Influence of UV-B radiation on nitrogen utilization by a natural assemblage of phytoplankton

A 7-day mesocosm experiment was conducted in July 1996 to investigate the e ffects of ambient UV-B radiation (UVBR) exclusion and two UVBR enhancements above ambient levels on NO3-, NH4+ and urea utilization in a natural plank ton community (< 240 mu m) from the Lower St. Lawrence Estuary, The phytopl ankton community was dominated by diatoms during the first 3 days and, afte rward, by flagellates and dinoflagellates. The results of 4-h incubations j ust below the water surface show that, compared with ambient UVBR condition s, UVBR exclusion generally increased NO3-, NH4+, and urea uptakes, During the last 4 days of the experiment, the percent increase in the specific upt ake rate of urea under excluded UVBR conditions varied between 17% and 130% and was a linear function of the ambient UVBR dose removed. During the fir st 3 days, the phytoplankton community dominated by diatoms was able to wit hstand UVBR enhancements without any perceptible effect on nitrogen uptake. However, during the post-diatom bloom period, UVBR enhancements resulted i n decreases in NO3-, NH4+, and urea uptake compared with ambient UVBR condi tions. The reduction of urea uptake under UVBR enhancements during the last 3 days varied between 23% and 64% and was linearly related to the enhanced UVBR dose. However, the different UVBR treatments did not affect the inter nal organic nitrogen composition (internal urea, free amino acids, and prot eins) of the phytoplankton community experiencing vertical mixing in the me socosms, The discrepancy between short-term uptake measurements at the surf ace and long-term effects iu the mesocosms emphasizes the importance of ver tical mixing on UVBR effects in natural ecosystems. This suggests that an i ncrease in ambient UVBR would have a minimal effect on nitrogen utilization by natural phytoplankton assemblages if these are vertically mixed.