Declining calcium concentration drives shifts toward smaller and less nutritious zooplankton in northern lakes.

Zooplankton community composition of northern lakes is changing due to the interactive effects of climate change and recovery from acidification, yet limited data are available to assess these changes combined. Here, we built a database using archives of temperature, water chemistry and zooplankton data from 60 Scandinavian lakes that represent broad spatial and temporal gradients in key parameters: temperature, calcium (Ca), total phosphorus (TP), total organic carbon (TOC), and pH. Using machine learning techniques, we found that Ca was the most important determinant of the relative abundance of all zooplankton groups studied, while pH was second, and TOC third in importance. Further, we found that Ca is declining in almost all lakes, and we detected a critical Ca threshold in lake water of 1.3 mg L-1 , below which the relative abundance of zooplankton shifts toward dominance of Holopedium gibberum and small cladocerans at the expense of Daphnia and copepods. Our findings suggest that low Ca concentrations may shape zooplankton communities, and that current trajectories of Ca decline could promote widespread changes in pelagic food webs as zooplankton are important trophic links from phytoplankton to fish and different zooplankton species play different roles in this context.

Improved Environmental Status: 50 Years of Declining Fish Mercury Levels in Boreal and Subarctic Fennoscandia.

Temporally (1965-2015) and spatially (55°-70°N) extensive records of total mercury (Hg) in freshwater fish showed consistent declines in boreal and subarctic Fennoscandia. The database contains 54 560 fish entries ( n: pike > perch ≫ brown trout > roach ≈ Arctic charr) from 3132 lakes across Sweden, Finland, Norway, and Russian Murmansk area. 74% of the lakes did not meet the 0.5 ppm limit to protect human health. However, after 2000 only 25% of the lakes exceeded this level, indicating improved environmental status. In lakes where local pollution sources were identified, pike and perch Hg concentrations were significantly higher between 1965 and 1990 compared to values after 1995, likely an effect of implemented reduction measures. In lakes where Hg originated from long-range transboundary air pollution (LRTAP), consistent Hg declines (3-7‰ per year) were found for perch and pike in both boreal and subarctic Fennoscandia, suggesting common environmental controls. Hg in perch and pike in LRTAP lakes showed minimal declines with latitude, suggesting that drivers affected by temperature, such as growth dilution, counteracted Hg loading and food web exposure. We recommend that future fish Hg monitoring sampling design should include repeated sampling and collection of pollution history, water chemistry, fish age, and stable isotopes to enable evaluation of emission reduction policies.

Climate warming feedback from mountain birch forest expansion: reduced albedo dominates carbon uptake.

Expanding high-elevation and high-latitude forest has contrasting climate feedbacks through carbon sequestration (cooling) and reduced surface reflectance (warming), which are yet poorly quantified. Here, we present an empirically based projection of mountain birch forest expansion in south-central Norway under climate change and absence of land use. Climate effects of carbon sequestration and albedo change are compared using four emission metrics. Forest expansion was modeled for a projected 2.6 °C increase in summer temperature in 2100, with associated reduced snow cover. We find that the current (year 2000) forest line of the region is circa 100 m lower than its climatic potential due to land-use history. In the future scenarios, forest cover increased from 12% to 27% between 2000 and 2100, resulting in a 59% increase in biomass carbon storage and an albedo change from 0.46 to 0.30. Forest expansion in 2100 was behind its climatic potential, forest migration rates being the primary limiting factor. In 2100, the warming caused by lower albedo from expanding forest was 10 to 17 times stronger than the cooling effect from carbon sequestration for all emission metrics considered. Reduced snow cover further exacerbated the net warming feedback. The warming effect is considerably stronger than previously reported for boreal forest cover, because of the typically low biomass density in mountain forests and the large changes in albedo of snow-covered tundra areas. The positive climate feedback of high-latitude and high-elevation expanding forests with seasonal snow cover exceeds those of afforestation at lower elevation, and calls for further attention of both modelers and empiricists. The inclusion and upscaling of these climate feedbacks from mountain forests into global models is warranted to assess the potential global impacts.

Phytoplankton biomass in northern lakes reveals a complex response to global change.

Global change may introduce fundamental alterations in phytoplankton biomass and community structure that can alter the productivity of northern lakes. In this study, we utilized Swedish and Finnish monitoring data from lakes that are spatially (135 lakes) and temporally (1995-2019, 110 lakes) extensive to assess how phytoplankton biomass (PB) of dominant phytoplankton groups related to changes in water temperature, pH and key nutrients [total phosphorus (TP), total nitrogen (TN), total organic carbon (TOC), iron (Fe)] along spatial (Fennoscandia) and temporal (25 years) gradients. Using a machine learning approach, we found that TP was the most important determinant of total PB and biomass of a specific species of Raphidophyceae - Gonyostomum semen - and Cyanobacteria (both typically with adverse impacts on food-webs and water quality) in spatial analyses, while Fe and pH were second in importance for G. semen and TN and pH were second and third in importance for Cyanobacteria. However, in temporal analyses, decreasing Fe and increasing pH and TOC were associated with a decrease in G. semen and an increase in Cyanobacteria. In addition, in many lakes increasing TOC seemed to have generated browning to an extent that significantly reduced PB. The identified discrepancy between the spatial and temporal results suggests that substitutions of data for space-for-time may not be adequate to characterize long-term effects of global change on phytoplankton. Further, we found that total PB exhibited contrasting temporal trends (increasing in northern- and decreasing in southern Fennoscandia), with the decline in total PB being more pronounced than the increase. Among phytoplankton, G. semen biomass showed the strongest decline, while cyanobacterial biomass showed the strongest increase over 25 years. Our findings suggest that progressing browning and changes in Fe and pH promote significant temporal changes in PB and shifts in phytoplankton community structures in northern lakes.

Dissolved organic carbon trends resulting from changes in atmospheric deposition chemistry.

Several hypotheses have been proposed to explain recent, widespread increases in concentrations of dissolved organic carbon (DOC) in the surface waters of glaciated landscapes across eastern North America and northern and central Europe. Some invoke anthropogenic forcing through mechanisms related to climate change, nitrogen deposition or changes in land use, and by implication suggest that current concentrations and fluxes are without precedent. All of these hypotheses imply that DOC levels will continue to rise, with unpredictable consequences for the global carbon cycle. Alternatively, it has been proposed that DOC concentrations are returning toward pre-industrial levels as a result of a gradual decline in the sulphate content of atmospheric deposition. Here we show, through the assessment of time series data from 522 remote lakes and streams in North America and northern Europe, that rising trends in DOC between 1990 and 2004 can be concisely explained by a simple model based solely on changes in deposition chemistry and catchment acid-sensitivity. We demonstrate that DOC concentrations have increased in proportion to the rates at which atmospherically deposited anthropogenic sulphur and sea salt have declined. We conclude that acid deposition to these ecosystems has been partially buffered by changes in organic acidity and that the rise in DOC is integral to recovery from acidification. Over recent decades, deposition-driven increases in organic matter solubility may have increased the export of DOC to the oceans, a potentially important component of regional carbon balances. The increase in DOC concentrations in these regions appears unrelated to other climatic factors.

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.

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.

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

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

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.

Recent advances in understanding and measurement of mercury in the environment: Terrestrial Hg cycling.

This review documents recent advances in terrestrial mercury cycling. Terrestrial mercury (Hg) research has matured in some areas, and is developing rapidly in others. We summarize the state of the science circa 2010 as a starting point, and then present the advances during the last decade in three areas: land use, sulfate deposition, and climate change. The advances are presented in the framework of three Hg "gateways" to the terrestrial environment: inputs from the atmosphere, uptake in food, and runoff with surface water. Among the most notable advances: These and other advances reported here are of value in evaluating the effectiveness of the Minamata Convention on reducing environmental Hg exposure to humans and wildlife.

Widespread diminishing anthropogenic effects on calcium in freshwaters.

Calcium (Ca) is an essential element for almost all living organisms. Here, we examined global variation and controls of freshwater Ca concentrations, using 440 599 water samples from 43 184 inland water sites in 57 countries. We found that the global median Ca concentration was 4.0 mg L-1 with 20.7% of the water samples showing Ca concentrations ≤ 1.5 mg L-1, a threshold considered critical for the survival of many Ca-demanding organisms. Spatially, freshwater Ca concentrations were strongly and proportionally linked to carbonate alkalinity, with the highest Ca and carbonate alkalinity in waters with a pH around 8.0 and decreasing in concentrations towards lower pH. However, on a temporal scale, by analyzing decadal trends in >200 water bodies since the 1980s, we observed a frequent decoupling between carbonate alkalinity and Ca concentrations, which we attributed mainly to the influence of anthropogenic acid deposition. As acid deposition has been ameliorated, in many freshwaters carbonate alkalinity concentrations have increased or remained constant, while Ca concentrations have rapidly declined towards or even below pre-industrial conditions as a consequence of recovery from anthropogenic acidification. Thus, a paradoxical outcome of the successful remediation of acid deposition is a globally widespread freshwater Ca concentration decline towards critically low levels for many aquatic organisms.

Stand age and fine root biomass, distribution and morphology in a Norway spruce chronosequence in southeast Norway.

We assessed the influence of stand age on fine root biomass and morphology of trees and understory vegetation in 10-, 30-, 60- and 120-year-old Norway spruce stands growing in sandy soil in southeast Norway. Fine root (< 1, 1-2 and 2-5 mm in diameter) biomass of trees and understory vegetation (< 2 mm in diameter) was sampled by soil coring to a depth of 60 cm. Fine root morphological characteristics, such as specific root length (SRL), root length density (RLD), root surface area (RSA), root tip number and branching frequency (per unit root length or mass), were determined based on digitized root data. Fine root biomass and morphological characteristics related to biomass (RLD and RSA) followed the same tendency with chronosequence and were significantly higher in the 30-year-old stand and lower in the 10-year-old stand than in the other stands. Among stands, mean fine root (< 2 mm) biomass ranged from 49 to 398 g m(-2), SLR from 13.4 to 19.8 m g(-1), RLD from 980 to 11,650 m m(-3) and RSA from 2.4 to 35.4 m(2) m(-3). Most fine root biomass of trees was concentrated in the upper 20 cm of the mineral soil and in the humus layer (0-5 cm) in all stands. Understory fine roots accounted for 67 and 25% of total fine root biomass in the 10- and 120-year-old stands, respectively. Stand age had no affect on root tip number or branching frequency, but both parameters changed with soil depth, with increasing number of root tips and decreasing branching frequency with increasing soil depth for root fractions < 2 mm in diameter. Specific (mass based) root tip number and branching density were highest for the finest roots (< 1 mm) in the humus layer. Season (spring or fall) had no effect on tree fine root biomass, but there was a small and significant increase in understory fine root biomass in fall relative to spring. All morphological characteristics showed strong seasonal variation, especially the finest root fraction, with consistently and significantly higher values in spring than in fall. We conclude that fine root biomass, especially in the finest fraction (< 1 mm in diameter), is strongly dependent on stand age. Among stands, carbon concentration in fine root biomass was highest in the 30-year-old stand, and appeared to be associated with the high tree and canopy density during the early stage of stand development. Values of RLD and RSA, morphological features indicative of stand nutrient-uptake efficiency, were higher in the 30-year-old stand than in the other stands.

Testing seasonal and long-term controls of streamwater DOC using empirical and process-based models.

Concentrations of dissolved organic carbon (DOC) in surface waters are increasing across Europe and parts of North America. Several mechanisms have been proposed to explain these increases including reductions in acid deposition, change in frequency of winter storms and changes in temperature and precipitation patterns. We used two modelling approaches to identify the mechanisms responsible for changing surface water DOC concentrations. Empirical regression analysis and INCA-C, a process-based model of stream-water DOC, were used to simulate long-term (1986--2003) patterns in stream water DOC concentrations in a small boreal stream. Both modelling approaches successfully simulated seasonal and inter-annual patterns in DOC concentration. In both models, seasonal patterns of DOC concentration were controlled by hydrology and inter-annual patterns were explained by climatic variation. There was a non-linear relationship between warmer summer temperatures and INCA-C predicted DOC. Only the empirical model was able to satisfactorily simulate the observed long-term increase in DOC. The observed long-term trends in DOC are likely to be driven by in-soil processes controlled by SO4(2-) and Cl(-) deposition, and to a lesser extent by temperature-controlled processes. Given the projected changes in climate and deposition, future modelling and experimental research should focus on the possible effects of soil temperature and moisture on organic carbon production, sorption and desorption rates, and chemical controls on organic matter solubility.

Mercury budget of a small forested boreal catchment in southeast Norway.

We present input and output fluxes of total mercury (Hg(tot)) and methyl mercury (MeHg) based on throughfall, litterfall and stream water samples from 2004/2005 from a small forested catchment (Langtjern) in Norway. Hg(tot) input via throughfall and litterfall was estimated to 6.7 and 2.7 microg m(-2) yr(-1), respectively, which is considerably lower than previously reported from other boreal catchments in Scandinavia. A likely cause for the low input flux is the sparseness and low productivity of the forest in the Langtjern catchment, with less atmospheric scavenging and lower litterfall fluxes than previously studied sites. In addition there has been a general decrease in mercury (Hg) in the atmosphere on the northern hemisphere in the last decade. The estimated output flux of Hg(tot) with surface water was 2.5 microg m(-2) yr(-1), which is comparable to what has been reported elsewhere. The ratio of Hg(tot) output flux to input flux was 26%, which is considerably higher than reported from other sites. This illustrates that catchment properties have greater importance for surface water export of Hg than the current atmospheric input. The estimated total soil pool of Hg(tot) in the catchment was 17.4 mg m(-2). This corresponds to roughly 8000 years of the current surface water output flux and 2000 years of the current input flux.

Projected stream water fluxes of NO3 and total organic carbon from the Storgama headwater catchment, Norway, under climate change and reduced acid deposition.

Fluctuations in the 20-year record of nitrate (NO3) and total organic carbon (TOC) concentrations and fluxes in runoff at the small headwater catchment Storgama, southern Norway, were related to climate and acid deposition. The long-term decline in NO3 related to reduced NO3 deposition and increased winter discharge, whereas the long-term increase in TOC related to reduced sulfur deposition. Multiple regression models describing long-term trends and seasonal variability in these records were used to project future concentrations given scenarios of climate change and acid deposition. All scenarios indicated reduced NO3 fluxes and increased TOC fluxes; the largest projected changes for the period 2071-2100 were -86% and +24%, respectively. Uncertainties are that the predicted future temperatures are considerably higher than the historical record. Also, nonlinear responses of ecosystem processes (nitrogen [N] mineralization) to temperature, N-enrichment of soils, and step-changes in environmental conditions may affect future leaching of carbon and N.

Effect of climate change on flux of N and C: air-land-freshwater-marine links: synthesis.

Projected climate change might increase the deposition of nitrogen by about 10% to seminatural ecosystems in southern Norway. At Storgama, increased precipitation in the growing season increased the fluxes of total organic carbon (TOC) and total organic nitrogen (TON) in proportion to the water flux. In winter, soil temperatures near 0 degrees C, common under a snowpack, induced higher runoff of inorganic nitrogen (N) and lower runoff of TOC. By contrast, soil temperatures below freezing, caused by little snow accumulation (expected in a warmer world), reduced runoff of inorganic N, TON, and TOC. Long-term monitoring data showed that reduced snowpack can cause either decreased or increased N leaching, depending on interactions with N deposition, soil temperature regime, and winter discharge. Seasonal variation in TOC was mainly climatically controlled, whereas deposition of sulfate and nitrate (NO3) explained the long-term TOC increase. Upscaling to the river basin scale showed that the annual flux of NO3 will remain unchanged in response to climate change projections.

Effect of climate change on fluxes of nitrogen from the Tovdal River basin, Norway, to adjoining marine areas.

The mass transport model TEOTIL was used to project nitrate (NO3) fluxes from the Tovdal River basin, southernmost Norway, given four scenarios of climate change. Forests, uplands, and open water currently account for 90% of the NO3 flux. Climate scenarios for 2071-2100 suggest increased temperature by 2-4 degrees C and precipitation by 3-11%. Climate experiments and long-term monitoring were used to estimate future rates of nitrogen (N) leaching. More water will run through the terrestrial catchments during the winter but less will run in the spring. The annual NO3 flux from the Tovdal River to the adjoining Topdalsfjord is projected to remain unchanged, but with more NO3 delivered in the winter and less in the spring. Algal blooms in coastal waters can be expected to occur earlier in the year. Major sources of uncertainty are in the long-term fate of N stored in soil organic matter and the impacts of forest management.

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 (BAFMeHg) 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 BAFMeHg 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 BAFMeHg 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 BAFMeHg 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.