Minimum Oxygen Concentration Research Articles

Analysis of the Influence of On-Board Temperature and Pressure Control System on Inert Gas Generating Performance of Hollow Fiber Membrane

Based on the typical on-board membrane separation system for fuel inerting, the mathematical models of temperature and pressure control subsystem and hollow fiber membrane were built with the coupled iterative numerical solution methods. While the temperature and pressure were controlled, the design area, nitrogen production effect of hollow fiber membranes were analyzed and compared with the situation of no temperature and pressure control in the full flight envelope by numerical simulation. Moreover, the root causes for those changes were found out. It shows that the method used in temperature and pressure control can increase the bleed air pressure and enhance the stability of temperature and pressure effectively. Compared with the situation of no temperature and pressure control, the temperature and pressure control method are able to decrease the demanded area of membrane by 42.3%∼51.7%. And there is no need to adjust the membrane area for condition of low nitrogen production efficiency. But it also decreases the highest nitrogen production efficiency by 9.4% and increase the minimum oxygen concentration by 0.65%. The rate of flow has the most significant effect on nitrogen production. These phenomena are caused by the effects of the rate of flow, pressure, temperature and membrane area.

Оxygen solubility in melts of Ni – Co system at complex deoxidation by aluminium and silicon

Alloys of Ni – Co system are widely used in industry. Oxygen is one of the harmful impurities in these alloys; in metal it is present in dissolved form or in the form of nonmetallic inclusions. Getting the finished metal with a minimum oxygen concentration is one of the main tasks of production process of these alloys. With the complex deoxidation of the metal melt, the activity of oxides resulting from the deoxidation process is less than one, due to this, with the same content of deoxidizing elements, it is possible to obtain a metal with a lower oxygen concentration, therefore, more deeply deoxidized. At joint deoxidation with two deoxidizers, a stronger deoxidizer takes a predominant part in the reaction, however, if oxides of the deoxidizing elements form chemical compounds, it contributes to participation of a weaker deoxidizer in deoxidation process. A thermodynamic analysis of the joint influence of aluminum and silicon on the solubility of oxygen in the melts of Ni – Co system has been performed. Deoxidation reaction products may be formed as mullite (3Al2O3·2SiO2 ), and kyanite (Al2O3·SiO2 ). Presence of silicon in the melt enhances the deoxidizing ability of aluminum: insignificantly in the case of formation of compound 3Al2O3·2SiO2 and significantly in the case of formation of compound Al2O3·SiO2 . Oxygen solubility curves in the case of formation of compound Al2O3·SiO2 pass through a minimum, the position of which depends on the content of aluminum in the melt and doesn’t dependent on silicon content. Aluminum content in the minimum points is insignificantly reduced from nickel to cobalt as in the case of melts of Ni – Co–Al. Further additives of aluminum lead to an increase in oxygen concentration. Areas of compounds Al2O3 , 3Al2O3·2SiO2 , Al2O3·SiO2 and SiO2 depending on aluminum and silicon contents in the melt are determined. In melts of the Ni – Co system, the deoxidizing ability of aluminum and silicon increases with an increase in cobalt content in the melt. However, silicon enhances the deoxidizing ability of aluminum the weaker the higher cobalt content.

Predicting Polymer Flammability Using Machine Learning Methods

Solid state batteries are a promising replacement for traditional liquid electrolyte batteries due to their superior stability and inflammability. Conventional lithium ion batteries use an organic liquid electrolyte, which is a potential hazard when it undergoes a decomposition reaction leading to an explosion. A solid, non- flammable electrolyte is a promising alternative. However, the chemical space of polymers is diverse and identifying new materials for electrolytes requires a critical screening criterion. Polymer flammability can be quantified using the limiting oxygen index (LOI), which is the minimum oxygen concentration necessary to sustain stable combustion. Polymers with an LOI greater than 21 are inflammable at room temperature and can be deemed safe for use in a battery. In this work, we predict the LOI for new polymers using a combination of data driven and machine learning methods. We have carefully curated a dataset of polymers and their associated LOI values, which are experimentally determined. A fingerprinting scheme is used to numerically represent the polymers from an atomistic to morphological length scale. We then use a gaussian progress regression model to map the polymers from their fingerprint space to their LOI. This model can then be used to rapidly predict the LOI of a polymer and its associated uncertainties. This study could rationally guide the design of solid polymer electrolytes which are thermally stable and inflammable.

Allometric Scaling of physiologically-relevant organoids

The functional and structural resemblance of organoids to mammalian organs suggests that they might follow the same allometric scaling rules. However, despite their remarkable likeness to downscaled organs, non-luminal organoids are often reported to possess necrotic cores due to oxygen diffusion limits. To assess their potential as physiologically relevant in vitro models, we determined the range of organoid masses in which quarter power scaling as well as a minimum threshold oxygen concentration is maintained. Using data on brain organoids as a reference, computational models were developed to estimate oxygen consumption and diffusion at different stages of growth. The results show that mature brain (or other non-luminal) organoids generated using current protocols must lie within a narrow range of masses to maintain both quarter power scaling and viable cores. However, micro-fluidic oxygen delivery methods could be designed to widen this range, ensuring a minimum viable oxygen threshold throughout the constructs and mass dependent metabolic scaling. The results provide new insights into the significance of the allometric exponent in systems without a resource-supplying network and may be used to guide the design of more predictive and physiologically relevant in vitro models, providing an effective alternative to animals in research.

Oxygen Index of Magnesium Powder

The combustion regimes were studied and the oxygen index for the bulk-density magnesium powder with a particle size of ~80 μm placed in a paper cup was determined in nitrogen–oxygen and argon–oxygen mixtures at pressures from 0.3 to 1 atm. It was shown that at a constant pressure of the mixture, there was a minimum concentration of oxygen at which sustained propagation of the combustion front was possible. The boundaries of the region of stable combustion of such a system in the pressure–oxygen concentration coordinates were determined. It was shown that, depending on the oxygen concentration, two combustion modes were possible: one-step mode far from the combustion limit and spin mode at the combustion limit. As the pressure or oxygen concentration in the mixture increased, the characteristic spin rotation time decreased and the propagation of the combustion front over the sample passed into a layer-by-layer mode. The dependence of the combustion front propagation velocity on the sample near the combustion limit on the mixture pressure and the oxygen concentration in its composition was determined.

Assessment of minimum oxygen concentrations for the growth of heat-resistant moulds

This study evaluated the effect of both gaseous and dissolved oxygen (O2) concentration (0 - 21%) on the growth of six heat-resistant moulds (HRMs) (Neosartorya and Byssochlamys spp.) previously isolated from high-acid fruit products. The study was performed in acidified potato dextrose agar (aPDA) with all six HRMs and with B. fulva and N. fischeri in strawberry, apple and orange juice-based media. At ≥ 0.15% O2, visible growth of the HRMs occurred within 3–6 days. Complete inhibition on aPDA did not occur even at very low levels of dissolved O2 (ca. 0.01% O2). With the exception of B. fulva, decrease of the O2 concentration to ≤0.03% resulted in significantly (p < 0.05) longer times to visible growth. The growth of N. laciniosa, N. fischeri, B. nivea and B. fulva was inhibited for 30 days when they were incubated under strict anaerobic conditions. As in aPDA, B. fulva and N. fischeri grew in the three fruit-based media at O2 concentrations ≥0.15%. Significantly slower (p < 0.05) growth was observed for N. fischeri in orange juice medium. Strategies to inhibit the growth of HRMs should therefore not be based entirely on establishing low headspace O2 levels. With this in mind, the effect of low O2 concentrations (<1%) should be studied in combination with other factors (hurdles) such as antioxidants, organic acids, sugars (aw), storage temperature and pasteurization intensity, in order to predict the growth inhibition of the HRMs.

Effects of ischaemic conditioning on tissue oxygen saturation and heart rate variability: an observational study.

ObjectiveIschaemic conditioning (IC) has organ-protective effects, but its clinical results have been inconsistent. Tissue oxygen saturation (StO2) and heart rate variability (HRV) reflect peripheral microcirculation and autonomic nervous system activity, but their changes during IC have not been well documented. We assessed StO2 and HRV during IC in patients undergoing cardiac surgery and healthy volunteers.MethodsTen patients undergoing cardiac surgery and 10 healthy male volunteers underwent remote IC (four 5-minute cycles of ischaemia/reperfusion) applied to the upper arm. Changes in StO2 at the thenar eminence and HRV according to the R-R intervals were recorded during IC.ResultsThe lowest StO2 during ischaemia significantly decreased in patients and significantly increased in volunteers. Among the HRV parameters, the low-frequency domain, which corresponds to sympathetic activity, significantly increased after IC in volunteers but not in patients. Other variables were similar between the groups.ConclusionsThese results suggest that the minimum tissue oxygen content is depleted during ischaemia in patients and preserved in healthy volunteers. Sympathetic nervous activity seems to increase after IC in healthy volunteers but remains unaffected in patients. Thus, IC may act differently between patients undergoing cardiac surgery and healthy subjects.

Oxygen consumption in recirculating nutrient film technique in aquaponics

Due to root respiration, a minimum oxygen concentration in the root zone of plants is necessary. To keep the plant healthy and fully efficient the concentration should not fall below a plant specific critical value. Especially in nutrient film technique, the oxygen concentration can deplete heavily during the daytime. Besides the root respiration also microorganisms, living in the root zone of the plants, consume oxygen and have to be considered. In double recirculating aquaponic systems (DRAPS) fish waste water is used for preparation of nutrient solution for plant production in hydroponics. The fish waste water, delivered by a recirculating aquaculture system (RAS), probably contains high amounts of microorganism which can compete with plants for oxygen.The present study was conducted to investigate the oxygen concentration of nutrient solutions used in both, conventional hydroponics and in DRAPS, respectively. Therefore, the oxygen concentration within the cultivation trenches of conventional hydroponics (control; nutrient solution prepared with fresh water, electrical conductivity (EC) 1.8 dS m−1) and DRAPS (nutrient solution prepared with fish waste water (AP), EC 1.8 dS m−1 (low) and EC 3.0 dS m−1 (high)) were investigated. To evaluate whether the differences in oxygen depletion might be almost due to the used process water, the oxygen consumption of the prepared nutrient solutions was investigated separately without plants.The oxygen concentration within the cultivation trenches followed a typical daily fluctuation pattern with a decreasing oxygen concentration during the day. As time passed from April to June the depletion during the day increased and was strongest in AP high. In June the oxygen concentration dropped partly to zero in AP high, while it dropped only to nearly 150 μmol L−1 and 60 μmol L−1 in control and AP low, respectively. The oxygen consumption of pure nutrient solution (without plants) was significantly different between AP high and control, which also affects the oxygen concentration within the cultivation trenches.

Thermodynamics of the Oxygen Solutions in Titanium-Containing Fe–Co–Cr Melts

A thermodynamic analysis of the influence of titanium on the oxygen solubility in Fe–Co–Cr melts at 1873 K is performed. The dependences of the oxygen solubility in Fe–Co–Cr melts on the titanium content are calculated. The titanium content at which the deoxidation reaction mechanism is changed (Cr2O3 $$ \rightleftarrows $$ Ti3O5) is as follows: 2.602 × 10–3% Ti for the Fe–10% Co–10% Cr alloy and 2.975 × 10–3% Ti for the Fe–20% Co–25% Cr alloy. A low titanium content insignificantly increases the oxygen concentration determined by the chromium content. At a higher titanium content (after the change of the mechanism of interaction of chromium and titanium with oxygen, when titanium determines the oxygen solubility in the melts), the oxygen concentration substantially decreases and, then, increases after passing a minimum point at a titanium content of ~0.9%. The minimum oxygen concentrations of these alloys are 1.8 × 10–3 and 3.9 × 10–3%, respectively.

Promising Processes for Producing Drop-in Biofuels and Petrochemicals from Renewable Feedstock (Review)

General types of renewable carbon sources have been considered to be suitable for production of drop-in biofuels with minimum oxygen content, fully compatible with conventional fuels. The methods for processing different feedstock types have been discussed, their main advantages and drawbacks are indicated. Preparation of drop-in fuels from vegetable oils and waste of biodiesel production is possible in the framework of current methods without significant capital investments. Processing the pyrolysis products of lignocellulose biomass is fraught with a number of technical and technological problems, being currently at the research-and-development step.

ОТНОШЕНИЕ КОЛЬЧАТЫХ И КРУГЛЫХ ЧЕРВЕЙ К КИСЛОРОДУ В ЗАПАДНОЙ ЧАСТИ СЕВЕРНОГО КАСПИЯ

Oxygen deficiency is caused by temperature and salinity stratification of water masses. Hypoxia was formed mainly on the border of the Northern Caspian - Middle Caspian, in shallow areas (up to 5 m depth), as well as in areas with fine-grained sediments. Representatives of annelid (Annelida) and round (Nemathelminthes) worms are characterized by high frequency. During the study period (2013-2017) the abundance and biomass of the study group varied widely. Oligochaeta was dominated by the total number, Hirudinea - by the total biomass. The maximum frequency was characteristic for polychaetes Hediste diversicolor . The dynamics of quantitative indicators of worms was inversely dependent on changes in the area of hypoxia: the expansion of the hypoxic area showed the decreased number of worms. The limits of tolerance to the oxygen content in individual species and classes of worms living in the north part of the Caspian Sea have been studied. The minimum concentration of oxygen recorded for the entire study period (1.29 ml/l; 21%) is found not critical for Hediste diversicolor and Hypaniola kowalewskii . The low concentration (1.76 ml/l; 31%) does not inhibit the development of Oligochaetas. The other representatives of Vermes group living in the western part of the Northern Caspian were found under the absolute oxygen content exceeding 4.06 ml/l and under the relative oxygen content exceeding 44%. A wide ecological valence for oxygen is characteristic for Hediste diversicolor , Hypaniola kowalewskii and Oligochaeta . Fabricia sabella , Caspiobdella tuberculosis and Archaeobdella esmonti demonstrate the narrow range of oxygen tolerance.

Assessment of flammability and explosion risks of natural gas-air mixtures at high pressure and high temperature

The flammability and explosion characteristics of natural gas (NG) and air mixtures at high pressure and high temperature are of great concerns for air injection IOR (improved oil recovery) process in oilfield and for natural gas combustion engines. In this study, an experimental setup is built up to measure the explosion limits (flammability limits) of methane and natural gas at elevated pressures and temperatures up to 20 MPa and 100 °C. The influences of pressure, temperature, and ignition energy and gas composition on the flammability limits, minimum oxygen content required for explosion (MinOC) and explosion energy have been investigated for methane and typical natural gas mixtures with air. High pressure and high temperature flammability limit models for natural gas-air mixtures have been proposed based on the experimental data. The experimental results show that the flammability limits of natural gas can be significantly extended at high temperature and high pressure, indicating a good flammability for combustion applications but high explosion risks in terms of the safety for the oilfield air injection process. At 100 °C and 20 MPa, the explosion limit range measured for methane in air is extended to 2.87%–64.40%vol, in contrast to the range of 4.95%–15.51%vol at the normal pressure and temperature (0.1 MPa and 25 °C), and the corresponding minimum oxygen content required for explosion can be reduced to as low as 5.74% from around 10% at the atmospheric conditions. The variation of the upper flammability limit (UFL) is more sensitive than that of the lower flammability limit (LFL) as pressure and temperature increase. Electrically heated tungsten wires were applied for the ignition, and it has been found that high ignition energy is required to ignite the gas mixtures at high pressure conditions. The peak explosion pressure (or explosion energy) is mainly depended on the composition of the flammable gas and oxygen, while it is slightly reduced at high temperature conditions and gradually increases with the initial pressure. Explosions with incomplete combustion reaction can occur at near UFL points with enormous amount of carbon black formed due to lack of oxygen, while the explosion energy can be significantly reduced at near LFL points because of lack of flammable gases. The increase of risk index at high pressure and high temperature indicates a larger explosion risk and safety concern.

Powder-epoxy resin/glass fabric composites with reduced flammability

This study investigates the possible enhancement of flame resistance in powder-epoxy resin/glass fabric composites. For this purpose, the halogen-free flame retardants containing phosphorous, nitrogen and aluminium were used. The total content of the fillers did not exceed 25 wt%. The laminates assessed for flame retardancy were designed specifically to be used as components of seats in public transport. Thermal resistance of the laminates and the surfaces of partially burned composites were also examined using thermogravimetric and scanning electron microscopy/energy-dispersive x-ray spectroscopy analyses, respectively. On the basis of the obtained results, it was found that the highest flame resistance (V-0 class, minimum oxygen concentration = 35.5% and maximum average rate of heat emission = 38.5 kW/m2 at an incident heat flux of 50 kW/m2) was identified in the laminates with matrix comprising 15 wt% aluminium diethyl phosphinate and 10 wt% melamine polyphosphate. In turn, the laminates with the matrix containing ammonium polyphosphate as the main component achieved only the V-1 flammability class.

Plasmatron for Coatings

The paper analyzes reasons for application of plasma torches of special design for deposition of heat-resistant layers of thermal barrier coatings that contribute to minimum oxygen content in the zone of spraying. It contains detailed description of design for a plasma torch with a nozzle that allows better protection of a plasma stream against atmospheric exposure.Heated inert gas (Ar) is fed inside the nozzle along its walls through holes in its end part. Air around a plasma jet is displaced, additional compression of the plasma occurs and due to this oxygen content is decreased in a coating and efficiency of material deposition is increased. An additional use of a tungsten insert in an electronic node makes it possible to create a pair of tungsten – tungsten in an electrode assembly and the life of the electrode assembly will be increased due to high erosion resistance, electrical conductivity, thermal conductivity and sufficient strength in 2–2.5 times while increasing spray performance through toughening regimes. The paper presents testing results of the developed plasma torch and its prototype for duration of continuous operation and intensity at 500 A current, 70 V voltage, nitrogen flow rate of 45 l/min (standard mode for deposition of oxide ceramics). Coatings from nickel–chromium–aluminum–yttrium powder have been obtained while using plasma torches with the same parameters of deposition and a comparative analysis of the technological properties have been made in the paper. The developed plasma torch has more qualitative characteristics of subsurface plasma coatings: increase in adhesive strength – by 1.4 times, material utilization ratio – by 1.3 times, micro-hardness – by 1.3 times, porosity decrease – by 2.3 times.

Induced geochemical reactions by compressed air energy storage in a porous formation in the North German Basin

A synthetic diabatic compressed air energy storage (CAES) based on an existing surface facility and using the Rhaetian sandstone formation in the North German Basin as porous storage reservoir is investigated for induced geochemical reactions. A daily storage cycle with a 6-h injection and a 6-h extraction phase in the early morning and late afternoon is applied. The chosen mineral assemblage shows the presence of pyrite in the Rhaetian sandstone, so that operating CAES in this porous formation introduces oxygen in the formation and induces geochemical reactions governed mainly by pyrite oxidation. A consistent geochemical reaction system for the Rhaetian sandstone is developed and kinetic batch simulations are used to quantify the changes due to the induced geochemical reactions in both stored air as well as storage formation.The injection of air into this geological formation leads to pyrite oxidation, changes in stored air composition, air pressure and formation properties. Results show that only a very small change up to 0.29% in the oxygen mole fraction is found within one storage cycle, which does not affect flammability of the stored air. Considering a longer residence time, the oxygen concentration in the stored air may drop below the minimum oxygen concentration for flame propagation and thus cannot be used for burning natural gas as required for a diabatic CAES. The pH of the formation fluid after 20-year cyclic daily operation can drop significantly below one near the gas wells increasing the risk of well corrosion, but with smaller effects at larger distances. However, mineral dissolution and precipitation found in the storage formation results in only minor increases of porosity and permeability with relative changes up to 1.0% and 5.0%, respectively. The uncertainties in mineral reactive surface area and pyrite oxidation kinetic strongly affect the rate of oxygen reduction and fluid acidification. Analysis of these parameters of the reservoir mineral phases from the target location, especially for pyrite, are therefore required for a reliable estimate of possible induced geochemical reactions and impacts.

Spatial structuring of zooplankton communities through partitioning of habitat and resources in the Bay of Bengal during spring intermonsoon

Despite advances in oceanographic investigations in the Indian Ocean, zooplankton ecology is still poorly known from the open waters of the Bay of Bengal. In order to understand the vertical and horizontal distribution of zooplankton in the Bay of Bengal during spring intermonsoon, stratified sampling was conducted using a multiple plankton net. We found that higher zooplankton biomass was often associated with mesoscale cold-core eddies. The biomass in the mixed layer was less in the central bay and the near absence of organisms beneath this layer was attributed to the very low dissolved oxygen levels, including suboxia in the northern extent. The copepod diversity was severely reduced here and only a few species specialized to thrive in minimum oxygen concentrations were found in this zone. However, Pleuromamma indica is another copepod that seems to have adapted to the oxygen minimum zone, as seen from its increased numbers and its strong vertical migrating ability to cross the low oxygen barrier. Highly diverse copepod assemblages comprising 129 identified species were found to be vertically structured, mostly through partitioning of habitat and food resources.

Redox induced sulfur-selenium isotope decoupling recorded in pyrite

This study presents the first combined S and Se isotope investigation of sulfide suited to explore differences in fractionation between these two redox sensitive isotope systematics as recorded in the same mineral. A case study of Cretaceous Navajún pyrite from the Mesozoic Cameros Basin, Spain, with known petrogenesis and geological context shows systematic decoupling at the microscale: Variable S isotope values within the analyzed pyrite coincide with rather constant Se isotope values and vice versa. These signatures were not generated during pyrite growth but record previous redox induced fractionations in fluids that each contributed both elements from two sources. It is likely that both S and Se isotope fractionation occurred during strong reduction from one fully oxidized source whereas only S but no Se isotope fractionation occurred during minor reduction following sulfide dissolution via H2O from another source. Subsequent mixing of these two H2S-H2Se fluids at different elemental S-Se ratios during incorporation into the pyrite can then explain the S-Se isotope variations in the investigated specimen. These inferences are in accordance with a larger range in the redox potential Eh of Se relative to S, resulting in coupled or decoupled Se and S isotope fractionation depending on the oxygen fugacity during the reduction process. If extended to other sulfides of diverse origin, for a given pH, combined Se and S isotope studies may allow to investigate the magnitude of redox variations and place more robust constraints on minimum and maximum oxygen concentrations in the source. We therefore suggest combined S-Se isotope analyses in sulfide as a new powerful proxy for studying Earth’s redox evolution beyond the bulk rock scale.

Flammability parameters of liquified natural gas

The use of liquefied natural gas (LNG) is constantly growing. However, safety issues regarding cryogenic storage and transportation systems are still to be fully resolved. In particular, the evaluation of the efficiency of inerting systems for low-temperature LNG vapour is essential. In this work, the variation of the flammability range (in terms of lower and upper flammability limits and minimum oxygen concentration) obtained by adding nitrogen to pure air for some representative LNG mixtures has been evaluated at ambient temperature and at temperatures below 0 °C by using a detailed kinetic model entitled KIBO, which has been proved to be reliable for the description of C0C4 reactions in oxidative conditions, and by the limiting burning velocity theory.Strong differences are reported among pure methane and natural gas mixtures for all the investigated temperatures. The effect of composition is therefore relevant. Furthermore, the obtained results suggest that the lower flammability limit is determined by thermal aspects at high temperature only, whereas at low temperature, kinetic limitations are more relevant.

Evaluation of safety parameters of light alkenes by means of detailed kinetic models

Abstract The oxidation of light alkenes is the core of the modern chemical industry and a pivotal point for several environmental and safety considerations. However, few works have dealt with the prediction of fire and explosion parameters for the definition of process conditions, for the design and safe handling of such reactive substances. In this work, flammability parameters for light alkenes (ethylene, propylene and the three butylene isomers) have been calculated by adopting the detailed kinetic mechanisms of the University of California, San Diego, integrated with C4 reactions by the Lawrence Livermore National Laboratory mechanism. The new model has been adopted for the definition of flammability limits, adiabatic flame pressure and temperature, maximum rate of pressure rise, gas deflagration index (KG), auto-ignition temperature and minimum oxygen concentration. Flammability regions for nitrogen and carbon dioxide dilution have also been reported.

Browning-related oxygen depletion in an oligotrophic lake

ABSTRACTIn recent decades, terrestrial dissolved organic matter (DOM) has increased in many northeastern North American and European lakes and is contributing to long-term browning. We used a long-term dataset (1988–2014) to study the consequences of browning-related decreased water transparency on dissolved oxygen dynamics in 2 small temperate lakes in Pennsylvania, USA, that differ in their dissolved organic carbon concentrations. The oligotrophic (“clearer”) lake has low productivity and historically oxygenated deep waters. The mesotrophic–slightly dystrophic (“browner”) lake also has relatively low productivity but historically anoxic deep waters. We examined whether browning coincided with changes in summer dissolved oxygen dynamics, with a focus on deep-water oxygen depletion. In the clearer lake, we found that minimum oxygen concentrations decreased by ∼4.4 mg L−1 over the 27-year period, and these changes were strongly associated with both decreased water transparency and increased water column stability. We also found a shallowing of the maximum dissolved oxygen depth by ∼4.5 m and anoxic conditions established in more recent years. In the browner lake, the metrics we used did not detect any significant changes in dissolved oxygen, supporting the prediction that vertical temperature and oxygen patterns in clearer lakes may be more sensitive to increasing DOM than darker lakes. Anoxia is traditionally considered to be a consequence of anthropogenic nutrient loading and, more recently, a warming climate. We show that browning is another type of environmental change that may similarly result in anoxia in oligotrophic lakes.