Increasing Dissolved Organic Carbon Redefines the Extent of Surface Water Acidification and Helps Resolve a Classic Controversy (original) (raw)
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Ambio, 2014
Long-term (1987-2012) water quality monitoring in 36 acid-sensitive Swedish lakes shows slow recovery from historic acidification. Overall, strong acid anion concentrations declined, primarily as a result of declines in sulfate. Chloride is now the dominant anion in many acid-sensitive lakes. Base cation concentrations have declined less rapidly than strong acid anion concentrations, leading to an increase in charge balance acid neutralizing capacity. In many lakes, modeled organic acidity is now approximately equal to inorganic acidity. The observed trends in water chemistry suggest lakes may not return to reference conditions. Despite declines in acid deposition, many of these lakes are still acidified. Base cation concentrations continue to decline and alkalinity shows only small increases. A changing climate may further delay recovery by increasing dissolved organic carbon concentrations and sea-salt episodes. More intensive forest harvesting may also hamper recovery by reducing...
Biogeosciences, 2011
Lysevatten, a lake in southwest Sweden, has experienced both acidification and recent changes in the amount of lake-water organic carbon (TOC), both causing concern across Europe and North America. A range of paleolimnological tools -diatom-inferred pH, inferred lake-water TOC from visible-near-infrared spectroscopy (VNIRS), multielement geochemistry and pollen analysis, combined with geochemical modeling were used to reconstruct the lake's chemistry and surroundings back to the most recent deglaciation 12 500 years ago. The results reveal that the recent anthropogenic impacts are similar in magnitude to the longterm variation driven by natural catchment changes and early agricultural land use occurring over centuries and millennia. The combined reconstruction of both lake-water TOC and lithogenic element delivery can explain the major changes in lake-water pH and modeled acid neutralizing capacity during the past 12 500 years. The results raise important questions regarding what precisely comprises "reference" conditions (i.e., free from human impacts) as defined in the European Water Framework Directive.
Recovery from acidification of lakes in Finland, Norway and Sweden 1990–1999
Hydrology and Earth System Sciences, 2001
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.
Environmental Science & Technology, 2007
Concentrations of dissolved organic carbon (DOC) in freshwaters have increased significantly in Europe and North America, but the driving mechanisms are poorly understood. Here, we test if the significant increase in TOC (total organic carbon, 90-95% DOC) in three acidsensitive catchments in Norway of 14 to 36% between 1985 and 2003 is related to climate, hydrology, and/or acid deposition. Catchment TOC export increased between 10 and 53%, which was significant at one site only. The seasonal variation in TOC was primarily climatically controlled, while the deposition of SO 4 and NO 3 snegatively related to TOCsexplained the long-term increase in TOC. We propose increased humic charge and reduced ionic strengths both of which increase organic matter solubilitysas mechanistic explanations for the statistical relation between reduced acid deposition and increased TOC. Between 1985 and 2003, ionic strength decreased significantly at all sites, while the charge density of TOC increased at two of the sites from 1-2 meq g -1 C to about 5 meq g -1 C and remained constant at the third site at 5 meq g -1 C. The solubility of organic matter is discussed in terms of the pHdependent deprotonation of carboxylic groups and the ionic strength-dependent repulsion of organic molecules.
Journal of Geophysical Research, 2010
1] In this study, we examine the impact of increasing concentrations of dissolved organic carbon (DOC) on the recovery from acidification for 66 lakes in Southern Sweden. The lakes are small, weakly buffered, and have all been affected by acidifying deposition. A majority of the lakes (∼75%) showed an increase in DOC concentrations between 1990 and 2008. The method used in this study was to model pH in 2008 from DOC, acid neutralizing capacity, pCO 2 (partial carbon dioxide pressure), and Al speciation, using both the DOC observed in 2008 and the "unelevated" DOC of 1990. Furthermore, we consider the indirect effects of increasing DOC on acidity, i.e., the ancillary effects from DOC on pCO 2 , Al transport and speciation, and release of base cations (BCs). Our results indicate that the DOC increase in the latest decades has retarded the recovery from acidification by 0.13 pH units (median for all lakes) and by more than 1 unit for individual lakes. The effects of elevated pCO 2 and BC concentrations accompanying the DOC increase compensated for each other for the average lake, whereas the effects of higher Al transport on pH were minor. The estimate of the amount of BCs released with the organic anions is however uncertain, and further studies on this topic are needed.
Scientific Reports, 2019
Dissolved organic carbon (DOC) concentrations and water colour are increasing in many inland waters across northern Europe and northeastern North America. This inland-water “browning” has profound physical, chemical and biological repercussions for aquatic ecosystems affecting water quality, biological community structures and aquatic productivity. Potential drivers of this “browning” trend are complex and include reductions in atmospheric acid deposition, changes in land use/cover, increased nitrogen deposition and climate change. However, because of the overlapping impacts of these stressors, their relative contributions to DOC dynamics remain unclear, and without appropriate long-term monitoring data, it has not been possible to determine whether the ongoing “browning” is unprecedented or simply a “re-browning” to pre-industrial DOC levels. Here, we demonstrate the long-term impacts of acid deposition and climate change on lake-water DOC concentrations in low and high acid-deposi...
A Comparison of MAGIC and Paleolimnological Predictions of Preindustrial pH for 55 Swedish Lakes
… science & technology, 2007
Two fundamentally different approaches to define reference conditions for acidification assessments are hydrogeochemical modeling and paleolimnological reconstructions. Both methods have been applied to calculate the preindustrial chemistry for 55 Swedish lakes in two independent studies. This paper investigates whether these methods give similar reconstructions of the preindustrial pH for these lakes. Special focus has been attached to the importance of total organic carbon concentrations and CO 2 partial pressure (pCO 2 ) in the conversion from ANC to pH in the hydrogeochemical modeling. With a uniform pCO 2 value for all the lakes of 0.63 matm, the mean absolute difference between pH from the hydrogeochemical model and the paleolimnological pH was +0.23 units (mean absolute difference 0.36 units). If instead a lake specific preindustrial pCO 2 is assumed, equal to contemporary pCO 2 , the mean difference in the predicted preindustrial pH between the two methods was reduced to +0.03 units (mean absolute difference 0.22 units). Statistical analyses indicated that with a lake specific pCO 2 , the difference between the reconstructions is smaller than 0.13 pH-units at a 95 % level of significance. The results of this study build confidence in the reliability of both methods, providing that lake-specific estimates of pCO 2 are used.