Abstract  Significant uncertainty exists in the magnitude and variability of ammonia (NH3) emissions. NH3 emissions are needed as input for air quality modeling of aerosols and deposition of nitrogen compounds. Approximately 85% of NH3 emissions are estimated to come from agricultural nonpoint sources, which are suspected to have a strong seasonal pattern. Because no seasonal information is available in current NH3 emission inventories for air quality modeling, the emissions are often distributed evenly over the year by default. Doing so can adversely affect air quality model-predicted concentrations of nitrogen-containing compounds, as shown here. We apply a Kalman filter inverse modeling technique to deduce monthly 1990 NH3 emissions for the eastern United States. The U. S. Environmental Protection Agency (USEPA) Community Multiscale Air Quality (CMAQ) model and ammonium (NH4+) wet concentration data from the National Atmospheric Deposition Program network are used. The results illustrate the strong seasonal differences in NH3 emissions that were anticipated, where NH3 emissions are more than 75% lower during the colder seasons fall and winter as compared to peak emissions during summer. The results also suggest that the current USEPA 1990 National Emission Inventory for NH3 is too high by at least 20%. This is supported by a recent USEPA study of emission factors that proposes lower emission factors for cattle and swine, which are two of the largest sources of NH3 emissions in the inventory.

Abstract  Previous inventories of ammonia emissions for the United States have not characterized the seasonal and geographic variations that are necessary for accurately predicting ambient concentrations of ammonium nitrate and ammonium sulfate aerosol. This research calculates the seasonal and geographic variation in ammonia emissions from dairy cows in the United States. Monthly, county-level emission factors are calculated with a process- based model of dairy farm emissions, the national distribution of farming practices, seasonal climate conditions, and animal populations. Annual, county-level emission factors are estimated to range between 13.1 and 55.5, with a national average of 23.9 kg NH3 cow(-1) yr(-1). The seasonal variation of the emission factor is estimated to be as high as a factor of seven in some counties. Emissions are predicted to be the highest in the spring and fall, because of high manure application rates during the spring planting and after the fall harvest. Summer emissions are higher than winter, resulting from the temperature dependence of housing and storage emissions. In the summer and winter, the majority of emissions are from animal housing. In the spring and fall, the majority of emissions are from field applied manure. The 5% and 95% confidence interval about the national annual average emission factor is between 18 and 36 kg NH3 COW I yr(-1). Uncertainties in farming practices contribute most to the total uncertainty, yet uncertainty in the timing of manure application, the quantity of manure and nitrogen excreted by cows, and the physical processes of volatilization affecting applied manure are also significant. (C) 2004 Elsevier Ltd. All rights reserved.

Abstract  Rates of atmospheric deposition of biologically active nitrogen (N) are two to seven times the pre-industrial rates in many developed nations because of combustion of fossil fuels and agricultural fertilization. They are expected to increase similarly over the next 50 years in industrializing nations of Asia and South America. Although the environmental impacts of high rates of nitrogen addition have been well studied, this is not so for the lower, chronic rates that characterize much of the globe. Here we present results of the first multi-decadal experiment to examine the impacts of chronic, experimental nitrogen addition as low as 10 kg N ha(-1) yr(-1) above ambient atmospheric nitrogen deposition (6 kg N ha(-1) yr(-1) at our site). This total input rate is comparable to terrestrial nitrogen deposition in many industrialized nations. We found that this chronic low-level nitrogen addition rate reduced plant species numbers by 17% relative to controls receiving ambient N deposition. Moreover, species numbers were reduced more per unit of added nitrogen at lower addition rates, suggesting that chronic but low-level nitrogen deposition may have a greater impact on diversity than previously thought. A second experiment showed that a decade after cessation of nitrogen addition, relative plant species number, although not species abundances, had recovered, demonstrating that some effects of nitrogen addition are reversible.

Abstract  Global energy use and food production have increased nitrogen inputs to ecosystems worldwide, impacting plant community diversity, composition, and function. Previous studies show considerable variation across terrestrial herbaceous ecosystems in the magnitude of species loss following nitrogen (N) enrichment. What controls this variation remains unknown. We present results from 23 N-addition experiments across North America, representing a range of climatic, soil and plant community properties, to determine conditions that lead to greater diversity decline. Species loss in these communities ranged from 0 to 65% of control richness. Using hierarchical structural equation modelling, we found greater species loss in communities with a lower soil cation exchange capacity, colder regional temperature, and larger production increase following N addition, independent of initial species richness, plant productivity, and the relative abundance of most plant functional groups. Our results indicate sensitivity to N addition is co-determined by environmental conditions and production responsiveness, which overwhelm the effects of initial community structure and composition.

Abstract  An important consequence of acidification is the mobilization of Al from the edaphic to the aquatic environment. Elevated Al levels in acidic waters may be toxic to fish. Eggs, larvae, and postlarvae of white suckers (Catostomus commersoni) and brook trout (Salvelinus fontinalis) were exposed in laboratory bioassays to pH levels 4.2 to 5.6 and inorganic Al concentrations of 0 to 0.5 mg l-1. Aluminum toxicity varied with both pH and life history stage. At low pH levels (4.2 to 4.8), the presence of Al (up 0.2 mg l-1 for white suckers; 0.5 mg l-1 for brook trout) was beneficial to egg survival through the eyed stage. In contrast, Al concentrations of 0.1 mg l-1 (for white suckers) or 0.2 mg l-1 (for brook trout) and greater resulted in measurable reductions in survival and growth of larvae and postlarvae at all pH levels (4.2 to 5.6). Aluminum was most toxic in over-saturated solutions at pH levels 5.2 to 5.4. The simultaneous increase in Al concentration with elevated acidity must be considered to accurately assess the potential effect of acidification of surface waters on survival of fish populations.

Abstract  Increased losses of nitrate from watersheds may accelerate the depletion of nutrient cations and affect the acidification and trophic status of surface waters. Patterns of nitrate concentrations and losses were evaluated in four forested watersheds (East Bear Brook Watershed, Lead Mountain, ME; Watershed 6, Hubbard Brook Experimental Forest, White Mountains, NH; Arbutus Watershed, Huntington Forest, Adirondack Mountains, NY; Biscuit Brook, Catskill Mountains, NY) located across the northeastern United States. A synchronous pattern was observed in nitrate concentrations of drainage waters from these four sites from 1983 through 1993. Most notably, high concentrations and high drainage water losses followed an anomalous cold period (mean daily temperature -11.4 to -16 C in December 1989) for all four sites. After high nitrate losses during the snowmelt of 1990, nitrate concentrations and fluxes decreased at all sites. These results suggest that climatic variation can have a major effect on nitrogen flux and cycling and may influence temporal patterns of nitrate loss in a region.

Abstract  Limited stream chemistry and macroinvertebrate data indicate that acidic deposition has adversely affected benthic macroinvertebrate assemblages in numerous headwater streams of the western Adirondack Mountains of New York. No studies, however, have quantified the effects that acidic deposition and acidification may have had on resident fish and macroinvertebrate communities in streams of the region. As part of the Western Adirondack Stream Survey, water chemistry from 200 streams was sampled five times and macroinvertebrate communities were surveyed once from a subset of 36 streams in the Oswegatchie and Black River Basins during 2003–2005 and evaluated to: (a) document the effects that chronic and episodic acidification have on macroinvertebrate communities across the region, (b) define the relations between acidification and the health of affected species assemblages, and (c) assess indicators and thresholds of biological effects. Concentrations of inorganic Al in 66% of the 200 streams periodically reached concentrations toxic to acid-tolerant biota. A new acid biological assessment profile (acidBAP) index for macroinvertebrates, derived from percent mayfly richness and percent acid-tolerant taxa, was strongly correlated (R2 values range from 0.58 to 0.76) with concentrations of inorganic Al, pH, ANC, and base cation surplus (BCS). The BCS and acidBAP index helped remove confounding influences of natural organic acidity and to redefine acidification-effect thresholds and biological-impact categories. AcidBAP scores indicated that macroinvertebrate communities were moderately or severely impacted by acidification in 44–56% of 36 study streams, however, additional data from randomly selected streams is needed to accurately estimate the true percentage of streams in which macroinvertebrate communities are adversely affected in this, or other, regions. As biologically relevant measures of impacts caused by acidification, both BCS and acidBAP may be useful indicators of ecosystem effects and potential recovery at the local and regional scale.

Abstract  Acidification has caused the loss or reduction of numerous Atlantic salmon (Salmo salar L.) populations on both sides of the North Atlantic. Acid deposition peaked in the 1980's and resulted in both chronically and episodically acidified rivers. At present, water quality is improving in all affected rivers due to reduced acid deposition. However, spring snow melt, heavy rainfall and sea salt episodes can still cause short term drops in pH and elevated concentrations of bioavailable aluminum. Technical malfunction in lime dozers will cause short termed episodic spates in the limed rivers. The current situation has prompted a need for dose-response relationships based on short term exposures of Atlantic salmon to assess the potential population effects of episodic acidification. Water quality guidelines for salmon have been lacking, despite a large number of experiments, all demonstrating dose-response relationships between water chemistry and fish health. We have summarized results from 347 short-term (< 14 days) exposures of salmon parr and smolt performed between 1990 and 2003 in Norway. The experiments have been performed as bioassays, where fish have been exposed in tanks fed river water, in tanks where the river water quality has been manipulated (added H+ and Al) and as Carlin-tagged smolt releases after pre-exposure to moderately acidic waters. The results from the various bioassays are compared to water quality limits proposed on basis of the relationship between water quality and population status/health in Norwegian rivers. The focus of this article is placed on chemical-biological interactions that can be drawn across experiments and exposure protocols. We propose dose-response relationships for acid neutralizing capacity (ANC), pH, cationic Al and gill accumulated Al, versus mortality in freshwater, effects on hypo-osmoregulatory capacity in seawater challenge tests and on smolt to adult survival in release experiments. The "no effect" dose depends on the life history stage tested and on the sensitivity of the biomarkers. Parr are more tolerant than smolt. Concentrations of Al that have no significant impact on freshwater life history stages can still have major population effects if they occur prior to smolt migration. While smolt can survive in freshwater for a prolonged period of time (> 10 days) at an Al dose resulting in a gill Al concentration of up to 300 mu g Alg(-1) dw, a 3 day exposure resulting in a gill Al accumulation in the range of 25 to 60 mu g Alg(-1) dw reduces smolt to adult survival in a dose related manner by 20 to 50%. For smolt to adult survival, the biological significant response is delayed relative to the dose and occurs first after the fish enters the marine environment. In addition to exposure intensity and timing, exposure duration is important for the setting of critical limits.

Abstract  Within-canopy profiles of ammonia (NH(3)) and measurements of the canopy turbulence characteristics were used to calculate the vertical source/sink density profile of NH(3) and sensible heat in a mature oilseed rape (Brassica napus) canopy. For the analysis, the inverse Lagrangian technique (ILT) based on localized near-field theory was used. Turbulence was measured with a standard ultrasonic anemometer, which, although not ideal for in-canopy work, is estimated to lead to a parameterization of the normalized standard deviation of the vertical wind component (sigma (w)/u*), which is +/-11% accurate for heights >0.16 m during the day.

The NH(3) profiles in the canopy consistently show largest concentrations at the ground caused by NH(3) release from decomposing litter leaves on the ground surface with values of up to 150 ng m(-2) s(-l) predicted by the ILT. The inverse Lagrangian source/sink analysis performs well for both sensible heat and NH(3), although it proves to be sensitive to the choice of the source/sink layers and becomes uncertain at the ground. Despite the large estimated ground level emission (26 g NH(3)-N ha(-1) per day), the analysis indicates that for the runs considered all NH(3) is recaptured by the lowest 0.7 m of the 1.38 m tall canopy, and that the bi-directional net exchange with the atmosphere is governed by the top 0.5 m, leading to a net emission from the canopy of 12 g NH(3)-N ha(-1) per day. Since measurements of apoplastic [NH(4)(+)] and pH indicate that no significant stomatal emission from foliage should have occurred, this suggests that the siliques were a further source of NH(3). (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract  The effects of simulated acid rain on dogwood anthracnose severity were evaluated in a series of greenhouse and field experiments over a 4-year period. In 1990 and 1991, Cornus florida seedlings received 10 weekly foliar applications of simulated rain adjusted to pH 2.5, 3.5, 4.5, and 5.5. They were then placed under mature dogwoods naturally infected with Discula de-structiva. In both years, the percent leaf area infected increased significantly as the pH of the simulated rain solution decreased. In 1992 and 1993, seedlings were wrapped in plastic bags below the root collar to permit separate application of simulated acid rain (pH 2.5) or normal rain (pH 5.5) to the foliage or the growing medium or both. Application of pH 2.5 rain to the growing medium increased disease severity. Foliar applications alone did not increase disease. These results suggest that changes in nutrient availability, rather than foliar damage, are responsible for the increase in anthracnose severity in dogwoods pretreated with simulated acid rain.

Abstract  Vehicle exhaust emissions are a dominant feature of urban environments and are widely believed to have detrimental effects on plants. The effects of diesel exhaust emissions on 12 herbaceous species were studied with respect to growth, flower development, leaf senescence and leaf surface wax characteristics. A diesel generator was used to produce concentrations of nitrogen oxides (NO(x)) representative of urban conditions, in solardome chambers. Annual mean NO(x) concentrations ranged from 77 nl l(-l) to 98 nl l(-1), with NO:NO(2) ratios of 1.4-2.2, providing a good experimental simulation of polluted roadside environments. Pollutant exposure resulted in species-specific changes in growth and phenology, with a consistent trend for accelerated senescence and delayed flowering. Leaf surface characteristics were also affected; contact angle measurements indicated changes in surface wax structure following pollutant exposure. The study demonstrated clearly the potential for realistic levels of vehicle exhaust pollution to have direct adverse effects on urban vegetation.

Abstract  The Clean Air Status and Trends Network (CASTNet) was established by the U.S. EPA in response to the requirements of the 1990 Clean Air Act Amendments. To satisfy these requirements CASTNet was designed to assess and report on geographic patterns and long-term, temporal trends in ambient air pollution and acid deposition in order to gauge the effectiveness of current and future mandated emission reductions. This paper presents an analysis of the spatial patterns of deposition of sulfur and nitrogen pollutants for the period 1990-2000. Estimates of deposition are provided for two 4-yr periods: 1990-1993 and 1997-2000. These two periods were selected to contrast deposition before and after the large decrease in SO2 emissions that occurred in 1995. Estimates of dry deposition were obtained from measurements at CASTNet sites combined with deposition velocities that were modeled using the multilayer model, a 20-layer model that simulates the various atmospheric processes that contribute to dry deposition. Estimates of wet deposition were obtained from measurements at sites operated bythe National Atmospheric Deposition Program. The estimates of dry and wet deposition were combined to calculate total deposition of atmospheric sulfur (dry SO2, dry and wet SO4(2-)) and nitrogen (dry HNO3, dry and wet NO3-, dry and wet NH4+). An analysis of the deposition estimates showed a significant decline in sulfur deposition and no change in nitrogen deposition. The highest rates of sulfur deposition were observed in the Ohio River Valley and downwind states. This region also observed the largest decline in sulfur deposition. The highest rates of nitrogen deposition were observed in the Midwest from Illinois to southern New York State. Sulfur and nitrogen deposition fluxes were significantly higher in the eastern United States as compared to the western sites. Dry deposition contributed approximately 38% of total sulfur deposition and 30% of total nitrogen deposition in the eastern United States. Percentages are similar for the two 4-yr periods. Wet sulfate and dry SO2 depositions were the largest contributors to sulfur deposition. Wet nitrate, wet ammonium, and dry HNO3 depositions were the largest contributors to nitrogen deposition.

Abstract  Biochemical and physiological experiments were conducted on pea plants (Pisum sativum) continuously exposed in growth chambers to SO2 gas for 18 days. S02 gas concentrations were 0.1, 0.15, and 0.25 ppm. In plants exposed to 0.1 and 0.15 ppm it was clearly demonstrated that there was a greater accumulation of inorganic sulfur, a reduced buffer capacity of the cells relative to H-ions, and a stimulation of glutamate dehydrogenase activity. The only macroscopic symptom seen was slight chlorosis of the older leaves. There was only a slight decrease in fresh and dry weights of these plants compared to the control plants whereas in the group of plants exposed to 0.25 ppm SO2 foliage necrosis was considerable. In addition, there was a marked reduction in the fresh and dry weights of the latter plants. However, the relationship among accumulated inorganic sulfur, reduced buffer capacity, and increased glutamate dehydrogenase activity as seen for the lower S02 concentrations was close. Accordingly, if might be possible to use these three parameters to diagnose S02 injury before any significant symptoms appear. In the case of severe SO2 injury there was a marked increase in glutamine and ammonia concentrations suggesting that these factors in addition to the above could be used in diagnosing severe SO2 injury. There was no significant difference between plants treated with 0.1 or 0.15 ppm SO2 and control plants in the contents of K, Ca, P, and N fractions. Therefore, these factors would not be useful in the early detection of SO2 injury.

Abstract  Recognizing the central role that science and engineering plays in air quality management and anticipating the next congressional reauthorization of the Clean Water Act and its amendments, the United States Congress directed the U.S. Environmental Protection Agency (EPA) to arrange for a study by the National Academy of Sciences (1) to evaluate from a scientific and technical perspective the effectiveness of the major air quality provisions of the Clean Air Act and their implementation by federal, state, tribal, and local government agencies; and (2) to develop scientific and technical recommendations for strengthening the nation's air quality management system with respect to the way it identifies and incorporates important sources of exposure to humans and ecosystems and integrates new understandings of human and ecosystem risks. In response, the National Research Council established the Committee on Air Quality Management in the United States, which prepared this report.

Abstract  An investigation was conducted over 20 weeks during winter to determine the effects of 6·8 pphm (194 μg m−3) SO2 and 6·8 pphm NO2, applied both singly and in combination, on the growth of Dactylis glomerata L. and Poa pratensis L. The combination of SO2 + NO2, and singly applied SO2, caused significant reductions in the leaf areas and all the dry weight fractions measured of both species. These were associated with reductions in the numbers of tillers and leaves produced by both species in the SO2 + NO2 treatment, but only by P. pratensis when SO2 alone was present. Singly applied NO2 had little effect on the growth of D. glomerata but caused significant reductions in leaf area and all the measured dry weight fractions of P. pratensis. There was some evidence to suggest that NO2 was more toxic during the mid-winter months when growth was slow. It is concluded that SO2 and NO2 in combination may be more toxic to plants than would be predicted by summing their individual effects on growth.

Abstract  A survey of 752 lakes in the mountainous areas of the western United States was conducted in fall 1985 by the U.S. Environmental Protection Agency in cooperation with the USDA-Forest Service. The survey was a stratified design with equal allocation among strata allowing population estimates of physical and chemical characteristics of lakes within the study area to be developed. The five subregions surveyed included California, Pacific Northwest, Northern Rockies, Central Rockies, and Southern Rockies. Almost 37% of the lakes in California had acid neutralizing capacity (ANC) 50 ueq/l, whereas only 4.6% of the lakes in the Southern Rockies had ANC 50 ueq/l. Only one acidic lake (ANC 0), associated with hot springs, was samples in the West. Ninety-nine percent of the lakes in the study area had pH values 6.0. Concentrations of many chemical constituents were low in western lakes compared to values measured in the Eastern Lake Survey. Over 26% of the western lakes had conductance measurements 10 uS/cmi the highest percentage of these dilute lakes was located in California. Evaluation of ion ratios and the relationship of base cations to ANC suggest that regional lake acidification has not occurred in the areas of the West.

Abstract  Quantitative predictions of the effects of acid deposition onterrestrial and aquatic systems require physically based, process-oriented models of catchment soil water and streamwater chemistry. A desirable characteristic of such models is that they include terms to describe the important phenomena controlling a system's chemical response to acidic deposition, yet be restricted in complexity so that they can be implemented on diverse systems with a minimum of a priori data. We present an assessment of a conceptual model of soil water and streamwater chemistry based on soil cation exchange, dissolution of aluminum hydroxide, and solution of carbon dioxide, all processes that occur in catchment soils and that have rapid equilibration times. The model is constructed using an “average” or lumped representation of these spatially distributed catchment processes. The adequacy of the model is assessed by applying it to 3 years of soil water and streamwater chemistry data from White Oak Run, Virginia, a second-order stream in the Shenandoah National Park. Soil properties predicted by the model are in good agreement with presently available measurements of those soil properties. The success of the model suggests that lumped representations of complex and spatially distributed chemical reactions in soils can efficiently describe the gross chemical behavior of whole catchments (e.g., pH, alkalinity, and major ionic concentrations in surface waters). Further assessment of the adequacy of this conceptual approach will require more detailed empirical knowledge of the soil processes being modeled, particularly soil cation exchange and the variability of soil CO2 partial pressures.

Abstract  The electrical conductivity of stream water draining from an unmanaged and undisturbed control watershed has been increasing rather steadily, about 0.03 mS m-1 yr-1, since 1971. During this period, NO3- and Ca2+ concentrations increased and were shown to mathematically account for the ionic contribution to conductivity; therefore, they are believed to be primarily responsible for the increase. However, the percentage of conductivity explained by the two ions was different over time. The percentage of conductivity attributable to NO3- increased in a pattern very similar to concentration. In contrast, the percentage of conductivity attributable to Ca2+ decreased slightly over time. The Ca2+ is believed to be pairing with the NO3- as the NO3- ions leach through the soil. While nitrification in mature stands can be strongly inhibited, limited nitrification, especially in forest gaps, and high anthropogenic inputs of NO3- probably were primary sources of the leached NO3-. Preferential adsorption of SO4(2-), rather than NO3-, on soil colloids is given as an explanation for the lack of retention of NO3- in the soil system and subsequent leaching to the stream.

Abstract  For more than half a century, the trophic status of water bodies has been of interest to limnologists and oceanographers alike. This report demonstrates the close, inverse relationship between 14 C-estimated primary productivity and transparency during the earliest stages of cultural eutrophication. As the population in the Tahoe basin has rapidly increased, Lake Tahoe has been characterized by an increase in primary productivity has averaged 5.6yr-1 for the last 28 yr. There has been a concomitant decline in transparency of 0.37 m yr-1. During winter months when transparency is highest, the average annual loss has been slightly greater (0.40 m yr-1). The average annual Secchi depth has decreased by 7 m during the last 19 yr of intensive conitoring. There has also been a significant increase in the ligth extinction coefficient. Photosynthetic efficiency has increased while there has been a gradual shrinkage of the euphotic zone. During the same period the total NO3- -N content of the lake has increased significantly, but total P content has not. Lake Tahoe primary production has become increasingly P sensitive during the last decade as N has accumulated in the system. A gradual increase in the N:P may prove to be a general evolutionary characteristic of oligotrophic lakes during the earliest stages of eutrophication. The importance of long-term studies in detecting gradual change that may be masked by considerable interannual variability is particularly evident from this study.

Abstract  Measurements have been made of the deposition of cloud droplets and dissolved chemical species to moorland vegetation. Measurements of liquid water fluxes have been made by the gradient technique using Knollenberg FSS probes and by a weighing lysimeter. Deposition velocities for cloud liquid water in the range 0.1 to 0.24gm-3 at a height of l.0 in were close to those for momentum. The FSS probes enabled the determination of deposition velocities as a function of droplet size. It was found that the deposition velocity increased with droplet radius from 2.5 Ám to 6.5 Ám radius. Between 6.5 and 12.5 Ám radius, deposition velocities exceeded those for momentum, but decreased sharply between 12.5 and 15.5 Ám. In periods of thin cloud (<0.1 gm-3), evaporation from the surface occurred despite continued deposition.

Abstract  The nature of nitrate (14797558) (NO3) in coarse continental environments was studied. NO3 aerosols were collected in two size fractions at a remote site in South Dakota, an urban site in Denver, a rural site in Louisiana near the Gulf Coast, a rural site in the Blue Ridge Mountains of Virginia, and an urban site in Detroit. Several different types of samples were used during the investigation. One sampler collected aerosols up to 30 micrometers (microm) in diameter while another collected aerosols less than 2.5microm in diameter. A manual dichotomous sampler was used to collect fractions less than 3.5microm and fractions between 3.5 and 15microm. Subsequent NO3 and sulfate (14808798) (SO4) determinations were made using ion chromatography. In South Dakota, Louisiana, Virginia, and Detroit, Michigan, the percent coarse NO3 fractions were 77, 76, 72, and 90 percent, respectively. However, in Denver only 18 percent of NO3 was collected in the coarse fractions. The SO4 was predominantly in the fine fraction at all sites. The author concludes that strong correlations between NO3 and the soil constituents aluminum (7429905), silicon (7440213), calcium (7440702), titanium (7440326), and iron (7439896) suggest that NO3 is associated with entrained soil dust.

Abstract  These Agency-wide guidelines are provided to improve the quality and consistency of EPA's ecological risk assessments. As a next step in a continuing process of ecological risk guidance development, the guidelines draw from a wide range of source documents including peer-reviewed issue papers and case studies previously developed by EPA's Risk Assessment Forum. The Guidelines expand on and replace the 1992 report Framework for Ecological Risk Assessment. EPA plans to follow the Guidelines with more detailed guidance in specific areas. A major theme of the guidelines is the interaction among risk assessors, risk managers, and interested parties at the beginning (planning and problem formulation) and end (risk characterization) of the risk assessment process. In problem formulation, the guidelines emphasize the complementary roles of each in determining the scope and boundaries of the assessment, selecting ecological entities that will be the focus of the assessment, and ensuring that the product of the assessment will support environmental decision making. The risk characterization section discusses estimating, interpreting, and reporting risks and applies an ecological perspective to recent Agency policy encouraging clear, transparent, reasonable, and consistent risk characterizations. The Guidelines emphasize that the interface between risk assessors, risk managers, and interested parties is critical for ensuring that the results of the assessment can be used to support a management decision. These Guidelines are not regulations and do not impose any new requirement on the regulated community. Rather, the Guidelines are internal guidance for EPA and inform the public and the regulated community regarding the Agency's approach to ecological risk assessment.