Long-term increase in dissolved organic carbon in streamwaters in Norway is response to reduced acid deposition

The role of dissolved organic carbon in Great Smoky Mountains National Park streams impacted by long-term acid deposition

Following reductions in acid deposition in the Great Smoky Mountains National Park (GRSM) since the 2000s, many streams remain acidic and the role of organic acids (OA-) remains unknown due to limited OA- data. This study investigated dissolved organic carbon (DOC) concentrations as a surrogate for OA- across GRSM and its relationships with watershed characteristics, seasons, flow, and stream chemistry. Baseflow water samples were collected from seven watersheds for 2 years and stormflow samples from three watersheds for 1 year. During baseflow, DOC concentrations ranged from < 0.04 to 2.29 mg L-1 with watershed medians between 0.61 and 1.00 mg L-1. Stormflow DOC concentrations ranged from 1.36 to 5.66 mg L-1. During the summer, median DOC concentrations were about twice that of the other three seasons. Stream DOC concentrations decreased with increasing elevation during baseflow but increased with increasing elevation during stormflow. Considering high elevations historically received greater acid deposition, this gradient between baseflow and stormflow suggests higher elevation streams are more impacted by OA-. Based on an OA-/DOC acidity model, it was estimated that during baseflow OA- was a minor contributor to stream acidity, in the order of 5.3 μeq L-1, however stormflow OA- was estimated at 52.5 μeq L-1, contributing to nearly half of stream acidity. Baseflow DOC was significantly correlated with pH and Ca2+, suggesting stream acidification/recovery is governed by base cations and Ca2+ availability. Furthermore, this study provides essential data for future research to evaluate stream DOC trends during acidification recovery and changes in biogeochemical processes.

Modelling the Effects of Forest use Change on Brownification of Finnish Rivers under Atmospheric Pressure

Browning of surface waters due to increased terrestrial loading of dissolved organic matter (DOM) is observed across the Northern Hemisphere. The effects influence several ecosystem services from freshwater productivity to water purification. Brownification is often explained by changes in large-scale anthropogenic pressures and ecosystem functioning (acidification, climate change, and land cover changes). This study examined the effect of forest use changes on water browning in Finland, considering the effects of global pressures. Our goal was to find the ecosystems and geographic areas that are most sensitive to environmental pressures that increase the loading of DOM. We were also looking for land use strategies that decrease browning. We combined mathematical watershed modelling to scenarios of climate change, atmospheric deposition, and forest use change. Changes included scenarios of forest harvest and protection on forest, that were derived from European Union's regulation. The study area covered 20 watersheds from south to north of Finland. In northern Finland brownification continue. In southern Finland global influence (atmospheric deposition, climate change) seem to weaken, giving more space for local forest use change having an influence on brownification. Forest use change was more influential in river basins dominated by organic soils than in mineral soils. Extending forest protection decreased brownification especially in areas where the influence of atmospheric pressure is decreasing. When forest protection is planned to provide a carbon storage and sequestration potential and to favor biodiversity, it has favorable effect on surface water quality as well.

Long-term dissolved organic carbon changes in Woronora drinking water system in Australia

Assessing historical records of DOC concentrations (DOC) in drinking water sources is important for water utilities to understand long-term planning for infrastructure needs. This study investigates 15-20 years of historical data of the Woronora water supply catchment in Australia inclusive of the water filtration plant (WFP), the lake from where the water was drawn for WFP supply, and the two primary river inputs. The DOC at each site ranged from 0.8 mg L-1 to 13.9 mg L-1, with the highest and lowest concentrations observed in Waratah Rivulet. The DOC in the lake and WFP significantly (p < 0.001) increased at annual change rates of 0.192 and 0.180 mg L-1 yr-1. However, Woronora River showed a ∼50% lower rate of DOC increase at 0.096 mg L-1 yr-1 (p < 0.001), while Waratah Rivulet showed no trend (p > 0.05). UV254 also showed increasing trends at Woronora River, Lake Woronora, and Woronora WFP, indicating an increase in aromatic DOC compounds in all three sites. Waratah Rivulet, however, transported more than 60% of the total DOC load into Lake Woronora due to high flow volumes (more than 65% of total annual system flow). Annual DOC load to the lake is positively correlated with annual rainfall (R2 > 0.92; p < 0.001). The higher percentage (>73%) of the samples had SUVA254 greater than 2 L mg -1 m-1 in all four sites indicating a dominance of hydrophobic DOC. The terrestrial plant-derived DOC has increased in Lake Woronora, predominantly influenced by historical rainfall magnitude. The results underscore the importance of considering the impact of increased DOC at the treatment plant intake for the planning and operation of the Woronora water supply system.

Widespread chemical dilution of streams continues as long-term effects of acidic deposition slowly reverse

Studies of recovery from acidic deposition have focused on reversal of acidification and its associated effects, but as recovery proceeds slowly, chemical dilution of surface waters is emerging as a key factor in the recovery process that has significant chemical and biological implications. This investigation uses long-term chemical records from 130 streams in the Adirondack region of New York, USA, to evaluate the role of ongoing decreases in conductance, an index of dilution, in the recovery of these streams. Stream chemistry data spanning up to 40 years (1980s-2022) showed that acid-neutralizing capacity has increased in 92% of randomly selected streams, but that harmful levels of acidification still occur in 37% of these streams. Conductance and Ca2+ concentrations decreased in 79% of streams, and SO42- concentrations in streams continued to show strong decreases but remained several times higher than concentrations in precipitation. These changes were ongoing through 2022 even though acidic deposition levels were approaching those estimated for pre-industrialization. Further dilution is continuing through ongoing decreases in stream SO42-. Nevertheless, Ca2+ continued to be leached from soils by SO42-, organic acids and NO3-, limiting the replenishment of available soil Ca2+, a prerequisite to stem further dilution of stream water.

Quantification of the effect of environmental changes on the brownification of Lake Kukkia in southern Finland

The browning of surface waters due to the increased terrestrial loading of dissolved organic carbon is observed across the northern hemisphere. Brownification is often explained by changes in large-scale anthropogenic pressures (including acidification, and climate and land-use changes). We quantified the effect of environmental changes on the brownification of an important lake for birds, Kukkia in southern Finland. We studied the past trends of organic carbon loading from catchments based on observations taken since the 1990s. We created hindcasting scenarios for deposition, climate and land-use change in order to simulate their quantitative effect on brownification by using process-based models. Changes in forest cuttings were shown to be the primary reason for the brownification. According to the simulations, a decrease in deposition has resulted in a slightly lower leaching of total organic carbon (TOC). In addition, runoff and TOC leaching from terrestrial areas to the lake was smaller than it would have been without the observed increasing trend in temperature by 2 °C in 25 years.

Spatial predictors and temporal forecast of total organic carbon levels in boreal lakes

Browning of Fennoscandian boreal lakes is raising concerns for negative ecosystem impacts as well as reduced drinking water quality. Declined sulfur deposition and warmer climate, along with afforestation, other climate impacts and less outfield grazing, have resulted in increased fluxes of Total Organic Carbon (TOC) from catchments to freshwater, and subsequently to coastal waters. This study assesses the major governing factors for increased TOC levels among several catchment characteristics in almost 5000 Fennoscandian lakes and catchments. Normalized Difference Vegetation Index (NDVI), a proxy for plant biomass, and the proportions of peatland in the catchment, along with surface runoff intensity and nitrogen deposition loading, were identified as the main spatial predictors for lake TOC concentrations. A multiple linear model, based on these explanatory variables, was used to simulate future TOC concentration in surface runoff from coastal drainage basins in 2050 and 2100, using the forecasts of climatic variables in two of the Shared Socio-economic Pathways (SSP): 1-2.6 (+2 °C) and 3-7.0 (+4,5 °C). These scenarios yield contrasting effects. SSP 1-2.6 predicts an overall decrease of TOC export to coastal waters, while SSP 3-7.0 in contrast leads to an increase in TOC export.

Effect of solar radiation on natural organic matter composition in surface waters and resulting impacts on drinking water treatment

Solar radiation experiments showed a shift in the composition of natural organic matter (NOM). Due to irradiation, the concentration of high molecular weight (HMW) molecules decreased, and that of the low molecular weight (LMW) fraction increased. Microbiological analyses showed that biodegradation was neglectable. To assess the consequences for water treatment processes, coagulation jar tests were performed by comparing the removal effectivity for NOM fractions from irradiated and unirradiated raw water. The degree of dissolved organic carbon (DOC) removal by coagulation was lower for irradiated waters. As primarily HMW organic compounds are removed by coagulation, the decrease in coagulation performance is attributed to the increase in the LMW concentration due to photochemical reactions induced by solar radiation. Flocs were about 15% larger for irradiated water. Possibilities to adapt water treatment to respond to changes in DOC composition and concentration are outlined. Ozonation-biofiltration is judged as the most promising treatment process to cope with climate change-related challenges in drinking water treatment.

A review of long-term change in surface water natural organic matter concentration in the northern hemisphere and the implications for drinking water treatment

Reduced atmospheric acid deposition has given rise to recovery from acidification - defined as increasing pH, acid neutralization capacity (ANC), or alkalinity in surface waters. Strong evidence of recovery has been reported across North America and Europe, driving chemical responses. The primary chemical responses identified in this review were increasing concentration and changing character of natural organic matter (NOM) towards predominantly hydrophobic nature. The concentration of NOM also influenced trace metal cycling as many browning surface waters also reported increases in Fe and Al. Further, climate change and other factors (e.g., changing land use) act in concert with reductions in atmospheric deposition to contribute to widespread browning and will have a more pronounced effect as deposition stabilizes. The observed water quality trends have presented challenges for drinking water treatment (e.g., increased chemical dosing, poor filter operations, formation of disinfection by-products) and many facilities may be under designed as a result. This comprehensive review has identified key research areas to be addressed, including 1) a need for comprehensive monitoring programs (e.g., larger timescales; consistency in measurements) to assess climate change impacts on recovery responses and NOM dynamics, and 2) a better understanding of drinking water treatment vulnerabilities and the transition towards robust treatment technologies and solutions that can adapt to climate change and other drivers of changing water quality.

Long-term rise in riverine dissolved organic carbon concentration is predicted by electrolyte solubility theory

The riverine dissolved organic carbon (DOC) flux is of similar magnitude to the terrestrial sink for atmospheric CO₂, but the factors controlling it remain poorly determined and are largely absent from Earth system models (ESMs). Here, we show, for a range of European headwater catchments, that electrolyte solubility theory explains how declining precipitation ionic strength (IS) has increased the dissolution of thermally moderated pools of soluble soil organic matter (OM), while hydrological conditions govern the proportion of this OM entering the aquatic system. Solubility will continue to rise exponentially with declining IS until pollutant ion deposition fully flattens out under clean air policies. Future DOC export will increasingly depend on rates of warming and any directional changes to the intensity and seasonality of precipitation and marine ion deposition. Our findings provide a firm foundation for incorporating the processes dominating change in this component of the global carbon cycle in ESMs.

Evaluation of the factors governing dissolved organic carbon concentration in the soil solution of a temperate forest organic soil

The widespread increase of dissolved organic carbon (DOC) in northern hemisphere surface waters have been generally attributed to the recovery from acidic deposition and to climatic variations. The long-term responses of DOC to environmental drivers could be better predicted with a better understanding of the mechanisms taking place at the soil level given organic forest soils are the main site of DOC production in forested watersheds. Here, we assess the long-term variation (25 years) of DOC concentration in the solution leaching from the soil organic layer (DOC_OL) of a temperate forest. Our results show that DOC_OL increased by 32 % (p < 0.001) during the period of study while the lake outlet DOC concentration did not show any changes. Weekly and annual models based on a simple set of explicative variables including throughfall DOC, throughfall precipitation, temperature, litterfall amounts and organic layer leachate calcium concentration (Ca_OL, taken as a proxy for soil solution ionic strength) explain between 17 and 58 % of the variance in DOC_OL depending on model structures and temporal scales. Throughfall DOC and Ca_OL were both positively related to DOC_OL in the models describing its variations at the weekly and annual scale. Temperature was positively correlated to DOC_OL, probably due to increased microbial activity, while precipitation had a negative effect on DOC_OL (only at the weekly scale), most probably due to a dilution effect. Contrary to our expectations, annual litterfall inputs had no impacts on annual DOC_OL variations. Overall, the results shows that DOC_OL control is a complex process implicating a set of environmental factors that are acting in different ways while no single variable alone can explain a large part of the variation in both, weekly or annual DOC_OL variations.

Drivers and variability of CO2:O2 saturation along a gradient from boreal to Arctic lakes

Lakes are significant players for the global climate since they sequester terrestrially derived dissolved organic carbon (DOC), and emit greenhouse gases like CO₂ to the atmosphere. However, the differences in environmental drivers of CO₂ concentrations are not well constrained along latitudinal and thus climate gradients. Our aim here is to provide a better understanding of net heterotrophy and gas balance at the catchment scale in a set of boreal, sub-Arctic and high-Arctic lakes. We assessed water chemistry and concentrations of dissolved O₂ and CO₂, as well as the CO₂:O₂ ratio in three groups of lakes separated by steps of approximately 10 degrees latitude in South-Eastern Norway (near 60° N), sub-Arctic lakes in the northernmost part of the Norwegian mainland (near 70° N) and high-Arctic lakes on Svalbard (near 80° N). Across all regions, CO₂ saturation levels varied more (6-1374%) than O₂ saturation levels (85-148%) and hence CO₂ saturation governed the CO₂:O₂ ratio. The boreal lakes were generally undersaturated with O₂, while the sub-Arctic and high-Arctic lakes ranged from O₂ saturated to oversaturated. Regardless of location, the majority of the lakes were CO₂ supersaturated. In the boreal lakes the CO₂:O₂ ratio was mainly related to DOC concentration, in contrast to the sub-Arctic and high-Arctic localities, where conductivity was the major statistical determinant. While the southern part is dominated by granitic and metamorphic bedrock, the sub-Arctic sites are scattered across a range of granitic to sedimentary bed rocks, and the majority of the high-Arctic lakes are situated on limestone, resulting in contrasting lake alkalinities between the regions. DOC dependency of the CO₂:O₂ ratio in the boreal region together with low alkalinity suggests that in-lake heterotrophic respiration was a major source of lake CO₂. Contrastingly, the conductivity dependency indicates that CO₂ saturation in the sub-Arctic and high-Arctic lakes was to a large part explained by DIC input from catchment respiration and carbonate weathering.

Three Decades of Changing Nutrient Stoichiometry from Source to Sea on the Swedish West Coast

European ecosystems have been subject to extensive shifts in anthropogenic disturbance, primarily through atmospheric deposition, climate change, and land management. These changes have altered the macronutrient composition of aquatic systems, with widespread increases in organic carbon (C), and declines in nitrogen (N) and phosphorus (P). Less well known is how these disturbances have affected nutrient stoichiometry, which may be a more useful metric to evaluate the health of aquatic ecosystems than individual nutrient concentrations. The Swedish west coast has historically experienced moderate to high levels of atmospheric deposition of sulfate and N, and eutrophication. In addition, coastal waters have been darkening with damaging effects on marine flora and fauna. Here, we present three decades of macronutrient data from twenty lakes and watercourses along the Swedish west coast, extending from headwaters to river mouths, across a range of land covers, and with catchments ranging 0.037–40,000 km2. We find a high degree of consistency between these diverse sites, with widespread increasing trends in organic C, and declines in inorganic N and total P. These trends in individual macronutrients translate into large stoichiometric changes, with a doubling in C:P, and increases in C:N and N:P by 50% and 30%, showing that freshwaters are moving further away from the Redfield Ratio, and becoming even more C rich, and depleted in N and P. Although recovery from atmospheric deposition is linked to some of these changes, land cover also appears to have an effect; lakes buffer against C increases, and decreases in inorganic N have been greatest under arable land cover. Our analysis also detects coherently declining P concentrations in small forest lakes; so called (and unexplained) “oligotrophication.” Taken together, our findings show that freshwater macronutrient concentrations and stoichiometry have undergone substantial shifts during the last three decades, and these shifts can potentially explain some of the detrimental changes that adjacent coastal ecosystems are undergoing. Our findings are relevant for all European and North American waters that have experienced historically high levels of atmospheric deposition, and provide a starting point for understanding and mitigating against the trajectories of long-term change in aquatic systems.

Climate change and mercury in the Arctic: Abiotic interactions

Dramatic environmental shifts are occuring throughout the Arctic from climate change, with consequences for the cycling of mercury (Hg). This review summarizes the latest science on how climate change is influencing Hg transport and biogeochemical cycling in Arctic terrestrial, freshwater and marine ecosystems. As environmental changes in the Arctic continue to accelerate, a clearer picture is emerging of the profound shifts in the climate and cryosphere, and their connections to Hg cycling. Modeling results suggest climate influences seasonal and interannual variability of atmospheric Hg deposition. The clearest evidence of current climate change effects is for Hg transport from terrestrial catchments, where widespread permafrost thaw, glacier melt and coastal erosion are increasing the export of Hg to downstream environments. Recent estimates suggest Arctic permafrost is a large global reservoir of Hg, which is vulnerable to degradation with climate warming, although the fate of permafrost soil Hg is unclear. The increasing development of thermokarst features, the formation and expansion of thaw lakes, and increased soil erosion in terrestrial landscapes are increasing river transport of particulate-bound Hg and altering conditions for aquatic Hg transformations. Greater organic matter transport may also be influencing the downstream transport and fate of Hg. More severe and frequent wildfires within the Arctic and across boreal regions may be contributing to the atmospheric pool of Hg. Climate change influences on Hg biogeochemical cycling remain poorly understood. Seasonal evasion and retention of inorganic Hg may be altered by reduced sea-ice cover and higher chloride content in snow. Experimental evidence indicates warmer temperatures enhance methylmercury production in ocean and lake sediments as well as in tundra soils. Improved geographic coverage of measurements and modeling approaches are needed to better evaluate net effects of climate change and long-term implications for Hg contamination in the Arctic.

Concentration of organic carbon in Finnish catchments and variables involved in its variations

The majority of the carbon worldwide is in soil. In a river catchment, the tight relationship between soil, water and climate makes carbon likely to be eroded and transported from the soil to the rivers. There are multiple variables which can trigger and accelerate the process. In order to assess the importance of the factors involved, and their interactions resulting in the changes in the carbon cycle within catchments, we have studied the catchments of 26 Finnish rivers from 2000 to 2019. These catchments are distributed all over Finland, but we have grouped them into three categories: southern, peatland and northern. We have run a boosted regression tree (BRT) analysis on chemical, physical, climatic and anthropogenic factors to determine their influence on the variations of total organic carbon (TOC) concentration. TOC concentration has decreased in Finland between 2000 and 2019 by 0.91 mg/l, driven principally by forest ditching and % old forest in the catchment. Old forest is especially dominant in the northern catchments with an influence on TOC of 40.5%. In southern and peatland catchments, average precipitation is an important factor to explain the changes in TOC whilst in northern catchments, organic fields have more influence.

Shifting stoichiometry: Long-term trends in stream-dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long-term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy-nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by-products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.

Cleaner air reveals growing influence of climate on dissolved organic carbon trends in northern headwaters

Surface water browning, the result of increasing concentrations of dissolved organic matter (DOM), has been widespread in northern ecosystems in recent decades. Here, we assess a database of 426 undisturbed headwater lakes and streams in Europe and North America for evidence of trends in DOM between 1990 and 2016. We describe contrasting changes in DOM trends in Europe (decelerating) and North America (accelerating), which are consistent with organic matter solubility responses to declines in sulfate deposition. While earlier trends (1990-2004) were almost entirely related to changes in atmospheric chemistry, climatic and chemical drivers were equally important in explaining recent DOM trends (2002-2016). We estimate that riverine DOM export from northern ecosystems increased by 27% during the study period. Increased summer precipitation strengthened upward dissolved organic carbon trends while warming apparently damped browning. Our results suggest strong but changing influences of air quality and climate on the terrestrial carbon cycle, and on the magnitude of carbon export from land to water.

Brownification on hold: What traditional analyses miss in extended surface water records

Widespread increases in organic matter (OM) content of surface waters, as measured by color and organic carbon (OC), are a major issue for aquatic ecosystems. Long-term monitoring programs revealed the issue of "brownification", with climate change, land cover changes and recovery from acidification all suspected to be major drivers or contributing factors. While many studies have focused on the impact and drivers, fewer have followed up on whether brownification is continuing. As time-series of OM data lengthen, conventional data-analysis approaches miss important information on when changes occur. To better identify temporal OM patterns during three decades (1990-2020) of systematic monitoring, we used generalized additive models to analyze 164 time-series from watercourses located across Sweden. Increases in OC that were widespread during 1990-2010 ceased a decade ago, and most color increases ceased 20 years ago. These findings highlight the need to reassess the understanding of brownification's spatial and temporal extent, as well as the tools used to analyze lengthening time series.

Spatial properties affecting the sensitivity of soil water dissolved organic carbon long-term median concentrations and trends

It is increasingly clear that increases in dissolved organic carbon in upland waters in recent decades have often been dominated by acid deposition, but reasons for substantial variation in rates of change remain unclear. This paper focuses on the extent to which spatial properties, such as variation in soil properties, atmospheric deposition and climate, affect the sensitivity of DOC concentrations in soil water. The purpose is to i) examine evidence for differences in site average concentrations and trends in soil water DOC between sites with contrasting ecosystem properties, i.e. vegetation cover and soil type, and ii) identify the wider combination of site characteristics that best explain variation in these DOC metrics between sites. We collated soil water and deposition chemistry, soil chemistry and meteorological data from 15 long-term UK monitoring sites (1992-2010) covering a range of soils, vegetation, climate and acid deposition levels. Mineral soils under forests showed the greatest range of long-term mean DOC concentrations and trends. Regression analysis indicated that acid and sea-salt deposition, and soil sensitivity to acidification were the factors most strongly associated with spatial variation in mean DOC concentrations. Spatial variation in DOC trends were best explained by Al saturation and water flux. Overall, the sensitivity of DOC release from soil to changes in pollutant deposition could be related to the type of vegetation cover and soils chemistry properties, such as Al saturation, divalent base cation content and hydrological regime. The identification of the ecosystem properties that appear most influential in modifying DOC production and responses to long-term drivers, helps elucidate potential mechanistic explanations for differences in DOC dynamics across seemingly similar ecosystems, and points to the importance of DOC mobility in regulating its dynamics.

Chemical recovery and browning of Nova Scotia surface waters in response to declining acid deposition

Declining emissions of sulfur and nitrogen have curtailed acid deposition across large areas of North America and Europe. This has allowed many lakes to recover from acidification, with decreases in sulfate, increases in pH, and increases in alkalinity. But reduced acid deposition has not always coincided with chemical lake recovery. Surface waters in Nova Scotia did not exhibit clear evidence of recovery as recently as 2007, due in part to increasing organic acidity and slow replenishment of base cations. In an updated assessment with data collected as recently as 2019, we analyze water chemistry representing 81 lakes and rivers and two precipitation monitoring stations over up to 41 years. We find that Nova Scotia surface waters are now exhibiting signs of chemical recovery. We estimated the linear decrease in precipitation sulfate and nitrate yield at up to 0.31 and 0.18 kg ha^-1 year^-2, respectively, and the linear increase in precipitation pH at up to 0.014 year^-1. Sulfate decreased in 60 of 62 lakes and 14 of 17 rivers (-0.0051 to -0.23 mg L^-1 year^-1), while pH increased in 55 of 64 lakes and 11 of 17 rivers (0.0015-0.072 year^-1). Apparent colour increased in 54 of 62 lakes and 13 of 17 rivers (0.0026-3.9 Pt-Co year^-1). We identified increasing aluminum trends in 46 of 61 lakes, and we show using size-exclusion chromatography that binding to organic and iron-based colloids may help to explain these trends. To the extent that increases in apparent colour are explained by chromophoric dissolved organic matter (DOM), they imply greater binding capacity for metals in surface waters, and greater capacity for DOM to stabilize metal (oxyhydr)oxide colloids.

Catchment effects of a future Nordic bioeconomy: From land use to water resources

In the future, the world is expected to rely increasingly on renewable biomass resources for food, fodder, fibre and fuel. The sustainability of this transition to bioeconomy for our water systems depends to a large extent on how we manage our land resources. Changes in land use together with climate change will affect water quantity and quality, which again will have implications for the ecosystem services provided by water resources. These are the main topics of this Ambio special issue on "Environmental effects of a green bio-economy". This paper offers a summary of the eleven papers included in this issue and, at the same time, outlines an approach to quantify and mitigate the impacts of bioeconomy on water resources and their ecosystem services, with indications of useful tools and knowledge needs.

Streamwater responses to reduced nitrogen deposition at four small upland catchments in Norway

Reduced emissions of nitrogen (N) in Europe have resulted in decreasing atmospheric deposition since 1990. Long-term data (1988-2017) from four small Norwegian catchments located along gradients in N deposition, rainfall, and organic carbon (C) show different responses to 25-30% reductions in N deposition during the same period. At three sites the decreased N deposition caused reduced leaching of nitrate to surface water, whereas the westernmost site showed no decrease, probably due to thin soils with low C:N ratio, poor vegetation cover and high precipitation. The loss of total N to streamwater constituted 30-50% of the N deposition. Losses via denitrification are unknown but assumed to be low, as a major fraction of the catchments are well-drained. Hence, the study sites seem to continue to accumulate N, presumably mostly in soil organic matter. Although atmospheric N deposition has declined, ambient loads might still exceed long-term sustainable levels in these vulnerable ecosystems.

Five decades of declining methylmercury concentrations in boreal foodwebs suggest pivotal role for sulphate deposition

Widespread declines in mercury (Hg) in fish in pristine lakes in Fennoscandia since the 1970s are unexplained. Interactions between climate, atmospheric deposition, and elemental cycling of carbon (C), sulphur (S) and Hg are complex and affect Hg bioaccumulation. A parallel significant decline in methyl-Hg (MeHg) concentrations in aquatic macroinvertebrates (Chironomidae) was found between 1976-78 and 2004-15 in an intensely studied, pristine boreal lake (Langtjern, boreal Fennoscandia). Monitoring at Langtjern demonstrated a four-fold decrease in aqueous sulphate concentrations (SO₄, 50-year record), significant lake browning (30-year records), increasing sediment Hg concentrations (50-year record), warming (45-year record) and increased runoff (40-year record). Contrasting Hg trends in biota (downward) and sediment (upward) indicated a disconnect between lake Hg loading and foodweb Hg bioaccumulation. We suggest that reduced SO₄-deposition has 1) constrained substrate availability for SO₄-reducing methylating bacteria (causing reduced foodweb MeHg exposure despite increased Hg loading to the lake), and 2), increased the binding affinity between aqueous organic matter and Hg species (leading to reduced MeHg bioavailability). The downward MeHg trend at the base of the foodweb at Langtjern is mirrored at higher trophic levels by strong declines in perch (Perca fluviatilis) and pike (Esox lucius) Hg concentrations in boreal Fennoscandia. A plausible explanation is that declining SO₄-deposition, rather than climate change or reduced atmospheric Hg, is currently driving reduced MeHg contamination in northern freshwater foodwebs.

Assessing the potential effect of extreme weather on water quality and disinfection by-product formation using laboratory simulation

Increased frequency and severity of extreme weather events (i.e., floods and droughts) combined with higher temperatures can threaten surface water quality and downstream drinking water production. This study characterized the effects of extreme weather events on dissolved organic matter (DOM) washout from watershed soils and the corresponding contribution to disinfection by-product (DBP) precursors under simulated weather conditions. A laboratory simulation was performed to assess the effects of temperature, drought, rainfall intensity, sea level rise, and acid deposition on the amount of DOM released from soil samples. DBP formation potentials (DBPFPs) were obtained to assess the effect of extreme weather events on DBP formation and drinking water quality. The results demonstrated that the dissolved organic carbon (DOC) and carbonaceous DBP levels increased with increasing temperature in a dry (drought) scenario. Regardless of the watershed from which a soil sample was obtained and the incubation temperature during rewetting or chlorination processes, the DOC and carbonaceous DBP levels also increased with increasing temperature. Brominated DBP formation was increased when bromide was present during the rewetting of soil, indicating the effect of sea level rise. When bromide was present during the chlorination of water for DBPFP tests, only the level of brominated DBPs increased. Acid deposition had various effects under different weather conditions. The results of heavy rainfall simulations suggested that water quality deteriorates at the beginning of an extreme rainfall event. Abundant DOM was washed out of soil, leading to a peak in the DBPFP level. The level of DOM in seepage water was less than that of the surface runoff water during rainfall. The situation was more severe when the rainfall came after a long drought and the drought-rewetting cycle effect occurred.

Effects of acidity on dissolved organic carbon in organic soil extracts, pore water and surface litters

Over the past 30-40 years, dissolved organic carbon (DOC) concentrations have increased in soil solutions and surface waters in many acid-sensitive areas of Europe and North America. This has been linked to recovery from acidification in response to decreasing levels of atmospheric pollution. Evidence from radiocarbon dating suggests that DOC in surface waters is typically derived from recently photosynthesised organic matter such as plant litter and exudates, yet there is little information on the pH-sensitivity of organic matter solubility, or its decomposition, in litter layers and in different organic soils. Therefore the purpose of this study was to determine a) the sensitivity of DOC to acidity in different surface layers and soil types, in order to b) improve understanding of the key sources contributing to the increasing DOC trend. Such information is vital for understanding site specific characteristics contributing to inconsistencies in DOC release between catchments, and for improving predictions of carbon fluxes and budgets. Based on data collected at four established field pH-manipulation experiments in upland areas of the United Kingdom, we examined the sources, composition and acid-sensitivity of DOC export from the litter and organic soils. We found that litter generated nearly three times more DOC than the organic soils, consistent with radiocarbon evidence that recent plant inputs are a major source of DOC. Furthermore, litter derived DOC had lower specific ultraviolet light absorbance (SUVA) than organic soil DOC, suggesting greater biodegradability, and was not acid sensitive. In contrast, organic soil DOC concentrations were more strongly related to experimentally manipulated pH, implying that the mobility of this DOC may be subject to physicochemical rather than biotic controls. Our results suggest that physicochemically mediated controls on organic matter solubility may be a key driver behind the widely observed increases in surface water DOC in areas undergoing recovery from acidification.

Unraveling long-term changes in lake color based on optical properties of lake sediment

A number of boreal surface waters have become browner over the last two decades. Recovery from acid rain is regarded as an important driver for this lake color increase, indicating a general browner lake color in preindustrial times. However, the lack of long-term monitoring data makes it challenging to unravel historical changes in lake color. In this study, we estimated long-term development in lake color (1800 to 2015) based on the optical properties of alkaline extractable dissolved organic matter (DOM) from sediment using UV-Vis and fluorescence spectroscopy. We found that the present lake color (2015) was significantly browner (four times higher in absorption coefficient) than for the period from 1800 to 1915 when lake color was at a lower and more stable level. Fluorescence excitation-emission matrices combined with parallel factor analysis (EEM-PARAFAC) indicate that terrestrially derived DOM was the main source of sediment DOM. However, the importance of in-lake source of DOM has significantly increased with time. The long-term trend in DOM burial was not consistent with the anthropogenic sulfur (S) deposition pattern. However, along with the increased sediment DOM, there has been increased precipitation, temperature and forest growth with time, which affect the production and degradation of DOM. Even though S deposition might have delayed the runoff of terrestrial DOM for a certain period, it comes in addition to other color-regulating factors. Thus, there is no single driver for the observed lake browning, but rather an interplay between different drivers varying in strength over time, such as afforestation, changes in areal use, declined S deposition, and increased temperature and precipitation.

Terrestrial organic matter increases zooplankton methylmercury accumulation in a brown-water boreal lake

Increases in terrestrial organic matter (tOM) transport from catchments to boreal lakes can affect methylmercury (MeHg) accumulation in aquatic biota both directly by increasing concentrations of aqueous MeHg, and indirectly through effects on MeHg bioavailability and on energy pathways in the lower food web. We carried out a detailed seasonal study of water chemistry, zooplankton diet, and MeHg accumulation in zooplankton in two lakes with contrasting tOM concentrations. Between-lake differences explained 51% of the variability in our water chemistry data, with no observed effect of season or sampling depth, contrary to our expectations. Higher tOM was correlated with higher aqueous Hg concentrations, lower areal pelagic primary productivity, and an increased contribution of terrestrial particles to pelagic particulate organic matter. Based on dietary marker analysis (δ¹³C, δ¹⁵N, and fatty acid [FA] composition), zooplankton diet was strongly linked to feeding mechanism, with dietary reliance on phytoplankton highest in the selective-feeding calanoid copepods, and lowest in filter feeding cladocerans. Zooplankton dietary reliance on phytoplankton and their concentrations of high-quality lipids, including polyunsaturated fatty acids, were higher in the clear-water lake than in the brown-water lake, where bacterial and terrestrial food sources were more prevalent. MeHg was highest in zooplankton from the brown-water lake, with highest concentrations in the 200-500 μm zooplankton size fraction for both lakes. Contrary to our expectations, there was no effect of season on zooplankton dietary markers or MeHg. Our results suggest that, overall, higher tOM results in higher MeHg concentrations in water and zooplankton, and reduces zooplankton dietary reliance on phytoplankton. Increased tOM thus leads to a decrease in the nutritional quality of zooplankton (i.e. higher MeHg concentrations, and lower concentrations of essential fatty acids), which may cascade up the food web with negative implications for higher trophic levels.

Effective depth controls the nitrate removal rates in a water supply reservoir with a high nitrate load

The Occoquan Reservoir is part of an indirect potable reuse system where a water reclamation plant (WRP) discharges a nitrified product water to prevent the onset of anaerobic conditions in the bottom sediments during the summer months. The elongated narrow shape of the reservoir combined with water temperature gradients in the inlet results in density currents that enhance the transport of nitrate from the surface to the bottom waters. The morphology of the reservoir also causes a longitudinal change in the ratio of water volume to sediment area, herein defined as the effective depth (Z_ED). Field observations revealed that first-order nitrate removal rate coefficients (k) varied inversely with Z_ED, suggesting that the upper reaches of the reservoir have a higher potential for nitrate removal compared to the areas closer to the dam. A similar relationship between k (d^-1) and Z_ED was confirmed during laboratory experiments. Differences in k values were attributed mainly to the change in the nitrate supply rate as a result of the increase in water volume flowing over a specific sediment area, which limited nitrate transport to the sediments. The low variability found between the mass transfer coefficients for nitrate (Coefficient of Variation = 0.25) suggested a nearly constant biotic nitrogen removal and confirmed that k values were mainly affected by changes in Z_ED. Finally, similarities in k values between field and laboratory samples with similar Z_ED values suggested that different segments of natural systems may be properly downscaled to laboratory-sized configurations for analytical purposes by means of the Z_ED concept.

Natural organic matter fractions and their removal in full-scale drinking water treatment under cold climate conditions in Nordic capitals

Drinking water treatment plants (DWTPs) designed to remove natural organic matter (NOM) are challenged as concentrations of NOM in raw waters are increasing. Here, we assess seasonal differences in NOM quality and quantity, from raw waters to the distribution network, at three large DWTPs in Oslo, Stockholm and Helsinki. Samples, collected during stable stratification in both winter and summer and during the autumnal turnover, were analysed for NOM concentrations and composition. The NOM was characterized by common routine parameters, size and content (TFF, LC-OCD, fluorescence) and biodegradability. The NOM concentration decreased to 2.5 mg/L (55%), 4.0 mg/L (48%) and 5.7 mg/L (76%) at the respective DWTPs in Oslo, Stockholm and Helsinki. The NOM in raw waters were predominantly in the largest size fraction (>50 kDa), in particular from Oslo. High MW fractions >50 kDa and humics remained the largest fractions with minimum 30% and maximum 80% of the total NOM. The BDOC in treated water <0.3 mg/L and the conditions in the distribution network imply low probability for bacteria regrowth. The multi-step treatment consisting of coagulation/flocculation, sedimentation, rapid sand filtration, ozonation and biological activated carbon filtration (BAC) was most effective in removing NOM. Coagulation/flocculation followed by sedimentation and sand filtration were critical, especially for the removal of biopolymers and humics, and somewhat for building blocks. The sand filtration provided up to 25% additional removal of biopolymers and below 7% removal of other fractions. The ozonation and BAC was more effective and removed 11% of biopolymers, and about 35% of building blocks and LMW neutrals.

The response of soil solution chemistry in European forests to decreasing acid deposition

Acid deposition arising from sulphur (S) and nitrogen (N) emissions from fossil fuel combustion and agriculture has contributed to the acidification of terrestrial ecosystems in many regions globally. However, in Europe and North America, S deposition has greatly decreased in recent decades due to emissions controls. In this study, we assessed the response of soil solution chemistry in mineral horizons of European forests to these changes. Trends in pH, acid neutralizing capacity (ANC), major ions, total aluminium (Al_tot ) and dissolved organic carbon were determined for the period 1995-2012. Plots with at least 10 years of observations from the ICP Forests monitoring network were used. Trends were assessed for the upper mineral soil (10-20 cm, 104 plots) and subsoil (40-80 cm, 162 plots). There was a large decrease in the concentration of sulphate (SO42-) in soil solution; over a 10-year period (2000-2010), SO42- decreased by 52% at 10-20 cm and 40% at 40-80 cm. Nitrate was unchanged at 10-20 cm but decreased at 40-80 cm. The decrease in acid anions was accompanied by a large and significant decrease in the concentration of the nutrient base cations: calcium, magnesium and potassium (Bc = Ca²⁺  + Mg²⁺  + K⁺ ) and Al_tot over the entire dataset. The response of soil solution acidity was nonuniform. At 10-20 cm, ANC increased in acid-sensitive soils (base saturation ≤10%) indicating a recovery, but ANC decreased in soils with base saturation >10%. At 40-80 cm, ANC remained unchanged in acid-sensitive soils (base saturation ≤20%, pHCaCl2 ≤ 4.5) and decreased in better-buffered soils (base saturation >20%, pHCaCl2 > 4.5). In addition, the molar ratio of Bc to Al_tot either did not change or decreased. The results suggest a long-time lag between emission abatement and changes in soil solution acidity and underline the importance of long-term monitoring in evaluating ecosystem response to decreases in deposition.

Mercury in fish from Norwegian lakes: The complex influence of aqueous organic carbon

Mercury (Hg) concentrations in water and biota are often positively correlated to organic matter (OM), typically measured as total or dissolved organic carbon (TOC/DOC). However, recent evidence suggests that higher OM concentrations inhibit bioaccumulation of Hg. Here, we test how TOC impacts the Hg accumulation in fish in a synoptic study of Methyl-Hg (MeHg) in water and total Hg (THg) in perch (Perca fluviatilis) in 34 boreal lakes in southern Norway. We found that aqueous MeHg (r² = 0.49, p < 0.0001) and THg (r² = 0.69, p < 0.0001), and fish THg (r² = 0.26, p < 0.01) were all positively related with TOC. However, we found declining MeHg bioaccumulation factors (BAF_MeHg) for fish with increasing TOC concentrations. The significant correlation between fish THg concentrations and aqueous TOC suggests that elevated fish Hg levels in boreal regions are associated with humic lakes. The declining BAF_MeHg with increasing TOC suggest that increased OM promotes increased aqueous Hg concentrations, but lowers relative MeHg bioaccumulation. A mechanistic understanding of the response from OM on BAF_MeHg might be found in the metal-complexation properties of OM, where OM complexation of metals reduces their bioavailability. Hence, suggesting that MeHg bioaccumulation becomes less effective at higher TOC, which is particularly relevant when assessing potential responses of fish Hg to predicted future changes in OM inputs to boreal ecosystems. Increased browning of waters may affect fish Hg in opposite directions: an increase of food web exposure to aqueous Hg, and reduced bioavailability of Hg species. However, the negative relationship between BAF_MeHg and TOC is challenging to interpret, and carries a great deal of uncertainty, since this relationship may be driven by the underlying correlation between TOC and MeHg (i.e. spurious correlations). Our results suggest that the trade-off between Hg exposure and accumulation will have important implications for the effects of lake browning on Hg transport, bioavailability, and trophodynamics.

National scale assessment of total trihalomethanes in Irish drinking water

Ireland reported the highest non-compliance with respect to total trihalomethanes (TTHMs) in drinking water across the 27 European Union Member States for the year 2010. We carried out a GIS-based investigation of the links between geographical parameters and catchment land-uses with TTHMs concentrations in Irish drinking water. A high risk catchment map was created using peat presence, rainfall (>1400 mm) and slope (<5%) and overlain with a map comprising the national dataset of routinely monitored TTHM concentrations. It appeared evident from the map that the presence of peat, rainfall and slope could be used to identify catchments at high risk to TTHM exceedances. Furthermore, statistical analyses highlighted that the presence of peat soil with agricultural land was a significant driver of TTHM exceedances for all treatment types. PARAFAC analysis from three case studies identified a fluorophore indicative of reprocessed humic natural organic matter as the dominant component following treatment at the three sites. Case studies also indicated that (1) chloroform contributed to the majority of the TTHMs in the drinking water supplies and (2) the supply networks contributed to about 30 μg L^-1 of TTHMs.

Runoff changes have a land cover specific effect on the seasonal fluxes of terminal electron acceptors in the boreal catchments

Climate change influences the volume and seasonal distribution of runoff in the northern regions. Here, we study how the seasonal variation in the runoff affects the concentrations and export of terminal electron acceptors (i.e. TEAs: NO₃, Mn, Fe and SO₄) in different boreal land-cover classes. Also, we make a prediction how the anticipated climate change induced increase in runoff will alter the export of TEAs in boreal catchments. Our results show that there is a strong positive relationship between runoff and the concentration of NO₃-N, Mn and Fe in agricultural catchments. In peaty catchments, the relationship is poorer and the concentrations of TEAs tend to decrease with increasing runoff. In forested catchments, the correlation between runoff and TEA concentrations was weak. In most catchments, the concentrations of SO₄ decrease with an increase in runoff regardless of the land cover or season. The wet years export much higher amounts of TEAs than the dry years. In southern agricultural catchments, the wet years increased the TEA export for both spring (January-May) and autumn (September-December) periods, while in the peaty and forested catchments in eastern and northern Finland the export only increased in the autumn. Our predictions for the year 2099 indicate that the export of TEAs will increase especially from agricultural but also from forested catchments. Additionally, the predictions show an increase in the export of Fe and SO₄ for all the catchments for the autumn. Thus, the climate induced change in the runoff regime is likely to alter the exported amount of TEAs and the timing of the export downstream. The changes in the amounts and timing in the export of TEAs have a potential to modify the mineralization pathways in the receiving water bodies, with feedbacks in the cycling of C, nutrients and metals in aquatic ecosystems.

Historical effects of dissolved organic carbon export and land management decisions on the watershed-scale forest carbon budget of a coastal British Columbia Douglas-fir-dominated landscape

BACKGROUND

To address how natural disturbance, forest harvest, and deforestation from reservoir creation affect landscape-level carbon (C) budgets, a retrospective C budget for the 8500 ha Sooke Lake Watershed (SLW) from 1911 to 2012 was developed using historical spatial inventory and disturbance data. To simulate forest C dynamics, data was input into a spatially-explicit version of the Carbon Budget Model-Canadian Forest Sector (CBM-CFS3). Transfers of terrestrial C to inland aquatic environments need to be considered to better capture the watershed scale C balance. Using dissolved organic C (DOC) and stream flow measurements from three SLW catchments, DOC load into the reservoir was derived for a 17-year period. C stocks and stock changes between a baseline and two alternative management scenarios were compared to understand the relative impact of successive reservoir expansions and sustained harvest activity over the 100-year period.

RESULTS

Dissolved organic C flux for the three catchments ranged from 0.017 to 0.057 Mg C ha^-1 year^-1. Constraining CBM-CFS3 to observed DOC loads required parameterization of humified soil C losses of 2.5, 5.5, and 6.5%. Scaled to the watershed and assuming none of the exported terrestrial DOC was respired to CO₂, we hypothesize that over 100 years up to 30,657 Mg C may have been available for sequestration in sediment. By 2012, deforestation due to reservoir creation/expansion resulted in the watershed forest lands sequestering 14 Mg C ha^-1 less than without reservoir expansion. Sustained harvest activity had a substantially greater impact, reducing forest C stores by 93 Mg C ha^-1 by 2012. However approximately half of the C exported as merchantable wood during logging (~176,000 Mg C) may remain in harvested wood products, reducing the cumulative impact of forestry activity from 93 to 71 Mg C ha^-1.

CONCLUSIONS

Dissolved organic C flux from temperate forest ecosystems is a small but persistent C flux which may have long term implications for C storage in inland aquatic systems. This is a first step integrating fluvial transport of C into a forest carbon model by parameterizing DOC flux from soil C pools. While deforestation related to successive reservoir expansions did impact the watershed-scale C budget, over multi-decadal time periods, sustained harvest activity was more influential.

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.

The effects of temperature and resource availability on denitrification and relative N2O production in boreal lake sediments

Anthropogenic environmental stressors (like atmospheric deposition, land use change, and climate warming) are predicted to increase inorganic nitrogen and organic carbon loading to northern boreal lakes, with potential consequences for denitrification in lakes. However, our ability to predict effects of these changes is currently limited as northern boreal lakes have been largely neglected in denitrification studies. The aim of this study was therefore to assess how maximum potential denitrification and N2O production rates, and the relationship between the two (relative N2O production), is controlled by availability of nitrate (NO3(-)), carbon (C), phosphorus (P), and temperature. Experiments were performed using the acetylene inhibition technique on sediments from a small, nutrient poor boreal lake in northern Sweden in 2014. Maximum potential denitrification and N2O production rates at 4°C were reached already at NO3(-) additions of 106-120μg NO3(-)-N/L, and remained unchanged with higher NO3 amendments. Higher incubation temperatures increased maximum potential denitrification and N2O production rates, and Q10 was somewhat higher for N2O production (1.77) than for denitrification (1.69). The relative N2O production ranged between 13% and 64%, and was not related to NO3(-) concentration, but the ratio increased when incubations were amended with C and P (from a median of 16% to 27%). Combined, our results suggests that unproductive northern boreal lakes currently have low potential for denitrification but are susceptible to small changes in NO3 loading especially if these are accompanied by enhanced C and P availability, likely promoting higher N2O production relative to N2.

From greening to browning: Catchment vegetation development and reduced S-deposition promote organic carbon load on decadal time scales in Nordic lakes

Increased concentrations of dissolved organic carbon (DOC), often labelled “browning”, is a current trend in northern, particularly boreal, freshwaters. The browning has been attributed to the recent reduction in sulphate (S) deposition during the last 2 to 3 decades. Over the last century, climate and land use change have also caused an increasing trend in vegetation cover (“greening”), and this terrestrially fixed carbon represents another potential source for export of organic carbon to lakes and rivers. The impact of this greening on the observed browning of lakes and rivers on decadal time scales remains poorly investigated, however. Here, we explore time-series both on water chemistry and catchment vegetation cover (using NDVI as proxy) from 70 Norwegian lakes and catchments over a 30-year period. We show that the increase in terrestrial vegetation as well as temperature and runoff significantly adds to the reduced SO₄-deposition as a driver of freshwater DOC concentration. Over extended periods (centuries), climate mediated changes in vegetation cover may cause major browning of northern surface waters, with severe impact on ecosystem productivity and functioning.

Boreal forest riparian zones regulate stream sulfate and dissolved organic carbon

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 (SO4(2)(-)) and dissolved organic carbon (DOC). A priori, declines in acid deposition and redox-mediated SO4(2)(-) 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, SO4(2-) concentrations are declining over time, whereas DOC is increasing. No temporal trends in SO4(2-) and DOC were observed in upslope mineral soils. SO4(2-) explained the variation of DOC in stream and RZ, but not in upslope mineral soil. Moreover, as SO4(2-) decreased with time, temporal variability of DOC increased. These observations indicate that: (1) SO4(2-) is still an important driver of DOC trends in boreal catchments and (2) RZ processes control stream SO4(2-) 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.

Hydro-Climatological Influences on Long-Term Dissolved Organic Carbon in a Mountain Stream of the Southeastern United States

In the past decade, significant increases in surface water dissolved organic carbon (DOC) have been reported for large aquatic ecosystems of the Northern Hemisphere and have been attributed variously to global warming, altered hydrologic conditions, and atmospheric deposition, among other factors. We analyzed a 25-yr DOC record (1988-2012) available for a forested headwater stream in the United States and documented two distinct regimes of stream DOC trends. From 1988 to 2001, annual mean volume-weighted DOC concentration (DOC, mg L) and annual DOC flux (kg ha yr) declined by 34 and 56%, respectively. During 1997 to 2012, the decline in DOC and DOC flux increased by 141 and 165%, respectively. Declining DOC from 1988 to 2001 corresponded to a decline in growing season runoff, which has the potential to influence mobilization of DOC from uplands to streams. Increasing DOC from 1997 to 2012 corresponded to increased precipitation early in the growing season and to an increase in the number and intensity of short-duration fall storms capable of mobilizing long-accrued DOC from forest litter and soils. In contrast, total annual runoff declined throughout the period. Rising air temperature, atmospheric acid deposition, and nitrogen depositions did not offer any plausible explanation for the observed bidirectional annual trends of stream DOC. Our study highlights the critical role of long-term datasets and analyses for understanding the impacts of climate change on carbon and water cycles and associated functions of aquatic and terrestrial ecosystems.

Increasing trends of dissolved organic nitrogen (DON) in temperate forests under recovery from acidification in Flanders, Belgium

We evaluated trends (2005-2013) and patterns of dissolved organic nitrogen (DON) and its ratio with dissolved organic carbon (DOC), DOC:DON in atmospheric deposition and soil solution of five Level II plots of the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) in Flanders, Northern Belgium. The primary aim was to confirm positive postulated trends in DON levels and DOC:DON under on-going recovery from acidification. The DON concentrations (0.95-1.41 mg L(-1)) and fluxes (5.6-8.3 kg ha(-1)y(-1)) in throughfall were about twice as high compared to precipitation in the open field (0.40-0.48 mg L(-1), 3.0-3.9 kg ha(-1)y(-1)). Annual soil profile leaching losses of DON varied between 1.2 and 3.7 kg ha(-1)y(-1). The highest soil DON concentrations and fluxes were observed beneath the O horizon (1.84-2.36 mg L(-1), 10.1-12.3 kg ha(-1)y(-1)). Soil solution concentrations and fluxes of DON showed significant increasing trends. Temporarily soil solution DOC:DON rose following an exceptionally long spring drought in 2007, suggesting an effect of drying and rewetting on DOM composition. Further research is needed to test the dependence of DON and DOC:DON on factors such as latitude, forest cover, length of the growing season, hydrology and topography. Nonetheless, even with considerable variation in soil type, level of base saturation, and soil texture in the five included ICP Forests Level II plots, all data revealed a proportionally larger positive response of DON flux than DOC to recovery from acidification.

Patterns and predictability in the intra-annual organic carbon variability across the boreal and hemiboreal landscape

Factors affecting total organic carbon (TOC) concentrations in 215 watercourses across Sweden were investigated using parameter parsimonious regression approaches to explain spatial and temporal variabilities of the TOC water quality responses. We systematically quantified the effects of discharge, seasonality, and long-term trend as factors controlling intra-annual (among year) and inter-annual (within year) variabilities of TOC by evaluating the spatial variability in model coefficients and catchment characteristics (e.g. land cover, retention time, soil type). Catchment area (0.18-47,000 km2) and land cover types (forests, agriculture and alpine terrain) are typical for the boreal and hemiboreal zones across Fennoscandia. Watercourses had at least 6 years of monthly water quality observations between 1990 and 2010. Statistically significant models (p<0.05) describing variation of TOC in streamflow were identified in 209 of 215 watercourses with a mean Nash-Sutcliffe efficiency index of 0.44. Increasing long-term trends were observed in 149 (70%) of the watercourses, and intra-annual variation in TOC far exceeded inter-annual variation. The average influences of the discharge and seasonality terms on intra-annual variations in daily TOC concentration were 1.4 and 1.3 mg l(-1) (13 and 12% of the mean annual TOC), respectively. The average increase in TOC was 0.17 mg l(-1)year(-1) (1.6% year(-1)). Multivariate regression with over 90 different catchment characteristics explained 21% of the spatial variation in the linear trend coefficient, less than 20% of the variation in the discharge coefficient and 73% of the spatial variation in mean TOC. Specific discharge, water residence time, the variance of daily precipitation, and lake area, explained 45% of the spatial variation in the amplitude of the TOC seasonality. Because the main drivers of temporal variability in TOC are seasonality and discharge, first-order estimates of the influences of climatic variability and change on TOC concentration should be predictable if the studied catchments continue to respond similarly.

Effects of photodemethylation on the methylmercury budget of boreal Norwegian lakes

Methylmercury (MeHg) concentrations in freshwater fish from southeastern Norway continue to increase, highlighting the need for a comprehensive understanding of MeHg sources, cycling, and degradation in the aquatic environment. The authors assessed the importance of photodemethylation in the MeHg budget of 4 Norwegian lakes. Photodemethylation rates were determined using incubation experiments with MeHg-spiked natural lake water. The authors determined full-spectrum exposure rates at all study sites and waveband-specific rates (photosynthetically active radiation, ultraviolet-A radiation, and ultraviolet-B radiation) at 1 clear-water (Sognsvann) and 1 humic (Langtjern) site. No significant differences in photodemethylation rates between the sites were found, and the authors' observed rates agreed with available literature for lake and wetland waters. The authors paired experimentally derived photodemethylation rates with lake-specific incident irradiation, light attenuation, and MeHg concentrations to estimate MeHg loss through photodemethylation for the study sites. For Langtjern, losses through photodemethylation equalled 27% of total annual inputs, highlighting the importance of photodemethylation in the MeHg budget. Furthermore, the authors assessed how changes in terrestrial dissolved organic carbon (DOC) exported to freshwaters and climate-driven reductions in ice cover duration may affect MeHg losses through photodemethylation. Results suggest that future increases in DOC may lead to higher aqueous MeHg concentrations in boreal lakes due to increased DOC-associated MeHg inputs paired with significant decreases in the loss of MeHg through photodemethylation due to increased light attenuation.

Simple models to estimate historical and recent changes of total organic carbon concentrations in lakes

Quantifying human impacts on the natural environment requires credible reconstructions of reference conditions. Anthropogenic acidification of surface waters is strongly influenced by total organic carbon (TOC) concentrations. Because both the degree of acidification and recovery are dependent on historical TOC concentrations, simple models to estimate changes in surface water TOC between reference conditions (1860) and the present day (2012) are needed. We used visible near infrared spectroscopy (VNIRS) of lake sediments to reconstruct reference condition TOC and long-term monitoring data to predict recent changes. Two empirical models were developed to predict: (i) historical TOC trends between reference conditions (1860) and peak acidification (1980) and (ii) trends in TOC between 1988 and 2012. The models were statistically robust with adj. R(2) of (i) 0.85 and (ii) 0.71, respectively. Models were driven by lake and catchment area, wetlands, historical sulfur deposition and water chemistry. Present day TOC concentrations are similar to VNIRS-reconstructed and modeled reference condition TOC in Swedish lakes. The results are valuable for understanding drivers of TOC changes in lakes and for more credible assessments of reference conditions needed for water management in Europe and elsewhere.

Long-term changes in acidity and DOC in throughfall and soil water in Finnish forests

The main objective of this study was to examine if any detectable trends in dissolved organic carbon (DOC), sulphate (SO4-S) concentrations and acid neutralizing capacity (ANC) in throughfall (TF) and soil water (SW) could be found during 1990-2010 and to relate them to recent changes in decreased acid deposition. The study was conducted in seven boreal coniferous forest sites: four of which are managed and three unmanaged forests sites. Generally, temporal trend showed a significant decrease in SO4-S concentrations in bulk precipitation (BP), TF and SW. At some of the sites, there was an increasing tendency in BP and TF in the DOC concentrations. This feature coincides with decreasing SO4-S concentration, indicating that SO4-S may be an important driver of DOC release from the canopy. However, a slightly increased temperature, larger senescing needle mass and consequently increased decaying activity in the canopy may partly explain the increasing trend in DOC. In SW, no consistent DOC trend was seen. At some sites, the decreased base cation concentrations mostly account for the decrease in the ANC values in SW and TF.

Environmental risk assessment of combined effects in aquatic ecotoxicology: a discussion paper

Environmental regulatory edicts within the EU, such as the regulatory framework for chemicals REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals), the Water Framework Directive (WFD), and the Marine Strategy Framework Directive (MSFD) focus mainly on toxicity assessment of individual chemicals although the effect of contaminant mixtures is a matter of increasing concern. This discussion paper provides an overview of the field of combined effects in aquatic ecotoxicology and addresses some of the major challenges related to assessment of combined effects in connection with environmental risk assessment (ERA) and regulation. Potentials and obstacles related to different experimental, modelling and predictive ERA approaches are described. On-going ERA guideline and manual developments in Europe aiming to incorporate combined effects of contaminants, the use of different experimental approaches for providing combined effect data, the involvement of biomarkers to characterize Mode of Action and toxicity pathways and efforts to identify relevant risk scenarios related to combined effects are discussed.