Content uploaded by Hjalmar Laudon
Author content
All content in this area was uploaded by Hjalmar Laudon on Aug 26, 2014
Content may be subject to copyright.
A preview of the PDF is not available
Recovery from episodic acidification delayed by drought and high sea salt deposition
Abstract and Figures
For the prediction of episodic acidification large uncertainties are connected to climatic variability and its effect on drought conditions and sea-salt episodes. In this study data on 342 hydrological episodes in 25 Swedish streams, sampled over 10 years, have been analyzed using a recently developed episode model. The results demonstrate that drought is the most important factor modulating the magnitude of the anthropogenic influence on pH and ANC during episodes. These modulating effects are especially pronounced in southern and central Sweden, where the historically high acid deposition has resulted in significant S pools in catchment soils. The results also suggest that the effects of episodic acidification are becoming less severe in many streams, but this amelioration is less clear in coastal streams subject to high levels of sea-salt deposition. Concurrently with the amelioration of the effects of episodic acidification, regional climate models predict that temperatures will increase in Sweden during the coming decades, accompanied by reductions in summer precipitation and more frequent storms during fall and winter in large areas of the country. If these predictions are realized delays in streams' recovery from episodic acidification events can be expected.
Figures - uploaded by Hjalmar Laudon
Author content
All figure content in this area was uploaded by Hjalmar Laudon
Content may be subject to copyright.
... Besides other environmental factors which are able to interfere with recovery, numerous studies from Scandinavia hint at a significant influence of sea salt deposition causing short-term acidification peaks that hinder long-term recovery of forested ecosystems from anthropogenic acidification (Hindar et al., 1995;Wright and Jenkins, 2001;Larssen and Holme, 2006;Skjelkvåle et al., 2007;Laudon, 2008;Akselsson et al., 2013). This so called 'sea salt effect' (Wiklander, 1975) is caused by cation exchange processes induced by an excessive input of sodium (Na + ). ...
... Various environmental factors are known to affect long-term recovery of anthropogenically acidified forest catchments including the 'sea salt effect' (Wiklander, 1975), with episodic sea salt input significantly hindering long-term recovery of coastal forest ecosystems (Larssen and Holme, 2006;Skjelkvåle et al., 2007;Laudon, 2008;Akselsson et al., 2013). By increasing the net charge in the soil solution, episodic input of sea salt is described by experimental as well as field studies to increase cation exchange processes in the soil, leading to temporary acidity flushes in the run-off especially in already acidified soils (Wiklander, 1975;Hindar et al., 1995;Skjelkvåle et al., 2007;Laudon, 2008). ...
... Various environmental factors are known to affect long-term recovery of anthropogenically acidified forest catchments including the 'sea salt effect' (Wiklander, 1975), with episodic sea salt input significantly hindering long-term recovery of coastal forest ecosystems (Larssen and Holme, 2006;Skjelkvåle et al., 2007;Laudon, 2008;Akselsson et al., 2013). By increasing the net charge in the soil solution, episodic input of sea salt is described by experimental as well as field studies to increase cation exchange processes in the soil, leading to temporary acidity flushes in the run-off especially in already acidified soils (Wiklander, 1975;Hindar et al., 1995;Skjelkvåle et al., 2007;Laudon, 2008). Furthermore, increasing concentrations of sodium are reported to enhance the leaching of important soil nutrients like K + , Mg 2+ and Ca 2+ (Larssen and Holme, 2006;Findlay and Kelly, 2011;Akselsson et al., 2013). ...
Atmospheric acidic depositions have strongly altered the functioning and biodiversity of Central European forest ecosystems. Most impacts occurred until the end of the 20th century but the situation substantially improved thereafter caused by legal regulations in the late 1980's to reduce acidifying atmospheric pollution. Since then slow recovery from acidification has been observed in forested catchments and adjacent waters. However, trends of recovery are inconsistent and underlying mechanisms diminishing recovery are still poorly understood.
We propose that the input of road salt can significantly affect acidity regime and acidification recovery of forest ecosystems.
By comparing the discharge hydro-chemistry and plant community composition of springs fed by forested catchments with and without high levels of salt input over two decades we observed a significant suppression of recovery and elevated levels of nutrient leaching (K+, Ca2 + and Mg2 +) in highly salt contaminated catchments.
We show that the pollution of near-surface groundwater (interflow) by road salt application can have lasting effects on ecosystem processes over distances of several hundred metres apart from the salt emitting road.
Link to the pdf-file: http://authors.elsevier.com/a/1RFXvB8ccV3~J
... Since the peak of the acidification debate, sulfur (S) and nitrogen (N) deposition in snow and rain across northern Sweden have continued to decline to levels on par with pre-industrial conditions (Fig 1). During this period only a few studies investigated the long-term effects of acidification, demonstrating that the anthropogenic component during snowmelt is strongly regulated by the winter S deposition levels (Laudon andHemond, 2002, Lawrence et al., 2008), and revealing interactions between acidification and long-term trends of DOC levels in streams (Erlandsson et al. 2011). Further, while earlier studies based on water chemistry trends predicted that episodic acidification would soon recover (Laudon and Bishop, 2002), other, ongoing environmental changes in the region (e.g., warming, increased drought 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A c c e p t e d M a n u s c r i p t frequency, and browning; Laudon and Sponseller 2018) have the potential to alter or mask such recovery trajectories. ...
... Droughts are known to cause large changes in stream water quality, including extreme SO4related acid surges during the wetting-up phase that have been shown to delay the recovery from episodic acidification both in Sweden (Laudon, 2008), and elsewhere (Mitchell et al. 2008). Consistent with this, SO4 concentrations were several times higher during the wettingup phase that followed severe summer droughts in our long-term record (Figure 6a). ...
Few environmental issues have resulted in such a heated policy-science controversy in Sweden as the 1990s acidification debate in the north of the country. The belief that exceptionally high stream acidity levels during hydrological events was caused by anthropogenic deposition resulted in a governmentally funded, multi-million dollar surface-water liming program. This program was heavily criticized by a large part of the scientific community arguing that the acidity of northern streams was primarily caused by naturally occurring organic acids. Here, we revisit the acid deposition legacy in northern Sweden two decades after the culmination of the controversy by examining the long-term water chemistry trends in the Svartberget/Krycklan research catchment that became a nexus for the Swedish debate. In this reference stream, trends in acidic episodes do show a modest recovery that matches declines in acid deposition to pre-industrial levels, although stream acidity continues to be overwhelmingly driven by organic acidity. Yet there are legacies of acid deposition related to calcium losses from soils, which are more pronounced than anticipated. Finally, assessment of these trends are becoming increasingly complicated by new changes and threats to water resources that must be recognized to avoid unnecessary, expensive, and potentially counterproductive measures to adapt and mitigate human influences. Here we make the argument that while the acidification era is ending, climate change, land-use transitions, and long-range transport of other contaminants warrant close monitoring in the decades to come.
... However, in nitrogen-rich areas, increased decomposition can lead to nitrate leaching (van Breemen et al. 1998;Wright and Jenkins 2001), which is an acidifying process. Other effects related to climate change that may affect recovery are changes in the frequency of sea-salt episodes (Akselsson et al. 2013;Skjelkvåle et al. 2007;Laudon 2008;Hindar et al. 1995), the frequency of drought (Laudon 2008), and changes in conditions affecting pests (Netherer and Schopf 2010). To conclude, temperature, soil moisture and other effects related to climate change are key factors in controlling soil and vegetation processes that may affect recovery from acidification. ...
... However, in nitrogen-rich areas, increased decomposition can lead to nitrate leaching (van Breemen et al. 1998;Wright and Jenkins 2001), which is an acidifying process. Other effects related to climate change that may affect recovery are changes in the frequency of sea-salt episodes (Akselsson et al. 2013;Skjelkvåle et al. 2007;Laudon 2008;Hindar et al. 1995), the frequency of drought (Laudon 2008), and changes in conditions affecting pests (Netherer and Schopf 2010). To conclude, temperature, soil moisture and other effects related to climate change are key factors in controlling soil and vegetation processes that may affect recovery from acidification. ...
Whole-tree harvesting, i.e. harvesting of stems, branches and tops, has become increasingly common during recent decades due to the increased demand for renewable energy. Whole-tree harvesting leads to an increase in base cation losses from the ecosystem, which can counteract recovery from acidification. An increase in weathering rates due to higher temperatures is sometimes suggested as a process that may counteract the acidifying effect of whole-tree harvesting. In this study the potential effect of increasing temperature on weathering rates was compared with the increase in base cation losses following whole-tree harvesting in spruce forests, along a temperature gradient in Sweden. The mechanistic model PROFILE was used to estimate weathering rates at National Forest Inventory sites at today’s temperature and the temperature in 2050, as estimated by two different climate projections. The same dataset was used to calculate base cation losses following stem-only and whole-tree harvesting. The calculations showed that the increase in temperature until 2050 would result in an increase in the base cation weathering rate of 20–33 %, and that whole-tree harvesting would lead to an increase in base cation losses of 66 % on average, compared to stem-only harvesting. A sensitivity analysis showed that moisture changes are important for future weathering rates, but the effect of the temperature change was dominating even when the most extreme moisture changes were applied. It was concluded that an increase in weathering rates resulting from higher temperatures would not compensate for the increase in base cation losses following whole-tree harvesting, except in the northernmost part of Sweden.
... Many of these climate-mediated mechanisms are influenced by landscape characteristics (Webster et al. 2000) and historical inputs of acidic deposition (Laudon 2008). In the catchments studied by Kerr et al. (2012), the proportion of variation in annual SO 4 2concentration explained by discharge ranged from over 70 % to little or none and was positively related to wetland coverage in the watershed. ...
... The sites in this analysis have an average maximum watershed elevation of 600 m, with *5 % above 1000 m and *30 % above 750 m. Historical deposition can influence climate-mediated change in SO 4 2and DOC (Laudon 2008), and could also account for the relationship between elevation and SO 4 2response to extreme weather. Because there was no obvious relationship between wetlands and SO 4 2concentrations during drought, the effects of elevation and hydrology are likely more important in regulating SO 4 2response to extreme dry years as compared to extreme wet years. ...
Interannual climate variability is expected to increase over the next century, but the extent to which hydroclimatic variability influences biogeochemical processes is unclear. To determine the effects of extreme weather on surface water chemistry, a 30-year record of surface water geochemistry for 84 lakes in the northeastern U.S. was combined with landscape data and watershed-specific weather data. With these data, responses in sulfate (SO42−) and dissolved organic carbon (DOC) concentrations were characterized during an extreme wet year and an extreme dry year across the region. Redundancy analysis was used to model lake chemical response to extreme weather as a function of watershed features. A response was observed in DOC and SO42− concentration in response to extreme wet and dry years in lakes across the northeastern U.S. Acidification was observed during drought and brownification was observed during wet years. Lake chemical response was related to landscape characteristics in different ways depending on the type of extreme year. A linear relationship between wetland coverage and DOC and SO42− deviations was observed during extreme wet years. The results presented here help to clarify the variability observed in long-term recovery from acidification and regional increases in DOC. Understanding the chemical response to weather variability is becoming increasingly important as temporal variation in precipitation is likely to intensify with continued atmospheric warming.
... Climate change resulting in increasing winter runoffs can thus increase the terrestrial export of NO 3 − and Al i even at a stable N deposition rate, as well as at a stable N-saturation status of the catchment. Similar reductions in summer precipitation and more frequent storms during fall and winter may delay surface water recovery from acidification (Laudon, 2008). These effects are further magnified after vegetation damage (e.g., after forest harvest or insect infestation) due to the diminished N uptake by vegetation and elevated NO 3 − concentrations in soils (Huber, 2005;Kaňa et al., 2015). ...
... These confounding effects are also assumed to become more marked with climate change, because increasing temperature results in more frequent outbreaks of insects and the large-scale dieback of infested stands (Mikkelson et al., 2013). All processes leading to elevated terrestrial NO 3 − export are important vehicles of Al i to receiving waters, delaying their chemical (and preventing their biological) recovery (Laudon, 2008). This effect is more serious for streams than for lakes with longer water residence time, where the microbial NO 3 − removal neutralizes H + proportionally to the NO 3 − input ( Fig. SI-6A). ...
Fluxes of major ions and nutrients were measured in the N-saturated mountain forest catchment-lake system of Čertovo Lake (Czech Republic) from 1998 to 2014. The lake has been rapidly recovering from atmospheric acidification due to a 90% decrease in sulphate (SO42-) deposition since the late 1980s and nitrate (NO3-) contribution to the pool of strong acid anion and leaching of dissolved organic carbon (DOC) have increased. Present concentrations of base cations, phosphorus (P), total organic N (TON), and ionic (Ali) and organically bound (Alo) aluminium in tributaries are thus predominantly governed by NO3- and DOC leaching. Despite a continuing recovery lasting 25 years, the Čertovo catchment is still a net source of protons (H+), producing 44 mmol m-2 yr-1 H+ on a catchment-area basis (corresponding to 35 μmol L-1 on a concentration basis). Retention of the deposited inorganic N in the catchment averages 20%, and ammonium consumption (51 mmol m-2 yr-1) and net NO3- production (28 mol m-2 yr-1) are together the dominant terrestrial H+ generating processes. In contrast, the importance of SO42- release from the soils on terrestrial H+ production is continuously decreasing, with an average of 47 mmol m-2 yr-1 during the study. The in-lake biogeochemical processes reduce the incoming acidity by ∼40%, neutralizing 23 μmol L-1 H+ (i.e., 225 mmol m-2 yr-1 on a lake-area basis). Denitrification and photochemical and microbial decomposition of DOC are the most important in-lake H+ consuming processes (50 and 39%, respectively), while hydrolysis of Ali (from tributaries and photochemically liberated from Alo) is the dominant in-lake H+ generating process. Because the trends in water chemistry and H+ balance in the catchment-lake system are increasingly related to variability in NO3- and DOC leaching, they have become sensitive to climate-related factors (drought, elevated runoff) and forest damage that significantly modify the leaching of these anions. During the study period, increased exports of NO3- (accompanied by Ali and base cations) from the Čertovo catchment occurred after a dry and hot summer, after forest damage, and during elevated winter runoff. Increasing DOC export due to decreasing acid deposition was further elevated during years with higher runoff (and especially during events with lateral flow), and was accompanied by P, TON, and Alo leaching. The climate-related processes, which originally "only" confounded chemical trends in waters recovering from acidification, may soon become the dominant variables controlling water composition in N-saturated catchments.
... The relationship between soil acidity and the leaching of H + , Al 3+ and base cations is complex, but it is often observed that the pH falls and Al 3+ increases and sea salt ions (Na + , Mg 2+ , and Cl − ) increase shortly after sea-salt episodes (Hindar et al., 1995;Pedersen and Bille-Hansen, 1995). Disturbances in acidification recovery as a result of sea-salt episodes have been observed in European and Swedish forests (Wright, 2008;Akselsson et al., 2013;Laudon, 2008). Wright observed that sea-salt episodes were responsible for more than one-third of the low ANC (ANC < −50 eq l −1 ) episodes, which were considered as strong indicator of acid episodes, at Birkenes in Norway during the period 1975-2004. ...
... Wright observed that sea-salt episodes were responsible for more than one-third of the low ANC (ANC < −50 eq l −1 ) episodes, which were considered as strong indicator of acid episodes, at Birkenes in Norway during the period 1975-2004. Akselsson et al. (2013) and Laudon (2008) pointed out that recovery from acidification was slow at some coastal forest sites in Sweden, as these sites are susceptible to sea-salt episodes. ...
A Norway spruce (Picea abies Karst) forest site in southwest Sweden was chosen to study the effects of storm disturbances over the period 1997–2009, during which two storms, ‘Lothar’ (December 1999) and ‘Gudrun’ (January 2005), affected the area. Monitored deposition data, soil water chemistry data and forest inventory data were compared with the predictions of an integrated ecosystem model, ForSAFE, in an effort to reveal and understand the effects of storms on acidification/recovery in forest soils. Both storms caused windthrow loss leading to increased nitrate and sulphate concentrations in soil water as a result of stimulated mineralization. Lothar led to increased concentrations of Na+, Mg2+, and Cl− in soil water due to sea-salt episode. No general sea-salt episode was seen following Gudrun, but small sea-salt episodes were observed in 2007 and 2008. Each sea-salt episode caused a temporary decrease of pH, and a subsequent recovery, but overall, the soil water pH decreased from 4.54 to 3.86 after Lothar. Modelling suggested that the site was recovering from acidification from 1990s, and would continue to recover in future. Both modelled and monitored data showed that storm caused disturbances in the recovery; monitored data even suggested that soil acidification happened due to storm disturbances. Sea-salt episode does not increase soil acidity in the long term, and will probably decrease the soil acidity by replenishing the base saturation. The modelled data also suggested that storms with only windthrow would not have effects on soil acidification recovery in the long term, but they may influence the soil fertility by losses of base cations.
... Al) leading to the subsequent elevated state of DOC solubility. Climatic episodes associated with droughts are more commonly reported for wetland soils, including peatlands (Clark et al., 2005;Eimers et al., 2008;Laudon, 2008). At the extreme scale of wetness, the MH soil shows different dominant processes associated with hydrological and redox controls on peatland stored S. Droughts and lowering of the water table at MH in 1995 and 2003 led to short-term suppression of DOC concentrations (Fig. 1). ...
Moorland carbon reserves in organo-mineral soils may be crucial to predicting landscape-scale variability in soil carbon losses, an important component of which is dissolved organic carbon (DOC). Surface water DOC trends are subject to a range of scaling, transport and biotic processes that disconnect them from signals in the catchment's soils. Long-term soil datasets are vital to identify changes in DOC release at source and soil C depletion. Here we show, that moorland soil solution DOC concentrations at three key UK Environmental Change Network sites increased between 1993–2007 in both surface- and sub- soil of a freely-draining Podzol (48 % and 215 % increases in O and Bs horizons, respectively), declined in a gleyed Podzol and showed no change in a Peat. Our principal findings were that: (1) considerable heterogeneity in DOC response appears to exist between different soils that is not apparent from the more consistent observed trends for streamwaters, and (2) freely-draining organo-mineral Podzol showed increasing DOC concentrations, countering the current scientific focus on soil C destabilization in peats. We discuss how the key solubility controls on DOC associated with coupled physico-chemical factors of ionic strength, acid deposition recovery, soil hydrology and temperature cannot readily be separated. Yet, despite evidence that all sites are recovering from acidification the soil-specific responses to environmental change have caused divergence in soil DOC concentration trends. The study shows that the properties of soils govern their specific response to an approximately common set of broad environmental drivers. Key soil properties are indicated to be drainage, sulphate and DOC sorption capacity. Soil properties need representation in process-models to understand and predict the role of soils in catchment to global C budgets. Catchment hydrological (i.e. transport) controls may, at present, be governing the more ubiquitous rises in river DOC concentration trends, but soil (i.e. source) controls provide the key to prediction of future C loss to waters and the atmosphere.
... Recovery has been slow in some areas (Driscoll et al., 2003;Evans et al., 2014;Futter et al., 2014) in part because the sulfur (S) legacy in the soil, predominantly accumulated in organic S pools (Wieder and Lang, 1988;Houle and Carignan, 1992;Giesler et al., 2005), can be oxidized to SO 4 2− and mobilized into surface waters after drought events (Aherne et al., 2008). Redox-mediated SO 4 2− pulses have been observed in many forest catchments (Laudon et al., 2004a;Eimers et al., 2008;Laudon, 2008;Vestin et al., 2008;Landre et al., 2009;Kerr et al., 2012) as well as in other ecosystems such as blanket bogs (Clark et al., 2005). However, it is not clear how long such pulses will continue as S pools are depleted in those areas and climate changes. ...
In boreal forest catchments, solute transfer to streams is controlled by hydrological and biogeochemical processes occurring in the riparian zone (RZ). However, RZs are spatially heterogeneous and information about solute chemistry is typically limited. This is problematic when making inferences about stream chemistry. Hypothetically, the strength of links between riparian and stream chemistry is time-scale dependent. Using a ten-year (2003−2012) dataset from a northern Swedish catchment, we evaluated the suitability of RZ data to infer stream dynamics at different time scales. We focus on the role of the RZ versus upslope soils in controlling sulfate (SO42−) and dissolved organic carbon (DOC). A priori, declines in acid deposition and redox-mediated SO42− pulses control sulfur (S) fluxes and pool dynamics, which in turn affect dissolved organic carbon (DOC). We found that the catchment is currently a net source of S, presumably due to release of the S pool accumulated during the acidification period. In both, RZ and stream, SO42− concentrations are declining over time, whereas DOC is increasing. No temporal trends in SO42− and DOC were observed in upslope mineral soils. SO42− explained the variation of DOC in stream and RZ, but not in upslope mineral soil. Moreover, as SO42− decreased with time, temporal variability of DOC increased. These observations indicate that: (1) SO42− is still an important driver of DOC trends in boreal catchments and (2) RZ processes control stream SO42− and subsequently DOC independently of upslope soils. These phenomena are likely occurring in many regions recovering from acidification. Because water flows through a heterogeneous mosaic of RZs before entering the stream, upscaling information from limited RZ data to the catchment level is problematic at short-time scales. However, for long-term trends and annual dynamics, the same data can provide reasonable representations of riparian processes and support meaningful inferences about stream chemistry.
The pH values of most natural, river, and lake waters are in the range of 6–9. There are natural and an-
thropogenic sources that may influence the acidification of surface inland waters. The former depend on
geological, geochemical, biological, and climatic factors. Since the 1960s, the problem of lake and river
acidification has mostly been related to anthropogenic emissions of chemical compounds that have contrib-
uted to acidification either through acid deposition (SO2, NOx) or via terrestrial chemical transformations
leading to H+ production (NH3/NH4+). This entry presents a brief history of the problem of river and lake
acidification. Moreover, it also focuses on the sources of this phenomenon with examples of natural and hu-
man-induced changes in freshwater pH. The environmental impact of acidification on surface inland waters
is also described with a brief discussion on possible solutions to this issue.
As part of the Episodic Response Project (ERP), we studied the effects of episodic acidification on fish in 13 small streams in the northeastern United States: four streams in the Adirondack region of New York, four streams in the Catskills, New York, and five streams in the northern Appalachian Plateau, Pennsylvania. In situ bioassays with brook trout (Salvelinus fontinalis) and a forage fish species (blacknose dace (Rhinichthys atratulus), mottled sculpin (Cottus bairdi), or slimy sculpin (Cottus cognatus), depending on the region) measured direct toxicity. Movements of individual brook trout, in relation to stream chemistry, were monitored using radiotelemetry. Electrofishing surveys assessed fish community status and the density and biomass of brook trout in each stream. During low flow, all streams except one had chemical conditions considered suitable for the survival and reproduction of most fish species (median pH 6.0-7.2 during low flow; inorganic Al < 60 mu g/L). ERP streams with suitable conditions during low flow, but moderate-to-severe episodic acidification during high flow, had higher fish mortality in bioassays, net downstream movement of brook trout during events, and lower brook trout density and biomass compared to nonacidic streams, and lacked acid-sensitive fish species (blacknose dace and sculpin). Movement of trout into refugia (areas with higher pH and lower Al) during episodes partially mitigated the adverse effects of episodes. Recolonization from alkaline tributaries or microhabitats can maintain low densities of fish in streams that experience severe acidic episodes, but it is not sufficient to sustain fish densities and community composition at levels expected in the absence of these episodes. Fish responses to acid-base chemistry were fairly consistent across regions. In general, trout abundance was reduced and acid-sensitive fish species were absent from ERP streams with median pH < 5.0-5.2 during high flow and inorganic Al > 100-200 mu g/L. We conclude that episodic acidification can have long-term effects on fish communities in small streams.
Major episodic acidifications were observed on several occasions in first-order brooks at Acadia National Park, Mount Desert Island, Maine. Short-term declines of up to 2 pH units and 130-mu-eq L-1 acid-neutralizing capacity were caused by HCl from soil solutions, rather than by H2SO4 or HNO3 from precipitation, because (1) SO4 concentrations were constant or decreased during the pH depression, (2) Cl concentrations were greatest at the time of lowest pH, and (3) Na:Cl ratios decreased from values much greater than those in precipitation (a result of chemical weathering), to values equal to or less than those in precipitation. Dilution, increases in NO3 concentrations, or increased export or organic acidity from soils were insufficient to cause the observed decreases in pH. These data represent surface water acidifications due primarily to an ion exchange "salt effect" of Na+ for H+ in soil solution, and secondarily to dilution, neither of which is a consequence of acidic deposition. The requisite conditions for a major episodic salt effect acidification include acidic soils, and either an especially salt-laden wet precipitation event, or a period of accumulation of marine salts from dry deposition, followed by wet inputs.
Sulphate deposition has decreased by about 60% in the Nordic countries
since the early 1980s. Nitrogen deposition has been roughly constant
during the past 20 years, with only a minor decrease in the late 1990s.
The resulting changes in the chemistry of small lakes have been followed
by national monitoring programmes initiated in the 1980s in Finland (163
lakes), Norway (100 lakes) and Sweden (81 lakes). These lakes are partly
a subset from the survey of 5690 lakes in the Northern European lake
survey of 1995. Trend analyses on data for the period 1990-1999 show
that the non-marine sulphate concentrations in lakes have decreased
significantly in 69% of the monitored lakes. Changes were largest in
lakes with the highest mean concentrations. Nitrate concentrations, on
the other hand, were generally low and showed no systematic changes.
Concentrations of non-marine base cations decreased in 26% of the lakes,
most probably an ionic-strength effect due to the lower concentrations
of mobile strong-acid anions. Acid neutralising capacity increased in
32% of the lakes. Trends in recovery were in part masked by large
year-to-year variations in sea-salt inputs and by increases in total
organic carbon concentrations. These changes were most probably the
result of climatic variations. Nordic lakes, therefore, show clear signs
of recovery from acidification. Recovery began in the 1980s and
accelerated in the 1990s. Reductions in sulphur deposition are the major
"driving force" in the process of recovery from acidification. Further
recovery can be expected in the next 10 years if the Gothenburg protocol
on emissions of acidifying pollutants is implemented.
Trends in anthropogenically driven episodic acidification associated with extended winter snow melt/rain episodes between 1983 and 1998 were investigated for two streams in Nova Scotia, Canada. The anthropogenic contribution to Acid Neutralization Capacity (ANC) was analysed using the Boreal Dilution Model (Bishop et al., 2000) modified by applying a sea-salt correction to all input hydrochemistry. The anthropogenic contribution to episodic ANC decline was statistically significant and strongly correlated with the decline in acid deposition, which decreased by approximately 50% during the period of record. Sensitivity analysis demonstrated that the BDM can be applied to surface waters with sea-salt contributions although the correction increases model uncertainty. Results of this study demonstrate the effectiveness of reduced emissions in North America in the last decades in decreasing the severity of episodic acidification in the Atlantic region of Canada.
A historical emission inventory for sulphur dioxide has been compiled for Europe covering the period 1880–1991. The estimated emissions have been used as input to the sulphur module of the EMEP/MSC-W acid deposition model. The aim was to show the way and the extent to which the historical development of anthropogenic sulphur dioxide emissions alone has affected the concentration and deposition fields of oxidised sulphur in Europe. Although acknowledged, effects exerted by the meteorological variability and the changing oxidising capacity of the atmosphere over the years have not been taken into consideration. Long-term emission estimates reveal that combustion of coal was the dominant emission source before World War II in all countries and combustion of liquid fuels thereafter in most. Releases from industrial processes were relatively small. National sulphur dioxide emissions peaked mainly in the 1960s and 1970s, whilst emission control measures resulted in gradual reductions in most countries in the 1980s. In Europe as a whole, coal combustion remained the major emission source throughout the century. Total anthropogenic releases increased by a factor of 10 between the 1880 s and 1970s when they peaked at approximately 55 million tonnes of sulphur dioxide, followed by a 30% decline in the 1980s. Uncertainties in national emission estimates due to uncertain sulphur contents in fossil fuels are within ± 30% for 22 out of 28 countries and ± 45% for the rest. The location of emission sources in Europe has shown over the years a progressive detachment from the coalfields towards a widespread distribution, accompanied in the last decades by considerable emission reductions over north-western and parts of central Europe and substantial increases in the south and south-east. Modelled air concentrations and depositions reflect to a great extent the emission pattern, revealing two- to six-fold increases between the 1880 s and 1970s. Maximum sulphur loadings are confined over parts of north-western and central Europe. Accumulated depositions over the period 1880–1991 in these areas reach 600 g (S) m −2 . Emissions are principally in the form of sulphur dioxide, so that comparable concentrations of particulate sulphate in low emission regions indicate the importance of long range transport. Assuming a constant ecosystem sensitivity throughout the period, depositions sufficient to cause ecosystem damage may have occurred before 1880 in many areas of north-western and central Europe. Nevertheless, in large parts of eastern and southern Europe depositions are still below these critical loads. DOI: 10.1034/j.1600-0889.1996.t01-2-00005.x
The Episodic Response Project (ERP) was an interdisciplinary study designed to address uncertainties about the occurrence, nature, and biological effects of episodic acidification of streams in the northeastern United States. The ERP research consisted of intensive studies of the chemistry and biological effects of episodes in 13 streams draining forested watersheds in the three study regions: the Northern Appalachian region of Pennsylvania and the Catskill and Adirondack Mountains of New York. Wet deposition was measured in each of the three study regions. Using automated instruments and samplers, discharge and chemistry of each stream was monitored intensively from fall 1988 through spring 1990. Biological studies focused on brook trout and native forage fish. Experimental approaches included in situ bioassays, radio transmitter studies of fish movement, and fish population studies. This paper provides an overview of the ERP, describes the methodology used in hydrologic and water chemistry components of the study, and summarizes the characteristics of the study sites, including the climatic and deposition conditions during the ERP and the general chemical characteristics of the study streams.
Sulphate concentrations in two headwater lakes and their major inflows were evaluated over an 18 year period (1980-81 to 1997-98) during which time sulphate bulk deposition declined by approximately 40%. The two lake catchments represent either end of the spectrum of acid sensitivity in the Muskoka-Haliburton region of Ontario. Between 1980 and 1998, sulphate concentrations in Harp and Plastic Lakes decreased, but the decrease was much less than expected (28% and 21% respectively) given the magnitude of change in deposition. Sulphate export in streams draining into the lakes greatly exceeded sulphate input to catchments in most years, which quantitatively explains the response of lake-sulphate concentration. Furthermore, temporal patterns in mean annual sulphate concentrations in streams were similar, and appeared to be related to climate factors. Specifically, catchment export of sulphate was greater and stream-sulphate concentrations were higher in years that had warm, dry summers, i.e. when streamflow in many catchments ceased for up to several weeks. Increased sulphate export from catchments resulted in higher sulphate concentrations in lakes, but the response of lake sulphate was not as immediate or dramatic as the response of stream sulphate to changes in catchment dryness. Factors that affect sulphate retention or export in catchments exert a strong influence on sulphate concentrations in lakes and streams and need to be considered when evaluating the response of surface water chemistry to changes in sulphate deposition.
The sulphate budget and a model for sulphate concentrations in streamwater have been established for a small (0.60 km2), gauged catchment in Telemark country, southern Norway. The annual input roughly balances the output; the average annual flux through the catchment for four years is about 3500 mg SO4m−2. Dry deposition is estimated to be 10% of the total sulphur input. The budget shows a marked seasonal pattern with net accumulation in the summer, net washout in the autumn, particularly in years with heavy autumn rains following dry summers, a second net accumulation due to the snowpack, and a pronounced net output during spring snowmelt. The concentrations in streamwater show minimum values (≈ 2 mg SO4l−1) at the end of melting, and maximum values (6–8 mg SO4l−1) at the start of melting and in the first runoff following dry summer periods. The model simulates main trends in the observed sulphate concentrations and is based on a simple reservoir hydrologic model. In the model, water-soluble sulphate accumulates in the reservoir representing the surface soils during dry periods because of dry deposition, evapotranspiration of summer rainfalls and mineralization. During wet summer periods, water-soluble sulphate is removed due to reduction or adsorption. The concentration pattern during snowmelt is largely dependent upon the fractionation of sulphate held by the snowpack. The model is particularly useful as a data interpretation tool.