The distribution of coccoid blue-green algae (Cyanobacteria) in the Menai Straits and the Irish Sea

Abstract

Direct counts of coccoid cyanobacteria were made by the autofluorescence technique over a period of two years in the waters of the Menai Straits and western Irish Sea. In both situations there was a seasonal fluctuation, from low numbers of around 103 cells ml-1 in winter to high numbers of around 105 cells ml-1 in summer. Numbers in the Straits were not affected by the strong tides in these waters but were correlated with water temperature. Three size classes of cyanobacterial cell were distinguished and of these the two smallest, having dimensions of 0·5-0·7×0·8 μm and 0·7-0·9×1·0-1·2 μm, preponderated in both inshore and offshore waters, together making up 75–80% of the total. A third group, 1-1·2×1·4-2·2 μm, was more abundant in inshore waters in which it occasionally equalled the smallest size group in abundance. Hydrographic sections across a tidal mixing front in the western Irish Sea showed that coccoid cyanobacteria were abundant in surface stratified, bottom stratified and mixed waters but tende...