Cottesloe Beach has reopened after being closed all day yesterday due to large masses of seaweed washing up on the shoreline.
Lifeguards have this morning put up flags next to the Cottesloe groyne and are on patrol.
The seaweed in the water is still very think from the pylon, heading north, while the seaweed bank on the shoreline has thinned out in patches.
The area of shore between the flags is nearly clear of seaweed.
Surf life savers who closed the popular beach yesterday said the seaweed and seagrass choking the water was so thick that swimmers could get held down in it.
Early morning swimmers at Cottesloe were greeted by beach closed signs yesterday after north-easterly winds trapped tonnes of weed against the groyne.
Surf Life Saving WA lifesaving operations supervisor Craig Bowley said the beach was closed because of the danger the thick weed posed to swimmers.
"It's so thick and it's so heavy that weaker swimmers would get held down in it," he said.
"They wouldn't be able to get to the back and the clear water."
Mr Bowley said cobblers could hide in the weed and deliver a painful sting if stood on.
Murdoch University marine biologist Mike van Keulen said the amount of weed was unusual but it was a natural phenomenon caused when seaweed was torn off rocky reefs and seagrass was ripped up during storms.
Perth's mild winter meant it was possible there was more offshore seaweed than usual.
One Cottesloe regular, who wished to be known only as Helen, said she had been going to the beach every day for 30 years and had never seen so much seaweed.
Rob Appleyard said he had swum at the beach every day for the past six or seven years but it was the first time he had not been able to go into the water because of the weed.
"It's the first time I've ever actually walked and seen signs where you can't swim because of seaweed and I can see why you can't," he said.
Town of Cottesloe chief executive Carl Askew said the council would monitor the weed but would try to allow it to clear naturally rather than clean the beach.
"One would expect that as the winds turn around and you get something from the south-west it'll go," he said.
Dr van Keulen said any remaining weed was usually buried naturally during spring and summer.
Marine biologists preferred beaches not to be cleaned because the nutrients released when the weed degraded fed the next generation of marine life.