Numerical lake models are a powerful tool to optimize water management and mitigate changes due to climate change. Hence, detailed implementation of lake specific processes is crucial to ensure optimal results. However, common numerical lake models have so far omitted the effect of thermobaricity despite its significant influence on deep water circulation in deep lakes. The thermobaric effect is based on the temperature dependence of the compressibility of water. As a consequence, deep water can be significantly colder than 4 degrees C and deep water renewal becomes complex. For a proper investigation, numerical models can be appropriate tools to display and understand such processes better. Inspired by Lake Shikotsu, which is an excellent example for the influence of thermobaricity, we developed a simplified 1D model for thermobaric effects. Here, we used in situ density to replace potential density for stability considerations such as the Brunt-V & auml;is & auml;l & auml; frequency. To prevent any competing influences and isolate thermobaric effects, we excluded any external forcing except for the surface temperature input. Accordingly, we excluded salinity, chose a cylindrical bathymetry without shallow areas, and omitted any inflows. Therefore, the model reproduced deep water circulation solely based on thermal forcing at the surface. We were able to identify key features of the deep water circulation: (1) cabbeling occurs at the intersection of the temperature profile with the Tmd (temperature of maximum density) line due to diffusion and induces thermobaricity driven deep water circulation, (2) this deep water circulation cell is detached from the surface, but can extend over hundreds of meters to the lake bed, (3) the deep water stays isothermal, (4) and after the winter stratification the temperature profile aligns with the Tmd line. Additionally, we investigated the influence of previous deep water renewal events and the current surface temperature on the deep water circulation. Our results emphasize the feasibility and necessity of the implementation of thermobaricity in numerical lake models by basing stability (Brunt-V & auml;is & auml;l & auml; frequency) on in situ density.

Abstract. The growing global demand for heating and cooling challenges decarbonization efforts, which requires a shift to renewable solutions. Inland waters, especially lakes and reservoirs, account for a significant portion of global land heat uptake, offering potential to help meet these demands. When used for heating, the resulting cooled lake could benefit from buffering climate-induced warming. However, existing lake models lack dedicated capabilities to simulate thermal energy extraction limiting the assessment of its environmental impacts on aquatic ecosystems. Here, we present a new thermal use module integrated into the General Lake Model (GLM) framework. The module features: (1) depth-flexible water extraction and injection at static or dynamic elevations; (2) user-defined operational parameters for flow rates, temperature spread, and heat flux; and (3) coupling with the Aquatic Ecodynamics (AED) library, enabling simulations of biogeochemical responses to thermal alterations. Heat transfer calculations are integrated with GLM's existing hydrodynamic and thermodynamic algorithms and maintain energy and mass conservation. We tested the module using a heat-extraction scenario (-2 K) for a drinking water reservoir in Germany with 19 years of monitoring data. The open-source module provides a new tool for both energy and lake management by enabling the evaluation of the thermal and ecological impacts of heat extraction.

Historically, regular mixing occurred predictably in Lake Bosumtwi in August. However, recent reports indicate a shift towards prolonged stratification, reduced seasonal mixing depth, and a decline in fish productivity, consistent with the effects of climate warming. This study aimed to investigate the perceived changes in the mixing and stratification regime by comparing historical (2004-2006) and current data to determine the magnitude of climate warming on the lake's hydrodynamic properties. From 2018 to 2020, daily measurements of meteorological variables, including temperature, wind speed, and rainfall within the crater, were recorded at an onshore weather station. Monthly data on water column temperature, dissolved oxygen, and electrical conductivity were collected, and Schmidt stability was calculated. Time series analyses suggest alterations in the stratification and mixing patterns of the lake induced by air temperature and wind speed forcing. The air temperature increased from 26.56 degrees C (SD = 3.13) to 27.28 degrees C (SD = 3.32) resulting in a 0.30 degrees C rise in the water column, greater thermal stability and a decline in dissolved oxygen concentration with depth. There appears to be a notable shift in the timing and intensity of the mixing period, transitioning from August to September, attributed to diminished convective cooling processes. Evidence indicates a transition from a holomictic state to meromictic. This transformation in the lake's hydrodynamics will affect nutrient renewal rates and primary productivity, with implications for water quality, fisheries, ecotourism, and livelihood activities vital to the lake communities.

Lentic waters integrate atmosphere and catchment processes, and thus ultimately capture climate signals. However, studies of climate warming effects on lentic waters usually do not sufficiently account for a change in heat flux from the catchment through altered inflow temperature and discharge under climate change. This is particularly relevant for reservoirs, which are highly impacted by catchment hydrology and may be affected by upstream reservoirs or pre‐dams. This study explicitly quantified how the catchment and pre‐dams modify the thermal response of Rappbode Reservoir, Germany's largest drinking water reservoir system, to climate change. We established a catchment‐lake modeling chain in the main reservoir and its two pre‐dams utilizing the lake model GOTM, the catchment model mHM, and the stream temperature model Air2stream, forced by an ensemble of climate projections under RCP2.6 and 8.5 warming scenarios. Results exhibited a warming of 0.27/0.15°C decade −1 for the surface/bottom temperatures of the main reservoir, with approximately 8%/24% of this warming attributed to the catchment warming, respectively. The catchment warming amplified the deep water warming more than at the surface, contrary to the atmospheric warming effect, and advanced stratification by about 1 week, while having a minor impact on stratification intensity. On the other hand, pre‐dams reduced the inflow temperature into the main reservoir in spring, and consequently lowered the hypolimnetic temperature and postponed stratification onset. This shielded the main reservoir from climate warming, although overall the contribution of pre‐dams was minimal. Altogether, our study highlights the importance of catchment alterations and seasonality when projecting reservoir warming, and provides insights into catchment‐reservoir coupling under climate change.

By hosting significant amounts of extra-atmospheric dissolved gases, including geogenic CO2 and CH4, volcanic lakes provide relevant ecosystem services through the key role the aquatic microbial community in mediating freshwater carbon fluxes. In view of elucidating the mechanisms governing the microbial spatial distribution and the possible implications for ecosystem functioning, we compared the hydrogeochemical features and the microbial community structure of two adjacent stratified volcanic lakes (Lake Grande - LG and Lake Piccolo - LP). Water chemistry, gases and their isotopic composition were coupled with microbial pigment profiling, cell counting, and phylogenetic analyses. LP showed transparent waters with low concentrations of chlorophyll-a and the occurrence of phycoerytrin-rich cyanobacteria. LG was relatively more eutrophic with a higher occurrence of diatoms and phycocyanine-rich cyanobacteria. Considering the higher concentrations of CO2 and CH4 in bottom waters, the oligotrophic LP was likely a more efficient sink of geogenic CO2 in comparison to the adjacent eutrophic LG. The prokaryotic community was dominated by the mixothrophic hgcI clade (family Sporichthyaceae) in the LG surface waters, while in LP this taxon was dominant down to -15 m. Moreover, in LP, the bottom dark waters harbored a unique strictly anaerobic bacterial assemblage associated with methanogenic Archaea (i.e. Methanomicrobiales), resulting in a high biogenic methane concentration. Water layering and light penetration were confirmed as major factors affecting the microbial distribution patterns. The observed differences in the geochemical and trophic conditions reflected the structure of the aquatic microbial community, with direct consequences on the dynamics of dissolved greenhouse gases.

Lake Bosumtwi in tropical Ghana has been known for its recurrent fish kills, but they have recently been reported to happen less frequently. The lake formed in a meteorite impact crater in Ghana, West Africa. It plays an important role for the local inhabitants for recreation and for fisheries. The lake is deep, and recent observations indicate that recirculation is incomplete. In general, the deep water is anoxic. Fish kills have been associated with the mixing events in the slightly colder rainy season. As unpleasant smells from the water during deep mixing had been reported, the question arose whether toxic gases that had accumulated in the deep water could be responsible; namely, hydrogen sulphide or large amounts of carbon dioxide were considered the most probable candidates. The analysis of the water properties, however, did not detect any hydrogen sulphide nor immensely large concentrations of carbon dioxide. On the contrary, the presence of large amounts of bound nitrogen could be substantiated. We hence concluded that most probably bound nitrogen was responsible for the fish kills on two paths (1) as bound nitrogen as ammonium forms toxic ammonia when mixed into high pH surface water and (2) depletes oxygen when it is oxidized in the surface waters.

Water quality and hypoxia in lakes and reservoirs are strongly associated with water renewal. Yet vertical water exchange is still not fully understood and challenging to evaluate in highly dynamic systems. Here, we applied a scaling approach using the vertical timescale, vertical water age (VWA), defined as time since a water parcel last touched the water surface. We established a 3D hydrodynamic-based age model to quantify the vertical water renewal in Xiangxi Bay, a tributary bay of the Three Gorges Dam. The integrated effects of hydrodynamic processes like stratification, intruding density currents from the mainstream, and upstream inflow on the vertical renewal were accounted for. Results indicated that the spatial-temporal distribution of VWA in Xiangxi Bay depended on stratification and forms of intruding density currents. Age was large in spring and summer, and small in autumn and winter, reaching a maximum of 25 days in April. The vertical water renewal was faster during bottom intrusions from the mainstream than during middle and surface intrusions. At times, the epilimnion contained old water due to circulations, and the hypolimnion contained young water due to upstream flushing. In contrast to natural lakes, the bottom water was often younger than overlying intermediate waters. This demonstrated that mixed layer depth was insufficient to fully capture the vertical exchange in riverine systems with significant surface/bottom intrusion. The findings suggested VWA as a quantitative measure of vertical water transport in highly dynamic systems and its usability for environmental water management.

Global warming is shifting the thermal dynamics of lakes, with resulting climatic variability heavily affecting their mixing dynamics. We present a dual ensemble workflow coupling climate models with lake models. We used a large set of simulations across multiple domains, multi-scenario, and multi GCM- RCM combinations from CORDEX data. We forced a set of multiple hydrodynamic lake models by these multiple climate simulations to explore climate change impacts on lakes. We also quantified the contributions from the different models to the overall uncertainty. We employed this workflow to investigate the effects of climate change on Lake Sevan (Armenia). We predicted for the end of the 21st century, under RCP 8.5, a sharp increase in surface temperature ( 4.3 ± 0.7 K ) and substantial bottom warming ( 1.7 ± 0.7 K ) , longer stratification periods (+55 days) and disappearance of ice cover leading to a shift in mixing regime. Increased insufficient cooling during warmer winters points to the vulnerability of Lake Sevan to climate change. Our workflow leverages the strengths of multiple models at several levels of the model chain to provide a more robust projection and at the same time a better uncertainty estimate that accounts for the contributions of the different model levels to overall uncertainty. Although for specific variables, for example, summer bottom temperature, single lake models may perform better, the full ensemble provides a robust estimate of thermal dynamics that has a high transferability so that our workflow can be a blueprint for climate impact studies in other systems.

Rising bubbles play a fundamental role in emitting greenhouse gases from shallow waters. Their size is crucial for bubble dissolution, gas exchange with the surrounding water, and the release of gases into the atmosphere. However, little is known about bubble sizes in shallow waters. To address this, we investigated bubble diameters in a 1.2 m deep fish pond, employing 2 methods: first, we measured the bubble size distributions by optical bubble sensors; second, we used an existing single bubble dissolution model to determine diameters representative for the respective bubble size distributions at the water surface based on measured bubble oxygen contents and dissolved oxygen concentrations. Results from optical bubble sensors were relatively similar at all sites; however, subsequent analysis revealed problems, particularly in detecting small bubbles under turbid, shallow water conditions. Model-derived bubble diameters ranged from 0.5 to 10.5 mm, varied spatially within the pond, and displayed diurnal fluctuations. With increasing bubble flux, bubble diameters increased; bubbles at feeding sites were larger than in the open water area. A detailed sensitivity analysis revealed that, depending on the bubble size distribution, the uncertainty of the model increases with increasing water depth. For a typical bubble diameter of 5 mm, the simple method can provide robust estimates of representative bubble size in waters shallower than 50 m.

Deoxygenation in the deep water of stratified lakes and reservoirs due to climate warming and human activities poses a threat to crucial limnetic ecosystem services. Metalimnetic oxygen minima (MOM) typically occurs during the mid to late stages of limnetic stratification. To investigate the mechanism of MOM during stably stratified periods in the Panjiakou Reservoir, northern China, four years of in-situ monitoring was conducted in parallel with longitudinal-vertical 2D modeling. The model simulated the hydrodynamic, water quality, algal, and DO processes in 2017 and 2020, and was calibrated and verified by the measurements. Three scenarios were designed to test the impact of hydrological processes and the high oxygen-consuming sedimentation zones (HOCSZ). We concluded that the stratification and the corresponding advection generated in the metalimnion are the driving forces of MOM in the reservoir, and the limnetic eutrophication-related benthic HOCSZ are the seedbeds of the metalimnetic hypoxia. This unique spatial setting driving the biogeochemical processing and MOM dynamics required a new generation of modelling, incorporating spatial inhomogeneities of sediment characteristics and a sophisticated resolution of hydrodynamics. Metalimnetic horizontal advection, hypolimnetic upwelling currents, sedimentation hot spots and the formation of high oxygen consumption zones became the major initiation processes leading to MOM. The numerical modeling not only represented the occurrence, development, and extinction of MOM but also comprehensively explained the spatial differences in the vertical morphology of MOM and its inter-annual variations. Our studies help to identify management strategies to mitigate the impacts of MOM on aquatic ecological security and drinking water quality, thus supporting the optimization of reservoir regulations.

Dissolved oxygen plays a central role for all organisms dwelling in water. However, the flux of oxygen by ebullition has not received much attention in environmental science. For a better quantitative understanding of the oxygen flux due to ebullition, we conducted a series of laboratory experiments, where we forced macrophytes to produce photosynthetic gas bubbles. Raising the CO2 concentration in the water greatly increased bubble formation. Depth was varied to compare the results with theoretically predicted composition of photosynthetic bubbles forming at minimum required gas pressure. Oxygen concentrations lay between this theoretical line as lower boundary (ca. 21% O2 at 0.3 m depth and 45% of O2 at 4.5 m) and 45% of oxygen as the purely empirical upper limit for all depths. As a consequence, no bubble formation was observed at depths below 4.5 m. Under light, green plants produce oxygen, when they grow. Submerged plants can produce bubbles under these conditions. Some of these bubbles rise to the surface and hence transport oxygen out of the water into the atmosphere. We studied this process by measuring the composition of such bubbles in a laboratory experiment. Adding carbonic acid to the water boosted the bubble production a lot. Depending of depth, there is a minimum of gases required for bubble formation, which yields a theoretical minimum concentration for oxygen. In all experiments, the oxygen concentration of bubbles was above atmospheric percentage of 21%, but always below 45%. Photosynthetic gas bubbles from submerged macrophytes contain other gases beyond oxygenThe depth-dependent lower limit can be derived from minimum gas pressure consideration for the formation of bubblesEmpirically we find an upper limit of 45% of oxygen in the collected gas bubbles

The dynamics of vertical mixing and the occurrence of basin-scale internal waves (internal seiches) in lakes and reservoirs are often classified and described based on the force balance of wind shear and horizontal pressure gradients resulting from wind-generated currents (the Wedderburn number). The classification schemes consider specific time scales that are derived based on a simplified vertical density distribution, a rectangular basin shape, and a constant water depth. Using field measurements and numerical simulations with a validated hydrodynamic model, we analyzed the transfer of energy from wind to the internal seiche field in a small reservoir. Our results demonstrate that the basin shape has a strong influence on the energy dissipation and on the transfer of energy to propagating high-frequency internal waves, thereby attenuating the generation of basin-scale internal seiches. Most of the energy loss of the internal seiche occurs at the sloping boundary, where the internal seiche is susceptible to shoaling and breaking. These findings suggest that the Wedderburn number can be used to predict the occurrence of internal seiche activity in continuously stratified systems. As the Wedderburn number and derived mixing classifications are widely applied also for the interpretation of observed ecological and biogeochemical processes, its application to basins with sloping bathymetry and complex shape should be critically scrutinized, and deviations from predicted dynamics, including the presence of hotspots of turbulent mixing, should be considered.

The molecular composition of dissolved organic matter (DOM) is of relevance for global carbon cycling and important for drinking water processing also. The detection of variation of DOM composition as function of time and space from a methodological viewpoint is essential to observe DOM processing and was addressed so far. High resolution concerning DOM quality was achieved with Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS). However almost none of the existing FTICR-MS data sets were evaluated addressing the fate of single mass features / molecular formulas (MFs) abundance during experiments. In contrast to former studies we analyze the function of MF abundance of time and space for such MFs which are present in all samples and which were formerly claimed as recalcitrant in not all but a great number of studies. For the first time the reactivity of MFs was directly compared by their abundance differences using a simple equation, the relative intensity difference (δRI). Search strategies to find out the maximum δRI values are introduced. The corresponding MFs will be regarded as key MFs (KEY-MFs). In order to test this new approach data from a recent photo degradation experiment were combined with monitoring surveys conducted in two drinking water reservoirs. The δRI values varied over one order of magnitude (more than five-fold). MFs like C9H12O6 and C10H14O6 revealed high biogeochemical reactivity as photo products. Some of the KEY-MFs were identical with MFs identified as disinfection byproducts precursors in recent studies. Other KEY-MFs were oxygen-rich and relatively unsaturated (poly-phenol-like) and hence relevant to flocculation procedures.

Aggregations of cyanobacteria in lakes and reservoirs are commonly associated with surface blooms, but may also occur in the metalimnion as subsurface or deep chlorophyll maxima. Metalimnetic cyanobacteria blooms are of great concern when potentially toxic species, such as Planktothrix rubescens (P. rubescens), are involved. Apparently, metalimnetic blooms of P. rubescens have increased in frequency and severity in recent years so there is a strong need to establish the external factors controlling its growth. We hypothesized that P. rubescens blooms in reservoirs can be managed by modifying the water withdrawal strategy and altering the light climate experienced by the algae. We tested our hypothesis in Rappbode Reservoir by establishing a series of withdrawal and light scenarios based on a calibrated water quality model (CE-QUAL-W2). Our scenarios demonstrated that metalimnetic water withdrawal reduced P. rubescens biomass in the reservoir. According to the simulation results we defined an optimal withdrawal volume to control P. rubescens blooms in the reservoir as approximately 10 million m3 during its blooming period. The numerical results also indicated that P. rubescens growth can be most effectively suppressed if the metalimnetic withdrawal is applied in the early stage of its rapid growth (i.e. before the occurrence of blooms). Additionally, the results showed that P. rubescens biomass gradually decreased with increasing light extinction and nearly disappeared when the extinction coefficient exceeded 0.55 m-1. Our results indicated that close linkages among in situ measurements, model simulations, empirical growth rate and flushing rate calculations could inform management strategies to minimise the harmful impacts of P. rubescens in water supplies. Such a strategy could be used in reservoir operational strategies as an adaptation way to offset the rise in P. rubescens populations that has been linked to climate change.

Due to their biological and chemical inertness, noble gases in natural waters are widely used to trace natural waters and to determine ambient temperature conditions during the last intensive contact with the atmosphere (equilibration). Noble gas solubilities are strong functions of temperature, with higher temperatures resulting in lower concentrations. Thus far, only common environmental conditions have been considered, and hence investigated temperatures have almost never exceeded 35 °C, but environmental scenarios that generate higher surface-water temperatures (such as volcanism) exist nonetheless. Recently published measurements of noble gas concentrations in Lake Kivu, which sits at the base of the Nyiragongo volcano in East Africa, unexpectedly show that the deep waters are strongly depleted in noble gases with respect to in-situ conditions, and so far no quantitative explanation for this observation has been provided. We make use of recently published noble gas solubility data at higher temperatures to investigate our hypothesis that unusually high equilibration temperatures could have caused the low measured noble gas concentrations by applying various approaches of noble gas thermometry. Noble gas concentration ratios and least squares fitting of individual concentrations indicate that the data agrees best with the assumption that deep water originates from groundwater formed at temperatures of about 65 °C. Thus, no form of degassing is required to explain the observed noble gas depletion: the deep water currently contained in Lake Kivu has most probably never experienced a large scale degassing event. This conclusion is important as limnic eruptions were feared to threaten the lives of the local population.

Noble gas concentrations in natural waters are widely used todetermine ambient temperature conditions during the last intensive contact withthe atmosphere (equilibration). Such applications require accurate solubilityfunctions, which so far are available only for the common environmentaltemperature range between (0 and 35)degrees C. Nonetheless, environmental scenariosthat generate higher surface-water temperatures (such as volcanism) exist. Previoussolubility measurements beyond similar to 35 degrees C are sparse or outdated and weredetermined through equilibration of water with pure noble gases. This canpotentially render them not suitable for environmental applications whereequilibration with atmospheric air is considered. We therefore conducted new measurements for the solubilities of helium, neon,argon, krypton, and xenon in deionized water equilibrated with atmospheric air at similar to 1 bar for temperatures ranging from (25 to 80)degrees C. These measurements were combined with data from the literature that were obtained in a similar manner andfitted with acommonly used function to determine new noble gas solubility functions valid from (0 to 80)degrees C. We estimate relative standarduncertainties with a 0.99 level of confidence between 0.015 and 0.030 for the new functions, which are thus suitable for theinvestigation of environmental high-temperature equilibration scenarios. For temperatures beyond 35 degrees C, the new functions deviate significantly from previous studies.

For many, 2020 was a year of abrupt professional and personal change. For the aquatic sciences community, many were adapting to virtual formats for conducting and sharing science, while simultaneously learning to live in a socially distanced world. Understandably, the aquatic sciences community postponed or canceled most in-person scientific meetings. Still, many scientific communities either transitioned annual meetings to a virtual format or inaugurated new virtual meetings. Fortunately, increased use of video conferencing platforms, networking and communication applications, and a general comfort with conducting science virtually helped bring the in-person meeting experience to scientists worldwide. Yet, the transition to conducting science virtually revealed new barriers to participation whereas others were lowered. The combined lessons learned from organizing a meeting constitute a necessary knowledge base that will prove useful, as virtual conferences are likely to continue in some form. To concentrate and synthesize these experiences, we showcase how six scientific societies and communities planned, organized, and conducted virtual meetings in 2020. With this consolidated information in hand, we look forward to a future, where scientific meetings embrace a virtual component, so to as help make science more inclusive and global.

is characterized by its very stable salt driven stratification and its high volcanogenic gas concentration. The constant gas input (CO2 and CH4) in combination with the stable water column might lead to disastrous degassing events, limnic eruptions, as happened before at the lakes Nyos and Monoun in Cameroon. To get insights to the water column dynamics and thus historic eruptions our aim is to date the age of the water at different depth using 39Ar Atom Trap Trace Analysis, short ArTTA.

Gases dissolved in the deep water of lakes can pose a hazard when extreme concentrations are reached. A sudden release of large amounts of gas can cost the lives of humans living in the neighbourhood, as happened at Lake Nyos in 1986. Since 2001, the gas risk at Lake Nyos has been mitigated by induced degassing, but the lake continues to be supplied by CO2, and a regular survey needs to be implemented to guarantee safe conditions. Frequent sampling of this remote lake requires an enormous effort, and many analytical techniques are very difficult to run at the lake site. In this contribution, we combined a commercially available sound speed sensor with a CTD (electrical conductivity, temperature, depth) probe to obtain an indirect but quantitative estimate of carbon dioxide concentrations with fine depth resolution (decimetre scale). Dissolved carbon dioxide increases sound speed but does not contribute to electrical conductivity. Hence the difference between measured and calculated (on the base of electrical conductivity, temperature and pressure) sound speed gives a quantitative indication of dissolved carbon dioxide. We infer the vertical distribution of dissolved CO2 and hence continue the survey of the progress of the intended degassing. In conclusion, we present an easy to implement method for very high CO2 concentrations in deep lakes, and we highly recommend the implementation of the sound speed-CTD probe combination at Lake Nyos and at other gas-laden volcanic lakes, as such an approach could safeguard the people living in the area with acceptable cost and effort for the operators. In this manner, alarming CO2 concentrations in deep parts of lakes can be detected in a timely fashion.