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Streams and rivers play an important role in the global carbon cycle. The origins of CO2 in streams are often poorly constrained or neglected, which is especially true for CO2 originating from heterotrophic metabolism in streambeds. We hypothesized that sediment movement will have a direct effect on stream metabolism, and thus, the aim of this study was to quantify the effect of moving bedforms on the production of CO2 in sandy streambeds. We conducted flume experiments where we used planar optodes to measure the distributions of O2 and CO2 under various streambed celerities. We combined these measurements with an assessment of bed morphodynamics and modeling to calculate O2 consumption and CO2 production rates. Our results indicate that sediment transport can strongly influence streambed metabolism and CO2 production. We found that bedform celerity controls the shape of the hyporheic zone and exchange flux, and is directly linked to the spatial and temporal distributions of O2 and CO2. It was also found that the most pronounced change in CO2 production occurred when the bed changed from stationary conditions to a slowly moving bed. A more gradual increase in O2 consumption and CO2 production rates was observed with further increase in celerity. Our study also points out that bedform movement causes hydraulic isolation between the moving and the non‐moving fraction of the streambed that can lead to a transient storage of CO2 in deeper sediments, which may be released in bursts during bed scour. European Union BMBF DFG Israel Science Foundation http://dx.doi.org/10.13039/501100003977

The main objective of this study was to analyze the spatial and temporal variability of dissolved organic matter (DOM) in an urban setting, identify its drivers and analyze the potential of DOM as a monitoring tool. We sampled at 32 sites within the city of Berlin (Germany) during 4 different seasons in 2016 and 2017. Sites included rivers, streams, ponds, and lakes. We here present the data used for the study, including the DOM composition parameters and the physico-chemical variables. DOM composition included the Absorbance/Fluorescence analysis, PARAFAC modelling and the indexes from the ultrahigh-resolution Fourier-Transform Ion Cyclotron Mass Spectrometry (FT-ICR-MS) measurements. The physicochemical variables included nutrients and land use. &nbsp

Regional-scale ecological processes, such as the spatial flows of material, energy, and organisms, are fundamental for maintaining biodiversity and ecosystem functioning in river networks. Yet these processes remain largely overlooked in most river management practices and underlying policies. Here, we propose adoption of a meta-system approach, where regional processes acting at different levels of ecological organization – populations, communities, and ecosystems – are integrated into conventional river conservation, restoration, and biomonitoring. We also describe a series of measurements and indicators that could be assimilated into the implementation of relevant biodiversity and environmental policies. Finally, we highlight the need for alternative management strategies that can guide practitioners toward applying recent advances in ecology to preserve and restore river ecosystems and the ecosystem services they provide, in the context of increasing alteration of river network connectivity worldwide. In a nutshell: • Rivers are hotspots of biodiversity and provide essential ecosystem functions and services, but face numerous threats globally • Understanding of how rivers are organized across spatial scales has progressed considerably over the past several decades, proving that regional-scale processes are vital for preserving population, community, and ecosystem dynamics • However, most existing river conservation, restoration, and biomonitoring practices focus on local-scale strategies and measures • To improve the management of river networks in the Anthropocene, we suggest additional metrics and assessment approaches that incorporate regional processes more effectively

International audience

Regional-scale ecological processes, such as the spatial flows of material, energy, and organisms, are fundamental for maintaining biodiversity and ecosystem functioning in river networks. Yet these processes remain largely overlooked in most river management practices and underlying policies. Here, we propose adoption of a meta-system approach, where regional processes acting at different levels of ecological organization – populations, communities, and ecosystems – are integrated into conventional river conservation, restoration, and biomonitoring. We also describe a series of measurements and indicators that could be assimilated into the implementation of relevant biodiversity and environmental policies. Finally, we highlight the need for alternative management strategies that can guide practitioners toward applying recent advances in ecology to preserve and restore river ecosystems and the ecosystem services they provide, in the context of increasing alteration of river network connectivity worldwide.

River networks are among Earth's most threatened hot-spots of biodiversity and provide key ecosystem services (e.g., supply drinking water and food, climate regulation) essential to sustaining human well-being. Climate change and increased human water use are causing more rivers and streams to dry, with devastating impacts on biodiversity and ecosystem services. Currently, more than a half of the global river networks consist of drying channels, and these are expanding dramatically. However, drying river networks (DRNs) have received little attention from scientists and policy makers, and the public is unaware of their importance. Consequently, there is no effective integrated biodiversity conservation or ecosystem management strategy of DRNs.A multidisciplinary team of 25 experts from 11 countries in Europe, South America, China and the USA will build on EU efforts to assess the cascading effects of climate change on biodiversity, ecosystem functions and ecosystem services of DRNs through changes in flow regimes and water use. DRYvER (DRYing riVER networks) will gather and upscale empirical and modelling data from nine focal DRNs (case studies) in Europe (EU) and Community of Latin American and Caribbean States (CELAC) to develop a meta-system framework applicable to Europe and worldwide. It will also generate crucial knowledge-based strategies, tools and guidelines for economically-efficient adaptive management of DRNs. Working closely with stakeholders and end-users, DRYvER will co-develop strategies to mitigate and adapt to climate change impacts in DRNs, integrating hydrological, ecological (including nature-based solutions), socio-economic and policy perspectives. The end results of DRYvER will contribute to reaching the objectives of the Paris Agreement and placing Europe at the forefront of research on climate change.

Experimental mesocosms and design; Modeling metabolism; Sigmoid model for GPP and ER; Statistical analysis of weekly measures of Ecosystem Metabolism; Modeling DOM composition

Global urbanization trends impose major alterations on surface waters. This includes impacts on ecosystem functioning that can involve feedbacks on climate through changes in rates of greenhouse gas emissions. The combination of high nutrient supply and shallow depth typical of urban freshwaters is particularly conducive to high rates of methane (CH4) production and emission, suggesting a potentially important role in the global CH4 cycle. However, there is a lack of comprehensive flux data from diverse urban water bodies, of information on the underlying drivers, and of estimates for whole cities. Based on measurements over four seasons in a total of 32 water bodies in the city of Berlin, Germany, we calculate the total CH4 emission from various types of surface waters of a large city in temperate climate at 2.6 ± 1.7 Gg CH4/year. The average total emission was 219 ± 490 mg CH4 m−2 day−1. Water chemical variables were surprisingly poor predictors of total CH4 emissions, and proxies of productivity and oxygen conditions had low explanatory power as well, suggesting a complex combination of factors governing CH4 fluxes from urban surface waters. However, small water bodies (area

Balkan rivers are experiencing a massive escalation in hydropower development unparalleled anywhere in Europe. This is largely prompted by the perceived need for renewable energy to fulfil a European Union Directive (2009/28/EC) through creating publicly subsidized investment opportunities. Decision- making processes sometimes lack evidence-based knowledge and ignore environmental and social impacts. This openly conflicts with Sustainable Development Goals since exceptionally widespread good hydromorphological status, high biodiversity, and significant potential for sustainable development prevail along rivers throughout the Balkans. Evidence-based planning integrating ecological, economic, social and cultural aspects is in some cases apparently not taking place. To address deficiencies in ecological data and risk assessment we here call for formation of a network of researchers motivated to produce scientific evidence for the ecological functioning of Balkan rivers and hydropower impacts. Our motivation is three- fold: (1) Demonstrating our wish to be heard as experts and our ability to produce scientific data that could support wise (trans)national decision-making ; (2) Assuming societal responsibility by pinpointing destructive impact and promoting conservation as senior experts while meaningfully fostering careers of young engaged colleagues ; and (3) Producing high- quality science with minimum means by collaboratively gathering data at a hitherto unachieved, sub-continental scale. We suggest campaigns aiming at collating data on selected key aspects of 4 domains of ecological knowledge: habitat properties, biodiversity, ecosystem functioning and ecosystem service provisioning. We search for senior partners and young scientists throughout the region and beyond, ready to contribute expertise, facilities and manpower – to produce ecological evidence at a scale matching the current plans for hydropower development.

River‐floodplain systems are characterized by changing hydrological connectivity and variability of resources delivered to floodplain water bodies. Although the importance of hydrological events has been recognized, the effect of flooding on CH4 concentrations and emissions from European, human‐impacted river‐floodplains is largely unknown. This study evaluates aquatic concentrations and emissions of CH4 from a highly modified, yet partly restored river‐floodplain system of the Danube near Vienna (Austria). We covered a broad range of hydrological conditions, including a 1‐yr flood event in 2012 and a 100‐yr flood in 2013. Our findings demonstrate that river‐floodplain waters were supersaturated with CH4, hence always serving as a source of CH4 to the atmosphere. Hydrologically isolated habitats in general have higher concentrations and produce higher fluxes despite lower physically defined velocities. During surface connection, however, CH4 is exported from the floodplain to the river, suggesting that the main channel serves as an “exhaust pipe” for the floodplain. This mechanism was especially important during the 100‐yr flood, when a clear pulse of CH4 was flushed from the floodplain with surface floodwaters. Our results emphasize the importance of floods differing in magnitude for methane evasion from river‐floodplains; 34% more CH4 was emitted from the entire system during the year with the 100‐yr flood compared to a hydrologically “normal” year. Compared to the main river channel, semi‐isolated floodplain waters were particularly strong sources of CH4. Our findings also imply that the predicted increased frequency of extreme flooding events will have significant consequences for methane emission from river‐floodplain systems.

The rapid erosion of biodiversity is among the biggest challenges human society is facing. Concurrently, major efforts are in place to quantify changes in biodiversity, to understand the consequences for ecosystem functioning and human wellbeing, and to develop sustainable management strategies. Based on comprehensive bibliometric analyses covering 134,321 publications, we report systematic spatial biases in biodiversity-related research. Research is dominated by wealthy countries, while major research deficits occur in regions with disproportionately high biodiversity as well as a high share of threatened species. Similarly, core scientists, who were assessed through their publication impact, work primarily in North America and Europe. Though they mainly exchange and collaborate across locations of these two continents, the connectivity among them has increased with time. Finally, biodiversity-related research has primarily focused on terrestrial systems, plants, and the species level, and is frequently conducted in Europe and Asia by researchers affiliated with European and North American institutions. The distinct spatial imbalances in biodiversity research, as demonstrated here, must be filled, research capacity built, particularly in the Global South, and spatially-explicit biodiversity data bases improved, curated and shared.

Within the project CONNECT we are establishing a collaborative network between experts in remote sensing (RS) and freshwater ecology to study connectivity and coherence of lake ecosystems in a regional context at unprecedented temporal and spatial resolution. The overall aim is to understand the yet unexplained variation in phytoplankton dynamics among river-connected German lowland lakes, many of which are presently classified as in poor to bad ecological status. These lakes often face a high risk of eutrophication, mass development of harmful algal blooms, and high production of greenhouse gases. We suggest if measured on adequate temporal and spatial scales much of the among-lake variation in phytoplankton dynamics to be explained by the strength of hydrological lake-to-lake and lake-to-catchment connectivity as modulated by lake depth and mixing regime. This may have profound implications for the maximum intensity, spatial range and regional-scale magnitude of eutrophication impacts. We will use (i) a large-scale experimental manipulation of lake connectivity, and (ii) an observational field campaign contrasting deep and shallow river-connected lakes, to challenge this research frontier by an innovative combination of automatic high- frequency in situ measurements with state of the art near-to-far RS technology. Climate change is expected to alter the hydrology, and thus the connectivity of lake-river systems. However, it is also predicted to increase extreme weather events leading to an increased input of nutrients as well as colored dissolved organic matter (cDOM). By providing data of high spatio-temporal coverage, CONNECT will provide basic high quality data to better understand mechanisms of eutrophication at the local and regional scale. Our data, thus, provide a valuable basis to improve current management of such river-connected lake ecosystems under future climate scenarios. To reach this ambitious goal, the project will (i) build a cross- disciplinary collaborative network of excellence, (ii) develop a mechanistic understanding of lake ecosystem functioning at local and regional scale, (iii) improve future environmental monitoring and interpretation of available data from inland waters, and (iv) support more effective integrated management of river-connected lakes to mitigate eutrophication impacts.

We established a new indoor mesocosm facility, 12 fully controlled “Planktotrons”, designed to conduct marine and freshwater experiments for biodiversity and food web approaches using natural or artificial, benthic or planktonic communities. The Planktotrons are a unique and custom-tailored facility allowing long-term experiments. Wall growth can be inhibited by a rotating gate paddle with silicone lips. Additionally, temperature and light intensity are individually controllable for each Planktotron and the large volume (600 L) enables high-frequency or volume-intense measurements. In a pilot freshwater experiment various trophic levels of a pelagic food web were maintained for up to 90 d. First, an artificially assembled phytoplankton community of 11 species was inoculated in all Planktotrons. After 22 d, two ciliates were added to all, and three Daphnia species were added to six Planktotrons. After 72 d, dissolved organic matter (DOM, an alkaline soil extract) was added as an external disturbance to six of the 12 Planktotrons, involving three Planktotrons stocked with Daphnia and three without, respectively. We demonstrate the suitability of the Planktotrons for food web and biodiversity research. Variation among replicated Planktotrons (n = 3 minimum) did not differ from other laboratory systems and field experiments. We investigated population dynamics and interactions among the different trophic levels, and found them affected by the sequence of ciliate and Daphnia addition and the disturbance caused by addition of DOM.

We established a new indoor mesocosm facility, 12 fully controlled Planktotrons, designed to conduct marine and freshwater experiments for biodiversity and food web approaches using natural or artificial, benthic or planktonic communities. The Planktotrons are a unique and custom-tailored facility allowing long-term experiments. Wall growth can be inhibited by a rotating gate paddle with silicone lips. Additionally, temperature and light intensity are individually controllable for each Planktotron and the large volume (600 L) enables high-frequency or volume-intense measurements. In a pilot freshwater experiment various trophic levels of a pelagic food web were maintained for up to 90 d. First, an artificially assembled phytoplankton community of 11 species was inoculated in all Planktotrons. After 22 d, two ciliates were added to all, and three Daphnia species were added to six Planktotrons. After 72 d, dissolved organic matter (DOM, an alkaline soil extract) was added as an external disturbance to six of the 12 Planktotrons, involving three Planktotrons stocked with Daphnia and three without, respectively. We demonstrate the suitability of the Planktotrons for food web and biodiversity research. Variation among replicated Planktotrons (n=3 minimum) did not differ from other laboratory systems and field experiments. We investigated population dynamics and interactions among the different trophic levels, and found them affected by the sequence of ciliate and Daphnia addition and the disturbance caused by addition of DOM.

While glaciers become increasingly recognised as a habitat for diverse and active microbial communities, effects of their climate change-induced retreat on the microbial ecology of glacier-fed streams remain elusive. Understanding the effect of climate change on microorganisms in these ecosystems is crucial given that microbial biofilms control numerous stream ecosystem processes with potential implications for downstream biodiversity and biogeochemistry. Here, using a space-for-time substitution approach across 26 Alpine glaciers, we show how microbial community composition and diversity, based on 454-pyrosequencing of the 16S rRNA gene, in biofilms of glacier-fed streams may change as glaciers recede. Variations in streamwater geochemistry correlated with biofilm community composition, even at the phylum level. The most dominant phyla detected in glacial habitats were Proteobacteria, Bacteroidetes, Actinobacteria and Cyanobacteria/chloroplasts. Microorganisms from ice had the lowest α diversity and contributed marginally to biofilm and streamwater community composition. Rather, streamwater apparently collected microorganisms from various glacial and non-glacial sources forming the upstream metacommunity, thereby achieving the highest α diversity. Biofilms in the glacier-fed streams had intermediate α diversity and species sorting by local environmental conditions likely shaped their community composition. α diversity of streamwater and biofilm communities decreased with elevation, possibly reflecting less diverse sources of microorganisms upstream in the catchment. In contrast, β diversity of biofilms decreased with increasing streamwater temperature, suggesting that glacier retreat may contribute to the homogenisation of microbial communities among glacier-fed streams.

Stream ecosystem metabolism modulates organic carbon (OC) cycling in rivers and regulates downstream OC fluxes. Climate-change scenarios predict precipitation to shift from snow towards rain in alpine ecosystems, however consequences of this shift to stream metabolism are poorly understood. Using 18-months of continuous metabolism estimates encompassing two winters with ‘high’ (1807 cm) and ‘low’ (407 cm) snow accumulation, we studied effects of snowmelt hydrology on stream ecosystem metabolism across 12 sites in an Alpine network (Ybbs River, Austria). Net ecosystem production (NEP) ranged from -41.3 to 15.7 g O2 m-2 d-1 across the network. Times of autotrophy coincided with spring snowmelt but the inter-annual comparison of spring metabolism indicated reduced snowmelt shifted metabolism towards heterotrophy. Catchment size and the variation of snowpack (‘high’ versus ‘low’) across the elevation gradient drove this shift in NEP. Furthermore, inter-annual carbon spiralling turnover lengths decreased, resulting in increased OC loss likely due differences in stream discharge and respiration. We suggest that metabolism in alpine streams respond to precipitation changes by shifting from autotrophy to heterotrophy during spring and potentially from CO2 sinks to sources.

Supplementary Information. (PDF 10383Â kb)

Intermittent rivers and ephemeral streams (IRES) encompass fluvial ecosystems that eventually stop flowing and run dry at some point in space and time. During the dry phase, channels of IRES consist mainly of dry riverbeds (DRBs), prevalent yet widely unexplored ecotones between dry and wet phases that can strongly influence the biogeochemistry of fluvial networks. DRBs are often overlooked because they do not strictly belong to either domain of soil or freshwater science. Due to this dual character of DRBs, we suggest that concepts and knowledge from soil science can be used to expand the understanding of IRES biogeochemistry. Based on this idea, we propose that DRBs can be conceptually understood as early stage soils exhibiting many similarities with soils through two main forces: i) time since last sediment transport event, and ii) the development status of stabilizing structures (e.g. soil crusts and/or vascular plants). Our analysis suggests that while DRBs and soils may differ in master physical attributes (e.g. soil horizons vs fluvial sedimentary facies), they become rapidly comparable in terms of microbial communities and biogeochemical processes. We further propose that drivers of DRBs biogeochemistry are similar to those of soils and, hence, concepts and methods used in soil science are transferable to DRBs research. Finally, our paper presents future research directions to advance the knowledge of DRBs and to understand their role in the biogeochemistry of intermittent fluvial networks., Earth-Science Reviews, 188, ISSN:0012-8252, ISSN:1872-6828

The role of inland waters for the global carbon cycle is now recognized and evidence increasingly suggests that stream ecosystems disproportionately contribute to the carbon cycle. Understanding the dynamics and drivers of stream water partial pressure of CO2 (pCO2) and CO2 evasion fluxes from streams to the atmosphere is imperative for assessing the role of climate change on the carbon cycle in stream ecosystems. Monitoring pCO2 over 3 years, we here report on the seasonal, diurnal, and event-driven dynamics of pCO2 in the hyporheic zone and stream water of an Alpine stream and assess possible drivers of these dynamics. Our findings suggest that both catchment-derived CO2 delivered by shallow groundwater into the stream and in-stream respiration continuously build up pCO2 in the hyporheic zone. Depending on stream water temperature and assumedly on primary production (inferred from photosynthetically active radiation), hyporheic CO2 contributes to stream water pCO2 and ultimately to CO2 outgassing to the atmosphere. Diurnal patterns of stream water pCO2 increasingly built up during extended base flow and streambed-scouring storms caused the collapse of these diurnal patterns. Post storm recovery of the diurnal pCO2 patterns was generally rapid. Our findings suggest that decreasing gas exchange velocity related to receding discharge drives recovery dynamics. We found that average CO2 outgassing fluxes during night exceeded those during day by up to 1.8 times. Our study highlights temperature and hydrology—key components of climate change—as major drivers of pCO2 dynamics in Alpine streams. They also underscore the necessity to consider day-night differences in CO2 outgassing fluxes to properly establish carbon budgets and regional estimates of CO2 outgassing to the atmosphere.

Surface waters contribute substantially to carbon dioxide (CO2) emissions to the atmosphere. However, global estimates remain uncertain due to methodological difficulties, such as in precisely estimating gas transfer in steep upland streams. Here, we addressed the question of what drives CO2 evasion from steep mountainous stream network of the European Alps by assessing the spatial and temporal variation of partial pressure of CO2 (pCO2) for 148 streams and the gas transfer coefficient for CO2 (kCO2) for 88 locations within this 254 km2 watershed. Results show that log kCO2 can be predicted reasonably well (r2 = 0.71, p

Recent studies highlight linkages among the architecture of ecological networks, their persistence facing environmental disturbance, and the related patterns of biodiversity. A hitherto unresolved question is whether the structure of the landscape inhabited by organisms leaves an imprint on their ecological networks. We analyzed, based on pyrosequencing profiling of the biofilm communities in 114 streams, how features inherent to fluvial networks affect the co-occurrence networks that the microorganisms form in these biofilms. Our findings suggest that hydrology and metacommunity dynamics, both changing predictably across fluvial networks, affect the fragmentation of the microbial co-occurrence networks throughout the fluvial network. The loss of taxa from co-occurrence networks demonstrates that the removal of gatekeepers disproportionately contributed to network fragmentation, which has potential implications for the functions biofilms fulfill in stream ecosystems. Our findings are critical because of increased anthropogenic pressures deteriorating stream ecosystem integrity and biodiversity.

Streamflow variability is a major determinant of basin-scale distributions of benthic invertebrates. Here we present a novel procedure based on a probabilistic approach aiming at a spatially explicit quantitative assessment of benthic invertebrate abundance as derived from near-bed flow variability. Although the proposed approach neglects ecological determinants other than hydraulic ones, it is nevertheless relevant in view of its implications on the predictability of basin-scale patterns of organisms. In the present context, aquatic invertebrates are considered, given that they are widely employed as sensitive indicators of fluvial ecosystem health and human-induced perturbations. Moving from the analytical characterization of site-specific probability distribution functions of streamflow and bottom shear stress, we achieve a spatial extension to an entire stream network. Bottom shear stress distributions, coupled with habitat suitability curves derived from field studies, are used to produce maps of invertebrate suitability to shear stress conditions. Therefore, the proposed framework allows one to inspect the possible impacts on river ecology of human-induced perturbations of streamflow variability. We apply this framework to an Austrian river network for which rainfall and streamflow time series, river network hydraulic properties, and local information on invertebrate abundance for a limited number of sites are available. A comparison between observed species density versus modeled suitability to shear stress is also presented. Although the proposed strategy focuses on a single controlling factor and thus represents an ecological minimal model, it allows derivation of important implications for water resource management and fluvial ecosystem protection. Key Points Hydrologic variability is a major control of invertebrate habitat suitability New analytical basin-scale approach for pdfs of ecohydrological key features Austrian river basin used for ecohydrological data-model comparison

Glaciers harbour diverse microorganisms, which upon ice melt can be released downstream. In glacier-fed streams microorganisms can attach to stones or sediments to form benthic biofilms. We used 454-pyrosequencing to explore the bulk (16S rDNA) and putatively active (16S rRNA) microbial communities of stone and sediment biofilms across 26 glacier-fed streams. We found differences in community composition between bulk and active communities among streams and a stronger congruence between biofilm types. Relative abundances of rRNA and rDNA were positively correlated across different taxa and taxonomic levels, but at lower taxonomic levels, the higher abundance in either the active or the bulk communities became more apparent. Here, environmental variables played a minor role in structuring active communities. However, we found a large number of rare taxa with higher relative abundances in rRNA compared with rDNA. This suggests that rare taxa contribute disproportionately to microbial community dynamics in glacier-fed streams. Our findings propose that high community turnover, where taxa repeatedly enter and leave the ‘seed bank’, contributes to the maintenance of microbial biodiversity in harsh ecosystems with continuous environmental perturbations, such as glacier-fed streams.

Streams receive substantial terrestrial deliveries of dissolved organic matter (DOM). The chromophoric (CDOM) fraction of terrestrial deliveries confers the brown colour to streamwater, often understood as browning, and plays a central role in aquatic photochemistry and is generally considered resistant to microbial metabolism. To assess the relevance of terrigenous DOM for carbon fluxes mediated by stream microorganisms, we determined the bioavailable fraction of DOM and microbial carbon use efficiency (CUE), and related these measures to partial pressure of CO2 in headwater streams spanning across a browning gradient. Fluorescence and absorbance analyses revealed high molecular weight and aromaticity, and elevated contributions from humic-like components to characterize terrestrial CDOM. We found that microorganisms metabolized this material at the cost of low CUE and shifted its composition (from fluorescence and absorbance) towards less aromatic and low-molecular weight compounds. Respiration (from CUE) was related to CO2 supersaturation in streams and this relationship was modulated by DOM composition. Our findings imply that terrigenous DOM is respired by microorganisms rather than incorporated into their biomass, and that this channelizes terrigenous carbon to the pool of CO2 potentially outgassing from streams into the atmosphere. This finding may gain relevance as major terrigenous carbon stores become mobilized and browning progresses.

1. Stream ecosystem metabolism integrates production and respiration of organic matter and plays a fundamental role in the global carbon (C) cycle. Several studies have identified distal and proximal physical controls, for example, land use and transient storage, or the effects of water chemistry, that is, organic matter and nutrient availability, on stream metabolism. In parallel, research on organic matter quality has identified conspicuous gradients of chemical composition, yet mostly without demonstrating any functional implications. 2. We hypothesise that organic matter holds a key position in a more comprehensive causal framework of stream ecosystem metabolism, and that a concurrent study can improve mechanistic understanding. Specifically, we here postulate that dissolved organic matter (DOM) quality, that is, its chemical composition, acts as a control of ecosystem respiration (ER) as much as it is a result of gross primary production (GPP). As such, DOM quality likely forms a central link between land use and stream metabolism, besides known physical controls including transient storage and light availability. 3. To examine these hypotheses, we studied 33 streams in north-eastern Austria, a region with diverse land use ranging from semi-natural, forested areas to agricultural areas and settlements. We analysed DOM composition by absorbance and fluorescence spectroscopy, including modelling excitation–emission matrices with parallel factor analysis. We then opposed these data to GPP and ER estimated by fitting a metabolism model to single-station diurnal oxygen records. 4. Structural equation modelling revealed land use as a control on light conditions, DOM composition and concentration and nutrient concentrations, which together ultimately shaped GPP and ER. In particular, humified, coloured and aromatic DOM of predominantly terrestrial origin was prevalent in coniferous forest catchments and increased stream ER. Agricultural and urban areas enriched streams with phosphorous and nitrogen, which increased ER and GPP. Besides nutrients, GPP seemed to be weakly correlated with light availability and – in contrast to our hypothesis – left only a weak imprint on DOM composition. 5. Land-use change is rated as the most pervasive human influence on natural ecosystems and our results highlight its impact on aquatic GPP and ER in streams. To understand the role of inland waters in the global C cycle will require mechanistic understanding of ecosystem metabolism, which notably includes organic matter quality as a hitherto underappreciated key player.

Methane (CH4) emissions from urban fresh water are starting to be addressed. CH4 as a greenhouse gas (GHG), enhances global warming of the earth. Any anthropogenic source of this greenhouse gas should be understood in order to reduce its atmospheric concentrations. On the other side, urban areas and expanding, and they modify surface waters. Recent literature has superficially approached the effect of fresh water modifications by cities on CH4 dynamics. This lack of information seems essential as most of the urban areas are established near freshwater sources. This doctoral dissertation aimed to investigate the dynamics of surface fresh water CH4 in urban areas. This doctoral dissertation uses two temperate cities, Berlin in Germany and Madison, Wisconsin, USA. Whole city scale in the case of Berlin is used to calculate the first freshwater footprint form a European city. This first study also served to identify drivers of CH4 emissions in the water chemistry and the surrounding land use to each waterbody type (lakes, ponds, rivers and streams) summing 32 sites and sampled over a whole year. This study pointed to small water bodies as the hotspots of methane emission rates, while in area, lakes account for more than half of the annual emission. Streams were studied in multiple spatial scales in the second study, performed in Madison. Three streams were analysed at fine scales of 10 to 500 m, with the same methodology as in the previous study in Berlin. The aim was to understand what is the spatial autocorrelation of CH4 emissions. This study would enable me thus to know the error of the single point measurement extrapolation from Berlin. I found high patchiness of methane in urban streams in the order of 1.2 km. This result pointed out that there was an error in the extrapolation footprint as we missed variability in the fluvial network, but still was only the second estimation done in the world. From both studies, I also found high nutrient states as the main surface water explanatory variable for methane emissions. The explanatory power of surface water chemistry or land uses was lower than the one found in the sediments. A third study was conducted in the same sampling sites from Berlin, looking at the methane production and sediment characteristics, with special emphasis on specific pollutants (e.g. heavy metals or Diclofenac). I didn’t find any effect of the pollutants. Most of the production was driven by the organic matter content and the trophic state. This matches the results from the water chemistry but points to the high organic sediment found in the city of Berlin. This results could be easily extrapolated to other cities as usually loads from runoff are high and flow regimes are low. This dissertation provides a whole city methane footprint from fresh water. This footprint is in the same range as the only reference in the literature. Methane emission rates found are higher than in natural homologous water bodies, reflecting the profound effect cities have urban surface waters. Small water bodies have the highest emission rates. Most systems in urban areas tend to have high organic sediment and be a potential source of this GHG. Stakeholders should consider these results and take actions on the organic load urban fresh water receives to avoid higher methane emissions. Methan ist als Treibhausgas an der globalen Erderwärmung beteiligt. Um die atmosphärische Methankonzentration reduzieren zu können, müssen die natürlichen und anthropogen beeinflussten Quellen bekannt sein. Als eine natürliche Quelle des Gases wurden Binnengewässer identifiziert. Jedoch können anthropogene Veränderungen der Gewässer zur Steigerung der Methanemission führen. Erst seit kurzem werden urbane Gewässer, die meist starkem anthropogenem Druck ausgesetzt sind, hinsichtlich der CH4-Emissionen untersucht. Diese Dissertation widmet sich der Untersuchung der CH4-Dynamik in Oberflächengewässern urbaner Räume. Die Studien wurden in zwei Städte temperierter Klimazonen, Berlin in Deutschland und Madison in Wisconsin, USA, durchgeführt. In Berlin wurde auf Basis eines gesamtstädtischen Ansatzes der erste Methan -„footprint“ einer europäischen Stadt ermittelt. In dieser ersten Studie wurden auch die die Emission regulierenden Parameter, innerhalb der Wasserchemie und der Landnutzung in den Einzugsgebieten untersucht. Die Berliner Gewässersysteme wurden in die Klassen ‚Seen, Teiche, Flüsse und Bäche‘ eingeteilt, aus denen zufällig insgesamt 32 Untersuchungsstelle ausgewählt wurden. Alle wurden je einmal während der vier Jahreszeiten innerhalb eines Jahres beprobt. Es zeigte sich, dass kleine Wasserkörper „hotspots“ der Methanemission pro Quadratmeter Gewässerfläche darstellen. Aufgrund der deutlich größeren Gesamtfläche der Seen, ist deren Gesamtemission deutlich höher und stellt mehr als die Hälfte der totalen jährlichen Emission dar. Im zweiten Teil der Studie wurde die räumliche Variabilität der CH4-Emission aus Flüssen in Madison, USA, studiert. In hoher Auflösung von 10 bis 500 m wurden drei Flüsse mit identischer Methodik wie in dem Berliner Teil der Studie, untersucht. Dadurch konnte aus den hoch aufgelösten Messungen auch auf die Genauigkeit der punktuellen Messungen in Berliner Flüssen geschlossen werden. Es zeigte sich eine deutliche räumliche Variabilität in Flussabschnitten von 1,2 km Länge. Diese Variabilität konnte in den Berliner Flüssen nicht berücksichtigt werden. Jedoch stellen die Ergebnisse einen ersten Datensatz dar, der ebenso wie der weltweit einzig publizierte zweite Datensatz in weiteren Studien verfeinert werden muss. In beiden Studien konnte die Methanemission vor allem durch den Nährstoffgehalt im Wasserkörper erklärt werden. Der dritte Teil der Dissertation wurde in den identischen Untersuchungsobjekten (32) durchgeführt. Die potentielle Methanproduktion in Sedimenten wurde in Inkubationsexperimenten ermittelt. Schadstoffe, besonders Schwermetalle und Organika wie Diclofenac, wurden als Regulatoren der mikrobiellen Methanproduktion analysiert. Es konnte jedoch kein Effekt nachgewiesen werden. Die Produktion wurde vor allem durch den Gehalt an organischem Material und den Trophiezustand reguliert. Das Ergebnis deckt sich mit dem Ergebnis der Wasserchemie. Da die Fracht an organischem Material und Nährstoffen in die Gewässer in den meisten Städten sehr hoch ist, bei niedrigen Fliessgeschwindigkeiten, ist eine relativ hohe potentielle Methanbildung in urbanen Gewässern vorauszusagen. Diese Dissertation liefert eine Angabe zur Gesamtemission von Methan aus allen aquatischen Systemen der Großstadt Berlin. Dieser „footprint“ ist ähnlich zu dem einzig publizierten einer anderen Metropole (Mexiko City). Die Emissionen sind höher als in vergleichbaren natürlichen Gewässern, was den Effekt der urbanen Besiedlung widerspiegelt. Kleine Gewässer zeigen die höchsten Emissionsraten. Die meisten urbanen Gewässer sind aufgrund der hohen Anteile an organischem Material in den Sedimenten potentielle Quellen des Treibhausgases Methan. Ziel planerischer Entscheidungen sollte eine Reduzierung der Frachten an organischem Material und an Nährstoffen in urbane Oberflächengewässer sein, um langfristig den Anteil der aquatischen Treibhausgasfreisetzung zu reduzieren.