Sections of the sea around Koh Larn turned green yesterday, September 30, after several days of heavy rain and unsettled weather in Pattaya, with officials saying a plankton bloom was the likely cause.
The discolouration was particularly noticeable around Tawaen Beach. Local officials described the occurrence as a normal rainy-season phenomenon and said similar blooms can appear following periods of intense rainfall.
Heavy rain can send large volumes of freshwater, sediment and nutrients from land into coastal waters. These conditions can encourage marine plankton to multiply rapidly, changing the appearance of the sea depending on the type and concentration of the organisms.
The Department of Marine and Coastal Resources (DMCR) identifies increased nutrient levels as an important factor in seawater discolouration. Nutrients washed from land during the rainy season can contribute to rapid phytoplankton growth, while sunlight, temperature, waves and currents can also affect where blooms form and accumulate.
The green water does not necessarily appear uniformly around the island, with conditions varying between beaches. Similar changes in seawater colour have been recorded around Koh Larn during previous rainy periods.
In July 2023, another bloom affected Tawaen Beach and other parts of Koh Larn. The DMCR identified Noctiluca scintillans during that earlier event, although that finding does not establish which plankton species is responsible for the current discolouration.
This plankton bloom in Koh Larn is expected to be temporary, with the water gradually returning to its usual colour as weather, currents and other conditions change. The process could take several days, although the timing can vary.
The green colour alone does not establish that sewage or another source of pollution is responsible for the current event.
Pattaya City is continuing to monitor conditions around the island. Koh Larn remains open to visitors, with the appearance of the water varying between different beaches as the bloom develops and fades.
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