Phuket Officials Explain Black Seawater at Patong Beach, Rule Out Wastewater Discharge

Authorities in Phuket have clarified the cause of the unusual black discoloration observed along parts of Patong Beach, confirming that the phenomenon is the result of a natural plankton bloom rather than wastewater pollution.

According to the Marine and Coastal Resources Research Center, laboratory analysis identified two species of phytoplankton — Chaetoceros sp. and Peridinium sp. Specialists emphasized that neither species produces toxins, meaning the event does not pose a poisoning risk.

Although the plankton is considered non-toxic, officials advise the public to avoid swimming while the water remains discolored, as direct contact may lead to minor skin irritation. Experts expect the bloom to disappear naturally within three to seven days as waves and ocean currents disperse the concentrated plankton.

Patong Municipality has also announced additional environmental monitoring measures to reassure residents and visitors. Officials will carry out random inspections of local businesses to ensure wastewater treatment systems are functioning properly and to prevent any illegal discharge into the sea.

Municipal authorities said they will continue working with relevant government agencies to closely monitor water quality and reduce conditions that could contribute to future plankton blooms.