Soils lose a large amount of carbon annually to freshwaters as dissolved organic matter (DOM), which, if degraded, can undermine climate change mitigation. The degradation state of DOM in aquatic ecosystems can reflect the distance from its source, with DOM increasingly dominated by similar compounds as degradation proceeds. However, the processes underlying the degradation of DOM and its generality across environments are poorly understood. Here we found DOM changed similarly along two soil-aquatic gradients irrespective of environmental conditions. We tracked DOM across soil depths and hillslope positions in forest headwater catchments using ultra-high-resolution mass spectrometry and related its composition to soil microbiomes and physical chemistry. Along both gradients, carbohydrate-like and unsaturated hydrocarbon-like compounds increased in mass, suggestive of microbial reworking of plant material. Most of the variation in the abundance of these compounds (>56%) was related to the expression of genes important for breaking down plant-derived carbohydrates. Our results highlight the value of high-resolution molecular data in understanding global carbon cycles, directly implicate microbial processing in shifting DOM towards universal compounds in soils, and suggest that this process is generalizable across ecosystems and spatiotemporal scales. This consistent degradation process could provide insights for estimating the state of DOM in different environments and inform the management of soil-to-stream carbon losses.

How to cite: Freeman, E., Emilson, E., Dittmar, T., Braga, L., Emilson, C., Goldhammer, T., Martineau, C., Singer, G., and Tanentzap, A.: Universal microbial reworking of dissolved organic matter along soil gradients, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3644, https://doi.org/10.5194/egusphere-egu24-3644, 2024.

Fluorescence spectroscopy is a rapidly evolving method for determining freshwater organic pollution. Historically, measurement was confined to the laboratory with a coarse temporal resolution. The development of field-deployable sensors has enabled in-situ, multi-peak monitoring - although challenges remain regarding fluctuating environmental conditions (e.g. pH and turbidity) that can impact on fluorometer accuracy and interpretation. This study aimed to use fluorescence spectroscopy (including in-situ sensors) to detect and differentiate sources of organic matter pollution in a predominantly groundwater fed, sewage-impacted, chalk stream.

High frequency monitoring (15 min resolution for 12 months) was undertaken at two sites on the River Chess, S. England. Two multi-parameter water quality sondes were installed above and below a Wastewater Treatment Works (WWTW) effluent outflow point in a mixed land use catchment (105 km²). Additional grab sampling was conducted during baseflow and stormflow for laboratory-based nutrient, spectrofluorimetric and bacterial analysis.

All sites had low turbidity (<10 NTU) and stable pH (7.7-7.8), during baseflow, ideal conditions for using in-situ fluorometers. Both the difference in wavelength intensity and the ratio of Peak T (Ex. 275/ Em. 350) to Peak C (Ex. 325/ Em. 470) could differentiate between sites, with an observable variation in response to diel cycles of effluent release downstream. The T:C ratio was able to characterize events with distinct hydrometeorological signatures (e.g. rainfall total, intensity, and antecedence), hence the ratio offers a feasible way of distinguishing between different sources of organic contamination in real-time. Relationships between fluorescence and nutrient/microbial concentrations varied in response to differing landcover (urban extent) and effluent contributions to bulk discharge. Effluent contributions also affected the strength of relationship between cultures and individual wavelength pairs, highlighting the importance of calibrating data for individual systems.

This study highlights that fluorescence is a valuable tool in both fingerprinting organic pollution and tracing the source across sites of contrasting landcover, and under varying hydro-climatological conditions that occur over event timescales. These findings provide the evidence base to develop a new method of detecting and understanding organic matter pollution events at a time scale that was previously unachievable.

How to cite: Gunter, H., Khamis, K., Bradley, C., Hannah, D. M., Heppell, C. M., Kelly, T., Nelson, R., Parry-Wilson, H., and Stevens, R.: Tracking organic matter pollution and bacteria using fluorescence-based approaches in a UK Chalk stream, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9065, https://doi.org/10.5194/egusphere-egu24-9065, 2024.

Streams and rivers emit substantial amounts of nitrous oxide (N₂O) and are therefore an essential component of global nitrogen (N) cycle. Permafrost soils store a large reservoir of dormant N that, upon thawing, can enter fluvial networks and partly degrade to N₂O, yet the role of waterborne release of N₂O in permafrost regions is unclear. Here we report N₂O concentrations and fluxes during different seasons between 2016 and 2018 in four watersheds on the East Qinghai-Tibet Plateau. Thawing permafrost soils are known to emit N₂O at a high rate, but permafrost rivers draining the East Qinghai-Tibet Plateau behave as unexpectedly minor sources of atmospheric N₂O. Such low N₂O fluxes are associated with low riverine dissolved inorganic N (DIN) after terrestrial plant uptake, unfavorable conditions for N₂O generation via denitrification, and low N₂O yield due to a small ratio of nitrite reductase: nitrous oxide reductase in these rivers. We estimate fluvial N₂O emissions of 0.432−0.463 Gg N₂O-N yr⁻¹ from permafrost landscapes on the entire Qinghai-Tibet Plateau, which is marginal (~0.15%) given their areal contribution to global streams and rivers (0.7%). However, we suggest that these permafrost-affected rivers can shift from minor sources to strong emitters in the warmer future, likely giving rise to the permafrost non-carbon feedback that intensifies warming.

How to cite: Zhang, L. and Stanley, E.: Unexpectedly small N2O emissions from alpine permafrost rivers on the East Qinghai-Tibet Plateau, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1448, https://doi.org/10.5194/egusphere-egu24-1448, 2024.

Cryosphere-fed rivers drain glacier, snow, and permafrost landscapes and are characterized by glacial, nival, pluvial and mixed hydrological regimes. Such river systems originate from high-mountain areas and transport water, sediment, nutrients, and organic carbon downstream, underpinning the freshwater and coastal ecosystems and supporting the lives of more than one-third of the world’s population.

In response to the amplified climate change, accelerating glacier-snow melt and permafrost thaw, the cryosphere-fed rivers are overall becoming warmer, wider and muddier associated with markedly increasing river turbidity and suspended sediment concentration. For instance, observational data from 28 headwater rivers in High Mountain Asia reveal that the river suspended sediment loads have been increasing at a rate of ~13% per decade since the 1950s, much faster than rate of increase of river water discharge (~5% per decade). Leveraging over 120 in-field observations and a sediment-climate elasticity model, we estimate that the present-day river suspended sediment load in High Mountain Asia is nearly two billion metric tons per year, and could more than double by 2050 under an extreme climate change scenario. Beyond High Mountain Asia, such warming-driven increases in river turbidity and suspended sediment concentrations have also widely featured in other cryospheric basins such as the Arctic, European mountains, and Andes.

The muddier rivers carry pollutants, nutrients, and organic carbon, thus affecting water quality and aquatic ecosystems in the cold regions and beyond. Increases in sediment-driven river turbidity can threaten river biotic conditions by blocking sunlight from reaching the streambed, limiting respiration, and deteriorating feeding conditions of benthic macroinvertebrates and fishes, thereby affecting habitat availability. Elevated turbidity can disturb habitats of macroinvertebrates and fishes by filling interstitial spaces between pebble and cobbles on the riverbed, thereby reducing the flow of oxygenated water through bed sediment that is essential to the survival of their eggs. The increased sediment supply especially the coarse sediment further magnifies river channel instability and migration, affecting fish habitats and carbon storage and release.

To better assess the impacts of changing climate on the functions and services of river ecosystems in strategically important cold regions, we highlight the pressing need to integrate multiple-sourced river observations, to develop empirical, physics-based, and AI-based river flux models, and to promote interdisciplinary scientific collaboration. The innovative system approach would best come from the creation of an interdisciplinary collaborative initiative, where climatologists, ecologists, glaciologists, permafrost scientists, hydrologists, civil engineers, and geomorphologists work together to establish an integrated cryosphere–water–sediment–carbon-ecology observation platform that facilitates the mechanism understanding and development of novel and powerful models. Furthermore, dialogues and collaboration between international scientists, stakeholders, local communities, and policymakers would help to bridge the gaps between state-of-the-art scientific findings and practicable adaptation strategies.

How to cite: Li, D., Zhang, T., Overeem, I., Kettner, A., Syvitski, J., Walling, D., Bookhagen, B., East, A., Best, J., Beylich, A., Koppes, M., Ni, J., and Lane, S.: Cryosphere-fed rivers in a warming climate, 1950-2050, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3133, https://doi.org/10.5194/egusphere-egu24-3133, 2024.

Nitrous oxide (N₂O) is a strong greenhouse gas with ozone layer destruction ability, and its atmospheric concentration has been increasing rapidly due to anthropogenic activities. N₂O reduction to dinitrogen (N₂), the last step of denitrification, was recognized as the only biological N₂O sink. Recently, diazotrophic N₂O assimilation to organic nitrogen in biomass by nitrogenase has been discovered in the eastern South Pacific Ocean and cultured diazotroph Crocosphaera and Trichodesmium. N₂O assimilation to organic nitrogen is thermodynamically more favored than N₂ fixation in higher N₂O concentration and cooler environments, but the distribution and detailed mechanism of this new N₂O sink are still unclear. We applied isotopic tracing experiments to validate and measure N₂O assimilation and built an enzymatic kinetics model for a mechanistic explanation. Cultured diazotroph Crocosphaera (WH8501) and Trichodesmium (IMS101) both showed evident N₂O assimilation rates of 0.751 nM N h^-1 for Crocosphaera at [N₂O]/[N₂] = 0.0075, 0.690 nM N h^-1 for Trichodesmium at [N₂O]/[N₂] = 0.01, and 0.481 nM N h^-1 for Trichodesmium at [N₂O]/[N₂] = 0.0005. Although N₂O assimilation was assumed to be carried out by nitrogenase, it was asynchronous with the diel rhythmicity of N₂ fixation. Field samples from the Pearl River Estuary did not demonstrate the presence of N₂O assimilation. Since N₂ fixation was absent as well, the isotopic tracer ⁴⁶N₂O barely introduced influences on nitrogen isotopic composition compared to photosynthesis and remineralization, indicating that N₂O assimilation is an insignificant N₂O sink in eutrophic estuarine waters. Our enzymatic kinetic model revealed that N₂ rather than N₂O dominated the overall growth rates of cultured diazotrophs. The model indicated the [N₂O]/[N₂] required for the presence of N₂O assimilation in isotopic tracing experiments and explained the absence of this process under natural N₂ concentration environments. The insights from this study may suggest new engineering methods to control N₂O emissions.

How to cite: Li, G. and Ji, Q.: N2O Assimilation, a New N2O Sink and Organic Nitrogen Source in Aquatic Ecosystems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4027, https://doi.org/10.5194/egusphere-egu24-4027, 2024.

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.

How to cite: Große, A., Perujo, N., Reisinger, A. J., Fink, P., Borchardt, D., and Graeber, D.: Reactive macronutrient ratios as predictors for nitrate cycling in stream ecosystems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5808, https://doi.org/10.5194/egusphere-egu24-5808, 2024.

A characteristic feature of macrotidal estuaries is the presence of a Maximum Turbidity Zone (TMZ), defined by a high concentration of suspended particles (>0.5 g.L^-1). It is maintained by frequent resuspension and deposition events, mainly influenced by waves, tidal currents, and river discharge. These cycles often result in enhanced organic matter degradation, generating a local dissolved oxygen (DO) demand, which can lead to drastic declines in DO and even hypoxic conditions (DO<2 mg.L^-1). In the Loire estuary (France), a macrotidal and turbid environment prone to summer hypoxia, the TMZ is a focal point of interest as it is the site of a persistent oxygen deficit in the inner estuary. To investigate the effects of particle reactivity on DO consumption within the inner estuary, we conducted 14 sampling campaigns between summer 2022 and summer 2023, covering a wide range of river discharge and temperature conditions. We selected two sampling stations: one subjected to freshwater influence and almost continuous presence of TMZ, and the second exposed to coastal ocean conditions. Suspended particles were collected at mid-tide and incubated in the laboratory under controlled conditions at 20°C with continuous stirring to maintain resuspension. DO concentrations were measured using optic sensors and incubations were stopped when 30% of the oxygen concentration was consumed. Nutrient and organic matter composition were investigated by pre- and post-incubation filtration to analyse ammonium, nitrate, phosphate, particulate organic carbon, and nitrogen (POC, PON). DO consumption rates reached maximum values in spring (52.2±0.1 µmol.g^-1.d^-1,42.6±0.4 µmol.g^-1.d^-1) at the upstream and downstream stations, respectively. Overall, the most downstream station had higher oxygen consumption rates due to the marine influence contributing to the input of fresher organic material compared to the upstream station where the presence of TMZ is associated with degraded material. These results emphasize the importance of the material source on oxygen consumption rates. Our discussion will focus on the degradation processes occurring within the TMZ and consider how the reactivity and source of suspended particles may play a role in influencing oxygen consumption patterns, potentially contributing to the development of hypoxic conditions within the estuary.

How to cite: Boukortt, N. E. I., Metzger, E., Bénéteau, E., Le Merrer, Y., Souchu, P., Sanchez, S., Barhdadi, M., Maillet, G., and Schmidt, S.: Impact of Particle Resuspension on Oxygen Consumption and Nutrient Cycling in a Turbid Estuary: Insights from the Loire Estuary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16364, https://doi.org/10.5194/egusphere-egu24-16364, 2024.

North American beavers commonly build dams and create ponds, which alter both the stream hydrology and biogeochemistry. Beaver ponds are common in headwaters of boreal and arctic watersheds of Canada, and while they cover only a small portion of watershed area, their position and biogeochemical influence may allow them to have a large impact on the downstream delivery of solutes and their dominant forms. Previous studies have suggested that boreal beaver ponds commonly act as methylmercury (MeHg) sources to downstream ecosystems, but this has not been studied in the wetland-rich areas of the Taiga Plains, western Canada. Since wetlands are also known as key watershed locations of MeHg production, our objective was to determine whether beaver ponds receiving water from wetland-rich areas still act as net sources of MeHg. We sampled water chemistry at the inflow and outflow of 20 beaver ponds over two years to evaluate Hg and MeHg changes. We determined that there was a net loss of MeHg in the beaver ponds (-34.4% on average), particularly during conditions when water residence time was long. This effect was greatly reduced in wet conditions when water was passing through the ponds more quickly. Net MeHg losses were greater when water entering the pond was already high in MeHg, whereas ponds receiving low MeHg concentrations were neutral or even acted as small sources. These decreases were also correlated with higher dissolved oxygen concentration and isotopic changes in surface water which suggests that aerobic microbial demethylation and photodemethylation may be contributing to net MeHg loss. Understanding the conditions that drive solute delivery from these ponds will allow local land managers to determine appropriate courses of action for beaver management and support well-informed water quality risk assessments.

How to cite: Lagroix, J., Olefeldt, D., and Hood, G. A.: I'll be dammed: Beaver ponds as sites for net loss of methylmercury along stream networks on the peatland-rich Taiga Plains, western Canada, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11993, https://doi.org/10.5194/egusphere-egu24-11993, 2024.

Dissolved organic matter (DOM) plays a vital role in river ecosystem function and therefore understanding its composition is key. DOM has important implications for nutrient cycling and riverine health and with this in mind, it is vital to gain a more comprehensive understanding of the composition of riverine DOM at a molecular level and how this varies across contrasting landscapes. There are many factors which will influence DOM signatures, from differences in climate, soil type/geology, land-use, as well as intensity and nature of anthropogenic activity. Through understanding the potential relationships between these factors and DOM composition, we can gain key information regarding both sources of riverine DOM within river catchments, aiding pollution mitigation strategies, and how signatures may vary under changing climate and/or land-use.

The analysis of DOM poses a significant analytical challenge due to its complexity, however the advances in mass spectrometry now allows detailed characterisation at molecular scale. This study examines the DOM composition across 56 UK field sites spanning contrasting landscapes, including four different geologies/soil types. Additionally, 18 effluents from UK sewage treatment works (STW) were investigated. River water samples were collected and an untargeted analysis carried out using direct-infusion high-resolution mass spectrometry (DI-MS) and the resultant DOM signatures across the samples were compared.

Principal component analysis (PCA) and hierarchical clustering analysis methodologies were applied and showed that the DOM molecular composition between sites could be distinguished according to landscape character. Specifically, the PCA analysis showed that contrasting geologies/soil types were separated by the derived Principle Component (PC) 2 while PC1 separated the riverine samples from the STW effluents in the analytical space. Explanatory variables including landcover, land-use and population density alongside bulk nutrient data were used to begin to elucidate the driving factors behind the PCs. In addition to differences in DOM signatures, further analysis of the molecular compositions identified anthropogenically derived organic compounds, for example, series of polypropylene glycol (PPG) and polyethylene glycol (PEG) oligomers, which were present in almost all landscapes across the UK, illustrating that they are now ubiquitous across riverine environments. Using these data, we can begin to provide generalisable information regarding the molecular composition of DOM across different UK landscapes.

How to cite: Lloyd, C., Pemberton, J., Johnes, P., Jones, D., Yates, C., Glanville, H., Brailsford, F., and Evershed, R.: Exploring dissolved organic matter (DOM) signatures across contrasting UK landscapes using a high-resolution mass spectrometry ‘fingerprinting’ approach., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17208, https://doi.org/10.5194/egusphere-egu24-17208, 2024.

The lateral transport of riverine carbon is a key component of the global carbon cycle, yet several aspects are poorly understood. In particular, the magnitude and nature of carbon cycle responses in freshwater aquatic networks to on-going climate and environmental change remain unclear. Addressing this issue requires assessment of temporal changes in riverine carbon dynamics and identifying the underlying factors that influence the fate of transported carbon. For example, long-term observations of river chemistry from the Swiss National River Monitoring and Survey Program have revealed a steady increase in dissolved inorganic carbon (DIC) concentrations in the major four Swiss rivers (Rhine, Rhone, Ticino, and Inn) over the past ~50 years, yet the cause of this increase remains unclear. Potential contributors include increased DIC inputs from bedrock weathering, soil organic matter (OM) respiration or OM remineralization within aquatic systems. All of these processes are potentially accelerated with increasing temperatures due to global warming, but they have markedly different implications with respect to carbon cycling and ecosystem dynamics. While sensor monitoring and remote sensing approaches are invaluable for creating high-resolution spatially and temporally resolved data, distinguishing specific source components requires ancillary information. In this context, radiocarbon (¹⁴C) measurements obtained through coordinated sampling programs can serve as a powerful complementary constraint on carbon sources, turnover and transport times.

Switzerland provides a unique opportunity to use radiocarbon to assess carbon provenance in alpine streams and rivers, thanks to its high diversity of watersheds spanning strong climatic, elevational, lithological, ecological as well as anthropogenic gradients. This diversity is expressed in a wide range of ¹⁴C signatures for particulate organic carbon (POC; Δ¹⁴C values, −446‰ to −158‰), for dissolved organic carbon (DOC; −377‰ to −43‰) and DIC (−301‰ to −40‰). We argue carefully designed parallel field sampling of streams and rivers and subsequent measurement of radiocarbon and ancillary geochemical parameters would aid in groundtruthing high-resolution sensor data. To illustrate the value of ¹⁴C measurements, we present a multi-year ¹⁴C time-series from the sub-alpine Sihl River system to highlight event- and seasonally-driven changes in the composition of riverine carbon POC, DOC, and DIC. We place these observations in the context of ¹⁴C measurements on a broad range of Swiss river systems to further investigate overarching controls on fluvial carbon export from alpine and sub-alpine watersheds. Such information can help the design of targeted sampling and measurement programs to complement sensor measurements in order to develop a comprehensive understanding of changing river carbon biogeochemical dynamics.

How to cite: Rhyner, T., Mittelbach, B., White, M., Broeder, L., Raymond, O., Haghipour, N., Brunmayr, A., Storck, F., Passera, L., Schwab, M., Hilton, R., Zobrist, J., and Eglinton, T.: Radiocarbon as a key constraint for prediction of river carbon biogeochemistry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9236, https://doi.org/10.5194/egusphere-egu24-9236, 2024.

River networks have been conceptualized as “leaky pipes” for carbon loss. However, there remains considerable uncertainty regarding where, when, and how carbon loss takes place along the aquatic continuum across hydroclimatic conditions. Recent modelling efforts have been developed to (1) connect river reaches with non- or semi-lotic systems including lakes, reservoirs, floodplains and wetlands, and (2) account for river network connectivity via quantification of ephemeral streamflow. These models, which use techniques such machine learning to scale from local measurements to high-resolution river network data products, enable the quantification of relative carbon loss fluxes in lentic vs. lotic systems across stream orders (“where”) within standardized hydroclimatic scenarios representing the full continua of flows and seasonal conditions possible within a watershed (“when”). These models further quantify carbon uptake via both biomineralization and photomineralization (“how”). We frame findings into an updated conceptual model of the Pulse-Shunt Concept, which builds on the representation of river networks as leaky pipes by correlating the “leakiness” with dependence on flow and stream order. We suggest that lakes and other lentic systems should be considered as reactivity “nodes” interspersed along mostly unreactive or passive river reaches. We additionally discuss how these river network modelling approaches can continue to be improved using sensor networks.

How to cite: Maavara, T., Aho, K., Brinkerhoff, C., Logozzo, L., Brown, L., McDowell, W., and Raymond, P.: Recent developments integrating connected non-lotic and ephemeral water bodies into the Pulse-Shunt Concept, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1498, https://doi.org/10.5194/egusphere-egu24-1498, 2024.

Streams and rivers are increasingly recognized as vital components of the global carbon cycle, especially in the context of climate change. To comprehensively understand their impact, it is essential to move beyond the study of individual reaches and consider the entirety of fluvial networks, including their terrestrial interactions. This holistic perspective is crucial for integrating fluvial networks into Earth System models and accurately assessing their role in the global carbon cycle.

In this context, we describe the Metabolic Regimes in Alpine Stream Networks Program (METALP, https://metalp.epfl.ch), an ecohydrological and biogeochemical monitoring study of high-mountain streams in the Swiss Alps, running since 2016. Employing a network of high-frequency sensors (10-min) paired with monthly grab sampling, METALP examines the hydrological, thermal, light, and carbon regimes of high-mountain streams. Initially focused on the metabolism of alpine streams, the project has evolved to explore long-term trends in ecosystem characteristics and functions, with a particular emphasis on understanding climate change impacts. This unique observatory has so far collected over 20 million usable data points, describing annual regimes of streamwater flow, temperature, sediment load, carbon fluxes, and ecosystem metabolism.

We present insights into the hydrologic and biogeochemical consequences of glacier loss, along with findings on dissolved organic carbon, gas exchange and CO₂ emissions, oxygen concentrations, and gross primary production. Building on these insights, we then delve into the unique challenges associated with long-term monitoring in high-mountain catchments. These include marked hydrologic variability, with flows ranging over several orders of magnitude, and the need for monitoring equipment to withstand high flows, sediment loads, and avalanches, and remain functional during low flow periods. Seasonal snow cover and the remoteness complicate sampling campaigns and sensor maintenance. Additionally, the oligotrophic nature of high-mountain streams, with low analyte concentrations, necessitates sensitive monitoring programs capable of detecting subtle changes. These challenges inherently lead to gaps in data, necessitating not only technical adaptations for monitoring under difficult conditions but also innovative modeling strategies for compensating data loss.

Finally, the METALP network, along with river networks located in different climatic regions (i.e., the Krycklan catchment in Sweden, the StreamPULSE project, or the Arctic Great Rivers Observatory), provides a broader perspective, enabling us to understand biogeochemical patterns and dynamics across multiple streams. This approach is crucial for constructing a comprehensive picture of stream biogeochemistry and its response to climate change.

How to cite: Deluigi, N. and Robison, A. L.: Deciphering alpine stream responses to climate change: lessons from the METALP monitoring network, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10927, https://doi.org/10.5194/egusphere-egu24-10927, 2024.

Coastal ecosystems are dynamic regions especially rich in diverse biological and geochemical interactions.  However, major gaps exist in our knowledge of the primary biogeochemical processes and the factors regulating their relative importance.  The study of the biogeochemical cycles of nitrogen and carbon in aquatic systems is important for understanding the fate of nutrients and other chemical components present there. Nitrous oxide (N₂O) and methane (CH₄), have important roles in these nitrogen and carbon biogeochemical processes as they are produced and cycled within coastal and ocean environments.  They are also significant greenhouse gases with major roles in climate change.  The gaps in our understanding of the distribution and dynamics of the underlying processes controlling their fluxes can be filled with the development and deployment of high-resolution spatial-temporal measurement methods.

We have developed a field deployable, real-time, in situ system to quantify dissolved greenhouse gases (N₂O and CH₄ and their isotopologues) in aquatic ecosystems including coastal wetlands.  This measurement system consists of i) an array of permeable, hydrophobic probes that can be brought under a partial vacuum without intrusion of liquid water; ii) a collection protocol for efficiently drawing dissolved gases into the sampling system without isotopic fractionation; and iii) an interface of the probe array and the extraction and sampling system with real time analytical instrumentation.  By integrating an Aerodyne tunable infrared laser direct absorption spectrometer (TILDAS) into the measurement system, we can achieve real time determination of concentration and isotopic abundances of N₂O and CH₄.

We have compared dissolved gases extracted from a variety of collected water samples including different tap water sources, ocean water, and wetland “swamp” water.  We observed higher N₂O in the tap water samples compared to the ocean waters.  Swamp water collected from two areas of the wetland (i.e., still and moving water zones) had elevated CH₄ and N₂O, with the still water having higher methane and lower N₂O than observed in water from area with movement.  We also compared dissolved N₂O isotopologues with headspace in dosing experiments, achieving excellent comparisons of the ¹⁵N₂O isotopic ratios (δ456, δ546) and site preference (SP = δ456- δ546) of dissolved N₂O with the headspace.  Laboratory results as well as plans for field demonstrations in coastal areas will be discussed.

How to cite: Shorter, J., Roscioli, J., Lunny, E., and Wankel, S.: A Real-time Monitoring System of Dissolved Nitrous Oxide, Methane and other Gases and their Isotopes in Aquatic Ecosystems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11259, https://doi.org/10.5194/egusphere-egu24-11259, 2024.

The freshwater riverine carbon budget has an unexplained imbalance (~1.5 Pg-C y⁻¹) between estimates of terrestrial C lateral imports and freshwater emissions. This imbalance may be resolved by investigating the source of freshwater CO₂ emissions. That is, what proportion of the excess CO₂ in rivers comes from lateral CO₂ inputs (external, allochthonous sources) versus from riverine respiration of organic matter (internal, autochthonous sources)? We address this question by developing a model to estimate the reach-scale dissolved inorganic carbon (DIC) mass balance using sub-daily time series of dissolved O₂ and CO₂. The approach extends the classical single station model for the estimation of stream metabolism based on O₂ observation by coupling the mass balance of DIC with the lateral input of water, O₂ and DIC, and the mass balance of total alkalinity. Here, we present the results of the model application to several study sites across varying discharge and carbonate chemistries. We further show the model's utility in estimating magnitudes of river metabolism, lateral DIC concentration, photosynthetic and respiratory quotients, and carbon flux to the atmosphere.

How to cite: Diamond, J. and Bertuzzo, E.: A coupled O2-CO2 model to understand CO2 source partitioning in flowing freshwaters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5918, https://doi.org/10.5194/egusphere-egu24-5918, 2024.

How to cite: Cheng, F., Park, J., Kumar, M., and Basu, N.: Disconnectivity matters: The outsized role of small ephemeral wetlands in landscape-scale nutrient retention, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14106, https://doi.org/10.5194/egusphere-egu24-14106, 2024.

In the polar deserts of Antarctica, meltwater from glaciers flows in streams for only about two months during the summer. As the water flows downstream and interacts with the sediment in the stream channel, weathering reactions increase the concentrations of dissolved constituents in the stream water, especially silica. In the McMurdo Dry Valleys, the glacial meltwater streams that flow during the austral summer are important biogeochemical links between the alpine and terminal glaciers and the lakes in the valley floors.  As part of the McMurdo Dry Valleys Long-Term Ecological Research (MCMLTER) project, 17 first and second order streams are monitored for flow and water quality, and diatom community composition in the perennial microbial mats on the streambed. This study found that in streams that are about 1 km long and have abundant microbial mats, the diatoms can take up enough silica to reduce the concentrations of dissolved silica to very low values (>/= 1 mg/L). In comparison, in longer streams Si concentrations are greater (2 mg/L and greater) due to the input of Si from weathering in the hyporheic zone. A previous study has found that diatom community composition in two short streams is significantly related to total flow during the austral summer, leading to a hypothesis that decreases in Si concentrations with increasing flow may favor smaller diatoms with less silicified frustules. We analyzed the 25-yr discharge and silica record for 10 streams using the Weighted Regressions on Time, Discharge, and Season (WRTDS) model to estimate mean 5-day silica concentrations for December through January. These analyses revealed that the shortest stream with the strongest relationship between flow and diatom community composition consistently exhibited minimum Si concentrations of ~ 0.5 mg/L at peak flow. In contrast, Si concentrations were higher and more stable throughout the summer for long streams that exhibit little variation in diatom community composition. These results suggest that Si uptake by diatoms can control both in-stream Si concentrations and diatom community composition.  Understanding the relationship between the diatoms in the mat communities and environmental change is useful for interpreting the record of the stream diatoms preserved in lake sediments and for considering future scenarios for the Dry Valleys.

How to cite: McKnight, D., Johnson, K., Heindel, R., and Jankowski, K. J.: Stream Length and Diatom Communities Control Si Dynamics in Glacial Meltwater Streams of the McMurdo Dry Valleys, Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14428, https://doi.org/10.5194/egusphere-egu24-14428, 2024.

QUANTOM aims to quantify how changes in quality and quantity of dissolved organic matter (DOM) supply alter the metabolic balance of rivers, i.e. the contribution of in-stream DOM degradation to CO2 emissions. QUANTOM will determine the coupling between land vegetation growth from satellite observation and DOM delivery and transformation in streams using in-situ sensor technology and whole stream metabolism. QUANTOM will characterise the molecular transformations (reactive pathways) of DOM, from riparian soils to the Barents sea, through the river continuum at control points (hot spots and hot moments) using carbon stable isotope ratios and FT-ICR-MS. QUANTOM will formalise mathematically our novel understanding into a parsimonious river basin model for DOM with in-stream processes. QUANTOM’s vision is to have a model applicable across the natural northern rivers around the globe and transform the way we see and study rivers.

We have completed three years of fieldwork in the river Tana (Northern Norway), draining 16,000 km2 of north boreal and sub-arctic landscapes and discharging in the Barents sea (70°N). We will present the outline of the project, our conceptual approach and preliminary results such as satellite and in-situ sensor data, carbon fluxes and metabolic balance of the river network.   

How to cite: Demars, B., McGovern, M., Jackson-Blake, L., Sample, J., Norling, M., Høgda, K., Karlsen, S., Dörsch, P., Stutter, M., Thornton, B., Junker, J., and D'Andrilli, J.: QUANTOM – QUANTification of dissolved Organic Matter and the metabolic balance in river networks: mechanisms and model simulations of CO2 emissions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20681, https://doi.org/10.5194/egusphere-egu24-20681, 2024.

Nitrogen and phosphorus inputs via rivers entering the North Sea showed maxima in the early 1980s. This led to eutrophication phenomena near the coast with high primary production and further negative consequences for the North Sea ecosystem.

Recent simulations with the ecosystem model ECOHAM for the North Sea, nested in the model NEMO-ERSEM for the Northwest European continental shelf, show that diatom and non-diatom driven productions behave differently with respect to decreasing eutrophication. In the southern and central North Sea, non-diatom production including calcifiers has indeed responded to the changes in nutrient supply via the rivers. However, diatom production in this region mostly remained stable and even increased in some cases.

A different picture emerges in the northern North Sea, where the reversal of the winter NAO index from high to lower values (1995/1996) was followed by a drastic collapse in the inflow of North Atlantic water. This also led to a cut in the nutrient supply. Here, both phytoplankton groups reacted similarly: from 1996, the primary production of both species declined and then recovered again from 1999.

Our results confirm the hypothesis of Desmit et al. (2019) that in the southern North Sea primary productivity responds to reduction in nutrient inputs with shifts in community structure, and in the northern North Sea with decrease in total productivity rates.

Reference:

Desmit, X., A. Nohe, A. V. Borges, T. Prins, K. De Cauwer, R. Lagring, D. Van der Zande and K. Sabbe (2019). Changes in chlorophyll concentration and phenology in the North Sea in relation to de-eutrophication and sea surface warming. Limnology and Oceanography 9999. DOI: 10.1002/lno.11351.

How to cite: Paetsch, J., Lessin, G., Artioli, Y., and Blackford, J.: Variability of annual primary production in the North Sea from 1983 to 2014: diatoms and non-diatoms show different trends, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10994, https://doi.org/10.5194/egusphere-egu24-10994, 2024.

Each year, rivers export more than one teragram of carbon out of Switzerland as dissolved inorganic carbon (DIC), integrating diverse atmospheric, terrestrial, and aquatic carbon sources over their catchments. However, the contributions of the different carbon sources to riverine DIC – and thus the implications of DIC dynamics for the global carbon balance and climate – remain uncertain. Building upon the 50-year dataset from the national long-term river monitoring network of Switzerland (NADUF), we attempt to predict the vertical CO₂ fluxes between rivers and the atmosphere, and to quantify catchment-scale DIC production through rock weathering, leaching of soil-respired CO₂, and mineralization of organic carbon during fluvial transport. Supported by the national network of groundwater monitoring sites (NAQUA) and soil sampling sites covering Switzerland, a Bayesian mixing model disentangles the sources of riverine DIC using measured data of carbon and water isotopes (¹⁴C, ¹³C, ²H, ¹⁸O), as well as ion concentrations. The exchanges between river DIC and atmospheric CO₂ across the air–water interface are predicted with a diffusion model, validated with measurements of the CO₂ flux and isotopes from in situ floating-chamber experiments. Our predictions of the DIC source contributions and the net CO₂ flux from rivers help to elucidate the role of DIC in the carbon balance of alpine and perialpine river catchments, and contribute towards closing the national carbon budget of Switzerland.

How to cite: Brunmayr, A., Rhyner, T., Geissbühler, D., Minich, L., White, M., Storck, F., Passera, L., Zimmermann, S., Moreno Duborgel, M., Laemmel, T., Mittelbach, B., Haghipour, N., Eglinton, T., Szidat, S., Hagedorn, F., and Graven, H.: Sources and fate of dissolved inorganic carbon in rivers of Switzerland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14012, https://doi.org/10.5194/egusphere-egu24-14012, 2024.

European ecosystems have been subject to extensive shifts in anthropogenic disturbance, primarily through atmospheric deposition, climate change, and land management. These changes have altered the macronutrient composition of aquatic systems, with widespread increases in organic carbon (C), and declines in nitrogen (N) and phosphorus (P). Less well known is how these disturbances have affected nutrient stoichiometry, which may be a more useful metric to evaluate the health of aquatic ecosystems than individual nutrient concentrations. The Swedish west coast has historically experienced moderate to high levels of atmospheric deposition of sulfate and N, and eutrophication. In addition, coastal waters have been darkening with damaging effects on marine flora and fauna. Here, we present three decades of macronutrient data from seven watercourses (plus additional lakes) along the Swedish west coast, including headwaters and river mouths, across a range of land covers, and with catchments ranging 0.037 – 40000 km².

We find a high degree of consistency between these diverse sites, with widespread increasing trends in organic C, and declines in inorganic N and total P. These trends in individual macronutrients translate into large stoichiometric changes, with a doubling in C:P, and increases in C:N and N:P by 50% and 30%, showing that freshwaters are moving further away from the Redfield Ratio, and becoming even more C rich, and depleted in N and P. These changes were not restricted to headwaters but were also evident in larger rivers and at river mouths. Although recovery from atmospheric deposition is linked to some of these changes, land cover also appears to have an effect; lakes buffer against C increases, and decreases in inorganic N have been greatest under arable land cover. Taken together, our findings show that freshwater macronutrient concentrations and stoichiometry have undergone substantial shifts during the last three decades, and these shifts can potentially explain some of the detrimental changes that adjacent coastal ecosystems are undergoing. Our findings are relevant for all European and North American waters that have experienced historically high levels of atmospheric sulphate and N deposition, and provide a starting point for understanding and mitigating against the trajectories of long-term change in aquatic systems.

How to cite: Peacock, M., Futter, M., Jutterström, S., Kothawala, D., Moldan, F., Stadmark, J., and Evans, C.: Three decades of changing nutrient stoichiometry from source to sea on the Swedish west coast, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6645, https://doi.org/10.5194/egusphere-egu24-6645, 2024.