On the northern coast of the Gulf of Thailand, stretches of water turned a vivid green, stretching along the shoreline while jet skis floated in the discoloured sea. Some parts of the island remained unchanged.

The colour comes from a type of plankton called Noctiluca, according to senior researcher Arvut Munhapon of Burapha University.

Blooms usually last two to three days, and when they end the mass die‑off can create hypoxic conditions, according to deputy director Anukul Buranapratheprat of Burapha University.

Anukul says hypoxia in the inner Gulf of Thailand has become more severe, with a wide hypoxic zone forming this year, too low in oxygen to support bottom‑dwelling marine life.

Monsoons, El Niño and warmer water may be intensifying the bloom and hypoxic conditions because warmer water holds less dissolved oxygen.

Recent flooding has increased freshwater runoff into the Gulf, adding to the bloom.

Beach food vendors report fewer customers, and many tourists avoid swimming because the water looks unusually green and they fear rashes. One visitor, Latipeya Thantibhapat, said she stayed out of the sea for fear of breaking out in a rash.

When oxygen levels drop, bottom‑dwelling organisms struggle to survive, which can disrupt the marine ecosystem and reduce the appeal of the coast for visitors and local businesses.