Coral reefs are expected to continue facing severe heat stress as rising temperatures cause the oceans to warm excessively. However, new research suggests that modifying coral feeding habits could help local populations avoid complete extinction.
A study focusing on two coral species native to Hawaii revealed that warmer waters, a result of climate change, are a major factor in coral bleaching – a process that leads to a loss of color in coral, significantly harming their health and growth.
The researchers also examined the effects of ocean acidification on heat-stressed corals. Ocean acidification occurs when seawater absorbs excess carbon dioxide and becomes more acidic.
Coral exposure to heat stress
Over the last decade, mass coral bleaching events have increased in both frequency and severity, leading to higher mortality rates among coral populations worldwide.
Study lead author Kerri Dobson completed the research as a graduate student in earth sciences at Ohio State University. She noted that this study offers hope by suggesting that some coral species may be more resilient to these extreme environmental changes.
“Each coral species responds differently to stress and employs different methods to recover from that stress. Now we know that those responses depend on several factors,” said Dobson, who is now a marine biology instructor at the University of Southampton.
Food boosts heat stress recovery
The study revealed that one of the most critical factors determining whether coral can recover from a stressful event is access to food.
Typically, corals get their nutrients from two sources: photosynthetic symbiotic algae and tiny aquatic organisms called zooplankton. The algae also provide the coral with their vibrant colors.
However, during heat stress, corals lose these algae, leaving them bleached and without a key food source.
Capturing the impact of coral heat stress
The researchers collected coral branch samples from bleached and unbleached parent colonies following two major heat-stress events in 2014 and 2015.
The samples were placed in special tanks where the researchers varied the amount of food provided and simulated ocean acidification rates to assess their impact on coral recovery.
“This is one of the first studies, to our knowledge, that has been able to capture the impact of natural thermal stress on coral in two consecutive years,” Dobson said.
Improving the resilience of corals
Thanks to this unique long-term data, the team found that feeding corals zooplankton after a bleaching event improved their resilience to warming temperatures, reducing mortality, promoting growth, and enhancing recovery.
Surprisingly, ocean acidification had little impact on the corals’ recovery, suggesting that some coral species may be capable of surviving in more acidic waters. Unfortunately, this also underscores that heat stress poses the greatest threat to coral reefs, Dobson noted.
“Heat stress affected the corals’ health much more than the simulated ocean acidification,” she said. “These heat-stress events function as selection forces, leaving only the more thermally tolerant coral that might be able to survive the stresses that we’re subjecting them to.”
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Ongoing research efforts
With the next few decades being critical in determining the fate of coral reefs, Dobson plans to continue studying the impact of thermal stress on coral ecology in other regions.
“When you do experiments with living animals in a natural setting, there’s always some degree of unpredictability, as we saw with the unexpected second heat-stress event we studied,” said Grottoli.
“Ultimately, you have to roll with it because the work matters, and sometimes the things you didn’t plan to learn are the parts that are the most interesting.”
The study is published in the journal Nature Communications Earth & Environment.
Earth.com, 25 September 2024. Press release.
