Effects of ultraviolet-B stressed diatom food on the reproductive output in Mediterranean Calanus helgolandicus (Crustacea; Copepoda)

Recent predictions anticipate a solar ultraviolet-B increase of 5-10% over temperate latitudes within the next 15 years. As Mediterranean waters are clear, enabling deep UVB penetration, we used controlled experiments to test the effects of equivalent levels of UVB on trophic plankton interactions. We grew the diatom Skeletonema costatum in the laboratory under daily 4 h UVB exposure in doses of 10% higher than today in contrast to control cultures (no UVB). UVB-exposed algae showed modification in cell structure, volume increase and delay in cell division, eventually leading to lysis and cell death after five days of daily 4 h exposure. Both control and UVB-stressed S. costatum cultures were used as food for wild Calanus helgolandicus females collected in the NW Mediterranean. Those fed on control diatoms produced three times more eggs, healthier offspring with fewer lethal naupliar deformities than females fed on UVB-exposed diatoms. Our results provide the first experimental evidence of indirect UVB effects on reproductive output through trophic response in marine plankton. The sensitivity of the S. costatum strain is discussed as well as the reduced reproductive output of the female copepods. We suggest that the low egg numbers and high naupliar deformity ratios are possibly a consequence of UVB-induced deficiency of essential components in the UVB-stressed maternal diet.