COMMUNITY RESPONSES TO UV-RADIATION - I - ENHANCED UVB EFFECTS ON BIOMASS AND COMMUNITY STRUCTURE OF FILAMENTOUS ALGAL ASSEMBLAGES GROWING IN A CORAL-REEF MESOCOSM

Four treatments (PAR; PAR + UVA; PAR + UVA + UVB enhanced by 20% from ambient levels; ambient levels of UVB) were performed using a combinat ion of metal halide lights, UV lamps and cutoff filters over developin g assemblages of filamentous algae in a coral reef mesocosm. Exposure to enhanced UVB initially reduced the standing crop by 67% as compared to the productivity of the assemblages grown under PAR and PAR + WA. Treatment reversal from PAR to enhanced UVB restricted temporarily the growth of the brown alga Ectocarpus rhodochondroides. While the spore s of this species are inhibited by UVB, the sporophytes seem to be cap able of adapting to UVB exposure. The effects of ambient UVB levels on biomass production and community composition were less pronounced, wh ile exposure to UVA did not affect productivity or community compositi on. All effects due to UVB exposure gradually diminished as succession progressed. Community composition and biomass production were fully r estored 1 to 2 weeks after the cessation of exposure to enhanced UVB.