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1. Locally Adapted Coral Species Withstand a 2-Week Hypoxic Event

   https://doi.org/10.3390/oceans6010005  

   Lucey, Noelle; César-Ávila, Carolina; Eckert, Alaina; Veintimilla, Paul; Collin, Rachel (March 2025, Oceans)

   NA (Ed.)

   One approach to improve long-term coral restoration success utilizes naturally stress-tolerant corals from the wild. While the focus has primarily been on thermal stress, low oxygen is a growing threat to coral reefs and restoration efforts should also consider hypoxia tolerance. Here we determine if Siderastrea siderea and Agaricia tenuifolia populations from a reef with a historical record of low oxygen exhibit evidence of local adaptation to hypoxic events, compared to populations from a reference reef. We employed a laboratory-based reciprocal transplant experiment mimicking a severe 14-night hypoxic event and monitored bleaching responses, photo-physiology, metabolic rates, and survival of all four populations during, and for two weeks following the event. In both species, we found the populations from the hypoxic reef either fully persisted or recovered within 3 days of the event. In contrast, the conspecific naïve populations from the well-oxygenated reference reef experienced bleaching and death. This showcases the vulnerability of naïve corals exposed to low oxygen but also suggests that corals from the hypoxic reef locally adapted to survive severe episodic hypoxia. Other reefs with past episodic low oxygen may also be home to corals with adaptation signatures to hypoxia and may be useful for restoration efforts. 

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2. Extent, patterns, and drivers of hypoxia in the world's streams and rivers

   https://doi.org/10.1002/lol2.10297  

   Blaszczak, Joanna R.; Koenig, Lauren E.; Mejia, Francine H.; Gómez‐Gener, Lluís; Dutton, Christopher L.; Carter, Alice M.; Grimm, Nancy B.; Harvey, Judson W.; Helton, Ashley M.; Cohen, Matthew J. (December 2022, Limnology and Oceanography Letters)

   Abstract Hypoxia in coastal waters and lakes is widely recognized as a detrimental environmental issue, yet we lack a comparable understanding of hypoxia in rivers. We investigated controls on hypoxia using 118 million paired observations of dissolved oxygen (DO) concentration and water temperature in over 125,000 locations in rivers from 93 countries. We found hypoxia (DO < 2 mg L⁻¹) in 12.6% of all river sites across 53 countries, but no consistent trend in prevalence since 1950. High‐frequency data reveal a 3‐h median duration of hypoxic events which are most likely to initiate at night. River attributes were better predictors of riverine hypoxia occurrence than watershed land cover, topography, and climate characteristics. Hypoxia was more likely to occur in warmer, smaller, and lower‐gradient rivers, particularly those draining urban or wetland land cover. Our findings suggest that riverine hypoxia and the resulting impacts on ecosystems may be more pervasive than previously assumed. 

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3. Zooplankton Community Response to Seasonal Hypoxia: A Test of Three Hypotheses

   https://doi.org/10.3390/d12010021  

   Keister, Julie E.; Winans, Amanda K.; Herrmann, BethElLee (January 2020, Diversity)

   Several hypotheses of how zooplankton communities respond to coastal hypoxia have been put forward in the literature over the past few decades. We explored three of those that are focused on how zooplankton composition or biomass is affected by seasonal hypoxia using data collected over two summers in Hood Canal, a seasonally-hypoxic sub-basin of Puget Sound, Washington. We conducted hydrographic profiles and zooplankton net tows at four stations, from a region in the south that annually experiences moderate hypoxia to a region in the north where oxygen remains above hypoxic levels. The specific hypotheses tested were that low oxygen leads to: (1) increased dominance of gelatinous relative to crustacean zooplankton, (2) increased dominance of cyclopoid copepods relative to calanoid copepods, and (3) overall decreased zooplankton abundance and biomass at hypoxic sites compared to where oxygen levels are high. Additionally, we examined whether the temporal stability of community structure was decreased by hypoxia. We found evidence of a shift toward more gelatinous zooplankton and lower total zooplankton abundance and biomass at hypoxic sites, but no clear increase in the dominance of cyclopoid relative to calanoid copepods. We also found the lowest variance in community structure at the most hypoxic site, in contrast to our prediction. Hypoxia can fundamentally alter marine ecosystems, but the impacts differ among systems. 

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4. Species Composition and Distribution of Jellyfish in a Seasonally Hypoxic Estuary, Hood Canal, Washington

   https://doi.org/10.3390/d12020053  

   Herrmann, BethElLee; Keister, Julie E. (February 2020, Diversity)

   Seasonal hypoxia (≤2 mg dissolved oxygen L−1) can have detrimental effects on marine food webs. Recent studies indicate that some jellyfish can tolerate low oxygen and may have a competitive advantage over other zooplankton and fishes in those environments. We assessed community structure and distributions of cnidarian and ctenophore jellyfish in seasonally hypoxic Hood Canal, WA, USA, at four stations that differed in oxygen conditions. Jellyfish were collected in June through October 2012 and 2013 using full-water-column and discrete-depth net tows, concurrent with CTD casts to measure dissolved oxygen (DO). Overall, southern, more hypoxic, regions of Hood Canal had higher abundances and higher diversity than the northern regions, particularly during the warmer and more hypoxic year of 2013. Of fifteen species identified, the most abundant—the siphonophore Muggiaea atlantica and hydrozoan Aglantha digitale—reached peak densities > 1800 Ind m−3 and 38 Ind m−3, respectively. M. atlantica were much more abundant at the hypoxic stations, whereas A. digitale were also common in the north. Vertical distributions explored during hypoxia showed that jellyfish were mostly in the upper 10 m regardless of the oxycline depth. Moderate hypoxia seemed to have no detrimental effect on jellyfish in Hood Canal, and may have resulted in high population densities, which could influence essential fisheries and trophic energy flow. 

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5. Independent losses of the Hypoxia-Inducible Factor (HIF) Pathway within Crustacea

   Graham, Allie M; Barreto, Felipe S (January 2020, Molecular biology and evolution)

   Metazoans respond to hypoxic stress via the Hypoxia Inducible Factor (HIF) pathway, a mechanism thought to be extremely conserved due to its importance in monitoring cellular oxygen levels and regulating responses to hypoxia. However, recent work revealed that key members of the HIF pathway have been lost in specific lineages (a tardigrade and a copepod), suggesting this pathway is not as widespread in animals as previously assumed. Using genomic and transcriptomic data from 70 different species across 12 major crustacean groups, we assessed the degree to which the gene HIFα, the master regulator of the HIF pathway, was conserved. Mining of protein domains, followed by phylogenetic analyses of gene families, uncovered group-level losses of HIFα, including one across three orders within Cirripedia, and in three orders within Copepoda. For these groups, additional assessment showed losses of HIF repression machinery (EGLN, VHL). These results suggest the existence of alternative mechanisms for cellular response to low oxygen, and highlight these taxa as models useful for probing these evolutionary outcomes. 

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