ON “red water” in the Isefjord (inner Danish waters) caused by the ciliateMesodjnium rubrum

Abstract

A bloom of the ciliate Mesodinium rubrum (Lohmann) causing “red water” in the Isefjord is described. The bloom lasted about 6 weeks and coincided with a period of warm and calm weather. It is shown that Cyclotrichium meunieri Powers, which has previously been shown to cause red water at the east coast of North America and in New Zealand waters, is identical with Mesodinium rubrum. The literature on red tides caused by Mesodinium rubrum is reviewed. They have previously been reported from nearly all continents but the bloom described here is the first to be reported from Scandinavian waters. It is shown that the blooms are correlated with warm and calm weather and mostly occur in late summer or autumn. There is no evidence showing that blooms of Mesodinium are toxic to marine life.