INFLUENCE OF STRATIFICATION AND IRRADIANCE REGIME ON SUMMER PHYTOPLANKTON COMPOSITION IN COASTAL AND SHELF SEAS OF THE BRITISH-ISLES

The effect of interaction between stratification and irradiance regimes on phytoplankton community structure was investigated in three shelf/coastal regions of the British Isles, each of which displayed ranges of vertical stability and light attenuation. Relationships between vertical stability, light penetration and community structure were indicated by the ordination of dominance (assessed using cell volume) of the main phytoplankton phyletic groups-diatoms, dinoflagellates and microflagellates-on a surface defined by a bulk stratification index (φ{symbol}) and water column depth, scaled by transparency (λh). Diatom- and dinoflagellate-dominated communities occupied distinctly different domains on the φ{symbol} - λh surface, diatoms being favoured in well-mixed water columns with high values for λh and dinoflagellates dominating where stratification was strong and λh was low. Microflagellates were not abundant in any of the study areas and showed no clear ordination on the φ{symbol} - λh surface. The domain of co-dominance of diatom and dinoflagellates on the φ{symbol} - λh surface was narrow with small changes in the irradiance or stratification regime resulting in a switch to diatom or dinoflagellate dominance. It is suggested that loss of non-motile diatom communities in strongly stratified water columns might be a strong selective force in favour of dinoflagel-lates. However, in water columns with intermediate stratification and optical properties, the outcome of competition may be decided by physiological attributes of the two groups with respect to growth in low and fluctuating irradiances. © 1990.