitrogen deposition in relation to soil and surface water acidification is not fully understood. Scientists in the United States and elsewhere are concerned about the long-term effects of nitrogen deposition on forests, grasslands, streams, and estuaries. These concerns involve deposition of organic nitrogen; oxides of nitrogen (NOX, HNO3, and NO3-) formed as a consequence of fossil fuel combustion; and, reduced forms of nitrogen, such as ammonium (NH4+) resulting from intensive animal husbandry, fertilizer use, and other activities. The effects of NOX emissions and nitrogen deposition include:
- Increased regional concentrations of oxides of nitrogen (including nitric oxide, NO) that drive the formation of photochemical smog;
- Degraded drinking water quality due to elevated nitrate levels in streams and groundwater from forested watersheds supplying water for human consumption (e.g., San Bernardino and San Gabriel Mountains and Los Angeles metropolitan area water supply);
- Increased atmospheric concentrations of greenhouse gases. A process whereby nitrogen deposition leads to nitrogen accumulation in soil and the forest floor, with a portion being re-emitted to the atmosphere from soil as trace gases (e.g., NO and N2O);
- Losses of soil nutrients through nitrate leaching. These nutrients include calcium and potassium that are essential for long-term soil fertility;
- Acidification of soils, streams and lakes in several regions of the United States, Canada, and Europe; and
- Increased transport of nitrogen by rivers into estuaries and coastal waters leading to eutrophication.
As reflected in the workshop findings below, a decade of research and scientific work has increased our understanding of the impact of nitrogen on surface water chemistry, soil chemistry, and forest ecosystems, but significant uncertainties remain (see box on workshop findings for nitrogen deposition). For example, there are significant questions regarding the fate of ammonium deposition in the environment, and its role in contributing to acidification.
Patterns in surface water chemistry
Patterns in forest response
A. Terrestrial Processes:
Acid Neutralizing Capacity and Base Cation Cycling
Rain water is normally weakly acidic (pH level of 5.2 to 5.4) due to
the presence of naturally occurring oxides of nitrogen and sulfur. However,
with the addition of strong acids from human activities, the mean pH of
rain is often in the range of 3.5 to 5. The degree to which acid deposition
results in the acidification of surface waters greatly depends on processes
occurring in the surrounding watershed. Greater acid inputs to a watershed
result in increased leaching of base cations through acid neutralizing
reactions in the soil. As water moves through a watershed, two important
chemical processes act to neutralize acids through the release of base
cations, positively charged ions such as calcium (Ca2+), potassium
(K+), sodium (Na+), and magnesium (Mg2+).
The first process is mineral weathering. Most calcium in soil is
bound within the mineral structure of rocks, and must be released by weathering,
a process whereby minerals gradually breakdown and dissolve over long time
periods. As calcium is released by weathering, it neutralizes acidity and
increases the pH level in soil water and surface waters. The second reaction
involves cation exchange in soils, a process by which hydrogen ions displace
other cations absorbed to the surface of soil particles, releasing
these cations to soil and surface water. The degree of acid neutralization
that has occurred can be determined by measuring the acid neutralizing
capacity (ANC) of surface waters (see box on ANC).
Acid neutralizing capacity (ANC) is a measure of the ability of a water
or soil sample to chemically neutralize acid inputs. Water with a
low ANC has little ability to neutralize acids, and extremely acidic waters
can have a negative ANC. A change in ANC alone does not necessarily
signal harmful effects on aquatic life because aquatic organisms are affected
primarily by changes in pH, aluminum, and calcium caused by acid precipitation.
Decreases in ANC are generally associated with decreases in pH and base
cation concentrations, and increases in concentrations of acid anions (sulfate,
nitrate and organic anions) and aluminum. Soil bases initially
act to buffer the ANC of soil water; however, their depletion can eventually
lead to decreases in soil ANC and soil and surface waters. ANC, therefore,
provides a useful index of acidification status, although it reflects both
natural acidification processes as well as acid deposition effects.
Base cation depletion is a cause for concern because of the role these
ions play in acid neutralization, and in the case of calcium, magnesium
and potassium, their importance as essential nutrients for tree growth
(see box on workshop findings for ANC and base cation response). Depletion
of base cations from the soil is the process that leads to aluminum mobilization,
which can have deleterious effects on fish and may interfere with the uptake
of calcium by tree roots in forest soils. Aluminum mobilization may have
contributed to the widespread decline of red spruce trees in the Northeast
and deficiency of calcium and magnesium in soils has been linked to high
mortality rates of sugar maple in western and central Pennsylvania. Research
results from the 1990's indicate that documented decreases in soil calcium
in the northeastern and southeastern United States are at least partially
attributable to acid deposition.
Some forest sites show depletion of labile pools of calcium and magnesium due to acid deposition (high agreement).
Some cation deficiencies are associated with increased mortality in sugar maples and red spruce, as has been demonstrated in western Pennsylvania, where soils are highly weathered and have low cation exchange capacity (moderate agreement).
Aluminum mobilization may have contributed to the widespread decline
of red spruce trees in the Northeast (moderate agreement).
Response and Recovery of Ecological Systems
Research regarding recovery of ecological systems remains in a relatively nascent stage, particularly as recovery relates to changes in ANC, base cation depletion, and aluminum concentration. Some workshop participants debated the term "recovery," questioning whether ecosystems can fully recover, whether recovery can be measured, how one judges when recovery is achieved, and whose standards should be used in making such judgments. Other workshop participants defined recovery as involving changes in chemical or biological indicators consistent with a return to pre-disturbance conditions, including:
- Increases in ANC or pH in acidified systems;
- Decreases in concentration of inorganic aluminum;
- Re-establishment of native fish species; and
- Increased biological integrity.
As discussed above, implementation of Title IV has resulted in reductions
of SO2 emissions in the United States. These emission reductions
have decreased the concentrations and fluxes of sulfate in precipitation
and surface waters in some regions, such as the northeastern United States.
Decreases in sulfate concentrations resulting from Title IV Phase I and
II emissions reduction programs are expected to lessen the deleterious
impacts of acidification on surface water chemistry and aquatic ecosystems.
It should be noted that even with the current reduction in SO2
emissions, acid sensitive regions such as the Adirondack and Catskill Mountains
of New York, and some areas of Maine, demonstrate little change in the
ANC of surface waters, possible continued depletion of base cations in
soil, and mobilization of inorganic aluminum into soil water. Recovery
of aquatic ecosystems also has not been observed in these regions.
Significant sublethal effects on fish have been observed in areas where data exist, indicating that the extent of affected ecosystems is broader than previously recognized (high agreement).
Widespread response to atmospheric deposition has not been detected in all ecosystems. This may be due to a lack of data, or it could be that data have not been synthesized in a way that allows evaluation of hypothesized effects. However, interactions of various stressors and atmospheric deposition have demonstrated effects for certain species and site combinations in forest ecosystems (high agreement):
Biological monitoring data in the United Sates are inadequate to track regional biological response to changes in emissions and deposition of sulfur and nitrogen (high agreement).
Biological recovery has been observed in heavily impacted areas outside of the U.S. (e.g., Sudbury, Canada), but neither large chemical changes nor biological recovery has been observed yet in the United States (high agreement).
Time to recovery is uncertain, and further reductions in acid deposition and/or longer response periods may be required to achieve chemical recovery (high agreement).
When acid deposition is decreased, chemical changes indicating recovery may be delayed in areas where cations have been depleted in watershed soils (high agreement).
Presently used ecological indicators of acidification and recovery are
adequate to measure aquatic responses to changes in deposition (high agreement).
Such indicators include:
- Reducing sulfur emissions was a major focus of the 1990 CAAA, Title IV. Although the results only reflect the first phase of the Acid Deposition Control Program, which began in 1995, implementation of Title IV has resulted in marked reductions of SO2 emissions in the United States. During the 1995-1998 period, for example, SO2 emissions from all utility units (including substitution and compensating units) affected by Phase I averaged 5.33 million tons/year, an average annual decrease of approximately 51% from those sources from 1980 levels. In some regions, such as the upper Mid-west and northeastern United States, these emissions reductions have decreased the concentrations and fluxes of sulfate (SO42-) in both deposition and surface waters.
- This reduction of sulfate in precipitation and surface waters, as a result of reduced SO2 emissions is linked with the recovery of ecosystems from the deleterious impacts of acidification. However, in some regions and ecosystems, current reductions in sulfate may be insufficient for recovery to occur. In regions such as New York State's Adirondack Mountains, the White Mountains of New Hampshire, and some sensitive lakes in Maine, the ecosystem response has been less than might have been expected as a result of SO2 emissions reductions and decreased sulfate wet deposition. In some locations, in fact, additional acidification has occurred in spite of decreasing sulfate concentrations in surface waters.
- Factors limiting the recovery of ecosystems from acidification include: 1) atmospheric deposition of nitrogen that increases the leaching of nitrogen as nitrate (NO3-) from ecosystems; 2) depletion of exchangeable base cations (calcium, magnesium, sodium, potassium); 3) contributions of natural acidity; and 4) unaccounted for inputs of sulfur (which may result from underestimates of dry deposition, or internal sources of sulfur such as weathering of minerals or mineralization of organic sulfur in soil). Given current knowledge of nitrogen deposition and its impact on ecological systems, the importance of nitrogen must be fully considered in formulating future air quality policies
- Monitoring the chemistry of atmospheric inputs and surface waters should be continued and enhanced, and include efforts to quantify dry deposition. After nearly twenty years of research and monitoring activities, there is still a lack of reliable estimates on total deposition on a regional basis. The inability to quantify total acid deposition to ecosystems limits the ability to measure progress towards the goals of the 1990 CAAA.
- Current monitoring efforts should be expanded through inclusion of biological indicators of ecosystem response. We can more fully judge ecosystem condition and evaluate the effectiveness of past and future clean air legislation only by undertaking selected biological assessments.
Exploring statistical relationships between multiple interacting environmental factors and long-lived organisms requires controlled experimentation and gathering of large numbers of observations over time across broad geographic regions. These requirements necessitate studying ecosystem responses to air pollutants over decades, rather than years, and across an array of ecosystem types. While some research sites and the efforts of research teams predate the NAPAP efforts of the 1980s, the data required for ecosystem studies at various temporal and spatial scales were largely unavailable at the time the NAPAP Report was published in 1991. Now, thanks in part to a decade of monitoring network implementation and long-term experimentation, there are more long-term data available with which to assess the effects of acid deposition on terrestrial and aquatic ecosystems.
Combining newly acquired data with long-term datasets developed over decades at individual research sites (e.g., Hubbard Brook, New Hampshire), incremental advances have been made over the last ten years in refining our understanding of the ecological effects of acid deposition, and the chemical and biological mechanisms through which those effects manifest themselves. However, as discussed in earlier sections of this Report, we do not fully understand acid deposition and its impacts on soils, surface water chemistry and biology, and forest ecosystems. As with most complex interactions, at least as many questions remain as have been answered. Workshop participants identified needs in the two broad areas of monitoring and research.
In order to evaluate the effectiveness of environmental policies and programs, a firm commitment is needed to long-term monitoring programs that help in assessing the status and trends of ecological systems, as well as discerning when and where recovery is evident. Monitoring data allow evaluation of the effectiveness of emission controls, exploration of dose-response relationships, validation of predictive models, and understanding of watershed processes. Recognizing that long-term monitoring data are essential and fundamental to future acid deposition research, several workshop participants noted during this discussion that "monitoring is research" (see box on workshop recommendations for monitoring needs). Thus, monitoring the chemistry of atmospheric inputs and surface waters must be continued and expanded. Continued long-term monitoring of wet deposition (including cloud and fog), dry deposition, streamflow, water chemistry, soil chemistry, and biological condition are essential for understanding and evaluating the response of ecological systems to acid deposition.
In addition, however, biological monitoring capabilities must be expanded.
We are developing a record of chemical response of surface waters and watersheds
to acid deposition, but we have little or no foundation to determine if
long-term changes in acid deposition are leading to long-term biological
responses. Present understanding of biological "trends" is not based on
biological data because sustained biological monitoring rarely occurs.
Instead, scientists must extrapolate biological effects from monitoring
of chemical indicators combined with an understanding of the ecological
effects of chemical changes. Based on their understanding of the reaction
of fish species to pH levels and inorganic aluminum concentrations, for
example, scientists might expect recovery of aquatic ecological systems
if they detect increasing pH and decreasing inorganic aluminum concentrations.
Chemical and biological interactions within the ecosystems, however, may
complicate simple cause and effect relationships like these. Moving beyond
such indirect analyses demands developing and refining biotic indicators
for evaluating acidification and recovery of aquatic ecosystems. Therefore,
including biological resources in monitoring activities is critical if
we are to truly advance our understanding of ecological effects of acid
Minimum monitoring needs:
1. Monitoring that is multi-scale, linked, and integrated;
2. Utilization of linked process sites and survey sites that are representative of affected areas;
3. Multiple variables:
One of the most important refinements of our understanding involves
the significance of nitrogen. Put simply, scientists are much more certain
than they were in 1991 that nitrogen plays an important role in acid deposition
and its ecological effects. However, the scientific community also realizes
that it is much more difficult to "close" the nitrogen budget than the
sulfur budget. In other words, discovering and accounting for sources,
sinks, and flows of nitrogen is a difficult and complex problem. In particular,
the inability to measure significant components of nitrogen deposition
(e.g., dry deposition of ammonia) is a basic and critical issue. These
questions can be addressed at long term and experimental research sites
already involved in this type of research (e.g., Hubbard Brook Experimental
Forest, Harvard Forest, Bear Brook). Thus, basic questions regarding nitrogen
deposition and its effects remain at the top of the research agenda.
Surface water chemistry
What is the fate of sulfur that has accumulated in soil from atmospheric sulfur deposition and what controls the leaching of sulfate following decreases in atmospheric sulfur deposition?
What is the fate of atmospheric nitrogen that has accumulated in soil? How much of the deposited nitrogen is denitrified, versus being accumulated in the watershed?
What factors (e.g., watershed hydrology) are controlling nitrogen retention and release from forested watersheds? To what extent are historical and current land use patterns contributing factors in determining the spatial variability of nitrogen leaching?
Can nitrogen retention be modeled on a regional scale to characterize regions sensitive to nitrogen loadings?
What are the possible complicating interactions with climate change (e.g., increased temperatures, CO2 increases and variations)?
What is the ecological response to anthropogenic acidification? If so, what is the rate of recovery and how is this coupled with chemical recovery?
What are the ecological effects of depletion of labile pools of calcium and magnesium from soil?
What is the rate of recovery of labile pools of calcium and magnesium in soil following decreases in acid deposition?
(Glossary terms are bolded in text)
Anions - Negatively charged molecule such as sulfate (SO42-) and nitrate (NO3-). In combination with hydrogen (H+), these molecules act as strong acids.
Acid neutralizing capacity (ANC) - A measure of the ability for water or soil to neutralize added acids. This is done by the reaction of hydrogen ions with inorganic or organic bases such as bicarbonate (HCO3-) or organic ions.
Acidification - refers to the loss of ANC or the lowering of pH.
Adsorb - To take up and hold (a gas, liquid, or dissolved substance) in a thin layer of molecules on the surface of a solid substance.
Atmospheric deposition - The processes by which chemical constituents move from the atmosphere to the Earth's surface. These processes include precipitation (wet deposition, such as rain, cloud or fog), as well as particle and gas deposition (dry deposition).
Biological mediation- A biological activity that counters the effects of another biological event.
Buffering capacity - The resistance of water or soil to changes in pH.
Base cations - Positively charged ions such as magnesium, sodium, potassium, and calcium that increase pH of water when released to solution through mineral weathering and exchange reactions.
Chronic acidification - Generally refers to surface waters that remain acidified (ANC<0) regardless of variations in hydrologic conditions (precipitation, stream flow, etc.).
Critical thresholds - The point at which a system switches to another state.
Dry deposition - The portion of total atmospheric deposition that is deposited on dry surfaces during periods of no precipitation as particles or in a gaseous form.
Labile - Chemically reactive or bioavailable element.
Leaching - Process by which water removes chemical solutes from soil through chemical reactions and the downward movement of water.
Mineral weathering - The physical and chemical breakdown of rocks that releases ions such as calcium and aluminum.
Nitrogen fixation - The process in which bacteria convert biologically unusable nitrogen gas (N2) into biologically usable ammonia (NH3) and nitrates (NO3-).
Non-Linear - A relationship between two factors that does not vary on a 1 to 1 basis.
Precipitation - Water in the form of rain, sleet, or snow (wet deposition).
Sublethal - Insufficient to cause death.
Total Deposition - Total atmospheric deposition is determined using both wet and dry deposition
Wet deposition - Wet deposition is the portion of total atmospheric deposition dissolved in cloud droplets and is deposited during rain or other forms of precipitation.
Al - Aluminum
Ca2+ - Calcium
CH4 - Methane
H+ - Hydrogen ion
HCO3- - Bicarbonate
HNO3 - Nitric acid
Mg2+ - Magnesium
K+ - Potassium
Na+ - Sodium
N2 - Diatomic nitrogen
N2O - Nitrous oxide
NH3 - Ammonia
NH4+ - Ammonium
NO - Nitric Oxide
NO2 - Nitrogen dioxide
NOX - Sum of NO and NO2
NO3- - Nitrate
O3 - Ozone
SO2 - Sulfur Dioxide
SO42- - Sulfate
Monday, March 1
I. Setting Policy, Management, and Scientific Context
12:30-12:45pm Introduction and Opening Remarks
Welcome-Mary Barber (Ecological Society of America)
Scope and purpose of meeting-Rich Pouyat (US Forest Service)
12:45-1:15pm Acid Rain Policy and Initial Results of Acid Rain Program-
Brian McLean (EPA)
Brief acid rain policy history
Current policy and management needs/questions
Emission trends (historic, future projections)
1:15-1:35pm Wet and dry deposition trends-
James Lynch (Pennsylvania State University)
1:35-2:00pm Historical Perspective On Research-
James Galloway (University of Virginia)
II. Atmospheric Deposition: Evaluating the Ecological Response
2:30- 2:45pm Present workshop framing questions
2:45-4:15pm Ecological Responses to Atmospheric Deposition
Trends in surface water chemistry and biology-
John Stoddard (EPA)
Trends in watershed/soil chemistry and biology-
Greg Lawrence (USGS)
Trends in forest response (USFS)
Walter Shortle (USFS)
4:15-4:30pm Integrated Assessment-Putting the Pieces Together
Steve Kahl (University of Maine)
A multi-ecosystem assessment of acid-base changes in Maine aquatic systems since 1982.
4:30-5:30pm Breakout session I: What have we learned about the effects of atmospheric deposition since the 1990 synthesis? How do the scientific conclusions of the 1990 NAPAP Report hold up in light of new data?
Trends in surface water chemistry and biology?
Trends in watershed/soil chemistry and biology?
Trends in Forest Response?
Tuesday, March 2
8:00-8:30am Continental Breakfast
8:30-10:30am Breakout session I: continued
10:30-11:30am Plenary: Breakout group I summary presentations and discussion
11:30-1:00pm How well can we predict ecological response to changes in atmospheric deposition?-
Experimental approaches- John Aber (University of New Hampshire)
Modeling approaches- Jack Cosby, Jr. (University of Virginia)
2:00-5:00pm Breakout Session II: What determines sensitivity of ecosystems to changes in atmospheric deposition? What is the potential for recovery of sensitive ecosystems?
Aquatic breakout group
Facilitator-Mark Nilles (USGS)
Rapporteur-Timothy Sullivan (E&S Environmental Chemistry)
Terrestrial breakout group
Facilitator-Rona Birnaum (EPA)
Rapporteur-Gary Lovett (IES)
Breakout group II summary presentations and discussion
Wednesday, March 3
- Trends in coastal watersheds (15 min). Report from ESA Narragansett workshop
(objectives, format, outcomes to provide model for workshop Report)
- Highlights from breakout group summary and presentations
- Develop workshop recommendations and report
Workshop Participants and Planning Group
Workshop Planning Group
- Stephanie Benkovic (EPA)
- Rona Birnbaum (EPA)
- Greg Lawrence (USGS)
- Mark Nilles (USGS)
- Richard Pouyat (USFS)
- Paul Ringold (EPA)
- Doug Ryan (USFS)
- Michael Uhart (NOAA)
- Kathie Weathers (Institute of Ecosystem Studies)
- John Aber (University of New Hampshire)
- Jill Baron (USGS, Natural Resource Ecology Lab)
- Douglas Burns (USGS)
- Don Cambell (USGS)
- Robbins Church (EPA)
- Jack Cosby (University of Virginia)
- Ellis Cowling (North Carolina State University)
- Brian Cumming (Queens University)
- Mark David (University of Illinois)
- Charley Driscoll (Syracuse University)
- Keith Eshlemann (University of Maryland)
- Jennifer Kramer (EPA)
- Kathy Fallon -Lambert (Hubbard Brook Research Foundation)
- Ivan Fernandez (University of Maine)
- Guy Fenech (Environment Canada)
- Mark Fenn (USFS)
- James Galloway (University of Virginia)
- Robert Goldstein (Electric Power Research Institute)
- John Hom (USFS)
- Rick Haeuber (Ecological Society of America)
- Dean Jeffries (National Water Research Institute, Environment Canada)
- Art Johnson (University of Pennsylvania)
- Steve Kahl (University of Maine)
- Kim Kirchner (University of California, Berkeley)
- Dennis Lemly (USFS)
- Orie Loucks (Miami University)
- James Lynch (Pennsylvania State University)
- Gary Lovett (Institute for Ecosystem Studies)
- Brian McLean (EPA)
- John Melack (University of California, Santa Barbara)
- Myron Mitchell (SUNY-Syracuse)
- Peter Murdoch (USGS)
- Steve Norton (University of Maine)
- Ellen Porter (USFW)
- Walter Shortle (USFS)
- John Stoddard (EPA)
- Tim Sullivan (E&S Environmental Chemistry)
- Helga Van Miegroet (Utah State University)
- Ann Watkins (EPA)
- Jim Wigington (EPA)
Copies of this report are available from: