Tourists were shocked by green seawater caused by plankton blooms on a Thai island.

Footage shows beach-goers on Koh Larn island in Chonburi province stepping into the sickly green soup of algae on October 9.

Last month on September 5, thousands of dead fish were washed onto the nearby Bang Saen beach during another plankton bloom, also in Chonburi province.

Thai marine experts have warned that the frequent plankton blooms occurring in the Gulf of Thailand may signify environmental degradation, as wastewater runoff provides plenty of nutrition for the plankton to flourish.

The organisms can produce toxins and deplete oxygen in the water, causing massive fish kills.

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.