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1. Anoxia begets anoxia: A positive feedback to the deoxygenation of temperate lakes

   https://doi.org/10.1111/gcb.17046  

   Lewis, Abigail_S_L; Lau, Maximilian_P; Jane, Stephen_F; Rose, Kevin_C; Be'eri‐Shlevin, Yaron; Burnet, Sarah_H; Clayer, François; Feuchtmayr, Heidrun; Grossart, Hans‐Peter; Howard, Dexter_W; et al (December 2023, Global Change Biology)

   Abstract Declining oxygen concentrations in the deep waters of lakes worldwide pose a pressing environmental and societal challenge. Existing theory suggests that low deep‐water dissolved oxygen (DO) concentrations could trigger a positive feedback through which anoxia (i.e., very low DO) during a given summer begets increasingly severe occurrences of anoxia in following summers. Specifically, anoxic conditions can promote nutrient release from sediments, thereby stimulating phytoplankton growth, and subsequent phytoplankton decomposition can fuel heterotrophic respiration, resulting in increased spatial extent and duration of anoxia. However, while the individual relationships in this feedback are well established, to our knowledge, there has not been a systematic analysis within or across lakes that simultaneously demonstrates all of the mechanisms necessary to produce a positive feedback that reinforces anoxia. Here, we compiled data from 656 widespread temperate lakes and reservoirs to analyze the proposed anoxia begets anoxia feedback. Lakes in the dataset span a broad range of surface area (1–126,909 ha), maximum depth (6–370 m), and morphometry, with a median time‐series duration of 30 years at each lake. Using linear mixed models, we found support for each of the positive feedback relationships between anoxia, phosphorus concentrations, chlorophyllaconcentrations, and oxygen demand across the 656‐lake dataset. Likewise, we found further support for these relationships by analyzing time‐series data from individual lakes. Our results indicate that the strength of these feedback relationships may vary with lake‐specific characteristics: For example, we found that surface phosphorus concentrations were more positively associated with chlorophyllain high‐phosphorus lakes, and oxygen demand had a stronger influence on the extent of anoxia in deep lakes. Taken together, these results support the existence of a positive feedback that could magnify the effects of climate change and other anthropogenic pressures driving the development of anoxia in lakes around the world. 

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2. Biochemical Barriers on the Path to Ocean Anoxia?

   https://doi.org/10.1128/mBio.01332-21  

   Giovannoni, S. (May 2021, bioRxiv)

   ABSTRACT The kinetics of microbial respiration suggest that, if excess organic matter is present, oxygen should fall to nanomolar levels, in the range of the Michaelis-Menten constants (Km). Yet even in many biologically productive coastal regions, lowest observed O2 concentrations often remain several orders of magnitude higher than respiratory Km values. We propose the Hypoxic Barrier Hypothesis (HBH) to explain this apparent discrepancy. The HBH postulates that oxidative enzymes involved in organic matter catabolism are kinetically limited by O2 at concentrations far higher than the thresholds for respiration. We found support for the HBH in a meta-analysis of 1137 O2 Km values reported in the literature: the median value for terminal respiratory oxidases was 350 nM, but for other oxidase types the median value was 67 μM. The HBH directs our attention to the kinetic properties of an important class of oxygen-dependent reactions that could help explain the trajectories of ocean ecosystems experiencing O2 stress. IMPORTANCE Declining ocean oxygen associated with global warming and climate change is impacting marine ecosystems across scales from microscopic planktonic communities to global fisheries. We report a fundamental dichotomy in the affinity of enzymes for oxygen. The importance of this observation has yet to be fully assessed, but it is predicted to impact the rate at which organic matter is oxidized in hypoxic ecosystems, and the types of organic matter that accumulate. Competition between intracellular enzymes for oxygen may also have impacted microbial strategies of adaptation to suboxia. 

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3. Anoxia decreases the magnitude of the carbon, nitrogen, and phosphorus sink in freshwaters

   https://doi.org/10.1111/gcb.16228  

   Carey, Cayelan C.; Hanson, Paul C.; Thomas, R. Quinn; Gerling, Alexandra B.; Hounshell, Alexandria G.; Lewis, Abigail S. L.; Lofton, Mary E.; McClure, Ryan P.; Wander, Heather L.; Woelmer, Whitney M.; et al (May 2022, Global Change Biology)

   Abstract Oxygen availability is decreasing in many lakes and reservoirs worldwide, raising the urgency for understanding how anoxia (low oxygen) affects coupled biogeochemical cycling, which has major implications for water quality, food webs, and ecosystem functioning. Although the increasing magnitude and prevalence of anoxia has been documented in freshwaters globally, the challenges of disentangling oxygen and temperature responses have hindered assessment of the effects of anoxia on carbon, nitrogen, and phosphorus concentrations, stoichiometry (chemical ratios), and retention in freshwaters. The consequences of anoxia are likely severe and may be irreversible, necessitating ecosystem‐scale experimental investigation of decreasing freshwater oxygen availability. To address this gap, we devised and conducted REDOX (the Reservoir Ecosystem Dynamic Oxygenation eXperiment), an unprecedented, 7‐year experiment in which we manipulated and modeled bottom‐water (hypolimnetic) oxygen availability at the whole‐ecosystem scale in a eutrophic reservoir. Seven years of data reveal that anoxia significantly increased hypolimnetic carbon, nitrogen, and phosphorus concentrations and altered elemental stoichiometry by factors of 2–5× relative to oxic periods. Importantly, prolonged summer anoxia increased nitrogen export from the reservoir by six‐fold and changed the reservoir from a net sink to a net source of phosphorus and organic carbon downstream. While low oxygen in freshwaters is thought of as a response to land use and climate change, results from REDOX demonstrate that low oxygen can also be adriverof major changes to freshwater biogeochemical cycling, which may serve as an intensifying feedback that increases anoxia in downstream waterbodies. Consequently, as climate and land use change continue to increase the prevalence of anoxia in lakes and reservoirs globally, it is likely that anoxia will have major effects on freshwater carbon, nitrogen, and phosphorus budgets as well as water quality and ecosystem functioning. 

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4. Anoxia elicits the strongest stimulatory protective response in insect low-oxygen hormesis

   https://doi.org/10.1016/j.cotox.2022.02.004  

   Campbell, Jacob B.; López-Martínez, Giancarlo (March 2022, Current Opinion in Toxicology)
5. Transcriptomic Responses of the Heart and Brain to Anoxia in the Western Painted Turtle

   https://doi.org/10.1371/journal.pone.0131669  

   Keenan, Sarah W.; Hill, Craig A.; Kandoth, Cyriac; Buck, Leslie T.; Warren, Daniel E.; Castoe, Todd Adam (July 2015, PLOS ONE)