Dun Laoghaire Rathdown County Council has said that the orange-coloured water at Dublin's Sandycove Beach is not connected to a wastewater overflow into Dublin Bay.
Swimming bans remain in place at four Dublin beaches after the second overflow at the Ringsend water treatment plant this month.
It comes as temperatures are expected to hit highs of 27C on Thursday and Friday.
The swimming bans are in place at Seapoint, Sandycove, the Forty Foot and Dollymount.
— Brian Costello (@BrianBREVL) June 25, 2019
Dun Laoghaire Rathdown County Council said the notices were put in place “as a precaution” after heavy rains on Sunday and Monday sent wastewater flowing into Dublin Bay.
The water quality has been tested at all four swimming spots and a decision on whether to life the bans will be taken as soon as the results are in.
— dlrcc (@dlrcc) June 24, 2019
After the bans were announced, locals in Dun Laoghaire complained that the water had turned a dark orange in places along the coast.
In a statement this evening however, the council said tests had shown that the colour comes from a non-toxic algae called from Noctiluca scintillans.
Great news that the orange slick is in fact an algae bloom. While we do welcome this, the substantive issue of sewage discharge resulting in bathing prohibitions and effecting other water users should not be forgotten. #dunlaoghaire #sailing pic.twitter.com/hzUrV7FtDE
— INSS Sailing (@IrelandSailing) June 25, 2019
It said the “benign, non-toxic, species” is not directly associated with the waste water overflows that led to the bans.
“We have been advised that this is a natural summer phenomenon in response to long day length, high nutrients and warm water,” it said.
“These blooms have been reported along the east coast for the past few weeks and are typical for this time of the year.”
We have taken samples of the orange coloured water reported at #Sandycove this morning.
Results show that this is not raw sewerage but is a micro-alga called #Noctiluca scintillans #bioluminescence which is non-toxic.
Read more here: https://t.co/EtcxwgivMG pic.twitter.com/nfBzHz7x0o
— dlrcc (@dlrcc) June 25, 2019
It said the algae emits bright sparkles of blue bioluminescence at night when it is disturbed by the waves.
Bathing prohibitions unrelated to the wastewater overflow are already in place at Sandymount and Merrion beaches due to “general water quality issues.”