There is a similar phenomena which occurs from now on in Llandudno Bay.
I have seen it for the last 4 years.
It is a cream coloured, foamy trail on the sea which emanates from the shore near the paddling pool, and extends accross the bay in a straight line, under the pier and beyond.
I thought it was pollution of some kind, but....
Those in 'the know' tell me it as an algae bloom and it disappears by mid August.