Plankton bloom turns waters off Thai island Ko Lan bright green, raising hypoxia concerns

Bright green discoloration along Ko Lan coast

The sea off the resort island of Ko Lan, situated near Pattaya in the northern Gulf of Thailand, turned a bright, pungent green this week. Dense patches of green water stretched along multiple coastal sectors, surrounding anchored boats and floating jet skis. Other sections of the shoreline retained their customary turquoise hue, establishing a stark visual contrast across the island. A four-member research team from the Marine Science Research Division at Burapha University arrived on site to investigate the phenomenon. The scientists gathered water samples and conducted on-site water quality measurements before transporting specimens back to their laboratory to catalogue the micro-organisms.

Plankton proliferation and runoff drivers

Field evaluations determined that the green discoloration was caused by a massive proliferation of single-celled organisms belonging to the Noctiluca genus. Plankton blooms are a recurring natural phenomenon in Thai waters, but researchers observe that their frequency and scale have intensified due to rising sea temperatures. The event followed weeks of severe rainfall and widespread flooding across central Thailand and the capital city of Bangkok, which channeled vast volumes of freshwater into the Gulf. This flood surge carried agricultural fertiliser runoff and nutrient-rich urban wastewater into coastal waters, providing an abundant nutrient supply that accelerated plankton growth.

Arvut Munhapon, head of marine environmental research at Burapha University, identified the microscopic organism responsible for the discoloration.

What we're actually seeing with the naked eye is a colour that comes from a type of plankton called Noctiluca.

Hypoxia and the emergence of dead zones

Noctiluca blooms generally persist for two to three days before entering a rapid decay phase. When the microscopic organisms die and sink toward the sea floor, their bacterial decomposition consumes dissolved oxygen from surrounding waters. This biological breakdown can trigger severe hypoxia, creating aquatic dead zones where oxygen levels fall too low to sustain marine organisms. Warming ocean waters hold less dissolved oxygen, an effect exacerbated by seasonal monsoon patterns and El Niño conditions in the region.

Anukul Buranapratheprat, deputy director of the Institute of Marine Science at Burapha University, noted that oxygen depletion has become increasingly pronounced in the inner Gulf.

We're looking at the endpoint, which is hypoxia... and hypoxia in the inner Gulf of Thailand, based on my own research, has genuinely gotten more severe.

Anukul stated that a broad hypoxic zone formed in the Gulf during the year, creating conditions incapable of supporting bottom-dwelling marine life.

Repercussions for local tourism and visitors

The pungent smell and dense discoloration prompted immediate disruptions for tourism operators and beachgoers across Ko Lan. Food vendors along the affected beaches reported a steep drop in customer foot traffic as visitors moved away from the green water. While some tourists relocated to unaffected beaches on opposite sides of the island, many chose to stay out of the water entirely. Although marine specialists noted that the Noctiluca bloom is non-toxic to humans, the unusual visual appearance and odor caused widespread hesitation among bathers.

Latipeya Thantibhapat, a 29-year-old tourist visiting from the northern city of Chiang Mai, explained why she chose to stay on the beach.

I did not dare enter the water. I was afraid I would get a rash.