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 communi ty, dominated by diatoms, was studied under different nutrient conditions i n a 9 day outdoor experiment on the Swedish west coast. The microalgal asse mblage was isolated from the sediment and resettled onto acid-cleaned sand placed in petridishes. The experimental treatments were: "ambient" or "enha nced" UVBR with no nutrient addition, "ambient" or "enhanced" UVBR with nut rient 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 ext racellular carbohydrate fractions were measured. Most UVBR effects were see n in treatments without nutrient addition. Exposure to enhanced UVBR result ed in statistically significant decreases in primary productivity, Chl a an d in the biomass of Haslea ostrearia and Nitzschia spp. The relative amount of carbon allocated to proteins increased when exposed to enhanced UVBR. T he effect of enhanced UVBR on microalgae subjected to nutrient addition was less pronounced, and observed for Chl a, algal intracellular storage of ca rbohydrates (glucans) and concentration of extracellular "colloidal" carboh ydrates. Enhanced UVBR + nutrients resulted in a significantly greater rati o between glucan and total carbohydrates and a decreased concentration of c olloidal carbohydrates compared to the ambient UVBR + nutrients treatment. These results indicate that availability of inorganic nutrients acts to mit igate the negative effects of UVBR on microphytobenthic communities and tha t UVBR acts as a selective force during early growth and succession.