The sea turned deep green amid an algae bloom off the coast of a southern Thai island.

The toxic-looking waters at Koh Chang island in Trant province sparked concern as authorities issued an advisory for tourists to avoid swimming at the beach on September 24.

Locals said they found many moon jellies washed ashore, but reported no foul odours in the area.

Pornchai Kemapuckpong, chair of the Tourism Council Office in Trat, said that the phenomenon occurs annually, when heavy rainfall leads to flash floods flowing into the sea.

He said the plankton bloom occurred in only a few areas and was generally harmless. However, visitors were advised to swimming as they might come into contact with poisonous jellyfish hidden by the murky waters.

The bloom was expected to clear up within two days if there was no substantial rainfall, Pornchai said.

On September 5, thousands of dead fish were washed onto Bang Saen beach during another plankton bloom in Chonburi province, also in southern Thailand.

Marine experts said decomposing dead plankton had depleted the oxygen of water, causing the massive fish kill.

Plankton, or microalgae, are tiny organisms that live in the water, both in the ocean and in freshwater bodies. They are unable to swim against currents, so they drift with the water.