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Hypoxia in coastal waters is a pressing ecological problem caused by continued eutrophication and climatic change that has widespread consequences for metazoan life and biogeochemical cycles. Numerous studies have investigated the controls on seasonal hypoxia formation and persistence in many of the world's large estuaries and coastal hypoxic zones, but far fewer studies have examined the controls on short-term oxygen variability that leads to diel-cycling hypoxia in shallow-water environments. We utilized a unique, comprehensive (181 stations) record of dissolved oxygen concentrations collected at shallow water sites (primarily < 2 m) at high frequency (15 min) throughout the estuarine complex of the Chesapeake Bay and its tributaries to quantify how internal and external variables co-varied with dissolved oxygen. We used a combination of time-series analysis, harmonic analysis, and machine learning (e.g., classification and regression trees (CART)) approaches to identify spatial patterns in major controls on oxygen variability and the duration of moderate hypoxia. We found that key controls on oxygen variability varied substantially over space. For example, photosynthetically active radiation (PAR) was a strong predictor of oxygen dynamics in the majority of mesohaline waters. In more fetch-exposed stations, wind strongly controlled hypoxic duration, but in eutrophic, inshore locations, chlorophyll a, or turbidity were often better predictors. Specifically, diel oxygen variability was muted in upstream regions characterized by high turbidity. The duration of low oxygen conditions, which we defined conservatively as less than 4.8 mg O2 L−1 (156 µM), was strongly controlled by temperature, and simple projections of regional warming and CART-derived oxygen thresholds suggest that the Bay could experience a 10% increase in this type of hypoxia duration by mid-to-late twenty-first century. The ratio of tidal to biological variability in oxygen was found to increase under conditions of higher turbidity, stronger wind, and lower salinity, but biological variability was typically a factor of two higher than tidal variability. Although chlorophyll-a generated high oxygen concentrations at some locations, those stations with exceptionally high chlorophyll a (> 30 µg L−1) were the most vulnerable to hypoxia. Because conventional water quality modeling frameworks are designed to capture hypoxia on relatively long time scales, these new insights can help inform updated oxygen models to support the management of shallow-water estuaries in the face of managed nutrient reductions and climate change. [ABSTRACT FROM AUTHOR]

Over 60 years of oceanographic observations from Ocean Station Papa (OSP) in the northeast Pacific indicate faster dissolved oxygen loss than the global average. The greatest negative trends in oxygen concentration occur on isopycnals in the upper water column (σθ = 26.1–26.8 kg m−3, ∼110–200 m) but have considerable uncertainty due to natural variability. In this paper, we use eight Argo profiling floats equipped with optode oxygen sensors to assess the 2008–2016 interannual variability of subsurface dissolved oxygen near OSP. We developed a method using high frequency Conductivity‐Temperature‐Depth data to correct optode profiles for slow response times and used reference profiles from the OSP time series to calibrate the optodes. Response time correction markedly improves subsurface bias caused by slow optode equilibration. Our analysis indicates that episodic shoaling of isopycnals can cause rapid reduction in dissolved oxygen concentration. Changes in ventilation, horizontal mixing, and water mass age are unlikely drivers for the rapid O2 loss events examined. We link dissolved oxygen loss during shoaling events to organic matter export, due to higher concentrations of organic matter and greater respiration rates at shallower depths. Reduced net community production during the "Blob" marine heatwave may have reduced the impact of the second shoaling event examined. Natural variations in dissolved oxygen in these layers provide context for uncertainty estimates of long‐term trends and insight toward the potential for future extreme oxygen minima from the combined impact of the long‐term decline and episodic shoaling. Plain Language Summary: Oxygen dissolved in the ocean is declining worldwide as temperatures increase. Widespread ecological stress and altered marine chemistry will be a likely result. The subarctic northeast Pacific Ocean has been losing oxygen even faster than the global average rate. In this study, we use data from ocean observing robots in this region to investigate factors causing episodes of faster oxygen depletion. To do this accurately, we developed and applied a method to correct the data for how slowly the oxygen sensor responds as it moves upward through the water column. We identify two periods of rapid oxygen loss in subsurface water layers of about 1.5 years each. Both episodes happen when regional wind patterns cause these water layers to move closer to the surface. When shallower, these layers experience faster oxygen consumption by respiration, because the supply of organic detritus from the surface is higher. Such oxygen depletion episodes, driven by periodic wind shifts, can be regarded as natural variations in the ocean's oxygen content and need to be considered when interpreting the potential for long‐term anthropogenic trends. Key Points: We observe two episodes of rapid oxygen loss in the upper pycnocline of the NE Pacific coincident with isopycnal shoalingWe propose enhanced respiration at shallower depths as the most likely driver of oxygen declines during these shoaling eventsHigh‐frequency Conductivity‐Temperature‐Depth data can be used to obtain float ascent velocities to correct oxygen optode lag [ABSTRACT FROM AUTHOR]

Dissolved oxygen (DO) is a vital water quality parameter with significant implications for aquatic life, serving as a key indicator of water pollution. The current research introduces two hybrid models utilizing Long Short-Term Memory (LSTM) networks, integrating the preprocessing methods of Discrete Wavelet Transform (DWT) and Complementary Empirical Mode Decomposition with Adaptive Noise (CEEMDAN). These models aim to efficiently model the DO process across five consecutive hydrologic stations situated along the Savannah River. Initially, a wavelet coherence (WTC) analysis was conducted to identify influential parameters for modeling, revealing that water temperature, discharge, mean water velocity, and turbidity exhibited the strongest correlations with dissolved oxygen. In single-gauge temporal modeling, the outcomes demonstrated that the T (IV) model, incorporating all the specified input parameters, achieved superior performance across all stations. Within all gauges, the fourth gauge exhibited the most favorable results using the hybrid CEEMDAN-LSTM method, attaining evaluation criteria of R = 0.991, DC = 0.976, and RMSE = 0.016. Furthermore, it was observed that the dissolved oxygen values from one day before the upstream gauge had a significant impact on predicting the dissolved oxygen concentration in the downstream gauge. In this strategy, the implementation of the CEEMDAN method results in a 35% increase in the modeling accuracy. [ABSTRACT FROM AUTHOR]

The purpose of aeration in water is to transfer or remove gases from the water. Dissolved oxygen is critical for living life. In water engineering, aeration and oxygen transfer using water jets are common. In this study, various venturi nozzles with air holes in the throat portion were manufactured and meaningful experiments were conducted to determine their oxygen transfer efficiency and aeration performance., the venturi nozzles with throat diameters 14, 17 and 20 mm were used in the experimental study. The ratio of the diameter of the air holes to the diameter of the throat portion is taken as 0.1, 0.2 and 0.3. Different experiments were carried out for nine different hole diameters. In addition, the ratio of the throat portion length to the throat portion diameter was taken as 1.00 and 0.50. For all nozzles, the ratio of outlet length to outlet diameter was taken as 1 and 2. In addition, comprehensive experiments were carried out for a venturi, considering the ratio of outlet length to outlet diameter as 1, 2, 3, 4 and 5. The experiments were also conducted for the circular nozzle to make comparisons. In this study, venturi nozzles gave better results than circular nozzles, and higher aeration performance values were obtained for the ratios 0.2 and 0.3, 1, 0.50 and 0.50. It was found that the aeration performance of the venturi nozzle was approximately 6.5 times higher than the air entrainment rate performance of the circular nozzle and 2.5 times higher in terms of oxygen transfer. [ABSTRACT FROM AUTHOR]

In water electrolysis experiments on hydrogen production, boiling effectively decreases the overpotential of the oxygen evolution reaction (OER) [Li et al. Int. J. Hydrog. Energy. 2022, 47(61):25499–510]. This study constructed a theoretical model to quantitatively explore the mechanism by which boiling reduces the OER overpotential. The model considers dissolved oxygen, liquid water, gaseous oxygen, and water vapor flowing through the porous transfer layer (PTL). The results showed that boiling increases the molar flux of gaseous oxygen and reduces the dissolved oxygen concentration. The PTL embedded in the catalyst layer (CL) increased the mass transfer resistance of dissolved oxygen and increased the dissolved oxygen concentration in the CL. Once boiling is superimposed on the OER, it attracts more dissolved oxygen to the water vapor bubbles and significantly decreases the dissolved oxygen concentration at the CL, thereby reducing the Nernst loss of the OER. • A dimensionless OER model with boiling is developed. • The model quantitatively clarifies the OER characteristics at boiling. • Boiling decreases dissolved oxygen concentration at the CL. • Current contribution ratio for the OER under bubbles rises at boiling. [ABSTRACT FROM AUTHOR]

All else equal, if the ocean's "biological [carbon] pump" strengthens, the dissolved oxygen (O2) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO2) concentrations of the ice ages. Vollmer et al. (2022, https://doi.org/10.1029/2021PA004339) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were "regenerated" (transported as sinking organic matter from the ocean surface to the interior) rather than "preformed" (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed 13C/12C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO2 storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange. Plain Language Summary: Recent studies have sealed the case that the concentration of oxygen was reduced in the deep ocean during the ice ages. Vollmer et al. (2022, https://doi.org/10.1029/2021PA004339) combine the oxygen results with data on deep ocean nutrient concentrations. They find that, relative to modern, more of the ice age deep ocean's nutrient reservoir arrived as sinking organic matter from surface waters, leading to more storage of carbon dioxide in the deep ocean. Moreover, they calculate that the entire observed drawdown in atmospheric carbon dioxide levels during ice ages can be explained by this strengthening of the ocean's "biological carbon pump." The findings bring the scientific community an important step closer to explaining why and how ice ages occur and end. The Southern Ocean, the ocean region around Antarctica, must have played a major role. However, it remains unclear whether the carbon dioxide was trapped in the Southern Ocean by changes in its circulation and biology or by limitations on air‐sea gas exchange across the Southern Ocean surface, such as might have occurred due to sea ice cover. Vollmer et al.'s reconstruction of the carbon isotopes in Southern Ocean surface waters may help to answer this question. Key Points: Proxies indicate less dissolved O2 in the ocean interior during ice ages than during interglacialsThis suggests an increase in CO2 storage by the ocean's biological pump adequate to explain the lower atmospheric CO2 of the ice agesReconstructed nutrient use implicates the Southern Ocean but does not yet resolve the roles of circulation, biology, and gas exchange [ABSTRACT FROM AUTHOR]

Hypoxic aquatic environments have increased in recent decades mainly by human activities that generate pollution. The objective of this work was to study the influence of the discharge of sewage effluents from Chascomús city on the concentration of dissolved oxygen (DO) in the Girado stream and its connection with Chascomús lake and analyze the impact on fish biodiversity. Four sites were selected to measure DO and temperature weekly for one year: discharge of sewage effluent, union of the sewage effluent with the Girado stream, Girado stream and Chascomús lake. Also, ichthyological samplings were carried out in a presumed hypoxic zone and a normoxic zone of the Girado stream. The results showed that there were two hypoxic zones corresponding to the sewage effluent and the union of the effluent with the Girado stream (~3 mg l−1). Also, two normoxic zones were identified, the course of the Girado stream and the union with Chascomús lake (~ 8 mg l−1). In these areas the water temperature varied according to the seasonality of the region (from 10 to 30 °C) without differences between zones. It is important to note that these areas received a constant flow of sewage effluent from Chascomús city (~ 885,600 l per day). In the fish sampling carried out in the 4 seasons of the year, 14 typical species of the Pampas lakes were captured with species more tolerant to low DO. Pejerrey were captured in a small number in both sites but with no signs of abnormalities in the gonads. [ABSTRACT FROM AUTHOR]

A 3D hydrodynamic model (EFDC) was applied to simulate dissolved oxygen (DO) and temperature (T) (two crucial parameters impacting water quality) throughout Lake St. Charles, a rural-urban shallow lake located North of Quebec City, Canada. Model outputs of T and DO corroborate observations at five monitoring stations within the lake. Simulated results indicated annual cycles of turnovers and stratifications and different behaviors for the deep and shallow basins. For the simulated years, the deep basin was stratified in summer and winter, while the shallow basin was mostly mixed throughout the year. The lake heat budget indicates that during summer with a long retention time, the thermal structure of the lake is principally controlled by net radiation, latent, and sensible heat fluxes. For the rest of the year, the inflow (from the main tributary, the Des Hurons River) and outflow are the main drivers of the lake's thermal structure. [ABSTRACT FROM AUTHOR]

Oceanic oxygen levels are decreasing significantly in response to global climate change; however, the microbial diversity and ecological functional responses to dissolved oxygen (DO) in the open ocean are largely unknown. Here, we present prokaryotic distribution coupled with physical and biogeochemical variables and DO gradients from the surface to near the bottom of a water column along an approximately 12,000-km transect from 13° N to 18° S in the Tropical Pacific Ocean. Nitrate (11.42%), temperature (10.90%), pH (10.91%), silicate (9.34%), phosphate (4.25%), chlorophyll a (3.66%), DO (3.50%), and salinity (3.48%) significantly explained the microbial community variations in the studied area. A distinct microbial community composition broadly corresponding to the water masses formed vertically. Additionally, distinct ecotypes of Thaumarchaeota and Nitrospinae belonging to diverse phylogenetic clades that coincided with specific vertical niches were observed. Moreover, the correlation analysis revealed large-scale natural feedback in which chlorophyll a (organic matter) promoted Thaumarchaeotal biomass at depths that subsequently coupled with Nitrospina, produced and replenished nitrate for phytoplankton productivity at the surface. Low DO also favored Thaumarchaeota growth and fueled nitrate production. [ABSTRACT FROM AUTHOR]

To investigate the factors affecting water quality in coastal regions with sea dike constructions, surface water outside a sea dike was monitored for six years from 2015 to 2020 in the Saemangeum region of Korea. Statistical analyses of the six years of high-frequency measurements revealed that the water quality in this system was predominantly governed by natural processes followed by pollutant inputs as the secondary influencing factor. Severe dissolved oxygen (DO) depletion was observed in the surface waters during warm periods, probably owing to the advection of DO-depleted water from elsewhere to the surface layer. Based on the apparent oxygen utilization (AOU)–pH relationship (r = 0.52, n = 1837), the maximum AOU (180 µM) led to a pH decrease from 8.04 to 7.50, which was considerably lower than the estimated value of 7.72. This extra pH drop was probably due to a reduction in the buffering capacity associated with increased CO2 in the water column originating from the atmosphere and in situ production, as well as local water column redox reactions associated with benthic inputs of reduced chemical species. Overall, persistent DO depletion with ongoing eutrophication/hypoxia could accelerate ocean acidification in Korean coastal waters, which could be more acute in coastal regions with artificial coastal constructions. [ABSTRACT FROM AUTHOR]

In this study, three-dimensional (3-D) ordinary kriging of dissolved oxygen (DO) concentrations was performed for a eutrophic reservoir based on 81 sampling points using Stanford Geostatistical Modeling Software (SGeMs). Potential hotspots (problematic zones in terms of water quality with high/low DO concentrations) not only at the surface but also in deeper layers of Porsuk Dam Reservoir (PDR) were evaluated. Moreover, 3-D distributions of DO, and specific conductivity (SC) were examined against the thermocline layer identified using the 3-D temperature data. Thermocline layer existed between 10 and 14 m below the surface based on 3-D temperature data. This result showed that the traditional approach of collecting samples from mid-depths may cause incomplete characterization and evaluation of water quality as thermocline layer may not coincide with mid-depth. Although the variation in SC values and temperatures above and below the thermocline layer were relatively homogeneous, this was not the case for DO. 3-D DO distribution suggested a better location for water withdrawal for domestic purposes. 3-D DO maps generated by predicting data at unmeasured locations at different depths could be used as input for 3-D water quality estimation in the reservoir through model simulations in future. Moreover, the outcomes can also be useful in the segmentation (physical configuration) of the water body for future water quality modeling studies. [ABSTRACT FROM AUTHOR]

Temperature and dissolved oxygen concentration are critical factors affecting the exchange of solutes between sediment and water; both factors will be affected by warming of lakes and thereby influence water quality. Temperature and oxygen responses of single solute fluxes are well known; however, not much is known about the interaction of temperature and oxygen in regulating the balance of different fluxes in the benthic environment. We analyzed benthic flux (mobilization and immobilization) data of various solutes (dissolved organic carbon (DOC), CH4, NO3−‐N, NH4+‐N, SRP, SO4−, Fe, Mn, and O2) collected from laboratory incubations of 142 sediment cores from 5 different reservoirs incubated under varying in situ temperature and oxygen conditions. Oxygen was the primary driver of benthic fluxes, while temperature and total organic content were secondary. Temperature effects on benthic fluxes were stronger under anoxic conditions which imply that warming will substantially increase the benthic fluxes if the sediment surface becomes anoxic. The varying temperature response of processes underlying the studied fluxes will result in a shift of their relative importance in the benthic environment, especially in shallow lakes that are more vulnerable to warming. For example, more anoxic conditions will shift the equilibrium between net sulfate reduction and methane release toward the latter. We also predict that physical effects of warming leading to hypolimnetic oxygen depletion, that is, stronger stratification and longer hypolimnetic confinement will increase the benthic mobilization of phosphorus, DOC, and methane into water and immobilization of sulfate by the sediments even in deep lakes. Plain Language Summary: Temperature and dissolved oxygen concentration control the release of undesirable components buried in lake or reservoir sediments, that is, nutrients, metals, and organic matter, which can cause water quality problems. We investigated the effects of rising temperature and levels of oxygen on the release of undesirable components by performing experiments using sediments and water from five different reservoirs. The sediments with a layer of water on top were incubated under different in situ temperature (low and high) and oxygen conditions (with and without). Our results show that the absence of oxygen was the main cause of the release of nutrients and metals. When there was no oxygen in the sediment and water, nutrients and metals were released from the sediment into the water and this effect increased when temperature was high. There is higher possibility that phosphorus, dissolved organic carbon, and methane will be released from sediments in some reservoirs as a result of global warming. Key Points: Solute fluxes from benthic lake sediments varied in response to temperature, with oxygen fluxes responding most stronglyTemperature effects on the magnitude of benthic fluxes were stronger under anoxic than oxic conditionsDirect temperature effects on reservoir water quality will be small compared to indirect effects through anoxia facilitation [ABSTRACT FROM AUTHOR]

Water quality impacts on water bodies such as reservoirs are strongly influenced by the hydrodynamics of the system. Although multiple models might be applied, they are limited by the simplification of the variables. In this study, a two-dimensional public domain model, CE-QUAL-W2, was adapted to test whether it would generate an accurate hydrodynamic simulation of the URRÁ Reservoir in Córdoba, Colombia, to understand water quality. The variables to be modeled were temperature and dissolved oxygen due to their importance in ecological terms. Thus, trial and error techniques were used to calibrate and validate the model, varying different parameters such as the wind shelter coefficient (WSC). Although the model accurately predicted the hydrodynamic part by having daily flow information, significant modifications to the eddy diffusivity coefficient were required to simulate acceptable longitudinal currents. This research shows that the CE-QUAL-W2 model fits adequately to tropical lentic systems. However, it is recommended that, for future studies, the modeling be adjusted using hourly data, especially in areas where inflow and boundary conditions are unstable. [ABSTRACT FROM AUTHOR]

Dissolved oxygen content is an important factor affecting eel culture. To improve the prediction accuracy of dissolved oxygen concentration in eel ponds, this paper proposes a model for predicting dissolved oxygen concentration in eel ponds based on the sparrow search algorithm (SSA) and long short-term memory neural network ( LSTM), after optimizing the hyperparameters of LSTM model using SSA algorithm, the dissolved oxygen in circulating water eel culture ponds was predicted. The results showed that the prediction accuracy of SSA-LSTM model was 96. 77%, which was 2. 09%, 3. 34% and 0. 55% higher than the control models LSTM, GRU and PSO-LSTM, respectively. The other indicators of this model, EMSE, EMAE and ERMSE, were 0. 67, 0. 53, and 0. 81, respectively, which also showed significant decreases compared with the control model. The results indicate that the SSA-LSTM model has good accuracy and robustness in predicting dissolved oxygen concentration in eel ponds, which can provide a basis for accurate regulation of water quality parameters in eel culture. [ABSTRACT FROM AUTHOR]

Hypolimnetic oxygen depletion during summer stratification in lakes can lead to hypoxic and anoxic conditions. Hypolimnetic anoxia is a water quality issue with many consequences, including reduced habitat for cold-water fish species, reduced quality of drinking water, and increased nutrient and organic carbon (OC) release from sediments. Both allochthonous and autochthonous OC loads contribute to oxygen depletion by providing substrate for microbial respiration; however, their relative importance in depleting oxygen across diverse lake systems remains uncertain. Lake characteristics, such as trophic state, hydrology, and morphometry are also influential in carbon cycling processes and may impact oxygen depletion dynamics. To investigate the effects of carbon cycling on hypolimnetic oxygen depletion, we used a two-layer process-based lake model to simulate daily metabolism dynamics for six Wisconsin lakes over twenty years (1995–2014). Physical processes and internal metabolic processes were included in the model and were used to predict dissolved oxygen (DO), particulate OC (POC), and dissolved OC (DOC). In our study of oligotrophic, mesotrophic, and eutrophic lakes, we found autochthony to be far more important than allochthony to hypolimnetic oxygen depletion. Autochthonous POC respiration in the water column contributed the most towards hypolimnetic oxygen depletion in the eutrophic study lakes. POC water column respiration and sediment respiration had similar contributions in the mesotrophic and oligotrophic study lakes. Differences in source of respiration are discussed with consideration of lake productivity, hydrology, and morphometry. [ABSTRACT FROM AUTHOR]

One of the essential issues in the quality management of water resources is the determination of the level of eutrophication and dissolved oxygen in dam reservoirs and other water bodies. It is possible to study changes in dissolved oxygen concentration using qualitative models. In this study, CE-QUAL-W2, a two-dimensional model of hydrodynamics and water quality capable of predicting the behavior of aquatic ecosystems, has been used to simulate dissolved oxygen concentration and eutrophication. As a case study, the water quality of the Yamchi dam reservoir, located in Ardabil Province, has been qualitatively investigated using the CE-QUAL-W2 model. Based on the geographical location of the Yamchi dam reservoir, it includes warm monomictic basins. According to the results, the dissolved oxygen concentration in the simulation period varied between 0 and 12 mg/L. In autumn and winter, this concentration was almost uniform at various depths. However, in summer, a sharp decrease in dissolved oxygen concentration, to less than 2 mg/L in the bottom layers, was observed, which is due to the presence of nutrients. Regarding eutrophication, the Yamchi dam reservoir is one of the eutrophic reservoirs, and practical strategies should be employed to improve these conditions. Recommended strategies include reducing the retention time of water in the reservoir, dewatering from the underlying layers during the thermal stratification period, and preventing the entry of agricultural drains into the inlet tributaries of the reservoir. [ABSTRACT FROM AUTHOR]

Dissolved oxygen depletion in the global ocean is well documented over several decades from the surface ocean to abyssal depths. This decline is especially prevalent in the Northeast Pacific. A significant decline in dissolved oxygen has been measured over 30 years at 4,000–4,100 m depth (Station M) beneath the California Current off central California. Three principal hypotheses examined the relationship of declining oxygen with biological and physical factors over the 30‐year time series. Annual resolution revealed Ekman pumping, coastal upwelling, particulate matter flux, and sediment community oxygen consumption having significant correlations with bottom water dissolved oxygen concentration. Coastal upwelling accounted for 65% of the annual variation in bottom water oxygen concentration. Stepwise regression yielded descriptive models of bottom water dissolved oxygen using coastal upwelling, wind stress and primary production variables. Is continued oxygen depletion in the Northeast Pacific indicative of abyssal regions in the world ocean? Plain Language Summary: The dissolved oxygen content of the global ocean has been declining over many decades. This decrease is well documented in the upper ocean particularly in the Northeast Pacific Ocean. To examine the change in dissolved oxygen at greater depths, we measured the bottom water oxygen concentration at 4,000–4,100 m depth at a single station in the Northeast Pacific over a 30‐year period from 1989 through 2019. We recorded a significant decline in dissolved oxygen at this abyssal station over three decades. We then examined possible biological and physical causes of this declining dissolved oxygen. Surface ocean Ekman pumping, coastal upwelling, particulate matter flux, and sediment community oxygen consumption on the seafloor revealed significant relationships with bottom water dissolved oxygen. Coastal upwelling overlying this station accounted for 65% of the annual variation in bottom water oxygen concentration. The question we are now poised to address is whether there is a similar long‐term decline in dissolved oxygen concentration at abyssal depths in other regions of the world ocean. Key Points: Oxygen concentration has significantly declined in abyssal waters of the NE Pacific over 30 yearsOxygen concentration in the NE Pacific correlates with physical and biological conditions from the surface ocean to the seafloor [ABSTRACT FROM AUTHOR]

Deviations of surface ocean dissolved oxygen (O 2) from equilibrium with the atmosphere should be rectified about twenty times more quickly than deviations of dissolved carbon dioxide (CO 2). Therefore, persistent O 2 disequilibria in the Labrador Sea, while CO 2 is close to equilibrium, has been a matter of interest to many previous works. Here we investigate this phenomenon by using a novel analytical technique, the 'CORS (Carbon Dioxide and Oxygen Relative to Saturation) method', and also by using more data than was available previously. We compare observations to results from a model we developed for the Labrador Sea which combines plankton ecology with biogeochemical cycling of oxygen, carbon and nitrogen. In contrast to earlier works which mostly considered individual factors in isolation, here we used the model, together with data, to distinguish between the varying influences of several processes potentially contributing to the long-lasting O 2 undersaturation: mixed layer depth, duration of mixed layer deepening, convection, entrainment and bottom water O 2 content. Our model experiments confirm that, for the same gas exchange rate, the effects on surface O 2 concentration differ significantly among the identified drivers. Our results suggest that prolonged surface O 2 undersaturation is not always dependent on the extreme winter mixed layer depths, but rather that even moderately deep mixed layers (e.g. 300 m), when prolonged and in conjunction with continuous entrainment of oxygen-depleted deep water, can also drive persistent surface O 2 anomalies. An implication of our results is that regions in the North Atlantic with maximum winter mixed layer depths of only a few hundred metres should also show persistent surface O 2 undersaturation. We further reveal that convection in deep water formation regions produces trendlines that do not pass through the origin of a plot of CO 2 vs. O 2 deviations which have previously been thought to indicate erroneous data. • Extreme winter mixed layer depths in the Labrador Sea drives its surface O 2 anomaly because of continuous entrainment of O 2 -poor deep waters. • Moderately shallow maximum winter mixing depths (e.g. 300 m) can also drive long-duration surface O 2 undersaturation. • Questionable autonomous data previously assumed to be erroneous may in fact be real. [ABSTRACT FROM AUTHOR]

The Southern Ocean (SO) plays a critical role in global ocean productivity and carbon cycling. Bio-Argo floats deployed in the Indian sector of the Southern Ocean provides new insights into the biogeochemical processes. Here we report significantly higher dissolved oxygen(DO) (~ 310 μmol/kg) in summer of 2014–2015 for one float (F1) and winter of 2014 in other float (F2) at sub-surface layer in the subantarctic region of the SO. The summer DO peak in F1 was 10% higher than those during the summer of succeeding year, while the winter DO peak in F2 was 20% higher than those during the winter of succeeding year. Temperature and dynamic height structure show that cyclonic eddies play an important role in the observed increase in the dissolved oxygen: the high DO is a manifestation of the co-occurrence of a cold core eddy which transported the cold oxygen rich water from deep to the surface during winter, while, during summer, the high chlorophyll below the mixed layer depth (MLD) also contributed to the elevated DO. Low apparent oxygen utilisation suggests that the observed high oxygen concentration was due to high production rates over the consumption. [ABSTRACT FROM AUTHOR]

Over a decade ago, oceanographers began installing oxygen sensors on Argo floats to be deployed throughout the world ocean with the express objective of better constraining trends and variability in the ocean's inventory of oxygen. Until now, measurements from these Argo-mounted oxygen sensors have been mainly used for localized process studies on air-sea oxygen exchange, biological pump efficiency, upper ocean primary production, and oxygen minimum zone dynamics. Here we present a four-dimensional gridded product of ocean interior oxygen, derived via machine learning algorithms trained on dissolved oxygen observations from Argo-mounted sensors and discrete measurements from ship-based surveys, and applied to temperature and salinity fields constructed from the global Argo array. The data product is called GOBAI-O2 for Gridded Ocean Biogeochemistry from Artificial Intelligence - Oxygen (Sharp et al., 2022; https://doi.org/10.25921/z72m-yz67; last access: 30 Aug. 2022); it covers 86% of the global ocean area on a 1° latitude by 1° longitude grid, spans the years 2004-2021 with monthly resolution, and extends from the ocean surface to two kilometers in depth on 58 levels. Two machine learning algorithms -- random forest regressions and feed-forward neural networks -- are used in the development of GOBAI-O2, and the performance of those algorithms is assessed using real observations and Earth system model output. GOBAI-O2 is evaluated through comparisons to the World Ocean Atlas and to direct observations from large-scale hydrographic research cruises. Finally, potential uses for GOBAI-O2 are demonstrated by presenting average oxygen fields on isobaric and isopycnal surfaces, average oxygen fields across vertical-meridional sections, climatological cycles of oxygen averaged over different pressure intervals, and a globally integrated oxygen inventory time series. [ABSTRACT FROM AUTHOR]

[Display omitted] Herein, a novel metal-free catalyst consisting of multiporous oxygen-rich carbon–nitrogen graphene-like nanosheets (O LAA -CN NSs) is first developed through a staged temperature-programmed calcination of l -ascorbic acid (LAA)-modified dicyandiamide precursor. It is found that the oxygen species from l -ascorbic acid (O LAA) are introduced into the graphene-like basic matrix and replace partial N atoms to form the C O C-R structure, leading to the non-uniform distribution of electrons on the catalyst surface, and the formation of electron-rich centers around the C O C microareas according to a series of characterization techniques. As a result, O LAA -CN NSs exhibits excellent performance for refractory pollutant removal in the presence of peroxymonosulfate (PMS) and dissolved oxygen. Some pollutants with complex structures are even completely degraded within only 1 min. The interface reaction mechanism is further revealed that PMS mainly acts as an active inducer to drive the electron donation of pollutants over O LAA -CN NSs. These electrons are finally utilized by dissolved oxygen to generate reactive oxygen species (ROS) through the interface process. This reaction system results in pollutants that can either be cleaved directly by surface oxidation process or degraded by the attack of the generated ROS, such as singlet oxygen (1O 2) and superoxide radicals (O 2 •−), through oxygen activation, which significantly reduces the resource and energy consumption in advanced wastewater treatment by harnessing the energy of pollutants and dissolved oxygen in the water. [ABSTRACT FROM AUTHOR]

This review discusses the relationship between dissolved oxygen and Legionella growth. Growth of Legionella in building water systems is considered to pose a health risk and controlling it could benefit public health by reducing the number of healthcare and community acquired Legionnaires' disease cases. Conditions known to encourage Legionella growth include low disinfectant concentrations, warm temperature, and water stagnation. Legionella is an obligate aerobe meaning its cellular respiration is inhibited under anaerobic conditions. Despite evidence from earlier published studies the importance of dissolved oxygen for the survival and growth of Legionella has received little attention from researchers and public health authorities. Modern measurement devices can reliably determine if dissolved oxygen concentrations in potable water and other building water systems are at levels capable of supporting Legionella growth or inhibiting its amplification. Removing dissolved oxygen from water can be achieved using gas transfer membrane contactors. Water with low dissolved oxygen levels interferes with Legionella's cellular respiration by oxygen starvation, whereas disinfectants achieve the same effect by interfering with oxygen transport across the cell membrane. Ultimately, the same effect on Legionella growth may be achieved by reducing dissolved oxygen as treatment with oxidants, while avoiding the drawbacks of corrosion and disinfectant byproducts. [ABSTRACT FROM AUTHOR]

The concentration of dissolved oxygen is an important indicator of water quality because aquatic life lives on the dissolved oxygen in the water. This can be achieved by using hydraulic structures because of substantial air bubble entrainment at these structures. Closed conduit aeration is a particular instance of this. In the present study, experiments were carried out to investigate the role of cross‐sectional geometry of high‐head gated conduit in oxygen transfer efficiency. In the first part of this study, the role of cross‐sectional geometry of high‐head gated conduit in air‐demand ratio was investigated. In the second part, the role of cross‐sectional geometry of high‐head gated conduit in oxygen transfer efficiency was investigated. Results pointed that the cross‐sectional geometry of the high‐head gated conduit had an important effect on the air‐demand ratio and the oxygen transfer efficiency. Moreover, a design formula was obtained for the conduit having the highest oxygen transfer efficiency relating the oxygen transfer efficiency to air‐demand ratio, Froude number, gate opening, conduit length and gate opening ratio. There was good agreement between the measured and the predicted values. The obtained results will be useful in future modelling processes and aid the practicing engineer in predicting oxygen transfer efficiency of high‐head gated conduits. [ABSTRACT FROM AUTHOR]

The decline of endangered freshwater pearl mussel (FPM, Margaritifera margaritifera) has been attributed to juvenile mortality caused by low concentrations of dissolved oxygen in the stream substrate resulting from fine sediments (siltation) that impede water exchange in the interstitial microhabitat of juveniles.If low oxygen concentration causes recruitment failure of FPMs, knowledge on the oxygen tolerance of juvenile FPMs is essential for the conservation of the species, as it will justify conservation efforts improving water exchange in the bottom gravel. However, the tolerance of low oxygen of FPM juveniles has not been directly studied.Juvenile FPMs (9–11 months old) were exposed in individual chambers equipped with optical oxygen measurement spots to different levels of dissolved oxygen at 19 °C and their viability was monitored for 10 days to assess the acute oxygen tolerance of juvenile FPMs. Oxygen concentration ranged between 8.8 and 6.2 mg L−1 in the high oxygen treatment (control), 5.0–0.4 mg L−1 in the medium treatment, and 1.3–0.04 mg L−1 in the low oxygen treatment (near‐anoxic conditions).Viability of juvenile FPMs depended on the concentration of available dissolved oxygen, such that all juveniles exposed to near‐anoxic conditions were classified as non‐viable, whereas all mussels exposed to high and medium concentrations were viable at the end of the 10 day experiment. Juveniles differed in their ability to tolerate near‐anoxic conditions, so that some individuals survived only 1 day and others survived up to 9 days.This study provides the first direct experimental evidence on the oxygen sensitivity of FPM juveniles and suggests that >10‐day events of very low dissolved oxygen at summer temperatures are fatal to juvenile FPMs, supporting the view that actions preventing low oxygen episodes in the substrate are essential for recruitment, and conservation, of FPMs. [ABSTRACT FROM AUTHOR]

Most amphibians from the temperate climate zones hibernate during the winter, and although incidental observations of winter activity are not rare, systematic data are still lacking. The study was conducted throughout the astronomical winter (December 2020 -- March 2021) in a pond near Sofia, Bulgaria. Funnel traps were set each week for the duration of the study and the ventral side of all captured newts was photographed to allow for individual identification. The following parameters were measured: dissolved oxygen in the water, air and water temperature. Results demonstrated that both male and female newts were active in the water throughout the winter period, even when the surface of the pond was frozen solid (at times, the ice was thick enough for a human to safely walk on the pond). As early as the beginning of January, males had very prominently developed crests. A total of 153 individual newts were captured, with 79 (51.63 %) of them recaptured at least once. Recaptures were used to calculate the maximum active population size, which was 142±9 (125-159) for males and 94±13 (69-118) for females. The number of captured newts each session correlated very closely with estimated active population size but there was no apparent correlation with the values of the measured parameters. [ABSTRACT FROM AUTHOR]

Aim: Temporal changes in cerebral regional oxygen saturation (crSO2) are useful for predicting return of spontaneous circulation (ROSC) in out-of-hospital cardiac arrest (OHCA) patients. However, little is known regarding the usefulness of peripheral regional oxygen saturation (prSO2) associated with OHCA patient outcomes. This study evaluated the association between temporal changes in prSO2 and ROSC in patients with OHCA.Methods: This was a prospective study at two tertiary emergency centres in Japan. We evaluated the relationship between ROSC and temporal changes in crSO2 and prSO2. The rSO2 sensor was attached to the patient's forehead and upper arm, and rSO2 was continuously measured until resuscitative efforts were terminated or until the patient with sustained ROSC (>20 min) arrived at the emergency department.Results: We included 145 patients with OHCA, of whom 35 achieved ROSC. Witness status (odds ratio [95% confidence interval]: 11.6 [3.13-58.1]) and ΔprSO2 (1.13 [1.06-1.24]) were significantly associated with ROSC in multiple logistic analysis. In the ROSC group, ΔprSO2 increased earlier than ΔcrSO2 during cardiopulmonary resuscitation. In the non-ROSC group, there was no significant difference between ΔcrSO2 and ΔprSO2, and neither increased before termination of resuscitation (TOR).Conclusion: We demonstrated for the first time that prSO2 is associated with ROSC in OHCA patients and showed that temporal changes in prSO2 could predict ROSC earlier than those in crSO2. Our findings could provide time to prepare early interventions after ROSC and assist in determining the TOR for OHCA patients in Japan. Further studies are needed to validate these findings. [ABSTRACT FROM AUTHOR]

A subsurface oxygen minimum layer (OML) in intermediate water is identified and characterized in Rivers Inlet, a fjord on British Columbia's central coast, using data from 1998 to 2018. The OML was observed in most years from May to September and was most persistent at the middle and head of the fjord. The Rivers Inlet OML develops in three stages: (i) in early spring, the cessation of winter storms from downwelling-favourable winds stops the ventilation of the water column; (ii) throughout spring and summer, the remineralization of organic matter, likely primarily phytoplankton, consumes oxygen in the intermediate waters; (iii) in late spring, deep-water renewal by oxygenated offshore water forms the base of the OML inside the inlet. The strength and persistence of the OML vary interannually, mainly due to variability in hemispheric-scale winds and primary production. In some years, the OML was hypoxic, which could influence the local marine ecosystem. Changes to downwelling, upwelling, or primary production in Rivers Inlet could affect the OML in the future. [ABSTRACT FROM AUTHOR]

The oxidation experiments of austenitic steel TP347HFG were carried out at 620°C under 25 MPa in supercritical water with the dissolved oxygen content of 0–2000 ppb. The oxidation rate of TP347HFG increased with the rise in dissolved oxygen content. A double-layered oxide scale composed of a Fe-rich outer layer and a Cr-rich inner layer developed on the TP347HFG steel surface. Haematite was detected in supercritical water with 2000 ppb dissolved oxygen while absent in deaerated supercritical water. The content of dissolved oxygen has a great impact on the formation and distribution of Cr2O3. A thicker and continuous Cr2O3 were observed in the grain boundary near the interface of oxide and the substrate in deaerated supercritical water. The Cr evaporation may be responsible for the oxidation resistance of TP347HFG steel in supercritical water with different dissolved oxygen content. The influence mechanisms of dissolved oxygen on oxidation rate and oxides composition were discussed in this paper. [ABSTRACT FROM AUTHOR]

Dissolved oxygen (DO) in Lam Takhong River gradually reaches zero value during the dry season on several occasions in the past decade causing the unsuitable quality for use as the raw water for Nakhon Ratchasima Town. Discharges of point sources and diffuse sources containing pollutants with organics and nutrients are the major cause of water quality deterioration in the river. To find the sources of impact on the water quality in the river, a one-dimensional steady-flow systems river water quality model, QUAL2Kw, was constructed and simulated. The model was calibrated and validated using the water quality data from 2008 to 2017 for the Lam Takhong River by seven monitoring stations. The modelling was applied to simulate various water quality parameters during the critical period to compare to the designated surface water quality criteria third class in Thailand (minimum dissolved oxygen at or above 4 mg/L; maximum biochemical oxygen demand (BOD), nitrate-nitrogen, and ammonia-nitrogen at or below 2.0, 5.0 and 0.5 mg/L, respectively). The study reach of the river flows 122 km from Lam Takhong Dam to the Mun River at Chaloem Phra Kiat district through the urban central area. Several segments of the river have been alarmed for many constituents with the dissolved oxygen impairment is the focus of the study. The scenarios of loads and upstream dissolved oxygen modification were conducted to assess the change of dissolved oxygen concentration. The result of the QUAL2Kw model showed that the decomposition of organic matter and a poor reaeration were the primary cause of the impairment. The local oxygenation causes fluctuations in dissolved oxygen levels along the river and the dissolved oxygen concentration decreases downstream of the river with some values fell the meet the fourth class of surface water quality criteria in Thailand (DO above 2 mg/L and BOD5<4 mg/L). The QUAL2Kw model is suitable for simulating the current and future river water quality and help water resources managers to issue the appropriate policy options for the Lam Takhong River. [ABSTRACT FROM AUTHOR]

Ocean deoxygenation threatens ocean productivity, carbon cycling and marine ecosystems. Shelf seas are highly dynamic regions, which contributes to their high productivity and also makes monitoring and constraining their oxygen status a challenge. Here, using the temperate Celtic shelf sea (April and July 2015) as a case study, we present high‐resolution ocean glider observations of turbulence and biogeochemical parameters, demonstrating the potential of these autonomous platforms for environmental monitoring. We estimate vertical turbulent oxygen fluxes be 25% higher in summer than in spring, due to the presence of subsurface chlorophyll and associated oxygen maxima at the seasonal thermocline. We demonstrate that glider‐based estimates were able to constrain similar bottom layer respiration rates as those derived from traditional ship‐based measurements. We suggest ocean gliders are useful monitoring tools that can aid sustainable management of shelf sea ecosystems. Plain Language Summary: Oxygen levels in the ocean are decreasing. Oxygen is needed by almost all life in the oceans, thus low oxygen levels can result in dramatic changes to marine ecosystems. The decrease in oxygen levels is particularly alarming in the coastal ocean or "shelf sea" (the region between the land and the deep open ocean), which supports the majority of global fisheries (over 90%). Therefore, there is both an urgent societal and an environmental need to better understand processes influencing oxygen levels in the coastal ocean, such as physical water circulation and mixing, and biological oxygen production and consumption. Here we present turbulent mixing data collected over 40 days in a typical shelf sea using an unmanned, autonomous underwater vehicle (AUV) called an ocean glider. We use this data combined with oxygen data to calculate the contribution of physical oxygen fluxes to the observed change in oxygen, and from this deduce how much of the change was driven by biology. We prove that AUVs may be used as an effective method for monitoring oxygen dynamics and that this can aid responsible marine management in shelf seas. Key Points: Glider observations are able to integrate physical and biogeochemical processes influencing oxygen depletion in shelf seasMixing and advection replenish respired oxygen in the bottom mixed layerThe presence of a subsurface oxygen maxima results in larger oxygen fluxes to the bottom mixed layer in summer compared to spring [ABSTRACT FROM AUTHOR]

The Labrador Sea in the North Atlantic Ocean is one of the few regions globally where oxygen from the atmosphere can reach the deep ocean directly. This is the result of wintertime deep convection, which homogenizes the water column to a depth of up to 2000 m and brings deep water undersaturated in oxygen into contact with the atmosphere. In this study, we analyze how the intense oxygen uptake during Labrador Sea Water (LSW) formation affects the properties of the outflowing deep western boundary current, which ultimately feeds the upper part of the North Atlantic Deep Water layer in much of the Atlantic Ocean. Seasonal cycles of oxygen concentration, temperature, and salinity from a 2-year time series collected by sensors moored at 600 m nominal depth in the outflowing boundary current at 53 ∘ N show a cooling, freshening, and increase in oxygen content of the water flowing out of the basin between March and August. Analysis of Argo float data suggests that this is preceded by an increased input of LSW into the boundary current about 1 month earlier. This input is the result of newly ventilated LSW entering from the interior, as well as LSW formed directly within the boundary current. Together, these results imply that the southward export of newly formed LSW primarily occurs in the months following the onset of deep convection, from March to August, and that this direct LSW export route controls the seasonal oxygen increase in the outflow at 600 m depth. During the rest of the year, properties of the boundary current measured at 53 ∘ N resemble those of Irminger Water, which enters the basin with the boundary current from the Irminger Sea. The input of newly ventilated LSW increases the oxygen concentration from 298 µmolL-1 in January to a maximum of 306 µmolL-1 in April. As a result of this LSW input, an estimated (1.60 ± 0.42) × 1012 molyr-1 of oxygen are added to the outflowing boundary current, mostly during spring and summer, equivalent to 50 % of the wintertime uptake from the atmosphere in the interior of the basin. The export of oxygen from the subpolar gyre associated with this direct southward pathway of LSW is estimated to supply 42 %–71 % of the oxygen consumed annually in the upper North Atlantic Deep Water layer in the Atlantic Ocean between the Equator and 50 ∘ N. Our results show that the formation of LSW is important for replenishing oxygen to the deep oceans, meaning that possible changes in its formation rate and ventilation due to climate change could have wide-reaching impacts on marine life. [ABSTRACT FROM AUTHOR]

Predicting changes in reservoir conditions from climatic warming is hindered by the paucity of long-term data on temperature and thermal and oxygen structure replicated across a range of reservoir sizes. The present study characterizes seasonal patterns in temperature, thermal structure, and dissolved oxygen availability in reservoirs, and evaluates how critical periods for aquatic organisms (i.e., periods of maximum temperature and minimum oxygen) for these features are affected by atmospheric temperature anomalies at different time lags. Temperature and dissolved oxygen were measured from May through October at 1 m intervals from surface to bottom for 10 reservoirs sampled between 14 and 21 years during 1995–2016. For most temperature and oxygen metrics July was the period of thermal maxima and oxygen minima across reservoirs, exceptions were thermocline depth, which was static from May through October, and bottom water temperature which peaked in August. Surface water temperature of reservoirs increased 0.6 °C for every 1.0 °C increase in atmospheric temperature in July independent of reservoir area. Although the percentage of water that was oxic in July decreased with increasing July air temperatures, it increased with warming air temperatures from February to April. These long-term data derived patterns highlight the importance of reservoir size and lag periods in building a framework for predicting climate-induced changes in the thermal and oxygen environments of reservoirs, which have important implications for water quality and ecosystem processes and the associated dynamics of reservoir flora and fauna. [ABSTRACT FROM AUTHOR]

Laboratory incubation of intact sediments is a popular and useful method for obtaining nutrient fluxes from lake sediments to the overlying water. However, one of the main problems facing this method is that the lake sediment core itself consumes oxygen from the overlying water due to the decomposition of organic matter. In this study, addition of headspace with controlled oxygen concentration and circulation of overlying water to the traditional intact sediment incubation method allows for incubation with a stable dissolved oxygen (DO) concentration in the overlying water. In 24 columns incubated for 179–269 h, the mean DO concentration in the overlying water ranged from 0.35 to 9.95 mgO2 L−1. In most cases, stability was high, with the standard deviation of the fluctuation in DO concentration less than 0.1 mgO2 L−1. The DO concentration of the overlying water could be adjusted by changing the partial pressure of oxygen in the headspace. This method supported a detailed description of the relationship between DO concentration and nutrient fluxes from sediments. The soluble reactive phosphorus fluxes and first‐order reaction constants of NO2− + NO3− fluxes increased rapidly when the DO concentration decreased below the thresholds of 1.5 and 3.0 mgO2 L−1, respectively, in Lake Biwa. The thresholds and precise relationships between DO concentration and nutrient fluxes provide useful information for predicting the effect of DO concentration decreases in the lake bottom on nutrient release from sediments. Our results indicate that this incubation method is a powerful tool for clarifying those relationships. [ABSTRACT FROM AUTHOR]

Long-term trends of bottom-water concentrations of dissolved oxygen (DO), total nitrogen, and total phosphorus at 42 sites in Tokyo Bay were analyzed using monthly data from 1983 to 2018 measured by local government agencies. The results were visualized as spatial distribution maps. Bottom-water DO concentrations varied seasonally. The area of hypoxic bottom water (DO < 2 mg L−1) was greatest in September of every year. Examination of interannual variations of the spatial distribution of bottom-water DO concentrations in September revealed that along the eastern side of the bay, hypoxic zones have been shrinking since the mid-2000s, but along the western side of the bay, the bottom water closest to shore is hypoxic in September even today. The areal percentage of hypoxic bottom water during September within inner Tokyo Bay has declined from 30% around 2007 to < 15% around 2015 and has not changed since then. A comparison of the spatial distributions of the frequencies of occurrence of red tides (chlorophyll-a concentrations ≥ 50 μg L−1) and hypoxic water masses during 8-year intervals revealed that areas with high percentages of red tides coincided with areas with high percentages of hypoxic bottom water. The implication is that the reduction of the size of the hypoxic zones along the eastern side of the bay was due to less frequent occurrence of red tides there. Reducing the inputs of nutrients to the bay as a result of policies aimed at reducing water pollution has, therefore, contributed to the mitigation of the hypoxia problem. [ABSTRACT FROM AUTHOR]

Ocean deoxygenation is an emerging hazard for marine ecosystems and a fingerprint of anthropogenic change. Interior ocean oxygen concentrations respond to changes in ventilation that supply oxygen and the demand of biogeochemical processes that consume oxygen. A better understanding of their regional importance would improve confidence in Earth System Model projections, which underpin ecosystem risk and vulnerability assessments. Using a hindcast reanalysis simulation, we find that oxygen trends between 1975 and 2014 in low‐oxygen zones along eastern margins are strongly affected by biogeochemical processes that alter oxygen demand, while oxygen‐rich regions of the open ocean are driven by ventilation changes. A similar regional distinction emerges among CMIP6 Earth System Models. Therefore, while biogeochemical functioning is an important source of uncertainty in low‐latitude, low‐oxygen regions, uncertainty in global trends is due to insufficient physical supply changes in Earth System Models, which is confirmed using repeat hydrographic section data in the southern mid‐latitudes. Plain Language Summary: Measurements of dissolved oxygen (O2) in the ocean since the 1960s have revealed a multidecadal decrease in ocean O2 levels that is a major concern. It represents a fingerprint of human‐induced climate change and is also an ecological hazard in the highly productive upwelling systems of the lower latitudes that host present‐day low‐O2 zones. Unfortunately, Earth System Models that are used to predict future changes fail to reproduce the magnitude of recent deoxygenation and are inconsistent in low‐O2 regions. This undermines our confidence in both reconstructions and future projections that form the basis of decision making. In this work, we use a suite of model simulations and observations over the 1975–2014 period to demonstrate that the biogeochemical response is important in the low‐O2 waters of the tropics and highly productive eastern margins, while physical supply is dominant elsewhere. Reducing our uncertainty in the low‐O2 regions will therefore rely partly on constraining the contribution of biogeochemical processes, while large‐scale trends are dependent on how models represent physical processes, which are chronically underestimated by models. Key Points: Observed and modeled mesopelagic oxygen trends are driven (85%) by apparent oxygen utilization that integrates oxygen supply and demandBiogeochemical processes shaping oxygen demand are important along eastern margins and some tropical areasLarge‐scale oxygen trends are driven by changing supply, and weak model trends are linked to misrepresented physical processes [ABSTRACT FROM AUTHOR]

Acidification can present a stress on organisms and habitats in estuaries in addition to hypoxia. Although oxygen and pH decreases are generally coupled due to aerobic respiration, pH dynamics may be more complex given the multiple modes of buffering in the carbonate system. We studied the seasonal cycle of dissolved oxygen (DO), pH, dissolved inorganic carbon, total alkalinity, and calcium ion (Ca2+) along the main channel of Chesapeake Bay from May to October in 2016. Contrary to the expected co‐occurrence of seasonal DO and pH declines in subsurface water, we found that the pH decline ended in June while the DO decline continued until August in mid‐Chesapeake Bay. We discovered that aerobic respiration was strong from May to August, but carbonate dissolution was minor in May and June and became substantial in August, which buffered further pH declines and caused the seasonal DO and pH minima mismatch. The rate of calcium carbonate (CaCO3) dissolution was not primarily controlled by the saturation state in bottom water, but was instead likely controlled by the supply of CaCO3 particles. The seasonal variability of Ca2+ addition in the mid‐bay was connected to Ca2+ removal in the upper bay, and the timing of high carbonate dissolution coincided with peak seasonal biomass of upper Bay submerged aquatic vegetation. This study suggests a mechanism for a novel decoupling of DO and pH in estuarine waters associated with CaCO3, but future studies are needed to fully investigate the seasonality of physical transport and cycling of CaCO3. [ABSTRACT FROM AUTHOR]

In the subsurface ocean, O2 depleted because of organic matter remineralization is generally estimated based on apparent oxygen utilization (AOU). However, AOU is an imperfect measure of oxygen utilization because of O2 air‐sea disequilibrium at the site of deepwater formation. Recent methodological and instrumental advances have paved the way to further deconvolve the processes driving the O2 signature. Using numerical model simulations of the global ocean, we show that the measurements of the dissolved O2/Ar ratio, which so far have been confined to the ocean surface, can provide improved estimates of oxygen utilization, especially in regions where the disequilibrium at the site of deepwater formation is associated with physical processes. We discuss applications of this new approach and implications for the current tracers relying on O2 such as remineralization ratios, respiratory quotients, and preformed nutrients. Finally, we propose a new composite geochemical tracer, [O2]bio* combining dissolved O2/Ar and phosphate concentration. Being insensitive to photosynthesis and respiration, the change in this new tracer reflects gas exchange at the air‐sea interface at the sites of deepwater formation. Plain Language Summary: Oxygen utilization in the ocean interior offers an insight into organic matter remineralization and the strength of the biological carbon pump. However, the oxygen concentration in the ocean interior is also impacted by additional biotic and abiotic processes occurring at the sites of deepwater formation and during transit in the ocean interior. In this study, we summarize, formalize, and model these processes to explore how decomposing the O2 signal at depth with new tools can provide a new insight and more refined budgets of the broad‐scale oceanic biogeochemical cycling of oxygen and nutrients. This is particularly important in light of the recent evidence that the role of physical processes in the regions of convective deepwater formation may currently be underestimated. Key Points: We propose the new tracer [O2]bio, estimated based on the dissolved O2/Ar ratio, to estimate oxygen utilization at depthWe assess the value in [O2]bio as a proxy for oxygen utilization in the ocean interior and gas exchange at sites of deepwater formationWhen physical processes dominate O2 disequilibrium at the regions of deepwater formation, the new tracer performs better than apparent oxygen utilization (AOU) [ABSTRACT FROM AUTHOR]

The dissolved oxygen (DO) concentration is an important index of water quality. It has a significant impact on the biodegradation process in water and the survival of aquatic organisms. Low DO concentrations often exist and may result in a deterioration in water quality and an increase in wastewater treatment expenses. An analysis of the oxygen transfer efficiency as correlated with the flow characteristics of a falling jet could help to solve these problems. Experiments related to oxygen transfer along a falling water jet were conducted, and the effects of the flowrate (Q), falling height (H) and initial DO concentration (c u) were considered. The results demonstrated that a falling jet in air could be an effective way of improving the DO concentration. The results also indicated that the oxygen transfer efficiency is linearly dependent on c u and has a negative exponential relation with H. To deeply analyze the oxygen transfer process, the jet disturbance process was divided into 3 parts for the first time based on flow features: a smooth zone, turbulent zone and breakup zone. The results showed that the jet outlet velocity had a limited influence on the smooth zone height but assisted in jet breakup. Thereafter, a correlation modeling DO reaeration along the falling jet considering jet disturbance characteristics was obtained. This correlation was also applied to predict the aeration efficiency in experimental cases from the literature and showed good agreement with the reported results. [ABSTRACT FROM AUTHOR]

Urbanization and subsequent expansion of wastewater treatment plant (WWTP) capacity has the potential to alter stream metabolic regimes, but the magnitude of this change remains unknown. Indeed, our understanding of downstream WWTP effects on stream metabolism is spatially and temporally limited, and monitoring designs with upstream-downstream comparison sites are rare. Despite this, and despite observed spatiotemporal variability in stream metabolic regimes, regulators typically use snapshot monitoring to assess ecosystem function in receiving streams, potentially leading to biased conclusions about stream health. To address these important practical issues, we assessed the spatiotemporal variability in stream metabolism at nine sites upstream and downstream of four WWTPs in a suburban stream. We used one year (2017–2018) of high-frequency dissolved oxygen (DO) data to model daily gross primary productivity (GPP) and ecosystem respiration (ER). We found that GPP was 1.7–4.0 times higher and ER was 1.2–7.2 times higher downstream of the WWTPs, especially in spring when light was not limited by canopy shading. Critically, we observed that these effects were spatially limited to the kilometer or so just downstream of the plant. These effects were also temporally limited, and metabolic rates upstream of WWTPs were not different from sites downstream of the plant after leaf-out at some sites. Across sites, regardless of their relation to WWTPs, GPP was positively correlated with potential incident light suggesting that light is the dominant control on GPP in this system. Temporal windowing of DO to proposed regulatory monitoring lengths revealed that the violation frequency of water quality criteria depended on both the monitoring interval and start date. We conclude that spatiotemporal variability in metabolism and DO are crucial considerations when developing monitoring programs to assess ecosystem function, and that evidence of WWTP effects may only arise during high light conditions and at limited scales. [ABSTRACT FROM AUTHOR]

Oxygen depletion in bottom waters of lakes and coastal regions is expanding worldwide. To examine the causes of hypoxia, we quantified the drivers of benthic oxygen uptake in Green Bay, Lake Michigan, USA, using 2 techniques, aquatic eddy covariance and sediment core incubation. We investigated benthic oxygen uptake along a gradient in C deposition, including shallow water near the riverine source of eutrophication and deeper waters of lower Green Bay where high net sediment deposition occurs. Time-averaged eddy covariance oxygen uptake was high near the source of eutrophication (11.5 mmol m−2 d−1) and at the shallower of the high deposition sites (9.8 mmol m−2 d−1). The eddy covariance technique revealed a decrease in benthic oxygen uptake with depth at the high deposition sites. These patterns were consistent with benthic uptake being driven by the deposition of autochthonous production. Additionally, eddy covariance revealed a nearly proportional relationship between benthic oxygen uptake and current velocity at all sites. Specifically, because of the lake seiche, water velocity typically varied 3× at a site and caused a 3× variation in benthic oxygen uptake. A summer storm also doubled bottom-water velocities and caused a further doubling of uptake to 28 mmol m−2 d−1. This high sensitivity of benthic oxygen uptake to seiche-driven water velocities indicates that redox conditions in surficial cohesive sediments are highly dynamic. [ABSTRACT FROM AUTHOR]

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• Small AGS (212–1000 μm) emitted higher N2O than large AGS (>1000 μm) at DO >2 mgO2/L. • Higher DO levels resulted in elevated N2O emissions for both small and large size fractions. • N2O reductase genes (nosZ) were more abundant in smaller granules. • Nitrification exhibited a positive correlation with N2O emission rates across all granule sizes. Aerobic granular sludge (AGS) at wastewater treatment plants (WWTPs) are known to produce nitrous oxide (N 2 O), a greenhouse gas which has a ∼300 times higher global warming potential than carbon dioxide. In this research, we studied N 2 O emissions from different sizes of AGS developed at a dissolved oxygen (DO) level of 2 mgO 2 /L while exposing them to disturbances at various DO concentrations ranging from 1 to 4 mgO 2 /L. Five different AGS size classes were studied: 212–600 µm, 600–1000 µm, 1000–1400 µm, 1400–2000 µm, and > 2000 µm. Metagenomic data showed N 2 O reductase genes (nosZ) were more abundant in the smaller AGS sizes which aligned with the observation of higher N 2 O reduction rates in small AGS under anaerobic conditions. However, when oxygen was present, the activity measurements of N 2 O emission showed an opposite trend compared to metagenomic data, smaller AGS (212 to 1000 µm) emitted significantly higher N 2 O (p < 0.05) than larger AGS (1000 µm to >2000 µm) at DO of 2, 3, and 4 mgO 2 /L. The N 2 O emission rate showed positive correlation with both oxygen levels and nitrification rate. This pattern indicates a connection between N 2 O emission and nitrification. In addition, the data suggested the penetration of oxygen into the anoxic zone of granules might have hindered nitrous oxide reduction, resulting in incomplete denitrification stopping at N 2 O and consequently contributing to an increase in N 2 O emissions. This work sets the stage to better understand the impacts of AGS size on N 2 O emissions in WWTPs under different disturbance of DO conditions, and thus ensure that wastewater treatment will comply with possible future regulations demanding lowering greenhouse gas emissions in an effort to combat climate change. [Display omitted] [ABSTRACT FROM AUTHOR]

Closely spaced CTD stations showed elevated oxygen within Monterey Submarine Canyon. Anomalously high (2–5 μmol kg−1) dissolved oxygen was found between 600–1,100 m in the O2 minimum, co‐located with a turbulence hotspot caused by convergence of upcanyon, semidiurnal internal tidal energy flux. Furthermore, the oxygen anomaly extended >10 km downcanyon at the same depth and isopycnals of a previously identified intrusion predicted from buoyancy conservation. We show that dissolved oxygen and fine suspended particles act as independent tracers to (a) validate previous microstructure observations of intense turbulence extending >400 m above the bed (mab) at the canyon hotspot, and (b) track boundary‐interior exchange driven by mixing in the form of isopyncal‐spreading of anomalies away from a near‐boundary source. This study demonstrates the use of oxygen, commonly measured with shipboard profiling, as a tool for tracking mixing and lateral dispersal. Plain Language Summary: Continental margins and complex topography at canyons play a critical role in mixing the main ocean thermocline and the vertical transport of heat and carbon. Monterey Submarine Canyon has a persistent hotspot of elevated internal wave energy dissipation between 1,200 and 700 m depth. Using previous data and new analyses we show that this mixing hotspot is also an elevated oxygen hotspot in the middle of the deep oxygen minimum zone (a basin‐wide feature), and that this elevated oxygen anomaly, along with suspended particles and mixed fluid, are expelled from the near‐boundary hotspot and flow offshore ∼10 km within an intrusion. Results improve understanding of how continental‐margin boundary layers remain stratified in the face of persistent energetic turbulence, and suggest potential use of oxygen in future studies of mixing and exchange between continental margins and ocean interior. Key Points: Elevated oxygen in deep oxygen minimum zone is caused by mixing in Monterey Canyon internal wave dissipation hotspotAnomalous oxygen and particles spread offshore in an intrusion, providing evidence for exchange driving by mixing and restratificationDissolved oxygen, often observable via CTD, may be useful for other turbulence and buoyancy‐flux studies when conditions are appropriate [ABSTRACT FROM AUTHOR]

The circulation patterns in the confluence of the North Atlantic subtropical and tropical gyres delimited by the Cape Verde Front (CVF) were examined during a field cruise in summer 2017. We collected hydrographic data, dissolved oxygen (O2) and inorganic nutrients along the perimeter of a closed box embracing the Cape Verde Frontal Zone (CVFZ). The detailed spatial (horizontal and vertical) distribution of water masses, O2 and inorganic nutrients in the CVF was analyzed, allowing for the independent estimation of the transports of these properties in the subtropical and tropical domains down to 2000 m. Overall, at surface and central levels, a net westward transport of 3.76 Sv was observed, whereas at intermediate levels, a net 3 Sv transport northward was obtained. We observed O2 and inorganic nutrient imbalances in the domain consistent with O2 consumption and inorganic nutrient production by organic matter remineralization, resulting in a net transport of inorganic nutrients to the ocean interior by the circulation patterns. [ABSTRACT FROM AUTHOR]

The distribution and concentration of dissolved oxygen (DO) play important roles in aerobic heterotroph activities and some slow chemical reactions, and can affect the water quality, biological communities, and ecosystem functions of rivers and lakes. In this work, the transport of high Schmidt number DO at the sediment-water interface of spanwise oscillating flow is investigated. The volume-averaged Navier-Stokes (VANS) equations and Monod equation are used to describe the flow in the sediment layer and the sediment oxygen demand of microorganisms. The phase-averaged velocities and concentrations of different amplitudes and periods are studied. The dependence of DO transfer on the amplitude and period is analyzed by means of phase-average statistical quantities. It is shown that the concentration in the sediment layer is positively correlated with the turbulence intensity, and the DO concentration and penetration depth in the sediment layer increases when the period and amplitude of the oscillating flow increase. Moreover, in the presence of oscillating flow, a specific scaling relationship exists between the Sherwood number/oxygen consumption of aerobic heterotrophs and the Reynolds number. [ABSTRACT FROM AUTHOR]

High‐resolution time series of dissolved oxygen (DO) have revealed different ecosystem energetics regimes across various stream types. Ecosystem energetic regimes are relevant to better understand the transformation and retention of nutrients and carbon in stream ecosystems. However, the patterns and controls of stream energetics in high‐mountain landscapes remain largely unknown. Here we monitored percent DO saturation (every 10 min) over 2 years in a glacier‐fed, krenal (groundwater‐fed) and a nival (snowmelt‐fed) stream as they are typical for the high mountains. We used daily Shannon entropy to explore the temporal dynamics of stream water DO and to infer information on the ecosystem energetics and on the potential drivers. We found that discharge modulated the drivers of DO variations at daily and seasonal scales. Elevated bed movement along with high turbidity and very high gas exchange rates drove the daily DO patterns in the glacier‐fed stream during snow and ice melt, whereas light seemed to drive DO dynamics in the krenal and nival streams. We found a window of favorable conditions for potential gross primary production (GPP) during the onset of the snowmelt in the glacier‐fed stream, whereas potential GPP seemed to extend over longer periods in the krenal and nival streams. Our findings suggest how the energetic regimes of these high‐mountain streams may change in the future as their biological and physical drivers change owing to climate warming. [ABSTRACT FROM AUTHOR]