Modelling impacts of atmospheric deposition and temperature on long-term DOC trends.

It is increasingly recognised that widespread and substantial increases in Dissolved organic carbon (DOC) concentrations in remote surface, and soil, waters in recent decades are linked to declining acid deposition. Effects of rising pH and declining ionic strength on DOC solubility have been proposed as potential dominant mechanisms. However, since DOC in these systems is derived mainly from recently-fixed carbon, and since organic matter decomposition rates are considered sensitive to temperature, uncertainty persists over the extent to which other drivers that could influence DOC production. Such potential drivers include fertilisation by nitrogen (N) and global warming. We therefore ran the dynamic soil chemistry model MADOC for a range of UK soils, for which time series data are available, to consider the likely relative importance of decreased deposition of sulphate and chloride, accumulation of reactive N, and higher temperatures, on soil DOC production in different soils. Modelled patterns of DOC change generally agreed favourably with measurements collated over 10-20years, but differed markedly between sites. While the acidifying effect of sulphur deposition appeared to be the predominant control on the observed soil water DOC trends in all the soils considered other than a blanket peat, the model suggested that over the long term, the effects of nitrogen deposition on N-limited soils may have been sufficient to raise the "acid recovery DOC baseline" significantly. In contrast, reductions in non-marine chloride deposition and effects of long term warming appeared to have been relatively unimportant. The suggestion that future DOC concentrations might exceed preindustrial levels as a consequence of nitrogen pollution has important implications for drinking water catchment management and the setting and pursuit of appropriate restoration targets, but findings still require validation from reliable centennial-scale proxy records, such as those being developed using palaeolimnological techniques.

Atmospheric deposition of phosphorus to land and freshwater.

We compiled published and newly-obtained data on the directly-measured atmospheric deposition of total phosphorus (TP), filtered total phosphorus (FTP), and inorganic phosphorus (PO4-P) to open land, lakes, and marine coasts. The resulting global data base includes data for c. 250 sites, covering the period 1954 to 2012. Most (82%) of the measurement locations are in Europe and North America, with 44 in Africa, Asia, Oceania, and South-Central America. The deposition rates are log-normally distributed, and for the whole data set the geometric mean deposition rates are 0.027, 0.019 and 0.14 g m(-2) a(-1) for TP, FTP and PO4-P respectively. At smaller scales there is little systematic spatial variation, except for high deposition rates at some sites in Germany, likely due to local agricultural sources. In cases for which PO4-P was determined as well as one of the other forms of P, strong parallels between logarithmic values were found. Based on the directly-measured deposition rates to land, and published estimates of P deposition to the oceans, we estimate a total annual transfer of P to and from the atmosphere of 3.7 Tg. However, much of the phosphorus in larger particles (principally primary biological aerosol particles) is probably redeposited near to its origin, so that long-range transport, important for tropical forests, large areas of peatland and the oceans, mainly involves fine dust from deserts and soils, as described by the simulations of Mahowald et al. (Global Biogeochemical Cycles 22, GB4026, 2008). We suggest that local release to the atmosphere and subsequent deposition bring about a pseudo-diffusive redistribution of P in the landscape, with P-poor ecosystems, for example ombrotrophic peatlands and oligotrophic lakes, gaining at the expense of P-rich ones. Simple calculations suggest that atmospheric transport could bring about significant local redistribution of P among terrestrial ecosystems. Although most atmospherically transported P is natural in origin, local transfers from fertilised farmland to P-poor ecosystems may be significant, and this requires further research.

The importance of the relationship between scale and process in understanding long-term DOC dynamics.

Concentrations of dissolved organic carbon have increased in many, but not all, surface waters across acid impacted areas of Europe and North America over the last two decades. Over the last eight years several hypotheses have been put forward to explain these increases, but none are yet accepted universally. Research in this area appears to have reached a stalemate between those favouring declining atmospheric deposition, climate change or land management as the key driver of long-term DOC trends. While it is clear that many of these factors influence DOC dynamics in soil and stream waters, their effect varies over different temporal and spatial scales. We argue that regional differences in acid deposition loading may account for the apparent discrepancies between studies. DOC has shown strong monotonic increases in areas which have experienced strong downward trends in pollutant sulphur and/or seasalt deposition. Elsewhere climatic factors, that strongly influence seasonality, have also dominated inter-annual variability, and here long-term monotonic DOC trends are often difficult to detect. Furthermore, in areas receiving similar acid loadings, different catchment characteristics could have affected the site specific sensitivity to changes in acidity and therefore the magnitude of DOC release in response to changes in sulphur deposition. We suggest that confusion over these temporal and spatial scales of investigation has contributed unnecessarily to the disagreement over the main regional driver(s) of DOC trends, and that the data behind the majority of these studies is more compatible than is often conveyed.

The United Kingdom Acid Waters Monitoring Network: a review of the first 15 years and introduction to the special issue.

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.

Trends in surface water chemistry of acidified UK freshwaters, 1988-2002.

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.

Long-term increases in surface water dissolved organic carbon: observations, possible causes and environmental impacts.

Dissolved organic carbon (DOC) concentrations in 22 UK upland waters have increased by an average of 91% during the last 15 years. Increases have also occurred elsewhere in the UK, northern Europe and North America. A range of potential drivers of these trends are considered, including temperature, rainfall, acid deposition, land-use, nitrogen and CO2 enrichment. From examination of recent environmental changes, spatial patterns in observed trends, and analysis of time series, it is suggested that DOC may be increasing in response to a combination of declining acid deposition and rising temperatures; however it is difficult to isolate mechanisms based on monitoring data alone. Long-term DOC increases may have wide-ranging impacts on freshwater biota, drinking water quality, coastal marine ecosystems and upland carbon balances. Full understanding of the significance of these increases requires further knowledge of the extent of natural long-term variability, and of the natural "reference" state of these systems.

Nitrate leaching as a confounding factor in chemical recovery from acidification in UK upland waters.

Over the period 1988-2002, data from 18 of the 22 lakes and streams in the UK Acid Waters Monitoring Network (AWMN) show clear trends of declining excess sulphate concentrations in response to reductions in sulphur deposition, but fewer trends in increasing pH or alkalinity. There has been no significant decline in the deposition of total nitrogen over the same period, and no sites show a trend in nitrate concentration. Peak nitrate concentrations have already surpassed excess sulphate on occasion in half of the AWMN sites. Furthermore, current understanding of terrestrial N saturation processes suggests that nitrate leaching from soils may increase, even under a constant N deposition load. Best-case projections indicate that nitrate will overtake sulphate as the major excess acid anion in many sites within 10 years, while worst-case predictions with steady-state models suggest that in the longer-term, nitrate could become the dominant excess acid anion in most of the UK.

Biological responses to the chemical recovery of acidified fresh waters in the UK.

We report biological changes at several UK Acid Waters Monitoring Network lakes and streams that are spatially consistent with the recovery of water chemistry induced by reductions in acid deposition. These include trends toward more acid-sensitive epilithic diatom and macroinvertebrate assemblages, an increasing proportional abundance of macroinvertebrate predators, an increasing occurrence of acid-sensitive aquatic macrophyte species, and the recent appearance of juvenile (<1 year old) brown trout in some of the more acidic flowing waters. Changes are often shown to be directly linked to annual variations in acidity. Although indicative of biological improvement in response to improving water chemistry, "recovery" in most cases is modest and very gradual. While specific ecological recovery endpoints are uncertain, it is likely that physical and biotic interactions are influencing the rate of recovery of certain groups of organisms at particular sites.

Reconstructing pre-acidification pH for an acidified Scottish loch: a comparison of palaeolimnological and modelling approaches.

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.

Temporal trends in spheroidal carbonaceous particle deposition derived from annual sediment traps and lake sediment cores and their relationship with non-marine sulphate.

Spheroidal carbonaceous particles (SCPs) provide an unambiguous indication of atmospherically deposited contamination from industrial sources. SCP data from a 12 year annual sediment trapping and coring programme at 14 lakes based on the UK Acid Waters Monitoring Network, were used to consider temporal trends in deposition and to compare these with measured non-marine sulphate fluxes. Results show good temporal coherence across a broad area of northern UK and that SCP deposition levels and are now at their lowest since the 1940s, in agreement with modelled sulphate data. SCP fluxes show reasonable linearity with measured non-marine sulphate depositional fluxes from the nearest UK Acid Deposition Monitoring Network sites, especially over the post-flue-gas desulphurisation period, but comparisons prior to 1972 are not possible due to lack of data. We speculate on whether palaeolimnological SCP data might be used to reconstruct the history of non-marine sulphate fluxes from industrial sources.

Regional scale evidence for improvements in surface water chemistry 1990-2001.

The main aim of the international UNECE monitoring program ICP Waters under the Convention of Long-range Transboundary Air Pollution (CLRTAP) is to assess, on a regional basis, the degree and geographical extent of the impact of atmospheric pollution, in particular acidification, on surface waters. Regional trends are calculated for 12 geographical regions in Europe and North America, comprising 189 surface waters sites. From 1990-2001 sulphate concentrations decreased in all but one of the investigated regions. Nitrate increased in only one region, and decreased in three North American regions. Improvements in alkalinity and pH are widely observed. Results from the ICP Waters programme clearly show widespread improvement in surface water acid-base chemistry, in response to emissions controls programs and decreasing acidic deposition. Limited site-specific biological data suggest that continued improvement in the chemical status of acid-sensitive lakes and streams will lead to biological recovery in the future.

Long-term variability in the deposition of marine ions at west coast sites in the UK Acid Waters Monitoring Network: impacts on surface water chemistry and significance for trend determination.

Eight lake sites in central and south-west Scotland, north-west England and north Wales, forming part of the UK Acid Waters Monitoring Network (UKAWMN), have been studied with regard to the influence of marine ions on surface water chemistry. Since monitoring began in 1988 these sites have exhibited large and long-term variation in Cl concentration, which are consistent between regions and can be linked to inter-annual variations in wet deposition. Through regression analysis against Cl, the response of other solutes to these fluctuations has been assessed. Sites show a highly consistent pattern of Na, and Mg retention during periods of high Cl, in accordance with the 'sea-salt' mechanism of marine cation adsorption onto soil exchange sites following large marine inputs. An associated displacement of cations with non-marine sources is also observed, with one or more of non-marine Ca, labile Al and hydrogen ions exhibiting a positive relationship with Cl at all sites. The relative extent to which these are released appears not to follow a simple relationship to site acidity, and may be linked to site/region-specific geology and soil characteristics. In addition, an inverse relationship between non-marine SO4 and Cl is observed at five of the sites, and the possibility is considered that a sea-salt related process, with soil retention and subsequent release, may also operate for SO4. A mechanism that might explain this process is suggested. The impact of marine inputs on non-marine solutes, including important indicators of acidification such as pH, labile Al and non-marine SO4, has clear implications for the detection of long-term trends in acidity status and is, therefore, of particular relevance to the UKAWMN. Due to their unpredictability, and the long timescale over which they operate, fluctuations caused by marine inputs may be difficult to separate from acid deposition related long-term trends. Evidence from a longer Cl time series from mid-Wales shows that fluctuations in concentration could be linked to the North Atlantic Oscillation and might therefore be expected to exhibit a similar, decal periodicity. Currently, the UKAWMN dataset only appears long enough to represent one climatic cycle. Consequently, and since few surface water chemistry datasets in the UK extend over more than a decade, it is important that: (a) trend analyses of current data from marine-impacted areas take account of possible marine input cycles; and (b) long-term monitoring is maintained into the future so that the impact of these cycles can be better quantified, and distinguished from anthopogenically-induced long-term changes.