Koh Larn sea turns green after rains

Sea water in parts of Koh Larn, off Pattaya in Chonburi province, has turned green following heavy rain and unsettled weather in the area.

The change, reported on 30 September 2026, surprised tourists visiting the island. Initial indications suggest a naturally occurring plankton bloom, in which microscopic marine organisms multiply rapidly and alter the colour of the water.

Heavy rain and runoff linked to colour change

Pattaya and nearby areas have recently experienced heavy rain, strong winds and continuing unstable weather. The conditions have increased the flow of fresh water, sediment and nutrients from land into the sea.

That runoff can create conditions favourable for plankton to multiply quickly. Depending on the species and concentration involved, the water can take on a green colour or other shades.

Thailand's Department of Marine and Coastal Resources says sea-colour changes occur when plankton populations increase rapidly. Nutrient levels in coastal waters, currents, temperatures and weather conditions can all play a part.

During the rainy season, water flowing from land can carry additional nutrients into coastal areas, contributing to such blooms.

Koh Larn is a popular day-trip destination from Pattaya, reached by boat across the bay, and weather conditions can affect both water quality and marine travel.

Water expected to gradually return to normal

Plankton blooms can occur naturally after heavy rainfall and fresh-water runoff into the sea. According to the information cited by authorities, water conditions will gradually return to normal.

The green water observed around parts of Koh Larn follows the recent period of monsoon weather affecting Pattaya and surrounding coastal areas.

Pictures courtesy of Amarin

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1 October 2026