Coral response to bleaching captured in unprecedented detail 

A team of international marine biologists has provided unprecedented insights into coral bleaching responses to stress using cutting-edge imaging technology. 

Their study, published on July 31 in the journal PLoS ONE, details the survival rates of coral colonies following multiple bleaching events around the island of Maui.

Patterns of coral growth during bleaching

The research, led by Scripps Oceanography’s Smith Lab, involved tracking 1,832 coral colonies using a time series of 3D coral reef models from Maui, collected between 2014 and 2021. 

This seven-year dataset offered a detailed annual view of the reefs, allowing the team to observe patterns of coral growth and survival through significant bleaching events in 2015 and 2019.

The study aimed to differentiate between natural selection, where only the hardiest corals survive, and acclimatization, where individual corals develop increased heat tolerance over time due to repeated exposure to heat stress. 

Some corals are more resilient 

Despite some corals bleaching and dying, others showed promising resilience, surviving both bleaching events without showing significant changes in growth over time. This resilience offers new hope for coral conservation and restoration strategies.

“This is one of the first studies to use this type of time series to look at multiple coral bleaching events and how the processes of acclimatization and selection play out,” said lead author Orion McCarthy, a recent graduate of Scripps Oceanography who conducted the research as a PhD student. 

“We found that older corals, which are more likely to have survived multiple bleaching events, could be a good source of outplants for coral restoration.”

Corals are increasingly threatened 

As ocean temperatures rise due to climate change, coral reefs are increasingly threatened by bleaching events. These events occur when corals, stressed by prolonged warming, expel the symbiotic algae living within their tissues. 

This expulsion causes the corals to turn white, or “bleach,” as they lose their primary source of food. If the elevated water temperatures persist, the affected corals may die. 

However, some corals can survive low- to moderate-strength bleaching events, reshaping the reef’s composition and its future responses to similar stressors.

Coral responses varied across bleaching events

In this study, the two measured bleaching events were of moderate strength, characterized by a sustained increase in sea-surface temperatures of more than one degree Celsius (1.8 degrees Fahrenheit) over several months. 

Despite experiencing similar thermal stress in both 2015 and 2019, coral responses varied. Some corals bleached and died after the first event, others did not bleach at all, some bleached during both events but survived, and others only bleached the first time. 

Interestingly, resilient corals were often found adjacent to more sensitive ones, despite being the same species and facing identical environmental conditions.

Acclimatization capabilities of some coral species

The research highlighted the acclimatization capabilities of certain coral species, particularly Porites lobata, which showed resilience to bleaching. Conversely, Pocillopora corals exhibited the lowest levels of survivorship. 

“Based on our observations, we recommend that restoration practitioners in Hawaii should focus on colonies of Porites and Montipora with a proven track-record of growth and survival,” advised McCarthy, who now works as a science lead for Sustainable Surf, a nonprofit focused on marine ecosystem restoration.

The team credited the innovative use of 3D imaging technology for their ability to detect and analyze acclimatization and selection processes.

The technology enabled detailed tracking of coral colonies that traditional in-water surveys would have missed. 

Comprehensive 3D representation of reefs

The 3D time series project, initiated by Scripps Oceanography marine biologist Jennifer Smith and her former PhD student Emily Kelly, employs photogrammetry to create detailed models of coral reefs. 

This technique involves capturing thousands of overlapping underwater images, which are then stitched together using advanced software to produce a comprehensive 3D representation of the reefs.

For the past decade, Smith and her team have used this technology to monitor fixed sites off Maui, as part of the 100 Island Challenge – a global initiative aimed at documenting coral reef changes over time using large-area imagery. 

Watching coral response to bleaching 

“This approach has revolutionized our ability to study long-term changes in coral reef communities at very fine scales,” said Smith, a professor of marine biology at Scripps Oceanography. 

“We can literally watch these systems change in 3D; we can watch corals grow and shrink and visualize how they respond to global stressors. We can use the knowledge gained from using this approach for education and outreach and our results can help to inform management and conservation action.”

McCarthy likened the study of coral responses to bleaching events to examining human health outcomes after successive pandemic-like events. 

By observing individual responses during a single event – such as the COVID-19 pandemic – researchers can identify traits associated with better or worse outcomes. 

A subsequent event provides further data on survivors’ resilience, similar to how researchers study coral responses to successive bleaching events.

Valuable foundation for coral conservation 

The study underscores the importance of addressing climate change, the primary driver of ocean warming and coral bleaching. Severe bleaching events pose a significant threat to coral reefs, potentially leading to widespread mortality. 

“Coral reefs are dynamic and bleaching isn’t necessarily going to kill every coral – at least not in the short term – so there is still cause for hope for these reefs and a need for active conservation,” said McCarthy. 

“Tools like 3D modeling are allowing us to get a more specific understanding of which corals are living and which ones aren’t, and we can use that information to help guide coral restoration efforts.”

Overall, this research represents a significant advancement in understanding coral resilience and provides a valuable foundation for developing effective coral conservation and restoration strategies in the face of climate change.

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