page_title: Coral bleaching
section_title: Recovery and macroalgal regime shifts
hierarchical_section_title: Coral bleaching / Recovery and macroalgal regime shifts
caption_attribution_description: English: Lemon damsel (Pomacentrus moluccensis), Pulau Aur, West Malaysia
context_page_description: Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with these algae, which are crucial for the health of the coral and the reef. The algae provides up to 90 percent of the coral's energy. Bleached corals continue to live but begin to starve after bleaching. Some corals recover.
  The leading cause of coral bleaching is rising water temperatures. A temperature about 1 °C above average can cause bleaching. According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global bleaching events killed coral on an unprecedented scale. In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral. In 2017, the bleaching extended into the central region of the reef. The average interval between bleaching events has halved between 1980 and 2016.
context_section_description: After corals experience a bleaching event to increased temperature stress some reefs are able to return to their original, pre-bleaching state. Reefs either recover from bleaching, where they are recolonized by zooxanthellae, or they experience a regime shift, where previously flourishing coral reefs are taken over by thick layers of macroalgae. This inhibits further coral growth because the algae produces antifouling compounds to deter settlement and competes with corals for space and light. As a result, macroalgae forms stable communities that make it difficult for corals to grow again. Reefs will then be more susceptible to other issues, such as declining water quality and removal of herbivore fish, because coral growth is weaker. Discovering what causes reefs to be resilient or recover from bleaching events is of primary importance because it helps inform conservation efforts and protect coral more effectively.
  A primary subject of research regarding coral recovery pertains to the idea of super-corals, otherwise referred to as the corals that live and thrive in naturally warmer and more acidic regions and bodies of water. When transplanted to endangered or bleached reefs, their resilience and irradiance can equip the algae to live among the bleached corals. As Emma Camp, a National Geographic Explorer, marine bio-geochemist and an ambassador for Biodiversity for the charity IBEX Earth, suggests, the super-corals could have the capability to help with the damaged reefs long-term. While it can take 10 to 15 years to restore damaged and bleached coral reefs, the super-corals could have lasting impacts despite climate change as the oceans rise in temperature and gain more acidity. Bolstered by the research of Ruth Gates, Camp has looked into lower oxygen levels and the extreme, unexpected habitats that reefs can be found in across the globe.
  Corals have shown to be resilient to short-term disturbances. Recovery has been shown in after storm disturbance and crown of thorns starfish invasions. Fish species tend to fare better following reef disturbance than coral species as corals show limited recovery and reef fish assemblages have shown little change as a result of short-term disturbances. In contrast, fish assemblages in reefs that experience bleaching exhibit potentially damaging changes. One study by Bellwood et al. notes that while species richness, diversity, and abundance did not change, fish assemblages contained more generalist species and less coral dependent species. Responses to coral bleaching are diverse between reef fish species, based on what resources are affected. Rising sea temperature and coral bleaching do not directly impact adult fish mortality, but there are many indirect consequences of both. Coral-associated fish populations tend to be in decline due to habitat loss; however, some herbivorous fish populations have seen a drastic increase due to the increase of algae colonization on dead coral. Studies note that better methods are needed to measure the effects of disturbance on the resilience of corals.
  Until recently, the factors mediating the recovery of coral reefs from bleaching were not well studied. Research by Graham et al. (2005) studied 21 reefs around Seychelles in the Indo-Pacific in order to document the long-term effects of coral bleaching. After the loss of more than 90% of corals due to bleaching in 1998 around 50% of the reefs recovered and roughly 40% of the reefs experienced regime shifts to macroalgae dominated compositions. After an assessment of factors influencing the probability of recovery, the study identified five major factors: density of juvenile corals, initial structural complexity, water depth, biomass of herbivorous fishes, and nutrient conditions on the reef. Overall, resilience was seen most in coral reef systems that were structurally complex and in deeper water.
  The ecological roles and functional groups of species also play a role in the recovery of regime shifting potential in reef systems. Coral reefs are affected by bioeroding, scraping, and grazing fish spe
caption: The lemon damselfish (Pomacentrus moluccensis) is a coral-associated species that has been shown to decline dramatically following coral bleaching.[107]
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