High Dissolved Oxygen Concentration Research Articles

Iodine enrichment in the groundwater in South China and its hydrogeochemical control

In North China, iodine-rich groundwater has been extensively studied, but few in South China. This study aimed to investigate the characteristics of iodine-rich groundwater in South China and identify potential contamination sources. The results revealed that the average concentration of iodine in groundwater was 890 µg/L, with a maximum concentration of 6350 µg/L, exceeding the permitted levels recommended by the World Health Organization (5–300 µg/L). Notably, the enrichment of iodide occurred in acidic conditions (pH = 6.6) and a relatively low Eh environment (Eh = 198.4 mV). Pearson correlation and cluster analyses suggested that the enrichment of iodide could be attributed to the intensified redox process involving Mn(II), iodine (I2), or iodate (IO3−) in the soil. The strong affinity between Mn(II) and I2/IO3− facilitated their interaction, resulting in the formation and mobilization of I− from the soil to the groundwater. Leaching experiments further confirmed that reducing substances (such as sodium sulfides, ascorbic acids, and fulvic acids) in the soil with low dissolved oxygen (DO) levels (< 1.0 mg/L) enhanced the dissolution of iodine species. Conversely, higher DO content (> 3.8 mg/L) promoted the oxidation of I− into I2 or IO3−, leading to its stabilization. This research provides new insights into the characteristics and mechanisms of I− enrichment in groundwater in South China, and emphasizes the significance of the redox reactions involving Mn(II) and I2/IO3−, as well as the influence of soil properties in regulating the occurrence and transportation of iodine species within groundwater systems.

Water and Sediment Quality Changes in Mangrove Systems with Shrimp Farms in the Northern Ecuadorean Coast

The environmental quality of mangroves is influenced by multiple factors, among which shrimp aquaculture currently plays a major role. This study describes the alterations of natural conditions of mangrove systems that house shrimp farms in the northern Ecuadorean coast. Water, sediment quality and the structure of benthic assemblages of four sectors with different proportions of mangroves and shrimp ponds are described. The samples were collected at the confluence of mangrove drainages or tidal creeks, as well as in the modified drainages for shrimp farm infrastructures towards navigable channels, during the dry and rainy seasons. Shrimp farm drainage water had a 17% higher dissolved oxygen concentration and 2.5 times higher total ammonium and phosphorus compared to mangrove drainage water. The sediment in the latter decreased their total organic matter and nitrogen content by 44% and 53%, respectively, slightly increasing the pH level and increasing the ammonium content by 93%. Furthermore, the redox profiles were different between the types of drainages. The soft-bottom benthic assemblages involved 56 species in the study area and exhibited a variety of sectoral structures, with better indicators of ecological status in sectors with fewer shrimp farms. Finally, improvements are suggested for monitoring the environmental quality of shrimp farms in Ecuadorean mangrove systems.

Hypoxia in the Bohai Bay, China: distributions, causes and mechanisms

The Haihe River system is a major conduit of land-based pollution into the Bohai Sea in North China, This study presented the first-ever observation of hypoxia in the Haihe River estuary, providing valuable insights into the complex interplay between physical and biochemical factors influencing hypoxia in estuarine systems. The research found seasonal variations in DO distribution in the estuaries of the Haihe and Yongdingxin Rivers. Hypoxia emerged in summer at the bottom of the Haihe River estuary and dissipated in autumn, with the minimum DO concentration of 1.85 mg/L recorded at the bottom of the HH-2 station. Within the hypoxic zone, the average DO concentration was 4.02 mg/L, and the average of DO saturation was less than 0.4. This study also identified the primary drivers of summer hypoxia as a combination of physical and biochemical factors, with water stratification due to hydrodynamic factors limiting exchange and reoxygenation between different water layers. In addition, the decomposition of organic matter in the bottom consumed a significant amount of dissolved oxygen.The significantly higher DO concentration and saturation in autumn compared to summer suggested the possible existence of seasonal hypoxia in the Haihe estuary. This study of hypoxia in the Haihe River estuary contribute to a better understanding of the factors that influence seasonal hypoxia. The findings have important implications for the management and remediation of hypoxia in the Haihe estuary and other estuarine systems.

Response of tomato root endophytic bacterial communities to water-oxygen coupling under micro/nanoaerated drip irrigation in slightly saline soils

The aims of this study were to explore the effects of micro/nanoaerated drip irrigation on the root endophytic bacterial communities of crops in saline soils and to offer new insights into how to increase the productivity of such soils by regulating these communities using water-oxygen coupling. A tomato cultivation experiment was conducted in mildly saline soil to evaluate the responses of the root endophytic bacterial community to the soil microenvironment created by micro/nanoaerated drip irrigation. The results revealed that the total relative abundance of phyla, which accounted for 54.80–80.31% of the root endophytic bacterial communities, differed significantly among treatments. The tomato root volume, soil available nitrogen (N) and soil electrical conductivity (EC) were the top three factors that had the greatest influence on the composition and functions of the root endophytic bacterial communities. The ability of these factors to explain the composition differences in the root endophytic bacterial communities reached 0.169, 0.158 and 0.154, respectively. The top three factors were significantly correlated with 10, 13 and 10 functional groups, respectively, of the root endophytic bacterial communities. Furthermore, the structural equation model (SEM) suggested that root endophytic bacterial community functions were significantly correlated with tomato yield (r = 0.71). The combination of the high dissolved oxygen concentration with sufficient and excessive irrigation (A30W1 and A30W1.2) significantly reduced the soil EC and increased the soil available N and root volume, thereby significantly improving the metabolic functions of the root endophytic bacterial communities and the tomato yields. The combination of moderate dissolved oxygen concentration with sufficient and excessive irrigation (A15W1 and A15W1.2) had moderate effects on the metabolic functions of the root endophytic bacterial communities, leading to medium tomato yields. In conclusion, different water-oxygen coupling treatments significantly affected the environmental factors in the root zone and then regulated the metabolic functions of the root endophytic bacterial communities, thus affecting the tomato yields.

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.

Nitrogen removal in moving-bed biofilm reactor plants at low temperatures: experiences from Norway.

Cold and diluted wastewater is a major challenge for nitrogen removal at wastewater treatment plants. The moving-bed biofilm reactor (MBBR) process has proven suitable for nitrogen removal under these conditions and several full-scale plants have been in operation for more than 20 years. These plants are still performing well, even at above design loads. Recently some of these plants have been evaluated for their low-temperature performance. In general, the results showed that temperatures down to 5 °C only had a minor impact on observed nitrification and denitrification rates. Higher dissolved oxygen concentrations can boost nitrification rates and are used as a tool to increase rates at low temperatures, thus partially compensating for the temperature effect. Post-denitrification rates were boosted by a controlled increase in carbon-to-nitrogen ratios at low temperatures. MBBR processes with combined pre- and post-denitrification are recommended for nitrogen-removal plants operating at low temperatures. Design recommendations and examples of flowsheets are given.

Response of phytoplankton to coastal upwelling: The importance of temporal and spatial scales

AbstractThe Western Iberian Coast (WIC) is characterized by a dynamic coastal upwelling system with interactions between oceanographic features and continental river run‐off. The present study investigates the spatio‐temporal variability in the concentrations of Chlorophyll a (Chl a) and turbidity, and their relationships with physical and other biogeochemical properties along the entire coast of Portugal, as well as the response of phytoplankton to coastal upwelling pulses. In situ data were obtained during four oceanographic campaigns conducted in spring (April–May 2019), autumn (October 2018 and 2019), and early spring (February–March 2020). Upwelling conditions were evaluated through an upwelling index and auxiliary data such as sea surface temperature and height. In the WIC, the in situ surface temperature and salinity have a clear spatial pattern, both increasing from North to South. In autumn 2018 the highest temperatures coincided with the weakening of spring–summer upwelling conditions. Overall, high Chl a was associated with lower temperatures and higher dissolved oxygen concentrations (e.g., spring 2019). During spring 2019, an increase in Chl a, especially in the northern area, was associated with previous upwelling conditions. In early spring 2020, ongoing upwelling conditions in the northern area were associated with high turbidity and nutrients close to the shoreline with no immediate biological responses. An increase in Chl a was visible in satellite imagery a few days later. These results highlight the importance of understanding the complexity of coastal processes in the WIC.

Towards a better and more complete understanding of microbial nitrogen transformation processes in the rhizosphere of subsurface flow constructed wetlands: Effect of plant root activities

Plant rhizosphere is the most active hotspot for microbial nitrogen (N) removal in constructed wetlands (CWs), but the effects of plant root activities (i.e., radial oxygen loss, ROL and root exudates, REs) on different microbial N transformation processes, including nitrification, denitrification, anaerobic ammonia oxidation (anammox) and dissimilatory nitrate reduction to ammonia (DNRA) have not been comprehensively studied. In this study, N removal performance and transformation processes in the rhizosphere of CWs were investigated using rhizobox, planar optode system, and metagenomic approaches. Results suggested nitrification was promoted in the rhizosphere, due to the high dissolved oxygen (DO) concentration caused by ROL and the humic acid-like substance in REs, making the rhizosphere a higher removal efficiency of NH4+-N than in the nonrhizosphere. Out of expectation, the denitrification process was not promoted by the REs, the potential “carbon source”, but inhibited by the high DO concentration in the rhizosphere and the phenol-like substance (a kind of biological denitrification inhibitor) in REs. In addition, both anammox and DNRA processes were, for the first time, found to be inhibited in the rhizosphere. The humic acid-like substance in REs and ammonia-oxidation production hydroxylamine inhibited the activity of anammox microorganisms and genes, and there was a symbiotic relationship between anammox microorganisms and DNRA microorganisms. This study provides better and more complete insights into the biogeochemical cycle of N occurred in the rhizosphere of CWs.

Effect of oxygen on pitting corrosion of copper pipe in fire sprinkler system

In this study, pitting corrosion in copper pipe used for sprinkler according to dissolved oxygen (DO) concentration was investigated through failure analyses, flow visualization test, DO consumption rate analysis, electrochemical tests, surface analyses, and corrosion simulation test. As a result of the failure analysis, all 10 leaks were due to pitting corrosion, and 8 leaks occurred in the branch pipe at the end of sprinkler system. As a result of flow visualization test, there were air pockets occupying 20% of the maximum total volume at the end of the branch pipe due to pressure drops. DO consumption rate analysis showed that air pocket acted as the oxygen source to maintain high DO concentration. As a result of electrochemical tests, surface analyses, and corrosion simulation, the corrosion rate of the higher DO concentration is about 10 times higher than that of lower DO concentration due to the increased oxygen reduction reaction. Through these results, it was confirmed that an air pocket was formed at the end of the pipe in the sprinkler system, and this air pocket became an oxygen source and accelerated galvanic corrosion between inside and outside of the pit during pitting propagation. This can be used as a guideline to prevent water leakage for pitting corrosion on copper pipe.

Combined effect of physico-chemical and microbial quality of breeding habitat water on oviposition of malarial vector Anopheles subpictus.

Mosquitoes prefer diverse water bodies for egg laying and larval survival. Present study was performed with an objective to characterize physico-chemical properties and microbial profiling of breeding habitat water bodies of Anopheles subpictus mosquitoes. A field survey was accomplished to check the presence of An. subpictus larvae to record per dip larval density in various breeding habitats throughout the year. Physico-chemical and bacteriological properties in relation to mosquito oviposition were assessed. Dissolved oxygen content, pH and alkalinity were found to have major impacts and ponderosity on the prevalence of An. subpictus larvae. Larval density showed significant positive correlation with dissolved oxygen content of water and significant negative correlation with pH and alkalinity of habitat water. Comparatively higher population (cfu/mL) of Bacillus spp. competent with starch hydrolyzing and nitrate reducing properties were recorded all the breeding habitat water bodies of An. subpictus. Higher amplitude of anopheline larvae was portrayed during monsoon and post-monsoon season in clear water with an inclining trend to high dissolved oxygen content and neutral pH. B. cereus, B. megaterium, B. subtilis and B. tequilensis prevalent in all habitat water bodies were marked as oviposition attractants of gravid An. subpictus mosquitoes. Microbial population played key roles in the modulation of physico-chemical parameters of habitat water with a view to enhance its acceptability by gravid mosquitoes in relation to their oviposition. Better understanding of the interactions along with the control of oviposition attractant bacterial strains from mosquito breeding habitats might contribute to the vector management programme.

Association of an endemic leuciscid, the Sandhills Chub, with microhabitat features and watershed‐level habitat characteristics

AbstractObjectiveHabitat alteration is the greatest threat to fishes globally, and freshwater fishes are particularly vulnerable to habitat alteration because freshwater ecosystems are more closely integrated with the human landscape than marine ecosystems. Headwater streams comprise the majority of stream length within a watershed and provide unique habitats for a variety of aquatic organisms, thus contributing to drainage wide species diversity by supporting many rare and endemic species. Unfortunately, because of their small size, headwater streams are particularly susceptible to the effects of habitat alteration and conversion of natural land cover to anthropogenically dominated landscapes. Therefore, understanding fish–habitat relationships is essential for conserving and restoring headwater species and their habitats. The Sandhills Chub Semotilus lumbee is a headwater specialist endemic to the Sandhills ecoregion in North Carolina and South Carolina. It has been extirpated from several locations in South Carolina; thus, quantitative information on abiotic habitat factors is needed to guide conservation and restoration efforts. The objective of this study was to produce a predictive model of Sandhills Chub occurrence using microhabitat features and watershed‐level habitat characteristics.MethodsLogistic regression was used to identify which habitat features were associated with the occurrence of Sandhills Chub. Habitat and fish data were collected between 2019 and 2020 at 115 sites within the South Carolina Sandhills.ResultSandhills Chub (total = 431) were collected at 41 out of 115 sites sampled. We observed that Sandhills Chub presence was positively associated with dissolved oxygen levels, instream cover, the percent of substrates between 6 and 11 mm, and elevation and was negatively related to the number of impoundments within each 12‐digit hydrologic unit code.ConclusionThe Sandhills Chub prefers streams that have high dissolved oxygen content, presence of instream cover, the substrates necessary to construct their pit‐ridge nests, low numbers of impoundments on the streams, and higher elevations within the Sandhills. Results from this study will provide important information to guide management decisions for the future conservation and restoration of Sandhills Chub.

The efficiency of full-scale subsurface constructed wetlands with high hydraulic loading rates in removing pharmaceutical and personal care products from secondary effluent

Constructed wetlands (CWs) are usually operated at low hydraulic load rates (HLRs) of < 0.5 m3/m2/d, and can efficiently remove pharmaceuticals and personal care products (PPCPs) from wastewaters. They however often occupy a large area of land, especially when treating the secondary effluent from wastewater treatment plants (WWTPs) in megacities. High-load CWs (HCWs) with an HLR ≥ 1 m3/m2/d, requiring smaller land areas, are a good option for urban areas. However, their performance for PPCP removal is not clear. In this study, we evaluated the performance of three full-scale HCWs (HLR: 1.0–1.3 m3/m2/d) to remove 60 PPCPs, and found they had a stable removal performance and a higher areal removal capacity than the previously reported CWs operated at low HLRs. We verified the advantages of HCWs by testing the efficiency of two identical CWs at a low HLR (0.15 m3/m2/d) and a high HLR (1.3 m3/m2/d) fed with the same secondary effluent. The areal removal capacity during the high-HLR operation was 6–9 times higher than that during the low-HLR operation. A high dissolved oxygen content, and low COD and NH4-N concentrations in the secondary effluent were critical for the robust PPCP removal by tertiary treatment HCWs.

Significant production of nitric oxide by aerobic nitrite reduction at acidic pH

The acidic (i.e., pH ∼5) activated sludge process is attracting attention because it enables stable nitrite accumulation and enhances sludge reduction and stabilization, compared to the conventional process at neutral pH. Here, this study examined the production and potential pathways of nitric oxide (NO) and nitrous oxide (N2O) during acidic sludge digestion. With continuous operation of a laboratory-scale aerobic digester at high dissolved oxygen concentration (DO>4 mg O2 L−1) and low pH (4.7±0.6), a significant amount of total nitrogen (TN) loss (i.e., 18.6±1.5% of TN in feed sludge) was detected. Notably, ∼40% of the removed TN was emitted as NO, with ∼8% as N2O. A series of batch assays were then designed to explain the observed TN loss under aerobic conditions. All assays were conducted with a low concentration of volatile solids (VS), i.e., VS<4.5 g L−1. This VS concentration is commensurate with the values commonly found in the aeration tanks of full-scale wastewater treatment systems, and thus no significant nitrogen loss should be expected when DO is controlled above 4 mg O2 L−1. However, nitrite disappeared at a significant rate (with the chemical decomposition of nitrite excluded), leading to NO production in the batch assays at pH 5. The nitrite reduction could be associated with endogenous microbial activities, e.g., nitrite detoxification. The significant NO production illustrates the importance of aerobic nitrite reduction during acidic aerobic sludge digestion, suggesting this process cannot be neglected in developing acidic activated sludge technology.

Correlation of Chlorophyll-a and Dissolved Oxygen in the Phytoplankton Abundance in Leyte Gulf, Philippines

Leyte Gulf is an important fishing ground in the Philippines that serves as a livelihood and food source for many coastal communities. This study aimed to determine the composition, abundance, and diversity of phytoplankton, examine the chlorophyll-a (chl-a) and dissolved oxygen (DO) concentration and distribution, and correlate the results of phytoplankton abundance to the examined water parameters in the Leyte Gulf. Samples were collected at 20 established oceanographic stations from April 24 to May 8, 2020, using 10 liters (L) of Niskin bottles at a depth of 10 meters (m), 25 m, 50 m, and 75 m. Phytoplankton were categorized into three classes, namely diatoms, dinoflagellates, and blue-green algae. Results of this study showed that diatoms were the most dominant class comprising about 89% abundance. Of all the identified phytoplankton taxa, Leptocylindrus spp., Rhizosolenia spp., and Coscinodiscus spp. were the most abundant. Leptocylindrus spp. was found to have a higher density in all sampling stations. Based on the diversity index and density-depth analysis, the phytoplankton abundance has a similar distribution in Leyte Gulf. A high concentration of phytoplankton abundance in the Leyte group was observed, wherein a high concentration of chl-a and DO occurred. Additionally, the status of water parameters throughout the study area was tolerable. Results further revealed that the relationship between the chl-a and DO was strongly positively correlated to phytoplankton. Therefore, continued monitoring of phytoplankton and other important water parameters within the gulf must be done.

A three-year study on the treatment of domestic-industrial mixed wastewater using a full-scale hybrid constructed wetland.

Three full-scale constructed wetlands (CWs), namely vertical flow (VFCW), surface flow (SFCW), and horizontal flow (HFCW) systems, were combined in a series process to form a hybrid CW, which was used for the treatment performance of domestic-industrial mixed wastewater and investigated over a three-year period. The hybrid CW demonstrated that it is effective and stable during the long-term treatment of high-loading mixed wastewater under different operation years, season changes, and technology processes, with the average removal efficiencies of suspended solids, chemical oxygen demand, biological oxygen demand, total nitrogen, ammonia nitrogen, nitrate nitrogen, and total phosphorous being 84, 40, 54, 54, 70, 40, and 46%, respectively. The effluent quality of the hybrid CW reached the highest discharge standard for wastewater treatment plants. First, a variety of pollutants from the mixed wastewater were effectively removed in the subsurface processes (VFCW and HFCW) via substrate adsorption and degradation of the attached biofilm. The higher dissolved oxygen content and oxygen transfer capacity values in the VFCW were favourable for the occurrence of aerobic pathways (such as nitrification and inorganic phosphorus oxidation). In addition, with the large consumption of oxygen in the previous process, the oxygen-enriching capacity of the SFCW processes, provided aerobic potential for the next stage. In particular, the plant debris in the SFCW temporarily increased the organics and suspended solids, further increasing the C/N ratio, which was beneficial for denitrification as the main nitrogen removal pathway in the HFCW.

Bioindicadores de calidad hídrica con diferente cobertura ribereña en la microcuenca Guapara, Ecuador

Agricultural activities produce changes in the margins of the stream banks, causing disturbances in water quality, and in the assemblage of aquatic insect communities. The objective of this research is to evaluate the changes in the structure and composition of the aquatic macroinvertebrate community, and physicochemical variables in relation to the land use of the Guapara river, Cotopaxi province, Ecuador. Samplings were conducted during the rainy season (December to February 2018-2019) in three streams with different riparian cover: agricultural, native forest, and forest plantations, where samples of aquatic insects and physicochemical parameters were collected in situ. A total of 461 aquatic insects corresponding to 7 orders and 25 families were collected. In general, the highest abundance in order and family was presented by Ephemeroptera with 34 %, and Leptophlebiidae with 21 % respectively. According to the Ephemeroptera, Plecoptera Trichoptera (EPT) index, the highest ecological condition was found in streams with native forest cover, and forest plantations respectively, and decreased in the stream with agricultural cover; while BMWP-Cr indicated water contamination in the three streams. Hydrobiosidae (Trichoptera) were associated with high turbidity values; while Gerridae (Hemiptera) were associated with high dissolved oxygen concentrations; in addition, Ptilodactylidae (Coleoptera) preferred the Q1-CA, and Q3-CF conditions, while Leptophlebiidae (Ephemeroptera) were associated with the stream of native forest. A negative influence of agricultural activity on the physicochemical parameters of the water and on the structure of the aquatic insect community assemblage was evidenced.

Patterns and Drivers of Dissolved Gas Concentrations and Fluxes Along a Low Gradient Stream

AbstractFreshwater ecosystems are globally significant sources of greenhouse gases (GHGs) to the atmosphere. Previous work has indicated that GHG flux in headwater streams is dominated by terrestrially derived gases, with in situ production limited by short organic matter residence times and high dissolved oxygen concentrations due to turbulent reaeration. However, low‐gradient headwater streams that contain pool structures with longer residence times may be conducive to the in situ production of GHG. These streams, and the longitudinal heterogeneity therein, are seldom studied. We measured continuous ecosystem metabolism alongside concentrations and fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in a low‐gradient third order stream in the North Carolina Piedmont. From autumn to the following spring, we characterized spatial and temporal patterns of GHG along an 8 km segment in the context of channel geomorphology, hydrology, and ecosystem metabolic rates using linear mixed effects models. We found that stream metabolism was responsible for most of the CO2 flux over this period, and that in‐channel aerobic metabolism was a primary predictor of both CH4 and N2O fluxes as well. Long water residence times, limited reaeration, and substantial organic matter from terrestrial inputs foster conditions conducive to the in‐stream accumulation of CO2 and CH4 from microbial respiration. Streams like this one are common in landscapes with low topographic relief, making it likely that the high contribution of instream metabolism to GHG fluxes that we observed is a widespread yet understudied behavior of many small streams.

Mechanism of arsenic release from realgar oxidation in the presence of dissolved oxygen: Effect of reactive oxygen species and light-induced transformation

Oxidative dissolution of realgar is an important cause of arsenic (As) pollution in waters and soils. Generally, environmental factors including dissolved oxygen (DO) and solar radiation can lead to the formation of reactive oxygen species (ROS) on As-bearing metal sulfide minerals, which significantly promotes mineral oxidation and As release. Realgar is a typical nonmetallic sulfide mineral with excellent photochemical properties. However, the possible formation mechanism of ROS and the effect of solar radiation on realgar oxidation and the subsequent As release remain elusive. Here, the processes of realgar oxidation and As release were studied under darkness and solar radiation, and the influence of DO and pH on the reaction process was also investigated. The decomposition of H2O at the sulfur-defect sites and the oxidation of crystal surface As(II) by DO on realgar led to the formation of ROS including H2O2 and OH, which play a critical role in realgar oxidation and As release. Under solar radiation, the reactions between electrons and O2 on light-excited realgar promoted the production of O2−; while the realgar oxidation and As release could be mainly ascribed to the transformation of realgar to pararealgar in the presence of DO. Under atmospheric conditions, the release ratio of total As in realgar suspension was 4.2% under darkness, and increased to 18.1% under solar radiation. Both higher DO concentration and pH facilitated the generation of ROS and light-induced transformation of realgar to pararealgar, thereby promoting As release. These results imply that the ROS generated on realgar can induce its oxidation and As release in underground aquifers and sediments, and solar radiation may be an important factor causing As pollution in realgar tailings. The findings improve the understanding of As pollution in sulfide mineral mining areas.

Divergent effects of biological and physical processes on dissolved oxygen and dissolved inorganic carbon dynamics on a eutrophied and hypoxic continental shelf

ABSTRACTDissolved oxygen (DO) and dissolved inorganic carbon (DIC) represent reciprocal sides of aquatic metabolism. In closed systems, absolute DO and DIC changes follow a ~ 1 : 1 ratio, but in open systems coupling depends on mixing, air–sea gas exchange, and anaerobic respiration. To quantify DO–DIC controls in coastal waters, we evaluated their concentrations and stable isotopes together with water quality and physical conditions across the hypoxic zone off the Louisiana continental shelf in July 2002 and July 2003. This area is characterized by high yet variable productivity and strong seasonal changes in vertical stratification and river discharge. Prior to 2003, this area was impacted by two tropical systems that could impact vertical stratification and mixing with offshore waters. The relationship between DO and DIC displayed three patterns: (1) In bottom waters, DO and DIC showed similar (opposing) trends in concentrations and stable isotopes in both years, largely associated with nutrients and algal biomass. (2) In 2002, DO and DIC concentrations in surface waters followed similar spatial patterns (reflecting algal biomass, salinity, and nutrients). However, due to different atmospheric gas exchange, DIC depletion exceeded DO supersaturation sixfold. Accordingly, short‐term (DO) and long‐term productivity (DIC) were on the same trajectory. (3) In 2003, DO and DIC were decoupled in surface waters. High DO concentrations correlated with algal biomass (recent production) while DIC concentrations followed salinity (past production). Understanding challenges and opportunities of using DO and DIC individually or combined is critical for quantifying metabolic patterns and capturing impacts of natural variability and climate change on eutrophic continental shelfs.

Vertical profile of dissolved oxygen and associated water variables in the Pasur-Rupsha estuary of Bangladesh

Vertical profile of dissolved oxygen (DO) and associated water variables were measured in the head of the Pasur-Rupsha estuary of Bangladesh. Water samples were collected from two stations at 0.60 m depth intervals during high and low tides from July, 2016 to January, 2017. DO concentration was ranged from 6.0 to 8.6 mg/l with maximum two units of variation in the profiles of the stations and demonstrated an inconsistent stratification pattern. The observed stratification pattern was remarkably triggered by a relatively high concentration of DO in the bottom layer than that of the surface layer during both the high and low tides in most of the study periods. High rate of freshwater flow at the surface; existence of estuarine gravitational circulation with 5–15 m depth profile; and lack of consumers in the bottom due to the high deposition of sediment particles were found responsible for the relatively high concentration of DO at bottom and low in surface layer of the estuary. Water temperature, salinity, pH, phytoplankton density and transparency were ranged from 22 to 32 °C, 0–6 ppt, 7.0 to 8.5, 1000–23000 ind/l and 13–17 cm respectively considering the depth profiles in the study periods. Temperature and plankton density were found significantly (P < 0.000) correlated with DO while the pH and salinity did not show any significant (P > 0.3) correlation. The ecosystem process associated with the vertical profile of DO explained in this study will provide considerable advances in understanding the ecosystem dynamics of the Pasur-Rupsha Estuary of Bangladesh.