Robert Morris and his wife Carolyn, who spend a month in Thailand every year, submitted the photograph to The Phuket News on Saturday.
Mr Morris wrote, “Yesterday we walked the beach and were horrified to find three very dense oil slicks almost onto the beach. We took some photos and would like to submit them in the hope of shaming whoever did this to a magnificent beach.”
In fact, according to Dr Ponsri Sutthanarak, Director of Regional Environment Office 15 in Phuket, this was not oil, but plankton bloom.
Sudden increases in the population of plankton – microscopically small organisms that are the basis of the ocean food chain – are quite common at this time of the year, she said, as heavy rain washes nutrients from the land into the sea, resulting in a sudden, very rapid increase in plankton numbers.
The plankton is not dangerous to people, though if the bloom is large enough, it can suck so much oxygen out of the water that fish and other sea animals suffocate.
After examining the Morris’s photo, she said, “Green tides happen when there is heavy rain which flushes organism materials into the ocean.
“This is not a large green tide so it won’t affect other animals or the environment. But a larger occurrence might affect animals [by depleting oxygen in the water], causing them to suffocate.
“It is natural. We had a brown tide last year at Patong beach last year.”
She warned that a green tide is a good reason for swimmers to stay out of the water. “It might cause itchiness. It should take around four or five days to clear.”
Different types of plankton cause blooms of different colours, including green, yellow, brown and red.
Additional reporting by Tanyaluk Sakoot