Abstract
Diurnal variation in the activity of the antioxidant enzymes catalase (CAT), glutathione transferase (GST) and superoxide dismutase (SOD) were studied in one arctic Daphnia tenebrosa and two alpine D. longispina populations. Both the alpine ponds were shallow and transparent, yet they were inhabited by different colour morphs due to carapace melanization (hyaline or melanic). Hyaline animals showed significantly higher activity of GST and SOD, but lower activity of CAT compared to the melanic morph. In the melanic morph significant diurnal variation was found only in GST activity with a minimum around noon. In the hyaline morph diurnal variation was observed in both CAT and SOD activity with a maximum at mid-day. Hyaline Daphnia may experience higher stress from UV-induced reactive oxygen species than animals with carapace melanization since the latter are well protected from harmful radiation due to their pigmentation. Compared to the alpine populations the arctic population displayed similar SOD activity, higher CAT and far lower GST activity, but the diurnal fluctuations were all small and insignificant.
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