Microalgae blooms proliferating in hot weather in Spain's Canary Islands are irritating beachgoers, who should avoid direct contact with them, local authorities said on Monday.
Tenerife in particular is awash with visitors at this time of year but some of those having an Atlantic dip have come out scratching themselves after brushing up against the tiny algae.
The spreading algae have produced a greenish brown hue in the waters off some beaches in the tourist haven.
"Since the end of June, we have seen episodes of massive efflorescence (or bloom) of microalgae, sometimes reaching as far as bathing beaches," Jose Juan Aleman, director of public health for the Canaries, told AFP.
The algae are a type of bacteria, trichodesmium erythraeum, also known as sea sawdust, said Aleman.
"Its proliferation is a natural, temporary phenomenon which is going to disappear" in due course, he added, suggesting global warming was helping the algae spread.
The bacterium "contains a toxin which can lead to skin irritation, dermatitis, hence one must avoid coming into contact with it in the water and on the sand."
With the islands last year welcoming more than 13 million foreign tourists, local authorities were keen to reassure sun-seekers.
"Generally, it has not been necessary to close the beaches," said Aleman.
However, AFP found that several have been closed to swimmers over recent weeks, including the popular Teresitas beach at Santa Cruz de Tenerife.
Marta Sanson, professor of plant biology at Tenerife's La Laguna university, told AFP that "ideal conditions are allowing proliferation of these microalgae".
Those include "an increase in water temperature" as well as a "dust cloud sweeping in off the Sahara which is rich in iron, a nutrient which micro-organisms like."