Abstract

Exclusion experiments on global UV (A and B) radiation and global UVB were performed in 460 1 mesocosms with plankton communities from the oligotrophic Andean lake Laguna Negra (33°35'S-70°04'W; 2700 m a.s.l.). The experiments were run for 30 days during the summers of 1991-1992 and 1992-1993, and for 48 days in 1993-1994. When UVB radiation was allowed to enter into the mesocosms (full sun), the population of Ankyra judayi (Chlorophyta) reached the highest density, suggesting that this species can endure high levels of UV radiation. Concurrently, an increase in chlorophyll a concentration was observed in this treatment. The cladoceran Chydorus sphaericus and the rotifer Lepadella ovalis were strongly inhibited by UVB. Conversely, UVB radiation had no effect on the survival of the different life stages of the calanoid copepod Boeckella gracilipes, suggesting a species-specific difference in the sensitivity to solar UVB radiation. Moreover, no reduction in the number of copepod eggs per female and the number of nauplii produced was observed. Apparently, herbivory does not strongly affect phytoplankton abundance. Moreover, the phytoplankton species composition changed in the different treatments over the time. Fragilaria construens and Fragilaria crotonensis were dominant in those mesocosms where UVB was excluded. Populations fluctuated depending on their life cycles and the period of time they were exposed to UVB radiation. It is important to define the time scale of exclusion experiments, because different conclusions about the influence of UVB irradiance result from short-, medium- or long-term exposures.