AN IN-SITU ENCLOSURE EXPERIMENT TO TEST THE SOLAR UVB IMPACT ON PLANKTON IN A HIGH-ALTITUDE MOUNTAIN LAKE - I - LACK OF EFFECT ON PHYTOPLANKTON SPECIES COMPOSITION AND GROWTH

The effect of solar UVB radiation on the growth and species compositio n of phytoplankton from a high-mountain lake (2417 m a.s.l.) was studi ed in situ for 16 days in two enclosures of 1 m(3), receiving either f ull sunlight or sunlight without UVB. A total of 20 species were ident ified in both enclosures, consisting mainly of dinoflagellates, chryso phytes and diatoms. During the experiment, there were no significant d ifferences in phytoplankton species composition between the two enclos ures. In both treatments, the abundance of phytoplankton increased con tinuously, and chlorophyll a changed by similar to 5-fold. We observed high fluctuations in the abundance of several species. However, these fluctuations occurred in both enclosures, and hence they were not rel ated to UVB radiation. Some species were affected by daily fluctuation s of radiation (UVA + photosynthetically active radiation). Cyclotella aff. gordonensis showed a robust positive correlation, whereas specie s of Gymnodinium were negatively correlated. For most species, the sen sitivity to radiation found during the experiment was consistent with their diurnal vertical distribution in the lake. Our findings suggest that the phytoplankton from this very clear lake (10% of the surface U VB radiation at 305 nm reached 9.6 m depth) were well adapted to the h igh UVB radiation characteristic of high-elevation sites.