Characterization of Critical Load Exceedances in Europe

The excess of acidic and eutrophying depositions over critical loads (critical load exceedances) is considered a measure for the risk of harmful effects on sensitive elements of the environment. The magnitude and the geographical distribution of critical load exceedances over Europe vary with the extent to which national emissions of sulfur dioxide, nitrogen oxide and ammonia are reduced. The scientific support of negotiations on emission reductions in the framework of the Convention on Long-range Transboundary Air Pollution (LRTAP) of the UN Economic Commission for Europe has been based on the integrated assessment of sources, including abatement costs, and risks to receptors (e.g. forests, lakes) using critical load exceedances. The shift from a single-pollutant (sulfur) protocol in 1994 to a multi-pollutant protocol in 1999 necessitated an extension of the methods by which critical load exceedances are computed and mapped. The focus changed from the protection of the most sensitive ecosystem against excessive deposition of one pollutant, to an assessment of the accumulated exceedance by more pollutants of all ecosystems. This paper presents and compares the different characterisations ("gap-closure methods") used in those negotiations. It is shown that the approach finally used has several appealing features, but treats the exceedance as a linear damage function, thus going beyond the critical load definition as a simple on-off limit value.     

Critical loads of sulfur and nitrogen for lakes II: Regional extent and variability in Finland

The concept of critical loads has been developed to assist in the design of environmentally sound abatement strategies for the emissions of acidifying compounds. In this paper the critical loads of S and N for lakes in Finland are computed and mapped, based on methods presented in an accompanying paper. The employed steady-state mass balance model allows the simultaneous evaluation of the reductions required of S and N deposition exceeding these critical loads. Special emphasis has been put on the presentation of the spatial variability and the uncertainty of the critical loads and their exceedances. The derived critical loads of S and N for lakes in Finland show a substantial spatial variability. The highest exceedance of critical loads is presently estimated in the south-east of the country, where up to 80% of the lakes show an exceedance of the critical loads of S. The evaluation of two emission scenarios shows that only “maximum feasible reductions” would be sufficient for protecting most Finnish lakes from the impacts of acidic deposition. The results of this study form a basis for setting national targets for emission reductions in Finland.     

Effects of Site Selection Strategy on Freshwater Critical Load Exceedances in Wales

Critical loads are used in international negotiations to reduce acid deposition resulting from emissions of sulphur and nitrogen compounds within Europe. For freshwater ecosystems, the First-order Acidity Balance (FAB) model is used to generate national maps of critical loads and exceedances for both sulphur (S) and nitrogen (N). In Wales, two survey datasets have been used to calculate critical loads and exceedances; one based on water bodies selected to be "most-sensitive" to acidification within a 10 km grid and the other based on a random selection of standing waters. Both datasets indicate that critical loads were exceeded in 1990 in a significant proportion of Welsh lakes and streams; 36% of sites in the grid-based survey and 31% of sites in the random survey. However, implementation of the Gothenburg Protocol would protect all but 6% of sites in the grid-based survey and all sites in the random survey. Assessment of the relative success of the Gothenburg Protocol in protecting Welsh freshwater ecosystems therefore depends on the site selection strategy employed.     

The use of critical load exceedances in abatement strategy planning

Critical loads for sulphur and nitrogen are defined to produce effective control strategies over Europe, such as those of the new sulphur protocol. To determine the critical load exceedances on the European scale it is necessary to simplify and generalize. The spatial variation on a scale smaller than the 150 × 150 km EMEP grid squares is considered for critical loads, via a cumulative frequency distribution and the 95 percentile for the grid square is determined. The deposition is assumed to be uniform over the area and the exceedance over the 95 percentile critical load is determined. In reality, the spatial variation is considerable for critical loads as well as for deposition. Calculations based on the frequency of local critical load exceedances have been made for two grid squares in southern Sweden. Local critical loads for acidity are compared to local deposition. Deposition variations due to pollution gradients within the square and to ecosystem structure have been considered. The results are similar for the two squares. The calculations based on local exceedances on 50×50 km grid squares and consideration to landuse variability, indicate that in order to protect 95% of the ecosystems in the square, emission reductions 25% greater than the large-scale European approach are needed. The effect of enhanced deposition at forest edges is of relatively small importance for the total exceedance.     

Watershed vs in-lake alkalinity generation: A comparison of rates using input-output studies

As a means of assessing the relative contributions of watershed (terrestrial) and in-lake processes to overall lake/watershed alkalinity budgets, alkalinity production rates for watersheds and low alkalinity lakes were compiled from the literature and compared. Based on net alkalinity production data, derived using wet or bulk deposition data, mean and median alkalinity production for 20 watersheds in North America and Europe were 89 and 69 meq m^?2 yr^?1 (range 20 to 235 meq m^?2 yr^?1). For a subset of 10 watersheds with dry deposition data, terrestrial alkalinity production neutralized an additional 35 meq m^?2 yr^?1 of acidic deposition. For 11 lakes, mean and median in-lake alkalinity generation were 99 and 88 meq m^?2 yr^?1 (range 22 to 240 meq m^?2 yr^?1). Analysis of data indicates that for the low alkalinity systems described here, areal alkalinity production rates for watersheds and lakes are approximately equal. This relationship suggests that watershed area to lake area ratio can be used as a convenient estimator of the relative importance of watershed and in-lake sources of alkalinity for drainage lake systems. For precipitation-dominated seepage lakes and other systems where hydrology limits soil-water contact, hydrologic flow paths and residence times can be of overriding importance in determining alkalinity sources. For regions dominated by drainage lakes with high watershed area to lake area ratios (such as the Northeastern U.S.), however, alkalinity budgets are dominated by watershed processes. Omission of in-lake alkalinity consideration for most lakes in such regions would have little impact on computed alkalinity budgets or on predicted response to changes in acidic deposition loadings.     

Integrated Modelling of Acidification Effects to Forest Ecosystems — Model Sonox

The model SONOX has been developed that replicates the sequence of events on the way from emission of sulfur and nitrogen to the potential risk to forest ecosystems as reflected by critical loads exceedance. The model produces a set of emission, deposition and critical load maps in various spatial resolutions as well as maps of critical loads exceedances dynamically generated in response to various sulfur and nitrogen emission scenarios. Optionally, also the share of forest areas protected or unprotected against acidification is spatially presented. The model provides a decision tool to develop strategies aimed at the abatement of excess sulfur and nitrogen emissions. The present version of the model has been tailored to and implemented for Polish conditions. This model has been used to analyse the trends in impacts of acid deposition on Polish forest ecosystems.     

Sulphate in Sudbury, Ontario, Canada, Lakes: Recent Trends and Status

Sulphur emissions from the Sudbury, Ontario, metal smelting industry have affected thousands of lakes in Ontario, Canada. Reductions in these emissions during the 1970's resulted in reduced lakewater SO₄ concentrations and other water quality changes in the 1970's and 1980's. Further declines in lakewater SO₄ concentrations have accompanied additional recent S emission reductions achieved by 1994. Recent (1997) SO₄ concentrations are still related to distance from the Sudbury smelters. A strong inverse relationship with distance is evident to about 45 km, and is most pronounced in lakes within about 20 km. In lakes beyond 45 km, dissolved organic carbon (DOC), which was correlated with hydrological response time and total phosphorus concentrations, was the best correlate with recent SO₄ concentrations, indicating that some slowly-flushing, oligotrophic lakes still exhibit a "Sudbury" effect. Most lakes beyond 45 km, however, showed SO₄ declines and recent SO₄ concentrations comparable to lakes around Dorset, ～200 km from Sudbury, suggesting that these lakes are now most affected by the long-range atmospheric transport of S.     

Modeling Effects of Changing Deposition and Forestry on Nitrogen Fluxes in a Northern River Basin

The application of a new, spatial nitrogen leaching/retention model N_EXRET to the Oulujoki river basin (22800 km²) in Finland is discussed. The model utilizes remote sensing-based land use and forest classification and evaluated export coefficients obtained from detailed small catchment studies. The present and future N depositions were estimated with the regional deposition model DAIQUIRI. Based on source apportionment, N deposition, forestry and agriculture each contribute 16–17% of the total export, with pronounced variation between the different sub-basins. The effect of changing forestry and deposition on N fluxes is assessed by using N deposition scenarios based on recent international emission reduction agreements.     

An Integrated Modeling Approach to Total Watershed Management: Water Quality and Watershed Assessment of Cheney Reservoir, Kansas, USA

Degradation of water quality is the major health concern for lakes and reservoirs in the central regions of the United States as a result of heavily devoted agricultural production. A vital key to the development of a reservoir management strategy is to identify nutrient loading that describes associated water quality conditions in reservoirs. This study integrated AnnAGNPS watershed and BATHTUB lake models to simulate actual lake water quality conditions of Cheney Reservoir, KS, and demonstrated the use of the coupled model for simulating lake response to changes in different watershed land use and management scenarios. The calibrated current-conditions model simulated in-lake reductions as much as 52% for TN, 48% for TP, and 70% for chlorophyll a due to conversion to native grass, and increases as much as 4% for TN, 9% for TP and 6% for chlorophyll a due to conversion of land from the Conservation Reserve Program (CRP) to cropland (15.5% of watershed). This model also demonstrated an increase in chlorophyll a (19%) as the lake sediment capacity was reached over the next century.     

Critical loads for nitrogen deposition for Great Britain

There is currently much interest in mapping critical loads for nitrogen deposition as part of a strategy for controlling nitrogen emissions. While nitrogen deposition may cause acidification and excess nutrient effects, the former were considered previously in studies of sulphur deposition. In the UK, work on developing nutrient nitrogen critical loads maps has used several methods and databases. Two approaches are described here, one a steady state calculation using a nitrogen saturation limit for soil systems, the other an empirical estimate of critical loads set to prevent changes to vegetation communities. The empirical method uses national species records and land cover data derived from satellite imagery. Maps drawn from the available data are dependent upon a number of factors which reflect the approach used. To apply the nutrient critical loads to a strategy for future abatement measures, the nutrient nitrogen values for soils have been incorporated within a “critical loads function” which takes into account both acidity and nutrient effects as related to deposition loads for sulphur and nitrogen. This function may be used with deposition data to identify the need for sulphur and nitrogen emission reductions.     

The role of nitrogen in acidification of Tatra Mountains lakes

An overall level as well as seasonal changes of acidification of Tatra Mountains surface waters have been studied. Measurements carried out in the Tatra Mountains National Park were concentrated on two lakes: Dlugi Staw and Zielony Staw, situated in the crystalline, granitic part of the mountains. These lakes differ from each other in altitude (1784 m and 1632 m a.s.l. respectively) as well as in quantity and variety of flora and fauna living in and around them. The wet deposition of acidifying compounds of sulphur and nitrogen, and their seasonal variability in the area of the lakes were measured and compared with the critical loads of sulphur and nitrogen for these lakes and their catchments. Results showed that the critical loads of both sulphur and nitrogen in Dlugi Staw were exceeded all over the year and in Zielony Staw the acid deposition was roughly equal to critical load. A surprisingly high concentration of nitrogen compounds in acid deposition, well above the absorption ability of both the lakes and their catchments were ascertained. Nitrogen retention coefficients calculated for the lakes were as follows: Dlugi Staw — approximately 10%, Zielony Staw — approximately 50%. Seasonal variability in nitrate ions concentration in the lakes' waters reflected their prominent acidification caused by nitrogen compounds corresponding to stages 2 and 3 in the scale proposed recently by Stoddard (Stoddard, 1994).     

The significance of in-lake production of alkalinity

Alkalinity production in terrestrial and aquatic ecosystems of Canada, the U.S.A., Norway and Sweden is calculated from either strong acid titrations or budgets for base cations and strong acid anions, using mass-balance budgets. Where alkalinity budgets for lakes and their catchments are calculated in acid-vulnerable geological settings, in-lake processes often contribute more to lake alkalinity than yield from terrestrial catchments. Nitrate and sulfate removal, and Ca exchange with sediments are the predominant alkalinity generating mechanisms in lakes. Nitrate and sulfte removal rates increase as the concentrations of NO^? ₃ and SO₄ ^2? in lake water increase, so that in-lake acid neutralizing capacity increases as acid deposition increases. Both processes occur in sediments overlain by oxic waters, at rates which seem to be controlled primarily by diffusion.     

Critical Loads of Acid Deposition for Wilderness Lakes in the Sierra Nevada (California) Estimated by the Steady-State Water Chemistry Model

Major ion chemistry (2000–2009) from 208 lakes (342 sample dates and 600 samples) in class I and II wilderness areas of the Sierra Nevada was used in the Steady-State Water Chemistry (SSWC) model to estimate critical loads for acid deposition and investigate the current vulnerability of high elevation lakes to acid deposition. The majority of the lakes were dilute (mean specific conductance?=?8.0 μS cm^?1) and characterized by low acid neutralizing capacity (ANC; mean?=?56.8 μeq L^?1). Two variants of the SSWC model were employed: (1) one model used the F-factor and (2) the alternate model used empirical estimates of atmospheric deposition and mineral weathering rates. A comparison between the results from both model variants resulted in a nearly 1:1 slope and an R ² value of 0.98, suggesting that the deposition and mineral weathering rates used were appropriate. Using an ANC_limit of 10 μeq L^?1, both models predicted a median critical load value of 149 eq ha^?1 year^?1 of H⁺ for granitic catchments. Median exceedances for the empirical approach and F-factor approach were ?81 and ?77 eq ha^?1 year^?1, respectively. Based on the F-factor and empirical models, 36 (17 %) and 34 (16 %) lakes exceeded their critical loads for acid deposition. Our analyses suggest that high elevation lakes in the Sierra Nevada have not fully recovered from the effects of acid deposition despite substantial improvement in air quality since the 1970s.     

A Strategic Appraisal of Options to Ameliorate Regional Acidification

Studies in the 1980's showed that there had been extensive loss of fish and invertebrates in lakes and streams in upland Wales due to acid deposition. A regional survey of these waters in 1995 showed that, despite large reductions in sulphur emissions, acidification was still widespread and no biological improvement was detectable. Policy options are needed which should include reductions in nitrogen as well as sulphur, the contribution of land use (especially conifer afforestation), liming and the introduction of species. This paper describes a screening study comprising the construction of an evaluation matrix, the use of decision workshops of expert stakeholders and trade-off analysis to identify possible combinations of options. The resulting output is being used to construct a regional strategy for achieving measurable ecosystem recovery within the next 10 years.     

Sublittoral Chironomids as Indicators of Acidity (Diptera: Chironomidae)

The sublittoral chironomid fauna of 22 lakes in Killarney Park, Ontario, Canada were examined for their response to different levels of acidification. Included in the analysis were naturally acidic lakes, lakes acidified by atmospheric deposition but now recovering, and unacidified circumneutral lakes. pH in the study lakes ranged from 4.6 to 7.7. No correlation was found between species richness and pH, nor between abundance and pH. Acid neutralizing capacity (ANC), the temperature at the sampling depth, pH, and dissolved organic carbon were the variables contributing significantly in a canonical correspondence analysis of the abundance data. ANC was the most important variable in describing the chironomid community, accounting for about 9% of the variance in the species data. This study is the first step in an effort to model the changes in the chironomid community of recovering acidified lakes and the results suggest that ANC may be an important predictor variable.     

The role of sea-salts in enhancing and mitigating surface water acidity

Enhancement of the acidity of fresh waters due to sea-salt “episodes” has been reported in western regions of Scotland, Ireland and Norway and eastern regions of Canada and the United States of America. In all cases these short-term pH depressions have been ascribed to cation-exchange processes in catchment soils whereby sodium ions displace acidic cations (H⁺, Al³⁺) resulting in a lowering of the Na/Cl ratio in run-off water. Studies of sea-salt episodes in different catchment types of similar sensitivity in Scotland show significant variations in chemical responses, especially in terms of acidification status. Catchments with high background salt content, in a low sulphur deposition area, show only a small increase in acidity with negligible aluminium release and most of the sodium retention matched by Ca and Mg release. Catchments in intermediate deposition areas, with low background salt levels, exhibit smaller sodium retention with increased leaching of acidity and labile aluminium. High non-marine S deposition sites, with intermediate background salt levels, reveal high levels of acidity and aluminium leaching associated with some calcium leaching and evidence of Mg retention. Mitigation of acidification occurs at sites with high background sea-salt levels (eg north west Scotland) where acidic deposition is selectively retained in catchment soils. Non-marine sulphate values in run-off are therefore much lower (often producing negative values) than those predicted from current S deposition values. Consequently such sites are presently producing false exceedances of freshwater Critical Loads when current S deposition values are used. Future reductions in S deposition will probably affect the adsorption characteristics at these sites with consequent effects on sulphate leaching.     

Determinants of Summer Nitrate Concentration in a Set of Adirondack Lakes,New York

Following reductions in the emission and deposition of sulfur compounds in the past decade, atmospheric deposition of nitrogen has become a focus of concern. Identification of watershed characteristics that mediate the effect of atmospheric nitrogen deposition can help evaluate the sensitivity of lakes to chronic and episodic nitrogen addition. Twenty four lakes in the southwestern portion of the Adirondack Park, New York, U.S.A., were classified into three N classes by cluster analysis of lakewater NO3- N concentration [N] during the summers of 1994–1996. The lake-N classes were best characterized as having (1) low [N] throughout the summer, (2) high [N] in early- but low [N] in late-summer, and (3) high [N] throughout the summer. The three lake-N classes were reconstructed perfectly by canonical discriminant analysis based mainly on lake average depth (AD), and lakewater concentrations of chlorophyll a [Chla] and SO4-S [S] in mid-summer. Increases in AD and [S], but decrease in [Chla] corresponded with a transition from low- to high-N classes.     

Cadmium and lead contamination of soils near plastic stabilizing materials producing plants in Northern Taiwan

Atmospheric mobilization and exchange at the air-water interface are significant features of biogeochemical cycling of Hg at the Earth's surface. Our marine studies of Hg have been extended to terrestrial aquatic systems, where we are investigating the tropospheric cycling, deposition and air-water exchange of Hg in the mid-continental lacustrine environs of northcentral Wisconsin. This program is part of a multidisciplinary examination into the processes regulating the aquatic biogeochemistry of Hg in temperate regions. Trace-metal-free methodologies are employed to determine Hg and alkylated Hg species at the picomolar level in air, water and precipitation. We have found Hg concentrations and atmospheric fluxes in these fresh water systems to be similar to open ocean regions of the Northern Hemisphere. A well constrained mass balance for Hg has been developed for one of the lakes, Little Rock Lake, which is an extensively studied clear water seepage lake that has been divided with a sea curtain into two basins, one of which is untreated (reference pH: 6.1) while the other is being experimentally acidified (current pH: 4.7). This budget shows that the measured total atmospheric Hg deposition (ca. 10 μg m⁻² yr⁻¹) readily accounts for the total mass of Hg in fish, water and accumulating in the sediments of Little Rock Lake. This analysis demonstrates the importance of atmospheric Hg depositional fluxes to the geochemical cycling and bioaccumulation of Hg in temperate lakes. It further suggests that modest increases in atmospheric Hg loading could lead directly to enhanced levels of Hg in biota. Analogous modeling for monomethylmercury (MMHg) is as yet limited. Nevertheless, preliminary data for the atmospheric deposition of MMHg indicate that this flux is insufficient. to account for the amounts of MMHg observed in biota. An in-lake synthesis of MMHg is implicated. The importance of volatile Hg which is principally in the elemental form, and its evasion to the atmosphere is also illustrated. We suggest that the in-lake production of Hg° can reduce the Hg (II) substrate used in the in-lake microbiological synthesis of MMHg.     

Assessing biological recovery of acid-sensitive lakes in Ontario,Canada

Information on breeding waterfowl, habitat and food chains, gathered from acid-sensitive lakes in Ontario, was used to develop a model of effects of acid deposition on waterfowl and their response to predicted sulphur dioxide (SO₂ emission reductions in eastern North America. The Waterfowl Acidification Response Modelling System (WARMS) is composed of an acidification model linked to fish and waterfowl models. WARMS uses pH, area, dissolved organic carbon, total phosphorus, and presence of fish to calculate estimates of pre-acidification, present and eventual steady-state values for pH, fish presence and waterfowl breeding parameters under proposed SO₂ emission scenarios. We used WARMS to estimate chemical and biotic responses to scenarios simulated in three regions of Ontario where biomonitoring studies are underway. For pH and fish presence, WARMS predicts the greatest improvements in the highly damaged Sudbury region, slight improvements in Algoma, and that the strongest proposed emission reductions will be required to maintain current conditions in Muskoka. For waterfowl, species-specific differences are evident among regions. We discuss implications of these assessments of biological recovery for watersheds in eastern Canada.     

Regionalization of critical loads under uncertainty

The steady-state model PROFILE was used to perform Monte Carlo simulations of critical loads of acidity and exceedances of forest soils for 128 sites in the province of Scania, southern Sweden. Statistical tests showed that 100 sites had normal distributed critical loads and exceedances and that the variance of these parameters was statistically equal for all sites. Pooled estimates of the standard deviation was 0.19 and 0.31 kmol_c ha^?1 yr^?1 for the critical loads and exceedances, respectively. Introduction of uncertainties, expressed as confidence intervals, in the cumulative distribution function for critical loads showed that overlaps between percentiles were substantial. The 5%-ile was systematically equal to the 57%-ile using a 67% confidence interval and equal to the 87%-ile when a 95% confidence level was chosen. The overlaps of percentiles cause a reduction of acidic deposition according to the mean value of the 5%-ile to protect only 68% of the ecosystem area with an 84% probability and not a guaranteed protection of 95% as if uncertainties did not exist. Thus, uncertainties make it possible to advocate reductions to levels of deposition below the 5%-tile of critical loads.