New coral bleaching index advances research to save coral reefs


By Jamie Leventhal Researchers at Northwestern University published the first coral bleaching response index on April 13 as part of their work to protect coral reefs from climate change.

A team of Northwestern professors and research fellows collaborated for over two years to combine data from historical surveys and records into a single index. Each coral reacts to changing ocean conditions differently, and this index allows scientists to analyze corals based on species and location.

“The data is not that easily accessible sometimes,” said Luisa Marcelino, a research assistant professor of molecular biology at Northwestern. “The biggest challenge was in fact that there are different communities of corals throughout the world and [the studies] are done by different people who have different criteria and protocols of collection. Now you can talk the same language.”


Due to rising ocean temperatures, coral reefs are dying off worldwide through a process called coral bleaching. Corals are naturally an off-white color because of their calcium carbonate skeletons, but they get their vibrant shades from algae that cover their surfaces. The algae act as a power center for the coral by converting sunlight into sugars, which are then transferred to the coral through tiny membranes. However, increased global temperatures from climate change and weather events like El Niño put “thermal stress” on the algae, causing them to be expelled from the coral’s surface.

“Once a coral is bleached, it has lost its food source,” said Northwestern civil environmental engineering post-doctoral fellow Timothy Swain. “The act of bleaching itself is injurious, and so the clock is now ticking. You’re either going to starve to death or succumb to the injuries from the bleaching.”

The index works by combining past and present data to form a standardized measure of vulnerability. This allows scientists to examine how resistant an individual coral is to coral bleaching. It’s a complicated system, as a coral’s defenses can vary not only by species, but also by location.

Acropora_coral_ffs“A particular community may have particular environmental conditions that are more prone to let the whole community bleach,” Marcelino said. “A volcanic island that is very isolated is probably going to flush out the temperature better than if it’s like an extended coastal shelf reef.”

Even within these reef communities, some corals are bleaching faster than others. This index is key for further research, as it allows scientists to pinpoint which corals are biologically more resistant to bleaching.

“It’s not just a tool you can use now, but it’s also a framework for including new data as it becomes available,” Swain said.

Coral reefs cover less than one percent of the Earth’s surface, but they contain more species per unit area than any other marine environment, according to the National Oceanic and Atmospheric Administration. In addition to sheltering about 4,000 species of fish, coral reefs benefit humans by acting as a buffer against tropical storms and hurricanes. They are also hugely important for many nations’ economies. In Australia alone, reef tourism brings in about $5 billion annually and employs almost 70,000 people.

Unfortunately, this index comes at a critical time for coral reefs. According to a report released by the Australian Research Council Centre of Excellence for Coral Reef Studies on April 20, areal and underwater surveys reveal that 93 percent of the Great Barrier Reef has been affected by coral bleaching. The report cites hotter temperatures in 2016 as the culprit behind the latest expansion of bleaching, which has been on-going since 1998.


“Typically the tropics already have higher levels of light, so the algae are already living with very high light stress. The more frequent the increases of temperature, the more [the corals] leave their best state of health,” Marcelino said.

Swain and Marcelino hope to use the bleaching index in the future as a way to better protect coral reefs from environmental threats, but they understand the severity of the situation.

“This March was the hottest March on record,” Swain said. “Every month we’re setting a new historical record. The coral ecosystem may be the first system lost to climate change.”

While they are concerned for the future status of coral reefs, they also are hopeful that a reduction in carbon emissions could save this ecosystem.

“They might not be completely lost if they have time to acclimate,” Marcelino said. “We are in the process of making things better, but the carbon dioxide already in the atmosphere will have repercussions for many years to come. But I wouldn’t say it’s the end for coral reefs just yet.”

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