Swimmers looking forward to a refreshing dip in the cool blue of the Øresund this week were greeted instead by red flags indicating waters were unfit to swim in.
A soupy, thick layer of algae slushed ashore from the northern coast of Zealand to the beaches in Copenhagen.
Lifeguards closed beaches since swimming in the waters could give stomach pains and irritated skin
'I've never seen anything like it,' Hanne Aarslev Sørensen, a biologist in the northern Zealand municipality of Græsted-Gilleleje told daily newspaper Berlingske Tidende. 'This is the first time I've seen such massive amounts of algae.'
She noted the algae could disappear as quickly as it arrived if the wind shifted directions and rain fell.
Visitors and residents on the Baltic island of Bornholm have also had to refrain from splashing in the sea. Warm temperatures and the low salt content in the sea surrounding the island have provided fertile breeding grounds for bacteria.
Health officials considered issuing a swimming prohibition for the entire island.