Temporal changes in effects of ambient UV radiation on natural communities of Antarctic marine protists

The effects of ambient solar UV radiation (280 to 400 nm) were determined using 3 nat- ural marine protist communities incubated in 650 l tanks (minicosms) for 13 to 14 d over the summer of 2002-2003 at Davis Station, Antarctica. Minicosms were exposed to ambient light that was vari- ously attenuated to give treatments of photosynthetically active radiation (PAR, ≥385 nm wave- length), PAR + UV-A radiation (315 to 385 nm), and PAR + UV-A + 4 different treatments of UV-B radiation (280 to 315 nm) that simulated a range of equivalent depths (ED) in the water column from 4.43 to 7.15 m. Results showed a seasonal progression in the response of microbial communities to UV radiation exposure. The first experiment in November showed that the microbial community was sig- nificantly inhibited in the PAR + UV-A-exposed treatment but this inhibition declined with increas- ing addition of UV-B radiation. The second experiment in December showed that UV-A or UV-B radi- ation had few significant effects. Like in Expt 1, some taxa were inhibited by PAR + UV-A or promoted by UV-B, but most were inhibited at the highest UV-B irradiances (≤4.43 m ED). The last experiment in January showed UV-B induced inhibition of all but one of the dominant taxa. The sea- sonal transition in UV wavelengths responsible for inhibition of protists may be due to ozone reduc- tion, the light history of protists, and/or changes in species composition. The increasing UV-B- induced inhibition we observed over the summer corresponded to a decline in ozone concentrations over Davis. This recurrent decline in ozone over Antarctica between January and April coincides with blooms of diatoms that appear to have low UV-B tolerance but are responsible for ~47% of annual primary production in Antarctic waters.