Effects of UVB radiation on marine phytoplankton communities

The impact of enhanced and reduced UVB radiation (UVBR) on pelagic ecosystems was studied during two mesocosm experiments in May and June/July 1994. The ambient UVBR exposure was either reduced with mylar foil or artificially enhanced with UVB fluorescent tubes. Developments in the phytoplankton communities were followed during 11 and 8 day periods using several structural and functional parameters. In the May experiment, enhanced UVBR significantly stimulated carbon dioxide fixation, activity of alkaline phosphatase and content of fatty acids. In the June-July experiment, the effects induced by changed UVBR were smaller with some indications of decreased algal biomass with enhanced UVBR. Several of the measured parameters indicated that the two experiments represented different stages in the plankton community development. In the May experiment, the community was in a development stage, moving from nutrient-replete to nutrient-depleted conditions, while the community in June/July was depleted of nutrients from the start. The stimulating effects of UVBR in May are suggested to be the secondary effects of a photochemically induced breakdown of dissolved organic matter, resulting in an increase in available nutrients.