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1. Changes in population depth distribution and oxygen stratification are involved in the current low condition of the eastern Baltic Sea cod (<i>Gadus morhua</i>)

   https://doi.org/10.5194/bg-18-1321-2021  

   Casini, Michele ; Hansson, Martin ; Orio, Alessandro ; Limburg, Karin ( January 2021 , Biogeosciences)

   null (Ed.)

   Abstract. During the past 20 years, hypoxic areas have expanded rapidly in theBaltic Sea, which has become one of the largest marine “dead zones” in theworld. At the same time, the most important commercial fish population ofthe region, the eastern Baltic cod, has experienced a drastic reduction inmean body condition, but the processes behind the relation betweendeoxygenation and condition remain elusive. Here we use extensive long-termmonitoring data on cod biology and distribution as well as on hydrologicalvariations to investigate the processes that relate deoxygenation and codcondition during the autumn season. Our results show that the depthdistribution of cod has increased during the past 4 decades at the sametime of the expansion, and shallowing, of waters with oxygen concentrationsdetrimental to cod performance. This has resulted in a progressivelyincreasing spatial overlap between the cod population and low-oxygenatedwaters after the mid-1990s. This spatial overlap and the actual oxygenconcentration experienced by cod therein statistically explained a largeproportion of the changes in cod condition over the years. These resultscomplement previous analyses on fish otolith microchemistry that alsorevealed that since the mid-1990s, cod individuals with low condition wereexposed to low-oxygen waters during their life. This study helps to shedlight on the processes that have led to a decline of the eastern Baltic codbody condition, which can aid the management of this population currently indistress. Further studies should focus on understanding why the codpopulation has moved to deeper waters in autumn and on analyzing the overlapwith low-oxygen waters in other seasons to quantify the potential effects ofthe variations in physical properties on cod biology throughout the year. 

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2. Metabolic prioritization of fish in hypoxic waters: an integrative modeling approach

   https://doi.org/10.3389/fmars.2023.1206506  

   Duskey, Elizabeth ( July 2023 , Frontiers in Marine Science)

   Marine hypoxia has had major consequences for both economically and ecologically critical fish species around the world. As hypoxic regions continue to grow in severity and extent, we must deepen our understanding of mechanisms driving population and community responses to major stressors. It has been shown that food availability and habitat use are the most critical components of impacts on individual fish leading to observed outcomes at higher levels of organization. However, differences within and among species in partitioning available energy for metabolic demands – or metabolic prioritization – in response to stressors are often ignored. Here, I use both a multispecies size spectrum model and a meta-analysis to explore evidence in favor of metabolic prioritization in a community of commercially important fish species in the Baltic Sea. Modeling results suggest that metabolic prioritization is an important component of the individual response to hypoxia, that it interacts with other components to produce realistic community dynamics, and that different species may prioritize differently. It is thus suggested that declines in feeding activity, assimilation efficiency, and successful reproduction – in addition to low food availability and changing habitat use – are all important drivers of the community response to hypoxia. Meta-analysis results also provide evidence that the dominant predator in the study system prioritizes among metabolic demands, and that these priorities may change as oxygen declines. Going forward, experiments and models should explore how differences in priorities within and among communities drive responses to environmental degradation. This will help management efforts to tailor recovery programs to the physiological needs of species within a given system. 

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3. Effects of hypoxia on the behavior and physiology of kelp forest fishes

   https://doi.org/10.1111/gcb.15076  

   Mattiasen, Evan G. ; Kashef, Neosha S. ; Stafford, David M. ; Logan, Cheryl A. ; Sogard, Susan M. ; Bjorkstedt, Eric P. ; Hamilton, Scott L. ( April 2020 , Global Change Biology)

   Forecasts from climate models and oceanographic observations indicate increasing deoxygenation in the global oceans and an elevated frequency and intensity of hypoxic events in the coastal zone, which have the potential to affect marine biodiversity and fisheries. Exposure to low dissolved oxygen (DO) conditions may have deleterious effects on early life stages in fishes. This study aims to identify thresholds to hypoxia while testing behavioral and physiological responses of two congeneric species of kelp forest fish to four DO levels, ranging from normoxic to hypoxic (8.7, 6.0, 4.1, and 2.2 mg O₂/L). Behavioral tests identified changes in exploratory behavior and turning bias (lateralization), whereas physiological tests focused on determining changes in hypoxia tolerance (pCrit), ventilation rates, and metabolic rates, with impacts on the resulting capacity for aerobic activity. Our findings indicated that copper rockfish (Sebastes caurinus) and blue rockfish (Sebastes mystinus) express sensitivity to hypoxia; however, the strength of the response differed between species. Copper rockfish exhibited reduced absolute lateralization and increased escape time at the lowest DO levels, whereas behavioral metrics for blue rockfish did not vary with oxygen level. Both species exhibited decreases in aerobic scope (as a function of reduced maximum metabolic rate) and increases in ventilation rates to compensate for decreasing oxygen levels. Blue rockfish had a lower pCrit and stronger acclimation response compared to copper rockfish. The differences expressed by each species suggest that acclimatization to changing ocean conditions may vary, even among related species that recruit to the same kelp forest habitat, leading to winners and losers under future ocean conditions. Exposure to hypoxia can decrease individual physiological fitness through metabolic and aerobic depression and changes to anti‐predator behavior, with implications for the outcome of ecological interactions and the management of fish stocks in the face of climate change.

    

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4. Shifts in the coral microbiome in response to in situ experimental deoxygenation

   https://doi.org/10.1128/aem.00577-23  

   Howard, Rachel D. ; Schul, Monica D. ; Rodriguez Bravo, Lucia M. ; Altieri, Andrew H. ; Meyer, Julie L. ( November 2023 , Applied and Environmental Microbiology)

   Biddle, Jennifer F. (Ed.)

   Global climate change impacts marine ecosystems through rising surface temperatures, ocean acidification, and deoxygenation. While the response of the coral holobiont to the first two effects has been relatively well studied, less is known about the response of the coral microbiome to deoxygenation. In this study, we investigated the response of the microbiome to hypoxia in two coral species that differ in their tolerance to hypoxia. We conductedin situoxygen manipulations on a coral reef in Bahía Almirante on the Caribbean coast of Panama, which has previously experienced documented episodes of hypoxia. Naïve coral colonies (previously unexposed to hypoxia) ofSiderastrea sidereaandAgaricia lamarckiwere transplanted to a reef and either enclosed in chambers that created hypoxic conditions or left at ambient oxygen levels. We collected samples of surface mucus and tissue after 48 hours of exposure and characterized the microbiome by sequencing 16S rRNA genes. We found that the microbiomes of the two coral species were distinct from one another and remained so after exhibiting similar shifts in microbiome composition in response to hypoxia. There was an increase in both abundance and number of taxa of anaerobic microbes after exposure to hypoxia. Some of these taxa may play beneficial roles in the coral holobiont by detoxifying the surrounding environment during hypoxic stress or may represent opportunists exploiting host stress. This work describes the first characterization of the coral microbiome under hypoxia and is an initial step toward identifying potential beneficial bacteria for corals facing this environmental stressor.

   Marine hypoxia is a threat for corals but has remained understudied in tropical regions where coral reefs are abundant. Though microbial symbioses can alleviate the effects of ecological stress, we do not yet understand the taxonomic or functional response of the coral microbiome to hypoxia. In this study, we experimentally lowered oxygen levels aroundSiderastrea sidereaandAgaricia lamarckicoloniesin situto observe changes in the coral microbiome in response to deoxygenation. Our results show that hypoxia triggers a stochastic change of the microbiome overall, with some bacterial families changing deterministically after just 48 hours of exposure. These families represent an increase in anaerobic and opportunistic taxa in the microbiomes of both coral species. Thus, marine deoxygenation destabilizes the coral microbiome and increases bacterial opportunism. This work provides novel and fundamental knowledge of the microbial response in coral during hypoxia and may provide insight into holobiont function during stress.

    

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5. Tolerance of aquifer stoneflies to repeated hypoxia exposure and oxygen dynamics in an alluvial aquifer

   https://doi.org/10.1242/jeb.225623  

   Malison, Rachel L. ; DelVecchia, Amanda G. ; Woods, H. Arthur ; Hand, Brian K. ; Luikart, Gordon ; Stanford, Jack A. ( August 2020 , The Journal of Experimental Biology)

   null (Ed.)

   ABSTRACT Aquatic insects cope with hypoxia and anoxia using a variety of behavioral and physiological responses. Most stoneflies (Plecoptera) occur in highly oxygenated surface waters, but some species live underground in alluvial aquifers containing heterogeneous oxygen concentrations. Aquifer stoneflies appear to be supported by methane-derived food resources, which they may exploit using anoxia-resistant behaviors. We documented dissolved oxygen dynamics and collected stoneflies over 5 years in floodplain wells of the Flathead River, Montana. Hypoxia regularly occurred in two wells, and nymphs of Paraperla frontalis were collected during hypoxic periods. We measured mass-specific metabolic rates (MSMRs) at different oxygen concentrations (12, 8, 6, 4, 2, 0.5 mg l −1 , and during recovery) for 111 stonefly nymphs to determine whether aquifer and benthic taxa differed in hypoxia tolerance. Metabolic rates of aquifer taxa were similar across oxygen concentrations spanning 2 to 12 mg l −1 ( P >0.437), but the MSMRs of benthic taxa dropped significantly with declining oxygen ( P <0.0001; 2.9-times lower at 2 vs. 12 mg l −1 ). Aquifer taxa tolerated short-term repeated exposure to extreme hypoxia surprisingly well (100% survival), but repeated longer-term (>12 h) exposures resulted in lower survival (38–91%) and lower MSMRs during recovery. Our work suggests that aquifer stoneflies have evolved a remarkable set of behavioral and physiological adaptations that allow them to exploit the unique food resources available in hypoxic zones. These adaptations help to explain how large-bodied consumers might thrive in the underground aquifers of diverse and productive river floodplains. 

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