Sydney beached by a crimson tide - The Australian

Red algal bloom at Clovelly Beach / Pic: Craig Greenhill Source: The Daily Telegraph

Red algal bloom at Clovelly Beach / Pic: Craig Greenhill Source: The Daily Telegraph

Karina Newmarch and her daughter Isabella check out the algal boom at Clovelly Beach / Pic: Craig Greenhill Source: The Daily Telegraph

The red aglae bloom forced the closure of Bondi Beach / Pic: Craig Greenhill Source: News Limited

A FEW worried tourists feared it was the aftermath of a shark attack. But what was turning the seas around Sydney into a blood-red tide yesterday - and closing some of the city's most popular beaches - was a danger of a different kind.

A thick algal bloom known as noctiluca scintillans - commonly called "red tide" or "fire in the sea" - capable of causing skin rashes and eye irritations, began washing up early yesterday morning.

Surfers first spotted the algae just after dawn at Bondi, with the number of sightings spreading throughout the morning - stretching from Wamberal on the Central Coast to Cronulla Beach in the city's south.

Bondi and Clovelly beaches were the first to close as a safety precaution as clouds of the algae, which contains high levels of ammonia, began washing ashore.

The bloom, a natural phenomenon and not toxic, was likely caused by higher water temperatures and more movement in ocean currents which led to an upwelling of colder nutrient-rich water.

With this weekend predicted to be one of Sydney's warmest for the year - with the mercury well into the 30s - it is hoped the major bloom will continue to break up.

But swimmers could be forced to stay out of the water if the rare algae continues to spread along the coast.

Bondi reopened in the afternoon after the algae bloom broke up sufficiently, with a spokeswoman for Waverley Council saying most of the algae had either washed up or broken up in the water.

Further south, Clovelly remained shut.

"There are some possible risks to human health from red algae including skin rashes and eye irritation, and for this reason the beach will remain closed until the algae dissipates," a Randwick Council spokeswoman said.

Signs have been installed at the affected beaches.

Marine biologist Fred Gurgel said most algal blooms last about a week and some species are "potentially dangerous" if people come into contact with them.

"It is potentially dangerous, it produces toxins and varies from people to people," Dr Gurgel said. "It should clear up in less than a week."

Office of Water spokesman James Muddle said tests conducted confirmed the unusual outbreak was not toxic.

Bondi local Michael Strum said the algae had stopped him swimming in the water.

"It looks like pink sludge, it's disgusting," he said.

Several tourists ignored lifeguards' warnings and could be seen swimming in the surf at Bondi yesterday.