Effects of ambient UVB radiation in a meiobenthic community of a tidal mudflat

Most studies involving UVB radiation effects in the marine environment have focused mainly on planktonic communities. Despite the importance of shallo w-water sediments as nursery and feeding areas for various taxa, investigat ions of the impact of UVB radiation on meiobenthic communities are still sc arce. A microcosm experiment was conducted over a 42 d period with a microb enthic community in a tidal mudflat of the Lower St. Lawrence Estuary, Cana da, in order to assess the influence of ambient UVB radiation on the abunda nce of ontogenic stages of harpacticoid copepods, on nematodes, and on chlo rophyll a and pheopigment concentrations. The microbenthic community within non-perturbed sediment cores was exposed to 2 different light regimes: nat ural irradiance (natural UVB) and reduced UVB. Sampling of the sediment cor es was carried out on a weekly basis. There was a significant decrease in c hlorophyll a concentrations in reduced UVB microcosms on Days 7 and 14, and no significant difference thereafter. A negative impact of UVB was observe d upon meiobenthic copepod nauplii and copepodites. Adult harpacticoid cope pods were significantly less abundant in the reduced UVB treatment on Day 7 . There was no significant difference in the abundance of adult harpacticoi d copepods between treatments at subsequent sampling dates and no effect on nematode abundance for the whole experiment. Our results suggest that natu ral UVB radiation may have a deleterious effect on the naupliar stages of h arpacticoid copepods, a key group of the meiobenthic compartment. Thus, a c hange in UVB radiation may greatly influence the dynamics of intertidal pop ulations of copepods by affecting their recruitment. Since meiofauna plays a key role in energy fluxes throughout the benthic food web, structural cha nges of harpacticoid populations induced by UVB radiation may significantly reduce nutrient transport at the water-sediment interface and the availabi lity of food in the upper trophic levels.