Jeju's underwater forests vanish as seaweed output plunges 97% in 15 years

- Agar seaweed down 99.4%, hijiki down 95.7%; abalone and sea snails have all but disappeared

- The 25°C isotherm has shifted 171 km northward over 23 years; subtropical corals now cover up to 55% of bare rock

Sargassum seaweed in Jeju waters. [Getty Images Bank]

Climate change is rapidly warming Jeju's coastal waters, erasing the "underwater forests" that form the foundation of the island's marine ecosystem. Seaweed output in Jeju's nearshore waters has plunged 97% in just 15 years, and tropical and subtropical corals are moving in to colonize the bare rock left behind — a shift researchers say amounts to a wholesale restructuring of the coastal ecosystem.

According to the Korea Institute of Ocean Science and Technology (KIOST), data compiled by the Ministry of Statistics show that seaweed production in Jeju's nearshore waters fell from 5,155 tons in 2010 to 154 tons in 2025 — a 97% collapse over 15 years.

The decline is even steeper for species once synonymous with Jeju. Agar seaweed plunged 99.4%, from 3,455 tons to just 21 tons over the same period, while hijiki dropped 95.7%, from 822 tons to 35 tons. Sargassum, which yielded roughly 230 tons in 2010, has for years produced output too small to register in official tallies.

The disappearance of underwater forests is not a problem confined to seaweed alone. These ecosystems serve as feeding grounds, spawning sites and habitat for a vast range of marine life, and as they collapse, the species that depend on them are vanishing too.

Abalone production fell 99.5%, from 184 tons in 1990 to just 1 ton in 2025. The small abalone known as "obunjagi" dropped 98.7%, from 159 tons in 1995 to 2 tons last year. Even the conch — a primary source of income for Jeju's haenyeo divers — declined 41.8%, from 2,466 tons in 2010 to 1,436 tons in 2025.

Experts point to rapidly rising sea temperatures as the central cause of Jeju's underwater forest collapse.

Sea surface temperatures around the Korean Peninsula are rising roughly 2.5 times faster than the global average. As the warm pool of equatorial water expands, more warm water is flowing toward Jeju along the Kuroshio and Tsushima currents.

KIOST's analysis of 23 years of satellite observation data found that the annual mean 25°C isotherm — the boundary marking tropical and subtropical waters — has shifted approximately 171 km northward, while the 20°C isotherm has moved about 97 km north.

High water temperatures around Jeju have moved beyond a temporary phenomenon and are becoming chronic. August average temperatures have exceeded 28°C for five consecutive years since 2022, and in September 2024 the sea recorded its highest temperature ever — more than 3°C above the historical average. Sea temperatures this past May were also the highest in 24 years.

"When the ocean surface warms, the warmer upper layer and the colder lower layer separate and stop mixing," said Son Young-baek, a principal researcher at KIOST. "Nutrients from the deeper layer can no longer reach the surface, and the ocean's overall productivity declines."

Large seaweeds such as sargassum and ecklonia — which need ample nitrogen and phosphorus to grow quickly — are left starved of nutrients. When prolonged high temperatures compound the problem, growth stalls and fronds begin to dissolve. Survival rates for spores and juvenile plants also fall, weakening the ecosystem's natural capacity to recover.

A chart showing the decline in seaweed production in Jeju. [Provided by the Ministry of Statistics]

Rising temperatures are also fueling a surge in the activity of seaweed-grazing species such as sea urchins and rabbitfish. Researchers say the combination of high temperatures, nutrient depletion and intensified grazing pressure is driving a vicious cycle in which whatever seaweed remains disappears ever more quickly.

Where the underwater forests have vanished, a different kind of change is taking hold.

A KIOST survey of waters off Bomok in Seogwipo, conducted between 2023 and 2025, found five species of tropical and subtropical stony coral on rocky seabed where seaweed had declined. The corals were covering between 29 and 55 percent of the surveyed rock surface.

Experts caution against interpreting this simply as corals filling the space left by seaweed. The arrival and establishment of new species in waters once dominated by seaweed may signal a fundamental reorganization of the ecosystem itself.

"To determine whether this is a temporary phenomenon or a sign that the ecosystem is fundamentally changing, we need to observe the same stretch of ocean repeatedly over a long period," said Kim Tae-hoon, a principal researcher at KIOST. "We need to look not only at the appearance of new species but also at their distribution, growth and decline, and how those relate to environmental changes such as water temperature."

"The sharp drop in seaweed production is the opening signal of change in Jeju's coastal ecosystem," said Choi Seon-gyeong, a senior researcher at KIOST. "We need long-term monitoring to understand the causes and pace of that change, and to build the scientific foundation for managing marine protected areas and preserving coastal ecosystems."

nbgkoo@heraldcorp.com