Lake Overturn Research Articles

Lake mixing regime selects apparent methane oxidation kinetics of the methanotroph assemblage

Abstract. In lakes, large amounts of methane are produced in anoxic sediments. Methane-oxidizing bacteria effectively convert this potent greenhouse gas into biomass and carbon dioxide. These bacteria are present throughout the water column, where methane concentrations can range from nanomolar to millimolar. In this study, we tested the hypothesis that methanotroph assemblages in a seasonally stratified freshwater lake are adapted to the contrasting methane concentrations in the epi- and hypolimnion. We further hypothesized that lake overturn would change the apparent methane oxidation kinetics as more methane becomes available in the epilimnion. In addition to the change in the methane oxidation kinetics, we investigated changes in the transcription of genes encoding methane monooxygenase, the enzyme responsible for the first step of methane oxidation, with metatranscriptomics. Using laboratory incubations of the natural microbial communities, we show that the half-saturation constant (Km) for methane – the methane concentration at which half the maximum methane oxidation rate is reached – was 20 times higher in the hypolimnion than in the epilimnion during stable stratification. During lake overturn, however, the kinetic constants in the epi- and hypolimnion converged along with a change in the transcriptionally active methanotroph assemblage. Conventional particulate methane monooxygenase appeared to be responsible for methane oxidation under different methane concentrations. Our results suggest that methane availability is one important factor for creating niches for methanotroph assemblages with well-adapted methane oxidation kinetics. This rapid selection and succession of adapted lacustrine methanotroph assemblages allowed the previously reported high removal efficiency of methane transported to the epilimnion to be maintained – even under rapidly changing conditions during lake overturn. Consequently, only a small fraction of methane stored in the anoxic hypolimnion is emitted to the atmosphere.

Growth and rapid succession of methanotrophs effectively limit methane release during lake overturn

Lakes and reservoirs contribute substantially to atmospheric concentrations of the potent greenhouse gas methane. Lake sediments produce large amounts of methane, which accumulate in the oxygen-depleted bottom waters of stratified lakes. Climate change and eutrophication may increase the number of lakes with methane storage in the future. Whether stored methane escapes to the atmosphere during annual lake overturn is a matter of controversy and depends critically on the response of the methanotroph assemblage. Here we show, by combining 16S rRNA gene and pmoA mRNA amplicon sequencing, qPCR, CARD-FISH and potential methane-oxidation rate measurements, that the methanotroph assemblage in a mixing lake underwent both a substantial bloom and ecological succession. As a result, methane oxidation kept pace with the methane supplied from methane-rich bottom water and most methane was oxidized. This aspect of freshwater methanotroph ecology represents an effective mechanism limiting methane transfer from lakes to the atmosphere.

Exploring Factors that Influence Collaborative Problem Solving Awareness in Science Education

This study designed a science course following collaborative problem solving (CPS) processes, and examined the effect on students’ CPS awareness. The Limnic Eruption CPS course was implemented using a Moodle system in a tenth-grade class. Considering the complex and coordinated nature of CPS, in order to improve CPS skills, it is important to identify what are related with the development of all sub-skills of CPS. Thus this study aimed to determine potential factors that affect the use of CPS skills in students’ motivational and behavioral dimensions. Multiple data sources including learning tests, questionnaire feedback, and learning logs were collected and examined by learning analytics approach. The relationships between students’ CPS awareness with their learning motivation and learning behaviors were explored. The research findings indicated a significant positive correlation between CPS awareness and certain learning motivation factors and learning behavior factors. Considering the students’ individual differences in learning abilities, we also compared the results of high and low performance groups. As a result, low performers’ learning motivation and learning behaviors were correlated with the social domain of CPS awareness, while those of high performers were correlated with their cognitive awareness.

Degassing Pit Lakes: Technical Issues and Lessons Learnt from the HERCO2 Project in the Guadiana Open Pit (Herrerías Mine, SW Spain)

The unique case of carbon dioxide accumulation in the bottom layer of the acidic lake formed in the Guadiana open pit (Herrerias mine, SW Spain) led to exceptionally high dissolved gas pressures (ca. 5.0 bar at depths of 50–70 m). This CO2 has formed by carbonate (calcite, dolomite) dissolution during decades of interaction of the acidic water with the rocks of the pit walls and associated mine tunnels. The elevated dissolved gas pressures were only comparable to those found in the volcanic crater lakes of Cameroon like Nyos and Monoun, which erupted in the 1980s and sadly triggered the loss of hundreds of human lives. This gas pressure was not far from the absolute (hydrostatic plus atmospheric) pressure limiting CO2 solubility at depth, thus making this pit lake potentially dangerous for the possibility of a hypothetical limnic eruption. Following detailed field and experimental studies aimed at deciphering the source and accumulation patterns of CO2 in the lake, a degassing project started in 2014, which followed an upscaling strategy with construction and installation of progressively bigger pipelines capable of extracting increasingly higher flow rates of CO2-charged water from the deep part of the lake. As of May 2019, the degassing systems have successfully extracted most of the CO2 originally present in the lake (i.e. ≈ 123,000 m3 CO2), which has been discharged to the atmosphere in a controlled and safe way. The fountains became progressively smaller and finally stopped, due to the decreased gas pressure at depth. The larger pipes will serve as a regulating system in the future. This paper describes the main findings encountered during the degassing project, some relevant technical issues observed during the last 5 years (2014–2019), and lessons learnt from the Guadiana pit lake experience.

A Comparison of the Diurnal Variation in Lake Surface Temperature for the Five Major Lakes of the Savannah River Basin

Satellite measurements of lake surface temperature can benefit several environmental applications such as estimation of lake evaporation, predictions of lake overturning, and meteorological forecasts. Using a one-dimensional lake simulation that incorporates satellite measurements of lake surface temperature, the average diurnal variation in lake surface temperature was obtained. The satellite measurements were obtained from the MODIS instrument aboard the Aqua and Terra satellites. Herein the functional form for the diurnal variation in surface temperature is presented for each of the five major lakes in the Savannah River Basin, which are located in South Carolina and Georgia: Lakes Jocassee, Keowee, Hartwell, Russell, and Thurmond. Differences in the diurnal variation in surface temperature between each of these lakes are identified and potential explanations for these differences are presented.

Reliable reference for the methane concentrations in Lake Kivu at the beginning of industrial exploitation

Abstract. Dissolved methane in Lake Kivu (East Africa) represents a precious energy deposit for the neighbouring countries, but the high gas loads have also been perceived as a threat by the local population. This is especially the case with respect to potential changes to the lake's stratification during planned industrial exploitation. Both of the above-mentioned issues require accurate and reliable measurements of dissolved gases and temporal changes to take responsible action. Previous data were unable to satisfactorily fulfil these requirements. Prior to our measurements, there was considerable disagreement about prognosticated new formation of methane. We show how measurement accuracy could be significantly improved by implementing equipment that is especially designed and modified for the complex gas conditions in Lake Kivu. Samples were taken from depths of 150 to 430 m to more reliably and more accurately determine the amount of dissolved methane and dissolved carbon dioxide. Beyond the provision of gas concentration profiles at the beginning of exploitation, this investigation can also potentially provide methods to survey the further evolution of gases over time. The use of gas-tight sampling bags produced highly reliable and accurate measurements. Our measurements confirmed the huge amount of stored methane, but clearly did not support the current belief of a significant recharge beyond diffusive loss. Direct measurements with a custom-made gas pressure sensor indicated no imminent danger due to limnic eruptions. However, a further survey of gas pressures is mandatory to detect changing conditions. Using sampling bags and a gas pressure sensor, we introduced reliable and highly accurate measurement approaches for the survey of the further development of gas concentrations. This equipment requires little effort with respect to calibration, which makes it suitable for use in the remote areas of Africa.

Managing the dangers in Lake Kivu – How and why

Lake Kivu is probably the World's largest natural freshwater digester of algae to produce biogas. Its resources in situ may generate power for generations. Extracting gas is essential to avert a future limnic eruption. Undisturbed, the reservoir formed by salinity-based chemoclines, keeps biogenic CH4 and CO₂ in solution. This is stored in lower strata of the lake.Gases accumulating ever closer to saturation levels, threaten to cause a future limnic eruption. An eruption as occurred at Lake Nyos in Cameroon in 1986 with 1746 casualties, can result if not prevented. But Lake Kivu has potential and inventory for consequences 1000 times larger.Based on novel hypotheses on vertical transport and the lake's history, we used multidisciplinary analyses of this situation. One can foresee its outcomes and recognise system constraints.Therefore, initiating gas extraction enables the vital outcomes; (a) society can enforce extraction methods that ensure prevention of future disasters while, (b) minimizing environmental impact, (c) maximizing useful energy output, and (d) developers pursuing economic projects. The key to safety is management of the chemoclines while producing gas. Achieving safety and production needs the right specification of plant design. For gas production facilities, it is designing to achieve what must be done technically.After our primary concern for public safety, we examine ways of minimizing any environmental impacts. Changes are caused by natural upwelling of saline meteoric water from lava strata into the deep monimolimnion of nutrient-rich water bodies. Raw gas extracted must be washed with water from the mixolimnion to make the gas fit for use in power-generation and domestic gas.For maintenance of chemoclines, we discuss how a fraction of the degassed water must be evacuated from the resource strata and re-injected into the mixolimnion to maintain chemoclines. The challenge lies in how to minimize this safety-driven impact on the mixolimnion from toxic effects of H₂S, from CO₂-induced acidity, and oxygen depletion by CH4 and H₂S.

A single limnic eruption at the origin of today's large-scale density structure of Lake Kivu

This work proposes that the mixed zones and haloclines in Lake Kivu are a result of the following changes: A drought triggered a limnic eruption around 4,000 yBP, which left behind a uniformly mixed lake. Increasing precipitation after the eruption generated the two upper zones by dilution, first by precipitation into the closed lake and subsequently by flushing when the lake overflowed to Lake Tanganyika. Sublacustrine and saline meteoric sources generated the first denser zone below the former holomictic zone from the eruption and later, magmatic CO2 was added to the saline sources, generating a denser bottommost zone.The continuously inflowing sublacustrine sources caused upward movement of the mixed zones in the lake, movements that continue to this day. If not stopped, this movement will in the future bring the gases from the lower strata closer to the surface and thus to the point of preventing utilization of accumulated CH4. If wanting to prevent future limnic eruptions, this would leave future generations with no other choice than venting the accumulating gases.Seven brown layers from the last 2,200 years in the sediments at Lake Kivu were thought to result from limnic eruptions. A re-evaluation of existing data indicates that these layers result from plumes of magmatic CO₂ emanating from fissures below the lake and entraining nutrient-laden bottom water to the mixolimnion, thus lowering the strata above the point of plume initiation. At unpredictable times and with unpredictable intensity, these plumes have thus for the last 2,000 to 3,000 years kept down the gas laden strata, thus contributing to preventing further abrupt limnic eruptions caused by gas accumulation. In addition, these CO₂ injections at varying depths and of varying intensities likely are a major contributor to onsets and disappearances of precipitating CaCO3 from the mixolimnion reaching the bottom sediments.

Detecting gas-rich hydrothermal vents in Ngozi Crater Lake using integrated exploration tools

Gas-rich hydrothermal vents in crater lakes might pose an acute danger to people living nearby due to the risk of limnic eruptions as a result of gas accumulation in the water column. This phenomenon has been reported from several incidents, e.g., the catastrophic Lake Nyos limnic eruption. CO2 accumulation has been determined from a variety of lakes worldwide, which does not always evolve in the same way as in Lake Nyos and consequently requires a site-specific hazard assessment. This paper discusses the current state of Lake Ngozi in Tanzania and presents an efficient approach how major gas-rich hydrothermal feed zones can be identified based on a multi-disciplinary concept. The concept combines bathymetry, thermal mapping of the lake floor and gas emission studies on the water surface. The approach is fully transferable to other volcanic lakes, and results will help to identify high-risk areas and develop suitable monitoring and risk mitigation measures. Due to the absence of a chemical and thermal stratification of Lake Ngozi the risk of limnic eruptions is rather unlikely at present, but an adapted monitoring concept is strongly advised as sudden CO2 input into the lake could occur as a result of changes in the magmatic system.

Final step of the 32-year Lake Nyos degassing adventure: Natural CO2 recharge is to be balanced by discharge through the degassing pipes

In wake of the lakes Monoun and Nyos disasters, the aim of the Nyos Organ Pipes Program was clearly to mitigate the hazard of a limnic eruption at both lakes in the coming decades. Due to the carbon dioxide natural recharge of the Nyos lake water, the self-siphon degassing pipes had been progressively scaled up since 1992 in order to liberate most of the CO2 content of the lake in a reasonable time. One of the pipes was also fine-tuned to exactly counterbalance for the CO2 recharge on the long term. In less than thirty years, both objectives have been met by a team of French scientists, engineers and technicians. Here we show that the continuous operation of no more than a single degassing pipe, as already installed in Lake Nyos since 2001 and refurbished in 2011, could match the natural CO2 recharge rate. The safeguarding, without any human intervention, of a visible water jet, equally feasible at Lake Monoun, will sustain the awareness of scientists and institutions in charge, and will perpetuate their commitment to decrease the risk of a new disaster among the Nyos and Monoun lakeside residents.

Vertical change in transparency of water at Lake Nyos, a possible indicator for the depth of chemocline

A limnic eruption in 1986 at Lake Nyos in Cameroon was caused by sudden release of magmatic CO2 dissolved in the lake water. For frequent measurements of CO2 in the lake water, we developed a simple method of CO2 monitoring using sound speed (SS) measurement. We measured the depth profile of sound speed at Lake Nyos in 2012, 2013, 2014, and 2015. Furthermore, in the 2014 and 2015 survey, we took movies of the underwater and the bottom of the lake using an underwater camera housed in a pressure-resistant container. The vertical change of transparency of water was checked by the visibility of reflectors set in front of the camera. A pressure sensor monitored the depth simultaneously. In both years, in addition to well-known red surface water, we discovered a cloudy layer near the upper chemocline around 100 m depth. The depth of chemocline is a good indicator of the change of total amount of CO2 in Lake Nyos. Because the transparency can be detected by a simple optical sensor, we can now apply our low-cost monitoring tool to find the depth of chemocline for Lake Nyos.

30 years of the Lakes Nyos and Monoun gas disasters: A scientific, technological, institutional and social adventure

The gas explosions of the mid 80's at Lakes Monoun and Nyos in Cameroon released huge quantities of magmatic CO2 gas that asphyxiated close to 1800 people and thousands of livestock in the surrounding villages, giving birth to a new type of natural disaster called limnic eruption which, interestingly, is both predictable and preventable. These unique events generated renewed interest in volcanic lake studies across the world, resulting in the creation of new associations such as the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) Commission on Volcanic Lakes (IAVCEI-CVL) and institutional reforms regarding civil protection and disaster management as a whole in Cameroon and beyond. Post scientific studies indicated not just gas buildup in the bottom layers of both lakes, but also identified the potential risk of collapse of the natural dam at the outlet of Lake Nyos. As an introduction to this special issue, this paper revisits these two lakes alongside the other Cameroon Volcanic Line (CVL) maars and appraises the scientific and technological achievements registered, discusses the phenomena of limnic eruption and lake turnover in some other Cameroonian lakes, and highlights the lessons learnt as well as perspectives.Design and test degassing experimentation started at Monoun in 1992 and at Nyos in 1995 and led to full-scale degassing with a single pipe at Nyos in January 2001 and at Monoun in 2003. Two more pipes were installed at Monoun in 2006 and at Nyos in 2011 so as to accelerate the degassing, which worked well and rendered the lakes safe by decreasing their CO2 contents to inoffensive levels. Moreover, the much dreaded potential risk of dam collapse and downstream flooding at Nyos has also been averted following the success of dam reinforcement works. In view of these achievements, the science underlying gas accumulation in lakes and subsequent similar catastrophic phenomenon is now understood. Lakes Nyos and Monoun are today infamous and have, over the past 30 years, not only attracted considerable scientific interest and media coverage but have above all, become natural laboratories for capacity development (design and testing of new investigation methods and equipment) and capacity building (infrastructural and human) which were largely boosted under a 5-year joint Japan Cameroon cooperation project code named ‘Satreps NyMo’.

Lago Albano, the “anti-Nyos-type” lake: The past as a key for the future

On 21 August 1986 almost 1800 people were asphyxiated by a CO2 cloud violently released by Lake Nyos, Cameroon. Post-1986 monitoring of Lake Nyos revealed that CO2 steadily accumulates in bottom waters through recharge from soda-rich aquifers. The 1986 lake roll-over event triggered scientific research on volcanic lakes, creating “Nyos biased” interpretations: the search for dissolved CO2 in other lakes might have led to over-interpretations regarding hazard assessment. In this study, a thorough review of the historical literature on Lago Albano over the past approx. 2800 years shows no clear evidence of Nyos-type bursts, contrary to previous ideas. In 1989 Lago Albano was affected by a large CO2 pulse, concomitant with a seismic swarm below Colli Albani volcano. Tracing back in historical literature, at least two similar anomalous degassing episodes occurred out of five seismic crises between 1829 and 1927. Partial temperature- and density-driven roll-over of the top-9 m of Lago Albano commonly releases accumulated CO2 each winter (Chiodini et al., 2012). This degassing dynamics avoids long-term CO2 accumulation in bottom waters, as is the case at permanently stratified lakes in the tropics, such as Lake Nyos.Here we show that Lago Albano is an “anti-Nyos-type" lake: sudden recharge and regularly periodic release of CO2 (Lago Albano) vs. steady recharge and sudden release of CO2 (Lake Nyos). Despite past evidence of hazardous events, (1) the intensive well pumping from the Albano aquifer might lower lake level, and hence decrease the CO2 saturation pressure threshold, and (2) the absence of cold winters, hence avoiding yearly lake roll-over favoring CO2 accumulation at bottom layers, are modern factors that rise the need to revise hazard assessment and future monitoring strategies. Despite the fact that CO2 concentration in bottom waters was far from saturation conditions during the last survey (May 2010; Chiodini et al., 2012), making a limnic eruption highly unlikely, it is necessary to know the saturation state of CO2 in bottom waters and physical lake stability at any time, in order to be prepared for a next anomalous co-seismic CO2 degassing event of unknown quantity near Lago Albano.

Using a decadal diatom sediment trap record to unravel seasonal processes important for the formation of the sedimentary diatom signal

Sediment trap studies and high frequency monitoring are of great importance to develop a deeper understanding of how seasonal environmental processes are imprinted in sediment signal formation. We collected whole year diatom assemblages from 2002 to 2014 with a sequential sediment trap from a varved boreal lake (Nylandssjön, Sweden) together with environmental and limnological parameters, and compared them with the corresponding diatom record of the annual laminated sediment. Our data set indicates a large year-to-year variability of diatom succession and abundance patterns, which is well reflected in the varved sediments. Specifically, Cyclotella glomerata dominated the annual sediment trap record (as well as in the corresponding sediment varves) in years with warmer air temperatures in March/April, and Asterionella formosa dominated the annual sediment assemblages as a consequence of years characterized by higher runoff before lake over-turn. Years succeeding forest clearance in the lake catchment showed marked increase in diatom and sediment flux. The DCA scores of the yearly diatom trap assemblages clearly resemble the lake’s thermal structure, which indicates that the relative abundance of major taxa seems primarily controlled by the timing of seasonal environmental events, such as above-average winter air temperature and/or autumn runoff and the current thermal structure of the lake. The high seasonal variability between environmental drivers in combination with the physical limnology leaves us with several possible scenarios leading to either an A. formosa versus C. glomerata dominated annual diatom sediment signal. With this study we highlight that short-term environmental events and seasonal limnological conditions are of major importance for interpreting annual sediment signals.

High-resolution modelling of atmospheric dispersion of dense gas using TWODEE-2.1: application to the 1986 Lake Nyos limnic eruption

Abstract. Atmospheric dispersal of a gas denser than air can threat the environment and surrounding communities if the terrain and meteorological conditions favour its accumulation in topographic depressions, thereby reaching toxic concentration levels. Numerical modelling of atmospheric gas dispersion constitutes a useful tool for gas hazard assessment studies, essential for planning risk mitigation actions. In complex terrains, microscale winds and local orographic features can have a strong influence on the gas cloud behaviour, potentially leading to inaccurate results if not captured by coarser-scale modelling. We introduce a methodology for microscale wind field characterisation based on transfer functions that couple a mesoscale numerical weather prediction model with a microscale computational fluid dynamics (CFD) model for the atmospheric boundary layer. The resulting time-dependent high-resolution microscale wind field is used as input for a shallow-layer gas dispersal model (TWODEE-2.1) to simulate the time evolution of CO2 gas concentration at different heights above the terrain. The strategy is applied to review simulations of the 1986 Lake Nyos event in Cameroon, where a huge CO2 cloud released by a limnic eruption spread downslopes from the lake, suffocating thousands of people and animals across the Nyos and adjacent secondary valleys. Besides several new features introduced in the new version of the gas dispersal code (TWODEE-2.1), we have also implemented a novel impact criterion based on the percentage of human fatalities depending on CO2 concentration and exposure time. New model results are quantitatively validated using the reported percentage of fatalities at several locations. The comparison with previous simulations that assumed coarser-scale steady winds and topography illustrates the importance of high-resolution modelling in complex terrains.

Lakes Nyos and Monoun Gas Disasters (Cameroon)—Limnic Eruptions Caused by Excessive Accumulation of Magmatic CO<sub>2</sub> in Crater Lakes

Lakes Nyos and Monoun Gas Disasters (Cameroon)—Limnic Eruptions Caused by Excessive Accumulation of Magmatic CO<sub>2</sub> in Crater Lakes

Methane storage and ebullition in monimolimnetic waters of polluted mine pit lake Vollert-Sued, Germany.

Reliable gas measurements from supersaturated deep waters still remain a challenge. However, good information is mandatory to investigate the limnic carbon cycle, assess the endangerment through limnic eruptions and evaluate a potential source of exploitable energy. We addressed these three points in a heavily polluted mine pit lake in Germany. We quantified the ebullition of methane from deep waters and the sediment below. Exposed to continuous percolation of gas bubbles, the deep (monimolimnetic) water had accumulated high concentrations of gas: directly measured gas pressures indicated the proximity to spontaneous ebullition. Consequently, the possibility of a limnic eruption was assessed by initiating a self-sustained flow through a vertical pipe. Despite the high gas pressures, the flow was slow and the endangerment was considered low. A sampling strategy with bags was developed to achieve a reliable measurement of gas content and gas composition in the monimolimnion. As a result, directly measured gas pressures could be confirmed and were nearly exclusively attributed to methane and nitrogen. Contrary to lakes that had shown limnic eruptions, carbon dioxide played a much subordinate role, and hence the driving force for a violent outburst of gases was missing. Nevertheless the amount of dissolved methane was remarkably high. This investigation closes with some estimates of the commercial value of the deposit and limiting conditions for a possible exploitation.

Stratification at the Earth's largest hyperacidic lake and its consequences

Volcanic lakes provide windows into the interior of volcanoes as they integrate the heat flux discharged by a magma body and condense volcanic gases. Volcanic lake temperatures and geochemical compositions therefore typically serve as warnings for resumed unrest or prior to eruptions. If acidic and hot, these lakes are usually considered to be too convective to allow any stratification within their waters. Kawah Ijen volcano, featuring the largest hyperacidic lake on Earth (volume of 27 million m3), is less homogeneous than previously thought. Hourly temperature measurements reveal the development of a stagnant layer of cold waters (<30 °C), overlying warmer and denser water (generally above 30 °C and density ∼1.083 kg/m3). Examination of 20 yrs of historical records and temporary measurements show a systematic thermal stratification during rainy seasons. The yearly rupture of stratification at the end of the rainy season causes a sudden release of dissolved gases below the cold water layer which appears to generate a lake overturn, i.e. limnic eruption, and a resonance of the lake, i.e. a seiche, highlighting a new hazard for these extreme reservoirs. A minor non-volcanic event, such as a heavy rainfall or an earthquake, may act as a trigger. The density driven overturn requires specific salinity-temperature conditions for the colder and less saline top water layer to sink into the hot saline water. Spectacular degassing occurs when the dissolved gases, progressively stored during the rainy season due to a weakened diffusion of carbon dioxide in the top layer, are suddenly released. These findings challenge the homogenization assumption at acidic lakes and stress the need to develop appropriate monitoring setups.

Surface CO 2 emission and rising bubble plumes from degassing of crater lakes in São Miguel Island, Azores

Abstract We report the first detailed study on the types and distributions of active subaqueous fumaroles and surface diffuse CO 2 degassing in the three main volcanic lakes of São Miguel Island (Sete Cidades, Fogo and Furnas), Azores archipelago, Portugal. The results of the surveys, carried out in May 2011 using a floating accumulation chamber and a dual beam 50 and 200 kHz echo sounder, revealed a very low surface CO 2 degassing at the three lakes, in the range of 32–608 kg d −1 . However, dense subaqueous degassing plumes were found in the north of the Furnas crater lake (7.5–9 plumes per 100 m 2 ), and moderate-density degassing in the Fogo (1.5–2 plumes per 100 m 2 ) and Sete Cidades crater lakes (1–1.5 plumes per 100 m 2 ). The echo sounder detected hydroacoustic signatures interpreted as acoustic flares, ‘puffing’ bubble plumes or walls of bubbles associated with numerous subaqueous fumaroles. The recorded echograms show that the bubbles rise at average speeds of between 19 and 30 cm s −1 at the bottom, with frequencies of release from 1–2 to 31 s. Most subaqueous fumaroles disappear due to the dissolution of CO 2 before reaching the lake surface. These dissolution processes are enhanced by the pH range observed in the three volcanic lakes ( c. 7–9). Observed dissolved CO 2 values indicate that the pressure of this gas in the three lakes remained much lower than the hydrostatic pressure and the risk of a limnic eruption is therefore negligible. We suggest that the rising levels of CO 2 from the subaqueous bubbles could constitute a critical fuel for subsurface phytoplankton layers, interpreted as horizontal acoustic layers with high backscattering values. The highest density of subaqueous bubbling correlates with recent submerged secondary craters formed around the caldera rims of the three Late Quaternary stratovolcano complexes of São Miguel Island. Our results emphasize the need to perform regular surface degassing studies as an important volcanic surveillance tool in the Azores archipelago.

Mathematical models for quantifying eruption velocity in degassing pipes based on exsolution of a single gas and simultaneous exsolution of multiple gases

After the limnic eruptions at Nyos and Monoun in the 1980s, degassing pipes were installed to reduce the continuous increase of CO2 at the bottom of these lakes. The degassing system consists of a vertical pipe from the lake bottom to the surface and a small pump located near the top of the pipe, which raises water in the pipe up to a level where it becomes saturated with gas, which in turn leads to volume expansion and eruption. This paper describes two new mathematical models for predicting eruption velocity in degassing pipes based on exsolution of a single gas and the simultaneous exsolution of multiple gases. The models were applied to the degassing system of lakes Nyos and Monoun, which contain two main gases, namely CO2 and CH4. Because the volume proportion of CH4 is significant only in Lake Monoun, the Lake Nyos test case considered the CO2 gas only, while as the Lake Monoun test case considered the simultaneous exsolution of CO2 and CH4. Good agreement between the results of the models and observed data is found for both test cases. The results for the eruption in Lake Monoun considering the two main gases measured in this lake (CO2 and CH4) were found to have a better agreement with the measurements compared to the model results obtained considering the main gas only (CO2).