Wetlands are often constructed to counteract eutrophication or brownification of downstream waters, although our understanding of their differences in efficiency and effects is still scarce. Therefore, we here used the upper and lower 95% confidence limits of the average retention capacity of 11 wetlands to represent efficient and inefficient wetlands, respectively. We then modelled future scenarios in water colour (brownification) and phosphorus (eutrophication) in a downstream lake. We show that efficient wetlands may delay eutrophication and brownification rates of downstream aquatic systems by up to 15 and 4 years, respectively, thereby providing a temporal gain in time that could be used to implement complementing measures. Inefficient wetlands, on the other hand, may instead deteriorate the water quality by increasing levels of phosphorus and water colour and thereby reducing the euphotic zone of downstream lakes. Our study underscores the difficulty of having realistic expectations of the efficiency of wetlands as tools in water management. We conclude that despite high variation in efficiency among wetlands, they may still hold a strong potential when properly designed and placed for mitigating both eutrophication and brownification. However, further evaluation or systematic monitoring after restoration or construction is needed to investigate optimal placement and design in relation to management objectives. © 2026 The Author(s). Ecohydrology published by John Wiley & Sons Ltd.

Climate change has intensified the mobility of dissolved organic matter (DOM) from land into aquatic ecosystemsleading to increased brownification and hypoxia. Constructed wetlands (CWs) offer a potential mitigationstrategy, yet optimal wetland design and water residence time (WRT) with respect to DOM removal remainsunderexplored. This study evaluates the effects of shallow and deep CWs on the degradation of DOM during twoseasons (June and November 2023) in Halmstad, Sweden. Small CWs were used for treatments which receivedorganic matter additions, and the impact of varying depths and WRT on DOM alteration and removal wereevaluated. DOM composition and degradation were assessed using fluorescence spectroscopy, bacterial activity(bacterial respiration, production and growth efficiency), and total organic carbon (TOC) analyses. We identifieda critical initial period (2 days) during which labile DOM is rapidly degraded by photochemical and microbialprocesses. Shallow CWs exhibited rapid initial breakdown of DOM but also a release of terrestrial like fractionsafter 2 days, potentially increasing downstream brownification. In contrast, deep CWs demonstrated sustainedDOM degradation and slower internal production, potentially reducing their contribution to downstreambrownification. Microbial processes dominated DOM degradation across both seasons, although photodegradationalso played a significant role during the summer months. Correlations between DOM compositionand bacterial dynamics underscore the role of labile substrates in driving carbon cycling efficiency. Thesefindings inform CW designs, advocating for hybrid approaches integrating shallow and deep systems in series tomaximize carbon removal, minimize brownification, and adapt to seasonal variability. © 2025 The Authors

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

Changes in rainfall patterns driven by climate change affect the transport of dissolved organic matter (DOM) and nutrients through runoff to freshwater systems. This presents challenges for drinking water providers. DOM, which is a heterogeneous mix of organic molecules, serves as a critical precursor for disinfection by-products (DBPs) which are associated with adverse health effects. Predicting DBP formation is complex due to changes in DOM concentration and composition in source waters, intensified by altered rainfall frequency and intensity. We employed a novel mesocosm approach to investigate the response of DBP precursors to variability in DOM composition and inorganic nutrients, such as nitrogen and phosphorus, export to lakes. Three distinct pulse event scenarios, mimicking extreme, intermittent, and continuous runoff were studied. Simultaneous experiments were conducted at two boreal lakes with distinct DOM composition, as reflected in their color (brown and clear lakes), and bromide content, using standardized methods. Results showed primarily site-specific changes in DBP precursors, some heavily influenced by runoff variability. Intermittent and daily pulse events in the clear-water mesocosms exhibited higher haloacetonitriles (HANs) formation potential linked to freshly produced protein-like DOM enhanced by light availability. In contrast, trihalomethanes (THMs), associated with humic-like DOM, showed no significant differences between pulse events in the brown-water mesocosms. Elevated bromide concentration in the clear mesocosms critically influenced THMs speciation and concentrations. These findings contribute to understanding how changing precipitation patterns impact the dynamics of DBP formation, thereby offering insights for monitoring the mobilization and alterations of DBP precursors within catchment areas and lake ecosystems. © 2024 The Author(s)

In this study, we examined transporter genes in metagenomic and metatranscriptomic data from a time-series survey in the temperate marine environment of the Baltic Sea. We analyzed the abundance and taxonomic distribution of transporters in the 3μm-0.2μm size fraction comprising prokaryotes and some picoeukaryotes. The presence of specific transporter traits was shown to be guiding the succession of these microorganisms. A limited number of taxa were associated with the dominant transporter proteins that were identified for the nine key substrate categories for microbial growth. Throughout the year, the microbial taxa at the level of order showed highly similar patterns in terms of transporter traits. The distribution of transporters stayed the same, irrespective of the abundance of each taxon. This would suggest that the distribution pattern of transporters depends on the bacterial groups being dominant at a given time of the year. Also, we find notable numbers of secretion proteins that may allow marine bacteria to infect and kill prey organisms thus releasing nutrients. Finally, we demonstrate that transporter proteins may provide clues to the relative importance of biogeochemical processes, and we suggest that virtual transporter functionalities may become important components in future population dynamics models.