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1. Asynchronous effects of heat stress on growth rates of massive corals and damselfish in the Red Sea

   https://doi.org/10.1371/journal.pone.0316247  

   Zahid, Fiza; Gajdzik, Laura; Korsmeyer, Keith E; Cotton, Jordyn D; Coker, Daren J; Berumen, Michael L; DeCarlo, Thomas M (January 2025, PLOS ONE)

   Mayfield, Anderson B (Ed.)

   Climate change is imposing multiple stressors on marine life, leading to a restructuring of ecological communities as species exhibit differential sensitivities to these stressors. With the ocean warming and wind patterns shifting, processes that drive thermal variations in coastal regions, such as marine heatwaves and upwelling events, can change in frequency, timing, duration, and severity. These changes in environmental parameters can physiologically impact organisms residing in these habitats. Here, we investigate the synchrony of coral and reef fish responses to environmental disturbance in the Red Sea, including an unprecedented combination of heat stress and upwelling that led to mass coral bleaching in 2015. We developed cross-dated growth chronologies from otoliths of 156 individuals of two planktivorous damselfish species,Pomacentrus sulfureusandAmblyglyphidodon flavilatus, and from skeletal cores of 48Poritesspp. coral colonies. During and immediately after the 2015 upwelling and bleaching event, damselfishes exhibited a positive growth anomaly but corals displayed reduced growth. Yet, after 2015–2016, these patterns were reversed with damselfishes showing a decline in growth and corals rebounding to pre-disturbance growth rates. Our results reveal an asynchronous response between corals and reef fish, with corals succumbing to the direct effects of heat stress, and then quickly recovering when the heat stress subsided—at least, for those corals that survived the bleaching event. Conversely, damselfish growth temporarily benefited from the events of 2015, potentially due to the increased metabolic demand from increased temperature and increased food supply from the upwelling event, before declining over four years, possibly related to indirect effects associated with habitat degradation following coral mortality. Overall, our study highlights the increasingly complex, often asynchronous, ecological ramifications of climate extremes on the diverse species assemblages of coral reefs. 

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2. Local conditions magnify coral loss after marine heatwaves

   https://doi.org/10.1126/science.abd9464  

   Donovan, Mary K.; Burkepile, Deron E.; Kratochwill, Chelsey; Shlesinger, Tom; Sully, Shannon; Oliver, Thomas A.; Hodgson, Gregor; Freiwald, Jan; van Woesik, Robert (May 2021, Science)

   null (Ed.)

   Climate change threatens coral reefs by causing heat stress events that lead to widespread coral bleaching and mortality. Given the global nature of these mass coral mortality events, recent studies argue that mitigating climate change is the only path to conserve coral reefs. Using a global analysis of 223 sites, we show that local stressors act synergistically with climate change to kill corals. Local factors such as high abundance of macroalgae or urchins magnified coral loss in the year after bleaching. Notably, the combined effects of increasing heat stress and macroalgae intensified coral loss. Our results offer an optimistic premise that effective local management, alongside global efforts to mitigate climate change, can help coral reefs survive the Anthropocene. 

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3. Increased dominance of heat-tolerant symbionts creates resilient coral reefs in near-term ocean warming

   https://doi.org/10.1073/pnas.2202388120  

   Palacio-Castro, Ana M.; Smith, Tyler B.; Brandtneris, Viktor; Snyder, Grace A.; van Hooidonk, Ruben; Maté, Juan L.; Manzello, Derek; Glynn, Peter W.; Fong, Peggy; Baker, Andrew C. (February 2023, Proceedings of the National Academy of Sciences)

   Climate change is radically altering coral reef ecosystems, mainly through increasingly frequent and severe bleaching events. Yet, some reefs have exhibited higher thermal tolerance after bleaching severely the first time. To understand changes in thermal tolerance in the eastern tropical Pacific (ETP), we compiled four decades of temperature, coral cover, coral bleaching, and mortality data, including three mass bleaching events during the 1982 to 1983, 1997 to 1998 and 2015 to 2016 El Niño heatwaves. Higher heat resistance in later bleaching events was detected in the dominant framework-building genus, Pocillopora, while other coral taxa exhibited similar susceptibility across events. Genetic analyses of Pocillopora spp . colonies and their algal symbionts (2014 to 2016) revealed that one of two Pocillopora lineages present in the region ( Pocillopora “ type 1”) increased its association with thermotolerant algal symbionts ( Durusdinium glynnii ) during the 2015 to 2016 heat stress event. This lineage experienced lower bleaching and mortality compared with Pocillopora “type 3”, which did not acquire D. glynnii . Under projected thermal stress, ETP reefs may be able to preserve high coral cover through the 2060s or later, mainly composed of Pocillopora colonies that associate with D. glynnii . However, although the low-diversity, high-cover reefs of the ETP could illustrate a potential functional state for some future reefs, this state may only be temporary unless global greenhouse gas emissions and resultant global warming are curtailed. 

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4. What makes a winner? Symbiont and host dynamics determine Caribbean octocoral resilience to bleaching.

   https://doi.org/10.1126/sciadv.adj6788  

   Coffroth, Mary Alice; Buccella, Louis A.; Eaton, Katherine M.; Lasker, Howard R.; Gooding, Alyssa T.; Franklin, Harleena (November 2023, Science Advances)

   Unlike reef-building, scleractinian corals, Caribbean soft corals (octocorals) have not suffered marked declines in abundance associated with anthropogenic ocean warming. Both octocorals and reef-building scleractinians depend on a nutritional symbiosis with single-celled algae living within their tissues. In both groups, increased ocean temperatures can induce symbiont loss (bleaching) and coral death. Multiple heat waves from 2014 to 2016 resulted in widespread damage to reef ecosystems and provided an opportunity to examine the bleaching response of three Caribbean octocoral species. Symbiont densities declined during the heat waves but recovered quickly, and colony mortality was low. The dominant symbiont genotypes within a host generally did not change, and all colonies hosted symbiont species in the genusBreviolum.Their association with thermally tolerant symbionts likely contributes to the octocoral holobiont’s resistance to mortality and the resilience of their symbiont populations. The resistance and resilience of Caribbean octocorals offer clues for the future of coral reefs. 

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5. Ocean currents magnify upwelling and deliver nutritional subsidies to reef-building corals during El Niño heatwaves

   https://doi.org/10.1126/sciadv.add5032  

   Fox, Michael D.; Guillaume-Castel, Robin; Edwards, Clinton B.; Glanz, J.; Gove, Jamison M.; Green, J. A.; Juhlin, E.; Smith, Jennifer E.; Williams, Gareth J. (June 2023, Science Advances)

   Marine heatwaves are triggering coral bleaching events and devastating coral populations globally, highlighting the need to identify processes promoting coral survival. Here, we show that acceleration of a major ocean current and shallowing of the surface mixed layer enhanced localized upwelling on a central Pacific coral reef during the three strongest El Niño–associated marine heatwaves of the past half century. These conditions mitigated regional declines in primary production and bolstered local supply of nutritional resources to corals during a bleaching event. The reefs subsequently suffered limited post-bleaching coral mortality. Our results reveal how large-scale ocean-climate interactions affect reef ecosystems thousands of kilometers away and provide a valuable framework for identifying reefs that may benefit from such biophysical linkages during future bleaching events. 

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