Eutrophication and flood risk are pressing issues of ecological and societal relevance. A key driver of eutrophication is the use of nitrogen (N) fertiliser in agriculture, resulting in N exports from land to water. Climate change increases the risk of flood events which can increase N exports and further worsen eutrophication. Created wetlands in agricultural areas are recognised as effective nutrient sinks, and the flood attenuating capabilities of wetlands are well established. However, the combination of these two ecosystem services in agricultural landscapes is understudied. This study examines how water flow buffering in created wetlands affects N removal and greenhouse gas (GHG) emissions. The study was performed in experimental wetlands of different designs (depth and size) subjected to intermittent or permanent flooding (gradual emptying between inflow events or a constant water level). Intermittently flooded wetlands removed less N than permanently flooded wetlands, especially during no-flow periods. The largest difference was found among deep wetlands, where the average N removal across the study was 0.19 g m−2 d−1 (25 %) in intermittently flooded wetlands and 0.27 g m−2 d−1 (38 %) in permanently flooded wetlands. No differences in the aqueous concentrations of N2O or CH4 were observed between wetlands with high or low water storage capacity, thus indicating similar emissions. This study highlights a risk of lowered N removal in created wetlands designed for flood attenuation. Further studies are needed on synergies and trade-offs when aiming to combine flood attenuation and N removal in created wetlands. © 2025 The Authors. Published by Elsevier Ltd.

Climate and land use changes can increase terrestrial runoff to aquatic systems, leading to brownification and eutrophication in northern boreal lakes. Brownification may boost bacterial respiration and production, while eutrophication can enhance primary production and algal blooms. However, their combined effects on basal producers and bacterial carbon utilization are less understood. This study explores the combined impacts of the two stressors: brownification and eutrophication on microbial dynamics in Lake Bolmen. Utilizing a field mesocosm experimental design, treatments received different combinations of organic matter (OM) and inorganic nutrients to simulate predicted future scenarios. Results showed that OM additions significantly increased bacterial production and respiration, regardless of nutrient additions. Nutrient additions enhanced bacterial production but did not affect respiration. Both nutrients and OM stimulated bacterial growth efficiency. Labile carbon from DOM was the main driver of higher bacterial respiration and short-term production increases. Fluorescence data indicated that the combination of brownification and eutrophication led to higher terrestrial DOM utilization than each stressor alone. The study suggests that future boreal lakes may become more heterotrophic, thus increasing CO2 release. These findings highlight the complex interactions between DOM and nutrients and underscore the importance of considering multiple stressors in lake management and mitigation strategies. © The Author(s) 2024

Increasing total organic carbon (TOC) concentrations concurrent with a darkening in water colour in boreal freshwaters is called brownification. This can affect crucial ecosystem services. Our study investigated constructed wetland optimisation regarding depth and water residence time (WRT) during different seasons to remedy dark water colour and high TOC concentrations, while retaining nitrogen removal. We conducted eleven-day experiments with deep-brown, shallow-brown and shallow-control treatments, in June and November 2023, using 18 small constructed wetlands at an experimental wetland area in southern Sweden. At the beginning of both experiments, the flow through the wetlands was halted and extracted peat was added to the brown treatments, to increase absorbance and TOC concentrations. Thereafter, changes in absorbance, TOC concentration and total nitrogen (TN) concentration were measured. A maximum TOC reduction of 25% was reached with a WRT of two days in summer and one day in autumn. A maximum absorbance reduction of 40% and 65% was reached with a WRT of one day in summer and two days in autumn, respectively. TN removal was not affected by TOC addition. We conclude that constructed wetlands increase water clarity and boost carbon degradation if their WRT, especially during summer, is sufficiently short. If WRT exceeds two to three days in summer, internal carbon production together with low oxygen levels and increased iron (Fe) mobilization, may instead increase downstream brownification. Our study shows that wetlands with a depth of 0.6 m and a short WRT of one to two days may mitigate the effects of brownification. © The Author(s) 2025

Wetland management maintains nitrogen (N) removal capacity in mature and overgrown constructed wetlands (CWs). We evaluated whether CW management by macrophyte harvesting, and subsequent installation of woodchips-based floating beds (WFBs) planted with Glyceria maxima and Filipendula ulmaria improved N removal. In sixteen heavily overgrown experimental CWs, we applied four treatments: i) only macrophyte harvesting, ii) 5% of the harvested-CW surface covered with WFBs, iii) 20% WFBs cover, and iv) a control treatment (heavily overgrown). N removal was determined in all wetlands at nine occasions. Plant biomass accrual, N assimilation, and denitrification genes nirS, nirK, nosZI and nosZII on plant roots and woodchips from WFBs were estimated. Macrophyte harvesting improved N removal of heavily overgrown CWs, whereas subsequent WFB installation only sometimes improved N removal. Mean N removal efficiencies (± standard deviation) overall were 41 ± 15 %, 45 ± 20 %, 46 ± 16 % and 27 ± 8.3 % for treatments i to iv, respectively. Relative biomass production, root length and root surface area for G. maxima (mean ± standard deviation: 234 ± 114 %, 40 ± 6.5 cm, 6308 ± 1059 cm2g-1, respectively) were higher than those for F. ulmaria (63 ± 86 %, 28 ± 12 cm, 3131 ± 535 cm2g-1, respectively) whereas biomass N assimilation was higher for F. ulmaria (1.8 ± 0.9 gNm−2 of WFB) than for G. maxima (1.3 ± 0.5 gNm−2 of WFB). Denitrification gene abundance was higher on plant roots than on woodchips while G. maxima hosted higher root denitrification gene abundance than F. ulmaria. We conclude that macrophyte harvesting improves N removal in heavily overgrown CWs. WFBs installation has the potential to support plant growth and denitrification in surface-flow constructed wetlands. Further studies need to evaluate the long-term effects of macrophyte harvesting and WFB installation on N removal in CWs. © 2024 The Authors

Wetlands in agricultural areas mitigate eutrophication by intercepting nutrient transports from land to sea. The role of wetlands for nutrient removal may become even more important in the future because of the expected increase in agricultural runoff due to climate change. Because denitrification is temperature dependent, wetland nitrogen (N) removal usually peaks during the warm summer. However, climate change scenarios for the northern temperate zone predict decreased summer and increased winter flows. Future wetlands may therefore shift towards lower hydraulic loading rate and N load during summer. We hypothesised that low summer N loads would decrease annual wetland N removal and tested this by examining 1.5–3 years of continuous N removal data from created agricultural wetlands in two regions in southern Sweden (East and West) during different periods. West wetlands showed relatively stable hydraulic loads throughout the year, whereas East wetlands had pronounced no-flow periods during summer. We compared East and West wetlands and tested the effects of several variables (e.g., N concentration, N load, hydraulic load, depth, vegetation cover, hydraulic shape) on annual absolute and relative N removal. We found no difference in annual N removal between East and West wetlands, even though summer N loads were lower in East than in West wetlands. A possible explanation is that stagnant water conditions in East wetlands suppressed decomposition of organic matter during summer, making more organic matter available for denitrification during winter. Absolute N removal in all wetlands was best explained by N load and hydraulic shape, whereas relative N removal was best explained by emergent vegetation cover and hydraulic shape. This study highlights the importance of design and location of agricultural wetlands for high N removal, and we conclude that wetlands in a future climate may remove N from agricultural runoff as efficiently as today. © 2023 The Authors

With increasing temperatures and precipitation, as well as land use changes in boreal regions, waterbodies are receiving larger inputs of coloured terrestrial humic substances. At the same time, nutrient inputs are increasing. This brownificationin combination with increasing nutrient levels has consequences for the aquatic food web in terms of species composition and energy transfer efficiency. In Lake Bolmen, Sweden’s 7ths largest lake, brownification additionally creates problems for drinkingwater production, since this lake is an important drinking water reservoir of southern Sweden. Lake monitoring data show a clear pattern of increasing brownification in Lake Bolmen over the preceding decades. To understand the consequences ofincreased browning and of increased nutrient inputs for Lake Bolmen’s food web on bacterial production, and phytoplankton and zooplankton species community composition and abundance, we conducted a 6-week mesocosm experiment during summer 2021. Brownification and nutrient ratios were manipulated. Measures of algal pigment concentrations show that browning has strong effects on algal pigment composition and thus probably on algal taxonomic composition. Our results suggest that brownification affects basic producer community composition in lakes, thus possibly changing community composition and biomass of higher trophic levels of the aquatic food web in boreal regions.

Jakobs sjö, an electricity dam of the Nissan River system (Southwest Sweden) is contaminated with large microplastics sedimented out at its shores. In 2021 a field survey of microplastics was carried out in the Nissan River with the Lagan River asreference. We sampled three locations in each river (upstream, lake/dam and downstream). Four replicate bottom and shore sediment samples were taken at each location. Dried samples were fractionated into three size categories (> 2 mm; 2 – 0.9mm; 0.9 – 0.55 mm) and microplastics per area and weight were estimated. We found a higher concentration of large shore microplastics in Jacobs sjö and downstream from it than upstream in the Nissan River. We found no large shore microplastics in the Lagan River system. Smaller size fractions of microplastics were omnipresent in shore and bottom sediments of both rivers. However, the smallest size category tended to be proportionately more dominant at upstream than at downstream locations. This could be explained by biofilm-microplastic floc formation and sedimentation as particles travel downstream. Concentrations of toxicants such as heavy metals can increase thousandfold on the surface of microplastics and since the five heavy metals most likely to sorb to microplastics in a freshwater environment (Cd, Cu, Ni, Pb and Zn) were all present in Jacobs sjö, we conclude that the high concentrations of microplastics in Jacobs sjö may pose a threat to aquatic life.

Wetland nitrogen (N) removal is temperature dependent and therefore generally highest during summer in the northern temperate zone. However, climate change scenarios predict more frequent summer droughts in these regions, resulting in lowered N transports during summer to wetlands created for interception of agricultural runoff. This may adversely affect annual wetland N removal, thus reducing the mitigative effects wetlands have on eutrophication. In this study, continuous flow-proportional sampling was performed in six agricultural wetlands located on the east coast, and three on the west coast, of southern Sweden. These two regions represent different climate conditions, where precipitation is lower and summer temperatures are higher on the east coast. Our results showed a pronounced no-flow period during summer in east coast wetlands, but not in west coast wetlands. No-flow periods only decreased N load and removal rate during summer but had no effect on annual N removal. Annual N removal was instead best explained by multiple regression with annual N load and hydraulic efficiency as predictors. This indicates that low wetland N removal during drier summers may be compensated by higher N removal during other seasons. A possible explanation is that annual N removal through denitrification is determined by the amount of organic carbon provided by wetland vegetation, and that organic carbon not utilized during summer, due to lack of nitrate and oxygen under no-flow conditions, will be available for denitrification during other seasons. In conclusion, climate change might not have the anticipated decreasing effect on wetland N removal.

A multitude of anthropogenic pressures deteriorate the Baltic Sea, resulting in theneed to protect and restore its marine ecosystem. For an efficient conservation,comprehensive monitoring and assessment of all ecosystem elements is of fundamentalimportance. The Baltic Marine Environment Protection Commission HELCOMcoordinates conservation measures regulated by several European directives. However,this holistic assessment is hindered by gaps within the current monitoring schemes.Here, twenty-two novel methods with the potential to fill some of these gaps andimprove the monitoring of the Baltic marine environment are examined. We asked keystakeholders to point out methods likely to improve current Baltic Sea monitoring. Wethen described these methods in a comparable way and evaluated them based ontheir costs and applicability potential (i.e., possibility to make them operational). Twelvemethods require low to very low costs, while five require moderate and two high costs.Seventeen methods were rated with a high to very high applicability, whereas fourmethods had moderate and one low applicability for Baltic Sea monitoring. Methodswith both low costs and a high applicability include the Manta Trawl, Rocket, SedimentCorer, Argo Float, Artificial Substrates, Citizen Observation, Earth Observation, theHydroFIARpH system, DNA Metabarcoding and Stable Isotope Analysis. © 2020 Mack, Attila, Aylagas, Beermann, Borja, Hering, Kahlert, Leese, Lenz, Lehtiniemi, Liess, Lips, Mattila, Meissner, Pyhälahti, Setälä, Strehse, Uusitalo, Willstrand Wranne and Birk.

Legislations and commitments regulate Baltic Sea status assessments and monitoring. These assessments suffer from monitoring gaps that need prioritization. We used three sources of information; scientific articles, projectreports and a stakeholder survey to identify gaps in relation to requirements set by the HELCOM's Baltic SeaAction Plan, the Marine Strategy Framework Directive and the Water Framework Directive. The most frequentlymentioned gap was that key requirements are not sufficiently monitored in space and time. Biodiversity monitoringwas the category containing most gaps. However, whereas more than half of the gaps in reports related tobiodiversity, scientific articles pointed out many gaps in the monitoring of pollution and water quality. Animportant finding was that the three sources differed notably with respect to which gaps were mentioned mostoften. Thus, conclusions about gap prioritization for management should be drawn after carefully consideringthe different viewpoints of scientists and stakeholders. © 2020 The Authors. Published by Elsevier Ltd.