In situ Exposure to Ultraviolet Radiation Alters the Depth Distribution of Daphnia

Damaging solar radiation was one of the first factors proposed to explain zooplankton diel vertical migrations (DVM), yet other factors such as temperature, food, and especially predation have typically been more widely studied and identified as the primary factors inducing DVM. This is true in spite of the fact that recent experiments have shown that ambient levels of solar ultraviolet radiation (UVR) are potentially lethal to zooplankton and negative phototactic behavior has been demonstrated in the laboratory. Here we present the first evidence from in situ experiments demonstrating that UVR wavelengths influence the vertical swimming behavior of zooplankton under full spectrum solar radiation. Cylindrical acrylic columns that transmitted or blocked UVR were used to examine the behavioral responses of Daphnia pulicaria to UVR. Experiments were conducted in the epilimnion of a high-UV system at midday and in late afternoon. In both experiments, a large proportion of individuals migrated downward in the presence of UVR, whereas those in the UVR-shielded treatments tended to remain closer to the surface.