Abstract Inland waters in the Northern Hemisphere are experiencing increased annual runoff due to higher overall precipitation as well as intensified short-term events such as heavy rainfall, floods and storms. These events affect the total loading and variability of inputs of allochthonous, coloured dissolved organic matter (cDOM) and inorganic nutrients into lakes. Previous studies have shown that increased total cDOM and inorganic nutrient loads affect phytoplankton biomass and metabolic rates, but it is unknown how the effects of different cDOM and nutrient pulse scenarios are modified by spatial and seasonal differences in lake characteristics. Here, we conducted a coordinated, standardized mesocosm experiment across three lakes with different ambient cDOM and nutrient concentrations. In two of these lakes, the experiment was implemented in two seasons. The same total amounts of cDOM, nitrate and phosphate were added to all mesocosms, but in pulses that differed in intensity and frequency. We found that pulse intensity and frequency affected chlorophyll a and phycocyanin concentrations and metabolic rates, i.e. gross primary production and respiration, differently. Specifically, more pronounced effects were found in response to the extreme pulse scenario compared to those with more frequent, smaller pulse additions. Furthermore, the effects were mainly temporary and varied more among lakes than between seasons. The clearest differences between the extreme and more gradual runoff scenarios were found in the lake with the lowest background cDOM and nitrate concentrations, likely because lower light limitation and possibly stronger initial N-limitation caused a faster response to the nutrient addition. Our results highlight that both antecedent lake conditions and characteristics of runoff events can affect phytoplankton biomass and metabolic rates and that comparative experimental approaches are needed to reveal the complexity of the responses.

Coastal lagoons are dynamic systems where the mode of nutrient input, continuous or pulsed, or a combination, can significantly affect plankton community structure and function. We conducted a spring field microcosm experiment in a Mediterranean lagoon to evaluate how three nutrient delivery regimes influence planktonic dynamics, using biomass measurements and extracellular enzyme activity (EEA) as functional indicators. Continuous nutrient additions promoted sustained phytoplankton and bacterioplankton growth, indicating bottom-up control and microbial stability. In contrast, pulse treatments saw a brief bacterioplankton bloom followed by delayed phytoplankton increase and a marked decline in zooplankton, suggesting disrupted trophic succession and food quality limitations. The pulse–continuous treatment yielded intermediate responses, confirming that delivery mode, not just nutrient load, modulates ecosystem processes. Elevated β-glucosidase (GLU) and leucine aminopeptidase (LAP) activity under nutrient addition, particularly in continuous treatments, signaled intensified carbon and nitrogen demand and active organic matter mineralization, even when nutrient concentrations appeared low. These findings suggest that, during stable spring conditions, microbial communities rapidly assimilate nutrients, potentially obscuring short-term biogeochemical changes from traditional concentration-based monitoring. Crucially, the timing and frequency of nutrient supply are as influential as total input, with continuous inputs supporting a more stable and efficient microbial loop. This work underscores the ecological importance of recognizing diffuse, low-level nutrient sources, such as groundwater seepage, which may be underrepresented in lagoon management strategies. Integrating functional indicators with delivery mode assessments can improve predictions of eutrophication risks and support more effective conservation planning in shallow coastal systems.

Human activities have significantly altered macronutrient concentrations in surface waters, impacting both ecological functions and water quality. Typically, research assesses this alteration and its effects from a single macronutrient perspective. Alternatively, we propose that macronutrient perspectives need to be integrated via a stoichiometric framework via carbon (C) : nitrogen (N) : phosphorus (P) ratios. These ratios may help to assess and improve natural attenuation at ecosystem and catchment level. From the C:N:P perspective, agricultural practices have resulted in a stoichiometric N surplus in temperate stream ecosystems, an issue of which German streams are a prime example. In contrast, Florida's streams are characterized by a P surplus relative to N and C due to high geological background P supply. Our study encompasses five streams in Germany and Florida, covering a wide range of C:N:P ratios, each characterized by distinct catchment characteristics. Here, we ask whether C:N:P ratios are the main driver of microbial nitrate-N uptake, irrespective of other differences between the two regions. Through streamside mesocosm and microcosm laboratory experiments employing an isotope tracer approach, we compared nitrate uptake. Additionally, we manipulated C:N:P ratios to assess the short-term effects on nitrate uptake and measured retention in the streamside mesocosm experiment. Enhancing our understanding of the interconnectedness of biogeochemical cycles enables the development of management recommendations for stoichiometric restoration in highly impacted stream ecosystems. This research contributes valuable insights towards sustainable practices and the preservation of aquatic ecosystems facing nutrient-related challenges and water security.

River floodplain systems are challenged by drought, which may trigger excess nutrient concentrations and greenhouse gas emissions. Increasingly frequent short-term droughts may exacerbate both problems by altering hydrological connectivity and thereby restructuring microbial communities and dissolved organic matter (DOM), which, in combination, may regulate sediment phosphorus and methane release. However, the combined effects of drought and connectivity on phosphorus and methane release via changes in DOM composition and microbial activity remain poorly understood. We incubated sediments from three floodplain sites along a hydrological connectivity gradient to the River Elbe and subjected them to two short-term drought intensities, corresponding to sediment moisture losses of 0.5–2.5% (moderate drought) and 19–21% (intense drought), followed by rewetting. Drought had surprisingly limited effects on phosphorus and methane release, while the site had a consistently higher impact and shaped the direction and magnitude of drought effects. Moreover, our results suggest that fluxes may be more pronounced at sites that were formerly well-connected to the river. Phosphorus was released under oxic conditions and was linked to heterotrophic microbial carbon use and humic-like DOM, implying that the effects of DOM-mediated microbial activity on phosphorus release need to be considered in future research efforts. Our findings suggest that long-term changes in hydrological connectivity, like lower discharge and changed DOM delivery, could have stronger effects on nutrient dynamics and microbial processes than short-term drought. Preserving floodplain connectivity is therefore critical to limiting nutrient and greenhouse gas release under climate change.

In cultural landscapes as in Central Europe, groundwater baseflow is often strongly enriched in nutrients (N; P) and reaches surface water bodies. Once approached, nutrients may lead to euthrophication within the receiving surface water and eventually in the ocean. Any intervening measures must rely on a fundamental knowledge about the amount of baseflow, the regions where it enters the water bodies and what is the ecological hazard resulting from it. Although groundwater usually differs chemically and isotopically from gaining surface water bodies, particularly in 1st order rivers like the Elbe, groundwater inflow is camouflaged due to the high dilution. Nevertheless, the present study addresses the spatial, temporal and volumetric determination and dynamics of groundwater discharge, its attributed nutrient contribution and the systematic analysis of its impact on benthic and planktonic eutrophication to the river Elbe. Therefore, water samples were taken from: the Elbe every 2 km along a 450 km long reach of the river, the inflowing streams, and wells and springs, representing the groundwater bodies along the river. the Elbe every 2 km along a 450 km long reach of the river, the inflowing streams, and wells and springs, representing the groundwater bodies along the river. All samples have been analysed for ionic composition, water isotopes nad nutrients. In a first step and on the base of: analysis of daily time series of hydraulic gradients between river- and groundwater levels, flux balances for river segments, inverse geochemical modelling of the river water composition, a Darcy approach based on the hydraulic conductivity of the upper aquifer and a model of the tritium dilution effect along the river water course, we were able to identify locations and the temporal dynamics of the interaction of groundwater with the river Elbe (Fig. 1). analysis of daily time series of hydraulic gradients between river- and groundwater levels, flux balances for river segments, inverse geochemical modelling of the river water composition, a Darcy approach based on the hydraulic conductivity of the upper aquifer and a model of the tritium dilution effect along the river water course, we were able to identify locations and the temporal dynamics of the interaction of groundwater with the river Elbe (Fig. 1). The groundwater samples were additionally analysed for radiocarbon, 3 He/ 4 He and anthropogenic gases (SF6, CFCs) to determine their average age, being important when it comes to management strategies to reduce the potentially harmful function of the groundwater. Since fertilization in agriculture is the major reason for the high nutrient load in groundwaters, to intervene it is important to know, whether the nitrate approaching the river stems from the current farming activities or from decades ago. Our results showed, groundwaters approaching the river are usually younger than 40 years, indicating modern regulatory measures such as German Fertilizer Regulation and the EU-WFD may have an impact on the nutrient load of the Elbe. To estimate how these groundwater-borne nutrient inflows affect the eutrophication of the river Elbe, we quantified the input and determined the impacts on local (benthic biofilms) and regional (phytoplankton) eutrophication in areas along the river, by monitoring biofilm growth on exposed artificial substratum in river segments and the abundance and composition of phytoplankton. The experimental benthos setup included sites where groundwater inflow was proven and others without proven groundwater inflow. Further, we differentiated between the types of aquifers and their hydraulic connection: unconsolidated Quaternary aquifers, which are supposed to let groundwater pass through the pore space and pre-Neogene hard rock aquifers, considered to restrict groundwater flow to faults and cracks. To also understand possible seasonal effects, the in-situ monitoring was repeated during springtime, summer and autumn, while effects on pelagic biomass production was monitored during summer only. As a result, along the 450 km stretch, groundwater contributes under low flow conditions daily up to 1.5 t PO 4 , which enter the river diffusely and significantly enhances the euthrophication risk, since the availability of phosphorus usually limits biomass production. Groundwater-P may hence result in an additional daily planktonic load of about 46 tons of particulate organic carbon, thereby contributing to eutrophication at the regional scale in the Elbe River. At the local benthic scale, analyses of biofilm community composition, biofilm macronutrients, and structural components react distinctly on the availability of groundwater and showed seasonal effects. Wherever groundwater inflow is high, as in regions dominated by unconsolidated, highly conductive Quaternary aquifers, benthic eutrophication takes place and most likely during autumn. The strong interaction of environmental factors in determining benthic eutrophication highlights the need to assess these factors in combination rather than in isolation. Our assessment of a large European 1 st order river with high eutrophication risk for the river itself and the German Bay provides the status-quo only and calls for systematic long-term monitoring of ground- and surface water composition as well as ecological indicators to evaluate the success of regulatory measures in respect to ecological quality in that complex and interacting system.

Nutrient dynamics in headwater streams are governed by benthic and hyporheic biofilms, with carbon (C) : nitrogen (N) : phosphorus (P) ratios driving the heterotrophic microbial biofilm development through nutrient limitation. Furthermore, heterotrophic responses to changes in C : N : P ratios are probably modulated by autotrophic responses to light and C : N : P ratios, which modify the amount and composition of photosynthetic exudates and increase competition for nutrients. Effects on functional properties like the use of organic compounds by the heterotrophic biofilm community are largely unknown. We conducted a stream mesocosm experiment with a factorial design with different C : N : P ratios and light availability levels to test direct and indirect effects on heterotrophic biofilm functioning via community‐level physiological profiles in benthic and hyporheic biofilms. When inducing a resource C : N : P ratio closer to heterotrophic microbial biomass C : N : P ratios, we found an increased functional diversity of metabolized substrates, especially in hyporheic biofilms. Furthermore, this alteration shaped substrate preferences toward less P‐containing substrates and more N‐containing substrates in early‐stage biofilms. Despite the absence of a direct impact of light on hyporheic biofilms, we detected a propagation of a benthic autotrophic effect into hyporheic biofilms. Light availability induced effects on hyporheic bacterial density and the use of phenolic compounds, amino acids, and carbohydrates. In benthic biofilms, only the use of carbohydrates was affected by light. These results emphasize the significant indirect effects of benthic autotrophs on the functionality of hyporheic microbial heterotrophs and suggest consequences of human impacts, such as nutrient inputs and clear‐cutting, on stream nutrient cycling.

This study investigates the adaptive response of streambed microbial biofilms to water scarcity, focusing on the role of extracellular polymeric substances (EPS) production across a gradient of hydrological conditions. Sediment samples from 37 streams in the north-eastern Iberian Peninsula, encompassing both permanent and intermittent flow regimes, were analysed for EPS-polysaccharide content, microbial biomass, chlorophyll-a, and biofilm function (carbon substrate utilization profiles). Drought conditions were characterized based on the number of dry days over the eight months preceding sampling. Results revealed that EPS production increased significantly in intermittent streams, particularly under long-term drought, reaffirming that EPS synthesis is a key microbial strategy to mitigate desiccation stress. Notably, when normalized to prokaryotic density, EPS content exhibited a significant positive correlation with drought duration, emphasizing the dominant role of heterotrophic bacteria over algae in EPS secretion. However, EPS content alone was not a universal indicator of water scarcity, which showed a large variability in permanently flowing streams. Functional profiling showed clear shifts in carbon substrate utilization associated with stream hydrology. Intermittent streams exhibited a broader metabolic range, and particularly a capacity to use phenolic compounds, suggesting an adaptation to terrestrial organic matter inputs. Contrary to expectations, functional diversity increased in drier conditions, challenging assumptions derived from controlled experiments and underscoring the resilience of Mediterranean microbial biofilm communities to drought. These findings provide empirical support for EPS-mediated drought adaptation in natural biofilms and highlight functional diversity as a potential mechanism maintaining ecosystem processes under increasing aridity due to climate change.

Phosphorus (P) dynamics at the sediment-water interface of aquatic ecosystems are receiving increasing attention due to their implications for water quality. P uptake by microbial biofilms can serve as a mechanism to control and mitigate the risk of eutrophication. Microbial biofilms capture P both intracellularly and extracellularly. While the significance of extracellular P entrapment in biofilms in engineered systems has recently been established, little is known about its dynamics in aquatic ecosystems. Current research on eutrophication control predominantly emphasises nitrogen, phosphorus or nitrogen-phosphorus ratio-based approaches, often overlooking the potential indirect influence of bioavailable dissolved organic carbon (DOC) on P uptake by heterotrophic microorganisms. In this study, we tested the effect of bioavailable DOC on P entrapment patterns in biofilms and in biofilm P-regulation mechanisms such as polyphosphate accumulation and alkaline phosphatase activity in semi-natural flow-through experimental flumes. Our results show that intracellular P entrapment is limited by bioavailable DOC, while extracellular P entrapment is independent of bioavailable DOC and has the potential to offset intracellular P saturation. We further demonstrate that DOC bioavailability influences benthic P cycling and that its implications extend into critical areas of ecosystem functioning such as river self-purification, competitive resource utilisation and organic P cycling.

Mediterranean coastal lagoons are increasingly affected by nutrient enrichment from both natural and anthropogenic sources. While most studies focus on total nutrient loads, little is known about how the mode of nutrient delivery, pulsed or continuous, affects plankton community dynamics. This study aimed to experimentally test how different nutrient input regimes influence zooplankton community structure and plankton biomass under microcosm conditions using natural lagoon water. Three nutrient regimes were applied: Pulse (P), Continuous (C), and Pulse–Continuous (PC), along with a Control (no addition), with total nutrient concentrations standardized across regimes. Biomass of bacterioplankton, phytoplankton, and zooplankton, as well as hydrochemical variables and species composition, were monitored over 28 days. P regimes led to reduced zooplankton biomass and impaired reproduction, while C and PC regimes maintained higher productivity and favored shifts in rotifer and copepod composition. To evaluate community response, we used the Z2 index, a disturbance metric developed for Mediterranean lagoons that quantifies the severity of environmental disturbance based on zooplankton assemblages. Z2 scores revealed a gradient aligned with nutrient delivery mode, with small but consistent increases in C and PC regimes. These findings highlight the potential long-term impacts of continuous low-level nutrient inputs in restructuring aquatic communities.

Anthropogenic inputs of reactive nitrogen (N) elevate nitrate–N (NO3-N) levels in streams, potentially shifting their dissolved organic carbon (DOC) to N to phosphorus (P) ratios (DOC:N:P) toward N excess. Meanwhile, changes in riparian vegetation can alter light availability. Together, these factors may influence NO3-N uptake by photoautotrophs and heterotrophs in surface (benthic) biofilms and by heterotrophs in subsurface (hyporheic) biofilms. Although these compartments may exhibit distinct rates and constraints on nutrient uptake and retention, the extent to which stoichiometric imbalances and light availability govern their macronutrient uptake remains largely unexplored. Here, we present results from a stream mesocosm experiment in which light availability and DOC:N:P were manipulated by adding labile DOC and inorganic P to create a physiologically more balanced stoichiometric composition of stream mesocosm water. We show (I) how the relative (macronutrient ratio) and absolute (particulate organic C, particulate N, and particulate P) macronutrient composition of benthic and hyporheic biofilms changes with different levels of light availability (20 and 90 µmol photons m−2 s−1) and different water DOC:N:P (350:940:1 and 73:40:1), (II) that benthic NO3-N uptake rates increased with addition of labile DOC and P, whereas light had only a minor effect, and (III) that higher NO3-N uptake rates due to labile DOC and P addition in benthic biofilms leads to higher N loss from biofilm biomass. This results in similar N retention times across treatments and highlights the importance of water column macronutrient stoichiometry as a predictor of in-stream N cycling.

Abstract. Phosphorus (P) entrapment by biofilms can be distinguished into the intracellular P entrapment (the P uptake by microbial cells) and the extracellular P entrapment via the extracellular polymeric substances. It is unknown how these two P entrapment pathways behave in natural ecosystems in which gradients of environmental drivers such as P, labile dissolved organic matter (DOM) and light occur. Another key aspect in P dynamics at the sediment-water interface is microbial activity. Microbially mediated mineralization of DOM and microbial activity in general is suspected to be a driver of internal P mobilization from the sediments. Here we brought such expectations into a real-world context by analysing the P entrapment patterns of benthic biofilms and the P release potential from the sediments along a longitudinal gradient in a third order river in front of the background of key microbial metabolic variables. The gradient consists of increasing P availability, DOM lability and light availability. We found a gradual shift in the dominance of P entrapment from higher intracellular P entrapment in the upstream biofilms to higher extracellular P entrapment in the downstream biofilms. This shift towards dominance of extracellular P entrapment was accompanied by an increase in the P release potential from the sediment. The increasing P release potential was also connected to high extracellular enzyme activity of alkaline phosphatase, an enzyme involved in the mineralization of P from organic compounds. We further found that a balanced ratio between intracellular and extracellular P was connected to a higher C metabolic diversity. All this evidence suggests an influence of the benthic biofilms on P dynamics at the sediment-water interface. This research advocates for a more integrated perspective that accounts for both intracellular and extracellular biofilm-mediated processes.

High loads of nutrients like phosphorus (P) persistently degrade water quality in floodplain waterbodies and cause eutrophication, i.e., elevated algae production. Despite continuous efforts to curb external nutrient loads entering freshwaters, internal P release from sediments frequently sustains eutrophication. Benthic microbial communities considerably impact nutrient turnover through the mineralisation of organic matter and may strongly affect P retention and release, which can be estimated via the P buffering potential. However, how these microbial processes shape the P buffering potential in hydrologically dynamic systems like floodplains is poorly understood. Here, we assessed the coupled effects of lateral hydrological connectivity, dissolved organic matter (DOM) quality and benthic microbial activity on the P buffering potential in a river-floodplain system. We examined seven sites in shallow floodplain waterbodies that were either connected or disconnected from the River Elbe (Magdeburg, Germany) after a summer contraction phase. Our findings show a significantly higher P buffering potential and thus, likely higher P release risk within connected sites, corroborated by distinct microbial community-level physiological profiles. The P buffering potential was positively correlated with more labile, low molecular weight DOM, increased autochthonous contribution, and a pronounced enzymatic degradation of hemicellulose (β-xylosidase activity). Our study underscores the pivotal role of hydrological connectivity, DOM quality and microbial enzyme activity in shaping sediment P buffering and potential P release in river-floodplain systems. Moreover, our results demonstrate the importance of critical yet often overlooked benthic sediment processes and microbe-organic matter interactions for P dynamics in floodplain waterbodies.

Groundwater inflow can be a significant source of nutrients for riverine ecosystems, which can affect eutrophication i.e., the elevated primary production and the corresponding accumulation of algal biomass. Experimental and modelling work has shown that benthic algae (autotrophic biofilms) in particular benefit, as they have direct access to the inflowing groundwater-borne nutrients. Primarily the supply of phosphorus (P) enhances pelagic algal biomass, as it is the limiting nutrient for primary production in most freshwater systems. In this study, we estimate the effect of groundwater inflow on overall eutrophication of a large, European lowland river and tested its seasonal effect on biofilms in particular. We calculated the effects on overall eutrophication during summer according to the estimated input of groundwater-borne P and the C:P stoichiometry of planktonic algae in the Elbe River. Our model indicated that these diffuse P inputs have the potential to significantly increase eutrophication. Groundwater-P can contribute up to 1.5 t/d PO4 over the investigated 450 km stretch of the Elbe River under low flow conditions. This would result in an additional planktonic load of about 46 t/d of particulate organic carbon, thereby contributing to eutrophication at the regional scale in this river. In contrast, at the local scale, biofilms were collected seasonally from artificial substrata exposed in the river either in hydrogeologically active areas with groundwater inflow, or in areas of varying hydraulic connectivity. Analyses of biofilm macronutrients, structural components and biofilm community composition show distinct effects of season, hydrogeology and groundwater inflow. The dominant predictors were season and the interaction between hydrogeology and groundwater. Benthic eutrophication is most likely to occur in autumn in areas of loose rock with high groundwater inflow. The strong interaction of environmental factors in determining benthic eutrophication highlights the need to assess these factors in combination rather than in isolation.

Microbial consortia in riverbed substrates and their extracellular matrix (biofilms) play a key role in phosphorus (P) entrapment. When P entrapment saturates, the benthic compartment changes from a P sink to a P source thus increasing eutrophication risk. P entrapment saturation is expected to differ between intracellular and extra- cellular P entrapment and between different magnitudes and durations of P inputs. We studied biofilm Pentrapment following short (48 h) and long (14 days) P loading events in stream bypass flumes supplied with a gradient of dissolved P concentrations. This allowed us to link local biofilm processes in sediments to potential effects on river self-purification, via quantifying the P removal efficiency in the flumes. We found that in shortterm events, biofilms develop intracellular mechanisms to cope with P inputs, while long-term events and high P inputs suppress the intracellular uptake mechanisms and increase the prevalence of extracellular entrapment. Specifically, long-term events lowered the threshold for intracellular P entrapment saturation, and decreased the ratio between intracellular and extracellular entrapment resulting in lower removal efficiency for dissolved phosphorus. Our results highlight the risk that aquatic ecosystems may face as the ratio of intracellular to extracellular P entrapment decreases, which may reduce their ability to deal with P inputs, thereby increasing risks of eutrophication.

Catchment urbanisation results in urban streams being exposed to a multitude of stressors. Notably, stressors originating from diffuse sources have received less attention than stressors originating from point sources. Here, advances related to diffuse urban stressors and their consequences for stream benthic communities are summarised by reviewing 92 articles. Based on the search criteria, the number of articles dealing with diffuse urban stressors in streams has been increasing, and most of them focused on North America, Europe, and China. Land use was the most common measure used to characterize diffuse stressor sources in urban streams (70.7 % of the articles characterised land use), and chemical stressors (inorganic nutrients, xenobiotics, metals, and water properties, including pH and conductivity) were more frequently reported than physical or biological stressors. A total of 53.3 % of the articles addressed the impact of urban stressors on macroinvertebrates, while 35.9 % focused on bacteria, 9.8 % on fungi, and 8.7 % on algae. Regarding ecosystem functions, almost half of the articles (43.5 %) addressed changes in community dynamics, 40.3 % addressed organic matter decomposition, and 33.9 % addressed nutrient cycling. When comparing urban and non-urban streams, the reviewed studies suggest that urbanisation negatively impacts the diversity of benthic organisms, leading to shifts in community composition. These changes imply functional degradation of streams. The results of the present review summarise the knowledge gained to date and identify its main gaps to help improve our understanding of urban streams.

Saline shallow lakes in arid and semi-arid regions frequently undergo drying episodes, leading to significant variations in salinity and water availability. Research on the impacts of salinity and drought on the structure and function of biofilms in hypersaline shallow lakes is limited. This study aimed to understand the potential changes of biofilms in playa-lake sediments during the drying process. Sediments were sampled at different depths (surface, subsurface) and hydrological periods (wet, retraction, and dry), which included a decrease in water activity (aw, the availability of water for microbial use) from 0.99 to 0.72. aw reduction caused a greater effect on functional variables compared to structural variables, indicating the high resistance of the studied biofilms to changes in salinity and water availability. Respiration and hydrolytic extracellular enzyme activities exhibited higher values under high aw, while phenol oxidase activity and prokaryote biomass increased at lower aw. This shift occurred at both depths but was more pronounced at the surface, possibly due to the more extreme conditions (up to 0.7 aw). The increased levels of extracellular polymeric substances and carotenoids developed at low aw may help protect microorganisms in high salinity and drought environments. However, these harsh conditions may interfere with the activity of hydrolytic enzymes and their producers, while promoting the growth of resistant prokaryotes and their capacity to obtain C and N sources from recalcitrant compounds. The resilience of biofilms in hypersaline lakes under extreme conditions is given by their resistant biochemichal structure and the adaptability of their microbial functioning.

Proper bed substrate selection is essential for the growth of microorganisms and plants, which are key elements in constructed wetlands (CWs) performance. This is even more important in CWs that treat irrigated agricultural drainage water characterised by a high nitrogen (N), but low carbon (C) and phosphorus (P) concentration. This nutrient imbalance compromises biota activity and limits water treatment processes. Three substrates were tested in a CW field-scale pilot plant for 2 years: gravel (100%) and two mixed substrates, gravel + 30% natural wetland soil and gravel + 10% biochar. While gravel is one of the most commonly used substrates in subsurface flow CWs, soil has hardly been employed, and field-scale biochar application studies are scarce. We analysed the effect of adding a C-rich substrate to gravel on both nutrient imbalance correction and the biotic performance of CWs. Adding natural soil or biochar increased P availability in beds. However, the dissolved organic C concentration in interstitial water was only enhanced by soil addition. Unlike gravel, microbial density and activity, and Phragmites australis plant growth, were higher in beds with soil, followed by those with biochar. A similar pattern was observed for plant tissue quality. Although biochar proved positive for biota, it had a short-term effect as a source of C and P. Therefore, employing a suitable gravel-soil mixture provides notable advantages for the irrigated agricultural water treatment challenge. Even after considering some limitations, our results represent an important step to design CWs that treat this wastewater type and for eutrophication control in sustainable agroecosystems.

Both gradual and extreme weather changes trigger complex ecological responses in river ecosystems. It is still unclear to what extent trend or event effects alter biodiversity and functioning in river ecosystems, adding considerable uncertainty to predictions of their future dynamics. Using a comprehensive database of 71 published studies, we show that event - but not trend - effects associated with extreme changes in water flow and temperature substantially reduce species richness. Furthermore, event effects - particularly those affecting hydrological dynamics - on biodiversity and primary productivity were twice as high as impacts due to gradual changes. The synthesis of the available evidence reveals that event effects induce regime shifts in river ecosystems, particularly affecting organisms such as invertebrates. Among extreme weather events, dryness associated with flow interruption caused the largest effects on biota and ecosystem functions in rivers. Effects on ecosystem functions (primary production, organic matter decomposition and respiration) were asymmetric, with only primary production exhibiting a negative response to extreme weather events. Our meta-analysis highlights the disproportionate impact of event effects on river biodiversity and ecosystem functions, with implications for the long-term conservation and management of river ecosystems. However, few studies were available from tropical areas, and our conclusions therefore remain largely limited to temperate river systems. Further efforts need to be directed to assemble evidence of extreme events on river biodiversity and functioning.

Biofilms in river sediments play a key role in P retention in aquatic ecosystems. Most studies in freshwater ecosystems focus mainly on the autotrophic component of biofilms but little is known about the role of heterotrophic components on P removal. It is known that DOC in some streams is of low bioavailability, hence, resulting in severe DOC and P co-limitation of heterotrophic biofilm growth which could then constrain P removal efficiency. How DOC limitation affects P removal efficiency in the benthic zone and how it modifies P thresholds (i.e. concentration from which the removal efficiency decreases) are still open questions.We performed an experiment in the MOBICOS (Streamside Mobile Mesocosms) in the Holtemme River (Germany) to study the role of labile DOC on P thresholds in P retention in the bed-sediment biofilm community. Our flume experiment followed a BACI design (before: no DOC addition; after: labile DOC addition at C:P molar ratios >100; control: basal P and DOC concentrations; impact: P concentrations ranging from 25 µg P/L to 420 µg P/L).Our results show that labile DOC increases the P removal efficiency of the system (i.e. P water mass balances in the flumes) and shifts P thresholds for P removal towards higher P concentrations meaning that at a given P concentration higher P removal efficiency is achieved if the system is supplied with labile DOC. Labile DOC activated the heterotrophic component in the flumes and benthic biofilms receiving labile DOC show higher bacterial density and higher P accumulation compared to the ones not receiving labile DOC.Our results demonstrate that the heterotrophic biofilm community plays a key role in in-stream phosphorus retention. As it relies on availability of labile DOC, the interaction of DOC and P dynamics need consideration in models for stream nutrient processing and retention.