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Abstract: We conducted complementary linear and non-linear statistical modelling of the first 20 years of hydrochemical data from the 22 lakes and streams of the UK Acid Waters Monitoring Network (AWMN) in order to assess temporal patterns and rates of change in indicators of the key drivers of surface water acidification and acidity status. Over the period 1988–2008, concentrations of non-marine sulphate (i.e. of anthropogenic origin) fell in line with reductions in non-marine sulphur deposition, the principal driver of acidification. Most of the decline was confined to the latter half of the 1990s. Whilst these reductions were substantial, concentrations in recent samples from the most contaminated sites remained several times higher than those in the most remote, low-deposition regions. Nitrate concentrations also declined slightly at several sites in northern England and Wales, possibly reflecting a recent regional reduction in N deposition. Further north, concentrations increased slightly in some Scottish sites, reflecting a continued dominance of climatic variation on nitrogen cycling in this region. A combination of unusually high rainfall and seasalt inputs in the early years, gradual long-term reductions in hydrochloric acid deposition, and later and more substantial reductions in sulphur deposition, appeared to account for relatively linear increases in an alkalinity-based estimate of Acid Neutralising Capacity ([AB-ANC]) with time. However, variation between sites in the size of the long-term trend in [AB-ANC] was most clearly linked to the size of the long-term reduction in sulphate concentration. The chemical characteristics of changes in [AB-ANC] across the network were dependent on rates of change in acid deposition, the extent to which sites have previously acidified, and average dissolved organic carbon concentration ([DOC]). In the most acidified waters, the response in acidity to reductions in acid deposition was dominated initially by large reductions in inorganic aluminium concentrations ([Alinorg]. Responses in pH have become clearer at these sites as [Alinorg] has begun to stabilise. Over the wider network, pH has been rising more rapidly in sites with lower concentrations of organic acids. A substantial proportion of the deposition-driven increase in [AB-ANC] at several sites is accounted for by increases in [DOC]. For the non-acidified but acid-sensitive waters in the far north and west, changes in [DOC] represent the only clear response to the small changes in sulphur deposition. In the more acidified sites, a reduction in the size of occasional extreme concentrations of Alinorg, that occur after periods of high rainfall or seasalt deposition events, has been more rapid than the declines in average levels, implying that at most sites the persistence of these events should not be seen as an additional barrier to ecological recovery. In a comparison of geographically paired sites with forested and moorland catchments, consistently higher levels of inorganic aluminium concentration and lower AB-ANC provided clear evidence that the former group had acidified to a greater extent. There were few significant differences in the rate at which non-marine sulphate concentration declined or AB-ANC increased and the forested sites therefore remain in a more acidified condition. [Copyright &y& Elsevier]

Increases in dissolved organic carbon (DOC) fluxes may relate to changes in sulphur and nitrogen pollution. We integrated existing models of vegetation growth and soil organic matter turnover, acid–base dynamics, and organic matter mobility, to form the ‘MADOC’ model. After calibrating parameters governing interactions between pH and DOC dissolution using control treatments on two field experiments, MADOC reproduced responses of pH and DOC to additions of acidifying and alkalising solutions. Long-term trends in a range of acid waters were also reproduced. The model suggests that the sustained nature of observed DOC increases can best be explained by a continuously replenishing potentially-dissolved carbon pool, rather than dissolution of a large accumulated store. The simulations informed the development of hypotheses that: DOC increase is related to plant productivity increase as well as to pH change; DOC increases due to nitrogen pollution will become evident, and be sustained, after soil pH has stabilised. [Copyright &y& Elsevier]

We present a new formulation of the acidification model MAGIC that uses decomposer dynamics to link nitrogen (N) cycling to carbon (C) turnover in soils. The new model is evaluated by application to 15–30 years of water chemistry data at three coniferous-forested sites in the Czech Republic where deposition of sulphur (S) and N have decreased by >80% and 40%, respectively. Sulphate concentrations in waters have declined commensurately with S deposition, but nitrate concentrations have shown much larger decreases relative to N deposition. This behaviour is inconsistent with most conceptual models of N saturation, and with earlier versions of MAGIC which assume N retention to be a first-order function of N deposition and/or controlled by the soil C/N ratio. In comparison with earlier versions, the new formulation more correctly simulates observed short-term changes in nitrate leaching, as well as long-term retention of N in soils. The model suggests that, despite recent deposition reductions and recovery, progressive N saturation will lead to increased future nitrate leaching, ecosystem eutrophication and re-acidification. [Copyright &y& Elsevier]

Abstract: We reconstruct the pre-acidification pH of the Round Loch of Glenhead for 1800 AD using three diatom-pH transfer functions and a diatom-cladocera modern analogue technique (MAT), and compare these palaeo-data with hindcast data for the loch using the dynamic catchment acidification model MAGIC. We assess the accuracy of the transfer functions by comparing pH inferences from contemporary sediment and sediment trap diatom samples from the lake with measured pH from the UK Acid Waters Monitoring Network. The results from the transfer functions estimate the pH in 1800 to have been between 5.5. and 5.7, the MAT approach estimates pH at 5.8 and the MAGIC hindcast (for 1850) is pH 6.1. Whilst we have no independent method of assessing which of these values is most accurate, the disagreement between the two approaches indicates that further work is needed to resolve the discrepancies. [Copyright &y& Elsevier]

Abstract: Analysis of water chemistry data from 15 years of monitoring at 22 acid-sensitive lakes and streams in the UK reveals coherent national chemical trends indicative of recovery from acidification. Excess sulphate and base cations exhibit significant decline, often accompanied by an increase in an alkalinity-based determination of acid neutralising capacity (AB-ANC) and, at fewer sites, a decline in hydrogen and labile aluminium. Acid neutralising capacity determined by “charge-balance” (CB-ANC) exhibits few trends, possibly due to compound errors associated with its determination. Trend slopes in excess sulphate correlate with those for base cations, hydrogen ion and AB-ANC, with between-site variability linked to catchment hydrology, sea-salt inputs and forestry. Nitrate concentrations have not changed significantly but show high sensitivity to varying climate. Trends in AB-ANC are influenced by significant increases in dissolved organic carbon, the cause of which it is vital to establish before trends in the former can definitively be attributed to decreasing acidic deposition. [Copyright &y& Elsevier]

Abstract: The United Kingdom Acid Waters Monitoring Network (AWMN) was established in 1988 to determine the ecological impact of acidic emissions control policy on acid-sensitive lakes and streams. AWMN data have been used to explore a range of causal linkages necessary to connect changes in emissions to chemical and, ultimately, biological recovery. Regional scale reductions in sulphur (S) deposition have been found to have had an immediate influence on surface water chemistry, including increases in acid neutralising capacity, pH and alkalinity and declines in aluminium toxicity. These in turn can be linked to changes in the aquatic biota which are consistent with “recovery” responses. A continuation of the current programme is essential in order to better understand apparent non-linearity between nitrogen (N) in deposition and runoff, the substantial rise in organic acid concentrations, and the likely impacts of forecast climate change and other potential constraints on further biological improvement. [Copyright &y& Elsevier]

Abstract: The dynamic hydrogeochemical model MAGIC has been applied extensively throughout Europe and North America and has proven to be a valuable tool for predicting the chemical response of catchment soils and surface waters to future changes in atmospheric pollution and land use. Here we present an application of MAGIC to 22 sites in the UK Acid Waters Monitoring Network (UKAWMN) that incorporated uncertainty in model calibration (using 20 years of surface water observations) to evaluate past acidification and prospects for future recovery in surface waters, soils and key ecological indicators. Simulated ANC in 1860 indicated that prior to industrialisation, all UKAWMN surface waters were above the critical acid neutralising capacity (ANC) limit of 20μeql–1, with the exception of a naturally acid site, underlain by granite with large expanses of bare rock, scree and eroded peat in Northern Ireland (Blue Lough); this site had a baseline ANC value <20μeql–1. The significant increase in acidic deposition from 1860 to its peak during the 1970s resulted in surface water acidification at 14 of the study sites (ANC<20μeql–1). The rate and magnitude of chemical recovery in soil is highly variable among sites owing to differences in factors such as soil mineral weathering and base saturation. Despite the significant reductions in sulphur and to a lesser extent nitrogen deposition since 1970, the simulated soil base saturation at all study sites either continued to decline or remained stable until the late 1980s, with marginal recovery detected at some sites in the past decade. Predictions were based on an emission reduction scenario (Gothenburg protocol) to 2020, and land use scenarios to 2050 based on approved Forestry Commission plans at the five afforested sites. Model predictions indicated that, in general, surface water acid status will continue to improve during the next decade and beyond under the Gothenburg protocol. Surface water recovery was primarily attributed to the significant decline in sulphate concentrations from a present day mean of 58.5μeql–1–43.8μeql–1 in 2020. The contribution of nitrate (NO3 –) leaching to the total acid status of surface water was small and predicted to decrease from 10.9μeql–1 in 2007 to 9.6μeql–1 in 2020. By 2100, NO3 – concentrations increased slightly to 11.8μeql–1 having a small confounding influence on the rate of chemical recovery at most sites in the network. The future response of soil base saturation to reductions in acid deposition and land use change was mixed and the difference in behaviour between sites is unclear. Whilst recent studies (Malcolm et al., 2014a,b; Monteith et al., 2014) have demonstrated that forest practices have contributed to the acidification of surface waters, in this study, there was no evidence from the model predictions that forested sites will follow a different recovery trajectory to moorland sites. Planned reductions in coniferous forest cover amounting to approximately 13% across the five afforested sites resulted in a slight increase in ANC and pH. Ecological predictions highlighted the sensitivity of three indicator species (Baetis Rhodani, Achnanthes minutissima and salmonids) to changes in the acid status of surface water in the past and into the future. There was clear evidence of biological recovery by 2015 with some sites returning close to their pre-industrial biological status (Allt a’Mharcaidh) while at others (including Loch Grannoch and River Etherow) predicted recovery was incomplete for one or more of the studied indicator species. [Copyright &y& Elsevier]

Abstract: Critical loads for acidification and eutrophication and their exceedances were determined for a selection of ecosystem effects monitoring sites in the Integrated Monitoring programme (UNECE ICP IM). The level of protection of these sites with respect to acidifying and eutrophying deposition was estimated for 2000 and 2020. In 2020 more sites were protected from acidification (67%) than in 2000 (61%). However, due to the sensitivity of the sites, even the maximum technically feasible emission reductions scenario would not protect all sites from acidification. In 2000, around 20% of the IM sites were protected from eutrophication. In 2020, under reductions in accordance with current legislation, about one third of the sites would be protected, and at best, with the maximum technically feasible reductions, half of the sites would be protected from eutrophication. Data from intensively monitored sites, such as those in ICP IM, provide a connection between modelled critical thresholds and empirical observations, and thus an indication of the applicability of critical load estimates for natural ecosystems. Across the sites, there was good correlation between the exceedance of critical loads for acidification and key acidification parameters in runoff water, both with annual mean fluxes and concentrations. There was also evidence of a link between exceedances of critical loads of nutrient nitrogen and nitrogen leaching. The collected empirical data of the ICP IM thus allow testing and validation of key concepts used in the critical load calculations. This increases confidence in the European-scale critical loads mapping used in integrated assessment modelling to support emission reduction agreements. [Copyright &y& Elsevier]

Abstract: The MAGIC model was used to evaluate the relative sensitivity of several possible climate-induced effects on the recovery of soil and surface water from acidification. A common protocol was used at 14 intensively studied sites in Europe and eastern North America. The results show that several of the factors are of only minor importance (increase in pCO2 in soil air and runoff, for example), several are important at only a few sites (seasalts at near-coastal sites, for example) and several are important at nearly all sites (increased concentrations of organic acids in soil solution and runoff, for example). In addition changes in forest growth and decomposition of soil organic matter are important at forested sites and sites at risk of nitrogen saturation. The trials suggest that in future modelling of recovery from acidification should take into account possible concurrent climate changes and focus specially on the climate-induced changes in organic acids and nitrogen retention. [Copyright &y& Elsevier]