The clarification was issued yesterday (June 24) after images of dark-coloured water circulating online sparked concern among residents and tourists.
According to Patong Municipality, water samples collected and analysed by the Marine and Coastal Resources Research Center found high concentrations of phytoplankton species Chaetoceros sp. and Peridinium sp., causing what is known as an “algal bloom’.
Municipal officials stressed that the species identified are not toxic to humans and that the blackened water is a natural phenomenon, not evidence of untreated wastewater entering the sea.
However, authorities advised beachgoers to avoid swimming in affected areas while the discolouration remains, warning that contact with the water may cause skin irritation.
The municipality said the bloom is expected to dissipate naturally within three to seven days, depending on wave action and sea currents.
The explanation comes amid longstanding concerns over water quality and algae-related problems at Patong Beach.
In April, repeated accumulations of what officials described as “seaweed” washed ashore along the beach, prompting clean-up operations and investigations into possible causes.
During an inspection in February, Phuket Governor at the time Nirat Pongsitthaworn was told that high phosphate levels from wastewater could be one factor contributing to algae growth, alongside favourable environmental conditions such as sunlight and sea temperatures.
At the time, provincial officials also investigated whether cruise ships anchored offshore might be playing a role, although no direct link was established.
The fact that the Patong wastewater treatment plant empties into the canal at the southern end of Patong Beach was not publicly recognised as a possible contributing factor.
In its latest statement, Patong Municipality said it would continue random inspections of businesses to prevent illegal wastewater discharges and ensure wastewater treatment systems are functioning properly.
Officials said they would also work with relevant agencies to closely monitor the situation until the seawater returns to normal.