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Due to the highly reductive capacity of nano zero-valent iron (nZVI) nanoparticles, the reduction of nitrate (NO3−-N) is prone to produce ammonia nitrogen (NH4+-N) as a by-product and has low selectivity for nitrogen gas (N2). Water and dissolved oxygen (DO) in the solution consume electrons from nZVI, decreasing the efficiency of NO3−-N reduction. In order to overcome the drawbacks of plain nZVI being used to remove NO3−-N pollution, nZVI-based multifunctional materials have been constructed to realize the selective conversion of NO3−-N to N2 as well as the efficient removal of NO3−-N. Therefore, advanced research on the reduction of NO3−-N by nZVI-based composites has been comprehensively reviewed. Strategies to improve NO3−-N reduction efficiency and N2 selectivity are proposed. Moreover, the shortcomings of iron-based nanomaterials in NO3−-N pollution control have been summarized, and some suggestions for future research directions provided. [ABSTRACT FROM AUTHOR]

Using the method of gravimetric and electrochemical measurements, the corrosion rate of St3 steel in the water basins of the Takhtakerpi and Shemkirchay hydroelectric power stations has been determined. To establish the expected corrosivity of water, the chemical composition and some physicochemical parameters of the above waters have been determined. As a result of long-term measurements carried out over one year, it has established that steel in the water basins of the Takhtakerpi and Shemkirchay hydroelectric power stations corrodes at a rate of 0.04 g/m² h and (0.07÷0.08) g/m² h, respectively. And in the atmosphere of these hydroelectric power plants, steel corrodes at a low rate, approximately an order of magnitude lower. Analysis of water taken from these hydroelectric power stations showed a very low content of corrosive ions, the concentration of which, especially in the Shemkirchay reservoir, corresponds to spring water. By taking polarization curves it has been established that the rate of steel corrosion is controlled by the concentration of dissolved oxygen in water. [ABSTRACT FROM AUTHOR]

This study aimed to assess the trophic status of Pegametan Bay, Bali, Indonesia, as influenced by floating net cage aquaculture activities during June 2019 (dry season) and March 2021 (wet season). The trophic status was assessed using 24 parameters developed from a literature review, and water samples were collected from four observation stations. Data were analyzed using field measurements and laboratory analyses. The results showed that dissolved oxygen ranged between 2.6–9.1 mg/l in June 2019 and 3.4–7.7 mg/l during the March 2021 measurement period, salinity (33.6–34.4 ppt and 31–34 ppt), turbidity (0.77–6.33 FNU and 1.2–3.42 FNU), water clarity (4.18–11.92 m and 4.44–8.44 m), Biological Oxygen Demand (10.5–26 mg/l and 1–17 mg/l), Chemical Oxygen Demand (3.68–11 mg/l and 15.5–46.5 mg/l), chlorophyll-a (0.11–0.24 mg/l and 0–0.56 mg/l), Total Dissolved Solids (20,400–22200 mg/l and 21,300–53400 mg/l), ammonia (0.038–0.178 mg/l and 0.01–0.3 mg/l), nitrate (0.02–0.706 mg/l and 0.01–0.03 mg/l), orthophosphate (0.014–0.174 mg/l and 0.04–1.59 mg/l, and culturable bacteria (0–1.74 ∙ 105 CFU/mL and 0–1.61 ∙ 108 CFU/mL). Pegametan Bay exhibited a significant decrease in water quality between June 2019 and March 2021, with mesotrophic and eutrophic trophic states. The parameters determining changes in the trophic state of Pegametan Bay were orthophosphate, TSS, organic content, E. coli, Salmonella, and Vibrio. This trophic state assessment can be used for early detection, monitoring, water quality control, and eutrophication control, thereby ensuring sustainable aquaculture activities and optimal marine resource production. [ABSTRACT FROM AUTHOR]

The golden mussel (Limnoperna fortunei) is an invasive bivalve that has established itself in several South American river systems, impacting ecosystem functioning. Reservoir cascades provide their larvae with the means of rapid dispersal, but the relationship between environmental variables and larval stage structure remains unclear. In this study, the density of three L. fortunei larval stages and quantitative detection using DNA are analyzed in a cascade of five reservoirs in the upper Uruguay River Basin and associated with spatiotemporal variation in environmental parameters. The analysis of L. fortunei eDNA presence and absence in freshwater systems appears to be a valuable mapping tool; however, no significant link was found between the eDNA magnitude and the overall larval density. The increase in larval density was related to the fluctuation of environmental parameters over a year, with the highest average larval densities observed in the CN and ITA reservoirs, though no significant difference was observed between the five reservoirs, where D-shaped larvae predominated. During winter, larval density decreased significantly, however, other variables also contribute to species activity and development in the upper Uruguay River Basin reservoirs and may be considered limiting factors. The relationships between environmental parameters were evaluated using a multivariate model. The interaction between reservoir area and precipitation, water temperature, electrical conductivity, and dissolved oxygen had a significant effect on larval density but showed specific influences on each larval stage. Any increase in density was regulated by dissolved oxygen and electrical conductivity content at all larval stages. Furthermore, total phosphorus affected the density of F1 and F3 larvae. The interaction between reservoir area and precipitation, nitrate content, phosphate concentrations, and water temperature had the most influence on the density of F2 and F3 larval stages; the F1 stage was mainly affected by calcium concentrations. The isolated effect of precipitation also contributed to the density of F2 and F3 larvae. Our findings shed light on the interaction between different phases of golden mussel larvae and the main nutrients found in reservoirs, which may be a determining factor in the rise in density of the non-native species in these systems. [ABSTRACT FROM AUTHOR]

The escalating demand for water and the increasing pollution of natural water bodies necessitate innovative solutions for wastewater treatment and reuse. This study investigated the potential of a horizontal subsurface flow (HSSF) constructed wetland to treat aquaculture wastewater for reuse. The system, planted with Taro (Colocasia esculenta) and sugarcane (Saccharum officinarum L.), received effluent from a recirculating aquaculture system (RAS) producing African Catfish (Clarias gariepinus). The study assessed the impact of varying hydraulic retention times (1-3 days) and flow rates (11-108 L/min) on water quality parameters, including dissolved oxygen, electrical conductivity, salinity, total dissolved solids, temperature, and pH. Results showed significant increase in dissolved oxygen (4.25-5.52 mg/L), while electrical conductivity (491-677 µS/cm), salinity (0.23-0.32 ppt), and total dissolved solids (237-332 mg/L) decreased considerably. Temperature (29.28-31.07 °C) and pH (7.57-7.59) remained stable and within acceptable ranges for reuse in African Catfish production. However, retention time and flow rate did not significantly affect treatment efficiency within the tested parameters. Further research is recommended to explore the impact of longer retention times, wider flow rate ranges, different plant species and substrate types, and microbial community analysis to optimize the system's performance and promote sustainable aquaculture practices. [ABSTRACT FROM AUTHOR]

Chlorophyceae is a class of plankton commonly discovered in aquaculture ponds in tropical aquatic regions. Therefore, this study aimed to analyze the abundance levels of Chlorophyceae in pen-culture ponds during the dry season using a dynamic modeling system. The method adopted was a causal ex-post facto design, and the observed parameters were water quality and plankton abundance. The results showed the presence of various plankton classes such as Chrysophyceae, Chyanophyceae, Chlorophyceae, Dynophyceae, and Oligotrichea, as well as the discovery of 17 plankton genera. Water quality parameters identified Total Dissolved Solids (TDS) concentrations, dissolved oxygen levels, temperature, pH levels, and salinity ranging from 3.06–7.37 mg·L-1, 4.39–6.30 mg·L-1, 26.21–31.17 °C, 7.7–8.1, and 10–25 ppt, respectively. The average plankton dominance index (D’) showed Chlorophyceae dominance at 4.55, followed by Chyanophyceae, Chrysophyceae, Dynophyceae, and Oligotrichea at 1.78, 0.08, 0.07, and 0.01, respectively, falling in the index category of 0.5 < D < 1. Furthermore, the average plankton abundance dominance level (pi) signified Chlorophyceae dominance at 7.11E-01, followed by Chyanophyceae, Chrysophyceae, Dynophyceae, and Oligotrichea at 4.68E-01, 1.30E-01, 8.05E-02, and 3.96–02, respectively. This implied that in pen-culture water, Chlorophyceae significantly dominated over other classes. Dynamic modeling results showed the abundance of Chlorophyceae in ponds A, C, and D to be relatively uniform (± 5.00E-02 cell/ ml) compared to pond B (± 0.10E-00 cell/ml). It was important to acknowledge that as Chlorophyceae dominance increased (0.10E-00–5.0E-02 cell/ml), the total plankton abundance in the water decreased contradictorily (0.01E00–100E-04 cell/ml) due to over-dominance. On the basis of the modeling description, Chlorophyceae sustained for 20 weeks during the culture period, with optimal growth observed from the 1st to the 5th week. In conclusion, this plankton class experienced dynamic fluctuations during the pen-culture cultivation period, and the life cycle stability was dependent on water quality and temperature. [ABSTRACT FROM AUTHOR]

Environmental contamination by potentially toxic elements (PTEs) poses a significant challenge, particularly in the metropolitan regions of developing countries. This issue arises from the high levels of pollution driven by industrial growth and the increased traffic from fossil fuel-powered vehicles. Even after the wastewater treatment in treatment plants, PTEs often persist, posing risks to stream structure and function. This form of pollution is persistent, long-term, and irreversible, presenting a significant challenge in terms of freshwater conservation. This study aimed to assess the water quality and PTE concentrations in urban streams in Santo André, SP, Brazil, to identify the PTEs relevant to stream pollution. We analyzed the water quality in seven catchments in the Santo André municipality, in the metropolitan region of São Paulo, Brazil. The samples were collected during the dry (2021) and rainy periods (2022), and the concentrations of potentially toxic elements (PTEs) were analyzed via inductively coupled plasma–mass spectrometry (ICP-MS). The results showed elevated electrical conductivity (429 ± 211 μS·cm) and low dissolved oxygen concentrations in the streams (2.3 ± 0.95 μg·L), indicating potential problems such as eutrophication and toxicity to aquatic organisms. PTE concentrations, particularly those of Mn (30.8 ± 22.3 μg·L), Fe (91.1 ± 72.1 μg·L), and Zn (38.1 ± 28.7 μg·L), were among the highest concentrations. Seasonal variations affected the PTE concentrations, with Cr and Fe predominating during the dry season and Zn increasing during the rainy season. Associations were found between the PTE concentrations and the water pH, indicating the importance of continuous monitoring and remediation efforts. [ABSTRACT FROM AUTHOR]

This study highlights the importance of managing fishpond effluent to minimize its impact on aquatic environments. Fish effluents are a major concern worldwide in aquatic environments. It also provides information on the impacts of effluents on the water quality index of streams when released into aquatic environments. Streams and fishponds were sampled bimonthly for two annual cycles, covering different sampling stations both upstream and downstream of the fishpond effluents at 50-m intervals. Water quality parameters associated with in situ conditions were determined in the field. In the laboratory, other physicochemical parameters were determined via proper standard protocols. The results of the study revealed that water quality parameters such as turbidity, total suspended solids, total solids, conductivity, total dissolved solids, dissolved oxygen, biological oxygen demand and organic matter recorded in water samples collected from 50 m upstream at the point of effluent discharge were significantly (p < 0.05) different from those recorded at other sampling stations. The WQI value of the discharge point was higher than the WQI recorded in both the upstream and downstream sections of the stream. The findings of this study showed that the point of discharge of fishpond effluents is particularly concerning for the receiving stream compared with other sampling points. This localized area has experienced marked deterioration in water quality, posing significant threats to the health and biodiversity of aquatic ecosystems. The effluent's impact, though slight when dispersed in the receiving waterbody, underscores the necessity for adequate management practices to mitigate its adverse effects. Effective strategies, including regular monitoring, effluent treatment, and sustainable pond management, are imperative to ensure the long-term health and resilience of the aquatic environment. [ABSTRACT FROM AUTHOR]

Dissolved oxygen concentration is one of the important indicators of water quality. In order to achieve low-cost and stable measurement of dissolved oxygen in water, this paper reported a single-optical-path dissolved oxygen sensor based upon quenching of single optical path fluorescence. A sol-gel method was used to prepare an oxygen sensitive membrane with tetraethyl orthosilicate (TEOS) and triethoxy(octyl)silane (octyl-triEOS) as the precursor embedded with tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride (Ru(dpp)3Cl2) to optimize the nitrogen-oxygen quenching ratio and improve the performance of the sensor. Optical stability experiments were also conducted. A single-optical-path structure was adopted to reduce complexity, and the fluorescence intensity was calculated using the fast Fourier transform (FFT). The number of calculations were effectively reduced by measuring and storing the fluorescence intensity at 0 mg/L in advance. Calibration experiments were performed on the sensor, and the oxygen sensitive membrane with the highest sensitivity was prepared by the control variable method. The results showed that the fluorescence dyes in the prepared oxygen sensitive membrane were uniformly dispersed, and a fitted Stern–Volmer equation was obtained with a linearity of 0.994 and a sensitivity of 0.115; moreover, the light stability was good. Performance testing and anti-interference experiments were conducted, and the results showed that the standard deviation was 0.076 mg/L and the relative standard deviation was 0.805%. The errors due to changes in salinity and pH were both less than 0.25 mg/L. The sensor offers high accuracy and anti-interference ability, and is suitable for water quality monitoring and marine science. [ABSTRACT FROM AUTHOR]

The production of oxygen through phytoplankton photosynthesis is a crucial phenomenon in the dynamics of marine ecosystems. A generic oxygen-phytoplankton interaction model is considered to comprehend its underlying mechanism. This paper investigates the discrete-time dynamics of oxygen and phytoplankton in aquatic ecosystems, incorporating factors that cause phytoplankton mortality due to external influences. We explore the conditions for the local stability of steady states concerning the oxygen content in dissolved water and phytoplankton density. The analysis reveals that the model undergoes a co-dimension one bifurcation, encompassing flip and Neimark–Sacker bifurcations, utilizing the center manifold theorem and bifurcation theory. To manage the chaos resulting from the Neimark–Sacker bifurcation, we apply the OGY feedback control method and a hybrid control methodology. Finally, we present numerical simulations to validate the theoretical discussion. [ABSTRACT FROM AUTHOR]

Plant litter imported from forest headwater streams is an important supply of material and energy to the downstream aquatic ecosystems, affecting the storages and fluxes of carbon and nutrients of the whole forest ecosystems. However, our knowledge on the dynamics of plant litter potassium (K) storage is still limited. Here, we monitored the spatial and temporal characteristics of plant litter K concentration and storage in a subtropical forest headwater stream for an entire year. Results showed that: (1) the annual average concentration and storage of K in plant litter were 1.4 mg/kg and 19.6 mg/m2, respectively, with an overall decreasing trend throughout the year and from the source of the stream to the mouth; (2) riparian forest type (broadleaved vs. mixed forests) and the presence of a tributary showed significant effects on twig K concentration and storage, but did not affect those of other types of plant litter (total, fine woody debris, leaf, reproductive part, and twig); (3) water depth and active channel width positively correlated with litter K storage, while stream gradient, water dissolved oxygen, and alkalinity negatively corrected to litter K storage. Our results clearly demonstrated the spatiotemporal dynamics of plant litter K storage in the subtropical forest headwater stream, which provides basic data and scientific basis for an in-depth understanding of nutrient fluxes and cycling along with ecohydrological processes in subtropical forest ecosystems. [ABSTRACT FROM AUTHOR]

In this study, Z-type ZnIn 2 S 4 @CuO composites with a core–shell structure were synthesized. Under visible light irradiation, ZnIn 2 S 4 @CuO generates electron-hole pairs, and the presence of an internal electric field (IEF) improves the charge transfer mechanism of the composites and promotes the separation of charge carriers. The electrons on the conduction band of ZnIn 2 S 4 combine with dissolved oxygen in water to generate •O 2 –, and the holes on the valence band of CuO combine with OH– and H 2 O in water to generate •OH, and the generated reactive substances attack atoms on the TC to cause its ring-opening degradation, and ultimately mineralize the TC into CO 2 and H 2 O. [Display omitted] • A novel method for efficient construction of binary composites is proposed. • Z-type heterostructure reduces the photogenerated carrier complexation efficiency. • Verification of the Z-structure charge transfer mechanism. • TC degradation pathways proposed in combination with HR-MC and FEDs. In this study, copper oxide (CuO) was prepared by the microwave-assisted hydrothermal technique subsequently, CuO was grown in situ onto different rare metal compounds to prepare Z-scheme heterojunctions to improve the degradation efficiency of tetracycline (TC) in water environments. Various characterization proved the successful synthesis of all composite materials, and the formation of tight heterojunction interfaces, among which, the core–shell structure ZnIn 2 S 4 @CuO exhibited excellent photocatalytic degradation capability. Research results indicated that the degradation efficiency of ZnIn 2 S 4 @CuO for TC (50 mg/L) in the water environment reached 95.8 %, and the degradation rate is 2.41 times and 12.93 times that of CuO and ZnIn 2 S 4 alone, respectively, the reason is because of the introduction of ZnIn 2 S 4 , Z-scheme heterojunction structures and internal electric field (IEF) is constructed and formed to extend the visible light response range of photocatalysts to improve electron-hole separation efficiency, and enhance charge transfer. In addition, ZnIn 2 S 4 @CuO-2 exhibited good stability and reproducibility, with no significant loss of activity after five cycles. Finally, the precise locations of free radical attack on TC were investigated by the combined use of high-resolution mass spectrometry (HR-MC) and frontier electron densities (FEDs), and a reasonable degradation pathway was provided. The results of this research provide a new and viable approach to overcome the limitations of conventional photocatalytic materials in terms of limited visible light absorption range and fast carrier recombination rates, which offers promising prospects for a wide range of applications in the field of wastewater purification. [ABSTRACT FROM AUTHOR]

Sea level rise (SLR) is a crucial indicator of climate change, primarily driven by greenhouse gas emissions and the subsequent increase in global temperatures. The impact of SLR, however, varies regionally due to factors such as ocean bathymetry, resulting in distinct shifts across different areas compared to the global average. Understanding the complex factors influencing SLR across diverse spatial scales, along with the associated uncertainties, is essential. This study focuses on the East Coast of the United States and Gulf of Mexico, utilizing historical SLR data from 1993 to 2023. To forecast SLR trends from 2024 to 2103, a weighted ensemble model comprising SARIMAX, LSTM, and exponential smoothing models was employed. Additionally, using historical greenhouse gas data, an ensemble of LSTM models was used to predict real-time SLR values, achieving a testing loss of 0.005. Furthermore, conductance and dissolved oxygen (DO) values were assessed for the entire forecasting period, leveraging forecasted SLR trends to evaluate the impacts on marine life, agriculture, and infrastructure. [ABSTRACT FROM AUTHOR]

Urban sprawl poses a significant threat to urban stream water quality due to impermeabilization, reduced vegetation cover, and the release of diffuse pollutants. This study evaluates water quality in seven catchments in Santo André, SP, considering seasonality. Nutrient concentrations and in situ measurements were taken during both dry and rainy seasons. Comparisons were made using Kruskal–Wallis and Mann–Whitney tests. Streams showed significant differences in relation to water quality parameters. The Carapetuba, Jundiaí, and Apiaí streams were most adversely affected, underscoring the need for urgent water quality intervention (water conductivity above 500 μS/cm, dissolved oxygen below 2 mg/L, total dissolved carbon above 50 mg/L, and total dissolved nitrogen above 25 mg/L). Significant differences were observed across seasons. The dry season showed elevated temperatures (above 25 °C) and increased total dissolved carbon (above 50 mg/L) and nitrogen concentrations (above 30 mg/L), indicating reduced dilution effects from rainfall and heightened organic contamination. Conversely, the wet season demonstrated lower nutrient concentrations, emphasizing seasonal dynamics. Sustained, long-term monitoring of urban streams in Santo André and the implementation of sewage collection and treatment in irregular settlements are recommended. These measures are essential to mitigate the adverse impacts of urban expansion on water quality. [ABSTRACT FROM AUTHOR]

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

Unearthing the effects of surface water and groundwater interactions on subsurface biogeochemical reactions is crucial for developing a more mechanistic understanding of carbon and energy flow in aquifer ecosystems. To examine physiological characteristics across groundwater microbial communities that experience varying degrees of interaction with surface waters, we investigated 10 springs and a river sink and rise system in north central Florida that discharge from and/or mix with the karstic upper Floridan aquifer (UFA). Groundwater with longer residence times in the aquifer had lower concentrations of dissolved oxygen, dissolved and particulate organic carbon, and microbial biomass, as well as the lowest rates of respiration (0.102 to 0.189 mgO2L-1d-1) and heterotrophic production (198 to 576 µgCL-1d-1). Despite these features, oligotrophic UFA groundwater (< 0.5 mgCL-1) contained bioavailable organic matter that supported doubling times (14 to 62 h) and cell-specific production rates (0.0485 to 0.261 pmolC per cell per hour) comparable to those observed for surface waters (17 to 20 h ; 0.105 to 0.124 pmolC per cell per hour). The relatively high specific rates of dissimilatory and assimilatory metabolism indicate a subsurface source of labile carbon to the groundwater (e.g., secondary production and/or chemoautotrophy). Our results link variations in UFA hydrobiogeochemistry to the physiology of its groundwater communities, providing a basis to develop new hypotheses related to microbial carbon cycling, trophic hierarchy, and processes generating bioavailable organic matter in karstic aquifer ecosystems. [ABSTRACT FROM AUTHOR]

Electrocatalytic activities for the oxygen reduction reaction (ORR) of Au electrodes modified by as prepared and size selected (0.45–1.0, 0.22–0.45, and 0.1–0.22 µm) h-BN nanosheet (BNNS), which is an insulator, were examined in O2 saturated 0.5M H2SO4 solution. The overpotential was reduced by all the BNNS modifications, and the smaller the size, the smaller the overpotential for ORR, i.e., the larger the ORR activity, in this size range. The overpotential was reduced by as much as ∼330 mV compared to a bare Au electrode by modifying the Au surface by the BNNS of the smallest size range (0.1–0.22 µm). The overpotential at this electrode was only 80 mV more than that at the Pt electrode. Both the rotation disk electrode experiments with Koutecky–Levich analysis and rotating ring disk electrode measurements showed that more than 80% of oxygen is reduced to water via the four-electron process at this electrode. These results strongly suggest and theoretical density functional theory calculations support that the ORR active sites are located at the edges of BNNS islands adsorbed on Au(111). The decrease in size of BNNS islands results in an effective increase in the number of the catalytically active sites and, hence, in the increase in the catalytic activity of the BNNS/Au(111) system for ORR. [ABSTRACT FROM AUTHOR]

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Introduction: Hypoxia is defined as a critically low-oxygen condition of water, which, if prolonged, can be harmful to fish and many other aquatic species. In the context of ocean salmon fish farming, early detection of hypoxia events is critical for farm managers to mitigate these events to reduce fish stress, however in complex natural systems accurate forecasting tools are limited. The goal of this research is to use a machine learning approach to forecast oxygen concentration and predict hypoxia events in marine net-pen salmon farms. Methods: The developed model is based on gradient boosting and works in two stages. First, we apply auto-regression to build a forecasting model that predicts oxygen concentration levels within a cage. We take a global forecasting approach by building a model using the historical data provided by sensors at several marine fish farms located in eastern Canada. Then, the forecasts are transformed into binary probabilities that indicate the likelihood of a low-oxygen event. We leverage the cumulative distribution function to compute these probabilities. Results and discussion: We tested our model in a case study that included several cages across 14 fish farms. The experiments suggest that the model can detect future hypoxic events with a commercially acceptable false alarm rate. The resulting probabilistic predictions and oxygen concentration forecasts can help salmon farmers to prioritize resources, and reduce harm to crops. [ABSTRACT FROM AUTHOR]

The literature has explored various methods for assessing minimum environmental flow. Implementing holistic approaches proves to be prohibitively expensive and impractical for many small and medium projects. Hence, desktop and cost-effective methods are commonly employed without an integrated decision-making system to justify the assessed values. This study introduces a systematic decision-making framework aimed at selecting the most suitable method for assessing the actual needs of river habitats. Employing a fuzzy technique known as the Order Preference Similarity to the Ideal Solution (FTOPSIS), the study considers factors such as physical, thermal, and dissolved oxygen habitat suitability, maximum habitat area, and water demand loss function to determine the most appropriate method among established ones, including the Tennant method, flow duration curve analysis method, wetted perimeter method, and physical habitat simulation method. The results prioritize physical habitat simulation, wetted perimeter by slope method, and flow indices of 70%, 75%, and 80% by flow duration curve analysis method as the optimal approaches for assessing minimum environmental flow. This proposed decision-making system offers a viable platform to explore the applicability of existing cost-effective methods for assessing minimum environmental flow. It also serves as an effective mechanism for reducing negotiations among stakeholders by comprehensively considering all relevant aspects in the environmental management of river ecosystem requirements. [ABSTRACT FROM AUTHOR]

Levels of dissolved oxygen in open ocean and coastal waters are decreasing (ocean deoxygenation), with poorly understood effects on marine megafauna. All of the more than 1000 species of elasmobranchs (sharks, skates, and rays) are obligate water breathers, with a variety of life‐history strategies and oxygen requirements. This review demonstrates that although many elasmobranchs typically avoid hypoxic water, they also appear capable of withstanding mild to moderate hypoxia with changes in activity, ventilatory responses, alterations to circulatory and hematological parameters, and morphological alterations to gill structures. However, such strategies may be insufficient to withstand severe, progressive, or prolonged hypoxia or anoxia where anaerobic metabolic pathways may be used for limited periods. As water temperatures increase with climate warming, ectothermic elasmobranchs will exhibit elevated metabolic rates and are likely to be less able to tolerate the effects of even mild hypoxia associated with deoxygenation. As a result, sustained hypoxic conditions in warmer coastal or surface‐pelagic waters are likely to lead to shifts in elasmobranch distributions. Mass mortalities of elasmobranchs linked directly to deoxygenation have only rarely been observed but are likely underreported. One key concern is how reductions in habitat volume as a result of expanding hypoxia resulting from deoxygenation will influence interactions between elasmobranchs and industrial fisheries. Catch per unit of effort of threatened pelagic sharks by longline fisheries, for instance, has been shown to be higher above oxygen minimum zones compared to adjacent, normoxic regions, and attributed to vertical habitat compression of sharks overlapping with increased fishing effort. How a compound stressor such as marine heatwaves alters vulnerability to deoxygenation remains an open question. With over a third of elasmobranch species listed as endangered, a priority for conservation and management now lies in understanding and mitigating ocean deoxygenation effects in addition to population declines already occurring from overfishing. [ABSTRACT FROM AUTHOR]

[Objective] The purpose of this paper is to study the optimal operation condition of AAO-MBBR mode of the new rural sewage treatment process and its treatment effect on pollutants. [Method] By comparing the pollutant removal rate under different residence time, different internal circulation ratio and different aerobic tank dissolved oxygen concentration, the optimal operation parameters of the equipment were screened out, and the equipment was set to operate under the optimal operating condition. The treatment effect on various pollutants was analyzed. [Result] The results showed that the improved AAO-MBBR process had the best removal effect when the hydraulic retention time was 8 h, the internal circulation ratio was 300% and the concentration of dissolved oxygen in aerobic pool was 2.5 mg/L. Under optimal operating conditions, the removal rates of SS, COD, BOD, NH3-N and TP of the equipment were 80%, 85%, 91%, 84.5% and 46%, respectively. And the treatment effect is significantly higher than that of other sewage treatment equipment. [Conclusion]The improved AAO-MBBR process has a good effect on pollutant removal in integrated wastewater treatment equipment. [ABSTRACT FROM AUTHOR]

The article investigates the purification and remediation of reverse osmosis concentrate (ROC) using the microalgae Chlorella vulgaris. The study examines the efficacy of Chlorella vulgaris in eliminating heavy metals, nutrients and other substances from ROC. It analyzes the physico-chemical characteristics of wastewater, including alkalinity, dissolved oxygen, potential of hydrogen (pH), total solids and concentrations of heavy metals, before and after treatment with Chlorella vulgaris.

The article assesses the water quality in Panvel Creek in Navi Mumbai, India using geographic information system (GIS). The study examines the pollution status of Panvel Creek and analyzes the physical and chemical properties and changes in water quality, including potential of hydrogen values, conductivity, dissolved oxygen levels, nitrate concentrations and phosphate levels during pre-monsoon, monsoon and post-monsoon.

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A study was carried out in 345 water ecosystems (rivers, streams, ditches, lagoons, ponds, puddles, rockpools, and artificial containers) throughout the autonomous community of Galicia (NW Spain) during different seasons between 2020 and 2023. The results revealed the first detections of Anopheles (Anopheles) petragnani Del Vechio, 1939 (n  = 185) and Culex (Culex) mimeticus Noè, 1899 (n  = 12) in the territory, allowing to update and expand the overall knowledge about their distribution and ecology. The breeding preferences of the species were analyzed considering habitat characteristics (land use, water body type, and degree of insolation), geographical variables (latitude, longitude, and altitude), and physical–chemical water parameters (temperature, pH, dissolved oxygen, electrical conductivity, salinity, total dissolved solids, and turbidity). In addition, the relationship between the occurrence of these species and other mosquitoes present in the study area was discussed. Anopheles petragnani is widely distributed in the region and shows breeding preferences for water bodies of fluvial origin, forest environments, and shaded situations. Culex mimeticus was detected sporadically breeding in a pond and in a river in the south of the region, both in industrial and agricultural land uses, and always exposed to the sun. Although the presence of these species may not currently pose a health risk in the region, it is important not neglecting their study since the adequate characterization of their larval biotopes is relevant regarding vector surveillance and control measures. [ABSTRACT FROM AUTHOR]

Dye is a substance that imparts colour onto textiles, fabrics, paper, leather, etc. and is not altered by washing, heating, and light. Dyes were extracted from plants, animals, and minerals until synthetic dyes came to the market, as synthetic dyes were more stable, readily available, and inexpensive. Despite being extremely important to the economy, they have been among the most significant global polluters. Textile dye industries have become the chief source of water pollution, with their effluents increasing the turbidity of water and reducing photosynthesis and dissolved oxygen levels. This leads to significant damage to aquatic biodiversity, threatening the survival of many species. Synthetic dyes have carcinogenic, mutagenic, and genotoxic effects on animals and human beings, posing a severe health risk. The degradation of dyes is essential for ensuring the sustainability of the environment for future generations. The traditional physicochemical means of dye treatment are not convenient because of the high solubility in water, cost of method utilisation and other disadvantages related to these techniques. To overcome the disadvantages of physicochemical treatment, biological methods or bioremediation can be used as an alternative. The objective of this review article is to study the mechanisms involved in the degradation of textile dyes by bacteria to obtain sustainable, economically and ecologically sound solutions for dye treatment. This paper will explain the various types of natural and synthetic dyes utilised in the textile industry, their chemistry, and how they affect water and soil ecosystems. The treatment of textile dye by various physicochemical methods and their advantages and disadvantages are also discussed. In bioremediation, the microorganisms utilize organic pollutants as a source of food or energy. Bioremediation uses biosorption and enzymatic activity for dye degradation, which does not disturb natural processes and is thus sustainable. The microorganisms secrete crucial extracellular and intracellular enzymes that carry out decolourisation and degradation through a series of events, which include hydrolysis, acidogenesis, and methanogenesis. We will discuss how aerobic and anaerobic microorganisms degrade these textile dyes through the process of biodegradation and bioaugmentation and how this technology provides a clean and eco-friendly method for removing textile dyes. [ABSTRACT FROM AUTHOR]

River water quality could be determined by understanding the capacity of pollutants in a water body. Fuzzy C-Means (FCM) is one of the fuzzy clustering methods for determining river water quality by measuring water quality parameters, that is, dissolved oxygen (DO) and total dissolved solids (TDS). The FCM algorithm is an effective fuzzy clustering algorithm for grouping data but often produces local and inconsistent optimal solutions due to the partition matrix's random initialisation process. Therefore, this study proposes to modify the FCM algorithm to be precise in the partition matrix initialisation process using several distance concepts. The purpose of the proposed algorithm modification is to get more consistent FCM clustering results and minimise stop iterations. The validation process for the clustering results uses the FCM algorithm, and the FCM modification algorithm uses three parameters, namely the Partition Coefficient Index (PCI), Partition Entropy Index (PEI) and Silhouette Score (SS). The experiments were conducted with three replications and using various distance concepts. The results showed that the number of iterations stopped in the FCM algorithm has different values for PCI, PEI, SS, and stop iterations and objective functions in each trial. On the contrary, the FCM modification algorithm has consistent PCI, PEI, and SS values, and the number of iterations stops with fewer iterations. Therefore, the modified algorithm for initialising the partition matrix can be used in the fuzzy C-means clustering algorithm. [ABSTRACT FROM AUTHOR]

The article focuses on the ecological factors affecting zooplankton diversity in the Ravi River and its implications for water quality. Topics include the dominant zooplankton species observed, the impact of environmental parameters like water temperature and turbidity on zooplankton distribution, and the influence of anthropogenic activities on river health and zooplankton diversity.

The article focuses on evaluating the seasonal changes in groundwater quality in the Al Hoceima region of northern Morocco, with an emphasis on physicochemical parameters and indicators of fecal contamination. Topics include seasonal variations in groundwater quality, analysis of physicochemical and bacteriological data, and the impact of environmental factors on water contamination.

The Siberian Arctic is warming rapidly, causing permafrost to thaw and altering the biogeochemistry of aquatic environments, with cascading effects on the coastal and shelf ecosystems of the Arctic Ocean. The Lena River, one of the largest Arctic rivers, drains a catchment dominated by permafrost. Baseline discharge biogeochemistry data is necessary to understand present and future changes in land-to-ocean fluxes. Here, we present a high-frequency, 4.5-year-long dataset from a sampling program of the Lena River's biogeochemistry, spanning April 2018 to August 2022. The dataset comprises 587 sampling events and measurements of various parameters, including water temperature, electrical conductivity, stable oxygen and hydrogen isotopes, dissolved organic carbon concentration and 14C, coloured and fluorescent dissolved organic matter, dissolved inorganic and total nutrients, and dissolved elemental and ion concentrations. Sampling consistency and continuity and data quality were ensured through simple sampling protocols, real-time communication, and collaboration with local and international partners. The data is available as a collection of datasets separated by parameter groups and periods at https://doi.org/10.1594/PANGAEA.913197 (Juhls et al., 2020b). To our knowledge, this dataset provides an unprecedented temporal resolution of an Arctic river's biogeochemistry. This makes it a unique baseline on which future environmental changes, including changes in river hydrology, at temporal scales from precipitation event to seasonal to interannual, can be detected. [ABSTRACT FROM AUTHOR]

Moving bed bioreactors are efficient for the removal of nitrogen effluents by simultaneous nitrification and denitrification (SND), which is particularly challenging for effluents with low C/N ratio due to insufficient carbon available for denitrification. The addition of external organic carbon in such cases increases treatment costs. Endogenous degradation of biomass offers a cost-effective and persistent source of carbon. Internal carbon leads to low denitrification rates and optimizing the biomass content leads to a sustainable solution for treating such effluents. The present study aims to optimize the process of SND using endogenous carbon by investigating the effect of operational parameters like dissolved oxygen, influent ammonium (NH4–N) loading, hydraulic retention time (HRT) and carrier filling ratio (CFR). The synthetic wastewater used in the study does not inherently contain any organic carbon, thus indicating the responsible carbon for denitrification to be of internal origin. Using a novel carrier for multispecies biofilm development, initial NH4–N concentration variation showed an insignificant effect on SND efficiency. At higher loading, nitrite accumulation due to incomplete denitrification was observed. The effect of CFR was investigated in a range of 20–35%, beyond which non-homogeneous mixing was caused in the reactor. Under controlled DO and temperature of about 5 mg/L and 28 ± 2°C, respectively, an optimum filling ratio of 35% was finalised, resulting in an SND efficiency of 97.9% at 22 h HRT for high influent NH4–N concentration of 500 mg/L. Bacteriological species identification revealed the coexistence of nitrogen-fixing, aerobic nitrifying and anaerobic denitrifying bacteria in the biofilm growing on the biocarrier. Mechanism of SND occurring in aerobic moving bed bioreactors: Laboratory-scale aerobic Moving Bed Bioreactor setup for a continuous study evaluating the influence of operational parameters on SND Occurrence of aerobic and anoxic zones in biofilm grown on the surface of novel carriers due to diffusional gradient of dissolved oxygen [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive approach to predicting short-term (for the upcoming 2 weeks) changes in estuarine dissolved oxygen concentrations via machine learning models that integrate historical water sampling, historical and upcoming 2-week meteorological data, and river discharge and discharge metrics. Dissolved oxygen is a critical indicator of ecosystem health, and this approach is implemented for the Neuse River Estuary, North Carolina, U.S.A., which has a long history of hypoxia-related habitat degradation. Through meticulous data preprocessing and feature selection, this research evaluates the predictions of dissolved oxygen concentrations by comparing a recurrent neural network with four other models, including a Multilayer Perceptron, Long Short-Term Memory, Gradient Boosting, and AutoKeras, through sensitivity experiments. The input predictors to our prediction models include water temperature, turbidity, chlorophyll-a, aggregated river discharge, and aggregated wind based on eight directions. By emphasizing the most impactful predictors, we streamlined the model-building processes and built a hindcast system from 2015 to 2019. We found that the recurrent neural network model was most effective in predicting the dissolved oxygen concentrations, with an R2 value of 0.99 at multiple stations. Different from our machine learning hindcast models that used observed upcoming meteorological and discharge data, an actual forecast system would use forecasted meteorological and discharge data. Therefore, an actual operational forecast may have lower accuracy than the hindcast, as determined by the accuracy of the predicted meteorological and discharge data. Nevertheless, our studies enhance our understanding of the factors influencing dissolved oxygen variability and set the basis for the implementation of a predictive tool for environmental monitoring and management. We also emphasized the importance of building station-specific models to improve the prediction results. [ABSTRACT FROM AUTHOR]

Microalgae-bacteria consortium for wastewater treatment has great potential for use in nutrient removal and biodiesel acquisition. In this study, the effect of four different light/dark regimes (6/18 h , 12/12 h , 18/6 h , 24/0 h) on biomass production and pollutant degradation during the treatment of oilfield wastewater by the algae-bacteria consortium was investigated. The maximum algal biomass (0.2 g/L) and total chlorophyll content (812.76 μgmg−1 cell dry weight−1) were obtained due to an appropriate illumination cycle (18 hlight/6 h dark) with enhanced dissolved oxygen content (5.87 mg/L), thus improving bacterial biomass (0.31 g/L). The ratio of microalgae to bacteria was essentially constant under different light/dark cycles. In addition, the highest removal rates of chemical oxygen demand (55.32%), ammonia nitrogen (50.75%), total nitrogen (64.53%), and total phosphorus (65.97%) were also obtained at the 18/6 h cycle, significantly different from the results of other light/dark cycles. The degradation efficiency of ammonia nitrogen by bacteria increased and then decreased with increasing light duration, while the removal rate of ammonia nitrogen by algae showed the opposite trend. The results indicated that the light and dark regimes had an important influence on the growth of the microalgae-bacteria consortium and their performance in treating oilfield wastewater. [ABSTRACT FROM AUTHOR]

The study investigated the impacts of dietary supplementation with two probiotics, PondToss® and LactéolFort®, on the growth performance and biochemical indices of the whiteleg shrimp (Litopenaeus vannamei) in clear water (CW) and biofloc technology (BFT) systems. Six different treatments were applied: T1 (CW without probiotics, control), T2 (BFT without probiotics), T3 (BFT with PondToss® at 0.5g g-1), T4 (BFT with PondToss® at 1g kg-1), T5 (BFT with LactéolFort® at 0.5gkg-1), and T6 (BFT with LactéolFort® at 1g kg-1). Over 90 days, shrimps with initial average weights of 3.9 to 4.10g were fed the experimental diets to satiety twice daily. The study found that neither rearing system nor probiotic treatment affected water temperature or dissolved oxygen (DO) levels. However, the treatments significantly influenced water pH, dissolved nitrogen forms (ammonia, nitrite, and nitrate), total suspended solids (TSS), total alkalinity (as CaCO3), and floc volume (VF). Biofloc and probiotic-enriched diets stimulated growth in terms of weight gain, specific growth rate (SGR), and survival rate. In biofloc conditions, shrimps fed with probiotics showed an increased feed intake and a better feed and protein utilization. Biochemical indices such as albumin, total protein, and globulin levels varied significantly among the groups, with the control group (T1) recording the lowest survival rate (66.89%), while the BFT and probiotic-enriched groups had higher survival rates, ranging from 87.35 (T6) to 90.21% (T2). The findings suggest that probioticsupplemented diets in BFT systems improve water quality, boost shrimp growth, enhance nutrient utilization, and positively affect biochemical characteristics. Thus, combining probiotics with biofloc technology appears to be a promising strategy for enhancing shrimp performance. [ABSTRACT FROM AUTHOR]

The Kuroshio-Oyashio confluence region of the Northwest Pacific Ocean is one of the most productive fishing areas, which is rich of fish resources. In order to understand the biodiversity of fish communities in the confluence region and to protect and exploit fishery resources properly' we used environmental DNA (eDNA) to analyze the fish diversity in the Kurishio-Oyashio confluence region during the summer. A total of 51 species of marine fishes (22 orders' 30 families and 44 genera) were detected among 22 water sampling stations using high-throughput sequencing analysis. The Myctophidae family was the richest in species, comprising 33.33% of the detected fish species. In addition' the primary species with higher relative abundance were Stenobrachius nannochir, Gonostoma atlanticum, Notoscopelus japonicus, Sardinops melanostictus, Takifugu poecilonotus, Ceratoscopelus warmingii, Hexagrammos agrammus, Scomber japonicus, Nematalosa japonica and Diaphus kuroshio. The alpha diversity among stations was obviously different, and there were higher biodiversity in low-latitude sections than that of high-latitude sections. Redundancy analysis showed that the content of dissolved oxygen was the main environment factor impacting the fish species composition. The results indicated that eDNA can be a useful method for monitoring fish diversity and spatial distribution in the Kuroshio-Oyashio confluence region. [ABSTRACT FROM AUTHOR]

This study tested the hypothesis of how the nanopore size of membranes and how the surface charge of nanobubbles responds to its pinch-off from the nanopore. This study also tested the hypothesis that nanobubbles that remain in solution after production may increase the dissolved oxygen content in water. The effect of membrane pore size, hydrodynamic conditions (gas and liquid flow rates), and physicochemical parameters (pH and temperature) on volumetric mass transfer coefficient (k L a) for oxygen nanobubbles formed by the nanopore diffusion technique was investigated. This study experimentally determined the k L a by carefully removing the dissolved oxygen by nitrogen purging from nanobubble suspension to examine the sole contribution of nanobubble dissolution in water to the reaeration. Scaling estimates indicate that the nanobubble pinch-off radius and nanopore radius have a power-law correlation and that nanobubble size declines with the nanopore size. This is in line with our experimental results. The surface charge of nanobubbles delays its pinch-off at the gas-liquid interface. Nanobubbles offered 3-4 times higher k L a than microbubbles. Standard oxygen transfer efficiency in water was found to be 78%, significantly higher than that in microbubbles. However, dissolving stable nanobubbles in water does not considerably increase dissolved oxygen levels. [ABSTRACT FROM AUTHOR]

The seasonal variation of physico-chemical parameters of backwater from Yashwant Sagar reservoir were investigated during 3rd January 2023 to 25th December 2023 by using standardised parameters to determine its quality. Seasonal values of water colour, odor, turbity, temperature, pH, total alkalinity, total hardness, dissolved oxygen, free CO2 were observed from 5 different stations of Yashwant Sagar dam backwater area of Karjat and Shrigonda are presented in this paper. It was found that physicochemical parameters of water were almost at optimum level. [ABSTRACT FROM AUTHOR]

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

The Arctic Ocean plays an important role in the regulation of the earth's climate system, for instance by storing large amounts of carbon dioxide within its interior. It also plays a critical role in the global thermohaline circulation, transporting water entering from the Atlantic Ocean to the interior and initializing the southward transport of deep waters. Currently, the Arctic Ocean is undergoing rapid changes due to climate warming. The resulting consequences on ventilation patterns, however, are scarce. In this study we present transient tracer (CFC‐12 and SF6) measurements, in conjunction with dissolved oxygen concentrations, to asses ventilation and circulation changes in the Eurasian Arctic Ocean over three decades (1991–2021). We constrained transit time distributions of water masses in different areas and quantified temporal variability in ventilation. Specifically, mean ages of intermediate water layers in the Eurasian Arctic Ocean were evaluated, revealing a decrease in ventilation in each of the designated areas from 2005 to 2021. This intermediate layer (250–1,500 m) is dominated by Atlantic Water entering from the Nordic Seas. We also identify a variability in ventilation during the observation period in most regions, as the data from 1991 shows mean ages comparable to those from 2021. Only in the northern Amundsen Basin, where the Arctic Ocean Boundary Current is present at intermediate depths, the ventilation in 1991 is congruent to the one in 2005, increasing thereafter until 2021. This suggests a reduced ventilation and decrease in the strength of the Boundary Current during the last 16 years. Plain Language Summary: The Eurasian Arctic Ocean, the region of the Arctic Ocean connected to the European and Asian continents, is an important pathway for recently ventilated water from the Nordic Seas. These waters are exported back to the North Atlantic following their travel through the Arctic Ocean. Ventilation describes the process of surface waters being transported into the interior ocean due to increasing density, which affects the underlying water masses. In this study we investigate how the ventilation patterns have evolved in the Eurasian Arctic Ocean over the past three decades, using transient tracer (CFC‐12 and SF6) measurements. We observed a significant change in the intermediate layer (250–1,500 m) with older waters found in measurements in 1991 and 2021 compared to 2005 and 2015. Moreover, our data suggest a slowdown in ventilation throughout the three decades in the northern Amundsen Basin, implying a decrease in the circulation time‐scale of the Arctic Ocean Boundary Current over the past 16 years. This has potentially important implications for the transport of, for example, heat, salt or oxygen from the Atlantic Ocean around the Arctic Ocean, and back. Key Points: Temporal variability of ventilation in the Eurasian Arctic Ocean during the past 30 years is estimated by observations of transient tracersWe found a slow down of the ventilation between 2005 and 2021 in the intermediate watersEvidence of multidecadal variability of ventilation in the intermediate waters of the Eurasian Arctic Ocean is present [ABSTRACT FROM AUTHOR]

While in situ toxicity testing with caged organisms has been used to assess surface water and sediment contamination, no successful application to benthic organisms exposed to highly contaminated groundwater plumes discharging to surface waters has been reported. The objective of this study was to demonstrate and evaluate this application using four sets of tests performed at three previously reported contaminated groundwater sites, which include one river site affected by volatile organic contaminant plumes, and two sites, one pond and one small urban stream, impacted by landfill plumes. The study examined multiple cage designs and orientations and two test organisms: an amphipod (Hyalella azteca) and midge larvae (Chironomus riparius; only one study). Cages were deployed for between 5 and 28 days and assessed for organism survival and growth. At all sites and for some deployment conditions, cages exposed to high contaminant concentrations in the plume footprint had greater mortality compared to those exposed to lower or background concentrations. Organism growth was less clear as a metric of toxicity. Vertically oriented cages typically showed high mortality to plume contaminants, but some were also affected by other non-target groundwater conditions (e.g., low dissolved oxygen, other contaminant sources), while horizontally oriented cages were rarely responsive to either groundwater influence. A hybrid cage design showed much promise in its single study. Useful observations on the test organisms and on potentially problematic site conditions were also made. The informed use of in situ toxicity cages could be an additional beneficial tool for groundwater contaminated site assessments. [ABSTRACT FROM AUTHOR]

Eutrophication has become a recurrent concern in reservoirs worldwide. This problem is intensified in tropical semiarid regions, where the reservoirs have high seasonal and annual variability of water level and volume. Therefore, an extensive understanding of the diel variation of water quality key-parameters can help improve management of such reservoirs. This study focuses on Castanhão reservoir with the largest multipurpose dam in the Brazilian semiarid. Its main water uses are irrigation, fish farming, and human supply. The reservoir faced a decline in water quality due to a prolonged drought period. While previous research has predominantly emphasized the seasonal dynamics of thermal and chemical stratification, our investigation provides diel assessments of multiple water quality parameters, including nutrient concentrations and phytoplankton abundance. Our primary objective is to compare seasonal and diel variations in stratification and nutrient distribution within the reservoir. Key findings reveal a diel cycle of thermal stratification, primarily during dry season, driven by higher wind speeds. This is corroborated by a significant negative correlation between wind speed and the relative water column stability index. In contrast, during the rainy season, the reservoir experiences continuous thermal stratification due to inflowing water being warmer than the reservoir's water temperature. Notably, a significant negative correlation between total phosphorus and chlorophyll-a, along with a two-fold increase of this nutrient throughout the day during the rainy season, underscores the influence of the phytoplankton community dynamics on the diel nutrient variation. Chemical stratification of dissolved oxygen occurred during dry and rainy seasons, indicating that even during the dry season, where there is no significant inflow, the internal nutrient loading can also significantly impact the water quality of a reservoir. This study advances the understanding of diel water quality dynamics in tropical semiarid reservoirs, shedding light on both climatic and anthropogenic influences on water resources. [ABSTRACT FROM AUTHOR]

Highlights : A clear negative correlation between the size of water clusters and the concentration of dissolved oxygen was observed. This implied that smaller clusters water exhibits higher concentrations of dissolved oxygen. Oxygen molecules are primarily existed at the surfaces or interfaces of water clusters and can rapidly traverse the gas liquid interface. A semi empirical formula relating the average number of water molecules in a cluster to 17O NMR half peak width was derived, demonstrating an approximate lin ear relationship. The structure of liquid water is primarily composed of three-dimensional networks of water clusters formed by hydrogen bonds, and dissolved oxygen is one of the most important indicators for assessing water quality. In this work, distilled water with different concentration of dissolved oxygen were prepared, and a clear negative correlation between the size of water clusters and dissolved oxygen concentration was observed. Besides, a phenomenon of rapid absorption and release of oxygen at the water interfaces was unveiled, suggesting that oxygen molecules predominantly exist at the interfaces of water clusters. Oxygen molecules can move rapidly through the interfaces among water clusters, allowing dissolved oxygen to quickly reach a saturation level at certain partial pressure of oxygen and temperature. Further exploration into the mechanism by molecular dynamics simulations of oxygen and water clusters found that oxygen molecules can only exist stably at the interfaces among water clusters. A semi-empirical formula relating the average number of water molecules in a cluster (n) to 17O NMR half-peak width (W) was summarized: n = 0.1 W + 0.85. These findings provide a foundation for exploring the structure and properties of water. [ABSTRACT FROM AUTHOR]

[Objective] The study was carried out to explore the effects of acute hypoxia on the survival of juvenile Eleutheronema tetradactylum and their spleen and heart tissue structure, with a view to providing technical support and theoretical references for the breeding and production of this species. [Method] Filling water with nitrogen to quickly overflow oxygen and reduce the content of dissolved oxygen in the water. Different levels of dissolved oxygen treatment were set in the experiment: normal oxygen group (6.33 ± 0.15 mg/L), moderate hypoxia group (3.99 ± 0.18 mg/L), and severe hypoxia group (2.05 ± 0.15 mg/L). Each treatment group was set up with 3 parallel devices. During the experiment, the flowing water was turned off, and the dissolved oxygen level in the water was monitored in real time by using a dissolved oxygen measuring instrument. The corresponding dissolved oxygen level was maintained by adjusting the size of the inflation valve. Using paraffin embedding, sectioning and hematoxylin eosin (HE) staining methods were used to observe the changes in tissue structure of the heart and spleen of juvenile E. tetradactylum. [Result] In terms of mortality rate, there was no mortality observed in the E. tetradactylum in the normal oxygen group; the mortality rates of juvenile fish under the stress in the moderate and severe hypoxia groups were 10.0%-13.3% at 23 hours, 30.0% - 40.0% at 26 hours, and 43.3%-53.3% at 29 hours. In terms of spleen tissue structure, compared with the normal oxygen group, the moderate hypoxia group showed an increase in white pulp and the number of lymphocyte in juvenile fish, sparse distribution of peripheral red pulp, and a decrease in the number of melanin macrophages; the red pulp area of juvenile fish in the severe hypoxia group expanded, with a decrease in the white pulp area and the number of lymphocytes, and melanin macrophages rarely appearing. In terms of heart tissue structure, the compact layer of heart tissue in juvenile fish in the moderate hypoxia group became thinner, the myocardial fibers in the sponge layer became thinner, and the spacing became wider, and partial myocardial fiber rupture phenomenon occurred; the degree of thinning of the compact layer and thinning of the myocardial fibers in the sponge layer of juvenile fish in the severely hypoxic group increased, and the degree of expansion of the interstitial space increased compared to the hypoxic group, with partial myocardial cell rupture, nucleus protrusion, myocardial fiber rupture, and disordered arrangement. [Conclusion] The study has shown that acute hypoxia stress can affect the survival rate of juvenile E. tetradactylum. The lower the dissolved oxygen level, the higher the mortality rate, and the degree of tissue structure damage increases with the level of hypoxia. [ABSTRACT FROM AUTHOR]

Headwater streams play a large role in aquatic greenhouse gas emissions. Carbon dioxide (CO2) and dissolved oxygen in streams often undergo changes through diel cycles. However, methane (CH4) and nitrous oxide (N2O) have unknown diel dynamics. Here, we reveal consistent patterns in CO2, CH4, and N2O over diel cycles and during flood events using high‐frequency continuous observations in a subtropical headwater stream. Diel cycles were most pronounced during baseflow. Increased nighttime discharge due to higher groundwater inputs enhanced gas transfer velocities and concentrations. Overall nocturnal emissions were 31%, 68%, and 32% greater than daytime for CO2, CH4, and N2O, respectively. Floods dampened diel signals. If both flood events and diel patterns are neglected, estimates of greenhouse gas emissions from headwaters may be greatly underestimated. Overall, CH4 and N2O emissions from headwater streams may be underestimated by ~ 20–40% due to a lack of observations during nighttime, floods, and in warmer climates. [ABSTRACT FROM AUTHOR]

Accurate determination of reaeration coefficient in lakes is a key factor for accurate modeling of dissolved oxygen. This study developed a new numerical model to determine the lake reaeration coefficient of Sawa lake, Iraq based on solving the transform mechanisms of dissolved oxygen without the using the empirical and laboratory methods. The model solved the gas transport at the water-air surface numerically to predict the dissolved oxygen and the reaeration coefficient simultaneously. Results showed that the lake reaeration coefficient was 0.001 day-1 at 20 °C with low root mean squared errors (0.138, 0.137, and 0.168 mg/L for the three years of simulation 2007, 2012, and 2017, respectively) and absolute mean errors (0.121, 0.114, and 0.145 mg/L in 2007, 2012, and 2017, respectively). It was noticed that the lake habitat was impacted causing unhealthy conditions (low reaeration rate), and there was upward dissolved oxygen movement due to the presence of positive trend in the yearly cycle of temperature. In addition, including this numerical approach along with other water quality models can improve the predictions of other water quality parameters. [ABSTRACT FROM AUTHOR]

The spread of industrial activities, especially electrical power generation stations on the banks of rivers, increases the pollution of the surrounding water environment as a result of the release of industrial water that is not treated adequately. Among these stations is the Al-Musayyib Thermal Power Station. The research focused on studying the quality of the water entering and leaving the station by installing two sensing stations. Remote monitoring of the Euphrates River, as the station contains several sensors, including measuring acidity, dissolved oxygen, turbidity, and the amount of total salts, in addition to a sensor for measuring refined fuel. These results are collected around the clock at a central station, where the results showed a decrease in the level of dissolved oxygen ranging between (3.5-5) RSD% 0.1-0.2 as well as an increase in The temperature of the excreted water ranging between from (30C°-33C°) RSD% 0.1-0.2, both of which negatively affect the environmental life of living organisms, as well as the appearance of a high percentage of pollutants resulting from refined fuel, ranging between (2.1-8.1) ppm, The findings will contribute to improving the efficiency of treatment plants to suit the environment surrounding the river, In order to preserve the water environment, it is necessary to place oil dispensers in the treatment units, as well as work to reduce the temperatures of the cooling water, which are at temperatures higher than the medical grades coming out of the station. [ABSTRACT FROM AUTHOR]

The formation of oxygen-depleted zones in the bottom waters is one of the most widespread phenomena in coastal areas. Upwelling episodes along India's west coast due to the southwest monsoon increase biological productivity, further lowering the dissolved oxygen in the upwelled waters, which intensifies annually between June and October. Here, we have determined the changes in the microbial community in response to the varying oxygen levels and other physicochemical parameters at the Candolim Time Series Station using highthroughput sequencing. Amplicon Sequence Variants across all the samples collected in different seasons were mostly affiliated to the phyla Proteobacteria, Actinobacteria, Bacteroidetes, Verrucomicrobia, Chloroflexi, Firmicutes and Planctomycetes, with the most dominant being Proteobacteria (21-41%). Statistical analysis revealed that microbial diversity differed significantly with changing DO, ammonia, nitrate and nitrite concentrations during different seasons. The microbial community shift due to seasonal hypoxia results in the differential biogeochemical cycling of essential nutrients, with certain years seeing redox conditions up to sulphate reduction while certain years seeing only nitrogen loss. Future global warming scenarios will serve as a big challenge for understanding the role of microbial diversity and its implications in the cycling of natural elements. [ABSTRACT FROM AUTHOR]