Results:

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.

Conclusions:

• 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.

Monitoring

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.

Research

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.

Policy Context

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 (NO_X), 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 NO_X 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.

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:

          Ecosystem responses to acid deposition: non-linear, case specific, and sub-lethal

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.

Sulfur Deposition

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.
 

Nitrogen Deposition

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 (NO_X, 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 NO_X 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.
 
 

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).
 
 

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.
 
 

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.
 
 

• 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 (SO₄^2-) 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 deposition.
 
 

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.