Past and future exceedances of nitrogen critical loads in Europe

Critical loads of acidity and nutrient nitrogen--simple measures of the sensitivity of ecosystems to deposition--have been widely used for setting emission reduction targets in Europe. In contrast to sulfur, the emissions of nitrogen compounds remain high in the future. This is also true for the exceedances of critical loads until 2010. Looking further into the future, climate change is likely to influence ecosystem sensitivity, and thus critical loads. It is shown that higher temperatures, changed precipitation patterns, and modified net primary production mainly increase critical loads, except in mountainous and arid regions. Using consistent scenarios of climate change and air pollution from a recently completed European study (AIR-CLIM), it is shown that the exceedances in 2100 of the critical loads are declining in comparison to 2010. However, exceedances of critical loads of nutrient nitrogen remain substantial, even under the most stringent scenario. This confirms the increasing role nitrogen plays in environmental problems in comparison to sulfur. Thus research should focus on the effects of nitrogen in the environment, especially under conditions of climate change, to support nitrogen-emission mitigating policies. This not only reduces acidification and eutrophication, but also helps curb the formation of tropospheric ozone.     

Country-dependent Characterisation Factors for Acidification in Europe - A Critical Evaluation (7 pp)

Goal, Scope and Background Country-dependent characterisation factors for acidification have been derived for use in life cycle assessments to describe the effect on ecosystem protection of a change in national emissions. They have recently also been used in support of European air pollution abatement policies and related cost benefit analyses. We demonstrate that the characterisation factors as calculated to date are unstable due to being derived from the non-smooth and highly varying part of the underlying emission-impact functions. The purpose of this paper is to discuss the currently available characterisation factors and to propose a modification that makes use of the full range of the underlying functions. Method The characterisation factors derived in this paper are based on updates of data used to support European air pollution agreements under the UNECE Convention on Long-range Transboundary Air Pollution (LRTAP) and the European Commission. The focus in this paper is on the analysis of characterisation factors for acidification. The analysis of characterisation factors for terrestrial eutrophication from nitrogen compounds is a simple extension of the methods described here. The analysis is conducted for 25 European nations, i.e. for 23 EU countries plus Norway and Switzerland; Cyprus and Malta are excluded due to lack of data on critical loads. Results and Conclusions We show that a linear model which is calibrated to emission changes of –50% is generally more reliable than characterisation factors which are based on emission changes of plus or minus 10%. Application of these characterisation factors are justified for emission reductions up to 30% in total European emissions, compared to 2000. This is within the range of currently agreed upon emission reductions in 2010 relative to 2000. Therefore, characterisation factors can be used in LCA as well as for the support of the revision of existing European air pollution agreements.     

Climate change and pollutant impacts on Scottish vegetation

Summary

By the 2050s the UK is projected to be about 1.6°C warmer, when the atmospheric CO₂ concentration will be 525 ppmv. These changes will have profound effects on the Scottish flora and fauna. Vegetation primary productivity will increase, except in dry regions, and the productivity of upland forest plantations may increase by several Yield Classes. The spread of plant species may be less than expected, but a number of slow-growing ‘stress-tolerant’ species, including montane/alpine species, are likely to be lost. Nitrogen deposited as a result of emission of NO_X from vehicles and NH₃ from agriculture is now a major source of acidity, and problems of acidification and eutrophication are linked. Despite reductions in sulphur emissions, critical loads of acid deposition are likely to be exceeded for soils in most of the Scottish uplands until at least 2005. Critical levels affecting tree growth may be exceeded where forests are in cloud for 10% of the time in areas of the Great Glen. Much of the Scottish uplands receives 25–30 kg N ha^-1 yr^-1, which may be causing change in species composition. Background tropospheric ozone concentrations are increasing. Much of the Scottish uplands experiences mean summer ozone concentrations exceeding those in southern England, but with fewer exceedances of critical levels. However, many crops and some sensitive native species are probably being adversely affected.     

Country-dependent Characterisation Factors for Acidification and Terrestrial Eutrophication Based on Accumulated Exceedance as an Impact Category Indicator (14 pp)

Background, Aims and Scope Several authors have shown that spatially derived characterisation factors used in life cycle impact assessment (LCIA) can differ widely between different countries in the context of regional impact categories such as acidification or terrestrial eutrophication. Previous methodology studies in Europe have produced country-dependent characterisation factors for acidification and terrestrial eutrophication by using the results of the EMEP and RAINS models and critical loads for Europe. The unprotected ecosystem area (UA) is commonly used as a category indicator in the determination of characterisation factors in those studies. However, the UA indicator is only suitable for large emission changes and it does not result in environmental benefits in terms of characterisation factors if deposition after the emission reduction is still higher than the critical load. For this reason, there is a need to search for a new category indicator type for acidification and terrestrial eutrophying in order to calculate site-dependent characterisation factors. The aim of this study is to explore new site-dependent characterisation factors for European acidifying and eutrophying emissions based on accumulated exceedance (AE) as the category indicator, which integrates both the exceeded area and amount of exceedance. In addition, the results obtained for the AE and UA indicators are compared with each other. Methods The chosen category indicator, accumulated exceedance (AE), was computed according to the calculation methods developed in the work under the United Nations Economic Commission for Europe (UNECE) Convention on Long-range Transboundary Air Pollution (LRTAP). Sulphur and nitrogen depositions to 150x150 km2 grid cells over Europe were calculated by source-receptor matrices derived from the EMEP Lagrangian model of long-range transport of air pollution in Europe. Using the latest critical load data of Europe, the site-dependent characterisation factors for acidification and terrestrial eutrophication were calculated for 35 European countries and 5 sea areas for 2002 emissions and emissions predicted for 2010. In the determination of characterisation factors, the emissions of each country/area were reduced by various amounts in order to find stable characterisation factors. In addition, characterisation errors were calculated for the AE-based characterisation factors. For the comparison, the results based on the use of UA indicator were calculated by 10% and 50% reductions of emissions that corresponded to the common practice used in the previous studies. Results and Discussion The characterisation factors based on the AE indicator were shown to be largely independent of the reduction percentage used to calculate them.. Small changes in emissions (≤100 t) produced the most stable characterisation factors in the case of the AE indicator. The characterisation errors of those characterisation factors were practically zero. This means that the characterisation factors can describe the effects of small changes in national emissions that are mostly looked at in LCAs. The comparison between country-dependent characterisation factors calculated by the AE and UA indicators showed that these two approaches produce differences between characterisation factors for many countries/areas in Europe. The differences were mostly related to the Central and Northern European countries. They were greater for terrestrial eutrophication because the contribution of ammonia emission differ remarkably between the two approaches. The characterisation factors of the AE indicator calculated by the emissions of 2002 were greater than the factors calculated by the predicted emissions for 2010 in almost all countries/sea areas, due to the presumed decrease of acidifying and eutrophying emissions in Europe. Conclusions and Recommendations. In this study, accumulated exceedance was shown to be an appropriate category indicator in LCIA applications for the determination of site-dependent characterisation factors for acidification and terrestrial eutrophication in the context of integrated assessment modelling. In the future, it would be useful to calculate characterisation factors for emissions of separate parts of large countries and sea areas in Europe. In addition, it would also be useful to compare the approach based on the AE indicator with the method of the hazard index, as recommended in the latest CML guidebook.     

Site-Dependent Life-Cycle Impact Assessment of Acidification

The lack of spatial differentiation in current life-cycle impact assessment (LCIA) affects the relevance of the assessed impact. This article first describes a framework for constructing factors relating the region of emission to the acidifying impact on its deposition areas. Next, these factors are established for 44 European regions with the help of the RAINS model, an integrated assessment model that combines information on regional emission levels with information on long-range atmospheric transport to estimate patterns of deposition and concentration for comparison with critical loads and thresholds for acidification, eutrophication via air; and tropospheric ozone formation. The application of the acidification factors in LCIA is very straightforward. The only additional data required, the geographical site of the emission, is generally provided by current life-cycle inventory analysis. The acidification factors add resolving power of a factor of 1,000 difference between the highest and lowest ratings, while the combined uncertainties in the RAINS model are canceled out to a large extent in the acidification factors as a result of the large number of ecosystems they cover The framework presented is also suitable for establishing similar factors for eutrophication and tropospheric ozone formation for regions outside Europe as well.     

Calculation of theoretical and empirical nutrient N critical loads in the mixed conifer ecosystems of southern California

Edaphic, foliar, and hydrologic forest nutrient status indicators from 15 mixed conifer forest stands in the Sierra Nevada, San Gabriel Mountains, and San Bernardino National Forest were used to estimate empirical or theoretical critical loads (CL) for nitrogen (N) as a nutrient. Soil acidification response to N deposition was also evaluated. Robust empirical relationships were found relating N deposition to plant N uptake (N in foliage), N fertility (litter C/N ratio), and soil acidification. However, no consistent empirical CL were obtained when the thresholds for parameters indicative of N excess from other types of ecosystems were used. Similarly, the highest theoretical CL for nutrient N calculated using the simple mass balance steady state model (estimates ranging from 1.4-8.8 kg N/ha/year) was approximately two times lower than the empirical observations. Further research is needed to derive the thresholds for indicators associated with the impairment of these mixed conifer forests exposed to chronic N deposition within a Mediterranean climate. Further development or parameterization of models for the calculation of theoretical critical loads suitable for these ecosystems will also be an important aspect of future critical loads research.     

Life cycle assessment of fuel ethanol derived from corn grain via dry milling

Life cycle analysis enables to investigate environmental performance of fuel ethanol used in an E10 fueled compact passenger vehicle. Ethanol is derived from corn grain via dry milling. This type of analysis is an important component for identifying practices that will help to ensure that a renewable fuel, such as ethanol, may be produced in a sustainable manner. Based on data from eight counties in seven Corn Belt states as corn farming sites, we show ethanol derived from corn grain as E10 fuel would reduce nonrenewable energy and greenhouse gas emissions, but would increase acidification, eutrophication and photochemical smog, compared to using gasoline as liquid fuel. The ethanol fuel systems considered in this study offer economic benefits, namely more money returned to society than the investment for producing ethanol. The environmental performance of ethanol fuel system varies significantly with corn farming sites because of different crop management practices, soil properties, and climatic conditions. The dominant factor determining most environmental impacts considered here (i.e., greenhouse gas emissions, acidification, eutrophication, and photochemical smog formation) is soil related nitrogen losses (e.g., N2O, NOx, and NO3-). The sources of soil nitrogen include nitrogen fertilizer, crop residues, and air deposition. Nitrogen fertilizer is probably the primary source. Simulations using an agro-ecosystem model predict that planting winter cover crops would reduce soil nitrogen losses and increase soil organic carbon levels, thereby greatly improving the environmental performance of the ethanol fuel system.     

Evaluating Critical Soil Acidification Loads and Exceedances for a Deciduous Forest at the Turkey Lakes Watershed

Critical soil acidification loads (CL) and related exceedances, base cation leaching, N leaching, and forest biomass growth were evaluated for a well-studied deciduous forest site within the Turkey Lake Watershed (TLW). The assessment was done by way of steady-state mass balance considerations of primary inputs for N, Ca, Mg, and K. Critical soil acidification rates were found to be high at TLW. These rates amounted to about 900 or 1400 eq/(ha yr) depending on the forest harvesting regime (selective harvest or maintainence of old-growth condition, respectively). The TLW soil substrate (till derived from basaltic bedrock) appeared to weather well, thereby buffering against natural and anthropogenic soil acidification. As a consequence, soil acidification exceedances were estimated to be relatively low for both the selective harvest and old-growth scenarios. In comparing overall S and N input/output data (atmospheric deposition data vs soil leaching losses), we found that the TLW site was essentially near or at S and N saturation. We also found that atmospheric deposition and soil leaching rates have been declining since about 1980. The figures for CL and exceedance varied to some extent depending on the quality of input data and related uncertainties. Estimated exceedances were increased when dry- as well as wet-deposition rates were considered. They varied depending on the yearly sulfate/nitrate/base-cation mix, and the definition of “acceptable acid leaching.” In addition, they were dependent on whether the forest was considered old growth or not. Received 5 October 1999; Accepted 1 November 2000.     

Developing an indicator-modelling approach to forecast changes in nitrogen critical load exceedance across Europe arising from agricultural reform

Atmospheric nitrogen (N) deposition above the critical load causes eutrophication with adverse impacts on biodiversity. Average Accumulated critical load Exceedance (AAE) is a measure of the amount of critical load exceedance and the area of habitat which is affected, and has been adopted in Europe as a pressure indicator for biodiversity. In Europe, AAE is calculated by the Coordination Centre for Effects (CCE) of the United Nations Economic Commission for Europe based on modelled nitrogen deposition and country-level reporting of critical load thresholds and ecosystem area. Due to differences in country-level reporting, AAE values for semi-natural habitats may show large differences across Europe. This paper therefore describes the development of a simpler approach to the modelling of habitat eutrophication. The eutrophication indicator model is applicable to all habitats for which empirical critical loads for nutrient nitrogen have been defined and is easily configured to assess impacts of modelled policies or scenarios on AAE. Outputs from the model showed a high correlation with published AAE data (R² = 0.80) and replicated broad spatial patterns in AAE, but under-estimated actual values by a factor of 2.5. This variation was traced primarily to the use of habitat-specific nitrogen deposition by the CCE which calculates higher nitrogen deposition to forest areas, but also to differences between countries in the critical load thresholds used and the areas of sensitive ecosystems reported. Sensitivity testing showed that exceedance calculations were particularly sensitive to the critical load threshold used, since nitrogen deposition across Europe lies in the middle of most critical load ranges. The indicator model was used to forecast AAE under two scenarios: socio-economic changes under a business as usual scenario to 2025, and the additional effect of agricultural reform where both direct support to farmers and market support were removed. Ammonia emissions showed a net decrease by 2025, with strong regional differentiation. Emissions increased in former Soviet bloc countries and decreased mainly in south west Europe and Finland. Agricultural reform had little additional impact on ammonia emissions. AAE was forecast to decrease by 49% by 2025, due mainly to reductions in emissions of oxidised N from industry and transport, independent of agricultural reform. However, as oxidised N decreases, the relative contribution of N from agricultural sources will increase, and policy measures which reduce ammonia emissions will have increasing relevance for reducing AAE.     

On the importance of incorporating forest edge deposition for evaluating exceedance of critical pollutant loads

The concept of critical load (CL) was defined to express the tolerance of natural and semi‐natural habitats for anthropogenic air pollution. Correct evaluation of the exceedance of critical loads is fundamental for the long‐term protection of ecosystems by limiting emissions of potential acidifying and eutrophying pollutants. For forest ecosystems, the exceedance of critical loads is often calculated using deposition data measured in the forest interior. However, several studies report forest edges acting as ‘hotspots’ of acidifying and nitrogen deposition, showing up to fourfold increases in atmospheric deposition compared to the forest interior. This paper estimates the relevance of considering the higher deposition load in forest edges for calculating exceedance of critical loads for nitrogen and potential acidifying deposition. If measures to control and reduce atmospheric deposition are based on mean deposition fluxes within forest stands, deposition reductions will not be enough for preventing adverse effects. In fact, emission reductions should be adjusted to deposition values at the forest edge, since these zones are most threatened. We thus conclude that there is an urgent need to reconsider the calculation of exceedance of critical loads, taking into account edge enhancement of deposition. This is an issue of high relevance, particularly in highly fragmented regions, such as Flanders (Belgium).     

Cost-effective emission abatement in europe considering interrelations in agriculture

Agriculture is an important source of ammonia (NH3), which contributes to acidification and eutrophication, as well as emissions of the greenhouse gases nitrous oxide (N2O) and methane (CH4). Controlling emissions of one of these pollutants through application of technical measures might have an impact (either beneficial or adverse) on emissions of the others. These side effects are usually ignored in policy making. This study analyses cost-effectiveness of measures to reduce acidification and eutrophication as well as agricultural emissions of N2O and CH4 in Europe, taking into account interrelations between abatement of NH3, N2O, and CH4 in agriculture. The model used is based on the RAINS (Regional Air pollution INformation and Simulation) model for air pollution in Europe, which includes emissions, abatement options, and atmospheric source-receptor relationships for pollutants contributing to acidification and eutrophication. We used an optimisation model that is largely based on the RAINS model but that also includes emissions of N2O and CH4 from agriculture and technical measures to reduce these emissions. For abatement options for agricultural emissions we estimated side effects on other emissions. The model determines abatement strategies to meet restrictions on emission and/or deposition levels at the least cost. Cost-effective strategies to reduce acidification and eutrophication in Europe were analysed. We found that NH3 abatement may cause an increase in N2O emissions. If total agricultural N2O and CH4 emissions in Europe were not allowed to increase, cost-effective allocation of emission reductions over countries in Europe changed considerably.     

Long-term sulphate and inorganic nitrogen mass balance budgets in European ICP Integrated Monitoring catchments (1990–2012)

Empirical evidence based on integrated environmental monitoring including physical, chemical and biological variables is essential for evaluating the ecosystem benefits of costly emission reduction policies. The international multidisciplinary ICP IM (International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems) programme studies the integrated effects of air pollution and climate change on ecosystems in unmanaged and calibrated forested catchments. We calculated site-specific annual input-output budgets for sulphate (SO₄) and total inorganic nitrogen (TINNO₃-N + NH₄-N) for 17 European ICP IM sites in 1990–2012. Temporal trends for input (deposition) and output (runoff water) fluxes and the net retention/net release of SO₄ and TIN were also analysed. Large differences in the input and output fluxes of SO₄ and TIN reflect important gradients of air pollution effects in Europe, with the highest deposition and runoff water fluxes at IM sites located in southern Scandinavia and in parts of Central and Eastern Europe and the lowest fluxes at more remote sites in northern European regions. A significant decrease in the total (wet + dry) deposition of non-marine SO₄ and bulk deposition of TIN was found at 90% and 65% of the sites, respectively. Output fluxes of non-marine SO₄ in runoff decreased significantly at 65% of the sites, indicating positive effects of the international emission abatement actions in Europe during the last 20 years. Catchments retained SO₄ in the early and mid-1990s, but this shifted towards a net release in the late 1990s, which may be due to the mobilization of legacy S pools accumulated during times of high atmospheric SO₄ deposition. Despite decreased deposition, TIN output fluxes and retention rates showed a mixed response with both decreasing (9 sites) and increasing (8 sites) trend slopes, and trends were rarely significant. In general, TIN was strongly retained in the catchments not affected by natural disturbances. The long-term annual variation in net releases for SO₄ was explained by variations in runoff and SO₄ concentrations in deposition, while a variation in TIN concentrations in runoff was mostly associated with a variation of the TIN retention rate in catchments. The net release of SO₄ from forest soils may delay the recovery from acidification for surface waters and the continued enrichment of nitrogen in catchment soils poses a threat to terrestrial biodiversity and may ultimately lead to a higher TIN runoff through N-saturation. Continued monitoring and further evaluations of mass balance budgets are thus needed.     

Temporal changes in nitrogen pollution in northeastern Estonia

During the last 5 decades the northeastern part of Estonia (the region where oil shale and the chemical industry are located) has been subjected to pollution with acidic compounds. In 1981-1988 the yearly mean nitrogen (N) deposition load was up to 11.1 kg ha(-1). This N pollution level combined with the deposition of sulphur (S) could have seriously endangered the environment, but the simultaneous emission of strongly alkaline fly ash restrained acidification processes. After 1989-1991 the situation changed, and in 1994-1996 the N deposition load in northeastern Estonia remained within the range of 2.6 to 6.6 kg ha(-1) year(-1) and that of S within 2 to 50 kg ha(-1) year(-1). Because the fly ash deposition is permanently decreasing, more sensitive lichens and mosses can be subjected to critical N+S loads in the future. The proportion of oil shale industry in total emission of NOx in Estonia from stationary sources equals approximately 65 to 75%. During 1996-2000 the yearly mean concentration of NO2 in the air of towns increased from 9 to 12 to 16 to 29 g m(-3). The emission of N compounds was mainly caused by N oxides in flue gases from power plants, as well as ammonia and carbamide discharges from chemical plants. In 1988-1990 the estimated yearly total emission of NOx (as NO2 equivalent) was about 18 to 18.6 thousand t and in 1994-2000, 9.9 to 11.8 thousand t.     

Long‐term temporal and spatial trends in eutrophication status of the Baltic Sea

Much of the Baltic Sea is currently classified as ‘affected by eutrophication’. The causes for this are twofold. First, current levels of nutrient inputs (nitrogen and phosphorus) from human activities exceed the natural processing capacity with an accumulation of nutrients in the Baltic Sea over the last 50–100 years. Secondly, the Baltic Sea is naturally susceptible to nutrient enrichment due to a combination of long retention times and stratification restricting ventilation of deep waters. Here, based on a unique data set collated from research activities and long‐term monitoring programs, we report on the temporal and spatial trends of eutrophication status for the open Baltic Sea over a 112‐year period using the HELCOM Eutrophication Assessment Tool (HEAT 3.0). Further, we analyse variation in the confidence of the eutrophication status assessment based on a systematic quantitative approach using coefficients of variation in the observations. The classifications in our assessment indicate that the first signs of eutrophication emerged in the mid‐1950s and the central parts of the Baltic Sea changed from being unaffected by eutrophication to being affected. We document improvements in eutrophication status that are direct consequences of long‐term efforts to reduce the inputs of nutrients. The reductions in both nitrogen and phosphorus loads have led to large‐scale alleviation of eutrophication and to a healthier Baltic Sea. Reduced confidence in our assessment is seen more recently due to reductions in the scope of monitoring programs. Our study sets a baseline for implementation of the ecosystem‐based management strategies and policies currently in place including the EU Marine Strategy Framework Directives and the HELCOM Baltic Sea Action Plan.     

Monitoring acid waters in the U.K.: an overview of the U.K. Acid Waters Monitoring Network and summary of the first interpretative exercise

1. The U.K. Acid Waters Monitoring Network (AWMN) was established in 1988. It comprises eleven lake and eleven stream sites located throughout the U.K. in areas sensitive to acidification. The principal objective of the AWMN is to provide a long-term, high-quality chemical, biological and palaeolimnological record which, in conjunction with the U.K. precipitation monitoring networks, will facilitate the assessment of trends within U.K. surface waters.
2. The first interpretation of results generated by the AWMN (April 1988–March 1993) has recently been completed and is summarized. During this period there have been no sustained changes in atmospheric deposition in the U.K. Trends are recognized in certain chemical and/or biological parameters at individual AWMN sites, but despite some (regional) patterns no regional trends toward increasing or decreasing acidification are apparent. The data comprise an excellent baseline with which future changes may be compared.
3. The methodology of the AWMN is reviewed and some amendments are suggested in the light of the first 5 years experience. These include: additional determinations for nitrogen species and total phosphorus; changes to the frequency of macrophyte and palaeolimnological sampling; new emphasis on data generated by sediment traps at lake sites; and the addition of extra sites to address better the full extent of acid-sensitive surface waters revealed by the national critical loads mapping programme, and to pay particular attention to the acidifying role of nitrogen.
4. The contribution of AWMN data to other national and international environmental monitoring programmes is highlighted. In addition, AWMN sites provide data to develop and validate critical loads models and dynamic models of acidification, and could also be used to monitor the impact of other air pollutants such as trace metals and persistent organic compounds on the freshwater environment.     

Monitoring acid waters in the U.K.: an overview of the U.K. Acid Waters Monitoring Network and summary of the first interpretative exercise

1. The U.K. Acid Waters Monitoring Network (AWMN) was established in 1988. It comprises eleven lake and eleven stream sites located throughout the U.K. in areas sensitive to acidification. The principal objective of the AWMN is to provide a long-term, high-quality chemical, biological and palaeolimnological record which, in conjunction with the U.K. precipitation monitoring networks, will facilitate the assessment of trends within U.K. surface waters.
2. The first interpretation of results generated by the AWMN (April 1988–March 1993) has recently been completed and is summarized. During this period there have been no sustained changes in atmospheric deposition in the U.K. Trends are recognized in certain chemical and/or biological parameters at individual AWMN sites, but despite some (regional) patterns no regional trends toward increasing or decreasing acidification are apparent. The data comprise an excellent baseline with which future changes may be compared.
3. The methodology of the AWMN is reviewed and some amendments are suggested in the light of the first 5 years experience. These include: additional determinations for nitrogen species and total phosphorus; changes to the frequency of macrophyte and palaeolimnological sampling; new emphasis on data generated by sediment traps at lake sites; and the addition of extra sites to address better the full extent of acid-sensitive surface waters revealed by the national critical loads mapping programme, and to pay particular attention to the acidifying role of nitrogen.
4. The contribution of AWMN data to other national and international environmental monitoring programmes is highlighted. In addition, AWMN sites provide data to develop and validate critical loads models and dynamic models of acidification, and could also be used to monitor the impact of other air pollutants such as trace metals and persistent organic compounds on the freshwater environment.     

Derivation and mapping of critical loads for nitrogen and trends in their exceedance in Germany

The term "critical load" means a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur, according to present knowledge. In the case of nitrogen, both oxidised and reduced compounds contribute to the total deposition of acidity, which exceeds critical loads in many forest ecosystems. These also cause negative effects through eutrophication. Critical loads of nitrogen were derived for forest soils (deciduous and coniferous forest), natural grassland, acid fens, heathland, and mesotrophic peat bogs. In Germany, a decrease in sulphur emissions over the past 15 years resulted in a reduced exceedance of critical loads for acid deposition. In the same period it was noted that reduction in the emissions of nitrogen oxides and ammonia remained insignificant. Therefore, emissions of nitrogen compounds have become relatively more important and will continue to threaten ecosystem function and stability. The risk of environmental damage remains at an unacceptable level. The German maps show the degree to which the critical loads are exceeded, and they present current developments and an expected future trend. Results indicate that recovery from pollutant stress occurs only gradually.     

Response of ecological indices to nutrient and chemical contaminant stress factors in Eastern Mediterranean coastal waters

Environmental data produced throughout monitoring activities in the framework of the implementation of Water Framework Directive 2000/60/EC (WFD) in Eastern Mediterranean (Greece) were used to assess the sensitivity and response of ecological indices against trace metals, eutrophication and multiple stress factors. The applied ecological indices include multi-metric eutrophication indices, a physicochemical status index applied for the first time in the Greek marine area, benthic indices, phytoplankton biomass index, and integrated status indices assessed through the application of the decision tree integration scheme. To investigate the exceedances in the eco-stoichiometric relationship between nutrients, considered a stressing factor, all physicochemical elements influenced directly or indirectly by eutrophication, such as nutrient concentrations, water transparency, oxygen saturation, particulates concentration, and sediment organic content, were related to ecological indices. Also, chemical contaminant stress factors represented by heavy metal concentrations in the water, as well as multiple stress factors represented by a pressure index, were related to ecological indices. A graphical visualization multivariate tool and statistical correlations were used to evaluate the sensitivity or explanatory power of the tested ecological indices against single and multiple stress factors. Results showed a strong response of all ecological indices to stress factors, although a diversification of sensitivity was evident. Primary production-related indices, i.e., macroalgae and chlorophyll-a indices, are more sensitive to particulates and nitrogen, while secondary production-related indices, i.e., benthic macroinvertebrates indices and eutrophication indices, including nutrients, are more sensitive to phosphates in the water column. The macroalgae index shows the strongest sensitivity to multiple stress factors. Among metals, mostly cadmium seems to match all indices⿿ performance. Nutrient relationships were shown as critical to eutrophication and ecological status.     

Critical Load Concept to Set Restoration Goals for Liming Acidif ied Nor wegian Waters

The concept of critical load is now widely used in the management of acidified waters. In southern Norway, acidification due to long-range transported air pollutants is one of the most serious environmental problems, affecting an area of 80,000 km². To preserve and restore biodiversity, Norwegian authorities have chosen liming as a temporary mitigation measure. Critical load estimates were used to estimate the material and financial needs for this large-scale program. In 1995, more than 2000 localities ranging from small lakes to large salmon rivers were limed. Liming costs in 1996 were $18 million (U.S.). Critical load estimates also formed the basis for international negotiations on sulfur emission reductions, resulting in the second sulfur protocol for Europe and North America in 1994. The critical load estimates indicated that acidified areas in Norway would be reduced to 35,000 km² after the year 2010, after the commitments of the sulfur protocols are met. By that time, estimated liming costs would be reduced by almost 40%. Lime treatment of River Tovdalselva, with a catchment area of 1885 km², is probably the largest integrated liming project in the world. In 1990 the critical load was exceeded in 98% of the Tovdalselva catchment. After the year 2010 the exceeded area may be reduced to 44% and the liming cost by two-thirds.     

Simulation of Critical Loads for Nitrogen for Terrestrial Plant Communities in The Netherlands

This paper describes a new method to derive nitrogen critical loads for vegetation, and its application to The Netherlands. An ‘inverted’ form of the soil chemical model SMART2 was used to estimate atmospheric nitrogen deposition at the critical conditions for 139 terrestrial vegetation types (associations) occurring in northwestern Europe, using an iterative search procedure. The critical conditions are the lower end of the pH range, and the upper end of the nitrogen availability range for each vegetation type. The critical load is assumed to be the nitrogen deposition that results in the critical conditions. The critical load values were subjected to a sensitivity and uncertainty analysis. Sensitivity analysis showed that the estimated critical N load mainly depends on the vegetation type and to a lesser extent on the soil type and the critical N availability. Of these variables N availability, which was estimated from Ellenberg’s indicator scale, contributes most to the uncertainty. The critical load averaged over all vegetation types and soil types is estimated to be 23 ± 7 kg N ha⁻¹y⁻¹. This is a rather reliable value because its uncertainty is small and it is in agreement with empirical estimates of critical loads. Critical loads per vegetation type are less reliable because they are not correlated to empirical values, although the ranges of simulated and empirical values usually overlap. At the site level, uncertainty becomes very large and it is not possible to determine critical loads with any practical significance. The uncertainties can only be reduced if more data become available on the abiotic response per species under field conditions, at least to nitrogen availability and soil pH.