Modelling the effect of climate change on recovery of acidified freshwaters: relative sensitivity of individual processes in the MAGIC model

Factors influencing the difference in dissolved ion inputs to the forest floor between deciduous and coniferous stands: comparison under high and low atmospheric deposition conditions

It is necessary to clear the relationship between physical and vegetation factors on the processes governing dissolved ion inputs to the forest floor to estimate correctly the values of atmospheric input to the forest. This study identified the factors influencing the differences in dissolved ion inputs to the forest floor between coniferous evergreen and broad-leaved deciduous species by analyzing the phenological variations of dry deposition and canopy exchange calculated by the canopy budget model under a high-deposition site near the city of Tokyo and a low-deposition site 84 km further away. At low-deposition site, vegetation factors such as capture efficiency did not explain the differences in Na+ or Cl- dry deposition. Leaf physiological characteristics influenced the differences in the Mg2+ and Ca2+ canopy leaching values, and phenology, leaf wettability, and diffusion processes from water film into leaves influenced the differences in NH4+ and NO3- input processes between tree types. At the high-deposition site, differences in the dry deposition of Na+, SO42-, Cl-, Mg2+, Ca2+, NH4+, and NO3- between tree types were influenced by differences in capture efficiency between coniferous and broad-leaved canopies in the leafed period and by the absence of leaves in deciduous species after leaf fall. These results indicated that atmospheric deposition affected the capture efficiency of coniferous trees for dry deposition and enhanced the difference of dissolved ion inputs to the forest floor between coniferous and deciduous species.

Diverse effects of accelerating climate change on chemical recovery of alpine lakes from acidic deposition in soil-rich versus scree-rich catchments

The current recovery of mountain lakes from atmospheric acidification is increasingly affected (both accelerated and/or delayed) by climate change. We evaluated long-term trends in the ionic composition of 30 lakes situated in the alpine zone of the Tatra Mountains, and compared the rates of their recovery with model (MAGIC) simulations done 20 years ago for the 2003-2020 period. The observed recovery was faster than the model forecast, due to greater reductions in acidic deposition than projected. Trends in water composition were further modified by climate change. Rising temperatures increased the length of the growing season and retention of inorganic N and SO₄^2- more in soil-rich compared with soil-poor catchments. In contrast, elevated precipitation and an increase in rainfall intensity reduced water residence time in soils, and consequently reduced N retention, especially in soil-poor catchments. It is likely that increases in rainfall intensity and annual number of days without snow, along with air temperatures fluctuating around the freezing point elevated the physical erosion of rocks, especially in high-elevation, steep, and scree-rich areas where rocks are not thermally insulated and stabilized by soils. Weathering of exposed accessory calcite in the eroded granodiorite bedrock was a source of Ca²⁺ and HCO₃^-, while S-bearing minerals likely contributed to lake water SO₄^2- and partly mitigated its deposition-related decrease in scree-rich catchments. The extent of climate effects on changes in the water composition of alpine lakes recovering from acidic deposition thus depended on elevation and cover of soil and scree in catchments. Our results highlight the need for incorporating dominant climate-related process into existing process-based models to increase their reliability in predicting the future development of lake water composition.

Assessing critical load exceedances and ecosystem impacts of anthropogenic nitrogen and sulphur deposition at unmanaged forested catchments in Europe

Anthropogenic emissions of nitrogen (N) and sulphur (S) compounds and their long-range transport have caused widespread negative impacts on different ecosystems. Critical loads (CLs) are deposition thresholds used to describe the sensitivity of ecosystems to atmospheric deposition. The CL methodology has been a key science-based tool for assessing the environmental consequences of air pollution. We computed CLs for eutrophication and acidification using a European long-term dataset of intensively studied forested ecosystem sites (n = 17) in northern and central Europe. The sites belong to the ICP IM and eLTER networks. The link between the site-specific calculations and time-series of CL exceedances and measured site data was evaluated using long-term measurements (1990-2017) for bulk deposition, throughfall and runoff water chemistry. Novel techniques for presenting exceedances of CLs and their temporal development were also developed. Concentrations and fluxes of sulphate, total inorganic nitrogen (TIN) and acidity in deposition substantially decreased at the sites. Decreases in S deposition resulted in statistically significant decreased concentrations and fluxes of sulphate in runoff and decreasing trends of TIN in runoff were more common than increasing trends. The temporal developments of the exceedance of the CLs indicated the more effective reductions of S deposition compared to N at the sites. There was a relation between calculated exceedance of the CLs and measured runoff water concentrations and fluxes, and most sites with higher CL exceedances showed larger decreases in both TIN and H⁺ concentrations and fluxes. Sites with higher cumulative exceedance of eutrophication CLs (averaged over 3 and 30 years) generally showed higher TIN concentrations in runoff. The results provided evidence on the link between CL exceedances and empirical impacts, increasing confidence in the methodology used for the European-scale CL calculations. The results also confirm that emission abatement actions are having their intended effects on CL exceedances and ecosystem impacts.

Dynamic Modeling and Target Loads of Sulfur and Nitrogen for Surface Waters in Finland, Norway, Sweden, and the United Kingdom

The target load concept is an extension of the critical load concept of air pollution inputs to ecosystems. The advantage of target loads over critical loads is that one can define the deposition and the point in time (target year) when the critical (chemical) limit is no longer violated. This information on the timing of recovery requires dynamic modeling. Using a well-documented dynamic model, target loads for acidic deposition were determined for 848 surface waters across Finland, Norway, Sweden, and the United Kingdom for the target year 2050. In the majority of sites ( n = 675), the critical ANC-limit was predicted to be achieved by 2050; however, for 127 sites, target loads were determined. In addition, 46 sites were infeasible, i.e., even a reduction of anthropogenic deposition to zero would not achieve the limit by 2050. The average maximum target load for sulfur was 38% lower than the respective critical load across the study lakes ( n = 127). Target loads on a large regional scale can inform effects-based emission reduction policies; the current assessment suggests that reductions beyond the Gothenburg Protocol are required to ensure surface water recovery from acidification by 2050.

Modelling study of soil C, N and pH response to air pollution and climate change using European LTER site observations

Current climate warming is expected to continue in coming decades, whereas high N deposition may stabilize, in contrast to the clear decrease in S deposition. These pressures have distinctive regional patterns and their resulting impact on soil conditions is modified by local site characteristics. We have applied the VSD+ soil dynamic model to study impacts of deposition and climate change on soil properties, using MetHyd and GrowUp as pre-processors to provide input to VSD+. The single-layer soil model VSD+ accounts for processes of organic C and N turnover, as well as charge and mass balances of elements, cation exchange and base cation weathering. We calibrated VSD+ at 26 ecosystem study sites throughout Europe using observed conditions, and simulated key soil properties: soil solution pH (pH), soil base saturation (BS) and soil organic carbon and nitrogen ratio (C:N) under projected deposition of N and S, and climate warming until 2100. The sites are forested, located in the Mediterranean, forested alpine, Atlantic, continental and boreal regions. They represent the long-term ecological research (LTER) Europe network, including sites of the ICP Forests and ICP Integrated Monitoring (IM) programmes under the UNECE Convention on Long-range Transboundary Air Pollution (LRTAP), providing high quality long-term data on ecosystem response. Simulated future soil conditions improved under projected decrease in deposition and current climate conditions: higher pH, BS and C:N at 21, 16 and 12 of the sites, respectively. When climate change was included in the scenario analysis, the variability of the results increased. Climate warming resulted in higher simulated pH in most cases, and higher BS and C:N in roughly half of the cases. Especially the increase in C:N was more marked with climate warming. The study illustrates the value of LTER sites for applying models to predict soil responses to multiple environmental changes.

Episodic acidification of 5 rivers in Canada's oil sands during snowmelt: A 25-year record

Episodic acidification during snowmelt is a natural phenomenon that can be intensified by acidic deposition from heavy industry. In Canada's oil sands region, acid deposition is estimated to be as much as 5% of the Canadian total and large tracks of northeastern Alberta are considered acid-sensitive because of extensive peatland habitats with poorly weathered soils. To identify the frequency, duration and severity of acidification episodes during snowmelt (the predominant hydrological period for delivery of priority pollutants from atmospheric oil sands emissions to surface waters), a 25-year record (1989 to 2014) of automated water quality data (pH, temperature, conductivity) was assembled for 3 rivers along with a shorter record (2012-2014) for another 2 rivers. Acidic episodes (pH<7, ANC<0) were recorded during 39% of all 83 snowmelt events. The severity (duration x magnitude) of episodic acidification increased exponentially over the study period (r²=0.56, P<0.01) and was strongly correlated (P<0.01) with increasing maximum air temperature and weakly correlated with regional land development (P=0.06). Concentrations of aluminum and 11 priority pollutants (Sb, As, Be, Cd, Cr, Cu, Pb, Se, Ag, Tl and Zn) were greatest (P<0.01) during low (<6.5) pH episodes, particularly when coincident with high discharge, such that aluminum and copper concentrations were at times high enough to pose a risk to juvenile rainbow trout (Oncorhynchus mykiss). Although low pH (pH<6.5) was observed during only 8% of 32 acidification episodes, when present, low pH typically lasted 10days. Episodic surface water acidification during snowmelt, and its potential effects on aquatic biota, is therefore an important consideration in the design of long-term monitoring of these typically alkaline (pH=7.72±0.05) rivers.

Effects of multiple stresses hydropower, acid deposition and climate change on water chemistry and salmon populations in the River Otra, Norway

Many surface waters in Europe suffer from the adverse effects of multiple stresses. The Otra River, southernmost Norway, is impacted by acid deposition, hydropower development and increasingly by climate change. The river holds a unique population of land-locked salmon and anadromous salmon in the lower reaches. Both populations have been severely affected by acidification. The decrease in acid deposition since the 1980s has led to partial recovery of both populations. Climate change with higher temperatures and altered precipitation can potentially further impact fish populations. We used a linked set of process-oriented models to simulate future climate, discharge, and water chemistry at five sub-catchments in the Otra river basin. Projections to year 2100 indicate that future climate change will give a small but measureable improvement in water quality, but that additional reductions in acid deposition are needed to promote full restoration of the fish communities. These results can help guide management decisions to sustain key salmon habitats and carry out effective long-term mitigation strategies such as liming. The Otra River is typical of many rivers in Europe in that it fails to achieve the good ecological status target of the EU Water Framework Directive. The programme of measures needed in the river basin management plan necessarily must consider the multiple stressors of acid deposition, hydropower, and climate change. This is difficult, however, as the synergistic and antagonistic effects are complex and challenging to address with modelling tools currently available.

Air-pollution emission control in China: impacts on soil acidification recovery and constraints due to drought

The Chinese government has established compulsory targets to reduce sulfur dioxide (SO2) and nitrogen oxide (NOx) emissions by 8% and 10%, respectively, during 2010-2015. In this study, the effect of the policy was evaluated by predicting the recovery of acidified forest soil in Chongqing, an area severely impacted by acid rain in southwest China. Since precipitation has decreased significantly in this area in recent years, the impact of drought on soil acidification was also considered. A dynamic acidification model, MAGIC, was used to predict future trends in soil chemistry under different scenarios for deposition reduction as well as drought. We found that the current regulation of SO2 emission abatement did not significantly increase soil water pH values, the Ca2+ to Al3+ molar ratio (Ca/Al), or soil base saturation to the level of 2000 before 2050. NOx emission control would have less of an effect on acidification recovery, while emission reduction of particulate matter could offset the benefits of SO2 reduction by greatly decreasing the deposition of base cations, particularly Ca(2+). Continuous droughts in the future might also delay acidification recovery. Therefore, more stringent SO2 emission control should be implemented to facilitate the recovery of seriously acidified areas in China.

Modeling past and future acidification of Swedish lakes

Decades of acid deposition have caused acidification of lakes in Sweden. Here we use data for 3000 lakes to run the acidification model MAGIC and estimate historical and future acidification. The results indicate that beginning in about 1920 a progressively larger number of lakes in Sweden fell into the category of "not naturally acidified" (∆pH > 0.4). The peak in acidification was reached about 1985; since then many lakes have recovered in response to lower levels of acid deposition. Further recovery from acidification will occur by the year 2030 given implementation of agreed legislation for emissions of sulphur (S) and nitrogen (N) in Europe. But the number of catchments with soils being depleted in base cations will increase slightly. MAGIC-reconstructed history of acidification of lakes in Sweden agrees well with information on fish populations. Future acidification of Swedish lakes can be influenced by climate change as well as changes in forest harvest practices.

Forest land cover continues to exacerbate freshwater acidification despite decline in sulphate emissions

Evidence from a multi-date regional-scale analysis of both high-flow and annual-average water quality data from Galloway, south-west Scotland, demonstrates that forest land cover continues to exacerbate freshwater acidification. This is in spite of significant reductions in airborne pollutants. The relationship between freshwater sulphate and forest cover has decreased from 1996 to 2006 indicating a decrease in pollutant scavenging. The relationship between forest cover and freshwater acidity (pH) is, however, still present over the same period, and does not show conclusive signs of having declined. Furthermore, evidence for forest cover contributing to a chlorine bias in marine ion capture suggests that forest scavenging of sea-salts may mean that the forest acidification effect may continue in the absence of anthropogenic pollutant inputs, particularly in coastal areas.

Spatiotemporal patterns of drivers of episodic acidification in Swedish streams and their relationships to hydrometeorological factors

This study examined the spatiotemporal patterns of episodic acidification in 87 weakly buffered streams in Sweden at a monthly sampling frequency during a ten-year study period (1998-2007). Time series of pre-industrial pH (pH(0)) were reconstructed from the acidification model Meta(MAGIC), and the acidification impact was defined by the difference between the pH(0) and the contemporary pH (i.e., DeltapH=pH(0)-pH(t)). Acidification episodes were defined as observations for which the pH(t) was at least 0.4 units lower than average, in combination with a pH at least 0.2 units higher than average. Thus, only occasions in which the stream water was both more acidic and more acidified than average were characterized as acidification episodes. For each observed episode, the primary cause was identified from one of the following five possible drivers: dilution, increase in sulfate, nitrate or organic acids, or sea salt deposition. In total, 258 episodes were observed during the study period. The study showed that streams that were acidified during baseflow (DeltapH>0.4), but not chronically acidic (pH>5.2), were subjected to regular episodic acidification. Dilution was the single most important cause and the main driver for 58% of the identified episodes. Increases in sulfate concentrations were also relatively common (26% of episodes), whereas increases in nitrate and organic acids as well as sea salt deposition were of minor importance. The total number of dilution-related acidification episodes within a year had a significant (p=0.005) positive correlation (r=+0.83) with the average annual precipitation. Occurrences of sulfate episodes were related to droughts during the preceding summers. While the number of streams that are susceptible to episodic acidification will decrease as a consequence of recovery from acidification, the hydrological and meteorological consequences of future climate change may make episodic acidification more common.

Modeling future acidification and fish populations in Norwegian surface waters

Despite great progress made in the past 25 years, acid deposition continues to cause widespread damage to the environment in Europe and eastern North America. Legislation to limit emissions of sulfur and nitrogen compounds in Europe is now under revision. The most recent protocol was based in part on the critical loads concept. The new protocol may also take into consideration the time delays between dose and response inherent in natural ecosystems. Policy decisions to reduce adverse effects on ecosystems entail a trade-off: quick response will require deeper cuts in emissions and thus higher costs, whereas lower costs with lesser cuts in emissions will give slower response. Acidification of lakes and damage to fish populations in Norway is used as an example. Under current legislation for emission reductions, surface waters will continue to slowly recover, but for many decades lakes in about 18% of Norway will continue to have water quality insufficient to support healthy populations of brown trout and other indicator organisms. Additional emission reductions can speed up the rate and degree of recovery.

Impact of climate change on three-dimensional dynamic critical load functions

Changes in climate and atmospheric deposition of base cations can alter the ionic composition of soil and surface waters, and therefore affect the structure and function of sensitive ecosystems. However, these drivers are not generally explicitly considered in the calculation of critical loads or dynamic critical loads to evaluate the recovery of ecosystems from elevated acidic deposition. Here we explore the importance of accounting for these changes in calculating dynamic critical loads for ecosystems. We developed three-dimensional dynamic critical load surfaces as a function of nitrate, sulfur, and base cation deposition under current and future climate change scenarios for the Hubbard Brook Experimental Forest, New Hampshire. This case study indicates that dynamic critical loads for nitrate and sulfur will be lower under conditions of potential climate change or decreases in base cation deposition. This analysis suggests that greater emission controls may be needed to protect sensitive forest ecosystems from acidic deposition under a future climate change or conditions of lower atmospheric deposition of base cations, particularly for watersheds experiencing elevated leaching losses of nitrate. This study should facilitate more informed policy decisions on emission control strategies and assessments of ecosystem recovery.

Modelling recovery from soil acidification in European forests under climate change

A simple soil acidification model was applied to evaluate the effects of sulphur and nitrogen emission reductions on the recovery of acidified European forest soils. In addition we included the effects of climate change on soil solution chemistry, by modelling temperature effects on soil chemical processes and including temperature and precipitation effects on nitrogen uptake and on leaching. Model results showed a strong effect of the emission reduction scenarios on soil solution chemistry. Using the Current Legislation (CLE) scenario, the forest area in Europe with soil solution Al/Bc >1 mol mol(-1) (a widely used critical limit) decreased from about 4% in 1990 to about 1.7% in 2050. Under Maximum Feasible Reductions (MFR), the exceeded area will be <1% in 2050. In addition, the area where limits for the nitrate concentration in soils are violated is predicted to be smaller under MFR than under CLE. Using the most stringent criterion for nitrate ([NO(3)] <0.3mg l(-1)), the area with nitrate concentrations in excess of the critical limit is about 33% in 2050 under CLE, but only 12% under MFR. Recovery, i.e. attaining non-violation of the criterion, is also much faster under MFR than under CLE. Climate change leads to higher weathering rates and nitrogen uptake in the model, but positive effects on recovery from acidification are limited compared to current climate, and differences between the A1 and B2 climate change scenarios were small. Target loads for 2050 exist for 4% of the area for Al/Bc=1 and for 12% of the area when using a criterion of ANC=0 for the soil solution. In about 30% of the area where meaningful target loads exists, the computed target load is lower than the deposition under MFR, and thus cannot be attained with current emission abatement technologies.

A method for upscaling soil parameters for use in a dynamic modelling assessment of water quality in the Pyrenees

Dynamic modelling of hydrochemistry is a valuable tool to study and predict the recovery of surface waters from acidification, and to assess the effects of confounding factors (such as delayed soil response and changing climate) that cause hysteresis during reversal from acidification. The availability of soil data is often a limitation for the regional application of dynamic models. Here we present a method to upscale site-specific soil properties to a regional scale in order to circumvent that problem. The method proposed for upscaling relied on multiple regression models between soil properties and a suite of environmental variables used as predictors. Soil measurements were made during a field survey in 13 catchments in the Pyrenees (NW Spain). The environmental variables were derived from mapped or remotely sensed topographic, lithological, land-cover, and climatic information. Regression models were then used to model soil parameters, which were supplied as input for the biogeochemical model MAGIC (Model for Acidification of Groundwater In Catchments) in order to reconstruct the history of acidification in Pyrenean lakes and forecast the recovery under a scenario of reduced acid deposition. The resulting simulations were then compared with model runs using field measurements as input parameters. These comparisons showed that regional averages for the key water and soil chemistry variables were suitably reproduced when using the modelled parameters. Simulations of water chemistry at the catchment scale also showed good results, whereas simulated soil parameters reflected uncertainty in the initial modelled estimates.

Streamwater acid-base chemistry and critical loads of atmospheric sulfur deposition in Shenandoah National Park, Virginia

A modeling study was conducted to evaluate the acid-base chemistry of streams within Shenandoah National Park, Virginia and to project future responses to sulfur (S) and nitrogen (N) atmospheric emissions controls. Many of the major stream systems in the park have acid neutralizing capacity (ANC) less than 20 microeq/L, levels at which chronic and/or episodic adverse impacts on native brook trout are possible. Model hindcasts suggested that none of these streams had ANC less than 50 microeq/L in 1900. Model projections, based on atmospheric emissions controls representative of laws already enacted as of 2003, suggested that the ANC of those streams simulated to have experienced the largest historical decreases in ANC will increase in the future. The levels of S deposition that were simulated to cause streamwater ANC to increase or decrease to three specified critical levels (0, 20, and 50 microeq/L) ranged from less than zero (ANC level not attainable) to several hundred kg/ha/year, depending on the selected site and its inherent acid-sensitivity, selected ANC endpoint criterion, and evaluation year for which the critical load was calculated. Several of the modeled streams situated on siliciclastic geology exhibited critical loads <0 kg/ha/year to achieve ANC >50 microeq/L in the year 2040, probably due at least in part to base cation losses from watershed soil. The median modeled siliciclastic stream had a calculated critical load to achieve ANC >50 microeq/L in 2100 that was about 3 kg/ha/year, or 77% lower than deposition in 1990, representing the time of model calibration.

A linked spatial and temporal model of the chemical and biological status of a large, acid-sensitive river network

Freshwater sensitivity to acidification varies according to geology, soils and land-use, and consequently it remains difficult to quantify the current extent of acidification, or its biological impacts, based on limited spot samples. The problem is particularly acute for river systems, where the transition from acid to circum-neutral conditions can occur within short distances. This paper links an established point-based long-term acidification model (MAGIC) with a landscape-based mixing model (PEARLS) to simulate spatial and temporal variations in acidification for a 256 km(2) catchment in North Wales. Empirical relationships are used to predict changes in the probability of occurrence of an indicator invertebrate species, Baetis rhodani, across the catchment as a function of changing chemical status. Results suggest that, at present, 27% of the river network has a mean acid neutralising capacity (ANC) below a biologically-relevant threshold of 20 microeq l(-1). At high flows, this proportion increases to 45%. The model suggests that only around 16% of the stream network had a mean ANC < 20 microeq l(-1) in 1850, but that this increased to 42% at the sulphur deposition peak around 1970. By 2050 recovery is predicted, but with some persistence of acid conditions in the most sensitive, peaty headwaters. Stream chemical suitability for Baetis rhodani is also expected to increase in formerly acidified areas, but for overall abundance to remain below that simulated in 1850. The approach of linking plot-scale process-based models to catchment mixing models provides a potential means of predicting the past and future spatial extent of acidification within large, heterogeneous river networks and regions. Further development of ecological response models to include other chemical predictor variables and the effects of acid episodes would allow more realistic simulation of the temporal and spatial dynamics of ecosystem recovery from acidification.

Climate variability and forecasting surface water recovery from acidification: modelling drought-induced sulphate release from wetlands

Climate-induced drought events have been shown to have a significant influence on sulphate (SO(4)(2-)) export from forested catchments in central Ontario, subsequently delaying recovery of surface waters from acidification. Field and modelling studies have demonstrated that water table drawdown during drought periods promotes oxidation of previously stored (reduced) sulphur (S) compounds in wetlands, with subsequent efflux of SO(4)(2-) upon re-wetting. Although climate-induced changes in processes are generally not integrated into soil-acidification models, MAGIC (Model of Acidification of Groundwater in Catchments) includes a wetland compartment that incorporates redox processes driven by drought events. The potential confounding influence of climate-induced drought events on acidification recovery at Plastic Lake, south-central Ontario (under proposed future S emission reductions) was investigated using MAGIC and two climate scenarios: monthly precipitation and runoff based on long-term means (average-climate scenario), and variable precipitation and runoff based on the past 20 years of observed monthly data (variable-climate scenario). The variable-climate scenario included several periods of summer drought owing to lower than average rainfall and higher then average temperature. Nonetheless, long-term regional trends in precipitation and temperature suggest that the variable-climate scenario may be a conservative estimate of future climate. The average-climate scenario indicated good recovery potential with acid neutralising capacity (ANC) reaching approximately 40 micromol(c)L(-1) by 2020 and 50 micromol(c)L(-1) by 2080. In contrast, the forecasted recovery potential under the variable-climate scenario was very much reduced. By 2080, ANC was forecasted to increase to 2.6 micromol(c)L(-1) from -10.0 micromol(c)L(-1) in 2000. Elevated SO(4)(2-) efflux following drought events (introduced under the variable-climate scenario) has a dramatic impact on simulated future surface water chemistry. The results clearly demonstrate that prediction of future water quality, using models such as MAGIC, should take into account changes or variability in climate as well as acid deposition.