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The impact of UV-B radiation (290-315 nm) on bacterial activity and abundance in coastal water was studied in mesocosm experiments in May 1994 and May 1995 at Kristineberg Marine Research Station, Sweden. Mesocosms (6 m3) containing natural pelagic communities were exposed either to ambient irradiation (AMB), ambient irradiation with enhanced UV-B (+UV) (0.7 W m-2 4h every day around noon), or ambient irradiation screened for UV-B (-UV). Bacterial activity in the mesocosms was measured by means of thymidine incorporation in short-term tests during incubations at ambient irradiation, at ambient irradiation with enhanced UV-B, and at ambient irradiation screened for UV-B. In +UV mesocosms, bacterial activity was significantly stimulated when incubated at ambient radiation. The stimulating effect was suggested to be due to an increase in carbon or nutrient supply through a photodegradation of recalcitrant dissolved organic material (DOM). Low attenuation coefficients for UV-B and PAR (400-700 nm) in the +UV mesocosms supported this hypothesis. The bacterial activity in +UV mesocosms, however, was inhibited when incubations were made at enhanced UV-B irradiation, implying that the bacteria had become more sensitive to UV-B radiation. The increased sensitivity to UV-B exposure in bacterial assemblages that already had been exposed and stressed by UV-B radiation is suggested to be due to an overburdening of the energy- consuming DNA repair mechanism. The data suggest that increased UV-B radiation, which might occur with ozone depletion, may both stimulate and suppress bacterial activity in coastal waters, implying that the net outcome of enhanced UV-B radiation could be an unchanged bacterial activity.

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 experi- ment, 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 experi- ments 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.