Effects of UVB radiation on a marine microphytobenthic community growing on a sand-substratum under different nutrient conditions

The effect of ultraviolet-B radiation (UVBR) on a microphytobenthic community, dominated by diatoms, was studied under different nutrient conditions in a 9 day outdoor experiment on the Swedish west coast. The microalgal assemblage was isolated from the sediment and resettled onto acid-cleaned sand placed in petridishes. The experimental treatments were: ambient or enhanced UVBR with no nutrient addition, ambient or enhanced UVBR with nutrient addition (N, P, Si). Enhanced UVBR (+15%) was provided by a computer-controlled system. Primary productivity, carbon allocation, chlorophyll a (Chl a) concentrations, composition of algae and pigments and intra- and extracellular carbohydrate fractions were measured. Most UVBR effects were seen in treatments without nutrient addition. Exposure to enhanced UVBR resulted in statistically significant decreases in primary productivity, Chl a and in the biomass of Haslea ostrearia and Nitzschia spp. The relative amount of carbon allocated to proteins increased when exposed to enhanced UVBR. The effect of enhanced UVBR on microalgae subjected to nutrient addition was less pronounced, and observed for Chl a, algal intracellular storage of carbohydrates (glucans) and concentration of extracellular colloidal carbohydrates. Enhanced UVBR + nutrients resulted in a significantly greater ratio between glucan and total carbohydrates and a decreased concentration of colloidal carbohydrates compared to the ambient UVBR + nutrients treatment. These results indicate that availability of inorganic nutrients acts to mitigate the negative effects of UVBR on microphytobenthic communities and that UVBR acts as a selective force during early growth and succession.