A Ko Lan plankton bloom discoloured sections of the sea after heavy rain. Researchers collected samples, while some visitors stayed out of the water.
Sections of the sea around Ko Lan, an island in Chon Buri province, turned bright green during the week described in a Reuters report published on October 2, 2026. The change did not affect every stretch of coast. Some visitors avoided swimming, while vendors near the beach reported fewer customers.
A local change, not an island-wide picture
The reports describe patches of green water rather than a uniform condition around the island. Reuters said some parts of the coast were affected while others remained unchanged. Local accounts also placed the discolouration in particular sections. That distinction matters: a report about one beach or coastal patch does not establish conditions across all of Ko Lan.
A researcher from Burapha University’s Institute of Marine Science told Reuters that the visible colour came from plankton of the genus Noctiluca. His team was collecting water samples, measuring water quality and taking samples back to the laboratory to identify species and build a clearer picture. The report did not present those laboratory checks as a completed result.
What researchers say may drive a bloom
Heavy rain and flooding may have helped trigger this event, but the reporting does not establish a single confirmed cause. Researchers described plankton blooms as natural, recurring events. They also said nutrient-rich runoff, including fertiliser and wastewater, can support rapid plankton growth. Those general factors should not be mistaken for a finding that a particular pollution source caused the Ko Lan plankton bloom.
The same caution applies to climate conditions. A Burapha University researcher said warmer seas and this year’s monsoon and El Niño conditions might intensify blooms or low-oxygen conditions, but he said further research was needed to confirm the connection. A separate study of a green Noctiluca bloom at Bangsaen in Chon Buri shows that researchers have examined similar events in the region. It does not identify the species or cause of the current event around Ko Lan.
The ecological concern comes after the bloom
The research concern extends beyond the water’s colour. A Burapha University scientist told Reuters that when a large quantity of plankton dies, its breakdown can consume oxygen. In some conditions, that process can produce hypoxia—water with too little oxygen to support marine life. This is a potential consequence described by the researcher, not confirmation that measurements found hypoxia around Ko Lan during this event.
The same researcher said his work suggests hypoxia in the inner Gulf of Thailand has become more severe and raised concern about a broad low-oxygen zone affecting marine life on the seabed. That is a wider regional concern, distinct from a measured finding at the island. Keeping those scales separate avoids turning an ecological warning into an unsupported claim about every beach.
What the reports mean for visitors
Some visitors stayed on shore or moved to areas where the water was unaffected. One visitor told Reuters she did not enter because the unusual colour made her worry about a rash. Beach vendors also said customer numbers had fallen. The reports document those choices and their local economic effect; they do not establish that the water caused a health problem.
A Reuters interviewee said blooms typically last two to three days. That is a general estimate, not a forecast for this event. A separate local report published on October 2 described clearer water, but it did not establish conditions across the island. The available reports therefore do not support treating any one observation as a current, island-wide bathing advisory.
