UV radiation and low calcium as mutual stressors for Daphnia

The cladocerans Daphnia magna and D. tenebrosa were exposed to daily 6-h ultraviolet (UV) radiation along a gradient of ambient Ca in the medium (0.5, 1.0, 5.0, and 10 mg Ca L-1) at nearly constant conductivity. Integrated irradiance over 300-400 nm was 35.95 W m(-2), corresponding to maximum outdoor intensities during midsummer: D. magna was most susceptible to UV radiation, but the UV susceptibility for both species increased significantly with decreased Ca content. Ca uptake (Ca-45) was apparently not influenced by short-term (3 h) UV exposure; thus, although lipid peroxidation and membrane damage are likely mechanisms for impaired ionic uptake, the overall role of low Ca may be reduced stress tolerance. While general conclusions an other species and taxa are premature, the data indicate that water hardness could be a major determinant to UV susceptibility among Ca-demanding zooplankton species like Daphnia.