Low Dissolved Oxygen Levels Research Articles

Understanding nitrogen removal and N2O emission mechanisms in an anaerobic-swing-anoxic-oxic (ASAO) continuous plug-flow system for low C/N municipal wastewater

This study aims to investigate effects of dissolved oxygen (DO) levels and aerated hydrodynamic retention time (HRT) on nitrogen removal and nitrous oxide (N2O) emissions in a novel anaerobic-swing-anoxic-oxic (ASAO) continuous plug-flow system for treating low carbon to nitrogen ratio municipal wastewater. The swing zones had varying DO levels and volumes, deciding the aerated HRT of the ASAO system. Results showed that low DO level (0.8–1.0 mg/L) and short aerated HRT led to high nitrogen removal performance (91.4 %–96.3 %) and low N2O emission factor (2.8 %). The simultaneous nitrification and denitrification (SND) in swing zones and endogenous denitrification in anoxic zones contributed to the nitrogen removal. Meanwhile, the SND and autotrophic denitrification processes were identified as the N2O sources. Low DO level enriched ammonia-oxidizing bacteria and enhanced the SND and autotrophic denitrification pathway. These findings suggest that the ASAO system is promising for reducing carbon emissions in municipal wastewater treatment.

Enhancing corrosion resistance of T91 F/M steel in liquid lead-bismuth (LBE) by slurry FeAl coating

A FeAl coating was fabricated on T91 steel via slurry aluminizing. The corrosion behavior of coated and uncoated samples was assessed in static LBE (lead-bismuth eutectic) with two both high and low dissolved oxygen concentrations at 550 °C. The coating was mostly intact and exhibited great corrosion resistance compared to the uncoated specimens regardless of the oxygen concentration. That is because the coating can form a protective alumina film on the surface at extremely low dissolved oxygen level. This coating is of significant engineering value in enhancing the corrosion resistance of Fe base alloy in LBE.

Insight into the effect of oxygen content on the corrosion behavior of X70 pipeline steel in a typical simulated soil solution by dissolution-diffusion-deposition model

In this work, traditional experimental methods were employed to investigate the effect of dissolved oxygen (DO) on the corrosion behavior of X70 pipeline steel in a low-temperature acidic-bentonite simulation soil solution. A novel model was utilized to describe the complex dissolution-diffusion-deposition process at the metal/solution interface. This model aims to enhance comprehension of the dynamics of the corrosion product layer of X70 pipeline steel under varying oxygen concentrations, as well as the effects of alloying elements (Fe, Cr and Cu) on the corrosion resistance. By integrating traditional experimental methods with calculation models, the results reveal a positive correlation between DO levels and the corrosion rate X70 pipeline steel. This relationship is attributed to the distinct characteristics of the corrosion product layers formed under different DO conditions. At low DO levels, the nucleation and growth rates of oxide/hydroxide are slower, leading to the formation of a denser, more protective corrosion product layer. Conversely, at high DO levels, the accelerated nucleation and growth rates produce larger oxide/hydroxide particles, resulting in a porous and less protective corrosion product layer. Furthermore, the variation in the protective qualities of the corrosion product layers under different DO conditions causes differences in corrosion morphology: X70 pipeline steel exhibits localized corrosion in low DO environments and more uniform corrosion under high DO conditions.

Genome-resolved metagenomics reveals the nitrifiers enrichment and species succession in activated sludge under extremely low dissolved oxygen

Nitrification, a process carried out by aerobic microorganisms that oxidizes ammonia to nitrate via nitrite, is an indispensable step in wastewater nitrogen removal. To facilitate energy and carbon savings, applying low dissolved oxygen (DO) is suggested to shortcut the conventional biological nitrogen removal pathway, however, the impact of low DO on nitrifying communities within activated sludge is not fully understood. This study used genome-resolved metagenomics to compare nitrifying communities under extremely low- and high-DO. Two bioreactors were parallelly operated to perform nitrification and DO was respectively provided by limited gas-liquid mass transfer from the atmosphere (AN reactor, DO < 0.1 mg/L) and by sufficient aeration (AE reactor, DO > 5.0 mg/L). Low DO was thought to limit nitrifiers growth; however, we demonstrated that complete nitrification could still be achieved under the extremely low-DO conditions, but with no nitrite accumulation observed. Kinetic analysis showed that after long-term exposure to low DO, nitrifiers had a higher oxygen affinity constant and could maintain a relatively high nitrification rate, particularly at low levels of DO (<0.2 mg/L). Community-level gene analysis indicated that low DO promoted enrichment of nitrifiers (the genera Nitrosomonas and Nitrospira, increased by 2.3- to 4.3-fold), and also harbored with 2.3 to 5.3 times higher of nitrification functional genes. Moreover, 46 high-quality (>90 % completeness and <5 % contamination) with 3 most abundant medium-quality metagenome-assembled genomes (MAGs) were retrieved using binning methods. Genome-level phylogenetic analysis revealed the species succession within nitrifying populations. Surprisingly, compared to DO-rich conditions, low-DO conditions were found to efficiently suppressed the ordinary heterotrophic microorganisms (e.g., the families Anaerolineales, Phycisphaerales, and Chitinophagales), but selected for the specific candidate denitrifiers (within phylum Bacteroidota). This study provides new microbial insights to demonstrate that low-DO favors the enrichment of autotrophic nitrifiers over heterotrophs with species-level successions, which would facilitate the optimization of energy and carbon management in wastewater treatment.

A novel process based on marine heterotrophic nitrification and aerobic denitrification bacteria for treating mariculture wastewater: Performance, biofilm characteristics and microbiome responses to dissolved oxygen

A novel reactor utilizing marine heterotrophic nitrification and aerobic denitrification (MHNAD) bacteria offers an effective approach for treating mariculture wastewater. However, the impact of dissolved oxygen (DO) concentration on its performance has been understudied. This research investigated how varying DO concentrations influence the performance, biofilm characteristics, and microbiome of the reactor when inoculated with MHNAD bacteria. The results demonstrated that reducing the DO concentration from 6 mg/L to 2 mg/L decreased the removal efficiency of chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) but significantly improved nitrate nitrogen (NO3−-N) removal. Higher DO concentrations (6 mg/L) promoted the production of extracellular polymeric substances (EPS), particularly proteins. Microbial analysis revealed that MHNAD bacteria, particularly the genus Zobellella, consistently dominated the bioreactor, though their abundance decreased at lower DO concentrations. Bugbase analysis indicated that biofilm formation and aerobic bacterial abundance decreased as DO concentration declined. Additionally, metabolic pathway analysis suggested that lower DO levels might inhibit glycolysis and TCA cycle processes, while also altering the primary nitrogen removal pathways.

Study on recirculating aquaculture system (RAS) in organic fish production

Abstract The growth of conventional aquaculture has created environmental issues due to excessive feeding, low dissolved oxygen level etc into the water body. Organic aquaculture is a recent development as a solution to these issues. It is a clean alternative to reduce pollution and to produce safer consumable food. In the past few decades, recirculating aquaculture systems (RAS) were introduced to maintain pond water quality through lesser water exchange, by focusing on water reuse after treatment. In this study, the technical viability of RAS was analyzed for introduction in organic aquaculture systems. Indian major carps (IMCs) were cultured through conventional methods in the three tanks of conventional system, while the same species was grown organically in another three tanks in organic system. The stocking density and physical conditions were kept same for both cultured systems. The RAS consisted of fish culture tank with an average volume of each tank was 165 m3, a screen filter, foam fractionator and trickling filter. The various water quality parameters, i.e., solid size distribution in water, and removal efficiencies in biological treatment of both systems were compared. The present study, particle size distribution of solids in water body was measured by filtration, followed by weighing of dry residue solids. Pore sizes of 1000μ, 100μ, 20μ and 3μ were employed for filtration. For organic tanks, a majority of solids are of size between 1 mm and 100 μm. The larger sized particles (&gt; 30 μm) constitute nearly 70 % of the solids and the trait differs from the generalized conclusions of that a major part of sediments will be of sizes less than 20 μm. For the conventional tanks, majority of solids are of size between 3 mm and 20 μm and TAN shows higher removal efficiencies for effluents from organic system rather than conventional system. The results showed that organic aquaculture causes lesser pollution load per weight of fish. The particle size distribution of organic water was better compatible to screening and sedimentation than conventional water. Also the filtration efficiencies in nitrifying trickling filters of both water bodies were comparable, with that of organic water slightly on the higher side. Thus recirculating water treatment systems are introduced in organic aquaculture.

Beyond denitrification: Alginate production by mucoid Pseudomonas aeruginosa using wastewater nitrate as electron acceptor

Microbial synthesis is an effective method for carbon and nitrogen recovery in the biological nitrogen removal process. The recycling of alginate-like exopolymers has attracted the attention of many researchers; however, the composition of the polymers and alginate differs. In this study, the capacity and mechanism of a mucoid Pseudomonas aeruginosa to remove nitrate and synthesize alginate were investigated. The results showed that the P. aeruginosa achieved higher nitrate removal efficiency (95.3 %) and alginate yield (298 mg/L) after 36 hours of microaerobic cultivation compare to aerobic condition. The optimal conditions for nitrogen removal and alginate recovery are 35 °C, pH 7 and C/N 15. The transcriptional activities of denitrifying genes in P. aeruginosa were upregulated at the low dissolved oxygen level. Although O2 may upregulate the expression of alginate synthesis key genes (algD, alg8 and alg44) by activating AlgZ-AlgR two-component signal transduction system, microaerobic condition facilitates P. aeruginosa proliferation and results in a higher alginate yield. Moreover, the P. aeruginosa strain was further coupled with an anoxic/aerobic reactor for deeper nitrogen removal, and the nitrate removal efficiency and alginate yield were maintained above 91 % and 100 mg/L, respectively. The conversion efficiency of sodium acetate to alginate reached 15 %. This work provides an economically and environmentally sustainable strategy for wastewater treatment and the recovery of high-value bioresource.

Polyculture of Genetically Improved Farmed Tilapia (GIF tilapia) and Penaeus vannamei Using Biofloc Technology – A Review

According to Food and Agriculture Organization of the United Nations (FAO), aquaculture has grown faster and its expansion aimed at meeting the increase of world fish demand, and preserving natural fish stocks. Currently, to produce fish in quantity and quality requires reduction of the environmental impact from aquaculture, through the improvement of culture systems. Disease is the major factor affecting the development and expansion in aquaculture. Losses due to disease in shrimp farming are high. Various approaches to minimize the impact of disease on production are possible. Another approach to keep the pathogen pressure low is polyculture of shrimp and finfish. This practice makes shrimp farming more sustainable by reducing the environmental impact and the incidence of shrimp disease. Antimicrobial peptides in the fish skin kill shrimp pathogens, keeping pathogen pressure of bacteria and viruses low. In polyculture, shrimps can eat tilapia faeces and unused fish feed, while tilapia filter phytoplankton, reducing the risk of low dissolved oxygen levels at night. In addition, shrimp bioturbation at the pond bottom returns nutrients to the water column, enhancing phytoplankton production and consequently the natural feed available for the tilapia. Biofloc technology (BFT) is one of the most applicable and promising systems for sustainable aquaculture development. This technology is essentially based on the recycling of nutrients via microorganisms, primarily (i) heterotrophic bacteria, which convert nitrogen compounds into microbial biomass, in addition to serves as a source of food for aquatic organisms, and (ii) chemoautotrophic bacteria, which convert ammonia to nitrite and nitrate.

Sezonowa i przestrzenna zmienność jakości wody w systemie jezior Lumina-Rosu w delcie Dunaju w Rumunii

The main aim of the present scientific paper is to assess water quality of the surface water of various lakes located in the Lumina-Rosu interdistributary depression, Danube Delta, Romania. As specific objectives, the paper considered to spot the main potential sources of contamination and their environmental impacts on aquatic ecosystems. Equally, the purpose of the article is to emphasize the importance of monitoring the water quality of the Danube Delta, for preserving these unique natural water resources and their ecosystem services (fresh water, biodiversity, flood control, recreation, nutrient cycling, fishing and other traditional activities, ecotourism). Surface water samples collected from several control sections situated on the main tributaries and canals, as well as from lakes were investigated physically (To - C, EC - μS/cm, TDS - mg/L, turbidity - NTU units, TSS - mg/L) and chemically (pH - units, DO - mg/L, N-NO2- - mg/L, N-NO3- - mg/L, P-PO43-- mg/L, Chla - mg/L, SiO2 - mg/L, TOC - mg/L, SO42- - mg/L, S2- - mg/L, H2S - mg/L and synthetic detergents - mg/L). Generally, the hydro-physical-chemical characteristics and water quality assessment of the surface waters of the lakes manifest the combined effect of both processes occurred in the catchment (weathering, sediment supply and transport), as well as the in situ lake processes (photosynthesis, sediment mixing, biogeochemical cycling, evaporation, eutrophication, productivity changes). Overall, the physical and chemical characteristics of the sampling sites investigated at high waters of the Danube are quite similar to those measured at low waters. The differences that mainly occurred within the lakes are more related to the limnological variables and the local environmental conditions. Anyway, a significant seasonal variation was noticed in the lakes, during the dry period, when different physical and chemical characteristics appeared to be influenced by high air temperature, lack of precipitation, low water level regime, low dissolved oxygen level regime, all related to climate change effects etc. Moreover, the anthropogenic factor that left its mark on some physical-chemical characteristics of water should not be omitted either. However, the results obtained within this study did not show such alarming values as might have dangerous effects on the investigated aquatic environments. The water quality information within this case study may be used for improving the understanding of the water quality issues and to better coordinate and plan for future monitoring activities in and around the Danube Delta environment. The results will contribute to updating the existing database with relevant information for a sustainable future of the Danube Delta and in similar areas that are subjected to such environmental challenges.

Physiological sensitivities to hypoxia differ between co-occurring juvenile flatfishes

Coastal hypoxia threatens estuarine nursery habitat for juvenile demersal fishes worldwide, and is expected to intensify with climate change. This study examines the physiological tolerances of hypoxia in two ecologically and economically important flatfish species that co-occur in a highly eutrophic estuary and biodiversity hotspot on California's central coast, the Elkhorn Slough. Juveniles of English sole (Parophrys vetulus) occur less frequently in areas of the slough where dissolved oxygen (DO) ≤ 4.0 mg/L O2 compared to speckled sanddabs (Citharichthys stigmaeus), suggesting that English sole may be more sensitive to hypoxia. We exposed both species to an ecologically relevant, acute six-hour exposure to six DO levels ranging from ambient to severely hypoxic: 8.0, 6.0, 4.0, 3.0, 2.0, and 1.5 mg/L O2 and measured known physiological and biochemical indicators of hypoxia stress. As DO declined, metabolic rate and aerobic scope decreased for English sole, while anaerobic activity (measured by L-lactate) and ventilation rate increased for both species. Biochemical responses to hypoxia were observed only at very low DO levels (≤ 4.0 mg/L O2), indicating that both species appear to have a higher tolerance for relatively short-term hypoxia than other teleost fishes. Speckled sanddabs increased ventilation rate and anaerobic activity at higher DO levels than English sole, suggesting that earlier onset of compensatory mechanisms may contribute to the greater relative hypoxia tolerance in sanddabs. Examining the link between physiological and ecological thresholds for hypoxia tolerance will help determine suitable nursery habitat areas and management targets for estuarine restoration, and aid in predictions of fishery success under eutrophication and climate change.

Nitrous oxide emissions and microbial communities variation in low dissolved oxygen and low carbon-to-nitrogen ratio anoxic-oxic wastewater treatment plant.

Dissolved oxygen (DO) levels and carbon-to-nitrogen (C/N) ratio affect nitrous oxide (N2O) emissions by influencing the physiological and ecological dynamics of nitrifying and denitrifying microbial communities in activated sludge systems. For example, Nitrosomonas is a common N2O producing nitrifying bacteria in wastewater treatment plants (WWTPs), and DO conditions can affect the N2O production capacity. Previous studies have reported N2O emission characteristics under adequate DO and C/N conditions in A/O WWTPs. However, in actual operation, owing to economic and managerial factors, some WWTPs have a long-term state of low DO levels in oxic tanks and low influent C/N. Research on N2O emission characteristics in low DO-limited and low C/N ratio WWTPs is limited. This study investigated N2O emissions and the corresponding shifts in microorganisms within an anoxic-oxic (A/O) WWTP over 9-month. Quantitative PCR was used to assess the abundance of ten functional genes related to nitrification and denitrification processes, and high-throughput sequencing of the 16S rRNA gene was employed to determine the composition change of microorganisms. The findings revealed that 1) the average N2O emission factor was 1.07% in the studied WWTP; 2) the DO-limited oxic tank primarily contributed to N2O; 3) NO2-, TOC, and C/N ratios were key factors for dissolved N2O in the aerobic tank; and 4) Nitrosomonas and Terrimonas exhibited a robust correlation with N2O emissions. This research provides data references for estimating N2O emission factors and developing N2O reduction policies in WWTPs with DO-limited and low C/N ratios.

Influence of water quality and seasonal variations on freshwater macroinvertebrate diversity and community structure in wastewater treatment ponds, Phetchaburi Province, Thailand

Wastewater originating from the Phetchaburi municipality undergoes treatment in a series of five distinct stages at the King’s Royally Initiated Leam Phak Bia Environmental Research and Development Project (LERD) in Phetchaburi province, Thailand. These stages involve a sedimentation (pond 1), three oxidation ponds (ponds 2 to 4), and a final stabilization pond (pond 5). These ponds serve as habitats for macroinvertebrates; consequently, their diversity and composition might be influenced by fluctuations in water quality and seasonal variations. The primary aim of this research was to analyze the diversity and species composition of macroinvertebrate communities concerning varying levels of organic contamination across the five wastewater treatment ponds at LERD. This investigation spanned three seasons: cold season (December 2019), rainy season (July 2020), and hot season (April 2021). The findings revealed that the diversity and species composition of macroinvertebrate communities displayed distinct alterations across multiple environmental gradients, especially identifying the significant influence of organic loading levels observed in ponds 1 to 5. The macroinvertebrate communities exhibited two distinct groupings, with the Chironomidae and Candonidae or ostracods prevailing prominently in ponds 1 and 2 (heterogenous environments). This prevalence was attributed to the high levels of detrital food and the robust resilience of chironomid larvae and ostracods to organic pollution, thriving even in environments characterized by low dissolved oxygen levels. Conversely, the prevalence of snails from the Thiaridae family in ponds 3 to 5 (homogenous environments) indicated improved water quality conditions, notably lower organic matter levels, and a higher dissolved oxygen content. In addition, the study identified seasonal variations in macroinvertebrates, likely influenced by the differing organic loading and environmental conditions. Thus, this research provided insights into the factors shaping macroinvertebrate communities in a wastewater treatment system.

Teasing out the Effects of Natural Stressors at Chemically Contaminated Sites.

Aquatic ecosystems are often impacted by a multitude of stressors, many of which are introduced by a combination of anthropogenic activities such as agricultural development, urbanization, damming, and industrial discharge. Determining the primary stressors responsible for ecological impairments at a site can be complex and challenging; however, it is crucial for making informed management decisions. Improper diagnosis of an impaired system can lead to misguided attempts at remediation, which can be both time consuming and costly. We focused on the development, implementation, and evaluation of methodologies that, in combination, allowed us to identify the primary stressors. These included a four-phase, weight-of-evidence (WOE) assessment including in situ Toxicity Identification and Evaluation (iTIE) testing, physicochemical and macrobenthos characterization, reciprocal sediment transplants, and laboratory and in situ toxicity testing. The contaminants of concern (COCs) at the site were elevated levels of ammonia, chloride, pH, and total dissolved solids in groundwater upwellings into a high-quality waterway. Reciprocal transplants of site sediments and nearby reference sediments and traditional benthic sampling showed impaired benthic indices and multiple stations around a contaminated industrial settling basin. Impaired stations had elevated COCs in groundwaters but exhibited a steep vertical concentration gradient, with concentrations decreasing near the sediment-surface water interface. We describe Phase 4 of the study, which focused on teasing out the role of dissolved oxygen sags in benthic macroinvertebrate responses. Extensive submerged and emergent macrophytes, algae, and cyanobacteria co-occurred at the impaired sites and increased throughout the summer. Laboratory testing suggested that ammonia and pH were possibly toxic at the sites, based on groundwater concentrations. The in situ toxicity testing, however, showed toxicity occurring even at stations with low levels of COCs concurrently with large diurnal fluxes in dissolved oxygen (DO). A final phase using a type of iTIE approach utilized limnocorrals with and without aeration and with in situ toxicity measures using Hyalella azteca. The Phase 4 assessment revealed that low DO levels were primarily responsible for impaired benthic communities, and COC upwellings were diluted at the sediment-water interface to nontoxic levels. These findings will allow for improved management decisions for more efficient and effective restoration activities. Environ Toxicol Chem 2024;43:1524-1536. © 2024 SETAC.

Hypoxia impairs cellular energy allocation in the juvenile horseshoe crab Tachypleus tridentatus

Although hypoxia is a serious environmental concern for marine ecosystems globally, its biological effects on the benthic biota remain mostly unclear for some endangered species. To provide an deep understanding of the possible effects of hypoxia on the tri-spine horseshoe crab Tachypleus tridentatus, the cellular energy allocation (CEA) approach was utilized to examine the cellular responses and adaption potential of horseshoe crabs. We examined the energetic responses of T. tridentatus under low dissolved oxygen level (2 mg O2/L). The horseshoe crabs first experienced 14 days of hypoxic stress, and then recovered in a normal dissolved oxygen environment for 7 days. On the 7th and 14th day of hypoxic exposure, the levels of available energy, electron transport system activity, protein, lipids, and carbohydrates were decreased in T. tridentatus (p < 0.05). All measured parameters in the hypoxic group partially or completely recovered after seven days of re‑oxygenation, reaching a level that was significantly up-regulated (p < 0.05) compared with the 14th day and non-significantly different from the 0th day exposure (p ˃ 0.05). In conclusion, hypoxic stress has adverse effects on the energy balance of juvenile T. tridentatus, but these adverse effects can be alleviated in a short recovery period. As a result, our findings provide novel perspectives on the physiology of T. tridentatus under hypoxia acclimation, which is essential information for establishing ideal conditions for the cultivation of this endangered species.

Modelling Nitrogen Removal in the Kibendera Wastewater Stabilization Ponds in Ruiru, Kenya

Application of waste stabilization ponds (WSP) in wastewater treatment in the tropical regions is primarily due to their affordability and relatively high treatment performance. Monitoring of 2-year nitrogen removal behavior in Kibendera WSP in Ruiru, Kenya, was undertaken between January 2021 and December 2022. The experimental work determined the concentrations and removal efficiencies of Ammonia, Nitrate, Nitrite, Organic Nitrogen and Total Nitrogen. Standard Methods for the examination of water and wastewater determined Nitrogen and Dissolved Oxygen (DO) concentrations. Based on the experimental data obtained, mass balance reaction rate models characterized the nitrogen transformation and removal behavior in the WSP. Whereas model calibration was achieved using observed data from January to December 2021, model validation was achieved using observed data from January to December 2022. Ammonia volatilization, sedimentation, mineralization, nitrification, denitrification and microbial ammonia uptake were the possible transformation and removal pathways. Whereas ammonia volatilization contributed the least to the overall nitrogen removal (0.01-0.02 mg/L.d), denitrification contributed the most (2.12-14.67 mg/L.d). Low DO levels and high ammonia concentrations were responsible for low nitrification rates and high microbial ammonia uptake respectively. Comparison between experimental and modelled effluent concentrations yielded correlation coefficients (&amp;lt;i&amp;gt;r&amp;lt;/i&amp;gt;) of 0.77 and 0.69 for ammonia and organic nitrogen respectively during the calibration period. The corresponding model validation &amp;lt;i&amp;gt;r&amp;lt;/i&amp;gt; values were 0.74 and 0.93 respectively. The good agreement between the model output and observed effluent concentrations implies that nitrogen removal prediction and optimization is possible. External aeration to spike DO concentration levels is necessary to enhance the long-term nitrification rates.

An environmental flow to an ephemeral creek increases the input of carbon and nutrients to a downstream receiving river

Although intermittent and ephemeral rivers lack surface flow for part of the year, they provide vital refuges for biota in otherwise dry semi-arid and arid landscapes. The hydrology of many such rivers has been altered due to river regulation and climate change. Environmental flows can be delivered to address the negative impacts of regulated flows, however there is limited knowledge of how dry ephemeral ecosystems respond following environmental flows. This study examined changes in water quality of the ephemeral Thule Creek in the southern Murray-Darling Basin, Australia, following delivery of environmental water from an irrigation canal. We also examined how the environmental flow influenced water quality of Wakool River that receives inflows from Thule Creek. Six sites in Thule Creek, three in Wakool River, and one in Yarraman irrigation channel (source water) were monitored for dissolved organic carbon (DOC), nutrients and dissolved oxygen (DO) once per week over 15 weeks from October 2019 to January 2020. The environmental flow resulted in high DOC concentrations (4.4–76 mg/L). Although low DO levels at sites in Thule Creek were recorded on some dates below the threshold for fish stress (&amp;lt; 4 mg/L) there were no fish kills observed during the environmental flow. The carbon-rich and nutrient-rich water (DOC &amp;gt;10 mg/L, total phosphorus up to 94 μg/L, total nitrogen up to 1,125 μg/L) was detected in the Wakool River downstream of Thule Creek confluence compared to the Wakool River upstream of Thule Creek confluence (DOC 6.6 mg/L, total phosphorus up to 64 μg/L, total nitrogen up to 805 μg/L) during the period when the environmental flow in Thule Creek was connected with the Wakool River. This research provides an example of how irrigation canal networks can be used to deliver environmental water to an ephemeral river to maintain refuges and contribute to the productivity of a receiving river further downstream. Careful management of the timing, volume and duration of environmental flows in arid or semi-arid landscapes is needed to avoid the development of poor water quality during, or following, the delivery of environmental water.

ASSESSING THE PERFORMANCE OF A POND SLUDGE REMOVER SET AND SLUDGE COMPOSITION

Bangladesh, a leading fish-producing country, relies on fish as a primary protein source, but poor management has led to a decline in fish quality. Ammonia and low dissolved oxygen levels adversely affect fish production and taste. Leftover pellet feed accumulating at the pond bottom decomposes releasing ammonia and forming sludge, the primary source of water pollution. High fish density exacerbates sludge buildup, necessitating effective removal for pond water preservation. The study addresses the critical issue of sludge accumulation in fish ponds, contributing to elevated gas and ammonia levels. A sludge remover set was constructed in Kaharole Upazila, Dinajpur, utilizing local materials and an electric motor. Testing three sludge collectors with perforated holes of 10 mm, 15 mm, and 20 mm revealed that the 15 mm perforated pipe was the most cost-effective, with a collection rate of 1.85 kg of sludge-water mixture per 5L. The sludge composition, analyzed at the Soil Resources Development Institute (SRDI), Dinajpur, indicated valuable nutrients like nitrogen (0.179%), phosphorus (54.05 ppm), potassium (0.28 meq/100g), and micronutrients, potentially beneficial for plant growth and agricultural fertilizer. The findings emphasize the vital role of sludge remover sets in increasing fish production by effectively managing sludge accumulation.

High-frequency dynamics of pH, dissolved oxygen, and temperature in the coastal ecosystems of the Tanga-Pemba Seascape: implications for upwelling-enhanced ocean acidification and deoxygenation

Ocean acidification, deoxygenation, and warming are three interconnected global change challenges caused by increased anthropogenic carbon emissions. These issues present substantial threats to marine organisms, ecosystems, and the survival of coastal communities depending on these ecosystems. Coastal upwelling areas may experience significant declines in pH, dissolved oxygen (DO), and temperature levels during upwelling events, making marine organisms and ecosystems in these areas more susceptible to ocean acidification and deoxygenation. Understanding the dynamics of pH, DO, and temperature in coastal upwelling areas is essential for evaluating the susceptibility of resident organisms and ecosystems to lower pH and DO conditions occurring during upwelling events. To accomplish this, we used the pH and the DO loggers to measure high-frequency data for pH and DO, respectively, over six months in the open ocean and for a 24-hour cycle within the mangrove, seagrass, and coral reef ecosystems of the Tanga-Pemba Seascape (T-PS) during the northeast monsoon season. Our findings revealed the occurrence of multiple upwelling events, with varying durations, that result in significant declines in pH, DO, and temperature within the seascape. This is the first study to confirm the occurrence of multiple upwelling events in the T-PS. Moreover, the study has revealed a pH threshold value of 7.43 for ocean acidification in the T-PS. This is the first study to report a threshold value for ocean acidification in coastal upwelling areas of the Western Indian Ocean (WIO). Furthermore, it revealed that the extremely low levels of pH that occurred during upwelling events were above the pH threshold value of 7.43 for ocean acidification, while the extremely low levels of DO fell below the oxygen threshold value of 4.6 mg/L for deoxygenation. During upwelling events, seagrass and coral reef ecosystems, but not mangrove ecosystems, demonstrated elevated mean hourly values of pH and DO compared to those of the open ocean. These findings show that marine organisms and ecosystems in the T-PS are frequently exposed to lower pH and DO conditions due multiple upwelling events. However, their susceptibility to these conditions is reduced to some extent by the presence of seagrass meadows within these interconnected systems.

Effects of environmental parameters on spatial and temporal distribution of marine microbial communities in the southern Black Sea

The Black Sea is a unique environment with strong and permanent vertical stratification, with a thin layer of oxic zone above and a permanent anoxic zone below. Few high-throughput genomic surveys have been conducted to examine microbiota in the Black Sea. Yet, there is no study on the seasonal and vertical variation in microbial community compositions, driving forces and mechanisms of community assembly. In this study, seasonal, vertical, and spatial microbial assemblages were studied in terms of diversity, abundance, and community structure using 16S rRNA metabarcoding. 16S rRNA metabarcoding confirmed seasonal changes in microbial communities and the presence of distinct microbial groups among different water layers. Taxa belonging to Cyanobiaceae contributed a large fraction of the total biomass and were the most abundant autotrophic bacteria found across the whole water column, including hydrogen sulfide-containing anoxic zone. Temperature, salinity, water density, conductivity, light, chlorophyll-a, O2, NO3, NH3, PO4, Si, and H2S had a significant influence on the vertical bacterial community assemblages. The copper mine discharge system at 180 m did not affect microbial community structure and composition. Temperature seemed to be a primary factor in the variance between shallow depths. In conclusion, the lack of light, low dissolved oxygen levels, and low temperature do not restrict microbial diversity, as proven by the higher diversity observed in deeper zones. Wastewater in Black Sea region may be discharged into the Black Sea to depth of 180 m or deeper without impacting microbial ecology.

Acidification and hypoxia in seawater, and pollutant enrichment in the sediments of Qi’ao Island mangrove wetlands, Pearl River Estuary, China

Mangrove forests are crucial in absorbing, storing, and purifying pollutants while maintaining ecological balance. A study was conducted in 2020 to investigate the biogeochemical processes of seawater and sedimentary environmental factors in the Qi'ao Island mangrove wetland. The study comprised two survey sections and ten survey stations within the mangrove forest and 16 large-scale survey stations in the adjacent sea area. During ebb tides, the mean concentrations of inorganic nitrogen and phosphate in Section D1 of the mangrove forest were 0.63 mg/L and 0.003 mg/L, respectively. These levels were significantly lower than the results observed in the adjacent sea area and Section D2 of the mangrove forest during flood tides. The mangrove forest efficiently absorbed and consumed nutrients, with the nitrogen-to-phosphorus ratio of consumed nutrients being notably higher than that typically utilized by plants during growth. We identified various biogeochemical processes, including nitrogen fixation, mineralization, nitrification, and denitrification, occurring within the mangrove forest. Seawater measurements in Section D1 during ebb tides showed the mean pH of 7.58 and dissolved oxygen levels of 4.52 mg/L, resulting in a dissolved oxygen saturation level of only 57.0 %. The low dissolved oxygen levels were attributed to organic matter degradation in the forest. Consequently, the longer the water retention time, the more obvious the trend of hypoxia and acidification was observed. In the adjacent sea area, the sedimentary environment was deemed healthy, with pollutants primarily originating from runoff and ship discharge from waterways and ports. However, within the mangrove forest, the sediments exhibited higher enrichment factors for organic carbon and sulfide, indicating significant pollution compared to the adjacent sea area. The sediments were conducive to the accumulation and burial of total Hg, Zn, and Cu, while other heavy metals did not show prominent deposition and enrichment. Notably, the enrichment factor of oils was as high as 4.37, leading to the formation of an oil pollution zone at the forest edge, and the enrichment of pollutants in sediment may inhibit the growth and expansion of mangroves. Overall, this study shed light on the occurrence of seawater acidification, hypoxia, and the behavior of sediment pollutants within the mangrove forest. The findings provide valuable insights to support efforts aimed at promoting and maintaining the ecosystem health of mangrove forests.