UV-B induced mortality and antioxidant enzyme activities in Daphnia magna at different oxygen concentrations and temperatures

Survival of adult Daphnia magna was assessed after acute (<96 h) exposure to UV312 under various temperatures (6, 12 and 18 degrees C) or oxygen concentrations (5.6, 8.5 and 14.1 mg O-2 l(-1)) in the laboratory. The surviving animals were screened for the enzymes catalase (CAT) and glutathione transferase (GST), which may protect against W-induced oxidative damage. In addition, the same two enzymes were assayed in separate experiments after acute exposure to UV312 (6 h, 0.014 mW cm(-2)) and the different levels of temperature and oxygen. No differences were observed in either CAT or GST activity after exposure to the three levels of oxygen, but there was a tendency for decreasing specific activity with decreasing temperature for both enzymes. CAT activity was not influenced by UV radiation, whereas GST activity displayed a slight increase. Oxygen concentration did not influence survival during UV exposure but, contrary to expectations, survival tests at different temperatures clearly showed that reduced temperature increased survival. The results indicate that temperature effects must be considered when comparing dose effect relationships in situ, and suggest that low temperature is not a major cause of UV susceptibility in cold-adapted alpine and Arctic populations of Daphnia.