Anoxic Area Research Articles

Using hydrochemistry and stable isotopes to character the karst water environment in a cave site, South China

For cave sites located within the seasonal fluctuation zone of groundwater, obtaining a comprehensive understanding of the hydrogeological conditions and hydrochemical environment is of utmost importance in assessing the preservation of cultural deposits. However, due to the heterogeneity of karst development, the hydrochemical environment of the overburden karst water-bearing unit may exhibit complex variations. In this study, the characteristics of the water environment in a foot cave system and its surrounding aquifer were investigated at the Zengpiyan site, employing hydrochemistry, hydrogen and oxygen isotope analysis, as well as sulfate isotope analysis, in conjunction with the hydrogeological background. Based on the exposure of the karst cave and the groundwater flow conditions, the water-bearing medium was categorized into five types. The aquifer exhibited either an oxidation or reduction condition, with a distinct “dissolved oxygen hole” observed in the saturated zone of the cave site. Ion concentration analysis revealed that the groundwater was influenced by the prior accumulation of cinders in the upstream region, leading to sulfate contamination within the foot cave system. However, during the groundwater movement from upstream to downstream, the attenuation rate of pollutants displayed significant variations. Notably, the sulfate content was unusually low within the site cave. Hydrogen and oxygen isotopes provided insights into the differing circulation and movement velocities of groundwater within the local environment. Additionally, sulfate isotopes confirmed the sources of sulfate and the occurrence of bacterial sulfate reduction within the site cave. Consequently, an environmental zoning classification was established based on these analyses. Although the concentration of dissolved ions appears low in the anoxic area, it does not imply a diminished environmental risk. Under reducing conditions, these pollutants can be converted into more aggressive gases, posing a substantial threat to the preservation environment of cave sites. Therefore, sufficient attention should be given to this aspect in order to ensure adequate protection.

Carbonate uranium isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event

The Toarcian Oceanic Anoxic Event (T-OAE; ~183 Mya) was a globally significant carbon-cycle perturbation linked to widespread deposition of organic-rich sediments, massive volcanic CO2 release, marine faunal extinction, sea-level rise, a crisis in carbonate production related to ocean acidification, and elevated seawater temperatures. Despite recognition of the T-OAE as a potential analog for future ocean deoxygenation, current knowledge on the severity of global ocean anoxia is limited largely to studies of the trace element and isotopic composition of black shales, which are commonly affected by local processes. Here, we present the first carbonate-based uranium isotope (δ238U) record of the T-OAE from open marine platform limestones of the southeastern Tethys Ocean as a proxy for global seawater redox conditions. A significant negative δ238U excursion (~0.4‰) is recorded just prior to the onset of the negative carbon isotope excursion comprised within the T-OAE, followed by a long-lived recovery of δ238U values, thus confirming that the T-OAE represents a global expansion of marine anoxia. Using a Bayesian inverse isotopic mass balance model, we estimate that anoxic waters covered ~6 to 8% of the global seafloor during the peak of the T-OAE, which represents 28 to 38 times the extent of anoxia in the modern ocean. These data, combined with δ238U-based estimates of seafloor anoxic area for other CO2-driven Phanerozoic OAEs, suggest a common response of ocean anoxia to carbon release, thus improving prediction of future anthropogenically induced ocean deoxygenation.

Long-term developments in seasonal hypoxia and response to climate change: A three-decade modeling study in the Ariake Sea, Japan

Hypoxia in the Ariake Sea, Japan, is steadily increasing in both duration and spatial coverage. Hypoxia, defined as dissolved oxygen (DO) below 3 mg/L, is strongly associated with the amplified frequency of extreme rainfall events driven by climate change, which poses a mounting threat to marine ecosystems on a global scale. In this study, we employed a general three-dimensional (3-D) hydrodynamic coastal model and a phytoplankton-based ecosystem model to identify the potential cause of seasonal hypoxic events over three decades. The results indicated a substantial decrease in bottom DO levels from 1992 to 2021, with the rate of increase in hypoxic area being 8 km2/yr (95 % CI: −0.38, 16.2) and the anoxic area increasing from almost non-existent to 100 km2. Notably, among various environmental drivers, increased river discharge was identified as a pivotal factor in the occurrence of hypoxia. Large-scale river discharge events can potentially increase water stratification, leading to the formation of hypoxia. River discharge volume and the duration of bottom hypoxia in the Ariake Sea were correlated. The duration of hypoxia was strongly associated with river discharge magnitude, with correlation coefficients ranging from 0.56 to 0.82 across six observational stations. Furthermore, analysis of varied simulated environmental factors over multiple years revealed diverse responses to climate change, indicating that the Ariake Sea is prone to experiencing a decline in its physical and water quality conditions.

Hydrodynamics, Diagenesis and Hypoxia Variably Drive Benthic Oxygen Flux in a River‐Reservoir System

AbstractBenthic oxygen flux with complex spatiotemporal variations is essential for the global budget of carbon dioxide and the regional security of water quality and ecology, but its dominant driver under different circumstances has yet to be identified. In this study, a parametric scheme of oxygen flux was proposed and validated with aquatic eddy correlation measurements and then coupled with a diagenesis model and a water environment model. The coupled model was applied to a river‐reservoir with significant environmental gradients in hydrodynamics, diagenesis, and hypoxia, which are three factors that competitively drive the variation in benthic oxygen flux. The results indicate that hydrodynamics dominate the flux in the riverine and thalweg areas, diagenesis is the dominant driver of the lacustrine and bank areas, and hypoxia shows dominance only in the hypolimnetic anoxic area. In general, diagenesis is the dominant driver of oxygen flux in river‐reservoirs, followed by hydrodynamics, both of which are more prominent than hypoxia. If the operated reservoir experiences a wet year, the dominance of hydrodynamics tends to increase, while that of diagenesis and hypoxia decreases. The three divers exhibit similar but more stable dominance in riverine systems than in reservoirs, while diagenesis becomes the exclusive driver of oxygen fluxes in lacustrine systems.

N2O dynamics in the hyporheic zone due to ripple migration

The migration rate of ripples along the streambed has a profound effect on the hyporheic exchange and nitrogen dynamics. However, how the denitrification intermediate N2O is released in mobile bedforms still remain unclear. In this study, the ripple domains are established based on the sediment stability criteria by using the mean channel flow velocity U and the median sediment grain size D50. A two-dimensional numerical model coupling hyporheic flow, ripple migration and N2O transformation is proposed to investigate how ripple celerity affects N2O dynamics in streambeds. The results show that there are different patterns for the release of N2O from migrating ripples with different medium grain sizes D50 (0.06–0.3 mm). N2O can be released via upwelling of anoxic pore water from the deeper area in immobile or slow-mobile ripples. However, rapid ripple migration and the resulting fast vertical flow reversal can suppress the net pore water displacement and lead to the formation of a redox seal, which separates the deeper anoxic area from the above oxic area and inhibits N2O emissions. The magnitude of the nutrients concentration gradient and the N2O flux between these two areas depend only on the dispersive and diffusive transport. Therefore, the controls of N2O emission flux hence shift from the pumping- dominated regime in stable ripples to a grain size dominated regime of the migrating ripples. The possible implications about the effects of hydrodynamic changes due to the migration of ripples are presented. Our results emphasize that knowledge of N2O production-consumption hotspots within streambeds is essential to understand the mechanisms of N2O emissions from the sediment–water interface.

A Regime Shift Toward a More Anoxic Environment in a Eutrophic Sea in Northern Europe

Dissolved oxygen in the sea is essential for marine fauna and biogeochemical processes. Decline in the sea water oxygen concentration is considered to be an effect of eutrophication, also exacerbated by climate change. The Baltic Sea is one of the most eutrophic seas in the world and is located in northern Europe. It is a vulnerable, brackish, semi-enclosed sea, suffering from high pressures from human activity. This leads to increased hypoxic and anoxic areas, which can be used as a measure of the environmental state. In the present study the extent of anoxic (O2 < 0 ml l–1) and hypoxic (O2 < 2 ml l–1) areas were estimated for the autumns in 1960–2019 using vertical profiles of observed oxygen concentrations in the Baltic proper and four sub-areas of the Baltic proper: the Bornholm Basin, the western, northern and eastern Gotland basins. From vertical profiles of observed salinity, the annual average of the halocline depths in the four sub-basins were estimated. The results imply regime shifts toward increased anoxic area extents in the Gotland basins around the turn of the 20th century. In autumn 2018, the extent of anoxic bottom areas in the Baltic Sea was record high since the start of the data series. During the later part of the studied period the depths of the halocline coincide with the depth of the hypoxia in the Gotland basins. This implies that in these basins a worst-case scenario for the extent of hypoxic areas seems to be reached.

Recent regime of persistent hypoxia in the Baltic Sea

Oxygen deficiency, in the form of hypoxia and anoxia, is a direct consequence of the eutrophication of the Baltic Sea. There is ongoing debate concerning the increasing extent of hypoxia. We analyse the integral metrics of hypoxia and anoxia: (1) temporal evolution of the hypoxic and anoxic area and volume, (2) the spatial distribution of the probabilities of hypoxia and anoxia occurrence in the Baltic Sea. The results are based on the state-of-the-art coupled physical and biogeochemical numerical model reanalysis data from Copernicus Marine Environment Monitoring Service for the period of 1993–2017. Statistical analysis showed that the variability of hypoxic and anoxic areas since the year 2000 represents stationary processes around their respective mean levels. From 2000 to 2017, the hypoxic area varies between 50000 and 80000 km2 and the anoxic area varies between 10000 and 50000 km2. Different methods and data sources indicate that the uncertainties of the estimates account for about 10000 km2. We suggest that the loss of stationarity of the time series of the hypoxic area would be an indication of the regime change of hypoxia development in the Baltic Sea. Probability distribution maps of hypoxia and anoxia provide detailed information about the persistency of hypoxia and anoxia in different parts of the Baltic Sea. The probability of hypoxia exceeds 0.9 in the eastern and western Gotland basins and in the deep area of the Bornholm basin. The Gulf of Finland and the shallower areas that connect different deep basins of the Baltic Sea exhibit seasonal and episodic hypoxia. The 80 m and 120 m isobaths are the approximate bathymetry limits of hypoxia and anoxia occurrence, respectively. Our study supports previous knowledge that hypoxia development is controlled to a large degree by the depth of the permanent halocline.

Estimates of internal loading of phosphorus in a western US reservoir using 3 methods

Burnet SH, Wilhelm FM. 2021. Estimates of internal loading of phosphorus in a western US reservoir using 3 methods. Lake Reserv Manage. 37:261–274. We used 3 methods to estimate internal loading of total and dissolved phosphorus (TP/DP) in 2 years in a stratified eutrophic western US reservoir with high internal loading of phosphorus. We calculated the internal load from (i) net increases of TP/DP from reservoir water column samples, (ii) gross TP mass balances, and (iii) gross TP/DP loading estimated from laboratory-incubated sediment cores and the anoxic reservoir area. We also present a cost comparison and spreadsheet to allow managers to estimate costs for each method. The highest internal loads were estimated in 2015 from the net in situ samples (999 mg/m2/yr) and the gross annual mass balance (850 mg/m2/yr). Gross estimates from sediment cores were 1.2× higher in 2015 than in 2014 due to differences in anoxic area resulting from a drought and record reservoir drawdown. Costs were highest for the mass balance method and lowest for the sediment core method. Given the small monetary difference between the sediment core and in situ methods, we suggest managers obtain in situ data to estimate internal loading because this yields a net estimate most reflective of whole-lake dynamics. Our data provide an important benchmark of internal loading in a human-made reservoir given the general paucity of such estimates in this large geographic region.

Uranium isotopes in marine carbonates as a global ocean paleoredox proxy: A critical review

The protracted oxygenation of the ocean-atmosphere system is one of the most fundamental changes to the Earth system through its history. The uranium isotopic composition (238U/235U, denoted as δ238U) of marine carbonates has been developed as a proxy to quantitatively track the timing, duration, and extent of global marine redox chemistry changes. This proxy has been applied to many critical evolutionary intervals in the last decade, significantly advancing our understanding of how life on Earth and its environment have co-evolved through geological history. Successful application of the uranium isotope paleoredox proxy requires a thorough understanding of the marine uranium budget, the processes by which seawater U-isotope signatures are recorded in marine carbonates, and the potential for alteration of these primary signatures by syn- and post-depositional diagenetic processes. Here, we provide a critical review of the U isotope proxy in marine carbonates with a focus on the current problems and areas where future work is needed to further develop this proxy. We also use a recently developed global C-P-U cycle model to illustrate that when the carbon cycle is perturbed by volcanic carbon injections, the ensuing transient relationship between seafloor anoxic area and δ238U can be complex and sometimes counter-intuitive.

Water Quality Characteristics and Distribution of Bacterial Communities During Thermal Stratification in the Miyun Reservoir

As an important urban drinking water source, reservoirs are a special type of water body formed by artificial dams. Water quality of reservoirs directly affects the residents' drinking water safety. In order to reveal the characteristics of stratification and vertical changes of bacterial communities in the Miyun Reservoir, a drinking water source of Beijing, vertical stratified samples were collected during the stable stratified period of the reservoir (autumn). The vertical distribution characteristics of bacterial communities in the Miyun Reservoir were studied by using 16S rDNA terminal restriction fragment length polymorphism (T-RFLP) and quantitative PCR. Cluster analysis and multivariate statistical analysis were used to reveal the response relationships between bacterial communities and environmental factors. The results were as follows. ①The thermocline of the Miyun Reservoir was located at a water depth of 20-30 m, and the water temperature range was 15-19℃. The cluster analysis data of the seven sampled water layers were divided into an aerobic area (upper layer) and anoxic area (lower layer). The temperature, dissolved oxygen, and pH gradually decreased below 15 m. The electrical conductivity, ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, and total nitrogen changed significantly after 15 m. The water quality showed obvious features in the vertical direction. ② The redundancy analysis (RDA) results showed that there were obvious vertical changes in the dissolved oxygen, pH, electrical conductivity, ammonia nitrogen, nitrate nitrogen, and nitrite nitrogen between the aerobic and anoxic water layer. Those factors were the main environmental factors affecting the vertical distribution of the bacterial communities in the Miyun Reservoir. ③ The total bacterial number fluctuated with changes in the water depth. The Shannon-Wiener index and the number of T-RFs of bacteria in the aerobic zone were significantly higher than those in the anoxic zone, which indicates that there was significant stratification in the distribution of bacterial communities in the water of the Miyun Reservoir in autumn. This study explored the effects of water stratification on reservoir water quality and bacterial communities, and the findings provide a scientific basis for predicting water quality changes and reservoir management.

Effects of oxygen and water content on microbial distribution in the polyurethane foam cubes of a biofilter for SO2 removal

The performance of a biofilter for off-gas treatment relies on the activity of microorganisms and adequate O2 and H2O. In present study, a microelectrode was applied to analyze O2 in polyurethane foam cubes (PUFCs) packed in a biofilter for SO2 removal. The O2 distribution varied with the density and water-containing rate (WCR) of PUFCs. The O2 concentration dropped sharply from 10.2 to 0.8mg/L from the surface to the center of a PUFC with 97.20% of WCR. The PUFCs with high WCR presented aerobic–anoxic–aerobic areas. Three-dimensional simulated images demonstrated that the structure of PUFCs with high WCR consisted of an aerobic “shell” and an anoxic “core”, with high-density PUFCs featuring a larger anoxic area than low-density PUFCs. Moreover, the H2O distribution in the PUFC was uneven and affected the O2 concentration. Whereas aerobic bacteria were observed in the PUFC surface, facultative anaerobic microorganisms were found at the PUFC core, where the O2 concentration was relatively low. O2 and H2O distributions differed in the PUFCs, and the distribution of microorganisms varied accordingly.

海面稀氧区的人工形成及其战术效能研究

为研发一种既能造成舰船人员窒息死亡又能毁坏舰船需氧动力系统的新型对舰攻击技术，提出了海面稀氧区的新技术思想，进而对其人工形成方法进行了研究。采用化学或物理的方法，在水下适当深度生成或释放特定气体，并通过气体的上浮和扩散，在海面局部区域造成氧气分压降低，从而导致氧浓度降低而形成稀氧环境。文章重点研究了海面稀氧区的人工形成原理，给出了特定气体的选取原则，对稀氧区的战术特点和作战效能进行了相应分析。研究结论可为海面稀氧区的实战应用提供理论支撑和技术基础，为新型对舰攻击技术的研发提供新的途径。 For developing a new type of attack technology on ship which can cause the ship personnel death by asphyxia and destroy ship aerobic dynamic system, the new technological thought of anoxic area over sea surface is proposed, then its artificial formation method is studied. By chemistry or physics method, the specific gas is generated or released in appropriate depth under water, through rise and diffusion of the gas, the oxygen partial pressure in local area over sea surface reduces, which results in decrease of oxygen concentration and formation of anoxic environment. The artificial formation principle of anoxic area over sea surface is studied mainly, the selection principle of specific gas is given, the tactical characteristics and operational effectiveness of anoxic area are analyzed accordingly. The research conclusion can provide theoretical support and technical basis for actual combat application of anoxic area over sea surface, also provide a new way to development of the new type of attack technology on ship.

Oxygen transfer dynamics and activated sludge floc structure under different sludge retention times at low dissolved oxygen concentrations

In activated sludge systems, the aeration process consumes the most energy. The energy cost can be dramatically reduced by decreasing the operating dissolved oxygen (DO) concentration. However, low DO may lead to incomplete nitrification and poor settling performance of activated sludge flocs (ASFs). This study investigates oxygen transfer dynamics and settling performances of activated sludge under different sludge retention times (SRTs) and DO conditions using microelectrodes and microscopic techniques. Our experimental results showed that with longer SRTs, treatment capacity and settling performances of activated sludge improved due to smaller floc size and less extracellular polymeric substances (EPS). Long-term low DO conditions produced larger flocs and more EPS per unit sludge, which produced a more extensive anoxic area and led to low oxygen diffusion performance in flocs. Long SRTs mitigated the adverse effects of low DO. According to the microelectrode analysis and fractal dimension determination, smaller floc size and less EPS in the long SRT system led to high oxygen diffusion property and more compact floc structure that caused a drop in the sludge volume index (SVI). In summary, our results suggested that long SRTs of activated sludge can improve the operating performance under low DO conditions.

Deep dynamic pools of phosphorus in the sediment of a temperate lagoon with recurring blooms of diazotrophic cyanobacteria

AbstractThe storage and release of phosphorus by sediments can act as an important control on the formation of noxious blooms of cyanobacteria in lakes and estuaries. Here we studied the uptake and release of phosphorus associated with iron oxides within sediments of a lagoon system affected by recurring summer blooms of the cyanobacterium Nodularia spumigena. Using an ascorbate extraction, we observed deep pools of iron oxide associated phosphorus (P) at concentrations of ∼5 μmol g−1 dry sed to a depth of ∼20 cm. This pool rapidly decreased with the onset of water column anoxia, leading to an integrated release of ∼300 mmol of phosphorus m−2 of sediment over 3.5 months. Scaling this flux over the periodically anoxic area of the lake and over the period of an N. spumigena bloom (November–December 2011) gave a broad mass balance agreement with the increase in total phosphorus within the water column and exported from the lagoon. Over this period we estimated that P release from the sediment would have allowed ∼130–187 tonnes of nitrogen to be fixed which agrees with previous estimates of nitrogen fixation. Upon reoxygenation of the water column, a regeneration of the deep iron oxide associated phosphorus pool was observed. This deep dynamic pool of phosphorus was most likely mediated by the burrowing activity of the polychaete Capitella capitata which was observed at the study site. This study underscores the potential importance of sediments colonized by deeply irrigating fauna to become a significant source of phosphorus during water column anoxia.

Characterization of phosphorus in marshland sediments by 31phosphorus nuclear magnetic resonance spectroscopy

Phosphorus turnover is vital to understand algal blooms and nutrient export in coastal waters; however, little is known about the composition and dynamic of sediment phosphorus in marshes. In this study, the relative composition of various organic phosphorus in sodium hydroxide (NaOH) extracts of freshwater and saltwater sediments in the Pelham Bay Park, New York City, the USA, were investigated, using solution 31phosphorus nuclear magnetic resonance Spectroscopy. The correlation between sodium hydroxide-extractable organic phosphorus and the organic matter content fitted to a linear model. Moreover, it is indicative that the species of organic phosphorus compounds in sediments are mainly dependent on salinity and the organic matter content in the sediments. The representative phosphorus compounds in the marshland sediments were glucose-6-phosphate nucleoside monophosphate and dihydroxyacetone phosphate. The composition of various phosphorous compounds was more complex in the saltwater sediments than in the freshwater sediments. Monoester phosphorus was the major component in most of the marshland, and diester phosphorus dominated the anoxic area.

Characteristics of greenhouse gas emission in three full-scale wastewater treatment processes

Three full-scale wastewater treatment processes, Orbal oxidation ditch, anoxic/anaerobic/aerobic (reversed A2O) and anaerobic/anoxic/aerobic (A2O), were selected to investigate the emission characteristics of greenhouse gases (GHG), including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Results showed that although the processes were different, the units presenting high GHG emission fluxes were remarkably similar, namely the highest CO2 and N2O emission fluxes occurred in the aerobic areas, and the highest CH4 emission fluxes occurred in the grit tanks. The GHG emission amount of each unit can be calculated from its area and GHG emission flux. The calculation results revealed that the maximum emission amounts of CO2, CH4 and N2O in the three wastewater treatment processes appeared in the aerobic areas in all cases. Theoretically, CH4 should be produced in anaerobic conditions, rather than aerobic conditions. However, results in this study showed that the CH4 emission fluxes in the forepart of the aerobic area were distinctly higher than in the anaerobic area. The situation for N2O was similar to that of CH4: the N2O emission flux in the aerobic area was also higher than that in the anoxic area. Through analysis of the GHG mass balance, it was found that the flow of dissolved GHG in the wastewater treatment processes and aerators may be the main reason for this phenomenon. Based on the monitoring and calculation results, GHG emission factors for the three wastewater treatment processes were determined. The A2O process had the highest CO2 emission factor of 319.3 g CO2/kg CODremoved, and the highest CH4 and N2O emission factors of 3.3 g CH4/kg CODremoved and 3.6 g N2O/kg TNremoved were observed in the Orbal oxidation ditch process.

Short-Cut Nitrification and Denitrification Treatment for High Strengh Ammonium Wastewater Using a Pilot-Scale “Anoxic-Anaerobic-Microaerobic-Aerobic (A<sub>2</sub>O<sub>2</sub>)” System

A pilot-scale nitrogen removal system composed of an anoxic area, an anaerobic area, a microaerobic area and an aerobic area (A2O2) was established to treat high strength ammonium wastewater from a nitrogenous fertilizer factory. The influences of key factors such as DO, pH, FA and temperature on partial nitrification in the microaerobic area were investigated. The results showed that in the DO concentration of 0.5~0.7mg/L, the stable and efficient nitrite accumulation could be realized. Meanwhile, the effluent NH3-N and TN quality was superior to the requirement of the《Synthetic ammonia industrial water contamination emission standard》(draft for comment). The optimal pH of 7.5~8.0 and the corresponding free ammonia (FA) concentration of around 0.95 mg/L limited the nitrobacteria activity selectively, thus favoring the stability of partial nitrification. The nitrite accumulation rate didn’t show obvious difference in wide temperature range (18~35°C) at given DO (0.5~0.7mg/L) and pH (7.5~8.0). But contrary to the usual partial nitrification obtained by high temperatures (>30°C), the effluent ammonium of A2O2system was substandard due to the inhibition of nitrobacteria activity caused by high temperature, which diminished the further removal of ammonium in the subsequent aerobic area.

Microbial Diversity of an Anoxic Zone of a Hydroelectric Power Station Reservoir in Brazilian Amazonia

Microbial diversity was evaluated in an anoxic zone of Tucuruí Hydroelectric Power Station reservoir in Brazilian Amazonia using a culture-independent approach by amplifying and sequencing fragments of the 16S rRNA gene using metagenomic DNA as a template. Samples obtained from the photic, aphotic (40m) and sediment (60m) layers were used to construct six 16S rDNA libraries containing a total of 1,152 clones. The sediment, aphotic and photic layers presented 64, 33 and 35 unique archaeal operational taxonomic units (OTUs). The estimated richness of these layers was evaluated to be 153, 106 and 79 archaeal OTUs, respectively, using the abundance-based coverage estimator (ACE) and 114, 83 and 77 OTUs using the Chao1 estimator. For bacterial sequences, 114, 69 and 57 OTUs were found in the sediment, aphotic and photic layers, which presented estimated richnesses of 1,414, 522 and 197 OTUs (ACE) and 1,059, 1,014 and 148 OTUs (Chao1), respectively. Phylogenetic analyses of the sequences obtained revealed a high richness of microorganisms which participate in the carbon cycle, namely, methanogenic archaea and methanotrophic proteobacteria. Most sequences obtained belong to non-culturable prokaryotes. The present study offers the first glimpse of the huge microbial diversity of an anoxic area of a man-made lacustrine environment in the tropics.

Temporal and spatial distribution of hypoxic/seasonal anoxic zone in Amvrakikos Gulf, Western Greece

Coastal hypoxia and anoxia have become a serious environmental problem in many coastal ecosystems, and recent evidence suggests an increase in the number of systems experiencing hypoxia globally and an increased frequency and duration of hypoxia in many historically hypoxic systems. Eutrophication, low physical energy and large freshwater input are some of the most important factors, which lead to the development of hypoxia in coastal areas. The Amvrakikos Gulf, located in western Greece, is one of the most important protected areas in Greece and in Europe and past studies indicated that some regions of the Gulf experience hypoxia. Despite the importance of the Amvrakikos Gulf ecosystem, little is known about key ecological, chemical, and physical processes in the Gulf and their relationship with oxygen depletion. In the present study, authors try to answer some important questions regarding the distribution and variation of the hypoxic zone. After a year of measurements, it was concluded that according to spatial distribution of DO, the gulf can be divided into two parts, the western and eastern areas, with important differences occurring between them. Within the western area, the water column was well oxygenated during winter and spring and hypoxic conditions occurred only in summer and autumn. The eastern area was hypoxic throughout the year and anoxic conditions occurred only during autumn. In conclusion, taking into account the author’s measurements and previous studies, the seasonally hypoxic eastern area in 1987 converted into a seasonally anoxic area in 2009 and the western part of the gulf, remained seasonally hypoxic, indicating the degradation of the gulf’s environmental state within the last 20 years.

Fluctuations of physicochemical characteristics in sediments and overlying water during an anoxic event: a case study from Lesina lagoon (SE Italy)

Coastal lagoons are vulnerable systems often affected by severe anoxic events, also as direct consequence of excessive accumulation of organic matter. During these events, also in function of their location and geomorphology, they are characterized by large temporal and spatial fluctuations of physical and chemical conditions. Results presented in this work are related to an anoxic event occurred in Lesina lagoon (SE Italy) in summer 2008. The main aim is to describe the short-term dynamics of physicochemical characteristics of both sediment and overlaying water, inside and outside the critical area of the anoxic event. Nine sampling campaigns were carried out on a weekly basis between the first week of July and the second week of September 2008. The water sampling was performed at three sites located in the western side (inside and outside area interested by crisis) and in the eastern zone of the lagoon, close to the communicating channel with the sea, while the sediment samples were collected in four stations, including two sites within crisis area and two outside. At each occasion T, S, pH, DO, nutrients and chl a were measured in water and TOC, TN, TP, TFe and ORP were performed on superficial sediments. In relation to the water, wide temporal fluctuations of physico-chemical parameters were observed for the site inside the anoxic area, which exhibited the highest concentrations of chl a at the beginning of the observation period, with a peak of biomass of 180 µg.L -1 followed by saturation of oxygen higher than 100%. A strong significant difference among sampling sites was observed in relation to nutrients; in particular, the site within the anoxic area exhibited broader concentration ranges of both SRP and TP than the other sites, while TN was in general higher in the site outside. Significant differences related to the sedimentary variables were also observed between anoxic and reference area, except for TP which was homogeneously distributed during the observation period in all the sites. Disturbed sites showed the highest contents of TOC at the beginning of the observation period (July 2008), TN contents almost two times lower and mean contents of TFe higher than those obtained in control sites. All sites were characterized by highly reducing conditions despite the well oxygenated conditions of the overlying waters. These findings highlight wider time-related fluctuations of physico-chemical parameters (for both sediments and overlying water) within anoxic area than in control area, but, at the same time, stress the significant role of the factor space (anoxic sites and control sites) for the observed physico-chemical diferences. Also, this integrated approach between sediments and water indicates sediment-related source of nutrients in the anoxic overlying water, suggesting that quick mineralization processes and nutrient release in mid-term happened mainly in disturbed area. The high biomass production observed along the overlying water within the anoxic area at the beginning of the observation period was related to this greater availability of nutrients in disturbed area.