Heleen de Wit

• Bergström, Ann Kristin; Paltsev, Aleksey; Hessen, Dag Olav; Kortelainen, Pirkko; Vuorenmaa, Jussi & Wit, Heleen de [Show all 15 contributors for this article] (2026). Global Change Reshapes Northern Lakes Towards Browner, More Nutrient-Depleted and Nitrogen-Limited Conditions With Contrasting Impacts on Phytoplankton Biomass. Global Change Biology. ISSN 1354-1013. 32(7), p. 1–19. doi: 10.1111/gcb.71008. Full text in Research Archive Show summary

  Global change is altering trade-offs between light and nutrient availability in northern lakes, with implications for biogeochemical cycling and ecosystem functioning. Using 29 years (1991–2019) of long-term monitoring from 169 Fennoscandian natural lakes, we quantified spatiotemporal change in browning (total organic carbon, TOC), dissolved inorganic nitrogen (DIN), total phosphorus (TP), and stoichiometry (TOC:TP and DIN:TP). We then related these trajectories to co-occurring trends in air temperature, precipitation, and atmospheric nitrogen (N) and sulfur (S) deposition to evaluate how climate change and deposition recovery jointly reconfigure lake chemistry across subregions. TOC increased widely and DIN declined across most lakes, whereas TP trends were mixed, producing a pervasive rise in TOC:TP and decline in DIN:TP over time. Mixed linear models indicated that TOC increases were most often associated with declining S deposition and increasing precipitation, while DIN was generally positively related to N deposition, with additional subregional roles for temperature and precipitation. As browning coincided with declining DIN:TP, lakes shifted toward darker conditions with greater prevalence of nitrogen and phosphorus co-limitation and nitrogen limitation relative to phosphorus limitation. In a Swedish subset of 74 lakes, chlorophyll-a trends were heterogeneous among subregions, indicating context-dependent biomass responses rather than a uniform phytoplankton signal. Together, these results show that ongoing climate change and reduced atmospheric deposition are reshaping carbon–nutrient coupling and nutrient-limitation regimes across northern lakes, with consequences for future primary production and energy transfer to higher trophic levels.

• Vogt, Rolf David; Sekkenes, Marianne Stave; Norling, Magnus Dahler; Austnes, Kari; Wit, Heleen de & Kaste, Øyvind (2026). Climate Effects on Water Chemistry in Acid-Sensitive Catchments. Water. 18(14). doi: https:/www.mdpi.com/2073-4441/18/14/1731. Full text in Research Archive Show summary

  Substantial declines in acidifying emissions across Europe have led to pronounced chemical recovery of Norwegian surface waters. In recent decades, however, changes in water chemistry have increasingly coincided with climate change, complicating the attribution of observed trends to individual drivers. This study assesses whether ongoing climate change has produced detectable effects on freshwater chemistry in Norway and how these effects vary among catchments with differing sensitivities to acidification. In this study, the Model of Acidification of Groundwater In Catchments (MAGIC), which is based on current understanding of the processes governing acid–base chemistry in soils and waters, was used to simulate the effects of declining acid deposition. Deviations between observed and modelled water chemistry were provisionally interpreted as climate-related effects. However, these residuals may also reflect model or parameter uncertainty and other unaccounted-for processes. The analysis draws on long-term monitoring data (1986–2022) from 59 acid-sensitive Trend Lakes distributed across Norway, together with four Field Research Stations (1986–2020) representing contrasting hydroclimatic and biogeochemical conditions. Temporal trends were evaluated using the Mann–Kendall test and Sen’s slope estimator, while relationships between inferred climate effects and climatic variables were examined using Pearson’s correlation analysis. Across the Trend Lakes, inferred climate effects were predominantly positive for acid-neutralising capacity (ANC) and weathering-derived cations, suggesting that climate change may contribute to accelerated chemical recovery, particularly in catchments less sensitive to acidification. The inferred climate effects varied substantially among the Field Research Stations. Higher temperatures were generally associated with enhanced recovery, possibly through intensified silicate weathering, whereas increased precipitation and runoff appeared to dampen recovery. Overall, the results suggest that climate change exerts a measurable influence on freshwater chemistry in Norway, although the magnitude and direction of the response are strongly modulated by catchment-specific characteristics. While previous studies have identified climate-related influences on individual chemical variables, quantitative attempts to separate climate- and acid-deposition-related effects across a large number of acid-sensitive catchments remain rare. Here, we use deviations between observed water chemistry and MAGIC simulations of acid deposition recovery as a screening approach to investigate whether climate-related signals can be detected at the national scale and whether these signals vary among catchments with differing sensitivities to acidification.

• Lu, Xiang; Clayer, Francois; Dörsch, Peter; Hessen, Dag Olav; Brandt, Uta & Ippach, Mats Rouven [Show all 8 contributors for this article] (2026). In-Lake GHG Production Constrained by Lake DOM Mass Balance in a Boreal Catchment. Water Resources Research. ISSN 0043-1397. 62(5). doi: 10.1029/2025WR041297. Full text in Research Archive
• Weldon, James; Aas, Wenche; Albiniak, Barbara; Augustaitis, Algirdas; Baužienė, Ieva & Capelli, Camilla [Show all 51 contributors for this article] (2026). A long-term ecosystem monitoring dataset from the ICP Integrated Monitoring network: biogeochemical data from 1977–2020 across 14 European countries. Scientific Data. 13(1). doi: 10.1038/s41597-026-07181-8. Full text in Research Archive Show summary

  Abstract The International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems (ICP IM) presents a comprehensive long-term dataset of ongoing integrated ecosystem monitoring from European forested catchments. The dataset encompasses measurements from 46 monitoring stations across 14 European countries, with temporal coverage mostly extending from the early 1990s to 2020 (48 sites are currently active). The integrated monitoring approach applies over 20 monitoring subprogrammes to simultaneously measure physical, chemical, and biological properties across multiple ecosystem compartments including atmosphere, precipitation, throughfall, soil water, groundwater, runoff water, soil, vegetation, and biota. All measurements follow standardised protocols detailed in the ICP IM Manual, ensuring data quality and comparability across sites and time periods. The dataset supports research on ecosystem responses to air pollution, climate change impacts, and biogeochemical cycling. Data are available under a Creative Commons By Attribution (CC BY) licence, providing valuable long-term environmental monitoring data for the scientific community.

• Futter, Martyn; Rankinen, Katri; Audet, Joachim; Wit, Heleen de; Farkas, Csilla & Forsius, Martin [Show all 16 contributors for this article] (2026). Northern Nordic river mouth N: P: Si stoichiometry shows limited evidence of Si depletion. Scientific Reports. 16. doi: https:/www.nature.com/articles/s41598-025-34052-w. Full text in Research Archive Show summary

  River mouth nitrogen: phosphorus: silicon (N:P:Si) stoichiometry can predict marine eutrophication potential. Furthermore, deviations from Redfield molar N: P:Si = 16:1:20 ratios can offer insight into limiting nutrients and risks for harmful algal blooms (HAB). Here, we present N:P:Si stoichiometry based on total P, dissolved inorganic N and dissolved Si for 88 coastal river mouths in Norway, Sweden and Finland between 2017 and 2024. Rivers ranged from ultra-oligotrophic to hyper-eutrophic. N and P concentrations increased from north to south, with no latitudinal Si gradient. Most rivers were either P or jointly NP depleted relative to Si, with no overall evidence of Si depletion. However, there was some evidence of seasonal Si depletion. We show how using multi-element approaches, e.g., the Index of Coastal Eutrophication Potential (ICEP) can help to guide management actions. Specifically, Using Si depletion relative to N and P concentrations as an indicator of HAB potential may offer a means to identify catchments where nutrient load reductions can have the largest benefit on marine ecosystem health. Such multi-element approaches can complement single nutrient assessments based on, e.g., the Carlson Trophic Status Index or Water Framework Directive status class assessments.

• Eklöf, Karin; Wit, Heleen de; Eckley, Chris S.; Eagles-Smith, Collin A.; Eggert, Susan L. & Mackereth, Robert W. [Show all 24 contributors for this article] (2025). The Effects of Forest Harvesting on Total and Methylmercury Concentrations in Surface Waters Depend on Harvest Practices and Physical Site Characteristics. Environmental Science and Technology. ISSN 0013-936X. 59(30), p. 15944–15955. doi: 10.1021/acs.est.5c02787. Full text in Research Archive Show summary

  Forest harvesting can lead to mercury (Hg) mobilization from soils to aquatic habitats and promote the transformation of inorganic Hg to highly neurotoxic and bioaccumulative methyl-Hg (MeHg). Multiple past studies reveal broad variation of stream water MeHg and total Hg (THg) concentration responses to forest harvesting, which has confounded messaging to forest and resource managers. To advance beyond divergent and sometimes contradictory findings, we synthesized information for 23 previously studied catchments in North America and Fennoscandia and compiled a uniform set of soil, landscape, and harvesting properties to identify forest management, riparian, and hillslope factors that influence responses of stream water MeHg and THg concentrations. From this synthesis, we found catchments with high soil moisture and organic soil layers >100 cm to be at highest risk for disturbance-induced increases in MeHg formation after harvest but not necessarily affecting concentrations of MeHg in stream waters. Instead, the combination of MeHg formation in soils along with factors that affect mobilization with runoff to streams most influenced how forest harvest affects MeHg concentrations in stream waters.

• Paltsev, Aleksey; Creed, Irena F.; Hessen, Dag Olav; Drakare, Stina; Lau, Danny C.P. & Vrede, Tobias [Show all 15 contributors for this article] (2025). Temporal Decoupling Between Total Organic Carbon and Iron in Lakes Linked to Interannual Changes in Precipitation. Global Biogeochemical Cycles. ISSN 0886-6236. 39(10). doi: 10.1029/2025GB008520. Full text in Research Archive Show summary

  Widespread increases in lake browning, which affects primary production, have been observed in northern lakes. While lake browning is attributed to increases in terrestrially derived total organic carbon (TOC) and total iron (Fe), Fe does not consistently correlate with increasing TOC over time. This temporal mismatch between TOC and Fe indicates that we still do not fully understand the causes of lake browning, especially in the context of gradually changing climatic conditions. In this study, we utilized Fennoscandian 30-year (1990–2020) time series data for 102 lakes to describe possible reasons for the temporal decoupling between TOC and Fe. Using Bayesian mixed-effects models and wavelet coherence analysis, we found evidence for differential responses of TOC and Fe concentrations to changes in precipitation, temperature, and sulfur deposition. While TOC appeared more sensitive to the effects of precipitation, temperature and sulfur deposition in individual lakes, Fe concentrations were impacted by complex interactions among these environmental variables. Although TOC and Fe increased in most lakes in response to increased temperature and precipitation, 41% of the lakes—typically with larger catchment-to-lake area ratios and shorter water residence times—exhibited a declining trend in Fe. This analysis encompasses lakes of both significant and non-significant changes over time. This decline in Fe was associated with short-timescale (2–4 years) increases in precipitation, leading to a temporal decoupling between Fe and TOC. Our findings suggest that Fe concentrations do not increase uniformly with rising temperatures and increased precipitation, especially in regions where sulfur deposition has declined due to atmospheric recovery policies.

• Knutson, Jacqueline Kay; Clayer, Francois; Dörsch, Peter; Westermann, Sebastian & Wit, Heleen de (2025). Water chemistry and greenhouse gas concentrations in waterbodies of a thawing permafrost peatland complex in northern Norway. Biogeosciences. ISSN 1726-4170. 22(15), p. 3899–3914. doi: 10.5194/bg-22-3899-2025. Full text in Research Archive Show summary

  Thermokarst ponds in thawing permafrost landscapes play a considerable role in greenhouse gas (GHG) emissions despite their small size, yet they remain underrepresented in Earth system models. Transitions from hydrologically isolated thermokarst ponds in peat plateaus to connected wetlands can substantially alter GHG dynamics. However, the processes and GHG impacts of these shifts are not well understood, particularly in the sporadic permafrost zones of Fennoscandia. To address this, we investigated water chemistry, dissolved organic matter (DOM) processing, and GHG fluxes over two years at the Iškoras site in northern Norway, where a degrading peat plateau includes both thermokarst ponds and an adjacent wetland stream. Thermokarst ponds exhibited low pH, high organic acidity, high oversaturation of dissolved carbon dioxide (CO2), and, especially, high concentrations of dissolved methane (CH4). The adjacent wetland stream, however, with near-neutral pH, showed lower CH4 concentrations and organic acidity but significantly higher CO2 emissions, despite moderate saturations driven by turbulence and bicarbonate replenishment. By contrast, CO2 emissions in ponds were primarily linked to DOM mineralization. Despite differences in chemistry, DOM mineralization rates were similar between ponds and the stream sites, suggesting that environmental factors like pH and microbial community differences counteract DOM lability variations. As permafrost decays and transitions from peat plateaus to wetlands, ponds as hotspots of CH4 emissions will disappear. However, total GHG fluxes across the peatland–wetland continuum will depend on wetland emissions, where CH4 emissions are usually considerable, and the fate of organic matter within the plateau. Lateral DOM fluxes may represent a significant loss of soil organic carbon (OC), highlighting the importance of hydrological connectivity in linking terrestrial and aquatic systems. This study emphasizes the need to account for the relationship between hydrological and chemical processes when assessing C and GHG fluxes in permafrost-impacted regions.

• Graves, Stephanie D.; Kidd, Karen A.; Arts, Michael T.; Braaten, Hans Fredrik Veiteberg; Wit, Heleen de & Borgå, Katrine [Show all 8 contributors for this article] (2025). Terrestrial carbon inputs drive methylmercury accumulation in zooplankton of boreal and subarctic lakes. Limnology and Oceanography. ISSN 0024-3590. 70(10), p. 2910–2923. doi: 10.1002/lno.70185. Full text in Research Archive Show summary

  Boreal and subarctic lakes are subject to the climate‐sensitive process of browning, whereby transport of terrestrial dissolved organic matter (tDOM) to lakes results in greater dissolved organic carbon (DOC) concentrations and associated darker water color. Increasing tDOM will increase mercury (Hg) transport to these lakes, but whether this leads to greater methylHg (MeHg) bioaccumulation in food webs remains unclear. We determined whether increasing DOC increased MeHg bioaccumulation in the lower food web (i.e., zooplankton) by measuring a suite of water chemistry characteristics (including aqueous MeHg and DOC) along with stable isotopes of C (δ 13 C) and N (δ 15 N), fatty acid (FA) profiles, and MeHg content of zooplankton from 16 Scandinavian boreal and subarctic lakes along a DOC gradient in the Fall of 2016. We found that both aqueous and zooplankton MeHg were positively correlated with DOC concentration, and that DOC and zooplankton MeHg both increased with the bacterial FA marker 18:1n‐7 and decreased with docosahexaenoic acid (DHA) : arachidonic acid and DHA : eicosapentaenoic acid ratios in zooplankton, which are indicators of diet or taxonomic composition. Zooplankton MeHg content was best predicted by δ 13 C and the FA 18:1n‐7, indicating that zooplankton MeHg bioaccumulation in zooplankton was associated with changes in their resource use along a DOC gradient. Our results suggest that lake browning will likely lead to an increase in MeHg bioaccumulation in zooplankton by affecting aqueous MeHg exposure and lower food web dynamics. In turn, this may lead to increased MeHg contamination in fish and other wildlife.

• Aslaksen, Iulie; Bryn, Anders; Clemmensen, Karina Engelbrecht; Wit, Heleen de; Emanuelsson, Urban & Garnåsjordet, Per Arild [Show all 21 contributors for this article] (2025). Afforestation on Nordic grasslands: Trade-offs and synergies for climate mitigation, biodiversity, and ecosystem services. Global Environmental Change (GEC) Advances. 4. doi: 10.1016/j.gecadv.2025.100015. Full text in Research Archive Show summary

  Afforestation of abandoned grasslands has been proposed as a global climate mitigation strategy, but the climate benefits of tree planting on grasslands remain contentious. Studies worldwide indicate that grassland soils have large potential for carbon storage, while semi-natural grasslands often support high biodiversity and provide multiple ecosystem services, including grazing resources, pollinator habitats, and aesthetic landscape values. In boreal and alpine regions of the Nordic countries, grasslands sustain extensive low intensity farming, contributing to milk and meat production and enhancing food self-sufficiency. Evaluating the impact of afforestation on climate mitigation requires a comprehensive assessment that, in addition to the carbon balance, considers both geophysical forcing (such as albedo and evapotranspiration) and the broader landscape-level effects on biodiversity in displaced ecosystems. The article postulates for policy to be inclusive of both biodiversity preservation and climate change mitigation. Such an approach should be grounded in evidence-based assessments of the ecological and climate-related impacts of afforestation on the biodiversity of semi-natural grasslands.

• Vogt, Rolf David; Garmo, Øyvind Aaberg; Austnes, Kari; Kaste, Øyvind; Haaland, Ståle Leif & Sample, James Edward [Show all 10 contributors for this article] (2024). Factors Governing Site and Charge Density of Dissolved Natural Organic Matter. Water. 16(12), p. 1–17. doi: 10.3390/w16121716. Full text in Research Archive Show summary

  Rising organic charge in northern freshwaters is attributed to increasing levels of dissolved natural organic matter (DNOM) and changes in water chemistry. Organic charge concentration may be determined through charge balance calculations (Org.−) or modelled (OAN−) using the Oliver and Hruška conceptual models, which are based on the density of weak acid functional sites (SD) present in DNOM. The charge density (CD) is governed by SD as well as protonation and complexation reactions on the functional groups. These models use SD as a key parameter to empirically fit the model to Org.−. Utilizing extensive water chemistry datasets, this study shows that spatial and temporal differences in SD and CD are influenced by variations in the humic-to-fulvic ratio of DNOM, organic aluminum (Al) complexation, and the mole fraction of CD to SD, which is governed by acidity. The median SD values obtained for 44 long-term monitored acid-sensitive lakes were 11.1 and 13.9 µEq/mg C for the Oliver and Hruška models, respectively. Over 34 years of monitoring, the CD increased by 70%, likely due to rising pH and declining Al complexation with DNOM. Present-day median SD values for the Oliver and Hruška models in 16 low-order streams are 13.8 and 15.8 µEq/mg C, respectively, and 10.8 and 12.5 µEq/mg C, respectively, in 10 high-order rivers.

• Vogt, Rolf David; Kaste, Øyvind; Thrane, Jan-Erik; Balkoni, Areti; Clayer, Francois & Schartau, Ann Kristin [Show all 7 contributors for this article] (2024). Er det mulig å enkelt skille effekter av klimaendringer og nedgang i sur nedbør på trender i vannkjemi? VANN. ISSN 0042-2592. 59(4), p. 287–305. Full text in Research Archive Show summary

  Studien undersøker om trender i klimavariablene nedbør, avrenning, temperatur og faktisk evapo- transpirasjon (AET) kan forklare endringer i vannkjemi i 78 norske innsjøer fra 1990 til 2020. AET har økt signifikant i nesten alle ned- børsfeltene, men det er store avvik mellom modellert og beregnet AET. Temperaturen har også steget signifikant i 79 % av stasjonene, mens nedbør og avrenning kun viser signifikan- te trender i henholdsvis 15 og 19 innsjøer. Dette reflekteres i betydningen av disse forklarings- variablene for endringer i vannkjemi. Forskjeller i klimatrender samvarierer i rom og tid med nedgangen i sur nedbør, noe som gjør det utford rende å skille effektene av disse driverne i enkle korrelasjonsanalyser. Forklaringsverdien av trender i klimaparameterne på trender i vannkjemien er i tillegg svake. Ved bruk av multivariat RDA finner vi imidlertid en ikke signifikant indikasjon på at økning i nedbør og avrenning kan ha en motsatt effekt av økning i temperatur på trender i vannkjemien.

• Aas, Elin Cecilie Ristorp; Wit, Heleen de & Berntsen, Terje Koren (2024). Modeling boreal forest soil dynamics with the microbially explicit soil model MIMICS+ (v1.0). Geoscientific Model Development. ISSN 1991-959X. 17(7), p. 2929–2959. doi: 10.5194/gmd-17-2929-2024. Full text in Research Archive Show summary

  Understanding carbon exchange processes between land reservoirs and the atmosphere is essential for predicting carbon–climate feedbacks. Still, considerable uncertainty remains in the representation of the terrestrial carbon cycle in Earth system models. An emerging strategy to constrain these uncertainties is to include the role of different microbial groups explicitly. Following this approach, we extend the framework of the MIcrobial-MIneral Carbon Stabilization (MIMICS) model with additional mycorrhizal groups and a nitrogen cycle that includes a novel representation of inorganic nitrogen sorption to particles via a Langmuir isotherm. MIMICS+ v1.0 is designed to capture and quantify relationships between soil microorganisms and their environment, with a particular emphasis on boreal ecosystems. We evaluated MIMICS+ against podzolic soil profiles in Norwegian forests as well as the conventional Community Land Model (CLM). MIMICS+ matched observed carbon stocks better than CLM and gave a broader range of C:N

• Basso, Stefano; Musolff, Andreas & Wit, Heleen de (2024). More Frequent Runoff and Connected Sources in Headwaters Promote Browning of Northern Freshwaters. Environmental Science and Technology Letters (ES&T Letters). 12(1), p. 51–56. doi: 10.1021/acs.estlett.4c00939. Full text in Research Archive Show summary

  Sustained browning of northern waters has prompted inquiries into the drivers of increasing concentrations of organic matter. While reduced sulfur deposition is a key cause, an increasing role of hydrologic mechanisms as a result of cleaner air and progressing climate change has been repeatedly suggested. How these controls act remains however unclear. Here we examine over 30 years of organic carbon concentration and discharge data from four reference streams located across Norway to pinpoint consistent hydrologic changes that may promote water browning. Stable slopes with changing intercepts of the concentration-discharge relations indicate that the influence of air pollution on soil solution chemistry is plausible, supporting available chemical explanations from a hydrologic perspective. Decreasing ratios of concentration to discharge variability, observed in autumn over the years, point to less spatial heterogeneity of the sources of organic carbon. A clear rise in the frequency of runoff events, which increases the opportunities for dissolved organic carbon to transit from soil to streams, also indicates higher hydrologic connectivity and more even mobilization of carbon sources. More connected sources and more frequent runoff events, which jointly enhance the likelihood of organic carbon reaching rivers, may thus contribute to the observed browning of northern waters.

• Wit, Heleen de; Clayer, Francois; Kaste, Øyvind & Norling, Magnus Dahler (2024). From anthropogenic toward natural acidification: Effects of future deposition and climate on recovery in a humic catchment in Norway. Ecological Research. ISSN 0912-3814. doi: 10.1111/1440-1703.12524. Full text in Research Archive Show summary

  Five decades of monitoring data (1974–2022) at the acidified forested catchment of Langtjern in southern Norway document strong chemical recovery and browning of surface water, related to changes in sulfur (S) deposition. Further recovery is likely to be impacted by future air quality and climate, through catchment processes sensitive to climate change, where the relative importance of these drivers of recovery is poorly known. Here, we explore the importance of the aforementioned drivers for recovery using the well-established process-oriented Model of Acidification of Groundwater In Catchments (MAGIC) with historical and projected deposition and climate from 1860 to 2100. New in MAGIC are (i) a solubility control of dissolved organic carbon (DOC) from S deposition, which allows inclusion of the role of organic acids in chemical recovery and (ii) climate-dependency of weathering rates. MAGIC successfully described observed chemical recovery and browning, and the change toward organic acid dominated acidification status. Hindcasts of pH predicted lower preindustrial pH than previously modeled with MAGIC (simulated without S-dependency of DOC solubility). Future deposition resulted in limited further recovery. Climate scenarios indicated a substantially wetter future, leading to increased base cation losses and slight surface water reacidification. A sensitivity analysis revealed that a 25%–50% increase of weathering rates was needed to reach preindustrial acid-neutralizing capacity in 2100, provided S deposition is reduced to a minimum. We predict that the limited chemical recovery from reduced S deposition will be counteracted by climate-driven reacidification from base cation losses, but that enhanced weathering rates could partly compensate these losses.

• Eklöf, Karin; Brömssen, Claudia von; Huser, Brian; Åkerblom, Staffan; Augustaitis, Algirdas & Braaten, Hans Fredrik Veiteberg [Show all 21 contributors for this article] (2024). Trends in mercury, lead and cadmium concentrations in 27 European streams and rivers: 2000–2020. Environmental Pollution (1987). ISSN 0269-7491. 360. doi: 10.1016/j.envpol.2024.124761. Full text in Research Archive Show summary

  Temporal trends for concentrations of mercury (Hg), lead (Pb) and cadmium (Cd) were evaluated from year 2000–2020 in 20 (Hg), 23 (Pb) and 11 (Cd) watercourses in remote forest catchments in Europe. Decreasing trends were observed in 15% (Hg), 39% (Pb) and 45% (Cd) of the watercourses during the period of evaluation. Decreasing trends were mainly observed between 2000 and 2005 for Hg and between 2000 and 2015 for Pb and Cd. For the last five years of the studied time period (2015–2020), more watercourses showed significant increasing, rather than decreasing Hg, Pb and Cd trends. This was interpreted as a legacy effect of metals still retained in catchment soils. The overall negative trends during the earlier part of the study period were likely driven by declining deposition of metals over Europe, especially for Pb and Cd. Other changes related to metal transport and chemistry may have contributed to the observed trends as well, including recovery from acidification and the ongoing browning of surface waters at northern latitudes. Here we found that organic carbon could explain the seasonal variation in Hg and Pb, but was not related the interannual trends. This study highlights the need for long-term monitoring and robust statistical methods that can detect multidirectional, long-term change in water chemistry.

• Pirk, Norbert; Aalstad, Kristoffer; Mannerfelt, Erik Schytt; Clayer, Francois; Wit, Heleen de & Christiansen, Casper Tai [Show all 9 contributors for this article] (2024). Disaggregating the Carbon Exchange of Degrading Permafrost Peatlands Using Bayesian Deep Learning. Geophysical Research Letters. ISSN 0094-8276. 51(10). doi: 10.1029/2024GL109283. Full text in Research Archive Show summary

  Extensive regions in the permafrost zone are projected to become climatically unsuitable to sustain permafrost peatlands over the next century, suggesting transformations in these landscapes that can leave large amounts of permafrost carbon vulnerable to post-thaw decomposition. We present 3 years of eddy covariance measurements of CH4 and CO2 fluxes from the degrading permafrost peatland Iškoras in Northern Norway, which we disaggregate into separate fluxes of palsa, pond, and fen areas using information provided by the dynamic flux footprint in a novel ensemble-based Bayesian deep neural network framework. The 3-year mean CO2-equivalent flux is estimated to be 106 gCO2 m−2 yr−1 for palsas, 1,780 gCO2 m−2 yr−1 for ponds, and −31 gCO2 m−2 yr−1 for fens, indicating that possible palsa degradation to thermokarst ponds would strengthen the local greenhouse gas forcing by a factor of about 17, while transformation into fens would slightly reduce the current local greenhouse gas forcing.

• Rankinen, Katri; Bernal, Jose E. Cano; Holmberg, Maria; Norling, Magnus; Schulz, Torsti & Mäkelä, Annikki [Show all 11 contributors for this article] (2024). Modelling the Effects of Forest use Change on Brownification of Finnish Rivers under Atmospheric Pressure. Environmental Management. ISSN 0364-152X. doi: 10.1007/s00267-024-02058-1. Full text in Research Archive Show summary

  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.

• Bergström, Ann-Kristin; Creed, Irena F.; Paltsev, Aleksey; Wit, Heleen de; Lau, Danny C. P. & Drakare, Stina [Show all 15 contributors for this article] (2024). Declining calcium concentration drives shifts toward smaller and less nutritious zooplankton in northern lakes. Global Change Biology. ISSN 1354-1013. 30(3). doi: 10.1111/gcb.17220. Full text in Research Archive Show summary

  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 m

• Paltsev, Aleksey; Bergström, Ann-Kristin; Vuorio, Kristiina; Creed, Irena F.; Hessen, Dag Olav & Kortelainen, Pirkko [Show all 15 contributors for this article] (2024). Phytoplankton biomass in northern lakes reveals a complex response to global change. Science of the Total Environment. ISSN 0048-9697. 940. doi: 10.1016/j.scitotenv.2024.173570. Full text in Research Archive Show summary

  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.

• Monteith, Donald T.; Henrys, Peter A.; Hruška, Jakub; Wit, Heleen de; Krám, Pavel & Moldan, Filip [Show all 12 contributors for this article] (2023). Long-term rise in riverine dissolved organic carbon concentration is predicted by electrolyte solubility theory. Science Advances. 9(3). doi: 10.1126/sciadv.ade3491. Full text in Research Archive Show summary

  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.

• Corman, Jessica R.; Zwart, Jacob A.; Klug, Jennifer; Bruesewitz, Denise A.; Eyto, Elvira de & Klaus, Marcus [Show all 17 contributors for this article] (2023). Response of lake metabolism to catchment inputs inferred using high-frequency lake and stream data from across the northern hemisphere. Limnology and Oceanography. ISSN 0024-3590. 68(12), p. 2617–2631. doi: 10.1002/lno.12449. Full text in Research Archive Show summary

  In lakes, the rates of gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP) are often controlled by resource availability. Herein, we explore how catchment vs. within lake predictors of metabolism compare using data from 16 lakes spanning 39°N to 64°N, a range of inflowing streams, and trophic status. For each lake, we combined stream loads of dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP) with lake DOC, TN, and TP concentrations and high frequency in situ monitoring of dissolved oxygen. We found that stream load stoichiometry indicated lake stoichiometry for C : N and C : P (r2 = 0.74 and r2 = 0.84, respectively), but not for N : P (r2 = 0.04). As we found a strong positive correlation between TN and TP, we only used TP in our statistical models. For the catchment model, GPP and R were best predicted by DOC load, TP load, and load N : P (R2 = 0.85 and R2 = 0.82, respectively). For the lake model, GPP and R were best predicted by TP concentrations (R2 = 0.86 and R2 = 0.67, respectively). The inclusion of N : P in the catchment model, but not the lake model, suggests that both N and P regulate metabolism and that organisms may be responding more strongly to catchment inputs than lake resources. Our models predicted NEP poorly, though it is unclear why. Overall, our work stresses the importance of characterizing lake catchment loads to predict metabolic rates, a result that may be particularly important in catchments experiencing changing hydrologic regimes related to global environmental change.

• Wit, Heleen de; Garmo, Øyvind Aaberg; Jackson-Blake, Leah Amber; Clayer, Francois; Vogt, Rolf David & Austnes, Kari [Show all 10 contributors for this article] (2023). Changing water chemistry in one thousand Norwegian lakes during three decades of cleaner air and climate change. Global Biogeochemical Cycles. ISSN 0886-6236. 37(2). doi: 10.1029/2022GB007509. Full text in Research Archive Show summary

  We present long-term changes in Norwegian lake water quality across regional gradients in atmospheric pollution, air temperature, hydrology, and vegetation using (a) a national representative lake survey carried out in 1995 and 2019 (ThousandLakes), and (b) an annual lake survey from acid-sensitive catchments (78 lakes, TrendLakes) from 1990 to 2020. Our analysis encompasses all major chemical constituents, for example, anions and cations, dissolved organic matter (DOM), nutrients, iron (Fe), and silicate (SiO2). During these decades, environmental changes included declines in sulfur (S) and nitrogen (N) deposition, climate warming, and increase in forest biomass. Strong chemical recovery from acidification is found, attributed to large reductions in atmospheric deposition, moderated by catchment processing from land use and climate change. Browning counteracted chemical recovery in some regions, while Ca increased unexpectedly. We suggest that increased weathering, from enhanced terrestrial productivity, is an important driver of increased Ca—substantiated by widespread, substantial increases in SiO2. Light- and nutrient-limitation has become more prevalent, indicated by higher DOM, lower nitrate (NO3), and lower NO3to total phosphorous ratios. Declines in lake NO3 occurred independently of N deposition, suggesting increased catchment N retention, possibly from increased terrestrial productivity. We conclude that decreased air pollution continues to be a dominant driver of long-term trends in lake chemistry, but climate-induced increase in terrestrial weathering processes, governed by increased biomass, is likely to have an increasing impact on future lake acidity, nutrient, and light status, that may cascade along the aquatic continuum from rivers to the coast.

• Pilla, Rachel M.; Williamson, Craig E.; Overholt, Erin P.; Rose, Kevin C.; Berger, Stella Angela & Couture, Raoul-Marie [Show all 16 contributors for this article] (2023). Deepwater dissolved oxygen shows little ecological memory between lake phenological seasons. INLAND WATERS. ISSN 2044-2041. 13(3). doi: 10.1080/20442041.2023.2265802. Full text in Research Archive Show summary

  Depletion of deepwater dissolved oxygen (DO) in lakes has become increasingly prevalent and severe because of many external stressors, potentially threatening human-derived ecosystem services ranging from drinking water quality to fisheries. Using year-round, high-frequency DO data from 12 dimictic lakes, we compared 3 measures of deepwater DO depletion during winter and summer: DO depletion rate, DO minimum, and hypoxia duration. Hypoxia (DO < 3 mg L−1) occurred in over half of the lakes and persisted an average of 83% longer in summer than in winter. While we found no difference in DO depletion rates between winter versus summer, these rates were significantly related to lake morphology in winter but trophic state in summer. In assessing cross-seasonal linkages, we found limited evidence for significant legacy effects in deepwater DO availability. Only fall mixing efficacy significantly responded to the previous summer’s minimum DO saturation, but it always reached moderate to high DO replenishment levels (>65%) regardless of the previous summer’s DO depletion severity. This lack of ecological memory in deepwater DO depletion across seasons suggests that deepwater DO largely resets during spring and fall mixing periods in most years in these dimictic lakes. Understanding the patterns and drivers in deepwater DO depletion in both winter and summer is a key step forward for predicting future chemical and biological consequences of seasonal DO depletion and managing lake ecosystem health, as well as the effects that climate change may have on these patterns.

• Parra, Nicolas Valiente; Eiler, Alexander; Allesson, Lina; Andersen, Tom; Clayer, Francois & Crapart, Camille Marie [Show all 13 contributors for this article] (2022). Catchment properties as predictors of greenhouse gas concentrations across a gradient of boreal lakes. Frontiers in Environmental Science. 10. doi: 10.3389/fenvs.2022.880619. Full text in Research Archive Show summary

  Boreal lakes are the most abundant lakes on Earth. Changes in acid rain deposition, climate, and catchment land use have increased lateral fluxes of terrestrial dissolved organic matter (DOM), resulting in a widespread browning of boreal freshwaters. This browning affects the aqueous communities and ecosystem processes, and boost emissions of the greenhouse gases (GHG) CH 4 , CO 2 , and N 2 O. In this study, we predicted biotic saturation of GHGs in boreal lakes by using a set of chemical, hydrological, climate, and land use parameters. For this purpose, concentrations of GHGs and nutrients (organic C, -P, and -N) were determined in surface water samples from 73 lakes in south-eastern Norway covering wide ranges in DOM and nutrient concentrations, as well as catchment properties and land use. The spatial variation in saturation of each GHG is related to explanatory variables. Catchment characteristics (hydrological and climate parameters) such as lake size and summer precipitation, as well as NDVI, were key determinants when fitting GAM models for CH 4 and CO 2 saturation (explaining 71 and 54%, respectively), while summer precipitation and land use data were the best predictors for the N 2 O saturation, explaining almost 50% of deviance. Our results suggest that lake size, precipitation, and terrestrial primary production in the watershed control the saturation of GHG in boreal lakes. These predictions based on the 73-lake dataset was validated against an independent dataset from 46 lakes in the same region. Together, this provides an improved understanding of drivers and spatial variation in GHG saturation in boreal lakes across wide gradients of lake and catchment properties. The assessment highlights the need to incorporate multiple explanatory parameters in prediction models of GHGs for extrapolation across the boreal biome.

• Forsius, Martin; Posch, Maximilian; Holmberg, Maria; Vuorenmaa, Jussi; Kleemola, Sirpa & Augustaitis, Algirdas [Show all 25 contributors for this article] (2021). Assessing critical load exceedances and ecosystem impacts of anthropogenic nitrogen and sulphur deposition at unmanaged forested catchments in Europe. Science of the Total Environment. ISSN 0048-9697. 753. doi: 10.1016/j.scitotenv.2020.141791. Full text in Research Archive Show summary

  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.

• Clayer, Francois; Thrane, Jan-Erik; Brandt, Uta; Dörsch, Peter & Wit, Heleen de (2021). Boreal Headwater Catchment as Hot Spot of Carbon Processing From Headwater to Fjord. Journal of Geophysical Research (JGR): Biogeosciences. ISSN 2169-8953. 126(12). doi: 10.1029/2021JG006359. Full text in Research Archive Show summary

  Boreal headwaters and aquatic sediments are significant transporters, stores and processors of terrestrial carbon (C) as well as emitters of greenhouse gases (GHGs), mainly CO2 and CH4. While terrestrial ecosystems are net sinks of atmospheric C, lateral fluxes of total organic and dissolved inorganic C (TOC, DIC) as well as GHG release can significantly reduce the land C sink. However, the fate of the laterally exported C remains often unresolved. Here, we combine datasets from high-frequency sensors, monitoring and modeling to produce a C budget for a typical boreal lake and its catchment and examine it in a regional context. We show that lake TOC removal is dominated by microbial metabolism which shows strong seasonal fluctuations following stratification and ice-cover dynamics. We estimate that 11.5 g C m−2 catchment yr−1 are exported from the catchment soils and wetlands, including 9.5 g of TOC. Only 5.4 g C m−2 catchment yr−1 reach the coast while over 50% is reemitted to the atmosphere (5.7 g C m−2 catchment yr−1). Part of the reemitted C originates from rapid turnover cycles and would not affect atmospheric CO2 and the land C sink. However, we show that the land C sink at Langtjern is reduced by >14% over the whole aquatic continuum, and by >10% within the headwater catchment. Our regional analysis suggests that headwater lake TOC mineralization is the main TOC loss along the aquatic continuum with burial being relatively small, and highlights the significance of small headwaters as intensive and fast-responding TOC processors.

• Wit, Heleen de; Stoddard, John L.; Monteith, Donald T.; Sample, James E.; Austnes, Kari & Couture, Suzanne [Show all 17 contributors for this article] (2021). Cleaner air reveals growing influence of climate on dissolved organic carbon trends in northern headwaters. Environmental Research Letters. 16(10). doi: 10.1088/1748-9326/ac2526. Full text in Research Archive Show summary

  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.

• Marttila, Hannu; Lepistö, Ahti; Tolvanen, Anne; Bechmann, Marianne; Kyllmar, Katarina & Juutinen, Artti [Show all 19 contributors for this article] (2020). Potential impacts of a future Nordic bioeconomy on surface water quality. Ambio. ISSN 0044-7447. 49, p. 1722–1735. doi: 10.1007/s13280-020-01355-3. Full text in Research Archive Show summary

  Nordic water bodies face multiple stressors due to human activities, generating diffuse loading and climate change. The ‘green shift’ towards a bio-based economy poses new demands and increased pressure on the environment. Bioeconomy-related pressures consist primarily of more intensive land management to maximise production of biomass. These activities can add considerable nutrient and sediment loads to receiving waters, posing a threat to ecosystem services and good ecological status of surface waters. The potential threats of climate change and the ‘green shift’ highlight the need for improved understanding of catchment-scale water and element fluxes. Here, we assess possible bioeconomy-induced pressures on Nordic catchments and associated impacts on water quality. We suggest measures to protect water quality under the ‘green shift’ and propose ‘road maps’ towards sustainable catchment management. We also identify knowledge gaps and highlight the importance of long-term monitoring data and good models to evaluate changes in water quality, improve understanding of bioeconomy-related impacts, support mitigation measures and maintain ecosystem services.

• Bishop, Kevin; Shanley, James B.; Riscassi, Ami; Wit, Heleen de; Eklöf, Karin & Meng, Bo [Show all 11 contributors for this article] (2020). Recent advances in understanding and measurement of mercury in the environment: Terrestrial Hg cycling. Science of the Total Environment. ISSN 0048-9697. 721. doi: 10.1016/j.scitotenv.2020.137647. Full text in Research Archive Show summary

  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:• The Arctic has emerged as a hotbed of Hg cycling, with high stream fluxes and large stores of Hg poised for release from permafrost with rapid high-latitude warming. • The bi-directional exchange of Hg between the atmosphere and terrestrial surfaces is better understood, thanks largely to interpretation from Hg isotopes; the latest estimates place land surface Hg re-emission lower than previously thought. • Artisanal gold mining is now thought responsible for over half the global stream flux of Hg. • There is evidence that decreasing inputs of Hg to ecosystems may bring recovery sooner than expected, despite large ecosystem stores of legacy Hg. • Freshly deposited Hg is more likely than stored Hg to methylate and be incorporated in rice. • Topography and hydrological connectivity have emerged as master variables for explaining the disparate response of THg and MeHg to forest harvest and other land disturbance. 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.

• Braaten, Hans Fredrik Veiteberg; Lindholm, Markus & Wit, Heleen A de (2020). Five decades of declining methylmercury concentrations in boreal foodwebs suggest pivotal role for sulphate deposition. Science of the Total Environment. ISSN 0048-9697. 714. doi: 10.1016/j.scitotenv.2020.136774. Full text in Research Archive Show summary

  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.

• Strand, Line Tau; Fjellstad, Wendy; Jackson-Blake, Leah Amber & Wit, Heleen A de (2020). Afforestation of a pasture in Norway did not result in higher soil carbon, 50 years after planting. Landscape and Urban Planning. ISSN 0169-2046. 207. doi: 10.1016/j.landurbplan.2020.104007. Full text in Research Archive Show summary

  Afforestation of marginal cultivated land is an internationally approved climate mitigation strategy, however, with uncertain implications for soil organic carbon (SOC) storage. We examined the effect of forest planting by measuring SOC at two adjacent sites: one with a Norway spruce forest planted in 1968 and one actively grazed pasture. Both sites had similar land-use history before forest planting, and they were as similar as possible in all other edaphic factors. There were no significant differences in SOC stocks down to 30 cm mineral soil, 7.15 and 8.51 kg C m−2 in the forest plantation and pasture respectively. Only a minimal build-up of an O horizon, less than 2 cm, was observed in the plantation. The SOC stocks of the plantation and pasture were not significantly different from that of a nearby old forest, 7.17 kg C m−2. When comparing these three land-uses we found that there were significant differences in the upper 10 cm of the soil with regard to other soil properties. Nitrogen (N) stock and pH were significantly lower in the old forest compared to the plantation, which again was significantly lower than that of the pasture. The opposite was the case for the C/N ratio. We conclude that there were no significant differences in SOC stocks in the upper 30 cm 50 years after afforestation with Norway spruce, but that there is still a legacy from the former cultivation that may influence both productivity and organic matter dynamics.

• Wit, Heleen A de; Lepistö, Ahti; Marttila, Hannu; Wenng, Hannah; Bechmann, Marianne & Blicher-Mathiesen, Gitte [Show all 12 contributors for this article] (2020). Land-use dominates climate controls on nitrogen and phosphorus export from managed and natural Nordic headwater catchments. Hydrological Processes. ISSN 0885-6087. 34(25), p. 4831–4850. doi: 10.1002/hyp.13939. Full text in Research Archive Show summary

  Agricultural, forestry‐impacted and natural catchments are all vectors of nutrient loading in the Nordic countries. Here, we present concentrations and fluxes of total nitrogen (totN) and phosphorus (totP) from 69 Nordic headwater catchments (Denmark: 12, Finland:18, Norway:17, Sweden:22) between 2000 and 2018. Catchments span the range of Nordic climatic and environmental conditions and include natural sites and sites impacted by agricultural and forest management. Concentrations and fluxes of totN and totP were highest in agricultural catchments, intermediate in forestry‐impacted and lowest in natural catchments, and were positively related %agricultural land cover and summer temperature. Summer temperature may be a proxy for terrestrial productivity, while %agricultural land cover might be a proxy for catchment nutrient inputs. A regional trend analysis showed significant declines in N concentrations and export across agricultural (−15 μg totN L−1 year−1) and natural (−0.4 μg NO3‐N L−1 year−1) catchments, but individual sites displayed few long‐term trends in concentrations (totN: 22%, totP: 25%) or export (totN: 6%, totP: 9%). Forestry‐impacted sites had a significant decline in totP (−0.1 μg P L−1 year−1). A small but significant increase in totP fluxes (+0.4 kg P km−2 year−1) from agricultural catchments was found, and countries showed contrasting patterns. Trends in annual concentrations and fluxes of totP and totN could not be explained in a straightforward way by changes in runoff or climate. Explanations for the totN decline include national mitigation measures in agriculture international policy to reduced air pollution and, possibly, large‐scale increases in forest growth. Mitigation to reduce phosphorus appears to be more challenging than for nitrogen. If the green shift entails intensification of agricultural and forest production, new challenges for protection of water quality will emerge possible exacerbated by climate change. Further analysis of headwater totN and totP export should include seasonal trends, aquatic nutrient species and a focus on catchment nutrient inputs.

• Kaste, Øyvind; Austnes, Kari & Wit, Heleen de (2020). Streamwater responses to reduced nitrogen deposition at four small upland catchments in Norway. Ambio. ISSN 0044-7447. 49, p. 1759–1770. doi: 10.1007/s13280-020-01347-3. Full text in Research Archive Show summary

  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.

• Hagman, Camilla Hedlund Corneliussen; Skjelbred, Birger; Thrane, Jan-Erik; Andersen, Tom & Wit, Heleen A de (2019). Growth responses of the nuisance algae Gonyostomum semen (Raphidophyceae) to DOC and associated alterations of light quality and quantity. Aquatic Microbial Ecology. ISSN 0948-3055. 82(3), p. 241–251. doi: 10.3354/ame01894. Full text in Research Archive Show summary

  In recent decades, the freshwater alga Gonyostomum semen (Ehrenberg) Diesing has increased in abundance and distribution, which has created problems regarding the use of lakes for drinking water and recreation. Increases in lake dissolved organic carbon (DOC; browning) and climate warming are among current hypotheses that explain the phenomenon of increased abundance of G. semen. A replicated, multifactorial laboratory experiment was performed in order to test the effects of DOC and light quality and irradiance on growth of G. semen in 96-well plates. Each well had a separate light source, creating a gradient of irradiance (10-140 µmol photons m-2 s-1) and wavelength (white, red, green and blue), combined with a DOC gradient (0-50 mg C l-1), all within representative ranges for the summer epilimnion in boreal waters. The experiment showed that growth of G. semen was significantly enhanced by the addition of DOC. No growth response relating to wavelengths was found, but where irradiance was inhibitory, a shading effect from DOC was evident. A composed response to increases in DOC suggests a versatility of G. semen that gives it a competitive advantage over other phytoplankton in humic lake ecosystems. Hence, further increases in abundance and bloom frequency of G. semen as browning continues in boreal lakes are to be expected.

• Braaten, Hans Fredrik Veiteberg; Åkerblom, Staffan; Kahilainen, Kimmo Kalevi; Rask, Martti; Vuorenmaa, Jussi & Mannio, Jaakko [Show all 15 contributors for this article] (2019). Improved Environmental Status: 50 Years of Declining Fish Mercury Levels in Boreal and Subarctic Fennoscandia. Environmental Science and Technology. ISSN 0013-936X. 53(4), p. 1834–1843. doi: 10.1021/acs.est.8b06399. Full text in Research Archive Show summary

  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.

• Weyhenmeyer, Gesa A; Hartmann, Jens; Hessen, Dag O; Kopáček, Jiří; Hejzlar, Josef & Jacquet, Stéphan [Show all 29 contributors for this article] (2019). Widespread diminishing anthropogenic effects on calcium in freshwaters. Scientific Reports. 9(1). doi: 10.1038/s41598-019-46838-w. Full text in Research Archive Show summary

  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.

• McGovern, Maeve; Evenset, Anita; Borgå, Katrine; Wit, Heleen de; Braaten, Hans Fredrik Veiteberg & Hessen, Dag Olav [Show all 9 contributors for this article] (2019). Implications of Coastal Darkening for Contaminant Transport, Bioavailability, and Trophic Transfer in Northern Coastal Waters. Environmental Science and Technology. ISSN 0013-936X. 53(13), p. 7180–7182. doi: 10.1021/acs.est.9b03093. Full text in Research Archive Show summary

  The increased transport of terrestrially derived organic material (tOM) from catchments to rivers and lakes is leading to browning of northern freshwaters.(1) Although the same environmental drivers can be expected to lead to widespread increases in tOM and subsequent darkening in adjacent coastal systems,(2) this has received considerably less attention, and long-term monitoring data for detecting these changes are scarce. This is despite expectations that darkening is likely to have a range of implications for coastal biogeochemistry and food-web ecology. tOM is accompanied by other catchment-derived materials including contaminants, inorganic particles, and nutrients. Together, these terrestrial inputs can affect cycling and food-web accumulation of contaminants, in particular, methyl mercury (MeHg) and persistent organic pollutants (POPs). Because of the human and ecosystem health risks associated with these contaminants, and to ensure informed policy decisions, there is a need for a better understanding of the drivers of darkening and their direct and indirect impacts on the transport, bioavailability, and trophic transfer of contaminants in northern coastal waters.

• Krzeminski, Pawel; Vogelsang, Christian; Meyn, Thomas; Köhler, Stephan; Poutanen, Heikki & Wit, Heleen de [Show all 7 contributors for this article] (2019). Natural organic matter fractions and their removal in full-scale drinking water treatment under cold climate conditions in Nordic capitals. Journal of Environmental Management (JEM). ISSN 0301-4797. 241, p. 427–438. doi: 10.1016/j.jenvman.2019.02.024. Full text in Research Archive Show summary

  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.

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• Austnes, Kari; Wit, Heleen de; Vogt, Rolf David; Arle, Jens; Balkoni, Areti & Franssen, Louise [Show all 26 contributors for this article] (2026). Forty years of studying effects of sulphur and nitrogen deposition in freshwaters in Europe and North America. Full text in Research Archive Show summary

  The International Cooperative Programme on Assessment and Monitoring of the Effects of Air Pollution on Rivers and Lakes (ICP Waters) was established in 1985 under the UNECE Convention on Long-range Transboundary Air Pollution (CLRTAP). An important basis for the work of ICP Waters is the long-term data submitted by participating countries, comprising over 500 sites in Europe and North America. The most recent trend analysis (1990-2020) showed declining sulphate and nitrate concentrations and increasing acid neutralising capacity (ANC) and pH, as expected given the general decline in sulphur and nitrogen deposition. There were clear regional differences, with the largest changes occurring in the more heavily affected regions. Trend trajectories of sulphate, ANC and pH indicated that chemical recovery has decelerated in Europe and accelerated in North America, which can be attributed to differences in the timing of regulatory measures. Nitrate declined to a lower extent than sulphate. This can be related to continued nitrogen retention within the catchments. Another ICP Waters analysis, targeted at nitrogen, showed that the strongest decline in nitrate concentrations occurred at sites with high nitrogen deposition. As a large share of sites show decreasing nitrate trends and an increase in the dissolved organic matter carbon to nitrogen ratios, there are few indications of widespread nitrogen saturation following long-term nitrogen deposition. Total organic carbon concentrations have increased, in line with the established connection between declining acid deposition and increased organic matter solubility. The increase has decelerated in Europe. Calcium concentrations declined less rapidly than sulphate concentrations, contrary to expectations based on charge balance. In low deposition regions, even an increase in calcium is observed. It is hypothesized that enhanced vegetation growth and longer growing seasons at less acid-sensitive sites can increase chemical weathering. Further chemical recovery was modelled at selected ICP Waters sites using the MAGIC model with different deposition scenarios. Current legislation is predicted to give considerable improvements in water chemistry in acidic lakes across Europe by 2040. Additional improvement from proposed revised legislation is limited, owing to relatively small additional deposition reduction and that sulphur deposition is already at a low level given current legislation.

• Vogt, Rolf David; Sekkenes, Marianne Stave; Norling, Magnus Dahler; Austnes, Kari; Wit, Heleen de & Kaste, Øyvind (2026). Climate effects on water chemistry in acid sensitive catchments. Full text in Research Archive Show summary

  Substantial declines in acidifying emissions across Europe have led to pronounced chemical recovery of Norwegian surface waters. In recent decades, however, changes in water chemistry increasingly coincide with climate change, complicating attribution of observed trends to drivers. This study assesses whether ongoing climate change has produced detectable effects on freshwater chemistry in Norway and how such effects vary among catchments with differing sensitivity to acidification. The Model of Acidification of Groundwater In Catchments (MAGIC) was used to simulate the effects of declining acid deposition. Deviations between observed and modelled water chemistry were tentatively interpreted as climate-related effects. Analyses were based on long-term monitoring data (1986–2022) from 59 acid-sensitive Trend Lakes distributed across Norway and four Field Research Stations representing contrasting hydroclimatic and biogeochemical conditions. Temporal trends were evaluated using Pearson correlation. Their relationships with climatic variables and their Mann-Kendall significant Sen’s time slopes were examined using regression and multivariate analyses. Across the Trend Lakes, climate effects were predominantly positive for acid neutralising capacity (ANC) and base cations, suggesting that climate change contributes to accelerated chemical recovery, at least in the less acidification-sensitive catchments. Climate effects varied substantially between the Field Research Stations, with increased temperature generally enhancing recovery, possibly through intensified weathering. In contrast, increased precipitation and runoff reduced recovery rates. Overall, the results suggest that climate change exerts a measurable influence on freshwater chemistry in Norway, strongly modulated by catchment characteristics.

• Wit, Heleen de; Berntsen, Terje Koren; Worner, Elisabeth-Christina; Jackson-Blake, Leah Amber; Clayer, Francois & lu, xiang (2025). Integration of terrestrial and aquatic C fluxes. Full text in Research Archive
• Wit, Heleen de; Jackson-Blake, Leah Amber; Berntsen, Terje Koren; Worner, Elisabeth-Christina; Lange, Holger & Clayer, Francois [Show all 7 contributors for this article] (2025). Integration of terrestrial and aquatic C fluxes. Case study Langtjern. Full text in Research Archive
• Sevillano, Ignacio; Granhus, Aksel; Fernandéz, Clara Antón; Wit, Heleen de; Schei, Fride Høistad & Jacobsen, Rannveig Margrete [Show all 14 contributors for this article] (2025). Effects on carbon dynamics of increased use of continuous cover forestry in Norwegian forests. Full text in Research Archive Show summary

  There is an increasing interest in continuous cover forestry (CCF) as an alternative to clearcutting to promote multi-objective forests and preserve continuous maintenance of forest cover. Here, we assessed the effect that an increased use of CCF harvesting methods (shelterwood and selection cutting) in Norwegian forests can have on carbon sequestration. Thus, we simulated CO2 uptake in Norwegian forest stands throughout the 21st century under three scenarios that represent different levels of clearcutting and CCF harvesting methods, keeping the annual harvest volumes constant across all scenarios. The three scenarios are: 1) Business-as-usual (reference scenario where 3.5% of the harvested volume is obtained using CCF harvesting methods); 2) Harvested volume using CCF harvesting methods is increased to 15%; 3) Harvested volume using CCF harvesting methods is increased to 25%. Increasing the proportion of CCF would increase CO2 removals in the long-term (2100), resulting in an additional uptake of nearly 32 and 24 Tg CO2 when increasing CCF up to 25% and 15%, respectively. However, the simulations also showed that to be able to harvest the same timber volume as in the reference scenario that reflects current practice, an increased proportion of CCF would also require logging on a larger proportion of the forest area. CCF could have also positive implications for certain aspects of biodiversity, such as species that require shaded conditions, but harvesting across a larger total area could negatively impact other animals, plants and fungi.

• Vogt, Rolf David; Kaste, Øyvind; Thrane, Jan-Erik; Balkoni, Areti; Clayer, Francois & Wit, Heleen de [Show all 7 contributors for this article] (2025). Distinguishing the Effects of Climate Change and Acid Rain Recovery on Water Chemistry Trends: A Challenging Task? doi: 10.3897/aca.8.e155377. Full text in Research Archive Show summary

  This study examines whether trends in climate variables—precipitation, runoff, temperature, and actual evapotranspiration (AET)—can help explain changes in water chemistry in 78 Norwegian lakes from 1990 to 2020. AET has increased significantly in nearly all catchments, though substantial discrepancies exist between modeled and calculated values. Temperature has risen at 79% of monitoring stations, whereas precipitation and runoff exhibit significant trends in only 15% and 19% of lakes, respectively. These patterns are reflected in the limited explanatory power of climate variables for water chemistry changes. Spatial and temporal correlations between climate trends and acid rain recovery further complicate distinguishing their individual effects in simple correlation analyses. Overall, climate variables show weak explanatory power for water chemistry trends. However, multivariate redundancy analysis (RDA) provides a non-significant indication that increases in precipitation and runoff may counteract the effects of rising temperatures on water chemistry.

• Vogt, Rolf David; Austnes, Kari; Gundersen, Cathrine Brecke & Wit, Heleen de (2025). Post-Acid Rain Recovery with Increasing Calcium Concentrations - Assessing relationships between trends in base cations relative to trends in sulphate, organic anions and bicarbonate. Full text in Research Archive Show summary

  This study assesses long-term trends in surface water chemistry using data from the Norwegian national monitoring and the ICP Waters programs that originally focused on the effects of reduced sulphuric acid deposition. As was anticipated, in acid-sensitive regions heavily effected by acid rain, the widespread and substantial decline in sulphate concentrations has led to a corresponding decrease in calcium concentrations. Conversely, in regions less impacted by acidification, calcium concentrations are increasing in a large number of sites, largely in response to rising bicarbonate levels. At these sites, the increase in bicarbonate contributes to a higher acid neutralizing capacity (ANC), which more than offsets the decline in sulphate. The most likely drivers of this increase in bicarbonate include a warming climate, which promotes longer growing seasons and increased biomass production, thereby enhancing soil respiration and CO2 exchange (i.e., CO₂ pump). This process increases weathering rates and the release of base cations such as calcium. As the decline in acid deposition has stabilized and surface water pH increases above 5.6 in formely strongly acidified areas, similar increases in calcium concentrations may eventually be observed there as well. However, statistically separating the effects of acid rain recovery from those of climate change on water chemistry remains a significant challenge. The observed calcium trends may have ecological implications. Interestingly, in regions most severely impacted by acidification, no significant correlation was found between the declining calcium concentrations and biodiversity indices. In contrast, in less-affected regions, strong negative correlations were observed - contradicting conceptual expectations and highlighting the need for further investigation into how aquatic ecosystems responds to changing chemical conditions.

• Vogt, Rolf David; Kaste, Øyvind; Thrane, Jan-Erik; Balkoni, Areti; Clayer, François & Wit, Heleen de [Show all 7 contributors for this article] (2025). Is it possible to easily distinguish the effects of climate change and the decline in acid rain on trends in water chemistry? Full text in Research Archive
• Dalen, Lars Sandved; Granhus, Aksel; Storaunet, Ken Olaf; Asplund, Ulrika Jansson & Wit, Heleen de (2024). Åpner opp for mer lukket hogst | Norsk Skogbruk. [Journal]. Norsk Skogbruk. Full text in Research Archive Show summary

  Hvordan vi hogger skog påvirker både karbonfangst, biologisk mangfold, vannkvalitet og hvor mye tømmer skogen leverer, viser en ny rapport utarbeidet av

• Dalen, Lars Sandved; Granhus, Aksel; Asplund, Ulrika Jansson & Wit, Heleen de (2024). Hvordan vi hugger skogen kan ha mye å si. [Internet]. forskning.no. Full text in Research Archive Show summary

  Hvordan vi hogger skog påvirker både karbonfangst, biologisk mangfold, vannkvalitet og hvor mye tømmer skogen leverer.

• Wit, Heleen de; Clayer, Francois; Kaste, Øyvind & Norling, Magnus Dahler (2024). Using MAGIC to describe the transition from anthropogenic to natural acidification in a humic catchment. Full text in Research Archive
• Wit, Heleen de & Austnes, Kari (2024). Effects of air pollution, climate and land use on aquatic ecosystems. Full text in Research Archive
• Vogt, Rolf David; Austnes, Kari; Wit, Heleen de & Gundersen, Cathrine Brecke (2024). Unexpected increases in Calcium - are we moving into a post-acid-rain era? Full text in Research Archive
• Vogt, Rolf David; Garmo, Øyvind Aaberg; Austnes, Kari; Kaste, Øyvind; Haaland, Ståle Leif & Sample, James Edward [Show all 10 contributors for this article] (2024). Factors Governing Site and Charge Density of Dissolved Natural Organic Matter (DNOM). Full text in Research Archive
• Dalen, Lars Sandved; Granhus, Aksel; Storaunet, Ken Olaf; Asplund, Ulrika Jansson & Wit, Heleen de (2024). Åpner opp for mer lukket hogst. [Journal]. Norsk Skogbruk. Full text in Research Archive
• Wit, Heleen de; Knutson, Jacqueline Kay; clayer, francois & Dörsch, Peter (2023). Contrasting water chemistry and dissolved gas concentrations in degrading thermokarst water bodies compared with surrounding wetland streams, in Iškoras, northern Norway. Full text in Research Archive
• Wit, Heleen de (2023). Effekter av skogsdrift på avrenning av næringsstoffer og mobilisering av kvikksølv. Full text in Research Archive
• Wit, Heleen de (2023). Overraskende utviklinger i vannkvalitet i NorgeBegynner vi å se effekter av klimaendringer? Full text in Research Archive
• Wit, Heleen de; Basso, Stefano & Balkoni, Areti (2023). Fare for økt nitrogenavrenning ved endret klima. Full text in Research Archive
• Vogt, Rolf David; Austnes, Kari; Wit, Heleen de & Gundersen, Cathrine Brecke (2023). Diverging trends in Ca and Mg under chemical recovery. Full text in Research Archive
• Wit, Heleen de (2022). Riverine transport and processing of organic carbon from headwaters to coast in subarctic Norway under global change. Full text in Research Archive
• Wit, Heleen de; Clayer, Francois; Jackson-Blake, Leah Amber; Vogt, Rolf David & Garmo, Øyvind Aaberg (2022). Repeated sampling of thousand Norwegian lakes reveals widespread changes in silica, calcium, aluminium and DOC. Full text in Research Archive
• Wit, Heleen de (2022). Riverine transport and processing of organic carbon from headwaters to coast in subarctic Norway under global change. Full text in Research Archive
• Wit, Heleen de (2022). Presentation of ICP Waters. Full text in Research Archive
• Knutson, Jacqueline; Clayer, Francois; Dörsch, Peter; Westermann, Sebastian & Wit, Heleen de (2022). Investigating hydrology and carbon cycling connections in peatland permafrost, northern Norway. Full text in Research Archive
• Vogt, Rolf David; Wit, Heleen de & Koponen, Kati (2022). Impacts of large-scale forest related NETPs on climate and ecosystem services in Nordic region . Full text in Research Archive
• Vogt, Rolf David; Wit, Heleen de & Koponen, Kati (2022). Impacts of large-scale forest related NETPs ​on climate and ecosystem services in Nordic region. Full text in Research Archive
• Hessen, Dag Olav; Wit, Heleen de & Vogt, Rolf David (2021). Mindre forsuring, mer skog og brunere kystvann – er det noen sammenheng? Naturen. ISSN 0028-0887. 145(5), p. 236–249. Full text in Research Archive
• Knutson, Jacqueline; Clayer, Francois; Norling, Magnus Dahler; Lepistö, Ahti; Marttila, Hannu & Futter, Martyn N. [Show all 8 contributors for this article] (2021). Hydrological controls of DOC export from Nordic headwater catchments. Full text in Research Archive
• Knutson, Jacqueline; Clayer, Francois; Brandt, Uta & Wit, Heleen de (2021). Investigating hydrology and carbon cycling in a permafrost palsa: preliminary data from Iskoras. Full text in Research Archive
• Wit, Heleen de; Braaten, Hans Fredrik Veiteberg; Gundersen, Cathrine Brecke; Krzeminski, Pawel & Vogt, Rolf David (2019). Long-term Mercury records in a boreal lake: lake Processing and the role of organic matter. Full text in Research Archive
• Wit, Heleen de (2019). Progress report ICP Waters. Full text in Research Archive Show summary

  Faglig rapportering om aktiviteter under ICP Waters. Bl.a. rapportering om kvikksølv i miljøet.

• Krzeminski, Pawel Konrad; Wit, Heleen de & Uhl, Wolfgang (2019). Natural organic matter variability and removal in main drinking water treatment plants of Northern European capital cities. Full text in Research Archive
• Wit, Heleen de (2019). Klimaendring og vannkvalitet. Kjemisk vannkvalitet – hva kan vi vente oss? Full text in Research Archive
• Gundersen, Cathrine Brecke; Krzeminski, Pawel; Martinez-Frances, Elena; Ong, Stevenson Harvey Lee; Færgestad, Eline Mosleth & Braaten, Hans Fredrik Veiteberg [Show all 8 contributors for this article] (2019). In-Lake Processing Influences DOM Biodegradability and Hg-cycling. Full text in Research Archive
• Braaten, Hans Fredrik Veiteberg; Åkerblom, Staffan; Wit, Heleen de; Poste, Amanda; Rask, Martti & Vuorenmaa, Jussi [Show all 8 contributors for this article] (2018). Mercury in freshwater fish from Fennoscandia (1965-2015) - temporal trends and potential environmental drivers. Full text in Research Archive
• Knutson, Jacqueline Kay; Wit, Heleen de; Westermann, Sebastian; Clayer, Francois & Hessen, Dag Olav (2026). Transport and Processing of Dissolved Organic Carbon in Peatland-Dominated Catchments in Subarctic Scandinavia. Universitetet i Oslo. Full text in Research Archive Show summary

  Northern subarctic peatland landscapes contain some of the largest terrestrial carbon reservoirs on Earth. Over thousands of years, cold climates, waterlogged soils, and slow decomposition have allowed plant material to accumulate and be stored as peat. These carbon stores, however, are not stable under a warming climate. In Arctic and subarctic regions, temperatures are rising faster than the global average, altering hydrology, ecosystem structure, and, where present, the stability of permafrost in many peatland systems. As permafrost thaws, it not only changes the ground surface, but also the pathways through which carbon moves in the landscape. The collapse of ice-rich permafrost in peat can create ponds, wetlands, and new drainage features, forming a dynamic network of aquatic environments. These waters can act both as sites where carbon is stored and transformed, and as conduits that transport carbon further through streams and rivers toward downstream and coastal ecosystems. Inland waters therefore play an important role by linking terrestrial carbon sources to aquatic systems while simultaneously transforming dissolved organic carbon (DOC) through microbial mineralization, photochemical reactions, and gas exchange with the atmosphere. In peatland-dominated landscapes, large quantities of DOC can be mobilized when hydrological flow paths connect organic soils to aquatic networks. Whether this carbon is retained, transformed, or transported down stream depends strongly on how water moves through the landscape and how connected different parts of the system are. This thesis investigates how permafrost degradation, hydrologic connectivity, and landscape composition interact to regulate DOC dynamics in peatland-dominated ecosystems of subarctic Scandinavia. The work combines detailed field investigations in a thawing peatland headwater catchment with regional analyses of DOC patterns across the Tana River basin. By linking observations from small-scale peatland systems to patterns at the river-basin scale, this thesis aims to improve understanding of how carbon moves through northern latitude landscapes. Investigations of the processes governing carbon dynamics were conducted in the Iškoras peatland complex on the Finnmarksvidda plateau in northern Norway, a degrading permafrost palsa mire within the sporadic permafrost zone. As one of the southernmost and most rapidly degrading permafrost peatland systems in Fennoscandia, Iškoras represents a landscape where thaw-driven hydrological and biogeochemical changes are already well advanced. This makes it a natural setting for studying processes that are likely to become more widespread in currently more stable permafrost regions as permafrost continues to thaw. Repeated measurements of water chemistry, dissolved greenhouse gases (GHG), and hydrological conditions were used to examine how carbon is mobilized, transformed, and transported across different parts of the system. The results show that permafrost degradation creates a mosaic of aquatic environments with distinct hydrochemical characteristics. Small ponds formed by thaw contain elevated DOC concentrations and high supersaturation of carbon dioxide (CO2) and methane(CH4), reflecting strong inputs of organic matter from thawing peat and active GHG production. In contrast, downstream stream waters show lower DOC concentrations and a stronger integration of ground water and catchment scale inputs. Hydrological conditions play a key role: when waters are isolated, DOC concentrations can increase, whereas in creased connectivity promotes dilution and downstream trans port. At the regional scale, analyses across 90 catchments of the Tana River basin show that wetland and peatland coverage strongly predicts DOC as well as iron concentrations, highlighting peatlands as major sources of DOC. The findings in this thesis demonstrate that DOC dynamics in subarctic peatland landscapes are governed by interactions between landscape structure, hydrological connectivity, and temperature-sensitive biogeochemical processes associated with permafrost thaw. As northern latitude regions continue to warm, these interactions will be crucial in determining whether carbon stored in peat soils is retained locally, released to the atmosphere, or transported through river networks.

• Hasegawa, Shun; Aas, Kjetil Schanke; Asplund, Ulrika Jansson; Dalsgaard, Lise; Wit, Heleen de & Hagenbo, Andreas [Show all 8 contributors for this article] (2025). Soil Carbon Dynamics and Biodiversity in Boreal Forests from Harvest Maturity to Old-Growth: A Research Overview. NIBIO. ISSN 9788217038771. Full text in Research Archive Show summary

  Norwegian forests cover 12 million hectares and are vital for carbon uptake and biodiversity, yet CO2 absorption has declined since 2010 due to increased harvesting, mortality and reduced growth as more forests surpass harvest maturity. With 45% now economically mature and 20% older than 120 years, the future carbon uptake of these stands is uncertain, particularly if they develop towards old-growth. Old-growth forests form without stand-replacing disturbances and have diverse structures and deadwood. Norwegian mature forests mostly originate from clear-cutting, so insights from primary old-growth must be applied with caution. After maturity, forests continue to sequester carbon but more slowly, with increasing storage in deadwood and soil. Soil carbon trajectories remain uncertain: disturbance often causes short-term losses followed by decades of accumulation. Microbial communities, especially fungi, influence long-term soil carbon, but data are limited. Norway uses the Yasso soil carbon model, which predicts continued soil carbon increases with age though at slowing rates; however, it simplifies key processes, and more advanced models are in development. Biodiversity supports carbon cycling, resilience and soil health, yet knowledge gaps persist. Climate change is expected to increase disturbances, raising long-term risks for older stands. The report highlights the need for improved monitoring, research and modelling to better understand carbon dynamics and resilience as forests age.

• Gundersen, Cathrine Brecke; Velasco, Manuel Toro; Velle, Gaute; Rogora, Michela; Hawley, Kate Louise & Fölster, Jens [Show all 18 contributors for this article] (2025). ICP Waters Programme Manual. Norsk institutt for vannforskning (NIVA). ISSN 9788257778187. Full text in Research Archive Show summary

  This report contains the latest updated version of the ICP Waters manual and presents guidance for monitoring of air pollution impacts in surface waters, covering acidification, nutrient enrichment and trace metal pollution. The chapters include general introduction to the LRTAP Convention and ICP Waters, chemical and biological effect monitoring of air pollution, and the design of a monitoring program. Then, organized according to sample type (e.g. water, groups of aquatic organisms, benthic macro-invertebrates, benthic diatoms, fish, and sediments) principles for monitoring of acidification, nutrient enrichment and trace metals pollution are treated subsequently, where appropriate. Finally, information on data reporting and inter-laboratory comparison tests is presented. The ICP Waters manual is relevant for surface water monitoring, not only under the LRTAP Convention, but also for the EU NEC Directive, the EU Water Framework Directive, the UNEP Minamata Convention, and for monitoring of arctic environments. -- Dette er en revidert versjon av rapport nr. 8047-2025.

• Aarønæs, Maja Stade; Grainger, Matthew; Søgaard, Gunnhild; Jansson, Ulrika; Wit, Heleen de & Lindemann, Christian [Show all 15 contributors for this article] (2025). Utvikling av framskrivinger og scenarioer for norsk natur. Norsk institutt for naturforskning (NINA). ISSN 9788242653505. Full text in Research Archive Show summary

  - Norge - Framskrivinger (Business as usual) - Scenarioer - Direkte og indirekte drivere - Modellering - Økologisk tilstand

• Skarbøvik, Eva; Jackson-Blake, Leah Amber; Sample, James Edward; Wit, Heleen de & Clarke, Nicholas (2024). Beregning av næringsstoffavrenning fra skogarealer og skogsdrift til Oslofjorden. Med vurdering av kunnskaps- og databehov. NIBIO. ISSN 9788217034988. Full text in Research Archive Show summary

  Effekten av skogbruk på eutrofibelastningen i Oslofjorden er anslått basert på tilførsler av løst uorganisk nitrogen (DIN). Dette anslaget er beheftet med en rekke usikkerheter som kan forbedres ved å bl.a. samle inn flere data om effekten av skogsdrift på vann, samt forbedre anslag over årlig hogstareal. Våre anslag antyder at tap av nitrogen fra skogsdrift til Oslofjorden ligger på nivå med tapet fra spredt avløp og bebyggelse. På side 6 finnes et utvidet sammendrag. The effect of forestry on the eutrophic pressure of the Oslo Fjord has been estimated based on fluxes of DIN (dissolved inorganic nitrogen). This estimate is subject to many uncertainties that can be improved by, among other things, collecting more data on the effect of forestry on water, as well as improving estimates of annual logging area. Our estimate suggests that losses of nitrogen to the Oslo Fjord from forestry are in the same range as losses from households not connected to sewage treatment plants and from settlements.

• Vogt, Rolf David; Arle, Jens; Austnes, Kari; Dam, Herman Van; Futter, Martyn & Fölster, Jens [Show all 20 contributors for this article] (2024). Trends and patterns in surface water chemistry in Europe and North America between 1990 and 2020, with a focus on calcium. Norsk institutt for vannforskning (NIVA). ISSN 9788257776671. Full text in Research Archive Show summary

  The report presents trends in major anions and cations, pH, TOC and bicarbonate in surface waters in Europe and North America from 1990 to 2020. Special attention is given to trends in calcium, which showed some unexpected increases. The trends in calcium are analysed in relation to changes in bicarbonates, organic anions, and deposition loads. The surface waters show strong signs of chemical recovery.

• Granhus, Aksel; Antón-Fernández, Clara; Wit, Heleen de; Hanssen, Kjersti Holt; Schei, Fride Høistad & Jacobsen, Rannveig Margrete Vigdisdatter [Show all 15 contributors for this article] (2024). Effekter på karbondynamikk, miljø og næring ved økt bruk av lukkede hogstformer. NIBIO. ISSN 9788217034995. Full text in Research Archive Show summary

  Miljødirektoratet har gitt NIBIO, NINA og NIVA i oppdrag å vurdere hvilken effekt økt bruk av lukkede hogstformer kan ha for karbonlagring i skog, biodiversitet, økologisk tilstand, vannkvalitet og skognæring. Oppdraget er utført ved hjelp av gjennomgang av relevant litteratur, data fra Landsskogtakseringen, modellbaserte simuleringer og ekspertvurderinger.

• Kaste, Øyvind; Schartau, Ann Kristin; Wit, Heleen de & Vogt, Rolf David (2023). Evaluation of the IKEU programme. An evaluation of the programme’s organization, its organs, financing and contributions. Norsk institutt for vannforskning (NIVA). ISSN 9788257776312. Full text in Research Archive Show summary

  IKEU (Integrated Studies of the Effects of Liming Acidified Waters) is the national monitoring programme for long-term effects of liming in Sweden. The main focus of IKEU is to study the long-term effects of liming on lakes and streams in order to improve the knowledge base for the liming activity. The main aim of this report is to evaluate the current programme structure and IKEU’s fulfilment of the four main objectives since the last evaluation in 2009. The evaluation includes an assessment of scientific quality, the usefulness of the long-term monitoring series produced by the programme, whether the programme produces relevant knowledge for decision making, and to what extent decisionmakers implement the knowledge produced by the programme.

• Moldan, Filip; Jutterström, Sara; Stadmark, Johanna; Akujärvi, Anu; Forsius, Martin & Holmberg, Maria [Show all 9 contributors for this article] (2023). NKL-1915 National Nitrogen Budgets in Scandinavia: consequences for climate change and for eutrophication – synthesis report. Report number C 814. IVL Swedish Environmental Research Institute. ISSN 9789178835676. Full text in Research Archive
• Velle, Gaute; Bodin, Christian Lucien; Arle, Jens; Austnes, Kari; Boggero, Angela & Bojkova, Jindriska [Show all 19 contributors for this article] (2023). Responses of benthic invertebrates to chemical recovery from acidification. Norsk institutt for vannforskning (NIVA). ISSN 9788257776176. Full text in Research Archive Show summary

  The report provides an assessment of biological recovery from acidification in freshwater environments in Europe. The report consists of two parts, a regional data analysis based on an international dataset of biological and water chemical records, and a collection of national contributions on monitoring and assessment of biological recovery in different countries. The regional analysis showed that 47% of all included rivers (21 sites, for the period 1994-2018) and 35% percent of all lakes (34 sites, for the period 2000 to 2018) showed significant increases in species richness. Correlations between species diversity and water chemical components (ANC, pH, SO4) were found, supporting that the biological responses were related to chemical recovery. Additionally, the composition of functional traits in rivers underwent significant changes over time. Both parts of the report demonstrate ongoing biological recovery from acidification in European acid-sensitive freshwater environments.

• Vogt, Rolf David; Wit, Heleen de & Koponen, Kati (2022). Case study on impacts of large-scale re-/afforestation on ecosystem services in Nordic regions. NEGEM project. Full text in Research Archive
• Bobbink, Roland; Loran, Christin; Tomassen, Hilde; Aazem, Khalid; Aherne, Julian & Alonso, Rocío [Show all 44 contributors for this article] (2022). Review and revision of empirical critical loads of nitrogen for Europe. German Environment Agency. Full text in Research Archive Show summary

  This report describes the scientific background and results from the review and revision of empirical critical loads of nitrogen that had been established for Europe in 2011 under the auspices of the UNECE Convention on Long-range Transboundary Air Pollution (LRTAP Convention). In 2020, the Coordination Centre for Effects started a project under the LRTAP Convention to bring empirical critical loads up to date. New relevant information from studies (2010 - summer 2021) on the impacts of nitrogen on natural and semi-natural ecosystems was incorporated in the existing European database on empirical critical loads of N (CLempN). The current review and revision used for the first time gradient studies to evaluate and determine the ClempN. The CLempN were structured according to the updated classification used within the European Nature Information System (EUNIS). Consensus on the results was reached in a UNECE expert workshop on empirical critical loads of nitrogen (26-28 October 2021, Berne, Switzerland), organised by the Swiss Federal Office for the Environment (BAFU), the Coordination Centre for Effects, and the B-WARE Research Centre. The results, as presented in Table 1 of the Executive Summary, show that in many cases the outer ranges of the empirical critical loads have decreased. The resulting 2021 European database includes both revised and newly established value ranges of CLempN for each EUNIS class. The outcomes of this report are of major importance for the protection of N-sensitive natural and semi-natural ecosystems across Europe. This knowledge is used to support European policies to reduce air pollution.

• Austnes, Kari; Hjermann, Dag Øystein; Sample, James Edward; Wright, Richard Frederic; Kaste, Øyvind & Wit, Heleen de (2022). Nitrogen in surface waters: time trends and geographical patterns explained by deposition levels and catchment characteristics. Norsk institutt for vannforskning (NIVA). ISSN 9788257774646. Full text in Research Archive Show summary

  The report presents a statistical analysis of controls (deposition, land cover, climate) on spatial variability and long-term trends (1992-2016) in surface water concentrations of nitrate (NO3) and carbon to nitrogen (C/N) ratios of dissolved organic matter (DOM) at the ICP Water sites in Europe and North America. Nitrogen (N) deposition has declined over large parts of Europe and North America. Surface water NO3 has decreased significantly at 46% of sites, increased at a few sites (4%) and has had no significant trend at the remaining sites. The C/N ratio of DOM has increased at 52% of the sites, decreased at 5% and has no trend at 43% of the sites. The greatest decline in surface water NO3 occurred at sites with high N deposition and high NO3 concentrations. The increase in C/N ratios of DOM is mainly related to increasing trends in total organic carbon (TOC) and seems largely unaffected by N deposition levels or trends. Data from most of the ICP Waters sites do not suggest that N saturation is occurring presently, but there is still a risk that climate change and ecosystem disturbances might cause higher N leaching in the future.

• Poste, Amanda; Kaste, Øyvind; Frigstad, Helene; Wit, Heleen de; Harvey, E. Therese & Valestrand, Louise [Show all 10 contributors for this article] (2021). The impact of the spring 2020 snowmelt floods on physicochemical conditions in three Norwegian river-fjord-coastal systems. Norsk institutt for vannforskning (NIVA). ISSN 9788257773878. Full text in Research Archive Show summary

  Due to very high snow accumulation in the winter of 2019/2020, the Norwegian Water Resources and Energy Directorate (NVE) in late May 2020 issued flood forecasts predicting extremely high flood risk for parts of northern Norway and high flood risk for rivers in south-eastern Norway with mountainous catchments. To capture impacts of these potentially extreme snowmelt floods on water chemistry in rivers and coastal waters, extra sampling campaigns were carried out in three selected river-fjord systems: Glomma river-Ytre Oslofjord-Skagerrak in southern Norway, and the Målselv river-Målselvfjord-Straumsfjorden and Tana river-Tanafjord systems in northern Norway. The overarching goal was to study the impact of the 2020 spring freshet (i.e. snowmelt flood) on river water chemistry, land-ocean fluxes, and physicochemical conditions in coastal waters. In this report we describe between-site differences and seasonal patterns in river and coastal water chemistry based on existing data from national river (2016–2020) and coastal (2017–2020) monitoring in the three study regions, and then use these to provide context for the data from the spring 2020-sampling campaign, with a focus on suspended particulate matter (SPM), dissolved organic carbon (DOC), and nutrients. This report also highlights the complexity of identifying climate change impacts along the terrestrial-freshwater-marine continuum and the need for tighter cross-disciplinary and cross-ecosystem integration between monitoring programmes.

• Thrane, Jan-Erik; Wit, Heleen de & Austnes, Kari (2021). Effects of nitrogen on nutrient-limitation in oligotrophic northern surface waters. Norsk institutt for vannforskning (NIVA). ISSN 9788257774165. Full text in Research Archive Show summary

  Nitrogen deposition has a potentially eutrophying impact on oligotrophic freshwaters. Here we present a data analysis and a literature review with a joint focus on nitrogen (N) limitation in oligotrophic lakes and rivers. The data analysis consists of a Nordic lake dataset where relations between algal biomass (chlorophyll a), water chemistry and N deposition are investigated, and an analysis of element stoichiometry (nitrogen : phosphorus ratios) in Norwegian natural rivers. The literature review includes international publications on N limitation from a wide variation of monitoring and experimental studies. The data analysis and literature both support that N limitation exists in surface waters that receive low N deposition and that elevated N deposition can shift an ecosystem from N to P limitation. The results are used to review and revise ranges of critical loads of nitrogen for surface waters, as a contribution to the ongoing review and revision of empirical critical loads under the Convention on Long-range Transboundary Air Pollution.

• Sandin, Leonard; Thrane, Jan-Erik; Persson, Jonas; Kile, Maia Røst; Bækkelie, Knut Andreas Eikland & Myrvold, Knut Marius [Show all 11 contributors for this article] (2021). Overvåking av referanseelver - Utprøving av klassifiseringssystemet for basisovervåking i referansevassdrag. Norsk institutt for vannforskning (NIVA). ISSN 9788257773762. Full text in Research Archive Show summary

  Overvåkingsprogrammet for referanseelver har pågått siden 2017, og 2020 var andre år med undersøkelser i de 33 vannforekomstene som første gang ble undersøkt i 2018. Vannforekomstene dekker mange ulike elvetyper i flere økoregioner. I tillegg til økologisk og kjemisk tilstandsklassifisering av lite påvirkede vannforekomster, har programmet som mål styrke datagrunnlaget for fastsettelse av referanseverdier; fange opp langsiktige endringer i vanntilstand; og teste ut metodikk for tilstandsklassifisering. Basert på en samlet vurdering av alle biologiske og fysisk-kjemiske kvalitetselementer oppnådde 13 av 34 stasjoner god økologisk tilstand, og ingen svært god tilstand. Om vi ser bort fra fiskeindeksen (som i dag ikke er godt egnet til klassifisering i alle typer vannforekomster) oppnår 12 vannforekomster god tilstand og 6 svært god tilstand. Moderat eller dårligere tilstand for bunndyrindeksen for organisk belastning (ASPT), forsuringsindeksen for påvekstalger (AIP), samt forhøyede nivåer av PCB7 i fisk, er hovedårsakene til at ikke flere vannforekomster oppnår miljømålet samlet sett. Det er knyttet relativt stor usikkerhet til flere av indeksene og den samlede klassifiseringen, og det er usikkert i hvilken grad mange av vannforekomstene faktisk avviker fra referanse-tilstand. Det ble observert lave konsentrasjoner av de fleste miljøgiftene, med unntak av kvikksølv og polybromerte difenyletere (PBDE), som lå over grenseverdiene for alle 11 vannforekomster der disse var målt. For PCB7 lå 4 av 11 vannforekomster over grenseverdiene og PFOS oversteg grenseverdiene i to vannforekomster.

• Magnussen, Kristin; Aarrestad, Per Arild; Austnes, Kari; Bakkestuen, Vegar; Wit, Heleen de & Fleisje, Erlend M. [Show all 8 contributors for this article] (2021). Verdsetting av luftforurensningens kostnader for miljø – en litteraturstudie. Menon Economics. Full text in Research Archive
• Kaste, Øyvind; Skarbøvik, Eva; Clarke, Nicholas & Wit, Heleen de (2021). Gjødsling av skog - vurdering av eksisterende hensynssone og tak for nitrogengjødsling på bakgrunn av ny kunnskap. Norsk institutt for vannforskning (NIVA). ISSN 9788257773991. Full text in Research Archive Show summary

  I 2014 ble det utgitt en rapport om skoggjødsling som klimatiltak, hvor det ut fra hensyn til vannmiljø ble stipulert en hensynssone for nitrogengjødsling av skog og et arealmessig tak for gjødsling innenfor denne sonen. Hensynssonen omfattet deler av Rogaland, Sørlandet og deler av Østlandet rundt Oslofjorden, hvor det ble definert et maksimalt tak på 25 000 daa som kunne gjødsles over en tidsperiode på 5 år. Denne rapporten har vurdert konklusjonene i 2014-rapporten på nytt, og etter en gjennomgang av relevante overvåkingsdata fram til i dag, samt nyere litteratur, finner vi ikke grunnlag for verken å lempe på eller å skjerpe begrensningene innenfor hensynssonen som ble definert da tilskuddsordningen ble lansert i 2016. For å bedre kunnskapsgrunnlaget er det behov for mer systematisk og langsiktig overvåking av effekter av skoggjødsling, ulike hogstmetoder og tiltak for å redusere belastningen på vannforekomster.

• Thrane, Jan-Erik; Persson, Jonas; Kile, Maia Røst; Bækkelie, Knut Andreas Eikland; Myrvold, Knut Marius & Garmo, Øyvind Aaberg [Show all 10 contributors for this article] (2020). Overvåking av referanseelver 2019. Basisovervåking i henhold til vannforskriften. Norsk institutt for vannforskning (NIVA). ISSN 9788257772208. Full text in Research Archive Show summary

  Rapporten presenterer resultatene fra 2019, som var andre år med overvåking i 40 vannforekomster. Vannforekomstene dekket alle økoregioner og mange ulike elvetyper, men det er knyttet relativt stor usikkerhet til en del av resultatene. Basert på en samlet vurdering av alle biologiske og fysisk-kjemiske kvalitetselementer oppnådde kun fem av 40 vannforekomster miljømålet om god eller svært god økologisk tilstand. Det var fiskeindeksen og forsuringsindeksen for påvekstalger som var bestemmende for at miljømålet ikke ble nådd for hoveddelen av vannforekomstene. Med hensyn til eutrofieringspåvirkning nådde alle vannforekomstene (bortsett fra ett leirvassdrag) miljømålet. Med unntak av fire vannforekomster ble miljømålet nådd med hensyn til organisk belastning (ASPT-indeksen for bunndyr). For forsuringsparameterne oppnådde kun 10 av 26 forsuringsfølsomme vannforekomster miljømålet. De fleste av de 16 som viste tegn til forsuring (og dermed ikke oppnådde miljømålet) lå på Sørlandet og i Midt-Norge. Det er behov for mer data og en sammenstilling av referanseverdier for de ulike forsuringsindeksene/parameterne før vi konkluderer med at såpass mange vannforekomster faktisk er forsuret. For fiskeindeksen er det tydelig at det er behov for mer data og en revidering av indeksen, da kun to vannforekomster havnet i svært god tilstand, og hele 22 vannforekomster ikke nådde miljømålet. Det ble observert lave konsentrasjoner av de fleste miljøgiftene i biota (fisk), med unntak av kvikksølv, polybromerte difenyletere og PCB7, som alle lå over grenseverdiene i alle vannforekomster der disse ble målt. I vann ble det i hovedsak målt lave konsentrasjoner av metaller, med unntak av de tre leirvassdragene, hvor enkelte prøver overskred grenseverdiene.

• Thrane, Jan-Erik; Wit, Heleen de; Blakseth, Tomas Adler; Skancke, Liv Bente & Garmo, Øyvind Aaberg (2020). Correcting for bias in freshwater total nitrogen concentrations obtained with a modified standard (NS4743) method. Norsk institutt for vannforskning (NIVA). ISSN 9788257772734. Full text in Research Archive Show summary

  Concentrations of total nitrogen (TotN) in freshwater monitoring programs were unreliable for a limited period, probably because of inadequate sample digestion when using a modified version of method NS 4743 (“method A”) for TotN analysis. The method resulted in underestimation of TotN, especially for humus-rich surface waters. Where re-analysis of samples with the original NS 4743 method (“method B”) is not possible, TotN may be corrected using a regression model fitted to ca. 1000 freshwater samples with analytical results from both method A (likely resulting in biased TotN concentrations; TotNold) and method B (resulting in presumably correct values; TotNnew). Here we present the dataset, the correction model, and a validation of the model using independent datasets. The regression model used TotNold and TOC and their interaction as predictors, and had an R2 of 0.956 and a root mean squared error (RMSE) of 31.87 μg N/L. The model predicted TotNnew well when tested on independent datasets from lakes and headwater streams. We recommend that the function is used to estimate TotN in long-term monitoring programs of lakes and headwater streams (1-3 order streams) where TotN was analysed with the modified standard method (method A), but not for rivers (>6-order streams) since the difference between TotN from the two methods was not significant for these water types.

• Garmo, Øyvind Aaberg; Kaste, Øyvind; Arle, Jens; Austnes, Kari; Wit, Heleen de & Fölster, Jens [Show all 18 contributors for this article] (2020). Trends and patterns in surface water chemistry in Europe and North America between 1990 and 2016, with particular focus on changes in land use as a confounding factor for recovery. Norsk institutt for vannforskning (NIVA). ISSN 9788257772147. Full text in Research Archive Show summary

  The report presents trends in sulphate, nitrate, chloride, base cations, ANC (acid neutralising capacity), pH and DOC at circa 500 ICP Waters sites in Europe and North America for the period 1990-2016. Time series were analysed for trends in annual median values, annual extreme values and change points, that indicate years with sudden changes in trend or level. Also provided is a brief overview of possible implications of land use change for recovery of acidified surface waters.

• Moe, Therese Fosholt; Persson, Jonas; Bækkelie, Knut Andreas Eikland; Myrvold, Knut Marius; Garmo, Øyvind Aaberg & Grung, Merete [Show all 9 contributors for this article] (2019). Overvåking av referanseelver 2018. Basisovervåking i henhold til vannforskriften. Norsk institutt for vannforskning (NIVA). ISSN 9788257771133. Full text in Research Archive Show summary

  Overvåking av referanseelver har vært gjennomført over to år, og i andre år (2018) ble 34 antatte referansevannforekomster undersøkt. Vannforekomstene dekket mange ulike elvetyper i økoregionene Finnmark, Vestlandet og Østlandet (resten av landet ble dekket med 47 vannforekomster i 2017). Det er knyttet usikkerhet til en del av resultatene, men med tanke på påvirkniger som fører til eutrofiering nådde alle vannforekomstene miljømålet, med unntak av to leirvassdrag. Det kan også se ut til å være noe organisk belastning i enkelte vannforekomster. For forsuringsparameterne var det 8 vannforekomster som ikke nådde miljømålet. Det er behov for mer data og en sammenstilling av de ulike forsuringsparameterne før vi kan konkludere med årsakene. For fiskeindeksen var resultatene for 2018 noe bedre enn i 2017, og 19 vannforekomster nådde miljømålet. Det er fortsatt behov for mer data og en revidering av denne indeksen. Det ble observert lave konsentrasjoner av de fleste miljøgiftene, med unntak av kvikksølv, polybromerte difenyletere og PCB7, som lå over grenseverdiene for alle 11 vannforekomster der disse var målt (i 10 av 11 for PCB7). Også nikkel og PFOS oversteg grenseverdiene for henholdsvis 1 og 2 vannforekomster.

• Austnes, Kari; Aherne, Julian; Arle, Jens; Čičendajeva, Marina; Couture, Suzanne & Fölster, Jens [Show all 21 contributors for this article] (2018). Regional assessment of the current extent of acidification of surface waters in Europe and North America. Norsk institutt for vannforskning (NIVA). ISSN 9788257770037. Full text in Research Archive Show summary

  The current status of surface water acidification related to air pollution in Europe and North America has been assessed using country reports, monitoring data, critical loads and exceedance data, acid sensitivity and deposition maps, and data reported under the European Commission’s Water Framework Directive (WFD). Acidification is still observed in many countries, but the extent and severity vary. Maps of acid sensitivity and deposition suggest that surface water acidification is present in regions and countries for which no data or reports were delivered for the current assessment. Existing national monitoring varies in the ability to assess the spatial extent of acidification and the recovery responses of acidified sites. The monitoring requirements under the European Union’s National Emission Ceilings Directive are expected to reverse the recent decline in the number of monitoring sites observed in some countries. The information reported under the WFD is currently of limited value in assessing the extent of acidification of surface waters in Europe. Chemical recovery in response to reductions in acid deposition can be slow, and biological recovery can lag severely behind. Despite large and effective efforts across Europe and North America to reduce surface water acidification, air pollution still constitutes a threat to freshwater ecosystems.

• Garmo, Øyvind Aaberg; Ulańczyk, Rafał & Wit, Heleen de (2018). Proceedings of the 34th Task Force meeting of the ICP Waters Programme in Warsaw, May 7-9, 2018. Norsk institutt for vannforskning (NIVA). ISSN 9788257770334. Full text in Research Archive Show summary

  Proceedings of the 34th Task Force meeting of the ICP Waters Programme in Warsaw, May 7-9, 2018. National contributions from Russia and Armenia.

• Moe, Therese Fosholt; Thrane, Jan-Erik; Persson, Jonas; Bækkelie, Knut Andreas Eikland; Myrvold, Knut Marius & Olstad, Kjetil [Show all 9 contributors for this article] (2018). Overvåking av referanseelver 2017. Basisovervåking i henhold til vannforskriften. Norsk institutt for vannforskning (NIVA). ISSN 9788257769949. Full text in Research Archive Show summary

  Overvåking av referanseelver startet opp i 2017, og 47 antatte referansevannforekomster ble undersøkt. Vannforekomstene dekket alle økoregioner og mange ulike elvetyper, men det er knyttet relativt stor usikkerhet til en del av resultatene. Med tanke på påvirkniger som fører til eutrofiering nådde alle vannforekomstene miljømålet, mens det kan se ut til å være noe organisk belastning i enkelte vannforekomster. For forsuringsparameterne var det 18 vannforekomster som ikke nådde miljømålet, med noe dårligere tilstand på Sørlandet enn i resten av landet. Det er behov for mer data og en sammenstilling av referanseverdier for de ulike forsuringsparameterne før vi konkluderer med at såpass mange vannforekomster faktisk er forsuret. For fiskeindeksen ble det tydelig at det er stort behov for mer data og en revidering av indeksen, da kun 3 vannforekomster havnet i svært god tilstand, og hele 35 vannforekomster ikke nådde miljømålet. Det ble observert lave konsentrasjoner av de fleste miljøgiftene, med unntak av kvikksølv og polybromerte difenyletere, som begge lå over grenseverdiene for alle vannforekomster der disse var målt. Også noen av de vannregionspesifikke stoffene oversteg grenseverdien for noen vannforekomster.

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