Sensitivity of winter phytoplankton communities from Andean lakes to artificial ultraviolet-B radiation

During July of 1999 sampling was carried out in five Andean takes to determ ine the sensitivity of winter phytoplankton communities to ultraviolet-B ra diation (UV-B, 280-320 nm). The studied lakes. Moreno, El Trebol, Nahuel Hu api, Gutierrez, and Morenito, located in the Patagonia region (41 degreesS, 71 degreesW, 800 m of altitude), had attenuation coefficients for UV-B tha t ranged from 0.36 m(-1) (Lake Moreno) to 2.8 m(-1) (Lake Morenito). The sa mples were inoculated with labeled carbon (NaH(14)CO3) and incubated in an illuminated chamber (UV-B = 0.35 W M-2, UV-A [320-400 nm] = 1.1 W m(-2), an d PAR [400-700 nm] = 10.8 W m(-2)) at 10 degreesC. The phytoplankton cells were exposed to UV radiation (280-400 nm) + PAR (quartz tubes), and to UV-A + PAR (quartz tubes covered with Mylar-D). The total duration of the exper iments was 4 h and two samples were taken from each treatment every hour. I n lakes Moreno, El Trebol, Nahuel Huapi and Gutierrez, the photosynthetic i nhibition increased linearly with UV-B doses, while in Lake Morenito just a slight relationship was observed. After receiving a dose of 1.25 kJ m(-2) (UV-B), phytoplankton from Lake Morenito had the highest cumulative photosy nthetic inhibition (44 %), whereas in Lakes Moreno, El Trebol, Nahuel Huapi and Gutierrez the inhibition was of 22, 11, 5, and 1 %, respectively. Howe ver, at the end of incubation period and after receiving doses of 5 kJ m(-2 ), the most inhibited phytoplankton cells were from Lake Moreno (70 %) and the most resistant (27 %) was that from Lake Gutierrez. The kinetics of inh ibition was different in each lake, and transparent lakes, with higher prop ortion of large cells, had higher inhibition rates. The results suggest tha t an increase in UV-B radiation (e.g., produced by a decrease in stratosphe ric ozone) would have a greater impact on microplankton from clear lakes, w hile pico- and nanoplankton from less transparent lakes will be less affect ed.