Seaside holiday-makers are about to have a bleak Christmas.
Environmentalists have warned that red tide has returned to most of the country’s east coast.
Worst affected is the Garden Route coast, with red tide sightings from Algoa Bay to Mossel Bay.
The experts predict extensive red tide formations continuing into next year.
The tourism industry fears the worst as millions of South Africans head to the coast this week for the festive season.
Beach-goers are advised not to eat shellfish or dead fish washed onto the shore.
Red tide is caused by microscopic phytoplankton rising to the surface, making the water reddish or brown.
Sea conditions are now optimal for the formation of red-tide blooms.
East coast tourism specialist Peter Myles said: “We hope it will clear before too many of the tourists arrive. There is not a lot anyone can do about it other than hope that nature does its job and it clears up quickly.”
Mandlakazi Skefile, CEO of Nelson Mandela Bay Tourism, said there should be no major effect on tourism “because reservations have already been made”.
“Although Nelson Mandela Bay is a coastal and beach destination there are alternative attractions and activities to experience.”
The SA Environmental Observation Network and the Institute for Coastal and Marine Research sampled several red tide blooms in Algoa Bay and St Francis Bay last week and confirmed that they had found the harmful dinoflagellate, Lingulodinium polyedrum.
Tommy Bornman, the network’s Elwandle coastal node manager, said this species had not been found along the coast since the summer of 2013-2014.
He said it might have returned as a result of climate change.
Bornman said that for now there was very little red tide in Algoa Bay, and it occurred mainly between Cape Recife and the Gamtoos River.
“It is then patchy between StFrancis Bay and Mossel Bay. It occurs along the entire coast [of the Garden Route].”
He said a slight southeasterly wind could spread the bloom.
Bornman said the water was safe for swimming.
Wildlife and Environment Society of SA spokesman Morgan Griffiths said: “The important thing is that no one eats anything that washes up onto the shore.”
He said the red tide might have been caused by plankton introduced by passing ships.
Garden Route National Park spokesman Nandi Mgwadlamba said SA National Parks rangers would have the results of tests on samples taken from areas from Tsitsikamma to Knysna by today.
Additional reporting by Yolandé Stander