A multi-agency investigation is under way after masses of yellow foam and dead sea creatures washed up on South Australian beaches, leaving more than 100 surfers reportedly infected. 

Metres of foam and dozens of dead fish and seahorses stretch across Waitpinga and Parsons beaches within the Newland Head Conservation Park in Victor Habor.

Surfers and beachgoers over the weekend suffered cold and allergy-like symptoms, including itchy eyes, blurred vision, coughing and breathing difficulties.

Anthony Rowland said he and more than 100 other surfers were infected while out in the water or the carpark on Saturday.

“We all experienced a sore throat, dry cough and irritated eyes. Some even reported blurred vision. It lasted 24 hours or so,” he said in a social media post.

“In the following hours/days there was visible evidence that something weird is in the water.

“A lot of yellow foam and dark green tidelines on the beach and then on Sunday, while collecting water samples, I noticed a lot of different species of fish dead on the beach including a handful of leafy sea dragons.”

The Department for Environment and Water has temporarily closed Waitpinga and Parsons beaches as the Department of Primary Industries and Regions (PIRSA), Environment Protection Authority (EPA) and SA Health all investigate.

The EPA has confirmed receiving the reports, including of red staining on the sand, and said crews will attend the site to collect water samples.

The event is believed to be due to a microalgal bloom — a harmful and toxic algal bloom caused by changes in temperature, light or rainfall.

“It is believed the event could be due to a microalgal bloom that has been driven by hot temperatures and still water and an ongoing marine heatwave, with temperatures currently 2.5 degrees warmer than usual, with little wind and small swell contributing to conditions,” an EPA spokesperson said. 

SA Health principal water quality adviser David Cunliffe said exposure to algae can cause general allergic-type responses including respiratory and eye irritation or skin rashes in some people and, if ingested, can cause stomach upsets or flu-like symptoms.

“These symptoms disappear when the exposure stops or following washing in clean water,” he said.

“People should avoid contact with discoloured fresh or marine water. If they do enter this water they should take care to wash in clean water afterwards.”

PIRSA will probe the cause of the dead fish and seahorses to rule out infectious and notifiable animal diseases and coordinate a response.

“Fish kill events can be brought on by a variety of causes including environmental factors such as temperature, rainfall (or lack of) and water quality including salinity or oxygen levels, or by disease or pollutants,” a PIRSA spokesperson said.