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The Gulf of Riga is a relatively shallow bay connected to the deeper central Baltic Sea (Baltic Proper) via straits with sills. The decrease in the near-bottom oxygen levels from spring to autumn is a common feature in the gulf, but in 2018, extensive hypoxia was observed. We analyzed temperature, salinity, oxygen, and nutrient data collected in 2018, along with historical data available from environmental databases. Meteorological and hydrological data from the study year were compared with their long-term means and variability. We suggest that pronounced oxygen depletion occurred in 2018 due to a distinct development of vertical stratification. Seasonal stratification developed early and was stronger in spring–summer 2018 than on average due to high heat flux and weak winds. Dominating northeasterly winds in early spring and summer supported the inflow of saltier waters from the Baltic Proper that created an additional deep pycnocline restricting vertical transport between the near-bottom layer (NBL) and the water column above. The estimated oxygen consumption rate in the NBL in spring–summer 2018 was about 1.7 mmolO2m-2h-1, which exceeded the oxygen input to the NBL due to advection and vertical mixing. Such a consumption rate leads to near-bottom hypoxia in all years when vertical mixing in autumn reaches the seabed later than on average according to the long-term (1979–2018) meteorological conditions. The observed increase in phosphate concentrations in the NBL in summer 2018 suggests a significant sediment phosphorus release in hypoxic conditions counteracting the mitigation measures to combat eutrophication. Since climate change projections predict that meteorological conditions comparable to those in 2018 will occur more frequently, extensive hypoxia would be more common in the Gulf of Riga and other coastal basins with similar morphology and human-induced elevated input of nutrients.

Aquatic hypoxia within the Laurentian Great Lakes has contributed to various adverse ecological consequences and stimulated research interest in recent decades. An analysis of published peer-reviewed journal articles from 2000 to 2020 demonstrates an increasing trend of studies related to hypoxia in the Laurentian Great Lakes. However, the majority of these studies (78%) focus on Lake Erie and in particular the well-documented hypolimnetic hypoxic conditions that develop in the central basin of Lake Erie. This hypoxic zone is relatively large (up to 1.5 million ha), has substantial ecological effects, and motivates monitoring programs and water quality improvement initiatives. Nonetheless, the hypoxic zone in the central basin of Lake Erie is only one of over twenty documented hypoxic zones in the Laurentian Great Lakes. Moreover, hypoxic conditions in the Great Lakes are quite diverse. Here, we define and characterize a four-fold classification of Great Lakes hypoxic conditions: 1) hypolimnetic hypoxia, 2) over-winter hypoxia, 3) diel hypoxia, and 4) episodic hypoxia. We suggest that Great Lakes research and monitoring programs should seek to more broadly document hypoxic conditions and develop models to predict the temporal and spatial occurrence of hypoxia. Such efforts are particularly timely as future climatic conditions contributing to warmer temperatures, longer and more intense stratified periods, increased spring nutrient loading and more variable allocthonous inputs are expected to exacerbate three of the four hypoxic conditions described for the Great Lakes (hypolimnetic, diel, and episodic hypoxia).

Submarine groundwater discharge (SGD) has received increasing attention by studies on coastal areas; however, its effects on biogeochemical zonation have not been investigated to date. The Yellow River Estuary (YRE) is a world class river estuary with high turbidity, and heavy human regulation. This study investigated how SGD is related to the benthic biogeochemistry of the YRE. Based on the distribution of several parameters (e.g., salinity, temperature, dissolved oxygen (DO) levels, pH, radium isotopes, and nutrients), the YRE was subdivided into six different zones, and the SGD fluxes within each zone were quantified and compared. The highest SGD flux was found in the northwest nearshore zone, where it was more than one order of magnitude higher than in the offshore zone. High SGD resulted in low DO and pH, but high nutrient levels in the benthic boundary layer. The southeast nearshore zone was also characterized by high SGD flux, but benthic waters were more oxic because of the dominating inputs by the Yellow River. These data suggest that such a zonation would help to understand benthic biogeochemical processes. High SGD may not only contribute to the estuarine nutrient budget, but may also contribute to the formation of hypoxia and acidification.

Marine bacterial community plays a vital role in the formation of the hypoxia zone in coastal oceans. Yet, their dynamics in the seasonal hypoxia zone of the Bohai Sea (BHS) are barely studied. Here, the 16S rRNA gene-based high-throughput sequencing was used to explore the dynamics of their diversity, structure, and function as well as driving factors during the gradual deoxygenation process in the BHS. Our results evinced that the bacterial community was dominated by Proteobacteria, followed by Bacteroidetes, Firmicutes, Actinobacteria, and Cyanobacteria, etc. The abundant subcommunity dominated in the number of sequences (49%) while the rare subcommunity dominated in the number of species (99.61%). Although abundant subcommunity accounted for most sequences, rare subcommunity possessed higher diversity, richness and their population dramatically changed (higher turnover) during the hypoxia transition. Further, co-occurrence network analysis proved the vital role of rare subcommunity in the process of community assembly. Additionally, beta diversity partition revealed that both subcommunities possessed a higher turnover component than nestedness and/or richness component, implying species replacement could explain a considerable percentage of community variation. This variation might be governed by both environmental selection and stochastic processes, and further, it influenced the nitrogen cycle (PICRUSt-based prediction) of the hypoxia zone. Overall, this study provides insight into the spatial-temporal heterogeneity of bacterial and their vital role in biogeochemical cycles in the hypoxia zone of the BHS. These findings will extend our horizons about the stabilization mechanism, feedback regulation, and interactive model inside the bacterial community under oxygen-depleted ecosystems.

MDFRC item.Hypoxic blackwater events are characterised by high levels of dissolved organic carbon in the water column,the metabolism of which depletes dissolved oxygen, which can cause fish and crustacean mortality. Understanding the drivers of and controls on hypoxic blackwater events is important in order to reduce the potential for detrimental water quality impacts from both managed and natural flows. After a decadelong drought in south-eastern Australia, a series of spring and summer flood events in 2010-2011 resulted in a large-scale hypoxic blackwater event in the southern Murray-Darling Basin that affected over 2000 km of river channels and persisted for 6 months. We examined the biogeochemistry and hydrology underpinning this extreme event and found that multiple drivers contributed to the development and persistence of hypoxic blackwater. Inundation of both forested and agricultural floodplains that had not been flooded for over a decade mobilised large stores of reactive carbon. Altered flow seasonality, due to a combination of climatic effects and river regulation, not only increased the risk of hypoxic blackwater generation but also shifted the proportion of bioavailable carbon that was returned to the river channels. Hypolimnetic weir discharge also contributed to hypoxia at some sites. These findings highlight the need for a whole-of-system perspective for the management of regulated river systems - especially in the face of a changing climate.

SUMMARY 1. Behavioural responses to changing environments affect community composition, so the identifica-tion of associations between environmental gradients, behavioural traits and physiological traitsmakes a significant contribution to the quest for trait-based rules of community change.2. We tested the hypothesis that fish morphology and lifestyle are associated with metabolic rate,hence oxygen demand, and behavioural response to gradual hypoxia [low dissolved oxygen (DO)],using respirometry and behavioural experiments. Three species fell along different points of thefast–slow lifestyle continuum: Melanotaenia fluviatilis, a pelagic fish adapted to endurance swimming,lies at the fast end of the lifestyle continuum, while Mogurnda adspersa, a benthic fish adapted to burstswimming, lies at the other end. The benthopelagic Hypseleotris sp. has an intermediate lifestyle.3. Standard and routine metabolic rates varied strongly among the species and were associated withlifestyle according to the inequality M. fluviatilis > Hypseleotris > M. adspersa. As DO declined,aquatic surface respiration behaviour also varied significantly among the species and indicated asensitivity to hypoxia described by the same inequality. As hypoxia ensued, changes in habitat werealso linked to lifestyle, but changes in activity level among species were not neatly correlated withlifestyle.4. Overall, our experiments imply that there are significant links between morphology, lifestyle,metabolism and behavioural response to hypoxia in these three species of fish.Keywords: animal behaviour, community ecology, ecophysiology, fresh water, functional traits

Hypoxic blackwater events occur when large amounts of organic material are leached into a water body (e.g., during floodplain inundation) and rapid metabolism of this carbon depletes oxygen from the water column, often with catastrophic effects on the aquatic environment. River regulation may have increased the frequency and severity of hypoxic blackwater events in lowland river systems, necessitating management intervention to mitigate the impacts of these events on aquatic biota. We examine the effectiveness of a range of mitigation interventions that have been used during large-scale hypoxic blackwater events in the Murray-Darling Basin, Australia and that may be applicable in other environments at risk from hypoxic blackwater. Strategies for hypoxia mitigation include: delivery of dilution flows; enhancement of physical re-aeration rates by increasing surface turbulence; and diversion of blackwater into shallow off-channel storages. We show that the impact of dilution water delivery is determined by relative volumes and water quality and can be predicted using simple models. At the dilution water inflow point, localized oxygenated plumes may also act as refuges. Physical re-aeration strategies generally result in only a small increase in dissolved oxygen but may be beneficial for local refuge protection. Dilution and natural re-aeration processes in large, shallow lake systems can be sufficient to compensate for hypoxic inflows and water processed in off-channel lakes may be able to be returned to the river channel as dilution flows. We provide a set of predictive models (as electronic supplementary material) for estimation of the re-aeration potential of intervention activities and a framework to guide the adaptive management of future hypoxic blackwater events.

Ocean deoxygenation is one of the major consequences of climate change. In coastal waters, this process can be exacerbated by eutrophication, which is contributing to an alarming increase in the so-called "dead zones" globally. Despite its severity, the effect of reduced dissolved oxygen has only been studied for a very limited number of organisms, compared to other climate change impacts such as ocean acidification and warming. Here we experimentally assessed the response of sponges to moderate and severe simulated hypoxic events. We ran three laboratory experiments on four species from two different temperate oceans (NE Atlantic and SW Pacific). Sponges were exposed to a total of five hypoxic treatments, with increasing severity (3.3, 1.6, 0.5, 0.4 and 0.13 mg O2 L-1 , over 7-12-days). We found that sponges are generally very tolerant of hypoxia. All the sponges survived in the experimental conditions, except Polymastia crocea, which showed significant mortality at the lowest oxygen concentration (0.13 mg O2 L-1 , lethal median time: 286 h). In all species except Suberites carnosus, hypoxic conditions do not significantly affect respiration rate down to 0.4 mg O2 L-1 , showing that sponges can uptake oxygen at very low concentrations in the surrounding environment. Importantly, sponges displayed species-specific phenotypic modifications in response to the hypoxic treatments, including physiological, morphological, and behavioural changes. This phenotypic plasticity likely represents an adaptive strategy to live in reduced or low oxygen water. Our results also show that a single sponge species (i.e., Suberites australiensis) can display different strategies at different oxygen concentrations. Compared to other sessile organisms, sponges generally showed higher tolerance to hypoxia, suggesting that sponges could be favoured and survive in future deoxygenated oceans.

Urbanized coral reefs are often chronically affected by sedimentation and reduced light levels, yet many species of corals appear to be able to thrive under these highly disturbed conditions. Recently, these marginal ecosystems have gained attention as potential climate change refugia due to the shading effect of suspended sediment, as well as potential reservoirs for stress-tolerant species. However, little research exists on the impact of sedimentation on coral physiology, particularly at the molecular level. Here, we investigated the transcriptomic response to sediment stress in corals of the family Merulinidae from a chronically turbid reef (one genet each of Goniastrea pectinata and Mycedium elephantotus from Singapore) and a clear-water reef (multiple genets of G. pectinata from the Gulf of Aqaba/Eilat). In two ex-situ experiments, we exposed corals to either natural sediment or artificial sediment enriched with organic matter and used whole-transcriptome sequencing (RNA sequencing) to quantify gene expression. Analysis revealed a shared basis for the coral transcriptomic response to sediment stress, which involves the expression of genes broadly related to energy metabolism and immune response. In particular, sediment exposure induced upregulation of anaerobic glycolysis and glyoxylate bypass enzymes, as well as genes involved in hydrogen sulphide metabolism and in pathogen pattern recognition. Our results point towards hypoxia as a probable driver of this transcriptomic response, providing a molecular basis to previous work that identified hypoxia as a primary cause of tissue necrosis in sediment-stressed corals. Potential metabolic and immunity trade-offs of corals living under chronic sedimentation should be considered in future studies on the ecology and conservation of turbid reefs.

Functional diversity metrics have been suggested as important indicators of ecosystem functioning. We investigated the effects of land use mediated by local environmental variables on the fish assemblages’ functional structure in 18 headwater streams distributed in the Upper Parana and Iguacu ecoregions. The species were characterized according to habitat use, food resource use, life history, and hypoxia tolerance. We calculated functional diversity indices [richness (FRic), evenness (FEve), divergence (FDiv), dispersion (FDis), diversity (RaoQ)] and functional rarity indices [distinctiveness (Disti), uniqueness (Uni)], and evaluated the effects of regional and local variables on these indices using generalized linear models. The results highlighted the occurrence of the most functionally distinct and unique species in the forested streams. Moreover, we observed higher FRic in streams sampled in argisol, with a high ammonia load and low habitat diversity. Forested and agricultural covers positively affected FDis and RaoQ. Urban cover had a negative effect on FDis and RaoQ, mainly due to the increase in generalist functional groups in urban streams. Our findings emphasize the importance of forested cover in maintaining specialized species in fish assemblages of streams and preserving functional diversity and ecosystem functioning. Furthermore, these results contribute to direct conservation and recovery measures in freshwater environments.

Oxygen concentrations in coastal waters have declined globally by 10% since the mid-twentieth century, and ocean warming will further reduce the solubility of oxygen in coastal habitats. Some nearshore reefs experience periodic hypoxic conditions due to eutrophication, especially during the wet season. Here, we determined the combined impacts of hypoxia and elevated temperature on the reef-building coral, Orbicella faveolata, by exposing corals to normoxic or hypoxic conditions and ambient or elevated temperatures. Oxygen consumption was monitored using closed-system respirometry. Corals within hypoxic conditions consumed 34% less oxygen relative to corals in normoxic conditions. Corals in the elevated temperature normoxic treatment experienced a 10% increase in oxygen consumption relative to the control. Corals exposed to both stressors simultaneously experienced a 62% reduction in oxygen consumption. These results suggest that increased temperature may exacerbate the negative effects of hypoxia on O. faveolata.

Located in the northern South China Sea, the Pearl River Estuary (PRE) is one of the most important estuaries in China and is surrounded by several megacities. Hypoxia mainly occurs in the bottom waters of the PRE during summer and is more prominent near the Humen outlet and the subestuary outside Modaomen and Jitimen. A well-validated three-dimensional (3-D) coupled physical-biogeochemical model was used to explore the changes in dissolved oxygen (DO) dynamics and hypoxic conditions in the PRE over an intra-annual cycle and elucidate the processes controlling the generation, development, and dissipation of hypoxia. In summer, oxygen consumption due to the high-intensity sediment oxygen demand (SOD) intensified by large inputs of riverine particulate organic carbon (POC) exceeded the DO supplemented by vertical diffusion (largely inhibited by strong fresh water–induced stratification), thus leading to a significant decrease in DO in the bottom waters; in other seasons, these DO source and sink terms were nearly balanced so that the bottom DO concentrations were maintained at higher levels. Moreover, the lag analysis shows that there is an approximately 2-month lag between riverine POC and SOD. By comparison, the low-oxygen area near Humen has a controlling mechanism distinct from that of the subestuary outside Modaomen and Jitimen. Specifically, the timely and sufficient oxygen supplement brought by vertical diffusion can replenish the bottom DO consumed by SOD in this region, and the formation and development of low-oxygen conditions (DO ≤ 4 mg L−1) is mainly affected by riverine low-oxygen inflows. In summary, our study clarified that the terrestrial organic pollutant input and low-oxygen water from the upper reaches have an important impact on the DO in different areas of the PRE, especially the low-oxygen area near Humen, which is controlled by the water quality of the upper reaches. This conclusion is of great significance for regional environmental management.

Hypoxia induced by eutrophication has become an important factor threatening the survival of coastal life such as Enhalus acoroides. The purpose of the current study was to explore the effect of hypoxia on photosystem II (PSII) of E. acoroides in the dark. The results showed that long-term dark hypoxia damages PSII activity of E. acoroides. The lower the oxygen content and the longer the hypoxic duration, the more seriously PSII was damaged and the less light-independent recovery parts of the damaged PSII. The damage to PSII caused by hypoxia was unrelated to ROS but related to respiration, because the respiration rate decreased with the decrease of oxygen content and PSII activity decreased significantly even at a normal oxygen content after the inhibition of aerobic respiratory pathway. Hypoxia reduced energy fluxes between the antennas and the RCs, and generated many inactive RCs, which significantly reduced the electron transfer efficiency of PSII. Severe hypoxia (2.65 mg L-1 oxygen content) caused chlorophyll degradation. The study demonstrated that hypoxia damages PSII of E. acoroides and inhibits PSII recovery in the dark. We suggested that hypoxia together with other environment stressors would be the key reason for the decline of E. acoroides meadows.

Oxygen is one of the important substances for the survival of most life systems on the earth, and plateau and underground burrow systems are two typical hypoxic environments. Small mammals living in hypoxic environments have evolved different adaptation strategies, which include increased oxygen delivery, metabolic regulation of physiological responses and other physiological responses that change tissue oxygen utilization. Multi‐omics predictions have also shown that these animals have evolved different adaptations to extreme environments. In particular, vascular endothelial growth factor (VEGF) and erythropoietin (EPO), which have specific functions in the control of O2 delivery, have evolved adaptively in small mammals in hypoxic environments. Naked mole‐rats and blind mole‐rats are typical hypoxic model animals as they have some resistance to cancer. This review primarily summarizes the main living environment of hypoxia tolerant small mammals, as well as the changes of phenotype, physiochemical characteristics and gene expression mode of their long‐term living in hypoxia environment., Small mammals have evolved mechanisms of hypoxia adaptation living under plateau and underground burrow conditions.

Marine ecosystems, particularly coastal environments, are rapidly changing due to anthropogenic impacts resulting in increased global climate change (ocean warming), ocean acidification, hypoxia, and eutrophication. On coral reefs, symbiont-bearing large benthic foraminifera (LBFs) can play a key role as reef constituents and carbonate producers, contributing up to 5% of reef-scale carbonate budgets. However, projected climate change, particularly ocean warming, has the potential to significantly alter the conditions in which marine organisms persist. While the response of LBFs to elevated thermal stress is well documented in laboratory studies, the potential influence of adaptation or acclimatization through prior environmental thermal history on this response remains largely unknown. In this study, specimens of Calcarina gaudichaudii, an LBF from the Penghu Islands, Taiwan, were collected from thermally variable intertidal and thermally stable subtidal (~ 6 m depth) environments representing thermal history. LBFs were then acclimated to laboratory conditions at ambient (25 °C) and elevated (28 °C) temperatures for three weeks, and subsequently exposed to control and heat stress treatments (25 °C, 28 °C, 30 °C, 33 °C) for an additional one week. Photosynthetic rates (determined through oxygen flux measurements) of C. gaudichaudii significantly decreased in specimens collected at subtidal depths acclimated at 25 °C when compared to those acclimated at 28 °C, whereas there was no effect of thermal history on respiration, indicating that symbiont and holobiont responses may differ in LBFs. Additionally, maximum photochemical efficiency (Fv/Fm) significantly decreased as a result of heat stress, although bleaching was not visually observed after one week. These results highlight the plastic responses of the algal microbiome and indicate that thermal history, acclimatization temperature, and heat stress interact to affect the physiological status of C. gaudichaudii. This study adds to the growing literature which highlights the larger implications of understanding thermal history as an important factor to consider to better understand how ecosystem processes (e.g., carbonate production) are altered on modern coral reefs.

Estuarine and coastal geomorphology, biogeochemistry, water quality, and coastal food webs in river-dominated shelves of the Gulf of Mexico (GoM) are modulated by transport processes associated with river inputs, winds, waves, tides, and deep-ocean/continental shelf interactions. For instance, transport processes control the fate of river-borne sediments, which in turn affect coastal land loss. Similarly, transport of freshwater, nutrients, and carbon control the dynamics of eutrophication, hypoxia, harmful algal blooms, and coastal acidification. Further, freshwater inflow transports pesticides, herbicides, heavy metals, and oil into receiving estuaries and coastal systems. Lastly, transport processes along the continuum from the rivers and estuaries to coastal and shelf areas and adjacent open ocean (abbreviated herein as “river-estuary-shelf-ocean”) regulate the movements of organisms, including the spatial distributions of individuals and the exchange of genetic information between distinct subpopulations. The Gulf of Mexico Research Initiative (GoMRI) provided unprecedented opportunities to study transport processes along the river-estuary-shelf-ocean continuum in the GoM. The understanding of transport at multiple spatial and temporal scales in this topographically and dynamically complex marginal sea was improved, allowing for more accurate forecasting of the fate of oil and other constituents. For this review, we focus on five specific transport themes: (i) wetland, estuary, and shelf exchanges; (ii) river-estuary coupling; (iii) nearshore and inlet processes; (iv) open ocean transport processes; and (v) river-induced fronts and cross-basin transport. We then discuss the relevancy of GoMRI findings on the transport processes for ecological connectivity and oil transport and fate. We also examine the implications of new findings for informing the response to future oil spills, and the management of coastal resources and ecosystems. Lastly, we summarize the research gaps identified in the many studies and offer recommendations for continuing the momentum of the research provided by the GoMRI effort. A number of uncertainties were identified that occurred in multiple settings. These include the quantification of sediment, carbon, dissolved gasses and nutrient fluxes during storms, consistent specification of the various external forcings used in analyses, methods for smooth integration of multiscale advection mechanisms across different flow regimes, dynamic coupling of the atmosphere with sub-mesoscale and mesoscale phenomena, and methods for simulating finer-scale dynamics over long time periods. Addressing these uncertainties would allow the scientific community to be better prepared to predict the fate of hydrocarbons and their impacts to the coastal ocean, rivers, and marshes in the event of another spill in the GoM.

Studies on hypoxia in Peter the Great Bay (Japan Sea) are reviewed. Seasonal hypoxia is observed in warm season at the bottom of three areas: Amur Bay, Ussuri Bay, and the southwestern part of Peter the Great Bay occupied by the Far-Eastern Marine Biosphere Reserve (FEMBR). Processes of the hypoxia forming are similar in all these areas. The main reason is the dissolved oxygen consumption by microbial degradation of organic matter within topographic depressions in conditions of limited ventilation because of strong summer stratification. The bottom depressions prevent horizontal water exchange and provide accumulation of organic and inorganic suspension, that is another factor important for development of hypoxia. The Amur Bay is the most subjected to hypoxia, being a semiclosed estuarine basin eutrophed by nutrients input from the Razdolnaya River and waste waters of Vladivostok city. The Ussuri Bay has better water exchange and less eutrophication, therefore there are scarce data about hypoxia in this area. FEMBR area has good water exchange and is only episodically influenced by nutrients discharge from the Tumen River, so hypoxia is observed there occasionally. Another consequence of microbial degradation of organic matter in these areas is acidification: pH decreased in 0.5 unit in the bottom water of the Amur Bay in eight decades from 1932 to 2013. Synchronism between regional and global processes of eutrophication, deoxygenation, and acidification of bottom waters is discussed.

Based on the analysis of element geochemistry and total organic carbon (TOC), this study investigates the main factors controlling organic matter (OM) enrichment, reconstructs the evolution process of the sedimentary environment, and proposes a dynamic OM enrichment model of the Jurassic Da’anzhai (D) Formation, Sichuan Basin. The results indicate that the Sichuan Basin was generally dominated by a warm and oxidizing sedimentary environment, but with some peculiarities, such as a hotter climate in the D1 member and more anoxic lake water in the D2a member. The sedimentary evolution of the Da’anzhai Formation can be divided into a fluctuating sedimentary stage, a stable sedimentary stage and a reef-building stage. The D2a member showed the strongest hypoxia, the weakest weathering, the largest amount of terrestrial inputs, and the highest TOC content. The TOC is positively correlated with reducing conditions and terrestrial inputs, negatively correlated with weathering. Based on these findings, it is suggested that the global climate in the Early Jurassic period had a complex regional effect and the global oceanic anoxic events of the Toarcian did not spread to the Sichuan Basin. Thus, the anoxic deep water, high terrestrial inputs, and weak weathering were conducive to rapid deposition and preservation of lacustrine OM.

Water quality standards based on sub-lethal effects and performance optima for aquatic organisms, rather than onset of mortality, are more ecologically relevant for management of species of conservation concern. We investigated the effects of hypoxia, temperature (with acclimation), nitrogenous chemical compounds, and chloride on Topeka shiners (Notropis topeka) by monitoring behavioral responses to a reduction in oxygen and, using swimming speed, determining thermal optima and onset of effect for concentrations of nitrogenous compounds and chloride. We found ASR50 (i.e., dissolved oxygen concentrations where 50% of fish use aquatic surface respiration) to be 1.65 mg/L and ASR90 to be 1.08 mg/L of dissolved oxygen. Optimum temperatures for the species ranged from 17.7 to 28.0 °C, while predicted 100% mortality ranged from 33.7 to 40.3 °C, depending on the temperature at which fish were acclimated prior to experiments. Ammonia and sodium chloride reduced swimming speed at concentrations below known LC50 values, while nitrite concentrations did not correspond with swimming speed, but rather, post-experiment mortality. This provides insight into where Topeka shiners can not only persist, but also thrive. Although swimming speed may not be a suitable metric for determining the effects of all contaminants, our focus on optima and sub-lethal effects over tolerance allows selections of the most suitable reintroduction site matching the species’ physiological profile.

The intrusion of the Kuroshio into the East China Sea (ECS) affects the development of hypoxia off the Changjiang (Yangtze) River estuary; however, quantitative analysis of its impacts is lacking. In this study, the Regional Ocean Modeling Systems (ROMS) model coupled with the Carbon, Silicate and Nitrogen Ecosystem (CoSiNE) model was used to investigate the relative importance of dissolved oxygen (DO) and different nutrients (silicate, nitrate, and phosphate) in the Kuroshio on hypoxia in the ECS. Results show that changes in DO concentrations in the Kuroshio modify the distribution and intensity of hypoxia through direct onshore transport by hydrodynamic processes. An increase in Kuroshio DO concentration by 25% or 50% would result in a decrease of the maximum hypoxia extent (MHE) in the ECS by 76% or 86%, respectively, while a 25% decrease in Kuroshio DO would increase the MHE by up to 219%. The contribution of DO in the Taiwan Strait is almost negligible. In contrast to Kuroshio DO, nutrients affect hypoxia in the ECS through onshore transport by hydrodynamic and biochemical processes. Changes in phosphate and nitrate concentrations by 25% in the Kuroshio would change the MHE by up to 30% and 18%, respectively, accompanied by apparent changes in surface chlorophyll-a concentrations. The effect of silicate on hypoxia is negligible because a 25% change in silicate concentrations in the Kuroshio would result in less than 1% change in the MHE. Our results reveal a hierarchical rank of importance for environmental variables in the Kuroshio (i.e., DO > phosphate > nitrate > silicate) in modifying the development of hypoxia in the ECS.

Anthropogenic climate change has resulted in warming temperatures and reduced oxygen concentrations in the global oceans. Much remains unknown on the impacts of reduced oxygen concentrations on the biology and distribution of marine fishes. In the Southern California Channel Islands, visual fish surveys were conducted frequently in a manned submersible at three rocky reefs between 1995 and 2009. This area is characterized by a steep bathymetric gradient, with the surveyed sites Anacapa Passage, Footprint and Piggy Bank corresponding to depths near 50, 150 and 300 m. Poisson models were developed for each fish species observed consistently in this network of rocky reefs to determine the impact of depth and year on fish peak distribution. The interaction of depth and year was significant in 23 fish types, with 19 of the modelled peak distributions shifting to a shallower depth over the surveyed time period. Across the 23 fish types, the peak distribution shoaled at an average rate of 8.7 m of vertical depth per decade. Many of the species included in the study, including California sheephead, copper rockfish and blue rockfish, are targeted by commercial and recreational fisheries. CalCOFI hydrographic samples are used to demonstrate significant declines in dissolved oxygen at stations near the survey sites which are forced by a combination of natural multidecadal oscillations and anthropogenic climate change. This study demonstrates in situ fish depth distribution shifts over a 15-year period concurrent with oxygen decline. Climate-driven distribution shifts in response to deoxygenation have important implications for fisheries management, including habitat reduction, habitat compression, novel trophic dynamics and reduced body condition. Continued efforts to predict the formation and severity of hypoxic zones and their impact on fisheries dynamics will be essential to guiding effective placement of protected areas and fisheries regulations.

Following a series of fish deaths in the lower Darling River in the austral summer of 2018–19, several field trials were undertaken over two summers to determine the efficacy of various aerators to mitigate the effects of hypoxia to prevent fish deaths. The aerators evaluated included: low-powered, solar bubble-plume diffusers; high-flow, single-port Venturi systems; pontoon-mounted multi-port Venturi systems; and an ultra-fine bubble generator. The effectiveness of bubble diffusers appears related to their power, the number of units used and their running time. Pontoon-mounted multi-port Venturi aerators and high-flow, single-port Venturi aerators were much more successful in maintaining oxygen levels and may have prevented fish deaths in the Darling River in early 2019. However, the Venturi aerators were expensive to run, consuming hundreds to thousands of litres of diesel fuel per week. Trial of an ultra-fine oxygen bubble pump showed promise because it supersaturated the water with dissolved oxygen while not elevating total gas pressure to levels that would cause gas-bubble trauma in fish. Although aerators may play a role in emergency responses to hypoxic fish deaths, they have limitations regarding their area of influence (at best thousands of square metres) and their cost to deploy and operate.

Climate change, river regulation and water extraction create the conditions where destratification-driven hypoxia will become more common in rivers. Preventing this and the fish deaths that can result requires options that prevent stratification and create oxygen refuges for fish. Here we discuss aeration and mixing approaches that may help prevent fish deaths when flow-related measures are not available. The options were evaluated based on efficacy, environmental risk and readiness to be deployed cost-effectively. The options either promote mixing, aeration or both. Bubble diffusers and paddle wheels used commonly in aquaculture are unlikely to aerate already hypoxic pools. However, if deployed before stratification occurs, they may promote mixing and maintain aeration. In comparison, pumps with Venturi tubes or ultrafine oxygen bubble condensers both mix and aerate, making them suitable for use once hypoxic events are underway. Water jets are low cost and could be deployed quickly. Dosing reaches with calcium peroxide may be useful for emergency aeration, but requires further safety and efficacy testing. Flow management that maintains fish refuges and storage reserves during drought is the best way to guard against fish deaths, but if storage releases are not available, there are options for creating and maintaining oxygen refuges to minimise ecosystem damage.

Deoxygenation has frequently appeared in coastal ecosystems over the past century due to the joint influence of increasing anthropogenically induced nutrient inputs and global warming. The semi-enclosed Bohai Sea is a typical system that is prone to deoxygenation, with regular hypoxia events consistently recorded in recent decades. Based on in-situ observation data collected in large-scale voyage surveys in the Bohai Sea during 2008–2017, the seasonal variability in dissolved oxygen (DO) and its controlling mechanisms were studied. The results indicated that in spring and autumn, the DO distributions exhibited similar spatial patterns in the surface and bottom layers, while in summer, its spatial distribution was characterized by large-scale oxygen-poor zones distributed off the Qinhuangdao Coast and the central southern Bohai Sea in the bottom layer. The controlling mechanisms of the DO distribution varied from season to season. Spring and autumn DO distributions were dominated by the seawater temperature. Under the combined effects of stratification and decomposition, the summer bottom DO exhibited dual-core distribution. On the one hand, stratification could greatly impede vertical mixing, resulting in reduced bottom DO replenishment. On the other hand, the increased bottom organic matter intensified the decomposition processes, inducing massive DO consumption and elevated dissolved inorganic nitrogen concentrations. In addition, the stronger stratification might be the reason for the more severe deoxygenation in the southern oxygen-poor zones in summer. Our study provides guidance for an in-depth understanding of the DO seasonality in the Bohai Sea and the mechanisms that modulate it and for the improvement of hypoxia forecasts in ocean models.

Understanding the impacts of extreme events is essential to effective fisheries management. During the summer of 2018–19 millions of native fish died in Lower Darling–Baaka River adjacent to Menindee, New South Wales, Australia. Hypoxia during a period of protracted low flow, triggered by climatic events, was responsible for the fish kills. From June 2019 to March 2020, further broader-scale fish kills occurred throughout ~600 km of the Darling–Baaka River as disconnected refuge pools contracted and water quality deteriorated. This study examined the status of the remnant fish populations, compared the fish assemblage of the Menindee fish death reach with an unaffected reach and monitored change of the fish community over 18 months after the initial fish kills. Significantly lower abundances of Murray cod (Maccullochella peelii), bony herring (Nematalosa erebi), carp gudgeon (Hypseleotris spp.) and freshwater prawn (Macrobrachium australiense) were captured within the Menindee fish death reach compared with the unaffected reach. Varied responses were observed in species abundances within the affected reach in the 18 months after the initial fish kills, attributed to the various life-history traits and reproductive strategies. The results presented highlight a fish community in continued stress. Continued monitoring will guide and track the effectiveness of recovery management interventions in the region.

Critical thermal maximum (CTmax ) is often used as an index of upper thermal tolerance in fishes; however, recent studies have shown that some fishes exhibit agitation or avoidance behavior well before the CTmax is reached. In this study, we quantified behavioral changes during CTmax trials in two Amazonian cichlids, Apistogramma agassizii and Mesonauta insignis. The thermal agitation temperature (Tag ) was recorded as the temperature at which fish left cover and began swimming in an agitated manner, and four behaviors (duration of sheltering, digging, activity, and aquatic surface respiration [ASR]) were compared before and after Tag . Both A. agassizii and M. insignis exhibited high critical thermal maxima, 40.8°C and 41.3°C, respectively. Agitation temperature was higher in M. insignis (37.3°C) than in A. agassizii (35.4°C), indicating that A. agassizii has a lower temperature threshold at which avoidance behavior is initiated. Activity level increased and shelter use decreased with increased temperatures, and patterns were similar between the two species. Digging behavior increased after Tag in both species, but was higher in A. agassazii and may reflect its substrate-oriented ecology. ASR (ventilating water at the surface film) was extremely rare before Tag , but increased in both cichlid species after Tag and was greater in M. insignis than in A. agassizii. This suggests that fish were experiencing physiological hypoxia at water temperatures approaching CTmax . These results demonstrate that acute thermal challenge can induce a suite of behavioral changes in fishes that may provide additional, ecologically relevant information on thermal tolerance.

Fossil fuel combustion and related accumulation of carbon dioxide (CO2) and other greenhouse gases in the atmosphere and oceans have contributed significantly to climate and ocean change. While coa...

For freshwater fishes, elevated water temperatures associated with climate warming and hypoxia can co-occur and are likely to interact as both affect oxidative metabolism. We quantified the effects of acclimation to elevated temperature and hypoxia on the thermal tolerance of pugnose shiner (Notropis anogenus), a cyprinid fish threatened in its Canadian range. In one experiment, pugnose shiner underwent two-week sequential acclimations to six increasing temperatures. Fish acclimated to warmer waters increased their critical thermal maximum (CTmax), while the agitation temperature (Tag) was 3.4 °C lower than the CTmax. In another experiment, fish were acclimated to three dissolved oxygen treatments (>95%, ∼56%, or ∼40% air saturation) for two weeks and tested under >95% and 40% conditions. CTmax was lower when measured under 40% for normoxia-acclimated fish but not for hypoxia-acclimated fish. Hypoxia-acclimated fish had higher Tag and smaller agitation windows than normoxia-acclimated fish, suggesting that hypoxia acclimation improves aspects of thermal tolerance. We examine the plasticity of thermal tolerance of pugnose shiner, showing that they may be more vulnerable to high temperatures compared with other non-imperiled Notropis species.

The Salish sucker (Catostomus sp. cf. catostomus) is a federally Threatened species in Canada, inhabiting small lowland streams along with juvenile coho salmon (Oncorhynchus kisutch) in southwestern British Columbia and adjacent Washington State. Experiments were conducted in two ponds to determine the effects of reduced flow on dissolved oxygen, growth of stocked Salish sucker and juvenile coho salmon (Oncorhynchus kisutch), and fish movement using passive integrated transponder (PIT) tags. Sequential flow reduction in the first experiment resulted in a significant decrease in dissolved oxygen (9.1 to 3.1 mg·L−1) and Salish sucker growth (23% decline), although lower dissolved oxygen and growth were confounded with decreased temperature. Flow reduction in a second split-pond experiment including coho salmon resulted in significantly lower dissolved oxygen (6.9 to 2.9 mg·L−1), a 51% reduction in growth of coho salmon, but no detectable change in Salish sucker growth. PIT tag detections demonstrated increased use by both species of a well-oxygenated refuge in the flow reduction treatment, indicating sublethal effects of hypoxia on fish behaviour and habitat use. These experiments demonstrate the induction of severe hypoxia by reduced flows under eutrophy, with the potential for negative impacts on Salish sucker and juvenile coho salmon growth and habitat use.

To understand the effects of widespread urbanization and agricultural impacts on recovery of Salish sucker (Catostomus sp. cf. catostomus), a federally threatened catostomid endemic to the lower Fraser Valley of British Columbia, we assessed (i) the current extent and effects of hypoxia on the distribution of Salish sucker and juvenile salmonids, (ii) potential drivers of hypoxia, and (iii) management options for hypoxia mitigation. Over 40% of sucker critical habitat experiences hypoxia (dissolved oxygen (DO) < 4 mg·L–1) by late summer, indicating widespread noncompliance with water quality guidelines. The strong positive relationship between seasonal hypoxia and temperature (R2= 0.53) and negative relationship with streamflow (R2= 0.78) indicates that hypoxia is driven by a synergy among low summer flows, elevated temperatures, and high primary production associated with nutrient enrichment (eutrophication). Sucker show strong selection against high water temperatures and weaker negative selection against low DO; juvenile salmonids show very strong selection against both high temperatures and low DO. Climate projections for declining summer flows and elevated temperatures indicate worsening trends in DO without intensive watershed-scale management to reduce nutrient loads, maintain or increase flows, and riparian restoration to lower stream temperatures, control primary production, and inhibit growth of invasive reed canary grass (Phalaris arundinacea).

The United States Harmful Algal Bloom and Hypoxia Research Control Act of 2014 identified the need for forecasting and monitoring harmful algal blooms (HAB) in lakes, reservoirs, and estuaries across the nation. Temperature is a driver in HAB forecasting models that affects both HAB growth rates and toxin production. Therefore, temperature data derived from the U.S. Geological Survey Landsat 5 Thematic Mapper and Landsat 7 Enhanced Thematic Mapper Plus thermal band products were validated across 35 lakes and reservoirs, and 24 estuaries.

We conducted hydrographic observations throughout the year to investigate seasonal variations of the hypoxic water mass distribution in the Upper Gulf of Thailand (UGoT). Hypoxic water masses were observed from June to November, with half of the UGoT occupied by hypoxic water in September. A hypoxic water mass appeared in the northeastern part of the UGoT in June and August, and moved westward over time. Low-salinity surface water moved from east to west as the rotational direction of surface circulation shifted with the reversal of monsoon winds. Westward movement of low-salinity water causes strong stratification in the northwestern part of the UGoT, leading to severe hypoxia. Numerical experiments showed high dissolved oxygen consumption rates around and offshore of river mouths, where hypoxic water is generated. This finding suggests that hypoxic water masses are transported to the south by physical processes. We examined how flooding affects hypoxic water mass formation. The volume of hypoxia in a flood year was approximately 2.5 times greater than in a normal year. In addition, hypoxia occurred in the dry season and extensive hypoxia was observed in the year after flooding. These results suggest that the hypoxic water mass persists for a long time after flooding.

We report a sudden explosive rise in abundance off southeastern China of a fish species that is hypoxia-tolerant, Bombay duck (Harpadon nehereus, Family Synodontidae), belonging to an Order (the Aulopiformes) encompassing overwhelmingly deep-sea fishes, but which predominantly occurs in coastal water. We suggest that this is made possible by the very high water content of its muscle and other tissues (about 90%, vs 75–80% for other coastal fish), which reduces its oxygen requirements and allows it to outcompete other fish in low-oxygen neritic and estuarine waters. We suggest that the extent to which this drastic shift in abundance and in the resulting community structure in a very extensive and extremely important coastal marine ecosystem offer evidence of growth of ocean deoxygenation along the western edge of the North Pacific Ocean. Also, we suggest that this episode may represent an instructive example of similar disruptions of established marine biological community structures and interlinked life support and natural resource systems that may be expected to emerge under current global trends of climate/environmental change.

13 pages, 4 figures, supporting Information https://doi.org/10.1111/ecog.05606.-- Data availability statement: Data available from the Dryad Digital Repository: https://dx.doi.org/10.5061/dryad.zs7h44j8s (Figuerola et al. 2021), Coastal hypoxia has become an increasingly acknowledged threat to coral reefs that is potentially intensifying because of increased input of anthropogenic nutrients. Almirante Bay (Caribbean Panama) is a semi-enclosed system that experiences hypoxia in deeper waters which occasionally shoals onto shallow coral reefs, suffocating most aerobic benthic life. To explore the long-term history of reefs in the bay we extracted reef matrix cores from two reefs that today experience contrasting patterns of oxygenation. We constructed a 1800-year-long record of gastropod assemblages and isotope compositions from six U-Th chronologically-constrained reef matrix cores. We extracted two cores from each reef at 3 m water depth and two additional cores from a deeper part (4.8 m) of the hypoxia-exposed reef. Results show that the deeper part of the hypoxic reef slowed in growth and stopped accreting approximately 1500 years BP while the shallow part of the reef continued to accrete to the present day, in agreement with a model of expanding hypoxia at this time. Our proxy-based approach suggests that the patterns of increasing herbivores and decreasing carbon isotope values in the deeper part of the hypoxic reef may have been driven by an increase in hypoxia via eutrophication caused by either natural changes or human impacts. Similar patterns in these paleoindicators occurred in the shallow part of the hypoxic reef during the last few decades. This suggests that the deep water hypoxia may be expanding to depths as shallow as 3 m and that shallow reefs are at greater risk due to increased human activity, This project and the research visit of BF to Texas A&M Univ. were financially supported by the Secretaría Nacional de Ciencia y Tecnología e Innovación (SENACYT; 47-2017-4-FID16-239), Sistema Nacional de Investigación (SENACYT), the Scholarly Studies Program (Smithsonian Institution), Smithsonian Tropical Research Institute (STRI), Texas A&M University and donations from M. Selin and family, J. Bilyk, V. and B. Anders, J. and M. Bytnar and the Young Presidents Organization (Los Angeles Gold Chapter). BF was supported by a STRI postdoctoral fellowship and the postdoctoral fellowships programme Beatriu de Pinós funded by the Secretary of Universities and Research (Government of Catalonia) and by the Horizon 2020 programme of research and innovation of the European Union under the Marie Skłodowska-Curie grant agreement no. 801370 (Incorporation grant 2019 BP 00183) and the Juan de la Cierva programme funded by the Ministry of Science and Innovation (Incorporation grant IJCI-2017-31478). With the institutional support of the ‘Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S)

Chemical pollution poses adverse effects on aquatic organisms, including altered gene expression. This study, therefore, investigated the hepatic expression of biomarkers of exposure and effect in Oreochromis niloticus inhabiting the drainage canal of a sugarbeet factory (an industrially polluted habitat) and the Nile River (a reference habitat). Compared to the Nile River, the drainage canal contained significantly elevated levels of heavy metals (Cd, Fe, Mn, Co, and Pb), nitrate, ammonia, nitrite, chemical oxygen demand, and biological oxygen demand. The liver of O. niloticus from the drainage canal accumulated significantly higher concentrations of the heavy metals. O. niloticus from the drainage canal had significantly higher hepatic expressions of genes related to oxidative stress (GSTa, GPx, and NRF2), metal toxicity (MT), endocrine disruption (Vtg), and hypoxia (HIF1a) and a significantly lower hepatic expression of the apoptosis-associated CAS gene. CYP1A (a detoxification biomarker) expression did not exhibit significant differences. These results provide insights into complex adaptive responses of O. niloticus to multiple chemicals and support the utilization of molecular biomarkers as warning signals for water pollution.

Synopsis Eastern oysters (Crassostrea virginica) have long been recognized as model organisms of extreme environmental tolerance, showing resilience to variation in temperature, salinity, hypoxia, and microbial pathogens. These phenotypic responses, however, show variability between geographic locations or habitats (e.g., tidal). Physiological, morphological, and genetic differences occur in populations throughout a species’ geographical range, which may have been shaped by regional abiotic and biotic variations. Few studies of C. virginica have explored the combined factors of physiological mechanisms of divergent phenotypes between locations and the genetic relationships of individuals between these locations. To characterize genetic relationships of four locations with aquacultured oysters along the North Carolina and Virginia coast, we sequenced a portion of cytochrome oxidase subunit I (COI) that revealed significant variation in haplotype distribution between locations. We then measured mitochondrial physiology and expression of the innate immunity response of hemocytes to lab acclimation and combined stress conditions to compare basal expression and stress response in oysters between these locations. For stress sensing genes, toll-like receptors had the strongest location-specific response to hypoxia and Vibrio, whereas mannose receptor and a stress-receptor were specific to hypoxia and bacteria, respectively. The expression of stress response genes also showed location-specific and stressor-specific changes in expression, particularly for big defensin and the complement gene Cq3. Our results further suggested that genetic similarity of oysters from different locations was not clearly related to physiological and molecular responses. These results are informative for understanding the range of physiological plasticity for stress responses in this commercially important oyster species. They also have implications in the oyster farming industry as well as conservation efforts to restore endangered native oyster beds.

Long-term trends of bottom-water concentrations of dissolved oxygen (DO), total nitrogen, and total phosphorus at 42 sites in Tokyo Bay were analyzed using monthly data from 1983 to 2018 measured by local government agencies. The results were visualized as spatial distribution maps. Bottom-water DO concentrations varied seasonally. The area of hypoxic bottom water (DO

Although the water quality in Hiroshima Bay has improved due to government measures, nutrient reduction has sharply decreased fisheries production. The law was revised in 2015, where the nutrient effluents from the sewage treatment plants were relaxed, yet no increase in fishery production was observed. Herein, we investigate the distribution of C, N, S, and P within Hiroshima Bay. Material loads from land and oyster farming activity influenced the C and S distributions in the bay sediments, respectively. Natural denitrification caused N reduction in areas by the river mouths and the landlocked areas whose sediments are reductive. The P content was high in the areas under aerobic conditions, suggesting metal oxide-bound P contributes to P accumulation. However, it was low in the areas with reducing conditions, indicating P is released from the sediments when reacting with H2S. In such reductive sediments, liberated H2S also consumes dissolved oxygen causing hypoxia in the bottom layer. It was estimated that 0.28 km3 of muddy sediment and 1.8 × 105 ton of P accumulated in Hiroshima Bay. There remains conflict between the ‘Legacy of Eutrophication’ in the sediment and ‘Cultural Oligotrophication’ in the surface water due to 40 years of reduction policies.

Changing ocean conditions, such as ocean acidification, hypoxia, and ocean warming, are impacting marine ecosystems and posing a variety of immediate and future challenges for natural resource mana...

Understanding what environmental drivers influence marine predator-prey relationships can be key to managing and protecting ecosystems, especially in the face of future climate change risks. This is especially important in environments such as the Black Sea, where strong biogeochemical gradients can drive marine habitat partitioning and ecological interactions. We used underwater video recordings in the north-eastern Black Sea in November 2013 to observe the distribution and behaviour of the Black Sea sprat (Sprattus sprattus phalericus, Risso 1827) and its zooplankton prey. Video recordings have shown that the Black Sea sprat S. sprattus phalericus tolerates severely hypoxic waters near the redoxcline. The school was distributed in the 33-96 m layer [oxygen concentration (O2 ) 277-84 μmol L-1 ]. Some individuals were observed to leave the school and descended 20 m deeper for foraging on copepods in the 119-123 m layer (O2 12-10 μmol L-1 ). Zooplankton appeared concentrated on the upper boundary of the suboxic zone (O2

Research focused on understanding the evolutionary factors that shape parity mode evolution among vertebrates have long focused on squamate reptiles (snakes and lizards), which contain all but one of the evolutionary transitions from oviparity to viviparity among extant amniotes. While most hypotheses have focused on the role of cool temperatures in favoring viviparity in thermoregulating snakes and lizards, there is a growing appreciation in the biogeographic literature for the importance of lower oxygen concentrations at high elevations for the evolution of parity mode. However, the physiological mechanisms underlying how hypoxia might reduce fitness, and how viviparity can alleviate this fitness decrement, has not been systematically evaluated. We qualitatively evaluated previous research on reproductive and developmental physiology, and found that (1) hypoxia can negatively affect fitness of squamate embryos, (2) oxygen availability in the circulatory system of adult lizards can be similar or greater than an egg, and (3) gravid females can possess adaptive phenotypic plasticity in response to hypoxia. These findings suggest that the impact of hypoxia on the development and physiology of oviparous and viviparous squamates would be a fruitful area of research for understanding the evolution of viviparity. To that end, we propose an integrative research program for studying hypoxia and the evolution of viviparity in squamates.

Understanding the impacts on the Brazilian semi-arid coast, which is a drought-prone area (>1000 km) in the tropical Atlantic, and how ecosystems survive and adapt to such extreme environments requires socioecological studies to create a theory for conservation. Here, we highlight five main ongoing changes in tropical semi-arid areas, namely (1) the decrease in rainfall rates due to climate change, which alters freshwater flows, alters water residence times, and promotes hypersalinity (>37) in low-inflow estuaries; (2) sea-level rise, groundwater hazards, increased erosion of beaches and nearshore mangroves, and landward mangrove forest expansion due to enhanced saline intrusion along river basins; (3) the decrease in land-ocean fluxes due to silting and closure of sandy bars in estuarine mouths; (4) warming and increased intensity and frequency of extreme events (e.g., heat waves, droughts, and sea swells); and (5) growing eutrophication and hypoxia, loss of vegetation cover and biodiversity due to urbanization, aquaculture (shrimp farming), agriculture, and land-use change, which includes building dams for water supply. The alteration of biogeochemical processes (“Arctic Paradox” hypothesis) and acidification that potentialize the impact of contaminants and nutrients is also highlighted. These impacts have effects on food and water security and multiple trophic levels, which should preferentially be studied through a long-term approach for advancing research. Based on these concerns, we propose key questions that should guide research in the context of the Decade of Ocean Science for Sustainable Development (2021–2030) to support science-based management actions in Brazil and other similar semi-arid areas worldwide.

The composition of metabolites in the coelomic fluid of the starfish Asterina pectinifera and sea urchin Mesocentrotus nudus was studied under normal and hypoxic conditions using NMR spectroscopy. Their change after keeping animals under hypoxia was shown. Sea urchins revealed the presence of anaerobic metabolism, which intensifies under stress in conditions of hypoxia. No similar mechanism of energy metabolism has been found for the starfish. Under conditions of hypoxia, a significant increase in lactate, which is observed in sea urchins, is not observed in the composition of the coelomic liquid of starfish. Asterine tissues contain a mixture of carotenoids in large quantities, the most well-known of which is astaxanthin. The starfish Asterina pectinifera can be used as a raw material for new medicinal, cosmetic, and food products.

Terrestrial isopods have evolved adaptations to reduce water loss, which is necessary for life in low humidity environments. However, the evolution of a waterproofed cuticle to prevent loss of water to the environment could also impede oxygen uptake from the environment. We therefore postulate an evolutionary trade-off between water retention and gas exchange in this group of soil animals. The outcome of this trade-off is expected to be affected by both differences across species (different types of lung) and differences within species (body size and resulting surface area to volume ratios). To test these ideas, we compared two sympatric isopods: Porcellio scaber and Oniscus asellus. While P. scaber possesses covered lungs typical for drier habitats, O. asellus has simple open respiratory fields which are in direct contact with external air. For each species, we assessed how individuals across a broad range in body size differed in their hypoxia and desiccation tolerance. In addition, we assessed how hypoxia and low humidity affected their thermal preference. We found clear effects of species identity and body size on tolerance to hypoxia and low humidity. Desiccation resistance was tightly linked to water loss rates (R2 = 0.96) and strongly resembled the interspecific pattern across 20 isopod species. However, our results did not support the postulated trade-off. Tolerance to hypoxia and low humidity covaried, both increasing with body size and being higher in P. scaber. Thermal preference was affected by both hypoxia and low humidity, but not by body size. Our study increases understanding of the ecophysiology of both species, which can be useful in explaining the geographical distribution and use of microhabitats of isopod species in a climate change context.

The present study was conducted to evaluate the effects of dissolved oxygen concentrations on growth, survival and gonadal development of freshwater pearl mussel (Lamellidens marginalis). Twenty adult L. marginalis were exposed to each of the five different concentrations of dissolved oxygen (DO) i.e (T0, no aeration), and T1, T2, T3 and T4 with one, two, three and four air stone aerators, respectively in fifteen glass aquaria each containing 10L of pond water. DO concentrations were significantly higher in all the treatments compared to the control but there were no significant differences among the treatments. There were significant differences in the growth and survival rates between control and treatments. The relative growth rates (%) were significantly higher in the highest oxygen concentration (T4) compared to other treatments. The survival rate was zero in control (T0), whereas it was 66.67%, 70.00%, 71.67% and 73.33%, respectively for T1, T2, T3 and T4. On day-30, oocytes in T0 were found to be smaller in size, with developing acini and no previtellogenic oocytes; whereas in T1, T2, T3 and T4, previtellogenic oocytes and larger acini were evident. On day-60, vitellogenic oocytes along with few under-developed oocytes started to detach from the acini walls, as well as some already started migration to germinal duct on day-90. This indicates that ovaries of all the treatments were ripe and were ready to spawn between day-60 and -90. On day-30, secondary spermatocytes were observed in testicular acini in case of T0, whereas more developed and dense spermatids were evident in T1, T2, T3 and T4. These results indicate that testes were in late development stages in T0, whereas they were ripe in all the treatments. The current findings indicate that hypoxia slows down growth, reduces survival rate and retards development of reproductive organs in L. marginalis.

Strongly stratified water structure and densely populated catchment make the Baltic Sea one of the most polluted seas. Understanding its circulation pattern and time scale is essential to predict the dynamics of hypoxia, eutrophication, and pollutants. Anthropogenic 236U and 233U have been demonstrated as excellent transient tracers in oceanic studies, but unclear input history and inadequate long-term monitoring records limit their application in the Baltic Sea. From two dated Baltic sediment cores, we obtained high-resolution records of anthropogenic uranium imprints originated from three major human nuclear activities throughout the Atomic Era. Using the novel 233U/236U signature, we distinguished and quantified 236U inputs from global fallout (43.3%-50.5%), Chernobyl accident (233U (7-15kg) from the atmospheric nuclear weapons testing, and pinpointed 233U peak signal in the mid-to-late 1950s as a potential time marker for the onset of the Anthropocene Epoch. This work also provides fundamental 236U data for Chernobyl accident and early discharges from civil nuclear facilities, prompting worldwide 233U-236U tracer studies. We anticipate our data to be a broader application in model-observation interdisciplinary research on water circulation and pollutant dynamics in the Baltic Sea.

In this study, we investigated the mechanism of eutrophication and hypoxia in the upper Gulf of Thailand from August 2014 to June 2015 based on field observation data, box model analysis, and the unscaled trophic status index (UNTRIX). Fresh water residence time derived from a simple box model was long (38.61 days) during the transition period between the southwest to northeast monsoon in September 2014. In contrast, fresh water residence time was short (2.63 days) during the late northeast monsoon in February 2015. Long residence time was related to the development of widespread strong hypoxia in near-bottom waters in over half of the gulf during the transition between the southwest and the northeast monsoon, when river discharge was also very large. UNTRIX is used to assess water trophic levels, and is based on water qualities including concentrations of chlorophyll-a (Chl-a), dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP). Hypertrophic and eutrophic conditions were observed at river mouths; their seasonal eutrophication was related to river discharge and circulation. Nutrients were mainly increased by river discharge. Water column stratification and long residence time were required for the development of severe hypoxia in the study area.