Sections of the sea around Koh Larn, off Pattaya, appeared green on September 30, 2026. Reports published through October 2 described a plankton bloom after heavy rain as the likely explanation, but the available coverage did not include final water-sample results or confirm the species responsible.
Where the sea around Koh Larn turned green
The discoloration was reported in parts of the coast around Koh Larn (also spelled Ko Lan), a tourist island off Pattaya in Chonburi province, in the northern Gulf of Thailand. Pattaya Mail said the green color was particularly visible near Tawaen Beach. Reports indicate it was not uniform around the island; they do not establish that every beach or the entire coastline was affected.
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Thairath English reported the event on September 30, while Pattaya Mail and Reuters, carried by Free Malaysia Today, published reports on October 1 and 2, respectively.
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What likely caused the green water?
Local reporting linked the color change to a suspected plankton bloom following heavy rain and unsettled weather. Rain runoff can carry freshwater, sediment and nutrients from land into coastal water. Plankton growth is also affected by nutrient levels, currents, temperature and weather, so the reports describe a plausible explanation rather than a confirmed cause for every affected stretch. Bangkok Post also reported a storm-fed plankton bloom around Koh Lan.
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Arvut Munhapon, a senior researcher and head of marine environmental research at Burapha University’s Institute of Marine Science, told Reuters: “What we’re actually seeing with the naked eye is a colour that comes from a type of plankton called Noctiluca,” His four-person team was collecting water samples, measuring water quality and taking samples to a laboratory to identify species and build a database. The coverage available through October 2 did not report the final species identification for the September 2026 event.
Why the 2023 species finding does not settle this event
Thaiger reported that Thailand’s Department of Marine and Coastal Resources identified Noctiluca scintillans during a 2023 Koh Larn bloom. That earlier identification does not establish which species was present in September 2026.
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Does green water mean pollution?
No. Green color alone does not prove sewage contamination or another pollution source. The reporting described a suspected plankton bloom and rain-related runoff, but it did not provide final test results that would establish the water quality at each beach. Freshwater, sediment and nutrients carried by runoff are possible contributors to bloom conditions; they are not, by themselves, proof of sewage.
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The coverage does not establish current official swimming advice for individual beaches or provide final water-sample results for the event. Reuters reported that some swimmers stayed out of the water and vendors noticed fewer customers; those observations describe visitor behavior, not a measured health risk.
Pattaya Mail advises visitors to avoid water with an unusual smell, heavy surface scum or many dead marine animals. That is the outlet’s precaution, not a beach-specific official clearance or warning. A visible change in water color is not enough to determine whether a particular spot is safe; follow any current instructions from local authorities and avoid water showing those warning signs.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What are the possible ecological effects?
When a plankton bloom dies off and decomposes, that process can consume oxygen in the water and contribute to hypoxic conditions. Burapha University deputy director Anukul Buranapratheprat told Reuters: “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,” His comment concerned a broader issue in the Gulf. The available reports do not establish that the Koh Larn event itself had created a measured hypoxic zone.
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