Monitoring the results of Canada/U.S.A acid rain control programs: some lake responses

Aquatic acidification by deposition of airborne pollutants emerged as an environmental issue in southeastern Canada during the 1970s. Drawing information from the extensive research and monitoring programs, a sequence of issue assessments demonstrated the necessity of reducing the anthropogenic emissions of acidifying pollutants, particularly sulphur dioxide (SO₂). The 1991 Canada-U.S. Air Quality Agreement (AQA) was negotiated to reduce North American SO₂ emissions by 40% relative to 1980 levels by 2010, and at present, both countries have reduced emissions beyond their AQA commitment. In response to reduced SO₂ emissions, atmospheric deposition of sulphate (SO₄ ^2–) and SO₄ ^2– concentrations in many lakes have declined, particularly in south-central Ontario and southern Québec. Sulphate deposition still exceeds aquatic critical loads throughout southeastern Canada however. Increasing pH or alkalinity (commonly deemed recovery) has been observed in only some lakes. Several biogeochemical factors have intervened to modify the lake chemistry response to reduced SO₄ ^2– input, notably release of stored SO₄ ^2– from wetlands following periods of drought and reduction in the export of base cations from terrestrial soils. Three examples from Ontario are presented to illustrate these responses. Significant increases in pH and alkalinity have been observed in many lakes in the Sudbury area of Ontario due to the large reductions in local SO₂ emissions; early-stage biological recovery is evident in these lakes. An integrated assessment model predicts that AQA emission reductions will not be sufficient to promote widespread chemical or biological recovery of Canadian lakes. Monitoring and modeling are mutually supporting assessment activities and both must continue.     

Extent of acidification in Southwestern Québec lakes

This paper presents the results from the first two areas covered by the statistically-oriented Québec Spatial Lake Acidity Monitoring Network (RESSALQ). It is used in combination with the existing LRTAP-Québec temporal network, the Québec precipitation sampling network (REPQ) and a dose-effect model (SIGMA/SLAM) in order to assess the global extent of damages related to acidity, to detect changes in water quality, and to measure the effects of wet sulphate deposition reduction and those of sulphate target loadings on lake acidity. Results obtained with this network were also used in combination with data issued from the Eastern Lake Survey, in order to establish the relative acidity and sensitivity status of lakes in Eastern North America. While Florida has the highest proportion of very acidic and very sensitive lakes, Québec has a higher overall proportion of sensitive and acidic lakes compared to other areas of the United States.     

Evaluation of the effects of acid precipitation in eastern Canada using the raison system

Evaluations have been made of the key chemical factors in the aquatic effects upon surface waters due to acidic precipitation in eastern Canada. The region of Canada east of the Manitoba/Ontario border was divided into 22 aggregates and assessments of inorganic and organic ion chemistry appraised relative to sulphate deposition rates and distributions. Aquatic sensitivity is largely dominated by the concentration, distribution and magnitude of SO _inf4 ^sup2- (sulphate) deposition and by the prevalent geology and derived soils found in each aggregate. The RAISON system provided an adaptable and highly flexible platform to evaluate interactively, multiple data sets of divergent characteristics. Attributes usually associated with geographical information systems are significantly augmented by quantitative numerical and stochastic capabilities that were used extensively in this study.     

Land district,ecophysiographic units and areas: The landscape mapping of the Ministère des Ressources naturelles du Québec

In 1985, the Ministère des Ressources naturelles established a Forest Ecological Survey Program for Southern Québec. One component of the program was the mapping of land districts, which provide a physiographically-based territorial reference system. The land districts have an average surface area of between 100 and 300 sq km. Since its inception, the department has further refined the methodologies and standardized the results. The current methodologies are based on inventory and analysis of the geographical distribution of permanent environmental components (e.g., relief, surficial deposits, geology, hydrography). The products include 1:50 000 maps of surficial deposits, 1:250 000 maps of the land districts and physiographic systems, and a computerized data bank.To date, 500 000 km square have been covered, and the estimated completion date for the work is 1999. To facilitate use of the products and to encourage the development of new applications, a guide for forest managers and other users has been recently published. The guide assists with forest management planning strategies based upon the physical environment, in particular the production and use of interpretation grids and maps. The products of the mapping process are, for many regions of Southern Québec, the only source of information on permanent environmental components.Currently, the Ministère des Ressources naturelles is planning to develop landscape maps of Southern Québec which identify ecophysiographic units (1:1 250 000) and ecophysiographic areas (1:2 500 000). These maps are based upon the integration of critical physiographic features and the synthesis of land regions.     

An overview of the acidification of lakes in Atlantic Canada

Analysis of water chemistry from a sample of lakes (1300) in Atlantic Canada has indicated that lakes in geologically sensitive portions of Nova Scotia and Newfoundland have been acidified due to the combined effects of natural organic acids and anthropogenically derived mineral acids. Principal component analysis of six measured variables (pH, Ca, Conductance, SO _inf4 ^sup* , Alkalinity, Colour) and one computed variable (Alk/Ca^*+Mg^*) for each province result in four components which retain at least 89% of the original variability. Cluster analysis of the four principal components resulted in 6 lake groups for New Brunswick and 8 groups for Nova Scotia and 7 groups for Newfoundland. Geographic ordination of these clusters indicates that there is good correspondence between cluster group and the underlying bedrock geology of the region.     

Ice Storm Damage and Early Recovery in an Old-Growth Forest

We quantified the damage causedby a major ice storm to individual trees in two1-ha permanent plots located at Mont St. Hilairein southwestern Québec, Canada. The storm, whichoccurred in January 1998, is the worst on recordin eastern North America; glaze ice on the orderof 80–100 mm accumulated at our study site. Allbut 3% of the trees (DBH 10 cm) lost at leastsome crown branches, and 35% lost more than halftheir crown. Damage to trees increased in the order: Tsuga canadensis, Betula alleghaniensis, Ostrya virginiana, Acer saccharum, Fagus grandifolia, Quercus rubra, Betula papyrifera, Acer rubrum, Tilia americana, and Fraxinus americana. Only 22% of the saplings and small trees (4 cm < DBH < 10 cm)escaped being broken or pinned to the ground byfalling material. Levels of damage generally weregreater in an exposed ridge top forest than in acove protected from wind. By August 1999 only53% of the trees had new shoots developing fromthe trunk or broken branches; among the moredominant canopy trees, Fagus grandifoliahad the least sprouting and Acer saccharumand Quercus rubra the most. We anticipateand will monitor both significant turnover in thetree community and some shift in composition ofthe canopy dominants.     

Trihalomethanes in Drinking Water of Greater Québec Region (Canada): Occurrence, Variations and Modelling

The levels of trihalomethanes (THMs) – the main species of by-product from water chlorination – were monitored in thedistribution systems of the five major drinking water utilitiesof the greater area of Québec City in order to investigate andmodel their occurrence on a spatial and seasonal basis. Data forTHMs and other water quality and operational parametersassociated with their formation were generated through a 16 monthsampling program involving several sites representing variablewater residence times, from the plant to the system extremity.The results demonstrate that the differences in measured THMlevels between the five utilities are mainly due to the variablequality of raw waters, the type of water treatment process beingused and the type and levels of applied disinfectant. Dependingon the utility, average THM levels were from 1.3 to 2.5 timeshigher in the system extremities than in the water leaving thetreatment plant. Also, average levels of THMs measured in summerat the distribution system extremities were, depending on theutility, from 2.5 to 5 times higher than the average levelsmeasured in winter. The seasonal differences were found to besignificantly greater than those observed by others in waterutilities in the United States and Europe and are explained inlarge part by the considerable changes, over the year, in thequality and temperature of surface waters in Southern Québec. Forthe five utilities under study, multivariate regression modelswere developed in order to predict spatial and seasonalvariations of THMs. Both residual chlorine demand and temperaturewere found to be better, statistically, as predictors for THMoccurrence. The usefulness of the developed models for routineand long term water quality management, as well as for assessmentof human exposure to THMs, are also discussed.     

Knowing, Mapping and Understanding st. Lawrence Biodiversity, with Special Emphasis on Bird Assemblages

Environment Canada and the Québec Department of the Environment, partners in the St Lawrence Vision 2000 Action Plan, set out to prepare a compendium of knowledge of the flora and fauna of the St Lawrence and to identify potential conservation sites. The resulting Portrait is an Internet site that presents the current knowledge base of the river's ecological and biological diversity (http://lavoieverte.qc.ec.gc.ca/faune/biodiv/index.html). The Portrait provides information on over 5,000 species of flora and fauna. On the website, you will find a detailed ecological analysis of the richness, rarity and vulnerability of several broad groups of plant and animal species. Furthermore, you will find a list of species for each of the 700 survey units and a distribution map for 2,500 species recorded along the St Lawrence., in atlas form, along with a detailed conservation plan. The plan encompasses the most unique and heterogeneous landscapes of the St Lawrence, some of which have no protection at present. The Portrait provides an overview of the sites that are currently protected by public agencies and private-sector organisations and identifies new sites of interest for conserving biodiversity and protecting species at risk. This paper exposes the content of this extensive compendium on the biodiversity of the St. Lawrence. For conciseness, it presents some of the analyses conducted on birds to illustrate a few of the analytical approaches that were used. Then, the information on species richness and concentration areas for priority species of vascular plants, breeding birds and herpetofauna will serve to identify the terrestrial sites of significance for biodiversity. Finally, a similar approach having been applied to the aquatic environnement, we will conclude with a conservation plan that identifies the terrestrial and aquatic ecosystems, and the geographic sites where the most important elements of St. Lawrence biodiversity are concentrated. Our analysis of the biodiversity of the most thoroughly studied taxa of the St. Lawrence clearly shows the importance of wetlands, particularly those located at the mouths of rivers or within archipelagos or delta complexes, such as the groups of islands and channels found at both ends of the Montréal Archipelago. These aquatic landscapes are sites of intense biological production, combining in a small geographical area spawning, nursery and feeding grounds for a large number of fish species and breeding, rearing and foraging areas for aquatic birds. Variable flooding conditions, associated with seasonal flooding or daily tidal fluctuations, create a complex mosaic of wetland and aquatic habitats. Although wetlands occupy only a small area in comparison with terrestrial habitats, they support a large number of rare plant and animal species in relation to their size. At present, 10% of the vascular flora and 27% of the herpetofauna of the St. Lawrence are at risk. In the case of reptiles and amphibians, the situation is especially worrisome because nearly all of the most threatened species live in a narrow band along the river corridor. Not only is this the sector that is under the greatest pressure from human development, very little public land remains here, making it difficult to create protected areas. Increased participation by non-governmental organisations and individuals, through private stewardship arrangements, is an essential precondition for completing the network of conservation areas in this part of the St. Lawrence. Along the estuary and the Gulf, habitat integrity has not been affected as much by the expansion of Québec's human population. This is a vast territory, and sites have been identified with a view to making up for the deficiencies in the present network of protected sites in terms of representing biodiversity.     

The predicted effect of SO2 emission controls on the water quality of eastern Canadian lakes

Changes in SO _inf4 ^sup2- deposition predicted to occur in response to implementation of announced SO₂ emission control programs in Canada and the U.S.A. have been used as input to water chemistry models thereby giving an estimate of the changes in lake acid neutralizing capacity (ANC) and pH that can be expected from these programs. Eastern Canada has been divided into 22 subregions for the purpose of this analysis. Relative to the current level (1982–86) of SO _inf4 ^sup2- deposition (Scenario 1), the effect of the Canadian SO₂ emission control program alone (Scenario 2) is compared to that obtained when controls are implemented throughout North America (Scenarios 3 and 4). SO₂ emission reduction will effect a shrinkage of the high wet SO _inf4 ^sup2- deposition field in NE North America such that under Scenario 4 conditions, almost no area will remain in Canada that receives >20 kg ha^-1 yr^-1. The greatest decrease in deposition and resulting change in lake chemistry occurs in southern Ontario and southwestern Quebec. ANC distributions shift to higher concentrations and the percentage of lakes having pH<6 decreases in these areas. The Atlantic Provinces will obtain only a minor benefit from the control programs, i.e. experiencing only a small decrease in deposition and improvement in water quality. High sensitivity of the terrain in many parts of Atlantic Canada means that large numbers of lakes will remain acidic (i.e. ANC<0) and/or have pH<6 (an important biological threshold) even after full implementation of the current plans for SO₂ control in Canada and the U.S.     

Testing the MAGIC acid rain model in highly organic,low-conductivity waters using multiple calibrations

Accurate predictions of acid precipitation effects on water resources are important in order to allow a better understanding of various pollution control strategy outcomes. Dynamic geochemical models have been developed to address this need, but have to be tested under a variety of environmental conditions to provide confidence in their predictions. The most commonly used aquatic acidification model in North America and Europe is the model of acidification of groundwater in catchments (MAGIC). Though extensively used, MAGIC has never been tested in catchments with extremely low ionic strength water and high in natural organic acids (NOAs) from wetlands, two conditions which are common in large parts of Canada. We calibrated the model for two catchments located in Nova Scotia, Canada, which had some of the most dilute freshwaters reported in the literature and very high NOA. We also evaluated the variability inherent in calibration data sets by using five separate data sets collected over a 15-year period at the same sites. We show good model simulations for the main cations and anions in catchment waters. However, modeling pH is more difficult in the highly organic waters and requires modification to the acid dissociation constants. Calculated acid neutralization capacity can also be more difficult to model due to the low ion content making small errors more important. In theory, multiple calibrations of a model at a same site should produce identical hindcasts and predictions. In reality, the multiple calibrations produced a series of similar, but not identical outcomes which give a probable range of past values and future outcomes. We feel that this practical approach to validation is a useful addition to the arsenal of model testing tools.     

An assessment of the relationship between potential chemical indices of nitrogen saturation and nitrogen deposition in hardwood forests in southern Ontario

Southern Ontario receives the highest levels of atmospheric nitrogen (N) deposition in Canada and there are concerns that forests in the region may be approaching a state of ‘N saturation’. In order to evaluate whether potential chemical indices provide evidence of N saturation, 23 hardwood plots were sampled along a modeled N-deposition gradient ranging from 9.3 to 12.8 kg/ha/year. All plots were dominated by sugar maple (Acer saccharum Marsh.) and foliar N and foliar δ¹⁵N were positively correlated with modeled N deposition. However, forest floor N content and the C:N ratio were unrelated to N deposition, but were instead related to soil pH and annual temperature; lower C:N ratios and higher N content in the forest floor were found at the most acidic sites in the cooler, northern part of the study region despite lower N deposition. Likewise, δ¹⁵N values in surface mineral soil and the ¹⁵N enrichment factor of foliage (δ¹⁵N foliage ? δ¹⁵N soil) are correlated to soil pH and temperature and not N deposition. Further, potential N mineralization, ammonification, and nitrification in Ontario maple stands were highest in the northern part of the region with the lowest modeled N deposition. Nitrogen cycling in soil appears to be primarily influenced by the N status of the forest floor and other soil properties rather than N deposition, indicating that chemical indices in soil in these hardwood plots may not provide an early indicator of N saturation.     

The use of leeches and logit log-linear contingency models to assess and monitor aquatic PCB contamination originating from mid-Canada radar line site 050

Abandoned military sites in northern North America are relics of the Cold War and sources of polychlorinated biphenyls (PCBs). In the late 1990s, the Canadian federal and provincial governments began the cleanup of the mid-Canada radar line in Ontario, Canada. The first site to be remediated was Site 050 (Fort Albany First Nation) in 2001; however, the community remains concerned that contaminants may have moved prior to, during, and after remediation into the Albany River directly adjacent to Site 050. Thus, the Albany River was monitored (1999, 2001, 2002) during the remediation process to determine if the cleanup itself further contaminated the aquatic compartment, using leeches (Haemopis spp.) as bioindicators. Few organochlorines were found in leeches at levels higher than the detection limit, aside from PCBs. Leech data from the present study indicated that PCB levels were significantly higher near Site 050 than the control site upstream, indicating point source contamination from Site 050. The temporal trend in leech contaminant data indicated an increase in PCB contaminant load from 1999 (pre-remediation) to 2001 (immediately post-remediation), but this difference was not statistically significant due to high variances. Nevertheless, logit log-linear contingency modeling did reveal that immediately after cleanup (2001), contaminants (CBs 99, 118, 128, 156, 170, 183) in leeches were detected significantly more frequent than expected. When taken together, leech body burden and frequency of detection data suggest that the remediation process itself further contaminated the aquatic environment, if only temporarily. Lastly, the removal of the terrestrial source of PCBs during remediation did remove the source of aquatic contaminants in that body burden of contaminants in leeches were significantly lower a year after cleanup.     

Estimation of Critical Loads of Acidity for Lakes in Northeastern United States and Eastern Canada

The New England Governors and Eastern Canadian Premiers (NEG/ECP) adopted the Acid Rain Action Plan in June 1998, and issued a series of action items to support its work toward a reduction of sulfur dioxide (SO₂) and nitrogen oxide (NO_x) emissions in northeastern North America. One of these action items was the preparation of an updated critical load map using data from lakes in the NEG/ECP area. Critical load maps provide a more complete index of the surface water sensitivity to acidification. Combined sulfur and nitrogen critical loads and deposition exceedances were computed using Henriksen's Steady-State Water Chemistry (SSWC) model. Results show that 28% of all 2053 lakes studied have a critical load of 20 kg/ha/year or less, making them vulnerable to acid deposition. Emission reductions, and more specifically SO₂ emission reductions have proven beneficial because critical loads were exceeded in 2002 for 12.3% of all studied lakes. Those lakes are located in the more sensitive areas where geology is carbonate-poor. Of these lakes, 2.9% will never recover even with a complete removal of SO₄ deposition. Recovery from acidification for the remaining 9.4% of the lakes will require additional emission SO₂ reductions.     

Mean Annual Temperature and Total Annual Precipitation Trends at Canadian Biosphere Reserves

This article examines instrumental climate records from a varietyof stations associated with the following Biosphere Reserves across Canada: (i) Waterton Lakes, (ii) Riding Mountain, (iii) Niagara Escarpment, (iv) Long Point, and (v) Kejimkujik (Candidate Biosphere Reserve). Annual series are generated fromdaily temperature and precipitation values. In addition, homogeneous data are used from other stations and regional records to supplement the records from the local biosphere stations. Long term trends are identified over the period of the instrumental record. In general, data from the interval 1900 to 1998 show cooler temperatures in the 1920's, warmingfrom the early 1940's into the early 1950's, cooling into the1970's, and subsequent warming. At many stations, 1998 is the warmest in the instrumental record. Comparisons with the regional data sets show good agreements between the temperatureseries. The 20th century warming is approximately 1.0 °C in the Riding Mountain area and 0.6 °Cin the Long Point, Niagara Escarpment, and Waterton Lakes areas.There has been slight cooling in the Kejimkujik area over the past half century. Precipitation data show increasing trends inthe Kejimkujik, Long Point, Niagara Escarpment, and Waterton Lakes areas with no long term trend in the Riding Mountain area. This work is part of the Canadian Biosphere Reserves Association (CBRA) Climate Change Initiative (CCI), designed to present climate change information to Biosphere Reserve communities to allow local organizations to understand climate change and adapt to potential impacts.     

Using Temporal Coherence to Determine the Response to Climate Change in Boreal Shield Lakes

Climate change is expected to have important impacts on aquatic ecosystems. On the Boreal Shield, mean annual air temperatures are expected to increase 2 to 4°C over the next 50 years. An important challenge is to predict how changes in climate and climate variability will impact natural systems so that sustainable management policies can be implemented. To predict responses to complex ecosystem changes associated with climate change, we used long-term biotic databases to evaluate how important elements of the biota in Boreal Shield lakes have responded to past fluctuations in climate. Our long-term records span a two decade period where there have been unusually cold years and unusually warm years. We used coherence analyses to test for regionally operating controls on climate, water temperature, pH, and plankton richness and abundance in three regions across Ontario: the Experimental Lakes Area, Sudbury, and Dorset. Inter-annual variation in air temperature was similar among regions, but there was a weak relationship among regions for precipitation. While air temperature was closely related to lake surface temperatures in each of the regions, there were weak relationships between lake surface temperature and richness or abundance of the plankton. However, inter-annual changes in lake chemistry (i.e., pH) were correlated with some biotic variables. In some lakes in Sudbury and Dorset, pH was dependent on extreme events. For example, El Nino related droughts resulted in acidification pulses in some lakes that influenced phytoplankton and zooplankton richness. These results suggest that there can be strong heterogeneity in lake ecosystem responses within and across regions.     

An Analysis of the Daily Radial Activity of 7 Boreal Tree Species, Northwestern Quebec

In the `Des Vieux Arbres' ecological reserve situated within northwestern Québec, 40 band dendrometers were installedon 7 of the major boreal tree species. The late Spring–early Summer daily radial activity registered in 1997 was related todaily weather variables. For each tree species, the daily mean i) cumulative radial increment and ii) radial activity indexedseries obtained by first-difference standardization were analyzed. The results indicate the existence of strong similarities among the 7 species. All showed strong synchronousfluctuations in radius during late winter and early spring. Thisperiod ended with a short but sharp increase in radial increments that marked the passage of water into the stem. Thisinitial swelling, less obvious in Pinus species was followed by a prolonged period of little change in radial activity. Meteorological data indicated that air temperature waspositively related to stem swelling during the late winter–earlyspring period. Both air and soil temperatures became negatively related to radial expansion once the passage of water has occurred in the stem. Starting in early June, all species registered a sustained increase in radial increments possiblyassociated with active cell division. After this, radial expansion was negatively related to air temperature and positively to rainfall.     

Canadian Environmental Effects Monitoring: Experiences with Pulp and Paper and Metal Mining Regulatory Programs

In Canada, Environmental Effects Monitoring (EEM) programs exist within two regulations: the Pulp and Paper Effluent Regulations and the newMetal Mining Effluent Regulations under the Canadian Fisheries Act. EEM provides a biological, effects-based feedback loop to assess the effectiveness of technology-based regulations in protecting receiving environments. The promulgation of the Pulp and Paper Effluent Regulations, in 1992, represented a significant step forward in the Canadian regulatory approach by incorporating directly into a regulation a requirement to assess the effects of effluent discharges on receiving environments using proven scientific monitoring methodologies. Similarly, an assessment of the aquatic impacts of mines resulted in recommendations to amend the Metal Mining Effluent Regulations, recently promulgated in 2002, and includes an EEM program as a science-based feedback loop. As such, these regulations recognize the possibility that national, technology-based standards may not necessarily protect all receiving environments because of the diversity and variability of both discharges and receiving sites across the country. Since that time, EEM has improved its flexibility by considering both advances in science and the uniqueness of monitoring sites across Canada to allow the most appropriate and cost-effective monitoring approaches at each site while maintaining national consistency. This paper discusses the use of monitoring under two Canadian regulations to assess effects on aquatic ecosystems. As well, the National EEM approach to maintaining up-to-date scientific practices in a national regulatory program is discussed using examples.     

Patterns and Trends in Southern Ontario Lake ice Phenology

An analysis is presented of 46 ice break up and 15 ice free season phenology data series obtained largely through volunteer monitoring efforts in Southern Ontario. Observations spanned the years 1853–2001. Available data included dates of ice formation and ice break up as well as the number of ice free days in a year. A high degree of temporal coherence in ice phenology between lakes was observed (137/365 pairwise correlations significant at P<0.05). Significant monotonic trends towards earlier break up dates and longer ice free seasons were observed across the region both in the entire series and in the last thirty years of data. Trends in longer series may be associated with the end of the Little Ice Age. The significantly longer ice free seasons and earlier ice break up dates observed in the study area have important implications for lakes in other parts of Canada where climate change effects are predicted to be more extreme than in South-Central Ontario.     

Status and Trends of Ontario's Sydenham River Ecosystem in Relation to Aquatic Species at Risk

The Sydenham River in southwestern Ontario is located in the Mixedwood Plains Ecozone, which supports the greatest diversity of flora and fauna in Canada. The river historically supported a rich aquatic community that included 80 fishes and 34 species of freshwater mussels. Fourteen aquatic species native to the river (8 fishes, 5 mussels and 1 turtle) have been designated as endangered, threatened, or of special concern by the Committee on the Status of Species at Risk in Canada (COSEWIC). A multi-agency Recovery Team was formed in 1999 to ensure the continued survival of these and other rare species in the river. The Sydenham River Recovery Team is the first in Canada to adopt an ecosystem approach to recovery planning for aquatic species. Information on land use patterns, water quality trends, the physical condition of the river, and the distributions of aquatic species at risk was synthesized to gain an understanding of the overall health of the river and its major anthropogenic stresses. Seven of the species at risk have declined in distribution or abundance, and three may be extirpated. The main threat to fishes and mussels is heavy loadings of sediment, nutrients, and possibly pesticides to the river via tile drainage and overland runoff from agricultural lands. A strategy that incorporates four overall approaches (management, stewardship, research and monitoring, and awareness and outreach) was developed to recover and protect this globally significant freshwater ecosystem.     

A Global Overview of Atmospheric Acid Deposition Fluxes

This paper presents a summary of globalacid deposition flux data taken from a globalassessment report on acid deposition prepared forUNEP/WMO (Whelpdale and Kaiser, 1996). There is a largevariation in the spacial coverage and reliability ofmonitoring around the world. Many more stationsmeasure wet deposition than collect appropriate datafor estimating dry deposition. The widespread regionswith highest precipitation concentrations anddeposition fluxes of sulphate and nitrate coincideclosely with the regions of highest density ofSO₂ and NO_x precursor emissions occurringprimarily in the mid-latitude, northern hemispherebelt where a large fraction of the worlds fossilfuels is consumed. Organic acids in precipitation makea minor contribution to acidity (<20%) inindustrial regions, but in the rest of the world theyare of same order, or even exceed, inorganic acids.Less is known about dry deposition, but it appears topredominate near strong emission sources with wetdeposition predominating farther downwind. The molarratio of the N/S contribution to acidic deposition isclose to 1.0 over large areas of Europe and NorthAmerica, but is highly variable elsewhere, beinghighest in equatorial regions due to biomass burningand lowest near smelters and other large sources of SO₂.