EFFECTS OF UVB RADIATION IN A MICROBENTHIC COMMUNITY OF A MARINE SHALLOW-WATER SANDY SEDIMENT

The role of ambient and enhanced ultraviolet-B radiation (UVB; 280 to 315 nm) in a natural sand-associated microbenthic community was studie d in a 3-week experiment by incubating intact sediment cores from a sh allow bay in an outdoor flow-through system with 27 aquaria. After sam pling of initial cores, the remaining cores tone per aquarium) were gi ven one of three treatments: no, ambient, and moderately enhanced UVB, and sampled, nine at a time, after 5, 12, and 19 d. The response of t he community was studied by analysing algal and meiofaunal composition and biomass, chlorophyll a content, composition of pigments and fatty acids, and content of UV-absorbing compounds (state variables), as we ll as carbon fixation and allocation, and bacterial productivity (rate variables). Among rate variables, significant effects of UVB-treatmen ts were found for carbon fixation and allocation, while bacterial prod uctivity was not affected. For state variables, a significant response was observed for the composition of microalgae and fatty acids, and f or chlorophyll a content. The effect of treatments was mainly observed as differences in development with time (two-way analysis of variance , treatment x time interaction). Towards the end of the experiment, th e no-UVB treatment most often differed from one or both of the two tre atments with UVB exposure, the latter showing lower values. There were marked successional changes in the community, irrespective of treatme nt. The microalgal community changed from being dominated by coccoid c yanobacteria and epipsammic diatoms to a dominance of epipelic diatoms and filamentous cyanobacteria. The pattern of carbon allocation, as w ell as an increased C/N ratio of the sediment, suggested limitation of growth, perhaps by nutrients, at the end of the experiment. This may possibly have acted synergistically with UVB exposure to create the tr eatment effects. The new knowledge gained from our experiment is that ambient UVB can exert a stress on the function of sand-associated micr obenthic communities in shallow waters and that this effect coincides with structural differences in the community.,More experiments in natu ral or semi-natural systems are needed to allow better prediction of m icrobenthic community-level responses to UVB.