Dissolved O2 Research Articles

Boosting Electrochemiluminescence Performance of a Dual-Active Site Iron Single-Atom Catalyst-Based Luminol-Dissolved Oxygen System via Plasmon-Induced Hot Holes.

Due to the commonly low content of biomarkers in diseases, increasing the sensitivity of electrochemiluminescence (ECL) systems is of great significance for in vitro ECL diagnosis and biodetection. Although dissolved O2 (DO) has recently been considered superior to H2O2 as a coreactant in the most widely used luminol ECL systems owing to its improved stability and less biotoxicity, it still has unsatisfactory ECL performance because of its ultralow reactivity. In this study, an effective plasmonic luminol-DO ECL system has been developed by complexing luminol-capped Ag nanoparticles (AgNPs) with plasma-treated Fe single-atom catalysts (Fe-SACs) embedded in graphitic carbon nitride (g-CN) (pFe-g-CN). Under optimal conditions, the performance of the resulting ECL system could be markedly increased up to 1300-fold compared to the traditional luminol-DO system. Further investigations revealed that duple binding sites of pFe-g-CN and plasmonically induced hot holes that disseminated from AgNPs to g-CN surfaces lead to facilitate significantly the luminous reaction process of the system. The proposed luminol-DO ECL system was further employed for the stable and ultrasensitive detection of prostate-specific antigen in a wide linear range of 1.0 fg/mL to 1 μg/mL, with a pretty low limit of detection of 0.183 fg/mL.

Ammonium load determines the performance of a continuous nitritation process and the related microbial communities even under excesses of O2 supply

AbstractBackgroundPartial oxidation of ammonium to nitrite, process known as nitritation, has important advantages over complete nitrification, including energy savings due to a reduced aeration, a lower organic carbon requirement for denitrification, a reduced production of sludge, and the potential use of nitrite in anammox and anoxic biogas purification technologies. Dissolved O2 (DO) concentration commonly has been regarded as the most important parameter to be controlled for achieving a successful nitritation process.ResultsThe present study showed that setting average DO values below 1 g O2 m−3 was insufficient to achieve a high and stable nitritation at ammonium loading rates between 2.5 to 5.1 g N‐NH4+ m−3 h−1. Under these conditions, the predominance of nitrite oxidizing bacteria (NOB) resulted in ammonium oxidation selectivity to nitrite from 5.2 ± 3.6% to 42.8 ± 21%. Increasing the ammonium loading to 10.1 gN‐NH4+ m−3 h−1 together with an average DO value of 0.9 ± 0.6 gO2 m−3 resulted in an ammonium oxidation efficiency of 94 ± 1.1% and ammonium oxidation selectivity to nitrite >90%.ConclusionContinuous nitritation was achieved in a single reactor provided with an easy‐to‐implement aeration strategy. The ammonium loading rate was the most important parameter to be controlled, which inhibited NOB above a threshold value. NOB inhibition was mediated mainly by free nitrous acid, and by free ammonia to a lesser extent. Increasing the ammonium loading rate led to an enrichment of ammonium oxidizing bacteria of the Nitrosomonas genus. © 2022 Society of Chemical Industry (SCI).

The University of West Florida Campus Ecosystem Study: Spatial and Temporal Variation in Water Quality at Thompson Bayou

Much of our understanding of factors influencing stream chemistry comes from studies of montane forests, whereas far less work has focused on streams of coastal areas that integrate a homogeneous, flat topography and interactions with the bodies of water into which they drain, especially involving tidal fluxes. Fewer still do so in the context of an urban interface, especially that of a college campus. This study assessed the water quality of Thompson Bayou, a freshwater stream entering the University of West Florida campus in a wetland after flowing through the urban property with impacted water quality. We measured temperature, pH, dissolved O2 (DO), and specific conductivity (SC) for one year at eight sites along Thompson Bayou from campus to the Escambia River. All variables, except temperature, varied spatially, with consistent increases in DO and SC toward the river of 10% and 75%, respectively. Variables exhibited temporal patterns of significant seasonal variation, especially temperature, increasing from a January minimum of 14 °C to a summer maximum of 28 °C. These results suggest that, in general, the biogeochemistry of coastal streams such as Thompson Bayou can be influenced by numerous factors, including (1) wetland processes, (2) interactions of the stream channel with forested uplands, and (3) tidal fluxes.

Plasmon-Boosted Fe, Co Dual Single-Atom Catalysts for Ultrasensitive Luminol-Dissolved O2 Electrochemiluminescence Detection of Prostate-Specific Antigen

Improving the sensitivity of electrochemiluminescence (ECL) systems is highly desired for in vitro ECL diagnosis and bio-detections due to the often-low content of biomarkers in diseases. And dissolved O2 (DO) as a co-reactant is considered superior to H2O2 in the most commonly used luminol ECL systems due to better stability and low biotoxicity, but it still suffers from low ECL performance due to the low reactivity of DO. In this study, an efficient luminol-DO ECL system was developed through the complexing of Fe, Co dual single-atom catalysts (D-SACs) supported by N-doped graphene with the luminol-capped Ag nanoparticles (AgNPs). Benefiting from the electronic interaction between Fe and Co metal sites in the relevant D-SACs and plasmon enhancement of AgNPs, the performance of the corresponding ECL system could be significantly boosted up to ≈677-fold under optimal testing conditions, comparable to the classic luminol-O2 system. Furthermore, the developed luminol-DO ECL system was successfully applied for the stable ultrasensitive detection of prostate-specific antigen (PSA) in a wide linear range of 1 fg/mL to 1 μg/mL, with a low limit of detection (0.98 fg/mL).

Insights on the capacitance degradation of polypyrrole nanowires during prolonged cycling

Polypyrrole (PPy) nanowires are widely used in supercapacitors due to their high faradaic pseudo-capacitance and specific surface areas. However, their rapid capacitance degradation is a key and broad issue that cannot be ignored. In this work, PPy/p-toluenesulfonate nanowires (PPy/pTS NWs) network films were electro-polymerized with different nanowire diameters (D) and film thicknesses (L). The effects of nanowire structure and the dissolved O2 (DO) in the media on the capacitance degradation behavior were investigated. With the cyclic voltammetry (CV) cycles increasing, the surface layer of PPy/pTS NWs was gradually damaged and dissolved with the loss of most doping anions, resulting in decreased specific capacitance. The high initial over-oxidation degree of PPy/pTS NWs and the high DO concentration significantly accelerated the degradation of PPy/pTS NWs during CV cycles. After the loss of most capacitance, PPy/pTS NWs network films were expanded with increased L but reduced D, displaying uniform degradation without fractured nanowires. The capacitance stability of PPy/pTS NWs could be enhanced by lowering the initial over-oxidation degree, inhibiting the loss of doping anions, and removing DO in the media.

Chromium(VI) removal from synthetic solution using novel zero-valent iron biochar composites derived from iron-rich sludge via one-pot synthesis

In this study, Fe-rich sludge obtained from coagulation–flocculation of swine wastewater was processed into zero-valent iron (ZVI) biochar composites through one-pot pyrolysis at 700 °C under anoxic (MBCN700) and hypoxic (MBCA700) conditions. X-ray diffraction analysis revealed that α-Fe0 and γ-Fe0 were the dominant Fe species in MBCN700 and MBCA700, respectively. The effects of different process parameters, including composite dosage, initial Cr(VI) concentration, pH, dissolved O2 (DO), and contact time on Cr(VI) removal from synthetic solutions were evaluated. The results showed that both the acidic initial pH and DO contributed to Cr(VI) removal; the highest removal rate was observed at pH 3. Based on the Langmuir isotherm model, the predicted maximum removal capacity of MBCA700 was greater than that of MBCN700, primarily because of its core-shell structure, less corrosive ZVI, and higher degree of graphitization. X-ray photoelectron spectroscopy analysis coupled with fitting of the kinetic (pseudo-second-order) and isothermal (Freundlich) data suggested that Cr(VI) removal by MBCN700 occurred via adsorption, FeC micro-electrolysis, and co-precipitation. In contrast, the removal mechanism for MBCA700 included adsorption, reduction, and electrostatic attraction. Thus, ZVI biochar composites can serve as potential remediators of Cr(VI)-contaminated wastewater.

Severe hypoxia exposure inhibits larval brain development but does not affect the capacity to mount a cortisol stress response in zebrafish.

Fish nursery habitats are increasingly hypoxic and the brain is recognized as highly hypoxia sensitive, yet there is a lack of information on the effects of hypoxia on the development and function of the larval fish brain. Here, we tested the hypothesis that by inhibiting brain development, larval exposure to severe hypoxia has persistent functional effects on the cortisol stress response in zebrafish (Danio rerio). Exposing 5 days post-fertilization (dpf) larvae to 10% dissolved O2 (DO) for 16 h only marginally reduced survival, but it decreased forebrain neural proliferation by 55%, and reduced the expression of neurod1, gfap and mbpa, markers of determined neurons, glia and oligodendrocytes, respectively. The 5dpf hypoxic exposure also elicited transient increases in whole-body cortisol and in crf, uts1 and hsd20b2 expression, key regulators of the endocrine stress response. Hypoxia exposure at 5dpf also inhibited the cortisol stress response to hypoxia in 10dpf larvae and increased hypoxia tolerance. However, 10% DO exposure at 5dpf for 16 h did not affect the cortisol stress response to a novel stressor in 10dpf larvae or the cortisol stress response to hypoxia in adult fish. Therefore, while larval exposure to severe hypoxia can inhibit brain development, it also increases hypoxia tolerance. These effects may transiently reduce the impact of hypoxia on the cortisol stress response but not its functional capacity to respond to novel stressors. We conclude that the larval cortisol stress response in zebrafish has a high capacity to cope with severe hypoxia-induced neurogenic impairment.

Mechanisms of sulfate formation in acidic hydrothermal sites of Iceland, Lassen, Valles Caldera, and Yellowstone: Implications for possible oxidation pathways in martian volcanic settings

Sulfate (SO42−) has been found in elevated concentrations on the surface of Mars and linked to oxidation of volcanically derived sulfur (S) in the past, with the best examples of hydrothermal S-rich deposits in Gusev crater. However, the oxidation mechanisms of volcanic S are unclear due to the lack of abundant molecular oxygen (O2) in the Martian atmosphere. To address this uncertainty, we investigated the mechanisms of hydrogen sulfide (H2S) oxidation to SO42− in O2-depleted acidic hydrothermal systems of Iceland and the United States (Valles Caldera, Lassen, and Yellowstone) as geochemical analogs. Approximately 50 water and sediment samples were collected for chemical and stable isotope analyses (δ34S, δ18O, δ2H). At the time of sampling, the measured dissolved O2 (DO) concentrations were low (0.01 to 1.03 mg/L) because of elevated temperatures (60 to 90 °C) and did not account for high concentrations of SO42− (up to 24,461 mg/L) in the acidic hot springs and mud pots. However, high concentrations of leachable iron (Fe) were present in the hydrothermal water and sediments (average of 158 mg/L and 71,302 mg/kg, respectively), implying that in addition to atmospheric O2, the Fe-driven oxidation of H2S via ferric Fe reduction to ferrous Fe is likely involved. Our laboratory experiments studying H2S oxidation carried out in a wide temperature range (25 °C, 50 °C, 90 °C) and low pH of 2 suggest that under acidic hydrothermal conditions, ferric Fe reduction involves stepwise oxidation of H2S with preferential formation of insoluble elemental S, which is later oxidized to SO42−. In contrast, the Gusev hydrothermal sediments on Mars appear to be enriched in Fe3+-sulfates with minor amounts of Ca-Mg-sulfates and negligible elemental S. This implies that the Gusev sediments must have experienced subsequent oxidation, most likely after the main episode of hydrothermal activity.

Rapid soil organic carbon decomposition in river systems: effects of the aquatic microbial community and hydrodynamical disturbance

Abstract. Different erosion processes deliver large amounts of terrestrial soil organic carbon (SOC) to rivers. Mounting evidence indicates that a significant fraction of this SOC, which displays a wide range of ages, is rapidly decomposed after entering the river system. The mechanisms explaining this rapid decomposition of previously stable SOC still remain unclear. In this study, we investigated the relative importance of two mechanisms that possibly control SOC decomposition rates in aquatic systems: (i) in the river water SOC is exposed to the aquatic microbial community which is able to metabolize SOC much more quickly than the soil microbial community and (ii) SOC decomposition in rivers is facilitated due to the hydrodynamic disturbance of suspended sediment particles. We performed different series of short-term (168 h) incubations quantifying the rates of SOC decomposition in an aquatic system under controlled conditions. Organic carbon decomposition was measured continuously through monitoring dissolved O2 (DO) concentration using a fiber-optic sensor (FireStingO2, PyroScience). Under both shaking and standing conditions, we found a significant difference in decomposition rate between SOC with aquatic microbial organisms added (SOC + AMO) and without aquatic microbial organisms (SOC − AMO). The presence of an aquatic microbial community enhanced the SOC decomposition process by 70 %–128 % depending on the soil type and shaking–standing conditions. While some recent studies suggested that aquatic respiration rates may have been substantially underestimated by performing measurement under stationary conditions, our results indicate that the effect of hydrodynamic disturbance is relatively minor, under the temperature conditions, for the soil type, and for the suspended matter concentration range used in our experiments. We propose a simple conceptual model explaining these contrasting results.

The Important Role of Dissolved Oxygen Supply Regulated by the Hydraulic Shear Force during the Biosynthesis of Iron Hydroxysulfate Minerals

The severity of environmental pollution from acid mine drainage (AMD) is increasingly garnering attention. In this study, the effects of hydraulic shear forces (achieved by regulating the shaking table’s rotation speed) on Fe2+ bio-oxidation and Fe3+ hydrolytic mineralization in an acidic 9K medium-FeSO4-Acidithiobacillus ferrooxidans system (simulated AMD) are investigated. Results reveal that a higher shaking speed favors a higher oxidation rate of Fe2+, whereas a very low or high shaking speed restricts the removal of Fe3+. Shaking table rotation speeds of 120–180 rpm were preferred for biomineralization treatment in the simulated AMD. As the initial concentration of Fe2+ in the system decreased from 9.67 to 0 g/L in 40 h, the dissolved O2 (DO) in the solution dropped to its lowest concentration after 20 h and then increased to its initial level between 40 and 120 h. However, the corresponding total Fe (TFe) precipitation efficiency increased with the increasing mineralization time after 40 h. The effect of O2 supply time on biomineralization revealed that DO was mainly used in Fe2+ bio-oxidation. After Fe2+ was completely oxidized by A. ferrooxidans, the precipitation efficiency of TFe was independent of the O2 supply.

Real-time dissolved carbon dioxide monitoring II: Surface aeration intensification for efficient CO2 removal in shake flasks and mini-bioreactors leads to superior growth and recombinant protein yields.

Mass transfer is known to play a critical role in bioprocess performance and henceforth monitoring dissolved O2 (DO) and dissolved CO2 (dCO2) is of paramount importance. At bioreactor level these parameters can be monitored online and can be controlled by sparging air/oxygen or stirrer speed. However, traditional small‐scale systems such as shake flasks lack real time monitoring and also employ only surface aeration with additional diffusion limitations imposed by the culture plug. Here we present implementation of intensifying surface aeration by sparging air in the headspace of the reaction vessel and real‐time monitoring of DO and dCO2 in the bioprocesses to evaluate the impact of intensified surface aeration. We observed that sparging air in the headspace allowed us to keep dCO2 at low level, which significantly improved not only biomass growth but also protein yield. We expect that implementing such controlled smart shake flasks can minimize the process development gap which currently exists in shake flask level and bioreactor level results.

Influence of the biological carbon pump effect on the sources and deposition of organic matter in Fuxian Lake, a deep oligotrophic lake in southwest China

Biological carbon pumping (BCP) is a key process in which dissolved inorganic carbon in terrestrial aquatic ecosystems is utilized by aquatic autotrophs for photosynthesis and transformed into autochthonous organic matter (AOC). However, the mechanisms underlying BCP and the amount of generated AOC deposited effectively, are still poorly understood. Therefore, we conducted a systematic study combining modern hydrochemical monitoring and a sediment trap experiment in Fuxian Lake (Yunnan, SW China), the second-deepest plateau, oligotrophic freshwater lake in China. Temperature, pH, EC (electrical conductivity), DO (dissolved O2), [HCO3−], [Ca2+], SIc, partial CO2 (pCO2) pressure, and carbon isotopic compositions of HCO3− (δ13CDIC) in water from Fuxian Lake all displayed distinct seasonal and vertical variations. This was especially apparent in an inverse correlation between pCO2 and DO, indicating that variations of hydrochemistry in the lake water were mainly controlled by the metabolism of the aquatic phototrophs. Furthermore, the lowest C/N ratios and highest δ13Corg were recorded in the trap sediments. Analyses of the C/N ratio demonstrated that the proportions of AOC ranged from 30% to 100% of all OC, indicating that AOC was an important contributor to sedimentary organic matter (OC). It was calculated that the AOC flux in Fuxian Lake was 20.43 t C km−2 in 2017. Therefore, AOC produced by carbonate weathering and aquatic photosynthesis could potentially be a significant carbon sink and may have an important contribution to solving the lack of carbon sinks in the global carbon cycle.

Positive cerium anomaly in the Doushantuo cap carbonates from the Yangtze platform, South China: Implications for intermediate water column manganous conditions in the aftermath of the Marinoan glaciation

Oxygenation exerted an important control on the emergence and diversification of metazoans in the aftermath of the Marinoan glaciation. However, the relationship between the oceanic dissolved O2 (DO) levels and early metazoan evolution remains equivocal. To provide a temporal and spatial reconstruction of the redox conditions for this critical interval, this study investigates the early Ediacaran cap carbonate deposits within intra shelf facies at three localities in the Three Gorges area, the Yangtze platform, South China. Trace and rare earth elements are sequentially extracted from the carbonate fraction for pristine temporal seawater signals. The Member II dolostones of the Duoshantuo Formation, which overlies the cap carbonates, show a negative Ce anomaly. No Ce anomaly is observed in the lower units of the cap carbonates. In contrast, positive Ce anomalies (Ce/Ce* > 1.3) in the uppermost part of the cap carbonates are observed in all studied sections. These positive Ce anomalies are accompanied by high Mn/Fe ratios (ranging from 5.04 to 14.96) and slightly positive MREE anomalies, suggesting that a Fe-Mn-(oxyhydro) oxide coparticipation occurred during the cap carbonate deposition. The positive Ce anomalies may result from the reductive dissolution of the Ce-enriched Mn-(oxyhydro) oxides across a Mn(IV)/Mn(II) redoxcline, in a distinct manganous water wedge that was sandwiched between well-oxygenated and anoxic ferruginous deep water layers. This wedge may have exhibited low oxygen conditions, with approximately 10 µM DO (dissolved oxygen), in comparison to the >90 µM DO expected in an oxic setting and 0 µM DO in an anoxic setting. Therefore, the presence of a positive Ce anomaly in the uppermost part of the cap carbonates indicates a new intermediate manganous condition and further elucidates the water column redox structure that existed following the end of the Marinoan glaciation. Early Ediacaran metazoans were possibly restricted to fully oxygenated conditions and were absent in the manganous or ferruginous-rich environments.

Temporal dynamics of dissolved inorganic nitrogen (DIN) in the aphotic layer of a coastal upwelling system with variable dissolved oxygen

Abstract Dissolved O2 (DO) concentration is critical to determining ecosystem functions such as organic matter respiration, which can favor fixed nitrogen loss and the accumulation of compounds such as NH4+. This dynamic is observed in central Chile's coastal upwelling system (36 °S), which presents seasonally O2 deficient waters and high biological productivity. Temporal dynamics for dissolved inorganic nitrogen (DIN: NO3−, NO2− and NH4+) are analyzed based on a ten year time series of monthly measurements of DO and DIN and a three year record of absolute DIN uptake rates (ρDIN), respective turnover rates (νDIN), and O2 utilization rates (OUR). Observed O2 deficit gradually increases from hypoxia to near anoxia as the system becomes more productive, favoring the accumulation of NO2− and NH4+. Three temporal phases within the aphotic layer were distinguished: (I) DO > 62 μmol L−1 (May to August), (II) 5 820 days). Integrated OUR gradually increased from phase I to III (from 225 to 422 mmol m−2 d−1), with DO pools replenished over 2.3 to 26 days. NH4+ regeneration rates ranged from 34 to 62 mmol m−2 d−1 and NH4+ pools were replenished within a few days. Variation in DO, which regulates N cycling, may explain the accumulation of N-species within the aphotic layer. Observed trends could be extrapolated to scenarios of upwelling-favorable winds, eutrophication and hypoxia.

Lethal and sublethal effects of novel terrestrial subsidies from an invasive shrub (Lonicera maackii) on stream macroinvertebrates

The biology of headwater streams is intimately linked to that of the surrounding terrestrial environment through organic matter subsidies. Lonicera maackii, an invasive shrub that is becoming abundant in headwater stream riparian areas, deposits substantial quantities of organic matter into the aquatic system. This organic material has allelopathic effects on terrestrial plants and insects, and a growing body of work suggests strong connections between L. maackii invasion and aquatic biota. Lonicera maackii deposits fruit and flowers in quantities and timings that are unique, and we tested the hypothesis that these subsidies would negatively affect survival and growth of laboratory-cultured Hyalella azteca and field-collected Anthopotamus verticis and Allocapnia spp. Invertebrates were exposed to a gradient of fruit (reference sediment + 0, 0.31, 0.62, 1.25, or 2.5 g dry mass [DM]) and flower (reference sediment + 0, 0.30, 0.60, 1.2, or 2.4 g DM) biomass in laboratory and field sediment exposure tests. Hyalella azteca survival was significantly reduced by exposure to L. maackii fruit in the laboratory and in the field exposures, and a negative effect was observed for A. verticis (p< 0.05). Lonicera maackii flower biomass was associated with negative effects on survival of H. azteca in the field and laboratory exposures and of A. verticis in the laboratory exposure. During the laboratory exposures, dissolved O2 (DO) and pH were <2 mg/L and 5.5, respectively. In the field exposures, DO and pH were comparable to stream conditions during fruit exposures, declining significantly with increasing flower biomass. Our results suggest that L. maackii fruit and flowers, novel subsidies in these systems, can negatively affect benthic organism survival and growth. Research focused on verifying this novel subsidy hypothesis for L. maackii and other species could enhance our understanding of invasion biology and terrestrial–aquatic linkages.

Coupled control of land uses and aquatic biological processes on the diurnal hydrochemical variations in the five ponds at the Shawan Karst Test Site, China: Implications for the carbonate weathering-related carbon sink

High-resolution hydrochemical data from five spring-fed ponds are presented to demonstrate the effect of different land uses and aquatic biological processes on the carbon cycle at a karst-analog test site. The results show that hydrochemical parameters including pH and the concentrations of HCO3−, Ca2+, NO3−, partial pressures of CO2 (pCO2) and dissolved O2 (DO) as well as carbon isotopic compositions (δ13C) of HCO3– in the pond water all displayed distinct diurnal variations, while those of the spring water itself were rather stable. The coupled dynamic behaviors of pCO2, DO and NO3− indicate a significant influence from the metabolism of submerged plants in the ponds. In the afternoon, when photosynthesis is the strongest, the pCO2 of the five pond waters was lower even than that of the ambient atmosphere, demonstrating the existence of a “biological carbon pumping (BCP) effect”, similar to that in the oceans. It was determined that, in October (autumn), the BCP fluxes in the five spring-fed ponds were 156±51tCkm−2a−1 in P1 (Pond 1 – adjoining a bare rock shore), 239±83tCkm−2a−1 in P2 (adjoining uncultivated soil), 414±139tCkm−2a−1 in P3 (adjoining land cultivated with corn), 493±165tCkm−2a−1 in P4 (adjoining grassland) and 399±124tCkm−2a−1 of P5 (adjoining brushland), indicating the potentially significant role of aquatic photosynthesis in stabilizing the carbonate weathering-related carbon sink. In addition, by comparing the DIC concentrations and fluxes of DIC transformed into autochthonous organic matter (AOC) in the five ponds, the so-called “DIC fertilization effect” was found in which more AOC is produced in pond waters with higher concentrations of DIC. This implies that the carbon cycle driven by aquatic biological processes can be regulated by changing land use and cover, the latter determining the DIC concentrations. Further, the rock weathering-related carbon sink is underestimated if one only considers the DIC component in surface waters instead of both DIC and AOC.

Environmental flow assessment in Andean rivers of Ecuador, case study: Chanlud and El Labrado dams in the Machángara River

The environmental flow is a management tool to mitigate the impact of a dam on ecosystems. The Ecuadorian law requires that the environmental flow remains 10% of mean annual flow in old hydroelectric stations, however advances on this area suggest this is not adequate. The objective of this research was to assess the impacts of a 10% release and establish environmental flow recommendations in the Machángara and Chulco rivers which have been dammed by the Chanlud and El Labrado hydroelectric plants, in the watershed of the Machángara River (southern Ecuador). During twelve months physical and chemical parameters, and aquatic macro-invertebrates were recorded. The analysis found significant differences in some parameters, indicating a decline in ecosystem condition and relations were found between the flow, the diversity of macro-invertebrates and the concentration of dissolved O2 (DO). However, determining whether these impacts are unacceptable is difficult because Ecuador does not have established criteria for required ecological condition. This deterioration in ecological conditions can be minimized by applying more modern methods of environmental flow assessment such as the basic flow methodology (BFM) that allows the variability of the river flow. In the Machángara River (3000–4000ma.s.l.) an environmental flow of 27–51% of the mean annual flow for the rainy season and 29–42% in the dry season were determined by using BFM. Whereas for the Chulco River (3000–4000ma.s.l.) the environmental flow was 15–45% of the mean annual flow for the rainy season and 15–36% for the dry season.

Pyrite oxidation in the presence of hematite and alumina: I. Batch leaching experiments and kinetic modeling calculations.

Pyrite is one of the most common and geochemically important sulfide minerals in nature because of its role in the redox recycling of iron (Fe). It is also the primary cause of acid mine drainage (AMD) that is considered as a serious and widespread problem facing the mining and mineral processing industries. In the environment, pyrite oxidation occurs in the presence of ubiquitous metal oxides, but the roles that they play in this process remain largely unknown. This study evaluates the effects of hematite (α-Fe2O3) and alumina (α-Al2O3) on pyrite oxidation by batch-reactor type experiments, surface-sensitive characterization of the oxidation layer and thermodynamic/kinetic modeling calculations. In the presence of hematite, dissolved sulfur (S) concentration dramatically decreased independent of the pH, and the formation of intermediate sulfoxy anionic species on the surface of pyrite was retarded. These results indicate that hematite minimized the overall extent of pyrite oxidation, but the kinetic model could not explain how this suppression occurred. In contrast, pyrite oxidation was enhanced in the alumina suspension as suggested by the higher dissolved S concentration and stronger infrared (IR) absorption bands of surface-bound oxidation products. Based on the kinetic model, alumina enhanced the oxidative dissolution of pyrite because of its strong acid buffering capacity, which increased the suspension pH. The higher pH values increased the oxidation of Fe2+ to Fe3+ by dissolved O2 (DO) that enhanced the overall oxidative dissolution kinetics of pyrite.

Land use affects temporal variation in stream metabolism

AbstractStream metabolism (gross primary production and ecosystem respiration) is increasingly used to assess waterway health because mean values are responsive to spatial variation in land use, but little is known about how human land use influences the temporal variability of stream metabolism. We investigated daily variation in dissolved O2 (DO) concentrations and calculated mean and within-season variation in gross primary production (GPP) and ecosystem respiration (ER) rates at 13 stream sites across a landuse intensity gradient in the Auckland region, New Zealand, over 9 y. Based on generalized linear mixed models, mean daily GPP (0.1–12.6 g O2 m−2 d−1) and ER (1.8–29.6 g O2 m−2 d−1) and seasonal variation in stream metabolism were significantly related to landuse intensity with higher variability associated with higher values of a landuse stress score. Overall, mean daily rates and day-to-day variation in GPP and ER were greatest in summer and least in winter. We recommend summer monitoring over a ...

Seasonal variability of the oxygen minimum zone off Peru in a high-resolution regional coupled model

Abstract. In addition to being one of the most productive upwelling systems, the oceanic region off Peru is embedded in one of the most extensive oxygen minimum zones (OMZs) of the world ocean. The dynamics of the OMZ off Peru remain uncertain, partly due to the scarcity of data and to the ubiquitous role of mesoscale activity on the circulation and biogeochemistry. Here we use a high-resolution coupled physical/biogeochemical model simulation to investigate the seasonal variability of the OMZ off Peru. The focus is on characterizing the seasonal cycle in dissolved O2 (DO) eddy flux at the OMZ boundaries, including the coastal domain, viewed here as the eastern boundary of the OMZ, considering that the mean DO eddy flux in these zones has a significant contribution to the total DO flux. The results indicate that the seasonal variations of the OMZ can be interpreted as resulting from the seasonal modulation of the mesoscale activity. Along the coast, despite the increased seasonal low DO water upwelling, the DO peaks homogeneously over the water column and within the Peru Undercurrent (PUC) in austral winter, which results from mixing associated with the increase in both the intraseasonal wind variability and baroclinic instability of the PUC. The coastal ocean acts therefore as a source of DO in austral winter for the OMZ core, through eddy-induced offshore transport that is also shown to peak in austral winter. In the open ocean, the OMZ can be divided vertically into two zones: an upper zone above 400 m, where the mean DO eddy flux is larger on average than the mean seasonal DO flux and varies seasonally, and a lower part, where the mean seasonal DO flux exhibits vertical–zonal propagating features that share similar characteristics than those of the energy flux associated with the annual extratropical Rossby waves. At the OMZ meridional boundaries where the mean DO eddy flux is large, the DO eddy flux has also a marked seasonal cycle that peaks in austral winter (spring) at the northern (southern) boundary. In the model, the amplitude of the seasonal cycle is 70 % larger at the southern boundary than at the northern boundary. Our results suggest the existence of distinct seasonal regimes for the ventilation of the OMZ by eddies at its boundaries. Implications for understanding the OMZ variability at longer timescales are discussed.