Optical Oxygen Sensor Research Articles

A microcontroller-based system for flexible oxygen control in laboratory experiments.

Environmental control systems are important tools for experimental researchers studying animal-environment interactions. Commercial systems for measurement and regulation of environmental oxygen conditions are relatively expensive and can not always be adapted to varying experimental applications. Here, I present a low-cost and highly flexible oxygen control system using Arduino microcontrollers in combination with a commercial optical oxygen sensor. Hardware and software examples are provided for three different applications: single-setpoint, sequential, and long-term dissolved oxygen (DO) control. All tested control systems created the desired DO conditions with high accuracy and repeatability across trials. The resources provided shown here can be adapted and modified to be used in a variety of experimental contexts.

Oxygen optodes on oceanographic moorings: recommendations for deployment and in situ calibration

Increasing interest in the deployment of optical oxygen sensors, or optodes, on oceanographic moorings reflects the value of dissolved oxygen (DO) measurements in studies of physical and biogeochemical processes. Optodes are well-suited for moored applications but require careful, multi-step calibrations in the field to ensure data accuracy. Without a standardized set of protocols, this can be an obstacle for science teams lacking expertise in optode data processing and calibration. Here, we provide a set of recommendations for the deployment and in situ calibration of data from moored optodes, developed from our experience working with a set of 60 optodes deployed as part of the Gases in the Overturning and Horizontal circulation of the Subpolar North Atlantic Program (GOHSNAP). In particular, we detail the correction of drift in moored optodes, which occurs in two forms: (i) an irreversible, time-dependent drift that occurs during both optode storage and deployment and (ii) a reversible and pressure-and-time-dependent drift that is detectable in some optodes deployed at depths greater than 1,000 m. The latter is virtually unidentified in the literature yet appears to cause a low-bias in measured DO on the order of 1 to 3 µmol kg−1 per 1,000 m of depth, appearing as an exponential decay over the first days to months of deployment. Comparisons of our calibrated DO time series against serendipitous mid-deployment conductivity-temperature-depth (CTD)-DO profiles, as well as biogeochemical (BGC)-ARGO float profiles, suggest the protocols described here yield an accuracy in optode-DO of ∼1%, or approximately 2.5 to 3 µmol kg−1. We intend this paper to serve as both documentation of the current best practices in the deployment of moored optodes as well as a guide for science teams seeking to collect high-quality moored oxygen data, regardless of expertise.

A portable multi-taxa phenotyping device to retrieve physiological performance traits

Organismal phenotyping to identify fitness traits is transforming our understanding of adaptive responses and ecological interactions of species within changing environments. Here we present a portable Multi-Taxa Phenotyping (MTP) system that can retrieve a suite of metabolic and photophysiological parameter across light, temperature, and/or chemical gradients, using real time bio-optical (oxygen and chlorophyll a fluorescence) measurements. The MTP system integrates three well-established technologies for the first time: an imaging Pulse Amplitude Modulated (PAM) chlorophyll a fluorometer, custom-designed well plates equipped with optical oxygen sensors, and a thermocycler. We demonstrate the ability of the MTP system to distinguish phenotypic performance characteristics of diverse aquatic taxa spanning corals, mangroves and algae based on metabolic parameters and Photosystem II dynamics, in a high-throughput capacity and accounting for interactions of different environmental gradients on performance. Extracted metrics from the MTP system can not only provide information on the performance of aquatic taxa exposed to differing environmental gradients, but also provide predicted phenotypic responses of key aquatic organisms to environmental change. Further work validating how rapid phenotyping tools such as the MTP system predict phenotypic responses to long term environmental changes in situ are urgently required to best inform how these tools can support management efforts.

Continuous oxygen monitoring to enhance ex-vivo organ machine perfusion and reconstructive surgery

Continuous oxygenation monitoring of machine-perfused organs or transposed autologous tissue is not currently implemented in clinical practice. Oxygenation is a critical parameter that could be used to verify tissue viability and guide corrective interventions, such as perfusion machine parameters or surgical revision. This work presents an innovative technology based on oxygen-sensitive, phosphorescent metalloporphyrin allowing continuous and non-invasive oxygen monitoring of ex-vivo perfused vascularized fasciocutaneous flaps. The method comprises a small, low-energy optical transcutaneous oxygen sensor applied on the flap's skin paddle as well as oxygen sensing devices placed into the tubing. An intermittent perfusion setting was designed to study the response time and accuracy of this technology over a total of 54 perfusion cycles. We further evaluated correlation between the continuous oxygen measurements and gold-standard perfusion viability metrics such as vascular resistance, with good agreement suggesting potential to monitor graft viability at high frequency, opening the possibility to employ feedback control algorithms in the future. This proof-of-concept study opens a range of research and clinical applications in reconstructive surgery and transplantation at a time when perfusion machines undergo rapid clinical adoption with potential to improve outcomes across a variety of surgical procedures and dramatically increase access to transplant medicine.

Exploring the effect of porphyrin chemical structure on the performance of polymer-based Pressure-Sensitive Paints

Pressure sensitive paints (PSPs) are a powerful optical oxygen sensor technique for measuring the surface pressure distribution upon an aircraft model in wind tunnel testing. Traditional PSPs have relied heavily on platinum(II)-5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorphenyl)-porphyrin due to its reasonable quantum yield of emission, high pressure sensitivity, resistance to photo oxidation, and commercial availability. This work investigates the effect of luminophore chemical structure on PSP performance by altering the degree, substitution pattern, and nature of halogen atom on the phenyl groups of ten different Pt(II) tetraphenyl porphyrins. From this initial screening it was found that platinum(II)-5,10,15,20-tetrakis-(3,5-bis(trifluoromethyl)phenyl)-porphyrin polymer PSPs had a substantially lower temperature sensitivity and a slightly higher pressure sensitivity. Using this new luminophore a Fluoro/Isopropyl/Butyl (FIB) polymer based PSP formulation was optimised whereby it was found that 3.2% w/v of FIB polymer and 800 μM concentration of platinum(II)-5,10,15,20-tetrakis-(3,5-bis(trifluoromethyl)phenyl)-porphyrin gave a low temperature sensitivity at 100 kPa for a single component polymer based PSP of 0.22%/K and a relatively high pressure sensitivity at 293 K of 0.846, exceeding current commercial PSP performance.

Spatially resolved quantification of oxygen consumption rate in ex vivo lymph node slices.

Cellular metabolism has been closely linked to activation state in cells of the immune system, and the oxygen consumption rate (OCR) in particular serves as a valuable metric for assessing metabolic activity. Several oxygen sensing assays have been reported for cells in standard culture conditions. However, none have provided a spatially resolved, optical measurement of local oxygen consumption in intact tissue samples, making it challenging to understand regional dynamics of consumption. Therefore, here we established a system to monitor the rates of oxygen consumption in ex vivo tissue slices, using murine lymphoid tissue as a case study. By integrating an optical oxygen sensor into a sealed perfusion chamber and incorporating appropriate correction for photobleaching of the sensor and of tissue autofluorescence, we were able to visualize and quantify rates of oxygen consumption in tissue. This method revealed for the first time that the rate of oxygen consumption in naïve lymphoid tissue was higher in the T cell region compared to the B cell and cortical regions. To validate the method, we measured OCR in the T cell regions of naïve lymph node slices using the optical assay and estimated the consumption rate per cell. The predictions from the optical assay were similar to reported values and were not significantly different from those of the Seahorse metabolic assay, a gold standard method for measuring OCR in cell suspensions. Finally, we used this method to quantify the rate of onset of tissue hypoxia for lymph node slices cultured in a sealed chamber and showed that continuous perfusion was sufficient to maintain oxygenation. In summary, this work establishes a method to monitor oxygen consumption with regional resolution in intact tissue explants, suitable for future use to compare tissue culture conditions and responses to stimulation.

Effects of temperature on metabolic rate during metamorphosis in the alfalfa leafcutting bee.

Spring conditions, especially in temperate regions, may fluctuate abruptly and drastically. Environmental variability can expose organisms to temperatures outside of their optimal thermal ranges. For ectotherms, sudden changes in temperature may cause short- and long-term physiological effects, including changes in respiration, morphology, and reproduction. Exposure to variable temperatures during active development, which is likely to occur for insects developing in spring, can cause detrimental effects. Using the alfalfa leafcutting bee, Megachile rotundata, we aimed to determine if oxygen consumption could be measured using a new system and to test the hypothesis that female and male M. rotundata have a thermal performance curve with a wide optimal range. Oxygen consumption of M. rotundata pupae was measured across a large range of temperatures (6-48°C) using an optical oxygen sensor in a closed respirometry system. Absolute and mass-specific metabolic rates were calculated and compared between bees that were extracted from their brood cells and those remaining in the brood cell to determine whether pupae could be accurately measured inside their brood cells. The metabolic response to temperature was non-linear, which is an assumption of a thermal performance curve; however, the predicted negative slope at higher temperatures was not observed. Despite sexual dimorphism in body mass, sex differences only occurred in mass-specific metabolic rates. Higher metabolic rates in males may be attributed to faster development times, which could explain why there were no differences in absolute metabolic rate measurements. Understanding the physiological and ecological effects of thermal environmental variability on M. rotundata will help to better predict their response to climate change.

Oligohexamethylene Guanidine Derivative as a Means to Prevent Biological Fouling of a Polymer-Based Composite Optical Oxygen Sensor.

The use of biocidal agents is a common practice for protection against biofouling in biomass-rich environments. In this paper, oligohexamethyleneguanidine (OHMG) polymer, known for its biocidal properties, was further modified with para-aminosalicylic acid (PAS) to enhance its properties against microorganisms coated with a lipid membrane. The structure of the product was confirmed by 1H NMR, 13C NMR, and FTIR spectroscopy. The values of the minimum inhibitory concentration (MIC) against Mycobacterium smegmatis ATCC 607 and Pseudomonas chlororaphis 449 were found to be 1.40 and 1.05 μg/mL, respectively. The synthesized substance was used as an additive to the polymer matrix of the composite optical oxygen sensor material. A series of samples with different contents of OHMG-PAS was prepared using a co-dissolution method implying the fabrication of a coating from a solution containing both polymers. It turned out that the mutual influence of the components significantly affects the distribution of the indicator in the matrix, surface morphology, and contact angle. The optimal polymer content turned out to be wt.3%, at which point the water contact angle reaches almost 122°, and the fouling rate decreases by almost five times, which is confirmed by both the respiratory MTT assay and confocal microscopy with staining. This opens up prospects for creating stable and biofouling-resistant sensor elements for use in air tanks or seawater.

Modulation of optical oxygen sensitivity of H2(TPP) by using Eu3+, Gd3+ and Yb3+ ions doped silicate-based bioactive glass powders immersed in simulated body fluid

Improving the performance of optical oxygen sensors can be accomplished by adding additives to the composition comprising an oxygen-sensitive probe encapsulated in a polymeric matrix. In this study, to advance the oxygen sensing ability, free meso-tetraphenyl porphyrin (H2(TPP)) was chosen as a luminophore. The sol-gel based 13–93 bioactive glass powders (BG) co-doped with ytterbium (Yb3+), gadolinium (Gd3+), and europium (Eu3+) and immersed in simulated body fluids (SBF) were also used as additives. After being immersed in SBF, bioactive glasses containing Eu3+, Gd3+, and Yb3+ were found to be able to convert hydroxyapatite corresponding to in vitro bioactivity tests. The optimum additive concentration for all glass samples is 3 wt%, based on photoluminescence-based properties. The luminescence dye was incorporated into poly(trimethylsilylpropyne) [poly(TMSP)] matrices in the form of a thin film along with bioactive glass additives to improve oxygen sensitivity. Emission-based intensity values of all porphyrin complexes have been followed as a signal drop during oxygen sensing measurements. The H2(TPP) along with the additive of BG:Eu3+ showed a 6.88 fold of I0/I ratio compared to the undoped and Gd3+ and Yb3+-doped BG forms in terms of the relative signal change. Large Stoke's shift (Ksv) values, high luminescence abilities, and linear spectral response make them promising candidates as oxygen sensing agents. It was concluded that prepared bioactive glass powders are suitable to use as additive materials for improving the oxygen sensitivity of meso-tetraphenyl porphyrin dye.

Evaluation of Usefulness of Yeast-Based Biological Phantom and Preliminary Study for Verification of Hypoxic Effect of Flash Radiotherapy

As a basic hypothesis for the effectiveness of flash radiation therapy, the effect of preserving normal tissue during flash radiation is due to the instantaneous chemical depletion of oxygen. A yeast-based biological phantom was created to verify the hypoxic effect of flash radiation therapy. A study to upgrade the previously developed X-Band LINAC to a flash irradiation mode is in progress, and a preceding study is conducted to evaluate the usefulness of a yeast-based biological phantom manufactured by analyzing the change in oxygen by irradiating a high dose in a general radiation therapy device. Freeze-dried yeast sample (Saccharomyces cerevisiae, S288C) is activated and sub-cultured. For mass production of yeast samples, yeast culture medium is prepared by adding yeast colonies to the ypd medium. This study was conducted to verify the hypoxic effect among the biological mechanisms that occur during flash radiation therapy at the basic stage, and the oxygen concentration change during general radiation irradiation was measured in real time using a DO (Dissolved oxygen) meter and fiber optic sensor designed to do that. To prevent scatter, which is a concern during flash irradiation, the fiber form was used, and precise experiments are possible as a non-invasive oxygen concentration measurement method. Based on 10MV of general radiation therapy device, high-dose radiation of 500-10,000 cGy is irradiated to measure real-time oxygen concentration change. As a result of irradiation with high-dose (500-10,000 cGy) radiation of general LINAC, it was confirmed that the oxygen concentration of the yeast culture medium decreased by 5.7-63.2%, and the usefulness of the biological phantom fabricated based on the yeast culture medium was evaluated. Prior to the analysis of oxygen concentration change in yeast cells during X-Band LINAC flash irradiation, a preliminary study was conducted at a high dose in a general LINAC to obtain a significant result of oxygen concentration change and confirm the usefulness of the yeast-based biological phantom. Prior research was conducted and verified as a general irradiation experiment using a yeast-based biological phantom manufactured based on a DO meter and a fiber optic oxygen sensor. After irradiation with high-dose radiation, the oxygen concentration of the yeast culture medium was measured 5 times, and it was confirmed that there was a change in oxygen concentration of 5.7-63.2%, verifying the usefulness and stability of the biological phantom. The usefulness of the yeast-based biological phantom for high doses was confirmed, and it is expected that the usefulness of the biological phantom for flash radiation can be verified by additionally measuring the change in oxygen concentration of the biological phantom according to the high dose rate in the future.

Engineered porosity for tissue-integrating, bioresorbable lifetime-based phosphorescent oxygen sensors

Versatile silk protein-based material formats were studied to demonstrate bioresorbable, implantable optical oxygen sensors that can integrate with the surrounding tissues. The ability to continuously monitor tissue oxygenation in vivo is desired for a range of medical applications. Silk was chosen as the matrix material due to its excellent biocompatibility, its unique chemistry that facilitates interactions with chromophores, and the potential to tune degradation time without altering chemical composition. A phosphorescent Pd (II) benzoporphyrin chromophore was incorporated to impart oxygen sensitivity. Organic solvent-based processing methods using 1,1,1,3,3,3-hexafluoro-2-propanol were used to fabricate: 1) silk-chromophore films with varied thickness and 2) silk-chromophore sponges with interconnected porosity. All compositions were biocompatible and exhibited photophysical properties with oxygen sensitivities (i.e., Stern-Volmer quenching rate constants of 2.7–3.2 × 104 M−1) useful for monitoring physiological tissue oxygen levels and for detecting deviations from normal behavior (e.g., hyperoxia). The potential to tune degradation time without significantly impacting photophysical properties was successfully demonstrated. Furthermore, the ability to consistently monitor tissue oxygenation in vivo was established via a multi-week rodent study. Histological assessments indicated successful tissue integration for the sponges, and this material format responded more quickly to various oxygen challenges than the film samples.

Phosphorescence or Delayed Fluorescence? Fate of Excitons in Core-Substituted Naphthalimide-Derived Emitters and Ratiometric Optical Oxygen Sensors

Phosphorescence or Delayed Fluorescence? Fate of Excitons in Core-Substituted Naphthalimide-Derived Emitters and Ratiometric Optical Oxygen Sensors

Development and Evaluation of Respirometric in Situ Corrosion Monitoring System

There is a growing need for a system to be used for real-time in-situ monitoring of corrosion rates. This system/method allows the determination of real-time corrosion rates under realistic exposure conditions and is capable of following changing exposure conditions in situ. This is realized by a combination of optical Oxygen sensor measurements with either gravimetric volume sensitive techniques or pressure sensor based techniques in a closed chamber. This study was therefore aimed at developing and evaluating a low-cost, real-time corrosion monitoring system, using copper (Cu) as a test sample. Materials used were sourced locally, the circuitry was designed and used to develop the system with incorporation of sensors that can monitor temperature, humidity and pressure within an airtight glass bottle and placed in a housing which was fabricated. The developed system was then evaluated using a piece of Cu exposed to 5% Sodium Chloride (NaCl). Hydrogen evolution reaction (HER) and Oxygen Reduction Reaction (ORR) within the closed chamber were monitored; and thus Ideal gas and Henry laws were adopted to calculate the amount of gas molecules, and convert them to cathodic reactions. The methods were carried out in accordance with existing literature and standard procedure. Results of evaluation of the system showed that, the more Oxygen is being consumed, the higher the corrosion. Mass loss validation measurements carried out at the end of exposure showed a good correlation with the total recorded cathodic charge. Immersion corrosion kinetics can be monitored non-destructively and in real-time. Manometric approach showed that HER leads to a pressure increase while ORR leads to a decrease in pressure. ORR monitoring is possible based on the amount of consumed O2 by manometric and sensor-based approaches; sensitive, non-destructive corrosion rate measurements are possible on Cu and could be monitored remotely.

On-line measurement of atmospheric oxygen by an open-path OA-ICOS based sensor with high accuracy at ambient pressure

In this paper, a compact optical oxygen (O2) sensor system was demonstrated based on off-axis integrated cavity output spectroscopy (OA-ICOS). Light from a distributed feedback laser near 761 nm was coupled into an open-path F-P cavity by off-axis configuration. To address the challenges of changing field environments for open-path configuration, the influence of pressure on the sensor system was studied. A linewidth enhancement factor (δ) was proposed to measure the ambient pressure and a pressure correction method was developed to improve the measurement accuracy. A minimum detection limit of 0.0038% was obtained for an optimum averaging time of 20 s under 760 Torr. Indoor and outdoor O2 concentration measurements were conducted and the sensing applicability of the sensors system at ambient pressure was verified. The demonstrated sensor system allows operation without a vacuum pump and a pressure controller combine with the self-developed laser driver and digital lock-in amplifier, which has advantages of reduced size, low power consumption, low cost and has wide application prospect for O2 sensing in field environments.

ВЛИЯНИЕ МАСЛА СЕМЯН ТЫКВЫ НА СКОРОСТЬ ПОТРЕБЛЕНИЯ КИСЛОРОДА ТКАНЬЮ ПРОСТАТЫ

Despite a long and successful experience with the use of pumpkin seed oil (PSO) in patients with chronic prostatitis (CP), the pharmacodynamics of PSO is not fully understood. We hypothesized that pumpkin seeds and PSO contain pharmacologically active substances that are able to influence on the rate of oxygen consumption (ROC) by the prostate tissue, which may be a determinant of their effectiveness in men with CP. The aim: to study the effect of eating pumpkin seeds, containing PSO, on the ROC of the prostate tissue in white outbred rats. Materials and methods. The control group consisted of 16 outbred male rats. The experimental group consisted of 6 white outbred male rats, which on the eve received, in addition to the standard diet, pumpkin seeds without restriction. Thus, 22 adult outbred male rats were used in the study. The ROC of the prostate tissue was measured by an optical sensor of dissolved oxygen with a DKTP-03 thermoelectric converter. A prostatectomy was performed under anesthesia. Immediately after that, prostate tissue weighing 0.5 g was placed in a beaker, filled with 15.0 ml of 0.9% sodium chloride solution at a temperature of 36.6 °C. An optical oxygen sensor DKTP-03 connected to an oxygen analyzer "Expert-009" was placed in a beaker and the tightness was created. The analyzer showed the concentration of dissolved molecular oxygen (CDO2) in a 0.9% solution of sodium chloride, expressed in mg / l. We recorded the CDO2 readings every minute from the beginning of the experiments (0 minutes) to their end (60 minutes). Results and conclusion. we have found for the first time that PSO statistically significantly increases the ROC in rat prostate tissue (p < 0.05).

Ratiometric optical oxygen sensor based on perovskite quantum dots and Rh110 embedded in an ethyl cellulose matrix

Ratiometric optical sensor gas sensing continues to develop optical sensing techniques and materials used in various industrial and environmental applications. This research focuses on a new ratiometric optical sensor using the development of new material of FAPbI3 perovskite QDs and a simple method to detect oxygen (O2) gas. FAPbI3 perovskite QDs are used as an indicator of oxygen gas, and rhodamine 110 (Rh 110) is a reference material in a ratiometric optical sensor. All of the sensing and reference materials are embedded in an ethyl cellulose (EC) matrix and coated on the surface of the filter paper. Using a UV LED with a central wavelength of 380 nm as the excitation light source, the emission spectra results show that the emission wavelengths of the oxygen-sensitive dye (O2) FAPbI3 perovskite QDs do not overlap with the Rh 110 reference signal. Thus, oxygen concentration can be measured using a ratiometric fluorescence reference-based approach. The sensing signal will be obtained in the presence of analyte gas in the ratiometric sensitivity of R0/R100, where R0 and R100 represent the luminescence intensity detected in 100% nitrogen and 100% oxygen concentrations, respectively. The experimental results show the optical oxygen sensor's sensitivity as R0/R100 = 12.7. In addition, the response time and recovery of the oxygen gas sensor produced are 75 s and 93 s, respectively. The use of a new type of FAPbI3 perovskite QDs material has been successfully developed in the optical ratiometric sensor for oxygen gas. The sensor proposed in this study has a low cost and easy fabrication process. The effect of spurious fluctuations in the excitation source intensity can be suppressed by the ratiometric optical sensing method.

Preparation of a high stability optical fiber oxygen sensor based on the bilayer sensitive membrane

A high stability optical fiber oxygen sensor based on the bilayer sensitive membrane was presented, in which the sensitive membrane includes the sol–gel layer containing an oxygen sensitive indicator [Ru(phen)3]Cl2 and the nano-porous cellulose acetate coating wrapped around the sol–gel layer. The bilayer sensitive membrane had good permeability and resistance to reaction, which could protect the sol–gel oxygen sensing layer while ensuring permeability and improving probe stability. The scanning electron microscope (SEM) was used to characterize the morphology and thickness of the layers. The fluorescence of [Ru(phen)3]Cl2 could be dynamic quench when the oxygen contacts the fluorescent indicator by passing through the holes on the layers. The developed optical fiber oxygen sensor based on this bilayer sensitive membrane shows a wide linear detection range from 5 to 100 % with highly stability.

Reducing biofouling on optical oxygen sensors; a simple modification enabling sensor cleaning via water splitting.

Biofouling is a major challenge in environmental sensing. Current mitigation strategies are often expensive, energy consuming or require toxic chemicals. In this contribution electrochemical biofouling control is evaluated as an alternative approach to reduce biofouling on an optical O2 sensor (optode). By using the outer stainless-steel sleeve of the optode as an electrode, water splitting increases the local pH and forms H2 bubbles close to the optode surface. As seen in a biofouling assay, the combination of those processes leads to biofilm removal when compared to a non-modified optode. The findings suggest that electrochemical biofouling control can be an attractive, low-cost alternative to current biofouling mitigation strategies and that this approach may not be limited to O2 optodes.

A high-performance optical trace oxygen sensor based on the room-temperature phosphorescence from palladium (II) octaethylporphyrin

A high-performance oxygen sensor is developed based on room temperature phosphorescence. The optimized sensing probe is fabricated by Sol-gel matrix doped with Palladium (II) octaethylporphyrin (PdOEP), and a sensitivity of the PdOEP probe reached 29.23 kPa−1. Dual-channel structure is designed to eliminate fluctuation of excitation source. One channel is used to detect oxygen sensing phosphorescence from PdOEP probe, and the other is used for receiving reference fluorescence signal, which is induced by 7-Diethylamino-4-methylcoumarin (C1). The C1 is insensitive to oxygen and is placed side by side with PdOEP probe, the spatial isolation of PdOEP and C1 probes could avoid the interference of different emissions. Photomultiplier tube equipped photon counter is employed as the sampling unit to promote the signal-to-noise ratio. The detection limit and indication error of measurement system reached 0.1 Pa and ± 1.5 %, respectively. In addition, the 1.9 μm thickness of probe ensures the system response within 5 s.

Enhanced oxygen consumption results in summertime hypoxia in Mikawa Bay, Japan.

In Mikawa Bay, where hypoxia occurs in the bottom layer during summer, six shipboard observations were conducted from the mouth to the head of the bay from May to August 2014 to investigate the spatiotemporal variation in the bottom layer oxygen consumption rate (OCR). The OCR was determined from the dark incubation of sample waters using an optical oxygen sensor, which showed a range of 5.7-38.3mmolm-3days-1. A high OCR was observed at the inner-most station, where higher concentrations of nutrients and chlorophyll a (Chl a) than at the other stations were found, and bottom hypoxic water appeared during the observation period after late June. These OCRs can deplete the oxygen dissolved in water within a week. The OCR showed a highly significant positive correlation with particulate organic carbon concentrations in the bottom water. Considering the relatively low carbon-to-nitrogen mole ratio (~ 6.4-7.6) and high carbon isotope ratio (between approximately - 20.2 and - 18.8‰) of particulate organic matter at the stations, the supply of fresh organic matter produced in the bay as opposed to the land may have affected the OCR by acting as a substrate for microbial aerobic respiration. High temporal resolution data from two automated observation buoys near the bay mouth and the inner area captured increases in Chl a at both sites in response to typhoon events, along with the subsequent appearance of bottom hypoxic water at the inner site and its expansion at the mouth. This supports our hypothesis that enhanced organic matter production due to nutrient input to the surface layer through vertical mixing would increase the bottom OCR, resulting in hypoxia. The apparent oxygen decline in the bottom layer from the buoy data was consistent with incubation-based OCRs during the observation period. Therefore, it is essential to model the OCR in numerical simulations of hypoxia, to which the variability characteristics that we revealed made significant contributions.