THREE beaches near Valencia city had to be closed for 24 hours from Sunday afternoon due to the appearance of a huge brown patch in the sea that triggered allergic reactions among bathers.
Up to eight kilometres of coastline off the towns of El Puig, Puçol and Pobla de Farnals were closed off, with coastguards raising the red flag banning bathers from entering the water until yesterday (Monday) afternoon.
A number of swimmers reported skin rashes, itchiness and other minor complications after coming into contact with the mysterious stain.
Initial fears of a possible toxic waste dump were promptly allayed by environmental experts working for the regional government, who rushed to the scene and took samples of the mysterious stain that stretched along the coast.
A preliminary report revealed that the brown patch was caused by a massive concentration of a type of microalgae known as Raphidophyceae, triggered by the high sea temperatures registered at this time of year.
It seems the warmth of the water reduces the amount of oxygen, which kills off the seaweed on the ocean floor and causes it to rise and decompose on the surface.
The same experts pointed out that similar incidents have been reported along the Valencian and Catalan coastlines in previous summers, and that it is completely natural and nothing to worry about.
By yesterday afternoon, aerial inspections of the area showed that the reddish-brown stain had all but disappeared and the water was deemed safe for bathing once again.
Although itchy and annoying, contact with the seaweed was not said to pose any serious health threat.