Impact of ultraviolet-B radiation on the development of phytoplankton communities in the eastern Atlantic sector of the Southern Ocean—results from on-deck model ecosystem experiments

Two model ecosystem experiments to assess the effects of UVB radiation (UVBR, 280–320 nm) on phytoplankton communities were performed on-deck during the SWEDARP 97/98 expedition to the eastern Atlantic sector of the Southern Ocean in January 1998. The first experiment was run in the Spring Ice Edge, around 60°S, and the second at the Antarctic Polar Front, around 50°S. The duration of the experiments were 12 and 10 days, respectively. Ambient UVBR was modified to three levels (i.e. treatments): NoUVB—shielded from UVBR (mylar foil); AMB—ambient UVBR; UVB+—enhanced UVBR. The development of the phytoplankton communities was followed by measuring primary production and composition of pigments and diatom species. start of the first experiment UVB+ had lower primary production per volume, but not when normalised to chlorophyll a. At the end there were no differences between treatments in primary production per volume but there were differences when normalised to chlorophyll a. In addition, a significant increase in the stimulation of primary production for UVB+ when excluding UVBR and in chlorophyll a concentration was found. These changes, together with changes in species composition, are assumed to be the result of an adaptation to the enhanced UVBR. During the second experiment, the effects from changes in UVBR were generally smaller but resembled to some extent the first experiment. The chlorophyll a concentration increased significantly in the UVB+ treatment at the end of the experiment and there was a strong tendency towards a reduction in primary production per chlorophyll a, but not per volume. The results are important when estimating the effects of increased UVBR as they show variability in short-term sensitivity, and also the capacity of phytoplankton communities to adapt to UVBR and retain primary production even in enhanced UVBR.