There have been several reports in recent years of the mass occurrence or blooms of the dinoflagellate Gyrodinium aureolum Hulburt in northern European Waters (Ballantine & Smith, 1973; Helm et al. 1974; Pingree et al. 1975; Pingree, Holligan & Head, 1977; Tangen, 1977). Gyrodinium aureolum is probably one of the most common ‘red-tide’ dinoflagellate species in these waters and some of the blooms of G. aureolum (ranging from 100 to 20000 cells ml-1) have been followed by the mass mortality of various fish and invertebrate species (Tangen, 1977). It has been suggested that the adverse effects of G. aureolum on marine organisms are caused by (a) the production of toxins and/or (b) the oxygen depletion during darkness due to dinoflagellate respiration and decomposition of cells.