Swimmers at Porthdinllaen beach feared they were swimming in sewage when the water around them turned brown on bank holiday weekend.

It is understood that there was a sewage leak at Bwlch, Morfa Nefyn for some days but the discolouration at Porthdinllaen was caused by algae, according to Natural Resources Wales (NRW).

An aerial photograph by James Tatham showed the discoloured water.

An eyewitness, who asked to remain anonymous, doubted algae as the cause of the dramatic change of colour of the water.

Speaking to the ‘Cambrian News’, the eyewitness said: “There was a sewage leak at Bwlch, Morfa Nefyn that had been going on for some days.

“You sometimes get algae this time of year, but there was definitely a sewage leak as one of the pumps failed at the public toilets at Bwlch.”

But a spokesperson for NRW moved to reassure people that sewage was not the cause of Porthdinllaen’s brown water colour.

Arfon Hughes, Environment Team Leader for Natural Resources Wales (NRW), told the ‘Cambrian News’: “Our officers received reports of potential pollution at Porthdinllaen beach over the bank holiday weekend.

“Upon inspection, this was found to be a type of non-toxic algae called Phaeocystis – one of the most common bloom-forming algae in our coastal waters. The blooms can form a brown, frothy scum that is harmless and may be influenced by recent sunlight and warmer temperatures.”

Arfon added: “Anyone with concerns about potential pollution can report them to us by calling our 24/7 incident communication line on 0300 065 3000 or by using our online incident form: https://naturalresources.wales/about-us/contact-us/report-an-incident/?lang=en.”

Bathers are becoming more familiar with ways to check if there has been sewage discharged. Surfers Against Sewage has an app and a live sewage map at https://datahq.sas.org.uk/sewage-data-hq/