Hydrobiologia 477: 15–30, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 15 Variations of antioxidant enzymes in Daphnia species and populations as related to ambient UV exposure Jan Borgeraas & Dag O. Hessen University of Oslo, Department of Biology, Section of Limnology, P.O. Box 1027, Blindern, N-0316 Oslo, Norway Tel: 22855761. Fax: 22854438. E-mail: jan.borgeraas@bio.uio.no Received 15 August 2000; in revised form 22 January 2002; accepted 6 February 2002 Key words: carotenoids, catalase, Daphnia, glutathione transferase, superoxide dismutase, UV Abstract To assess their role in photoprotection in the crustacean zooplankton Daphnia spp., activities of the antioxidants catalase (CAT), superoxide dismutase (SOD), glutathione transferase (GST) and content of carotenoids (CAR) were studied in 4 Daphnia species from a total of 50 populations. Included in the survey were alpine and lowland populations of both pigmented and non-pigmented D. longispina, rock-pool and laboratory clones of D. magna and Arctic populations of the D. pulex/D. middendorffiana complex. Most of the surveyed populations inhabit shallow ponds, and are thus highly exposed to UV-radiation during the summer. The results are primarily dis- cussed in relation to antioxidants as possible protection against UV-radiation. D. pulex, D. middendorffiana and D. longispina occurred both as hyaline and pigmented (carapace melanization) morphs. The most notable inter- species differences were the high activity of CAT in the D. magna lab-clone, the low activity of GST in melanic animals from the D. pulex group and the high activity of SOD in D. longispina from a lowland humic pond. Contrary to expectations, we found no differences in antioxidant capacity between melanic and non-pigmented alpine D. longispina. Intraspecies variation in antioxidant activities was studied in relation to pond characteristics. Among the alpine populations of D. longispina there was a significant positive correlation between absorbance (300 nm) of the pond water and CAT activity, which could be related to ambient levels of photoinduced hydro- gen peroxide production in these small water bodies. The data from this comparative study provide insight in photo-protective mechanisms in cladocera, and constitute a valuable basis for future research on pro-oxidant and antioxidant processes in alpine and arctic freshwater zooplankton. Introduction UV-radiation appears to be a potential hazard to zoo- plankton, and harmful effects of UV-B (290–320 nm) radiation has been detected to depths of 6 m in clear lakes (Williamson et al., 1994). Moreover, the ozone reductions over the northern hemisphere have res- ulted in an increase in the UV-B portion of solar radiation reaching the earth’s surface (Kerr & McEl- roy, 1993). Also reduced inputs of dissolved organic carbon (DOC) caused by increased drought and tem- perature, may cause strong increases in downwelling UV-radiation (Schindler at al., 1996). In freshwater ecosystems, UV-B radiation will generally be atten- uated more rapidly with depth than in oceans due to higher concentrations of UV-absorbing dissolved humic substances. On the other hand, the average depth of lakes worldwide is <10 m and in clear oligo- trophic lakes >10% of incident UV-B radiation may still be present at 10 m depth (Hessen, 1993; Kirk, 1994). Daphnia spp. inhabit a variety of freshwater habitats, including highly UV-exposed ponds such as coastal rock-pools and alpine ponds and are often key species in these localities. Many of these localities have high water transparency and are very shallow (<1 m) offering no depth refugium for the animals. The success of Daphnia in such habitats indicates that these organisms must have evolved efficient mech- anisms for minimizing and correcting UV-induced damage.