Marine aloricate ciliate red tides in a temperate Irish sea lough

Abstract

Although red tides are typically formed by dinoflagellate blooms, aloricate ciliates in the genus Mesodinium also produce striking red blooms throughout the world. In mid-March 2015, we confirmed the occurrence of dense blooms of Mesodinium major in Lough Hyne Marine Nature Reserve, a fully marine sea lough in County Cork, SW Ireland. Daytime aggregations of these highly mobile ciliates occurred in the surface and subsurface waters of the lough, particularly along the western shore and in the Goleen. Fine-scale comparisons of dissolved oxygen (DO) and water temperature values indicated that the surface water supporting dense blooms was slightly but significantly more oxygenated and warmer than closely juxtaposed water without visible blooms. Coarse-scale comparisons, however, showed no significant difference in DO or temperature between bloom and non-bloom areas, although there were highly significant differences in the two parameters by water depth and location within the lough. Although this is the first formal report of red-water ciliate blooms for Lough Hyne and County Cork, it is not the first known occurrence; earlier spring red tides at the lough were assumed (probably in error) to be caused solely or primarily by dinoflagellates and/or loricate ciliates (tintinnids).