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

Abstract

The effect of solar UVB radiation on the growth and species composition of phytoplankton from a high-mountain lake (2417 m a.s.l.) was studied in situ for 16 days in two enclosures of 1 m-3, receiving either full sunlight or sunlight without UVB. A total of 20 species were identified in both enclosures, consisting mainly of dinoflagellates, chrysophytes and diatoms. During the experiment, there were no significant differences in phytoplankton species composition between the two enclosures. In both treatments, the abundance of phytoplankton increased continuously, and chlorophyll a changed by ~5-fold. We observed high fluctuations in the abundance of several species. However, these fluctuations occurred in both enclosures, and hence they were not related to UVB radiation. Some species were affected by daily fluctuations of radiation (UVA + photosynthetically active radiation). Cyclotella aff. gordonensis showed a robust positive correlation, whereas species of Gymnodinium were negatively correlated. For most species, the sensitivity 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 UVB radiation at 305 nm reached 9.6 m depth) were well adapted to the high UVB radiation characteristic of high-elevation sites.