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Reservoir drawdown areas—where sediment is exposed to the atmosphere due to water-level fluctuations—are hotspots for carbon dioxide (CO2) emissions. However, the global extent of drawdown areas is unknown, precluding an accurate assessment of the carbon budget of reservoirs. Here we show, on the basis of satellite observations of 6,794 reservoirs between 1985 and 2015, that 15% of the global reservoir area was dry. Exposure of drawdown areas was most pronounced in reservoirs close to the tropics and shows a complex dependence on climatic (precipitation, temperature) and anthropogenic (water use) drivers. We re-assessed the global carbon emissions from reservoirs by apportioning CO2and methane emissions to water surfaces and drawdown areas using published areal emission rates. The new estimate assigns 26.2 (15–40) (95% confidence interval) TgCO2-C yr−1to drawdown areas, and increases current global CO2emissions from reservoirs by 53% (60.3 (43.2–79.5) TgCO2-C yr−1). Taking into account drawdown areas, the ratio between carbon emissions and carbon burial in sediments is 2.02 (1.04–4.26). This suggests that reservoirs emit more carbon than they bury, challenging the current understanding that reservoirs are net carbon sinks. Thus, consideration of drawdown areas overturns our conception of the role of reservoirs in the carbon cycle.

Headspace analysis of CO2 frequently has been used to quantify the concentration of CO2 in fresh water. According to basic chemical theory, not considering chemical equilibration of the carbonate system in the sample vials will result in a systematic error. By analysing the potential error for different types of water and experimental conditions, we show that the error incurred by headspace analysis of CO2 is less than 5 % for typical samples from boreal systems which have low alkalinity ( 1000 µatm). However, the simple headspace calculation can lead to high error (up to −300 %) or even impossibly negative values in highly undersaturated samples equilibrated with ambient air, unless the shift in carbonate equilibrium is explicitly considered. The precision of the method can be improved by lowering the headspace ratio and/or the equilibration temperature. We provide a convenient and direct method implemented in an R script or a JMP add-in to correct CO2 headspace results using separately measured alkalinity.

River sediments falling dry at low water level are sources of CO2 to the atmosphere. While the general relevance of CO2 emissions from dry sediments has been acknowledged and some regulatory mechanisms identified, knowledge on mechanisms and temporal dynamics is still sparse. Using a combination of high frequency measurements and detailed studies we thus aimed to identify processes responsible for CO2 emissions and to assess temporal dynamics of CO2 emissions from dry sediments at a large German river. CO2 emissions were largely driven by microbial respiration in the sediment. Observed CO2 fluxes could be explained by patterns and responses of sediment respiration rates measured in laboratory incubations. We exclude groundwater as a significant source of CO2 because potential evaporation rates were too low to explain CO2 fluxes by groundwater evaporation. Furthermore, CO2 fluxes were not related to radon fluxes, which we used to trace groundwater derived degassing of CO2. CO2 emissions were strongly regulated by temperature resulting in large diurnal fluctuations of CO2 emissions with emissions peaking during the day. The diurnal temperature – CO2 flux relation exhibited a hysteresis which highlights the effect of transport processes in the sediment and makes it difficult to identify temperature dependence from simple linear regressions. The temperature response of CO2 flux and sediment respiration rates in laboratory incubations was identical. Also deeper sediment layers apparently contributed to CO2 emissions because the CO2 flux was correlated with the thickness of the unsaturated zone, resulting in CO2 fluxes increasing with distance to the local groundwater level and with distance to the river. Rain events lowered CO2 emissions from dry river sediments probably by blocking CO2 transport from deeper sediment layers to the atmosphere. Terrestrial vegetation growing on exposed sediments largely increased respiratory sediment CO2 emissions. We show that the regulation of CO2 emissions from dry river sediments is complex. Diurnal measurements are mandatory and even CO2 uptake in the dark by phototrophic micro-organisms has to be considered when assessing the impact of dry sediments on CO2 emissions from rivers.

Meteorological Drivers of Mass and Energy Exchange between Inland Waters and the Atmosphere (MEDIWA) Die genaue Quantifizierung des Stoff- und Energieaustausches zwischen Binnengewässern und der bodennahen Luftschicht ist von großer wissenschaftlicher und praktischer Bedeutung. So sind zum Beispiel genaue Abschätzungen der auftretenden Verdunstungsraten essentiell für die Steuerung und Betriebsführung von Talsperren und Stauseen sowie für eine erfolgreiche Realisierung von Flutungsvorhaben in Bergbaufolgelandschaften. In gleicherweise sind Kenntnisse des Wärmewärmehaushalts bzw. des Wärmeaustauschs eines Gewässers mit der Atmosphäre sowie ein exaktes Wissen über den an der Wasseroberfläche stattfindenden Gasaustauschs wichtig, um die wasserwirtschaftlichen Steuermöglichkeiten effizient zur Optimierung und Verbesserung der Wassergüte in Gewässern einsetzen zu können. Exakte, direkte Messungen des an der Wasseroberfläche stattfindenden Stoff- und Energieaustauschs sind möglich, aber sehr teuer und technisch extrem aufwendig. Daher werden Stoff- und Energieflüsse (z.B. Verdunstungsraten) meist mit Hilfe von verschiedenen numerischen Modellen aus einfacher zu messenden limnologischen und meteorologischen Variablen abgeschätzt. Die Genauigkeit derartig bestimmter Austauschraten ist jedoch sehr stark abhängig von (i) der Komplexität des eingesetzten Modells, (ii) der Parametrisierung der Modellgleichungen und (iii) der Repräsentativität und Genauigkeit der zur Verfügung stehenden Eingangsdaten. In der Praxis treten dadurch oftmals Konfliktsituationen auf, da die für den Einsatz genauer, komplexer Modelle notwendigen Umweltmessdaten in vielen Anwendungsfällen nicht oder nicht in ausreichender Qualität bereitstehen. Dieser Sachverhalt führt in Konsequenz dazu, dass der Stoff- und Energieaustausch eines Gewässers meist nur sehr überschlägig quantifiziert werden kann. Ergänzend ist festzuhalten, dass auch komplexe Modelle nach wie vor erhebliche Defizite und Schwächen in der Nachbildung der in Gewässern ablaufenden Prozesse haben. Insgesamt sind die Möglichkeiten zur Modellierung des Stoff- und Energiehaushalts von Gewässern somit stark verbesserungsbedürftig. Mit dem Ziel Stoff- und Energieflüsse von Binnengewässern auf Grundlage allgemein verfügbarer Umweltgrößen genauer modellieren und prognostizieren zu können, sollen in dem Projekt -MEDIWA- die an der Wasseroberfläche stattfindenden Austauschprozesse intensiv messtechnisch untersucht werden. Auf Grundlage der gewonnenen Messdaten sollen Methoden entwickelt werden, um die Verlässlichkeit von Modellabschätzungen zu verbessern. Komplexe Zusammenhänge und Prozessketten sollen aufgelöst und durch mathematische Gleichungen beschreibbar gemacht werden. Im besonderem Fokus steht dabei die Analyse und quantitative Beschreibung, wie die zeitliche und räumliche Variabilität meteorologischer Steuergrößen den Stoff und Energiehaushalt eines Gewässers beeinflusst. In unserem Tagungsbeitrag wollen wir das Projekt vorstellen und erste Ergebnisse der gegenwärtig noch laufenden Messungen präsentieren. Insbesondere wollen wir die Aspekte der räumlichen Heterogenität meteorologischer Steuergrößen auf den Stoff- und Energiehaushalt eines Gewässers diskutieren und im Hinblick auf die Zielvorhaben des Projektes erörtern.

Freshwater microbes play a crucial role in the global carbon cycle. Anthropogenic stressors that lead to changes in these microbial communities are likely to have profound consequences for freshwater ecosystems. Using field data from the coordinated sampling of 617 lakes, ponds, rivers, and streams by citizen scientists, we observed linkages between microbial community composition, light and chemical pollution, and greenhouse gas concentration. All sampled water bodies were net emitters of CO2, with higher concentrations in running waters, and increasing concentrations at higher latitudes. Light pollution occurred at 75% of sites, was higher in urban areas and along rivers, and had a measurable effect on the microbial alpha diversity. Genetic elements suggestive of chemical stress and antimicrobial resistances (IntI1, blaOX58) were found in 85% of sites, and were also more prevalent in urban streams and rivers. Light pollution and CO2 were significantly related to microbial community composition, with CO2 inversely related to microbial phototrophy. Results of synchronous nationwide sampling indicate that pollution-driven alterations to the freshwater microbiome lead to changes in CO2 production in natural waters and highlight the vulnerability of running waters to anthropogenic stressors.

Despite streams and rivers play a critical role as conduits of terrestrially produced organic carbon to the atmosphere, fluvial CO

The eddy-covariance (EC) technique is used to determine mass and energy fluxes between the Earth’s surface and the lower atmosphere at high temporal resolution. Despite the frequent and successful use of the EC technique at terrestrial sites, its application over water surfaces is rare. We present one season of EC measurements conducted on the Rappbode Reservoir, Germany’s largest drinking water reservoir. A floating observation platform in the centre of the reservoir is used for observations of fluxes that were unaffected by surrounding land surfaces and therefore representative of the actual water–atmosphere exchange. The temporal patterns of sensible heat flux are inverted compared to land sites, since the maxima and the minima occur at night and day respectively. The latent heat flux and the evaporation are unexpectedly low for a site where evaporation is not limited by the availability of water. The daily totals in summer and autumn are only 50% and 75% of the potential evaporation assessed by the FAO grass-reference evaporation, respectively. Measurement uncertainties and the effects of the energy balance closure are ruled out as potential factors, so that low values appear to be a general feature of large water surfaces. The observed carbon dioxide fluxes are characterized by distinctive diurnal variations in a typical range for lakes and reservoirs. However, the methane fluxes are low compared to other inland waters.

Artificial water bodies like ditches, fish ponds, weirs, reservoirs, fish ladders, and irrigation channels are usually constructed and managed to optimize their intended purposes. However, human-made aquatic systems also have unintended consequences on ecosystem services and biogeochemical cycles. Knowledge about their functioning and possible additional ecosystem services is poor, especially compared to natural ecosystems. A GIS analysis indicates that currently only ~ 10% of European surface waters are covered by the European Water Framework directive, and that a considerable fraction of the excluded systems are likely human-made aquatic systems. There is a clear mismatch between the high possible significance of human-made water bodies and their low representation in scientific research and policy. We propose a research agenda to build an inventory of human-made aquatic ecosystems, support and advance research to further our understanding of the role of these systems in local and global biogeochemical cycles as well as to identify other benefits for society. We stress the need for studies that aim to optimize management of human-made aquatic systems considering all their functions and to support programs designed to overcome barriers of the adoption of optimized management strategies. MK and PSK were financially supported by the German Research Foundation (DFG) (project TregaTa, KO 1911/6- 1). JH was financially supported by the Grant Agency of the Czech Republic, project no. 17-09310S. RM was supported by project C-HydroChange funded by the Spanish Ministry of Economy, Industry and Competitiveness (CGL2017-86788-C3-2-P). AL was supported by Estonian Research Council Grants PSG 32 and IUT 21-2 of the Estonian Ministry of Education and Research. This study benefitted from the collaborative environment of the GLEON network. MK and PSK were financially supported by the German Research Foundation (DFG) (project TregaTa, KO 1911/6- 1). JH was financially supported by the Grant Agency of the Czech Republic, project no. 17-09310S. RM was supported by project C-HydroChange funded by the Spanish Ministry of Economy, Industry and Competitiveness (CGL2017-86788-C3-2-P). AL was supported by Estonian Research Council Grants PSG 32 and IUT 21-2 of the Estonian Ministry of Education and Research. This study benefitted from the collaborative environment of the GLEON network.

Despite substantial advances in quantifying greenhouse gas (GHG) emissions from dry inland waters, existing estimates mainly consist of carbon dioxide (CO

Understanding river-sea-systems requires a thorough understanding of processes that span different Earth system compartments. To overcome issues related to the interaction of different scientific disciplines and compartments, such as different measurement and calibration standards, quality control approaches and data formats for specific environmental parameters, joint measurement campaigns have been initiated within the Helmholtz Association’s MOSES (Modular Observation Solutions for Earth Systems) project. Following multiple senor comparison and intercalibration campaigns in 2019, MOSES’ Hydrological Extremes event chain working group initiated joint field campaigns in summer 2020 covering the Elbe river from the Czech-German border to the tidal Elbe and further on into the estuary and the German Bight.The fundamental objective was to establish scientifically sound and resilient multi-ship applicable sampling procedures and to create reference data for the main environmental parameters for future investigation of extreme events such as flooding and drought and their overall impact on the catchment region and the adjacent estuarine area of a large European fresh water / marine system. The campaign involved four research vessels, four research centers and spanned nearly two months. Measurements included standard hydrological and oceanographic parameters, as well as quantities relevant to the nutrient and carbonate system. Furthermore, selected water quality indicators and atmospheric measurements were performed. In the fresh water section of the Elbe river measurements were taken while drifting with the water mass. In the tidal section of the river sampling was done against the ebb current while in the North Sea a grid covering a large part of the German exclusive economic zone (EEZ) was sampled.We detected a longitudinal increase of phytoplankton biomass along the 585 km freshwater part of the river towards the tidal system. In contrast, concentrations of dissolved nitrate and phosphate decreased to low values due the uptake by planktonic algae. The concentration of dissolved CO2 decreased caused by increasing photosynthesis while the concentration of methane increased along the river stretch, particularly in the most downstream part when sedimentation of phytoplankton increased the organic load of sediments. The tidal part of the transect showed a strong influence of Hamburg harbor on almost all quantities, while downstream towards the estuary, the effects of the tidal cycle dominated variabilities. In the marine area, elevated chlorophyll concentrations were mainly found near the west coast of Schleswig-Holstein, probably mostly influenced by the Eider river outflow or the adjacent tidal flats. While most of the measured parameters showed an expected behavior relative to their individual compartments, the transfer of quantities between the compartments revealed rather complex and sometimes difficult to understand behaviors and patterns, especially when considering a functional quantitative analysis. The first results of this trans-compartment campaign showed that a quantitative understanding of the fate and dynamics of water constituents across compartments from the spring to the sea needs enhanced scientific collaboration and awareness to finally come to a better integrated understanding of physical, biogeochemical and biological processes from the local to the global scale.

Surface waters are known to be significant sources of greenhouse gases (CH4 and CO2), but our understanding of large scale patterns is still incomplete. The greenhouse gases in rivers originate both from in-stream processes and interactions with the catchment. For coastal seas, rivers are suspected to be one of the main source of greenhouse gases, while the role of the interjacent tidal flats is still ambiguous. Especially the reaction of the entire system on terrestrial hydrological extremes such as low flow situations are still under consideration. The functional understanding of such events and their impacts on the water chemistry along its transition pathway in the terrestrial and limnic compartment as well as in the coastal marine environment is crucially needed for the evaluation of its relevance in the Earth system. As part of a MOSES campaign (Modular Observation Solutions for Earth Systems) spanning disciplines as well as earth system compartments we investigated the aquatic as well as the atmospheric compartemt in and above the Elbe River from inland waters through the tidal section of the river and the estuary to the North Sea with the goal to explore spatial heterogeneity of CO2 and CH4 concentrations in the water and in ambient air above the water during a low water period in summer 2020. Overall, dissolved CH4 concentrations ranged over three orders of magnitude. Along the freshwater part of the transect, dissolved CH4 increased and weirs and harbors appeared to be hot spots of elevated CH4 concentrations both for the dissolved and atmospheric phase. We observed a longitudinal gradient of CO2 in the river which was closely linked to primary production. In the estuary and the marine part, dissolved CH4 concentrations of the transect were determined by the variability of temperature and salinity. Correlations with other water parameters revealed the complex regulation of dissolved CH4 concentrations along the freshwater-seawater continuum. For atmospheric CH4 above the North Sea, wind direction and wind speed proved to be crucial. Besides the typical diurnal fluctuations of atmospheric CO2 and CH4, an observed link between dissolved and atmospheric concentrations has to be further clarified.

Despite substantial advances in quantifying greenhouse gas (GHG) emissions from dry inland waters, existing estimates mainly consist of carbon dioxide (CO2) emissions. However, methane (CH4) may also be relevant due to its higher Global Warming Potential (GWP). We report CH4 emissions from dry inland water sediments to i) provide a cross-continental estimate of such emissions for different types of aquatic systems (i.e., lakes, ponds, reservoirs, and streams) and climate zones (i.e., tropical, continental, and temperate); and ii) determine the environmental factors that control these emissions. CH4 emissions from dry inland waters were consistently higher than emissions observed in adjacent uphill soils, across climate zones and in all aquatic systems except for streams. However, the CH4 contribution (normalized to CO2 equivalents; CO2-eq) to the total GHG emissions of dry inland waters was similar for all types of aquatic systems and varied from 10 to 21%. Although we discuss multiple controlling factors, dry inland water CH4 emissions were most strongly related to sediment organic matter content and moisture. Summing CO2 and CH4 emissions revealed a cross-continental average emission of 9.6 ± 17.4 g CO2-eq m−2 d−1 from dry inland waters. We argue that increasing droughts likely expand the worldwide surface area of atmosphere-exposed aquatic sediments, thereby increasing global dry inland water CH4 emissions. Hence, CH4 cannot be ignored if we want to fully understand the carbon (C) cycle of dry sediments.

Background Groundwater is typically over-saturated in CO2 with respect to atmospheric equilibrium. Irrigation with groundwater is a common agricultural practice in many countries, but little is known about the fate of dissolved inorganic carbon (DIC) in irrigation groundwater and its contribution to the CO2 emission inventory from land to the atmosphere. We performed a mesocosm experiment to study the fate of DIC entering agricultural drainage channels in the North China Plain. Specifically, we aimed to unravel the effect of flow velocity and nutrient on CO2 emissions. Results All treatments were emitting CO2. Approximately half of the DIC in the water was consumed by TOC production (1-16%), emitted to the atmosphere (14-20%), or precipitated as calcite (CaCO3) (14-20%). We found that DIC depletion was stimulated by nutrient addition, whereas more CO2 evasion occurred in the treatments without nutrients addition. On the other hand, about 50% of CO2 was emitted within the first 50h under high flow velocity. Thus, in the short term, high nutrient levels may counteract CO2 emissions from drainage channels, whereas the final fate of the produced biomass (burial versus mineralization to CO2 or even CH4) determines the duration of the effect. Conclusion Our study reveals that both hydrology and biological processes affect CO2 emissions from groundwater irrigation channels. The estimated CO2 emission from total groundwater depletion in the North China Plain is up to 0.52 ± 0.07 Mt CO2 y−1. Thus, CO2 emissions from groundwater irrigation should be considered in regional CO2 budgets, especially given that groundwater depletion is expected to acceleration in the future.

Headspace analysis of CO2 frequently has been used to quantify the concentration of CO2 in freshwater. According to basic chemical theory, not considering chemical equilibration of the carbonate system in the sample vials will result in a systematic error. In this paper we provide a method to quantify the potential error resulting from simple application of Henry's law to headspace CO2 samples. By analysing the potential error for different types of water and experimental conditions we conclude that the error incurred by headspace analysis of CO2 is less than 5 % for samples with pH

The exchange of oxygen between lakes and the atmosphere is assumed to be driven by diffusion. Here we show that lakes can emit significant amounts of O2 by emerging gas bubbles—a process called ebullition. We found very high proportions of 17 ± 10% O2 (maximum 34%) in emerging gas bubbles in two shallow eutrophic reservoirs. In the studied reservoirs, O2 emission by ebullition was of similar magnitude as diffusive O2 fluxes. By reanalyzing previous studies, we show that the process is ubiquitous and probably quantitatively relevant in many places. We present evidence that O2 in bubbles originates both from photosynthetic oxygen production and hence bubble formation in the oxic water and from stripping by emerging methane bubbles. Ebullition can turn lakes undersaturated in respect to the atmosphere into a net O2 source. Neglecting O2 ebullition leads to an overestimation of lake internal respiration.

Many inland waters exhibit complete or partial desiccation, or have vanished due to global change, exposing sediments to the atmosphere. Yet, data on carbon dioxide (CO2) emissions from these sediments are too scarce to upscale emissions for global estimates or to understand their fundamental drivers. Here, we present the results of a global survey covering 196 dry inland waters across diverse ecosystem types and climate zones. We show that their CO2 emissions share fundamental drivers and constitute a substantial fraction of the carbon cycled by inland waters. CO2 emissions were consistent across ecosystem types and climate zones, with local characteristics explaining much of the variability. Accounting for such emissions increases global estimates of carbon emissions from inland waters by 6% (~0.12 Pg C y−1). Our results indicate that emissions from dry inland waters represent a significant and likely increasing component of the inland waters carbon cycle., Many inland waters seasonally or permanently dry up, thus exposing sediments to the atmosphere. Here the authors show that a substantial amount of CO2 is emitted from these dry sediments, increasing current inland water carbon flux estimates by 6%.

Inland waters and reservoirs in particular are significant sources of methane to the atmosphere. However, little information is available on the extent to which organic carbon from terrestrial vegetation or from internal photosynthesis fuels the methane production. This limits our ability to constrain methane emissions efficiently. We studied the isotopic composition (13C, 14C) of pelagic and sedimentary carbon sources in two small German reservoirs. The methane was enriched by radiocarbon with isotopic ranges (∆14C 5‰ to 31‰) near to fresh terrestrial organic carbon (OC, 17‰ to 26‰). In contrast, potential source OC produced by internal photosynthesis was characterized by negative ∆14C values (−30‰ and −25‰) as derived from signatures of inorganic carbon in the reservoirs. The particulate OC in stream supplies (terrestrial OC) was also 14C depleted in almost all cases, but highly variable in ∆14C (−131‰ to 42‰). Although the import of terrestrial OC was lower than the amount of OC produced by reservoir-internal photosynthesis, we conclude that the methane production was predominantly fuelled by catchment vegetation. The utilized terrestrial OC was of contemporary origin, fixed within years to decades before sampling and supplemented with reservoir-internal or aged terrestrial OC. Our results indicate that terrestrial biomass is an important driver of methane production in reservoirs receiving significant imports of terrestrial OC.

Natural neutralization of acidic mining lakes is often limited by organic matter. The knowledge of the sources and degradability of organic matter is crucial for understanding alkalinity generation in these lakes. Sediments collected at different depths (surface sediment layer from 0 to 1 cm and deep sediment layer from 4 to 5cm) from an acidic mining lake were studied in order to characterize sedimentary organic matter based on neutral signature markers. Samples were exhaustively extracted, subjected to pre-chromatographic derivatizations and analyzed by GC/MS. Herein, molecular distributions of diagnostic alkanes/alkenes, terpenes/terpenoids, polycyclic aromatic hydrocarbons, aliphatic alcohols and ketones, sterols, and hopanes/hopanoids were addressed. Characterization of the contribution of natural vs. anthropogenic sources to the sedimentary organic matter in these extreme environments was then possible based on these distributions. With the exception of polycyclic aromatic hydrocarbons, combined concentrations across all marker classes proved higher in the surface sediment layer as compared to those in the deep sediment layer. Alkane and aliphatic alcohol distributions pointed to predominantly allochthonous over autochthonous contribution to sedimentary organic matter. Sterol patterns were dominated by phytosterols of terrestrial plants including stigmasterol and β-sitosterol. Hopanoid markers with the ββ-biohopanoid "biological" configuration were more abundant in the surface sediment layer, which pointed to higher bacterial activity. The pattern of polycyclic aromatic hydrocarbons pointed to prevailing anthropogenic input. Pyrolytic makers were likely to due to atmospheric deposition from a nearby former coal combustion facility. The combined analysis of the array of biomarkers provided new insights into the sources and transformations of organic matter in lake sediments.

Rising concentrations of dissolved organic carbon (DOC) are negatively affecting the water quality in several drinking water reservoirs. The presence of beaver dams can influence surface water quality on a catchment scale. In recent years, beavers have been re-introduced in numerous locations in Central Europe. We investigated whether the presence of beaver dams in the catchment of a German drinking water reservoir impacts DOC quantity and quality in the streams entering the Wehebach reservoir in Germany. By comparing water quality upstream and downstream of beaver dams during three low discharge situations we did not find a significant effect of dams both on DOC quantity and quality. The analysis of long term monitoring data at the gauges showed that beaver dams had a negligible effect on the DOC load to the reservoir. DOC quantity was closely linked to iron concentration in the streams. Co-precipitation with iron minerals was an effective process removing DOC from the stream-water. By analyzing fluorescence excitation emission indices we show that beaver dams did not have a clear effect on DOC quality. We conclude that the presence of beaver dams has only small effects on water quality and is not a problem for water quality in the downstream drinking water reservoir.

Although the occurrence of metalimnetic oxygen minima (MOM) in lakes during summer stratification was described by early limnologists, not much is known about the processes leading to its formation. Generally, dissolved oxygen (DO) consumption and net transport contribute to the observed DO decrease, but the latter was rarely considered in former studies. To examine the importance of both processes to MOM development in Lake Arendsee, Germany, we measured DO concentration and temperature with high spatial and temporal resolution during a complete stratification period. Vertical turbulent diffusivities were estimated from temporal changes of the heat content of the lake. The MOM development was not caused by locally enhanced DO consumption rates, but rather by a combination of vertical gradients in DO concentration and consumption. A vertical DO mass balance revealed that net DO transport into the metalimnion was in the same order of magnitude as DO consumption in the MOM. We found that DO consumption was governed by microbial respiration and the vertical variations of DO depletion rates in the metalimnion could be explained by a minor contribution of temperature and a higher contribution of turbidity, implying that the downward flux of particulate organic carbon promoted the MOM development. The intensive metalimnetic respiration in lakes forming a MOM can be expected to accelerate nutrient cycling close to the photic zone and thus, may further stimulate primary production.

The trend of increasing dissolved organic carbon (DOC) in surface waters motivated us to gain a mechanistic understanding of DOC exchange at the sediment water interface in lakes. We quantified seasonal DOC and solute fluxes under different redox conditions, in a small drinking water reservoir using sediment core incubations. Processes governing benthic DOC exchange were microbial production of DOC and interaction with mineral surfaces. Mobilization of DOC in anoxic sediments seemed to be closely linked to reductive dissolution of ferric minerals as shown by the strong positive correlation (r2 = 0.99) between DOC and Fe fluxes. Oxidized surface sediments were an efficient DOC trap where DOC was bound to ferric minerals. Redox conditions appeared to be the primary regulator of the DOC exchange, resulting in sedimentary uptake of DOC (−1.8 mmol m−2 d−1) only under oxic conditions. DOC production was regulated by temperature, leading to higher DOC fluxes (up to 2.4 mmol m−2 d−1) in summer. The sediment was a net sink of DOC (−0.2 tonnes yr−1) but this was small compared with the annual DOC load (15 tonnes yr−1) of the reservoir. However, the benthic DOC flux was about 10–20% of the dissolved carbon flux at the sediment–water interface, making it a significant process in the lake internal carbon cycle. Climate change is supposed to promote reducing conditions at the bottom of lakes, which might increase benthic DOC production in the future.

During summer drought, Mediterranean fluvial networks are transformed into highly heterogeneous landscapes characterized by different environments (i.e., running and impounded waters, isolated river pools and dry beds). This hydrological setting defines novel biogeochemically active areas that could potentially increase the rates of carbon emissions from the fluvial network to the atmosphere. Using chamber methods, we aimed to identify hot spots for carbon dioxide (CO2) and methane (CH4) emissions from two typical Mediterranean fluvial networks during summer drought. The CO2 efflux from dry beds (mean ± SE = 209 ± 10 mmolCO2m-2 d-1) was comparable to that from running waters (120 ± 33 mmol m-2 d-1) and significantly higher than from impounded waters (36.6 ± 8.5 mmol m-2 d-1) and isolated pools (17.2 ± 0.9 mmol m-2 d-1). In contrast, the CH4 efflux did not significantly differ among environments, although the CH4 efflux was notable in some impounded waters (13.9 ± 10.1 mmol CH4 m-2 d-1) and almost negligible in the remaining environments (mean\0.3 mmol m-2 d-1). Diffusion was the only mechanism driving CO2 efflux in all environments and was most likely responsible for CH4 efflux in running waters, isolated pools and dry beds. In contrast, the CH4 efflux in impounded waters was primarily ebullition-based. Using a simple heuristic approach to simulate potential changes in carbon emissions from Mediterranean fluvial networks under future hydrological scenarios, we show that an extreme drying out (i.e., a four-fold increase of the surface area of dry beds) would double the CO2 efflux from the fluvial network. Correspondingly, an extreme transformation of running waters into impounded waters (i.e., a twofold increase of the surface area of impounded waters) would triple the CH4 efflux. Thus, carbon emissions from dry beds and impounded waters should be explicitly considered in carbon assessments of fluvial networks, particularly under predicted global change scenarios, which are expected to increase the spatial and temporal extent of these environments This research was funded by the Spanish Ministry of Economy and Competitiveness through the projects CGL2011-30474-C02-01 and CGL2014-58760-C3-1-R

The exchange of dissolved organic carbon (DOC) between sediment and water likely has an impact on aquatic DOC quantity and quality. Therefore, we developed a method to measure the flux of DOC between sediment and water. We constructed Plexiglas sediment core covers, which enable the stirring of the overlying water, continuous measuring of the oxygen concentration, adding and/or withdrawal of water, bubbling and microsensor measurements at the sediment-water interface. Sediment cores from two pre-dams of drinking water reservoirs were incubated in a set-up allowing the measurement of benthic fluxes of DOC and other solutes, including oxygen, dissolved nutrients and metals. Dissolved organic carbon fluxes were low and influenced by the redox conditions in the bottom water. We tested whether UV absorption (UV254) can be used as a fast method to monitor DOC changes in the core incubations. There was a good correlation between UV254 and DOC only at shallow, oxic sites. In anoxic waters, UV254 was strongly influenced by iron. The fluxes of DOC and other solutes, especially oxygen, differed by more than 1 order of magnitude. Thus, a two-step incubation, measuring first the fast fluxes followed by a 14-day incubation, is recommended. Sulfate reduction appeared to be the most important process of organic matter mineralisation under anoxic conditions, while aerobic respiration and, to a lesser extent, denitrification dominated under oxic conditions. We present a set-up that allows measuring the flux of DOC as well as of other solutes between sediment and water. UV254 can be used as a proxy for DOC at oxic sites after establishing a site-specific calibration. Special care has to be taken to maintain redox conditions constant and to prevent oxygen depletion or contamination by atmospheric oxygen. The flux of DOC might be coupled to the reductive dissolution of iron minerals.

The quality of dissolved organic carbon (DOC) affects both carbon cycling in surface waters and drinking water production. Not much is known about the influence of environmental conditions on DOC quality. We studied the effect of redox conditions on the chemical composition of DOC in a drinking water reservoir by Fourier transform ion cyclotron resonance mass spectrometry in combination with sediment core incubation experiments under manipulated redox conditions. We observed clear differences in DOC quality among oxic epilimnion, anoxic hypolimnion, and sediment porewater. Sediment porewater showed relatively high intensities of polyphenol-like components with H/C ratios of1 and O/C ratios of0.6. Consistent with this, anoxic incubation of a sediment core resulted in an accumulation of these components in the overlying water. The observed pattern of DOC quality change can be explained by redox-dependent adsorption/desorption of DOC on iron minerals. Under oxic conditions, the polyphenol-like components bind on freshly formed iron hydroxides, a process that affects both DOC stability in surface waters and treatability during drinking water production.

Two morphologically similar pre-dams (Hassel and Rappbode pre-dams, Harz Mountains, Germany) of a large reservoir with different land use in their catchments were compared with respect to their physical, chemical, and biological properties in order to test if differences can be attributed to the different land use. In addition, local geology, soil types, and topography were evaluated in detail to distinguish its influence from that of present-day land use. Despite a remarkable similarity in physical variables and stratification, the annual development of hydrochemistry and biology differed between Rappbode and Hassel pre-dam. While the Hassel pre-dam received much higher nitrogen and phosphorus inputs from the catchment, the Rappbode pre-dam received more dissolved organic carbon. The quality of dissolved organic carbon also differed between the two catchments, indicating different sources. The higher residence time of Hassel pre-dam amplified the effects of these inputs on the trophic state of both pre-dams. The phytoplankton communities in summer were dominated by diatoms in the Rappbode pre-dam and by cyanobacteria in the Hassel pre-dam. In conclusion, land use appeared to be an important driver for the observed lake characteristics. However, it was itself strongly related to soil types and topography, which on the other hand influences the residence time of water within the catchments.

Temporary watercourses that naturally cease to flow and run dry comprise a notable fraction of the world’s river networks, yet estimates of global carbon dioxide (CO 2 ) emissions from watercourses do not consider emissions from these systems when they are dry. Using data from a sampling campaign in a Mediterranean river during the summer drought period, we demonstrate that the CO 2 efflux from dry watercourses can be substantial, comparable to that from adjacent terrestrial soils and higher than from running or stagnant waters. With an up-scaling approach, we show that including emissions from dry watercourses could increase the estimate of CO 2 emissions from watercourses in our study region by 0.6–15%. Moreover, our results tentatively illustrate that emissions from dry watercourses could be especially important in arid regions, increasing the estimate of global CO 2 emissions from watercourses by 0.4–9%. Albeit relatively small, the contribution of dry watercourses could help to constrain the highly uncertain magnitude of the land carbon sink. We foresee that in many areas of the world, the expected increase in the extent of temporary watercourses associated with future global change will increase the relevance of CO 2 emissions from dry watercourses.

Anaerobic microbial processes in reservoirs have a strong influence on water quality, and these processes are influenced by inputs from the catchment. Two morphologically similar pre-dams of the large Rappbode reservoir system in Germany were studied for oxygen consumption, carbon dioxide production, nitrate reduction, manganese and iron reduction, sulfate reduction, and methanogenesis. Rates were obtained by calculations from hypolimnion size and hypolimnetic water chemistry and from incubation of sediment samples (manganese and iron reduction, sulfate reduction, and methanogenesis only). Length of anoxic conditions (anoxic factor) and anaerobic process rates were generally higher for Hassel pre-dam, a water body with 58% grasslands and agriculture in its catchment. Rappbode pre-dam had higher iron reduction rates. Oxygen consumption and sulfate reduction (and nitrate reduction for Hassel pre-dam) were the most important respiratory processes. While hypolimnion chemistry was similar and showed mainly temporal differences in seasonal development, the pre-dam sediments differed considerably with roughly twofold higher microbial biomass and 102–104 fold higher counts of sulfate-reducing prokaryotes in Hassel pre-dam compared to Rappbode pre-dam. These differences can be explained by the higher nutrient input, longer water retention time, and consequently higher biomass production in Hassel pre-dam. The view of microbial activities against known loads of solutes also implies that the quality of allochthonous organic matter and particulate fluxes likely governs respiratory processes in the pre-dams by providing electron donors and acceptors.

We compared oxygen fluxes measured simultaneously at the pelagic and benthic oxycline in a lake and analyze their relation to hydrodynamic forcing conditions. While the mean oxygen fluxes did not differ significantly among both sites, the fluxes were highly variable in time. Short energetic periods contributed disproportionately to the overall oxygen flux above both the benthic and pelagic oxycline. In the pelagic region, mean fluxes across the oxycline were limited by low diffusivities (7 3 1028 m2 s21) and were one to two orders of magnitude smaller than fluxes above the oxycline (0.5 and 32 mmol m22 d21, respectively). A one-dimensional transport model was used to estimate sources and sinks of oxygen potentially causing this imbalance. The model results indicate that 92% of dissolved oxygen transported into the oxycline is used by the respiration of organic material imported into the oxycline from the epilimnion; chemical oxygen consumption associated with the upward flux of reduced substances is negligible. Our findings indicate that under such conditions, dissolved oxygen consumption and therewith mineralization within the oxycline can be comparable with the corresponding rates occurring in the sediments of eutrophic lakes with an oxic hypolimnion.

Production of H2S by bacterial sulphate reduction in the water column of Cueva de la Mora pit lake (SW Spain) may favour an important self-mitigating capacity with respect to metal pollution. The interaction of bacterially produced H2S with dissolved metal(loid)s provokes the precipitation of various sulphides, which in turn form a 10 m-thick turbidity layer below the redoxcline. The main goal of this study was to identify the main factors driving the formation of these sulphide precipitates and their impact on the pit lake water quality. Given the severe geochemical conditions found in this lake (low pH, high metal concentrations), our findings may be relevant for a wide spectrum of environmental systems where BSR-based biotechnologies are applied. The water column has been studied by spatially resolved samplings and detailed chemical analyses, physico-chemical profiling and geochemical modelling. The suspended particulate matter forming the turbidity has been chemically and mineralogically characterized. Our results suggest that the low pH (3.0–4.0) and the high metal concentrations are not inhibitory for the sulphate-reducing bacteria (SRB). The intensity of sulphide precipitation seems to be closely related with the activity of SRB, which in turn appear to depend on the availability of organic carbon produced in the photic zone. The relative location of oxygen and iron gradients and the redoxcline thickness are influencing the development of the sulphate reducing zone. The most common sulphides are CuS, As2S3 and ZnS, though formation of some other minor phases (PbS, CdS, NiS) has been also detected and/or deduced by geochemical calculations. Metal sulphide precipitation has provoked a complete removal of Cu from the monimolimnion (though it is still present at concentrations of 5.5 mg/L in the mixolimnion) and a net decrease in highly toxic trace metals (Cd, Pb, U, Th) and metalloids (As, Sb).

Greenhouse gas emissions from dry aquatic sediments are probably globally relevant. However, they are difficult to measure because of the often rocky substrate and the dynamic nature of the habitat. Here we tested the performance of different materials to seal a closed chamber to stony ground both in laboratory and field experiments. Using on-site material consistently resulted in elevated fluxes. The artefact was caused both by outgassing of the material and production of gas. The magnitude of the artefact was site dependent – the measured CO2 flux was increased between 10 and 208 %. Errors due to incomplete sealing proved to be more severe than errors due to non-inert sealing material. Pottery clay as sealing material provided a tight sealing of the chamber to the ground and no production of gases was detected. With this approach it is possible to get reliable gas fluxes from hard-substrate sites without using a permanent collar. Our test experiments confirmed that CO2 fluxes from dry aquatic sediments are similar to CO2 fluxes from normal soils.

Globally, reservoirs are a significant source of atmospheric CO

Purpose Microbial sulfate reduction is an alkalinity-producing process and potentially supports the neutralization of acidic mine pit lakes. In many acidic lakes the process does not occur. Sulfate-reducing bacteria are known to be pH sensitive. There are, however, several reports of sulfate reduction occurring in the sediment of acidic lakes. To find out why sulfate reduction occurs in some acidic lakes but not in others, we conducted a field experiment.

Neutralization of acidic mine pit lakes by biotechnological means results in the production of labile metal-sulfides. These reaction products can theoretically be stored sustainably in the lake, provided reducing conditions are maintained at the lake bottom. In a field mesocosm experiment, we tested, if reducing conditions can be maintained in an acidic mine pit lake by the addition of a complex organic substrate. An enclosure of 30 m diameter was covered by a floating foil, and whey was repeatedly added to the water column to stimulate microbial respiration. A suspension of whey was successfully mixed into the enclosure by means of a boat motor. Whey was completely dissolved and subsequently consumed by microbial respiration in the water column. This resulted in oxygen consumption leading to anoxic conditions. About 10 mmol m−2 d−1 oxygen permanently entered the enclosure from the atmosphere, while a minor amount of oxygen was produced by primary production. By careful monitoring and repeated additions, it was possible to keep the bottom of the enclosure permanently anoxic, even during mixing periods in autumn and spring. Fe3+, however, was not reduced significantly. A laboratory experiment revealed that microbial iron reduction was inhibited by both low concentrations of organic substrates and low temperature. Since FeIII is a potential oxidizing agent, it is questionable, if the stability of metal-sulfides in acidic mine pit lakes can be increased by the addition of complex organic substrates.

Ferric iron reduction was studied in a pilot-scale enclosure experiment for passive biological remediation of an acidic mine pit lake in Lusatia, Germany. The metabolic properties of prokaryotes involved in Fe(III) reduction may be important for the outcome of biological remediation, as chemolithotrophic Fe(III) reduction can counteract the desired pH increase, but heterotrophic Fe(III) reduction will provide the necessary Fe(II) for precipitation of sulfide minerals following sulfate reduction. Therefore, vertical profiles of sediment parameters related to iron and sulfur cycling were determined in conjunction with viable counts of different ferric iron-reducing micro-organisms using selective media. Findings were compared to an untreated reference site. The addition of organic matter stimulated ferric iron reduction and sulfate reduction in the enclosure and led to elevated pH and accumulations of ferrous iron and reduced sulfur compounds. Numbers of neutrophilic heterotrophic Fe(III) reducers increased during treatment, those of acidophilic heterotrophic Fe(III) reducers remained similar, and those of acidophilic chemolithotrophic Fe(III) reducers decreased. Zones of ferric iron-reducing activity corresponded well with microbial depth profiles; however, viable counts of neutrophilic or acid-tolerant Fe(III) reducers must have been underestimated based on the corresponding observed activity levels. Ferric iron reduction by chemolithotrophic acidophiles seemed to be of minor importance, so a lowering of pH values due to Fe(III) reducing activity is unlikely.