Average Dissolved Oxygen Concentration Research Articles

Landsat data reveal lake deoxygenation worldwide

Dissolved oxygen (DO) is a fundamental requirement for the survival of aquatic organisms, which plays a crucial role in shaping the structure and functioning of aquatic ecosystems. However, the long-term DO change in global lakes remains unknown due to limited available data. To address this gap, we integrate Landsat data and geographic features to develop DO estimation models for global lakes using machine learning approaches. The results demonstrated that the trained random forest (RF) model has better performance (R2 = 0.72, and RMSE = 1.24 mg/L) than artificial neural network (ANN) (R2 = 0.66, and RMSE = 1.39 mg/L), support vector machine regression (SVR) (R2 = 0.62, and RMSE = 1.45 mg/L) and extreme gradient boosting (XGBoost) (R2 = 0.72, and RMSE = 1.29 mg/L). Then, we used the trained RF model to reveal the DO long-term (1984–2021) change in surface water (epilimnetic) of 351,236 global lakes with water area ≥ 0.1 km2. The results show that the average epilimnetic DO concentration of global lake was 9.72 ± 1.07 mg/L, with higher DO in the polar regions (latitude > 66.56 °) (10.87 ± 0.54 mg/L) and lower in the equatorial regions (−5 ° < latitude < 5 °) (6.29 ± 0.63 mg/L). We also find widespread deoxygenation in surface water of global lakes, with a rate of − 0.036 mg/L per decade. Meanwhile, the number of lakes and surface area that experiencing DO stress are continuously increasing, with rate of 39 and 212.85 km2, respectively. Our results offer a comprehensive dataset of DO variation spanning nearly 40 years, furnishing valuable insights for formulating effective management strategies, and enhancing the maintenance of the health of aquatic ecosystems.

Dissolved Oxygen Recovery in the Oxygen Minimum Zone of the Arabian Sea in Recent Decade as Observed by BGC‐Argo Floats

AbstractThe Arabian Sea (AS) hosts the world's thickest and most intense oxygen minimum zone (OMZ), and previous studies have documented a dramatic decline of dissolved oxygen (DO) in the northeastern AS in recent decades. In this study, using the recently released data from Biogeochemical‐Argo floats, we found a surprising trend of recovery in deoxygenation within the core region of the OMZ in the AS (ASOMZ) since 2013. The average DO concentration increased by approximately threefold, from ∼0.63 μM in 2013 to ∼1.68 μM in 2022, and the thickness of the ASOMZ decreased by 13%. We find that the weakening of Oman upwelling resulting from the weakening of the summer monsoon is the main driver of oxygenation in the ASOMZ. In addition, the reduction of primary production linked to warming‐driven stratification reinforces deoxygenation recovery at depth.

Dissolved Oxygen Inversion Based on Himawari-8 Imagery and Machine Learning: A Case Study of Lake Chaohu

Dissolved oxygen (DO) concentration is a widely used and effective indicator for assessing water quality and pollution in aquatic environments. Continuous and large-scale inversion of water environments using remote sensing imagery has become a hot topic in water environmental research. Remote sensing technology has been extensively applied in water quality monitoring, but its limited sampling frequency necessitates the development of a high-frequency dynamic water quality monitoring model. In this study, we utilized Lake Chaohu as a case study. Firstly, we constructed a dynamic water quality inversion model for monitoring DO concentrations using machine learning methods, with Himawari-8 (H8) satellite imagery as input data and DO concentrations in Lake Chaohu as output data. Secondly, the developed DO concentration inversion model was employed to estimate the overall grid-based DO concentration in the Lake Chaohu region for the years 2019 to 2021. Lastly, Pearson correlation analysis and significance tests were performed to examine the correlation and significance between the estimated grid-based DO concentration and the ERA5 reanalysis dataset. The results demonstrate that the Random Forest (RF) model performs best in DO concentration inversion, with a high R2 score of 0.84, and low RMSE and MAE values of 0.69 and 0.54, respectively. Compared to other models, the RF model improves average performance with a 38% increase in R2, 13% decrease in RMSE, and 33% decrease in MAE. The model accurately predicts DO concentrations. Furthermore, the inversion results reveal seasonal differences in DO concentrations in Lake Chaohu from 2019 to 2021, with higher concentrations in spring and winter, and lower concentrations in summer and autumn. The average DO concentrations in the northwest, central-south, and northeast regions of Lake Chaohu are 10.12 mg/L, 9.98 mg/L, and 9.96 mg/L, respectively, with higher concentrations in the northwest region. Pearson correlation analysis indicates a significant correlation (p &lt; 0.01) between DO concentrations and temperature, surface pressure, latent heat flux from the atmosphere to the surface, and latent heat flux from the surface to the atmosphere, with correlation coefficients of −0.615, 0.583, −0.480, and 0.444, respectively. The results verify the feasibility of using synchronous satellites for real-time inversion of DO concentrations, providing a more efficient, economical, and accurate means for real-time monitoring of DO concentrations. This study has practical value in improving the efficiency and accuracy of water environmental monitoring.

Sediment Pollution and Dredging Effect of Waiqinhuai River

The Waiqinhuai River is an important urban landscape flood channel in Nanjing, which has been seriously polluted by industrial and domestic sewage for many years. To fully understand the characteristics of Waiqinhuai river sediment pollution and provide a decision-making basis for dredging, the sediment interface microenvironment and nutrient content of river sediment collected from method-typical sections in the upper, middle, and lower reaches were determined, and the organic index and pollution index methods were used to evaluate the sediment pollution condition. We also simulated the effect of desilting on the reduction of endogenous release in sediment according to pollutant characteristics of vertical distribution. The results showed that the average dissolved oxygen concentrations at the upper, middle, and lower sediment interfaces were 4.62, 3.25, and 3.41 mg·L-1, respectively; the concentrations were exhausted at 4.4, 3.5, and 5.5 mm, respectively, which were typical characteristics of urban river pollution. The average contents of TN, TP, and OM in the surface sediment of the investigated reach were 1734 mg·kg-1, 1337 mg·kg-1, and 4.82%, respectively. The organic pollution index of TN and OM in the sediment was 0.48 on average, which was at the clean level, whereas the individual pollution index of TP was 3.18 on average, which was at the severe pollution level. The results of simulating the dredging depth of 30 cm showed that the release rates of SRP and iron divalent were reduced by 42%-82% and 88%-96%, respectively, whereas the release rate of ammonia nitrogen was increased. The results of DGT determination and phosphorus speciation analysis showed that the phosphorus activity of surface sediment decreased significantly after desilting, and DGT-P and Mobile-P decreased by 9%-11% and 1%-35%, respectively, compared with those in the control treatment. These results indicated that the sediment of Waiqinhuai River was seriously polluted by endogenous phosphorus, and desilting could reduce the release of endogenous phosphorus to a certain extent, which may be an important method for improving the water quality of the Waiqinhuai River.

Pilot Study of Oxic–Anoxic Process under Low Dissolved Oxygen for Nitrogen Removal from Low COD/N Tropical Wastewater

Conventionally, nitrification in biological nitrogen removal (BNR) requires high dissolved oxygen (DO) concentrations (&gt;2 mg L−1), making the process energy intensive. Recent studies have shown that efficient ammonium removal and energy reduction can be realized by operating the nitrification at low DO concentrations (&lt;1 mg L−1). In this study, the low-DO oxic anoxic (low-DO OA) process was operated in a pilot-scale sequencing batch reactor (SBR) over 218 days to evaluate the feasibility of nitrogen removal from low chemical oxygen demand-to-nitrogen ratio (COD/N) tropical municipal wastewater. The results revealed that the low-DO OA process attained high removal efficiency for ammonium (97%) and total nitrogen (TN) (80%) under an average DO concentration of 0.6 mg L−1. The effective TN removal efficiency is attributed to the occurrence of simultaneous nitrification–denitrification (SND) under low DO conditions. Further batch tests revealed that slowly biodegradable COD (sbCOD) in tropical wastewater can support denitrification in the post-anoxic phase, resulting in a high TN removal rate. Compared with high DO concentrations (2 mg L−1), low DO conditions achieved 10% higher TN removal efficiency, with similar ammonium and COD removal efficiency. This study is crucial in promoting the energy efficiency and sustainability of wastewater treatment plants treating low COD/N wastewater.

Controllable generation of multi-row bubbly flow in coupled microstructures and its boosting of photocatalytic decarboxylative carbonylation

Continuous-flow microreactor technology has emerged as a powerful driver of process innovation in photocatalysis. However, microfluidic schemes that are ideal for gas–liquid photoreactions are still limited by the low specific interfacial area. Herein, we invent for the first time a coupled microstructure comprising sudden contraction, shearing, and sudden expansion parts, which can generate microbubbles with smaller sizes under lower flow rates compared with the conventional T-junction. Based on bubble dynamics studies, a predictive model for the bubble size, with prediction errors of less than 10%, is established. Interestingly, the generated microbubbles can be induced by the expansion flow to form a multi-row bubbly flow. Additionally, predictive models for the row number are proposed to guide its controlled production. Computational fluid dynamics simulations show a 22.8-fold increase in specific interfacial area for the five-row bubbly flow compared to the segmented flow. Moreover, the average dissolved oxygen concentration at a given time increases by 34.0 times. Further, photocatalytic decarboxylative carbonylation limited by oxygen mass transfer is employed as the model reaction, demonstrating the excellent performance of multi-row bubbly flow in enhancing the gas–liquid photocatalysis, and the stronger enhancement at higher row number. Subsequently, the phototransformation achieves 96.3% yield within 19 min at a six-row bubbly flow, and the space–time yield reaches 6.82 gL–1 h−1, thus indicating the high efficiency of the developed microdevice. The multi-row bubbly flow enabled by this coupled microstructure provides a new process window for continuous, efficient, and controllable gas–liquid photocatalysis.

Intracellular electron competition in response to the oxygen pressure of the aerobic denitrification process in an O2-based membrane biofilm reactor (MBfR) for nitrate removal

This study quantitatively investigated the effect of dissolved oxygen (DO) concentration on aerobic denitrification, and showed the mechanism of aerobic denitrification from the perspective of electron competition by cultivating Pseudomonas stutzeri T13, a typical aerobic denitrifier, in an oxygen-based membrane biofilm reactor (O2-based MBfR). The experiments showed that when the O2 pressure increased from 2 to 10 psig , the average effluent DO concentration during steady-state phases increased from 0.02 to 4.23 mg/L, and the corresponding mean NO3−-N removal efficiency slightly decreased from 97.2 % to 90.9 %. Compared to the maximum theoretical flux of O2 in various phases, the actual O2 transfer flux increased from a limited status (2.07 e− eq m−2 d−1 at 2 psig) to an excessive status (5.58 e− eq m−2 d−1 at 10 psig). The increase of DO inhibited the electron availability for aerobic denitrification, which decreased from 23.97 % to 11.46 %, accompanying the increased electron availability for aerobic respiration from 15.87 % to 28.36 %. Unlike the napA and norB genes, the expression of the nirS and nosZ genes was significantly affected by DO, with the highest relative fold-changes of 6.5 and 6.13 at 4 psig O2, respectively. The results contribute to clarifying the mechanism of aerobic denitrification from the quantitative perspective of electron distribution and the qualitative perspective of gene expression, which benefits the control and practical application of aerobic denitrification for wastewater treatment.

Longer duration of seasonal stratification contributes to widespread increases in lake hypoxia and anoxia.

The concentration of dissolved oxygen (DO) is an important attribute of aquatic ecosystems, influencing habitat, drinking water quality, biodiversity, nutrient biogeochemistry, and greenhouse gas emissions. While average summer DO concentrations are declining in lakes across the temperate zone, much remains unknown about seasonal factors contributing to deepwater DO losses. It is unclear whether declines are related to increasing rates of seasonal DO depletion or changes in seasonal stratification that limit re-oxygenation of deep waters. Furthermore, despite the presence of important biological and ecological DO thresholds, there has been no large-scale assessment of changes in the amount of habitat crossing these thresholds, limiting the ability to understand the consequences of observed DO losses. We used a dataset from >400 widely distributed lakes to identify the drivers of DO losses and quantify the frequency and volume of lake water crossing biologically and ecologically important threshold concentrations ranging from 5 to 0.5mg/L. Our results show that while there were no consistent changes over time in seasonal DO depletion rates, over three-quarters of lakes exhibited an increase in the duration of stratification, providing more time for seasonal deepwater DO depletion to occur. As a result, most lakes have experienced summertime increases in the amount of water below all examined thresholds in deepwater DO concentration, with increases in the proportion of the water column below thresholds ranging between 0.9% and 1.7% per decade. In the 30-day period preceding the end of stratification, increases were greater at >2.2% per decade and >70% of analyzed lakes experienced increases in the amount of oxygen-depleted water. These results indicate ongoing climate-induced increases in the duration of stratification have already contributed to reduction of habitat for many species, likely increased internal nutrient loading, and otherwise altered lake chemistry. Future warming is likely to exacerbate these trends.

Variation characteristics of dissolved oxygen and metabolic rate in lakes during the ice-covered period

冰封期湖泊与大气的气体交换受冰盖阻碍，影响湖泊溶解氧含量，进而影响湖泊水质.为探究冰封期湖泊溶解氧和新陈代谢速率变化特征及影响因素，本研究通过监测典型季节性冰封湖泊不同深度的溶解氧（DO）、水温和光合有效辐射（PAR），结合水质检测结果，分析冰封期湖泊DO变化及代谢速率影响因素，对湖泊日新陈代谢速率计算方法进行改进并估算冰封期湖泊新陈代谢速率.结果表明：2021年1—3月间，岱海DO浓度平均值为15.49 mg/L，并出现昼夜变化和分层现象.DO在冰封期变化趋势呈现出先逐渐升高，然后保持稳定，进入消融期（2021年3月2—11日）分层现象逐渐消失的规律.岱海冰封期净初级生产力和生态系统呼吸速率平均值分别为0.11和0.10 mg/（L·d），在水温未出现分层时净初级生产力呈现较高水平；之后水温沿水深方向出现分层，净初级生产力明显下降；当冰层融化后，PAR显著上升，净初级生产力又逐渐恢复至0.10 mg/（L·d）.统计分析表明，岱海冰封期DO与水温、PAR、总氮等变化具有一定相关关系，且由于湖泊流域生态环境条件及冰封期物候特征不同，岱海与内蒙古其他湖泊相比，冰封期DO变化趋势存在一定差异.;In ice-covered periods, the gas exchange between lakes and the atmosphere is blocked by ice sheets. The dissolved oxygen concentration and consequently the water quality of the lakes are thus affected. To explore the metabolic rates and variation characteristics of the dissolved oxygen, and its influencing factors in typical seasonal ice-covered lakes, dissolved oxygen concentration, water temperature and photo-synthetically active radiation (PAR) were monitored at different depths of the lakes. In combination with the water quality monitoring results, the variations of the dissolved oxygen and the influencing factors of the metabolic rates were analyzed to optimize the calculation methods of the everyday metabolic rates, thus to estimate the metabolic rates of the ice-covered lakes. The results showed that the average dissolved oxygen concentration in Lake Daihai was 15.49 mg/L, and diurnal variation and stratification had appeared. The dissolved oxygen concentration initially increased gradually and then stabilized from Jan. to Mar. 2021, the ice-covered period. The stratification phenomena disappeared from Mar. 2nd to 11th, 2021, the thawing period. The average net primary productivity and ecosystem respiration rate of Lake Daihai were 0.11 mg/(L·d) and 0.10 mg/(L·d) respectively in the ice-covered period. The net primary productivity appeared to be higher when the water temperature was not stratified. Along with the rise of the water temperature, the water temperature began to stratify along the depth of water, and the net primary productivity decreased significantly. After the ice had melted, the PAR increased significantly, and the net primary productivity gradually increased back to 0.10 mg/(L·d). Statistical analysis demonstrated that water temperature, PAR and total nitrogen had exerted significant effects on the dissolved oxygen concentration during the ice-covered period in Lake Daihai. Due to the different ecological environment conditions and phenological characteristics of the ice-covered period in the lake basin, compared with other lakes in Inner Mongolia, Lake Daihai has certain differences in the trend of DO changes during the ice-covered period.

Dissolved oxygen variation in the North China Plain river network region over 2011–2020 and the influencing factors

The 2011–2020 data of dissolved oxygen (DO) and nutrients—total phosphorus (TP), solution reactive phosphorus (SRP), total nitrogen (TN), ammoniacal nitrogen (NH3–N), and chemical oxygen demand (CODCr)—in the North China Plain river network region were analyzed. Moreover, the DO variation trend and the influencing factors were investigated. The results showed that between 2011 and 2020, the DO concentration steadily increased from <3 mg L−1 to >5 mg L−1. Negative relationships were observed between the DO and CODCr (R = −0.34, p < 0.01), TN (R = −0.41, p < 0.01), NH3–N(R = −0.40, p < 0.01), TP (R = −0.28, p < 0.01), and SRP (R = −0.19, p < 0.01), indicating that a reduction in the nutrient input promoted the increase in the DO concentration in the past decade. The DO concentration in the rainy season was lower than that in the dry season, which suggests that nonpoint-source pollution caused by heavy storm runoff was the main factor affecting the water quality. The average DO concentration in the suburban rivers (4.88 mg L−1) was higher than that in urban rivers (3.41 mg L−1). Furthermore, comprehensive analysis indicated that the loss of riparian buffer, river solidification, pollutant input, and sluice dam operations are the main factors affecting DO concentration decrease and water quality deterioration. Finally, measures for water improvement and DO recovery in the river network region, namely pollutant input control, reduction in the impact of water conservancy projects, and river ecological restoration, were examined. Overall, this study shows that water quality has improved over the past decade. The study results provide a reference for the continuous improvement of water quality and the continuous recovery of DO in river network regions.

The influence of aeration control and temperature reduction on nitrogen removal and microbial community in two anammox‐based hybrid sequencing batch biofilm reactors

AbstractBACKGROUNDTwo hybrid biofilm reactors, an integrated fixed‐film activated sludge–sequencing batch reactor (IFAS‐SBR) and a hybrid SBR with a mixture of activated sludge and granules, were used to reach enhanced nitrogen removal based on the anaerobic ammonium oxidation (anammox) process under intermittent aeration.RESULTSIFAS‐SBR reached a more stable performance after a rapid start‐up period of 25 days, and it showed a more reliable performance than the other reactor. The highest nitrogen removal efficiency (NRE) was up to 99% based on NH4+‐N concentration and 91% based on total nitrogen concentration (TIN). A nitrogen removal rate of 0.01 to 0.15 kgN m−3 d−1 was achieved throughout the operation based on the TIN. The set‐point of maximum dissolved oxygen (DO) at the peak of each aeration cycle was maintained at 1.2 ± 0.1 mg L−1. The optimum working range for daily average DO concentration was 0.2–0.7 mg L−1. Cold temperatures caused a decline of NRE, but NRE &gt; 50% for temperatures above 13 °C was achieved.CONCLUSIONSResults implied a higher abundance of Candidatus brocadia species of anammox bacteria in the biofilm samples and Denitratisoma genus in the flocs. The most substantial contribution of shared taxa among all samples at class level was Betaproteobacteria and Gammaproteobacteria, and the least abundant class was Nitrospira. Overall, results implied that IFAS‐SBR is a robust configuration for anammox‐based nitrogen removal in sidestream, and controlled intermittent aeration could suppress nitrite oxidation and help to regain system efficiency.

Gas\u2013liquid mass transfer characteristics of aviation fuel scrubbing in an aircraft fuel tank

Dissolved oxygen evolving from aviation fuel leads to an increase in the oxygen concentration in an inert aircraft fuel tank ullage that may increase the flammability of the tank. Aviation fuel scrubbing with nitrogen-enriched air (NEA) can largely reduce the amount of dissolved oxygen and counteract the adverse effect of oxygen evolution. The gas–liquid mass transfer characteristics of aviation fuel scrubbing are investigated using the computational fluid dynamics method, which is verified experimentally. The effects of the NEA bubble diameter, NEA superficial velocity and fuel load on oxygen transfer between NEA and aviation fuel are discussed. Findings from this work indicate that the descent rate of the average dissolved oxygen concentration, gas holdup distribution and volumetric mass transfer coefficient increase with increasing NEA superficial velocity but decrease with increasing bubble diameter and fuel load. When the bubble diameter varies from 1 to 4 mm, the maximum change of descent rate of dissolved oxygen concentration is 18.46%, the gas holdup is 8.73%, the oxygen volumetric mass transfer coefficient is 81.45%. When the NEA superficial velocities varies from 0.04 to 0.10 m/s, the maximum change of descent rate of dissolved oxygen concentration is 146.77%, the gas holdup is 77.14%, the oxygen volumetric mass transfer coefficient is 175.38%. When the fuel load varies from 35 to 80%, the maximum change of descent rate of dissolved oxygen concentration is 21.15%, the gas holdup is 49.54%, the oxygen volumetric mass transfer coefficient is 44.57%. These results provide a better understanding of the gas and liquid mass transfer characteristics of aviation fuel scrubbing in aircraft fuel tanks and can promote the optimal design of fuel scrubbing inerting systems.

A COMPARATIVE STUDY OF THE TYPE OF GROUNDWATER FOR WESTERN MOSUL REGIONS

Three water springs were selected in the western part of the city of Mosul (AinQana belongs to Al-Ayadiyah sub-district, Ain Abu Maria village, and AinSubashi in the center of Tal Afar district) to study some physical and chemical characteristics (temperature, electrical conductivity, acidic function, as well as negative and positive ions) and the biological characteristics of algae. The results of the study showed that the change in water temperature was little during the study period, and the highest value of the electrical connection reached (2557, 2488, and 1817) microsiemens / cm in Subashi, Qana, and Abu Maria sites, respectively. Whereas, the average dissolved oxygen concentration ranged between (0.0 to 3.6) mg / L at Qana and Subashi sites respectively. The total basal values ranged between (128) mg / L as CaCO3 at the site of Qana and (154) mg / L as CaCO3 at the site of Subashi. The results showed that the sodium concentration values were greater than the potassium concentration values in the three sites. Sixteen species of algae that belong to the algae section were diagnosed as bluish-green algae, green algae, and red algae. Oscillatoria was diagnosed in AinQana and the absence of the species belonging to the two algae sections, green and red algae in AinQana.

Increase in sedimentary organic carbon with a change from hypoxic to oxic conditions

In the Seto Inland Sea, Japan, chemical oxygen demand has increased over recent decades, while average dissolved oxygen concentrations in the bottom water have increased. In this study, we investigated responses of organic carbon (OC) in hypoxic sediment to changes of redox conditions using experimental columns containing sediment and overlying water. Surface sediment showed an increase in OC along with the change to an aerobic condition. Microbial community analysis showed a predominance of sulfur-oxidizing bacteria (SOB) such as Sulfurovum sp. in the sediment. This dominance could account for the increased OC. Additionally, the dissolved organic carbon (DOC) concentration in the overlying water increased. Further experiments using sandy sediment showed that biodegradation of Sulfurimonas denitrificans was associated with DOC release. These results show that a change in the sedimentary environment (increase in dissolved oxygen) increased the sedimentary OC and DOC of overlying water by stimulating certain autotrophic bacteria, especially the SOB.

A novel pilot-scale IFAS-MBR system with low aeration for municipal wastewater treatment: Linkages between nutrient removal and core functional microbiota

In this study, we proposed a novel IFAS-MBR with low aeration for the treatment of real municipal wastewater. With biocarriers packed in the anoxic tank, the pilot-scale IFAS-MBR operated with average dissolved oxygen concentrations of 0.56 mg/L in the oxic tank. Over 110 days of operation, highly efficient nutrient removal was achieved with the total nitrogen (TN) and phosphorus (TP) removal efficiencies of 78.1 ± 7.2% and 93.7 ± 5.8%, respectively. The average effluent concentrations of TN and TP reached 5.4 and 0.26 mg/L, respectively. Meanwhile, the removal efficiency of COD reached 95.3 ± 1.3% in the system, and the concentrations of COD decreased from 31.9 ± 3.7 (sludge supernatant) to 12.7 ± 1.6 mg/L (permeate) after membrane filtration. Microbial community analysis showed that Nitrosomonas (0.32%) and Nitrospira (1.85%) in activated sludge were the main drivers of the nitrification process, while various denitrifying bacteria in activated sludge and biofilms were responsible for nitrate reduction in the anoxic tank. Candidatus Accumulibacter (0.34%) and Dechloromonas (1.31%) primarily contributed to denitrifying phosphorus uptake in the anoxic tank. Furthermore, these organisms (i.e., core functional microbiota) exhibited stable levels over the entire operation. The highly enriched hydrolytic fermentation bacteria drove community succession, and the remarkable functional robustness of microbial communities in activated sludge and biofilms favored nutrient removal. Overall, the novel IFAS-MBR system provides an energy-efficient MBR alternative owing to its highly efficient performance and low operating costs enabled by low aeration rates and the absence of an external carbon source.

Surface Water Pollution by Untreated Municipal Wastewater Discharge Due to a Sewer Failure

The study is an attempt to assess the pollution impact on the aquatic ecosystem related with an emergency discharge of untreated municipal wastewater from the “Czajka” wastewater treatment plant (WWTP) in Warsaw. The present case study is based on the analysis of available monitoring data for chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (N-NH4), and total phosphorus (TP) in the effluent mixing zone (MZ), the stretch of the river and before the discharge point. Data analysis was supported by a basic statistical analysis based on Pearson’s correlation coefficient. The results proved the importance of efficient and reliable nutrient removal technologies used in modern WWTPs. A statistically significant correlation was achieved between the COD (r = 0.567) and TP (r = 0.714) discharged loads and their concentration in the MZ. However, no significant correlation has been identified with TN and N-NH4. Furthermore, the dissolved oxygen (DO) deficits in the MZ were observed within 7 days of the discharge period resulting in an average DO concentration decrease from 8.4 to 7.1 mgO2/L. The river stretch has not been affected by DO deficits while the average observed DO concentration 30 km behind the discharge point was 9.1 mgO2/L. The analysis results present the pollutants assimilation capacity of a river ecosystem and its real reaction to sudden excessive nutrient loads discharge.

Spatial-Monthly Variations and Influencing Factors of Dissolved Oxygen in Surface Water of Zhanjiang Bay, China

Dissolved oxygen (DO) is one of the most important factors for maintaining a healthy marine ecosystem. The information of DO in large estuaries or bays with large entrances has been widely studied, while it is relatively limited for a bay with a narrow entrance which is vulnerable to human activities. The Zhanjiang Bay, located in the northwestern South China Sea, has a very narrow entrance and suffers from strong anthropogenic activities and obvious seasonal variations in environmental parameters. In this study, we analyzed the spatial and monthly variations of DO, apparent oxygen utilization (AOU), percent oxygen saturation (DO-saturation), and related environmental parameters in the surface water of Zhanjiang Bay to find out the factors controlling the dynamics of DO. Different from many other coastal ecosystems, DO concentrations in the Zhanjiang Bay reached minimum values in late spring and early autumn. The phytoplankton bloom in summer months, which was related to the high concentrations of nutrients brought by rainfall-induced terrestrial inputs, contributed to that phenomenon. Though high chlorophyll a (Chl a) concentrations were observed in both the summer months and December, the DO-saturation values were relatively low and AOU values were relatively high in summer months. Rainfall-induced terrestrial discharge in summer months, which had high concentrations of chemical oxygen demand, contributed much to that phenomenon. The average DO concentrations and DO-saturation values in a hydrological year decreased seaward, and AOU values increased seaward, indicating the anthropogenic influence from terrestrial input. The highest annual average Chl a concentration, relatively high annual average DO-saturation value and relatively low annual AOU value were observed near the Donghai Dam. This indicated that the construction of Donghai Dam has significant influences on the environment of Zhanjiang Bay.

Summertime Oxygen Depletion and Acidification in Bohai Sea, China

Increasing attention has recently been drawn to oxygen depletion and ocean acidification in coastal seas and their impacts on the marine ecosystem. Intensive organic matter degradation combined with weak water exchange is considered to be mainly responsible for bottom water oxygen depletion and acidification. Hypoxia and acidification in the bottom water of Bohai Sea have been frequently observed in summer seasons over the past decade. In this study, monthly surveys were conducted along an inshore-offshore transect in the north Bohai Sea from May to August in 2017 and 2018. The physical structure of the water column in the study area was characterized by a well-mixed surface layer and an essentially homogeneous deep layer that were separated by a strong pycnocline over the period from mid-Jun to late August in both years. Dissolved oxygen (DO), pH and dissolved inorganic carbon (DIC) in the water column were also split into two layers by the pycnocline and varied little in each layer. No significant interannual variations were observed for the temporal progressions of DO, pH and DIC in the bottom water. Average DO concentration and pH in the bottom water linearly decreased with time from May to August while DIC presented an opposite trend. DO became increasingly unsaturated from mid-June to late August when DO decreased to 96 µmol L -1 and pH to 7.74. The molar ratio of net oxygen consumption to DIC production was 1.42, similar to the Redfield ratio (1.30). Pelagic respiration (> 60%) linked to degradation of fresh organic matter prevailed over sediment respiration (< 40%) in contributing to the oxygen depletion and acidification observed in this shallow water body. The duration and intensity of oxygen depletion and acidification in the study area were strongly affected by stormy weather events such as typhoons.

Exploring the linkage between bacterial community composition and nitrous oxide emission under varied DO levels through the alternation of aeration rates in a lab-scale anoxic-oxic reactor

Dissolved oxygen (DO) level is crucial in shaping bacterial community and impacts biological nitrogen removal and nitrous oxide (N2O) emission. Online gaseous and off-line dissolved N2O under varying DO levels through aeration rate alternations were measured in lab-scale anoxic-oxic reactors. It showed that sharp changes in DO levels caused immediate N2O emission increase, while the total average gaseous N2O emission stabilized at 0.011%, 0.046%, 0.308% and 0.229% of influent nitrogen as DO in oxic tanks averaged at 0.58, 1.67, 3.2 and 6.12 mg/L, respectively. Process with an average DO concentration of 1.67 mg/L had the highest microbial diversity and relative abundances of potential denitrifers and ammonia-oxidizing bacteria (NOB), while the least ammonia-oxidizing bacteria (AOB) were detected, which contributed to efficient nitrogen removal and minor N2O emission. In conclusion, regulation and control of denitrifiers, AOB and NOB with the determination of a proper DO set point is feasible for N2O mitigation.

Effect of aeration and oxygenation on growth and survival of rainbow trout in a commercial serial-pass, flow-through raceway system

Serial-pass raceway systems are characterized by reduced water quality as water passes from upper to lower raceways. We previously showed that rainbow trout reared in 1st use (source spring) and 3rd use (after 2 raceway passes) water exhibited significantly better growth and survival compared to those in 5th use water (after 4 raceway passes) and directly related to dissolved oxygen (DO) concentration. Our objective was to determine if increasing the DO in 5th use water by aeration or oxygenation to levels in 3rd use water could restore growth and survival of rainbow trout. We stocked rainbow trout (126 ± 9.3 g mean weight) into tanks receiving either 1st use, 3rd use, or 5th use water with and without supplemental air or oxygen (five treatments at four tanks each). Average DO concentration was highest in 1st use (92.0% saturation) followed by 3rd use (79.4%) and 5th use (52.1%) water. In 5th use water, use of aeration (68.4%) or oxygenation (71.7%) raised the DO levels similar to 3rd use water. Growth performance (SGR and FCR) and survival were significantly lower for trout in 5th use compared to 1st and 3rd use water. Aeration and oxygenation of 5th use water restored growth to that observed for trout in 3rd use water. Improvements were limited for survival with 3rd use water (88.8%) showing higher values than 5th use with oxygenation (74.2%) or aeration (53.1%). These treatments were still significantly better than survival observed for trout in untreated 5th use water (35.0%). Dissolved oxygen appears to be the primary water quality parameter limiting performance in this system. However, due to dissimilar survival in 3rd and 5th use water with aeration or oxygenation, other water parameters (e.g. total dissolved solids or turbidity), which increased in 5th use water, may also contribute to the reduced growth and survival.