Early life stage mechanisms of an active fish species to cope with ocean warming and hypoxia as interacting stressors

Ocean's characteristics are rapidly changing, modifying environmental suitability for early life stages of fish. We assessed whether the chronic effects of warming (24 °C) and hypoxia (<2-2.5 mg L-1) will be amplified by the combination of these stressors on mortality, growth, behaviour, metabolism and oxidative stress of early stages of the white seabream Diplodus sargus. Combined warming and hypoxia synergistically increased larval mortality by >51%. Warming induced faster growth in length and slower gains in weight when compared to other treatments. Boldness and exploration were not directly affected, but swimming activity increased under all test treatments. Under the combination of warming and hypoxia, routine metabolic rate (RMR) significantly decreases when compared to other treatments and shows a negative thermal dependence. Superoxide dismutase and catalase activities increased under warming and were maintained similar to control levels under hypoxia or under combined stressors. Under hypoxia, the enzymatic activities were not enough to prevent oxidative damages as lipid peroxidation and DNA damage increased above control levels. Hypoxia reduced electron transport system activity (cellular respiration) and isocitrate dehydrogenase activity (aerobic metabolism) below control levels. However, lactate dehydrogenase activity (anaerobic metabolism) did not differ among treatments. A Redundancy Analysis showed that ∼99% of the variability in mortality, growth, behaviour and RMR among treatments can be explained by molecular responses. Mortality and growth are highly influenced by oxidative stress and energy metabolism, exhibiting a positive relationship with reactive oxygen species and a negative relationship with aerobic metabolism, regardless of treatment. Under hypoxic condition, RMR, boldness and swimming activity have a positive relationship with anaerobic metabolism regardless of temperature. Thus, seabreams may use anaerobic reliance to counterbalance the effects of the stressors on RMR, activity and growth. The outcomes suggests that early life stages of white seabream overcame the single and combined effects of hypoxia and warming.

Physical Dissolution Combined with Photodynamic Depletion: A Two-Pronged Nanoapproach for Deoxygenation-Driven and Hypoxia-Activated Prodrug Therapy

Hypoxia may enhance the chemoresistance of cancer cells and can significantly compromise the effectiveness of chemotherapy. Many efforts have been made to relieve or reverse hypoxia by introducing more oxygen into the tumor microenvironment (TME). Acting in a diametrically opposite way, in the current study, a novel nanocarrier was designed to further exhaust the oxygen level of the hypoxic TME. By creating such an oxygen depleted TME, the hypoxia-selective cytotoxin can work effectively, and oxygen exhaustion triggered chemotherapy can be achieved. Herein, deoxygenation agent, FDA-approved perfluorocarbon (PFC) and photosensitizer indocyanine green (ICG) for oxygen depletion, along with the hypoxia-activating drug tirapazamine (TPZ), were coincorporated within the poly(lactic-co-glycolic acid) (PLGA) nanoemulsion (ICG/TPZ@PPs) for the treatment of hypoxic tumors. Following hypoxia amplifying through physical oxygen dissolution and photodynamic depletion in tumors, hypoxic chemotherapy could be effectively activated to improve multitreatment synergy. After achieving local tumor enrichment, PFC-mediated oxygen dissolution combined with further ICG-mediated photodynamic therapy (PDT) under near-infrared (NIR) laser irradiation could induce enhanced hypoxia, which would activate the antitumor activity of codelivered TPZ to synergize cytotoxicity. Remarkably, in vivo experimental results exhibited that deoxygenated ICG/TPZ@PPs-based photothermal therapy (PTT), PDT, and hypoxia activated chemotherapy have an excellent synergistic ablation of tumors without obvious side effects, and therefore, a broad prospect of application of this nanocarrier could be expected.

Trade-induced displacement of impacts of global crop production on oxygen depletion in marine ecosystems

In our globalized world, local impacts of agricultural production are increasingly driven by consumption in geographically distant places. Current agricultural systems strongly rely on nitrogen (N) fertilization to increase soil fertility and crop yields. Yet, a large portion of N added to cropland is lost through leaching / runoff potentially leading to eutrophication in coastal ecosystems. By coupling data on global production and N fertilization for 152 crops with a Life Cycle Assessment (LCA)-based model, we first estimated the extent of oxygen depletion occurring in 66 Large Marine Ecosystems (LMEs) due to agricultural production in the watersheds draining into these LMEs. We then linked this information to crop trade data to assess the displacement from consuming to producing countries, in terms of oxygen depletion impacts associated to our food systems. In this way, we characterized how impacts are distributed between traded and domestically sourced agricultural products. We found that few countries dominate global impacts and that cereal and oil crop production accounts for the bulk of oxygen depletion impacts. Globally, 15.9 % of total oxygen depletion impacts of crop production are ascribable to export-driven production. However, for exporting countries like Canada, Argentina or Malaysia this share is much higher, often up to three-quarters of their production impacts. In some importing countries, trade contributes to reduce pressure on already highly affected coastal ecosystems. This is the case for countries whose domestic crop production is associated with high oxygen depletion intensities, i.e. the impact per kcal produced, such as Japan or South Korea. Next to these positive effects trade can play in lowering overall environmental burdens, our results also highlight the importance of a holistic food system perspective when aiming to reduce the oxygen depletion impacts of crop production.

Non-interactive effects drive multiple stressor impacts on the taxonomic and functional diversity of atlantic stream macroinvertebrates

Freshwaters are considered among the most endangered ecosystems globally due to multiple stressors, which coincide in time and space. These local stressors typically result from land-use intensification or hydroclimatic alterations, among others. Despite recent advances on multiple stressor effects, current knowledge is still limited to manipulative approaches minimizing biological and abiotic variability. Thus, the assessment of multiple stressor effects in real-world ecosystems is required. Using an extensive survey of 50 stream reaches across North Portugal, we evaluated taxonomic and functional macroinvertebrate responses to multiple stressors, including marked gradients of nutrient enrichment, flow reduction, riparian vegetation structure, thermal stress and dissolved oxygen depletion. We analyzed multiple stressor effects on two taxonomic (taxon richness, Shannon-diversity) and two trait-based diversity indices (functional richness, functional dispersion), as well as changes in trait composition. We found that multiple stressors had additive effects on all diversity metrics, with nutrient enrichment identified as the most important stressor in three out of four metrics, followed by dissolved oxygen depletion and thermal stress. Taxon richness, Shannon-diversity and functional richness responded similarly, whereas functional dispersion was driven by changes in flow velocity and thermal stress. Functional trait composition changed along a major stress gradient determined by nutrient enrichment and oxygen depletion, which was positively correlated with organisms possessing fast-living strategies, aerial respiration, adult phases, and gathering-collector feeding habits. Overall, our results reinforce the need to consider complementary facets of biodiversity to better identify assembly processes in response to multiple stressors. Our data suggest that stressor interactions may be less frequent in real-word streams than predicted by manipulative experiments, which can facilitate mitigation strategies. By combining an extensive field survey with an integrative consideration of multiple biodiversity facets, our study provides new insights that can help to better assess and manage rivers in a global change context.

Deoxygenation turns the coastal Red Sea lagoons into sources of nitrous oxide

Direct measurements of dissolved N₂O concentrations, fluxes and saturation percentages undertaken for the first time in two coastal lagoons - Al-Shabab and Al-Arbaeen, along the east coast of the Red Sea, revealed the region as a significant source of N₂O to the atmosphere. The exacerbated dissolved inorganic nitrogen (DIN) from various anthropogenic sources led to substantial oxygen depletion in both the lagoons, which turned to bottom anoxia at Al-Arbaeen lagoon during the spring season. We assume that the accumulation of N₂O is caused by nitrifier-denitrification in the hypoxic/anoxic boundaries. In fact, the results indicated that oxygen-depleted bottom waters favoured denitrification when the oxygenated surface waters recorded nitrification signals. Overall, the N₂O concentration ranged from 109.4 to 788.6 nM (40.6-325.6 nM) in spring and 58.7 to 209.8 nM (35.8-89.9 nM) in winter in the Al-Arbaeen (Al-Shabab) lagoon. The N₂O flux ranged from 647.1 to 1763.2 μmol m^-2 day^-1 (85.9 to 160.2 μmol m^-2 day^-1) and 112.5 to 150.8 μmol m^-2 day^-1 (76.1 to 88.7 μmol m^-2 day^-1) in the spring and winter respectively, in the Al-Arbaeen (Al-Shabab) lagoons. The ongoing developmental activities may worsen the current situation of hypoxia and associated biogeochemical feedbacks; therefore, the present results underline the need for continuous monitoring of both lagoons to restrict more severe oxygen depletion in future.

A multi-stable isotopic constraint on water column oxygen sinks in the Pearl River Estuary, South China

Bottom water oxygen depletion is a central concern in estuaries and coastal oceans worldwide. However, a mechanistic understanding and quantitative diagnosis of different oxygen-consuming processes are less clear. In this study, a multi-stable isotope approach is developed to delineate the role of oxygen respiration and nitrification contributing to total oxygen consumption in the Pearl River Estuary (PRE), a large eutrophic estuary in south China. The approach highly couples with analysis of the carbon isotope composition of dissolved inorganic carbon (δ¹³C-DIC) and with stable nitrogen isotope analysis in ammonium (δ¹⁵N-NH₄⁺) and nitrate (δ¹⁵N-NO₃^-). In all seasons, relatively low DO concentrations were observed in the upper reach and, to some extent, in the outer estuary during summer, while high concentrations of DO were found in the transition zone between the inner and outer estuary. On the basis of isotopic differentiation, our data reveal that much more depleted δ¹³C-DIC is coincident with DIC additions and low oxygen in the upper reach and inner estuary during most seasons. This is most likely a consequence of organic carbon (OC) degradation via aerobic respiration. Based on the carbon isotopic mass balance of DIC and the stoichiometry ratio of -ΔDO/ΔDIC, we found that the OC degradation dominates the total oxygen consumption in the upper reach, as well as in the inner estuary during summer (48.3%-93.5%). In addition, nitrification is another key process in contributing to total oxygen loss in the upper reach, as supported by the well-coupled variations of δ¹⁵N of NH₄⁺ and NO₃^- and apparent oxygen utilization (AOU). Using the formerly determined N isotopic fractionation and observed δ¹⁵N variation, we estimated that nitrification could account for 35.3%-44.1% and 28.5%-31.6% of the total oxygen consumption in the upper reach during winter and summer, respectively, while its contribution to total oxygen loss is minor in the inner and outer estuary. Overall, this study demonstrates the potential of the multi-stable isotopic approach to assess oxygen sink partitioning in large human-perturbed estuaries.

Seasonal evolution and controlling factors of bottom oxygen depletion in the Bohai Sea

A coupled physical-biogeochemical model is used to investigate the seasonal evolution and controlling factors of oxygen depletion in the Bohai Sea (BS). Comparisons show that the model reproduces observed spatiotemporal variations of important physical and biogeochemical variables well. Bottom oxygen in the BS shows an annual cycle with significant drawdown in summer and enhanced replenishment in fall. Two oxygen-depleted regions off Qinhuangdao (QHD) and the Yellow River estuary (YRE) develop separately and experience higher oxygen depletion rates and longer durations of low-oxygen conditions. The evolution of oxygen depletion is primarily controlled by stratification and biological oxygen consumption but is also modulated by lateral transport. Strong stratification is established earlier than oxygen depletion and maintains its development. The biological oxygen consumption determines the two oxygen-depleted regions under stratified conditions. Lateral transport influenced by anticyclonic circulations favors an expansion of oxygen depletion off QHD but alleviates oxygen depletion off the YRE.

A Radiation Biological Analysis of the Oxygen Effect as a Possible Mechanism in FLASH

The delivery of radiation at an ultra-high dose rate (FLASH) is an important new approach to radiotherapy (RT) that appears to be able to improve the therapeutic ratio by diminishing damage to normal tissues. While the mechanisms by which FLASH improves outcomes have not been established, a role involving molecular oxygen (O2) is frequently mentioned. In order to effectively determine if the protective effect of FLASH RT occurs via a differential direct depletion of O2 (compared to conventional radiation), it is essential to consider the known role of O2 in modifying the response of cells and tissues to ionising radiation (known as 'the oxygen effect'). Considerations include: (1) The pertinent reaction involves an unstable intermediate of radiation-damaged DNA, which either undergoes chemical repair to restore the DNA or reacts with O2, resulting in an unrepairable lesion in the DNA, (2) These reactions occur in the nuclear DNA, which can be used to estimate the distance needed for O2 to diffuse through the cell to reach the intermediates, (3) The longest lifetime that the reactive site of the DNA is available to react with O2 is 1-10 μsec, (4) Using these lifetime estimates and known diffusion rates in different cell media, the maximal distance that O2 could travel in the cytosol to reach the site of the DNA (i.e., the nucleus) in time to react are 60-185 nm. This calculation defines the volume of oxygen that is pertinent for the direct oxygen effect, (5) Therefore, direct measurements of oxygen to determine if FLASH RT operates through differential radiochemical depletion of oxygen will require the ability to measure oxygen selectively in a sphere of <200 nm, with a time resolution of the duration of the delivery of FLASH, (6) It also is possible that alterations of oxygen levels by FLASH could occur more indirectly by affecting oxygen-dependent cell signalling and/or cellular repair.

May oxygen depletion explain the FLASH effect? A chemical track structure analysis

BACKGROUND AND PURPOSE

Recent observations in animal models show that ultra-high dose rate ("FLASH") radiation treatment significantly reduces normal tissue toxicity maintaining an equivalent tumor control. The dependence of this "FLASH" effect on target oxygenation has led to the assumption that oxygen "depletion" could be its major driving force.

MATERIALS AND METHODS

In a bottom-up approach starting from the chemical track evolution of 1 MeV electrons in oxygenated water simulated with the TRAX-CHEM Monte Carlo code, we determine the oxygen consumption and radiolytic reactive oxygen species production following a short radiation pulse. Based on these values, the effective dose weighted by oxygen enhancement ratio (OER) or the in vitro cell survival under dynamic oxygen pressure is calculated and compared to that of conventional exposures, at constant OER.

RESULTS

We find an excellent agreement of our Monte Carlo predictions with the experimental value for radiolytic oxygen removal from oxygenated water. However, the application of the present model to published radiobiological experiment conditions shows that oxygen depletion can only have a negligible impact on radiosensitivity through oxygen enhancement, especially at typical experimental oxygenations where a FLASH effect has been observed.

CONCLUSION

We show that the magnitude and dependence of the "oxygen depletion" hypothesis are not consistent with the observed biological effects of FLASH irradiation. While oxygenation plays an undoubted role in mediating the FLASH effect, we conclude that state-of-the-art radiation chemistry models do not support oxygen depletion and radiation-induced transient hypoxia as the main mechanism.

Determining the parameter space for effective oxygen depletion for FLASH radiation therapy

There has been a recent revival of interest in the FLASH effect, after experiments have shown normal tissue sparing capabilities of ultra-high-dose-rate radiation with no compromise on tumour growth restraint. A model has been developed to investigate the relative importance of a number of fundamental parameters considered to be involved in the oxygen depletion paradigm of induced radioresistance. An example eight-dimensional parameter space demonstrates the conditions under which radiation may induce sufficient depletion of oxygen for a diffusion-limited hypoxic cellular response. Initial results support experimental evidence that FLASH sparing is only achieved for dose rates on the order of tens of Gy/s or higher, for a sufficiently high dose, and only for tissue that is slightly hypoxic at the time of radiation. We show that the FLASH effect is the result of a number of biological, radiochemical and delivery parameters. Also, the threshold dose for a FLASH effect occurring would be more prominent when the parameterisation was optimised to produce the maximum effect. The model provides a framework for further FLASH-related investigation and experimental design. An understanding of the mechanistic interactions producing an optimised FLASH effect is essential for its translation into clinical practice.

Prolonged high biomass diatom blooms induced formation of hypoxic-anoxic zones in the inner part of Johor Strait

The Johor Strait has experienced rapid development of various human activities and served as the main marine aquaculture area for the two countries that bordered the strait. Several fish kill incidents in 2014 and 2015 have been confirmed, attributed to the algal blooms of ichthyotoxic dinoflagellates; however, the cause of fish kill events after 2016 was not clarified and the causative organisms remained unknown. To clarify the potential cause of fish kills along the Johor Strait, a 1-year field investigation was conducted with monthly sampling between May 2018 and April 2019. Monthly vertical profiles of physical water parameters (temperature, salinity, and dissolved oxygen levels) were measured in situ at different depths (subsurface, 1 m, 5 m, and 8 m) depending on the ambient depth of the water column at the sampling stations. The spatial-temporal variability of macronutrients and chlorophyll a content was analyzed. Our results showed that high chlorophyll a concentration (up to 48.8 μg/L) and high biomass blooms of Skeletonema, Chaetoceros, Rhizosolenia, and Thalassiosira were observed seasonally at the inner part of the strait. A hypoxic to anoxic dead zone, with the dissolved oxygen levels ranging from 0.19 to 1.7 mg/L, was identified in the inner Johor Strait, covering an estimated area of 10.3 km². The occurrence of high biomass diatom blooms and formation of the hypoxic-anoxic zone along the inner part Johor Strait were likely the culprits of some fish kill incidents after 2016.

Absence of hypoxia events in the adjacent coastal waters of Grijalva-Usumacinta river, Southern Gulf of Mexico

Globally, oxygen concentration in many coastal areas is depleting. River nutrient discharges may produce hypoxia events. The Southern Gulf of Mexico receives the discharges of the Grijalva-Usumacinta River System, the second largest in the Gulf of Mexico. To evaluate the influence of river discharges on dissolved oxygen concentrations in the receiving coastal ecosystem, we studied the variation of physicochemical variables in the water column. During the dry season, the influence of the river waters to the coastal area is scarce, but during the rainy season the river plume reached ~9 km offshore. The lowest concentration of dissolved oxygen (3.6 mg L^-1) was observed within the river plume. We concluded that, in the studied area, hypoxia events (oxygen concentrations ≤ 2 mg L^-1) would occur during the rainy season, low winds and in deeper waters (>80 m depth).

Multiple factors dominate the distribution of methane and its sea-to-air flux in the Bohai Sea in summer and autumn of 2014

The Bohai Sea is well-known as a source of atmospheric methane (CH₄). However, the main regulate factors of the spatiotemporal distribution of CH₄ and its sea-to-air flux remain largely unknown. In this study, the observed CH₄ concentration ranged from 4.8 to 32.7 nmol/L and 3.1 to 15.2 nmol/L in August and November of 2014, respectively. The main factors that influence the distribution of CH₄ and its sea-to-air flux were stratification, solubility, and current structure for the mid-west depression basins, the permanent well-mixed seawater column and CH₄ source strength for the centre shallow ridge zone, and the upwelling for the east depression basin, respectively. Meanwhile, wind also plays an important role in sea-to-air CH₄ flux in the study area except the centre shallow ridge zone. Upwelling made the east depression basin the most intensive source of CH₄, with a flux of 2 to 4 times higher than the other sub-regions.

Metalimnetic oxygen minimum and the presence of Planktothrix rubescens in a low-nutrient drinking water reservoir

Dissolved oxygen is a key player in water quality. Stratified water bodies show distinct vertical patterns of oxygen concentration, which can originate from physical, chemical or biological processes. We observed a pronounced metalimnetic oxygen minimum in the low-nutrient Rappbode Reservoir, Germany. Contrary to the situation in the hypolimnion, measurements of lateral gradients excluded the sediment contact zone from the major sources of oxygen depletion for the metalimnetic oxygen minimum. Instead, the minimum was the result of locally enhanced oxygen consumption in the open water body. A follow-up monitoring included multiple chlorophyll a fluorescence sensors with high temporal and vertical resolution to detect and document the evolution of phytoplankton. While chlorophyll fluorescence sensors with multiple channels detected a mass development of the phycoerythrin-rich cyanobacterium Planktothrix rubescens in the metalimnion, this species was overlooked by the commonly used single-channel chlorophyll sensor. The survey indicated that the waning P. rubescens fluorescence was responsible for the oxygen minimum in the metalimnion. We hypothesize that pelagic processes, i.e., either oxygen use through decomposition of dead organic material originating from P. rubescens or P. rubescens extending its respiration beyond its photosynthetic activity, induced the metalimnetic oxygen minimum. The deeper understanding of the oxygen dynamics is mandatory for optimizing reservoir management.

Eutrophication assessment in the transit area German Bight (North Sea) 2006-2014 - Stagnation and limitations

The eutrophication status of the German Bight (North Sea) has been assessed the third time since 1998 according to the OSPAR-Comprehensive Procedure between 2006 and 2014. Since the 1980s nutrient discharges and atmospheric nitrogen deposition had declined significantly but chlorophyll a and nutrient concentrations remained above assessment levels inshore and in inner coastal waters, reflecting continuing eutrophication. Recently local river discharges stagnated or increased again and total nitrogen remained above a reduction target of 200 μM. Most nutrients and conversion products were imported by a coastal current, passing the German Bight. Organic matter was trapped in offshore bottom waters in the ancient Elbe valley, causing repeated annual oxygen minima (<6 mg/L) and a classification as Problem Area. Effects of national reduction measures are limited in the transit area German Bight because improvements in open coastal waters require international efforts, based on comprehensive analyses.

A microfluidic oxygen sink to create a targeted cellular hypoxic microenvironment under ambient atmospheric conditions

Physiological oxygen levels within the tissue microenvironment are usually lower than 14%, in stem cell niches these levels can be as low as 0-1%. In cell cultures, such low oxygen levels are usually mimicked by altering the global culture environment either by O₂ removal (vacuum or oxygen absorption) or by N₂ supplementation for O₂ replacement. To generate a targeted cellular hypoxic microenvironment under ambient atmospheric conditions, we characterised the ability of the dissolved oxygen-depleting sodium sulfite to generate an in-liquid oxygen sink. We utilised a microfluidic design to place the cultured cells in the vertical oxygen gradient and to physically separate the cells from the liquid. We demonstrate generation of a chemical in-liquid oxygen sink that modifies the surrounding O₂ concentrations. O₂ level control in the sink-generated hypoxia gradient is achievable by varying the thickness of the polydimethylsiloxane membrane. We show that intracellular hypoxia and hypoxia response element-dependent signalling is instigated in cells exposed to the microfluidic in-liquid O₂ sink-generated hypoxia gradient. Moreover, we show that microfluidic flow controls site-specific microenvironmental kinetics of the chemical O₂ sink reaction, which enables generation of intermittent hypoxia/re-oxygenation cycles. The microfluidic O₂ sink chip targets hypoxia to the cell culture microenvironment exposed to the microfluidic channel architecture solely by depleting O₂ while other sites in the same culture well remain unaffected. Thus, responses of both hypoxic and bystander cells can be characterised. Moreover, control of microfluidic flow enables generation of intermittent hypoxia or hypoxia/re-oxygenation cycles.

STATEMENT OF SIGNIFICANCE

Specific manipulation of oxygen concentrations in cultured cells' microenvironment is important when mimicking low-oxygen tissue conditions and pathologies such as tissue infarction or cancer. We utilised a sodium sulfite-based in-liquid chemical reaction to consume dissolved oxygen. When this liquid was pumped into a microfluidic channel, lowered oxygen levels could be measured outside the channel through a polydimethylsiloxane PDMS membrane allowing only for gaseous exchange. We then utilised this setup to deplete oxygen from the microenvironment of cultured cells, and showed that cells responded to hypoxia on molecular level. Our setup can be used for specifically removing oxygen from the cell culture microenvironment for experimental purposes and for generating a low oxygen environment that better mimics the cells' original tissue environments.

Fungal mitochondrial oxygen consumption induces the growth of strict anaerobic bacteria

Fungi are commonly encountered as part of a healthy oral ecosystem. Candida albicans is the most often observed and investigated fungal species in the oral cavity. The role of fungi in the oral ecosystem has remained enigmatic for decades. Recently, it was shown that C. albicans, in vitro, influences the bacterial composition of young oral biofilms, indicating it possibly plays a role in increasing diversity in the oral ecosystem. C. albicans favored growth of strictly anaerobic species under aerobic culture conditions. In the present study, the role of mitochondrial respiration, as mechanism by which C. albicans modifies its environment, was investigated. Using oxygen sensors, a rapid depletion of dissolved oxygen (dO₂) was observed. This decrease was not C. albicans specific as several non-albicans Candida species showed similar oxygen consumption. Heat inactivation as well as addition of the specific mitochondrial respiration inhibitor Antimycin A inhibited depletion of dO₂. Using 16S rDNA sequencing, it is shown that mitochondrial activity, more than physical presence of C. albicans is responsible for inducing growth of strictly anaerobic oral bacteria in aerobic growth conditions. The described mechanism of dO₂ depletion may be a general mechanism by which fungi modulate their direct environment.

Potamodromous fish movements under multiple stressors: Connectivity reduction and oxygen depletion

Rivers are impacted by multiple stressors that can interact to create synergistic, additive or antagonistic effects, but experimental studies on fish encompassing more than one stressor are seldom found. Thus, there is the need to study stressors through multifactorial approaches that analyse the impact of fish exposure to multiple stressors and evaluate fish sensitivity to stressor combinations. Some of the most common impacts to Mediterranean rivers are of two natures: i) water abstraction and ii) diffuse pollution. Therefore, the present study aims at studying the responses of potamodromous fish facing combinations of: 1) a primary stressor (two levels of connectivity reduction due to water scarcity), and 2) a secondary stressor (three levels of oxygen depletion due to increase organic load - of anthropogenic nature). Schools of five wild fish from a cyprinid species (Luciobarbus bocagei) were placed in a flume, equipped with see-through sidewalls to allow for behavioural analysis, and subjected to different combinations of the stressors. Results show that at the unconnected level the primary stressor (lack of connectivity) overrode the effect of the secondary stressor (oxygen depletion), but when connectivity existed oxygen depletion caused a reduction of fish movements with decreasing oxygen concentrations. This multifactorial study contributes to improved prediction of fish responses upon actual or projected pressure scenarios.

Potential impact of an exceptional bloom of Karenia mikimotoi on dissolved oxygen levels in waters off western Ireland

In the summer of 2005 an exceptional bloom of the dinoflagellate Karenia mikimotoi occurred along Ireland's Atlantic seaboard and was associated with the mass mortality of both benthic and pelagic marine life. Oxygen depletion, cellular toxicity and physical smothering, are considered to be the main factors involved in mortality. In this paper we use a theoretical approach based on stoichiometry (the Anderson ratio) and an average K. mikimotoi cellular carbon content of 329pgCcell^-1 (n=20) to calculate the carbonaceous and nitrogenous oxygen demand following bloom collapse. The method was validated against measurements of biochemical oxygen demand and K. mikimotoi cell concentration. The estimated potential oxygen utilisation (POU) was in good agreement with field observations across a range of cell concentrations. The magnitude of POU following bloom collapse, with the exception of three coastal areas, was considered insufficient to cause harm to most marine organisms. This indicates that the widespread occurrence of mortality was primarily due to other factors such as cellular toxicity and/or mucilage production, and not oxygen depletion or related phenomena. In Donegal Bay, Kilkieran Bay and inner Dingle Bay, where cell densities were in the order of 10⁶cellsL^-1, estimated POU was sufficient to cause hypoxia. Of the three areas, Donegal Bay is considered to be the most vulnerable due to its hydrographic characteristics (seasonally stratified, weak residual flow) and hypoxic conditions (2.2mgL^-1 O₂) were directly observed in the Bay post bloom collapse. Here, depending on the time of bloom collapse, depressed DO levels could persist for weeks and continue to have a potentially chronic impact on the Bay.

Reconstructing ecosystem functions of the active microbial community of the Baltic Sea oxygen depleted sediments

Baltic Sea deep water and sediments hold one of the largest anthropogenically induced hypoxic areas in the world. High nutrient input and low water exchange result in eutrophication and oxygen depletion below the halocline. As a consequence at Landsort Deep, the deepest point of the Baltic Sea, anoxia in the sediments has been a persistent condition over the past decades. Given that microbial communities are drivers of essential ecosystem functions we investigated the microbial community metabolisms and functions of oxygen depleted Landsort Deep sediments by metatranscriptomics. Results show substantial expression of genes involved in protein metabolism demonstrating that the Landsort Deep sediment microbial community is active. Identified expressed gene suites of metabolic pathways with importance for carbon transformation including fermentation, dissimilatory sulphate reduction and methanogenesis were identified. The presence of transcripts for these metabolic processes suggests a potential for heterotrophic-autotrophic community synergism and indicates active mineralisation of the organic matter deposited at the sediment as a consequence of the eutrophication process. Furthermore, cyanobacteria, probably deposited from the water column, are transcriptionally active in the anoxic sediment at this depth. Results also reveal high abundance of transcripts encoding integron integrases. These results provide insight into the activity of the microbial community of the anoxic sediment at the deepest point of the Baltic Sea and its possible role in ecosystem functioning.

Mathematical Modelling of Plankton-Oxygen Dynamics Under the Climate Change

Ocean dynamics is known to have a strong effect on the global climate change and on the composition of the atmosphere. In particular, it is estimated that about 70% of the atmospheric oxygen is produced in the oceans due to the photosynthetic activity of phytoplankton. However, the rate of oxygen production depends on water temperature and hence can be affected by the global warming. In this paper, we address this issue theoretically by considering a model of a coupled plankton-oxygen dynamics where the rate of oxygen production slowly changes with time to account for the ocean warming. We show that a sustainable oxygen production is only possible in an intermediate range of the production rate. If, in the course of time, the oxygen production rate becomes too low or too high, the system's dynamics changes abruptly, resulting in the oxygen depletion and plankton extinction. Our results indicate that the depletion of atmospheric oxygen on global scale (which, if happens, obviously can kill most of life on Earth) is another possible catastrophic consequence of the global warming, a global ecological disaster that has been overlooked.

Effects of rainfall patterns on water quality in a stratified reservoir subject to eutrophication: Implications for management

The seasonal variation of hydrological conditions caused by shifting rainfall patterns observed in recent years has significant effects on water quality. High-volume inflows following heavy rainfall events that significantly disturb stratification lead to increased dissolved oxygen (DO) at the bottom of the reservoir, inhibiting the release of nutrients from sediments and causing a rapid reduction of algal biomass in the reservoir. However, the duration and extent of these effects depend not only on the frequency and intensity of heavy rainfall events but also on the period of thermal stratification in the reservoir. The effects of heavy rainfall events on water quality during three typical stratification periods of the reservoir were systematically investigated using extensive field data. The continuous heavy rainfall that occurred in September 2011 (stratification began to diminish) completely mixed the reservoir and produced a high concentration of DO along with a low phytoplankton concentration throughout the reservoir until stratification occurred the following year. Conversely, several days were required for anoxic conditions (in the hypolimnion) and cyanobacterial blooms to reappear after the storm runoff that occurred during the stable period of stratification (August 2012). In addition, the heavy rainfall that occurred in May 2013 accelerated the formation of an anoxic zone at the bottom of the reservoir and promoted cyanobacterial blooms due to the high nutrient input and the increased water temperature after the storm runoff ended. Water-lifting aerators (WLAs) were employed in the Shibianyu Reservoir to inhibit algal growth and to control the release of nutrients. Based on our field observations and theoretical analyses, optimized management strategies are recommended to improve water quality in the reservoir under different rainfall patterns at a reduced cost.

Seasonal changes in infaunal community structure in a hypertrophic brackish canal: Effects of hypoxia, sulfide, and predator-prey interaction

We conducted a one-year survey of macrozoobenthic community structure at 5 stations in a eutrophic canal in inner Tokyo Bay, focusing on the impacts of hypoxia, sediment H2S, and species interaction in the littoral soft-bottom habitats. Complete defaunation or decreasing density of less-tolerant taxa occurred under hypoxia during warmer months, especially at subtidal or sulfidic stations; this was followed by rapid recolonization by opportunistic polychaetes in fall-winter. Sedimentary H2S increased the mortality of macroinvertebrates under hypoxia or delayed population recovery during recolonization. The density of several polychaetes (e.g., Pseudopolydora reticulata) declined in winter, coincident with immigration of the predator Armandia lanceolata. This suggests that absence of A. lanceolata under moderate hypoxia enabled the proliferation of prey taxa. We conclude that oxygen concentration, sediment H2S, and hypoxia-induced changes in species interactions are potential drivers for spatiotemporal changes in macrozoobenthic assemblage structure in hypoxia-prone soft-bottom communities.