Negative effects of UVB-irradiated phytoplankton on life history traits and fitness of Daphnia magna

SUMMARY 1. We tested the effect of ultraviolet‐B (UVB)‐irradiated phytoplankton on life history characteristics of Daphnia magna. Two phytoplankton species were used, Chlamydomonas reinhardtii and Cryptomonas pyrenoidifera. The phytoplankton species were cultured under photosynthetically active radiation (PAR) conditions, and under PAR supplemented with ultraviolet‐A and ultraviolet‐B radiation, and fed to Daphnia. 2. Life history traits of Daphnia were negatively affected when fed on UVB‐irradiated Cryptomonas. Size at maturity was depressed and fewer juveniles with lower fitness were produced in the UVB treatments. In the Chlamydomonas experiment, no significant effects were found. 3. The cause of the observed UVB effects is likely to be constraints in food quality. Ultraviolet‐B radiation thus has the potential of inhibiting energy transfer from the first to the second trophic level.