Effects of UVB radiation on marine phytoplankton communities

The impact of enhanced and reduced UVB radiation (UVBR) on pelagic ecosyste ms was studied during two mesocosm experiments in May and June/July 1994. T he ambient UVBR exposure was either reduced with mylar foil or artificially enhanced with UVB fluorescent tubes. Developments in the phytoplankton com munities were followed during 11 and 8 day periods using several structural and functional parameters. In the May experiment, enhanced UVBR significan tly stimulated carbon dioxide fixation, activity of alkaline phosphatase an d content of fatty acids. In the June-July experiment, the effects induced by changed UVBR were smaller with some indications of decreased algal bioma ss with enhanced UVBR. Several of the measured parameters indicated that th e two experiments represented different stages in the plankton community de velopment. In the May experiment, the community was in a development stage, moving from nutrient-replete to nutrient-depleted conditions, while the co mmunity in June/July was depleted of nutrients from the start. The stimulat ing effects of UVBR in May are suggested to be the secondary effects of a p hotochemically induced breakdown of dissolved organic matter, resulting in an increase in available nutrients.