Marine heatwaves can decimate the oldest and youngest coral, raising concerns about the reproductive future of reefs

The coral in Moorea, French Polynesia, was bleached after a major marine heatwave. The dark tips show partial mortality. Credit: Kelly Speare

Nature can surprise you. The waters around Moorea, French Polynesia, experienced a massive marine heatwave in the late 18th century. UC Santa Barbara researchers were able to investigate coral bleaching.

Scientists surveyed coral around the island during and after the heat wave, recording which colonies survived and which died. They found that the largest coral colonies were the hardest hit by high ocean temperatures. The baby corals were decimated by the heatwave. According to the trends detailed in Global Change Biology, heatwaves could restructure the size distribution of corals on reefs.

"We had a big marine heatwave event, and we found that it kills the corals that make the babies, and the corals that are the babies, and the ones that have just settled on the reef," said senior author Deron Burkepile. The corals that are disproportionately responsible for reef recovery.

The shallow-water corals are symbiotic. They receive energy from their partner's photosynthesis. When the coral is stressed, the partnership can break down. The coral's white skeleton can be seen if the algae leave or are expelled. If the stress continues, the colony will die.

The authors wrote that the marine heatwave that began in December and continued through May was one of the most intense in the past 30 years.

Kelly Speare is at Moorea. Credit: Allison Aplin.

The researchers decided to look at how the corals responded to the event. Divers surveyed two genera of coral at six sites on the island. The researchers categorized the colonies into three size classes based on their diameter.

The results showed that larger corals were more likely to die than smaller corals. The event killed up to 70% of the largest colonies of the coral genera Pocillopora and Acropora.

Scientists had identified this pattern in previous events, but no one had documented it at a large scale before. The team was not expecting what they found. Burkepile said they just wanted to see what was happening. We took data at many different sites around the island, and this pattern fell into our laps. It was a surprise.

The team used coral settlement tiles to study the survival of coral recruits. Speare spent hours looking through a microscope to survey nearly 65 tiles. The team discovered that the heatwave wiped out nearly all of the baby coral. The number of recruits who died following the heatwave was much higher than the previous two years. The coral population was truncated at both ends of the size spectrum.

It's still an open question as to what mechanisms are driving these trends despite the strength of the findings. Large colonies may be more susceptible to bleaching. Smaller colonies may do this more efficiently than larger ones due to their geometry. Larger corals may have a harder time getting rid of free radicals and toxins when they get stressed by temperature. They may be more likely to kick out their symbionts.

The coral polyps look like brown dots. Credit: Kelly Speare

Many of the coral species are indistinguishable to the naked eye. It's possible that the large corals the team saw belonged to more sensitive species than the smaller corals currently present on the reef. Moorea researchers are looking into this.

It's possible that coral's microbiology may change as they grow. "All of these mechanisms are interdependent," Burkepile said, "so many may be operating at once."

Speare believes that different mechanisms are vulnerable to baby corals. She said that the early life stage is sensitive to all kinds of stresses. These recruits are just getting started and usually consist of a few small animals. One idea is that small corals may struggle to regain their symbiotic partners as they get older.

This is the first study to track the survival of coral recruits. The baby corals are difficult to study because they are hard to follow. The studies on baby corals have been small-scale.

Losing the largest and smallest corals could have a devastating effect on the reef. Larger corals have greater reproductive capacity because they are colonies of hundreds or thousands of individual animals. "If we shift to a community of corals that is much smaller, that community is going to produce far fewer baby corals," Speare explained.

Corals make colorful fluorescent proteins to regulate their light environment. When the coral is bleached, they don't have the same color as their symbionts. Credit: Kelly Speare

The researchers used a mathematical model to estimate how the disappearance of large colonies might affect this. The authors estimated that the bleaching event reduced overall fecundity on the reefs. The mortality of baby coral compounded this effect. Coral recruits and the largest, most fecund corals may be disproportionately impacted by coral bleaching events.

Scientists can model and forecast the impacts of marine heatwaves in the future by understanding these patterns. This will teach them about how the recovery of coral reefs is influenced by the transport of coral larvae between locations. This could help researchers figure out which reefs are more resistant to climate change.

There is still much to learn about this phenomenon. The team isn't sure if larger colonies were more likely to bleach or if they were more likely to succumb once they had bleached. There will be more data timepoints from another large event.

Speare said that they are trying to understand whether or not local factors like fishing pressure and pollution influence how well communities do during and after bleaching. The Burkepile lab has found that coral reefs can survive in the face of climate crisis.

The findings reinforced the seriousness of the climate crisis. The impact of the bleaching event on the differently sized corals was surprising to us.

He said that climate change has more surprises in store for us, and that the more of them we can avoid, the better off we will be.

Kelly E. Speare and her colleagues studied the effects of marine heatwaves on the recovery of a coral reef. 10.1111 is the date.

The Global Change Biology journal has information.

Marine heatwaves can decimate the oldest and youngest coral, raising concerns about the reproductive future of reefs.

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