Coral reefs ‘can recover quickly after bleaching’

[NEW YORK] Scientists say they have found evidence that coral reefs in remote or protected areas can quickly recover from mass coral bleaching events.

Two years after the Chagos Archipelago bleaching event in 2015-2016, researchers found the reefs shrinking, with coral cover and carbonate production down by more than 70 per cent. But on returning in 2021, the researchers found the reefs on a path to recovery, though the speed varied from place to place, says a paper published 24 March in Limnology and Oceanography.

Twelve reefs of the Chagos islands in the Indian Ocean were studied before and after the bleaching event to assess the impact of climate change on reef functions, says Ines Lange, an author of the study and postdoctoral research fellow with a project funded by the Bertarelli Programme in Marine Science.

“The protected status of the Chagos Archipelago has not prevented mass coral mortality during the heating event but helped the reefs to recover faster than in many other regions”

Ines Lange, Bertarelli Programme in Marine Science

Coral reefs are shallow-water ecosystems that play important roles in the tropics, housing rich biodiversity and supporting fisheries, coastal protection and tourism. About 275 million people live within 30 kilometres of a coral reef.

Bleaching is caused by warmer water temperatures which result to corals expelling their symbiotic algae and turn white. Corals can survive bleaching and recover their multi-coloured forms, but extreme heatwaves may result in large-scale mortality.

Lange notes that where key coral species returned quickly the underlying physical reef structure had stayed intact. Particularly important, she says, is the fact that the study shows that “in remote or protected areas without local impacts such as pollution or fishing, coral reef communities and their functions can recover quickly, even after large-scale disturbances”.

Lange also says that key coral species such as tabular and branching Acropora corals helped increase coral cover and calcium carbonate production. “High local fish biomass and lack of disturbances such as pollution from land likely also supported the fast recovery,” she adds.

“The protected status of the Chagos Archipelago has not prevented mass coral mortality during the heating event but helped the reefs to recover faster than in many other regions,” Lange says, highlighting the need for critical local management.

She believes “a full recovery of reefs across the Chagos Archipelago over the next few years is likely if the region is spared from re-occurring marine heating events”.

To quantify the reef recovery and determine the health of the corals, Lange used the ‘reef budget’ method which measures net coral production against reef erosion over time. The method was devised by Chris Perry, a professor at the Tropical Coastal Geoscience Programme at the University of Exeter and co-author of the study.

According to the researchers, carbonate budgets are important indicators of a reef’s ability to provide habitat to marine life, protect shorelines from wave energy and help reef islands to keep up with future sea-level rise.

“While there is increasing data on the impacts of mass coral bleaching on reef communities, our understanding of their recovery potential and knowledge on spatial variation of key contributing processes is limited,” says Lange. “The reefs of the Chagos Archipelago in the central Indian Ocean are geographically very remote and thereby offered the opportunity to study the natural responses of reef communities.”

The study shows that coral reefs can recover quickly and that carbonate budgets — the balance between biologically-driven constructive and destructive forces on a reef — allow us to see how some groups of organisms respond or recover from a disturbance and to understand how the entire system operates at the functional level, says Lorenzo Alvarez-Filip, a researcher at the Biodiversity and Reef Conservation Laboratory Reef Systems Unit, at UNAM Mexico, Puerto Morelos, Mexico.

“This set of semi-pristine conditions is increasingly rare across our oceans, particularly on the coastal water. Therefore, it is likely that observing similar recovery rates on other reef systems would be necessary first to control local sources of stress such as pollution, sedimentation or eutrophication that are often associated with coastal human development,” Alvarez-Filip said.

“There has been concern that coral reefs might be unable to recover functions of growth, accretion, and carbonate production which are essential for preventing reef island erosion and supporting fisheries,” says Ronan Roche, a researcher at the School of Ocean Sciences at Bangor University, Wales.

“The research shows that this is an initial stage of recovery which is uneven across these remote coral reefs and emphasises their susceptibility to any future bleaching impacts associated with global climate change,” says Roche.

This piece was produced by SciDev.Net’s Asia & Pacific desk.