Ko Myeong-hyo loved the vibrant color and thriving life of South Korea's soft corals when she first began diving off the coast of Jeju island six years ago, looking to harvest seafood for sale.
"Watching the coral garden collapse and turn white, it was so disorienting," Ko says now, after two years of rising temperatures and harsher weather brought by climate change have led to many corals slumping over and dying.
"I was afraid the whole sea might be collapsing."
The "slumping" phenomenon is showing up again this year, says the Paran Ocean Citizen Science Center, a group of divers who monitor the island's UNESCO-listed soft corals, mostly around uninhabited islets south of Jeju.
Roughly half of one cluster off the islet of Beomseom has been struggling in past months, the group says.
Unlike hard corals supported by rigid calcium carbonate skeletons, soft corals rely on internal fluid pressure to keep upright. Environmental changes can disrupt that pressure, leading them to sag or collapse.
"If another large-scale 'slumping' happens, corals partially recover and then a lot break off and deteriorate again, and that keeps repeating, their capacity to recover just keeps declining," said researcher Kim Taihun.
Kim and Anna Beate Jöst, a colleague at the Korea Institute of Ocean Science and Technology (KIOST), documented Jeju's 2024 soft coral slumping event for the first time in a May article in the journal Scientific Reports.
In an email, Korea Heritage Service, which manages the soft coral colonies designated a national monument, said it has monitored the situation this year and will keep up long-term research to identify the reasons for the changes.
The Oceans Ministry, which oversees marine protected species and areas such as soft corals, added that it plans detailed investigation to gauge the scale of damage.
EL NINO COULD WORSEN THIS YEAR'S EVENT
Kim said this year's event looked similar to that of 2024, but could worsen should high water temperatures persist through September, and the El Nino weather phenomenon may trigger typhoons and heavy rains.
But he called for consistent and quantitative monitoring of the loss of soft corals through slumping, citing a lack of research support.
Climate change has hit corals globally, with perhaps the world's most famous colonies on Australia's Great Barrier Reef getting the most attention.
A record heatwave has parched the Korean peninsula this year while heavy rains have pelted the south.
Coral researchers say such weather conditions, as well as water flowing down from China's Yangtze River to affect the area's salinity, are potential triggers for "slumping".
Jeju's waters have given safe harbor to soft coral communities, sensitive to environmental conditions and living on the edge of tolerable conditions in the northernmost limit of coral distribution in the East China Sea.
Soft coral functions as the nursery underpinning Jeju's marine ecosystem, and soft coral communities reside in the UNESCO-listed Jeju Island Biosphere Reserve.
"The seaweed sea, the coral sea, is the foundational ecosystem underlying Jeju's marine ecology," said Yoon Sanghoon, an environmental expert at Paran.
"So if this base ecosystem collapses, there's a real possibility the entire marine ecosystem could collapse."
Over the past decade, marine heatwaves around Korea have extended almost four times longer, at 83.3 days, weather authorities say.
RIPPLING IMPACT
Paran, formed in 2023 by Jeju residents concerned by marine changes, brings together office workers, traditional divers and documentary filmmakers who flock there for monthly dives.
They plunge to the coral bed to take photograph and document changes they provide to scientists.
While Ko, a third-generation Jeju "haenyeo" or "sea woman", can dive to a depth of as much as 8 m (26 ft) relying on her own breath, she carries an oxygen tank on coral dives to greater depths.
The warming waters have affected the seabed in other ways, said her mother, 70-year-old Son Choon-sook, who began diving 45 years ago, when species such as turban shells, abalone and sea cucumber were abundant and thick seaweed festooned the rocks.
"I've worked at the sea for about 45 years, earned money, and sent my children to school on it," she said. "Now the catch has run dry, and it makes me very sad."
The thinning catch of the past few years follows summers that make the water too hot for species to spawn, she added, and her workday has shrunk to three hours from five formerly, as there is less to gather, but partly also for reasons of health.
Ko said her most recent dives in the summer have worried her again. "Rather than the beautiful coral shapes, I feel like I'm seeing more sick coral now," she added.
