Zooplankton behavioral responses to solar UV radiation vary within and among lakes

Zooplankton taxa exhibit varying tolerances to ultraviolet radiation(UVR), with rotifers and copepods tending to be more UV tolerantthan cladocerans, regardless of lake UV transparency. On thebasis of these differences, we hypothesized that UV avoidancebehavior would be greatest in zooplankton with low UV tolerance,particularly in high-UV systems. Both downward and upward movementsof zooplankton were examined in cylindrical acrylic columnsthat either transmitted full sunlight or blocked UV-B and shortwavelength UV-A radiation. Columns were suspended verticallyin the surface waters of a high- and low-UV lake at varyingtimes of day and cloud cover. In the high-UV lake, the cladoceranDaphnia catawba displayed a distinct avoidance of the surfacewaters in the UV+ treatment but often showed a preference forthe surface in the UV– treatment. Copepods were less responsiveto UVR with calanoids displaying a somewhat greater UV avoidancethan cyclopoids. In the low-UV lake, no behavioral differenceswere observed among UV treatments. These results suggest thatUVR may influence the vertical distribution and habitat partitioningof certain zooplankton taxa in high-UV lakes while other factors,such as predation and food limitation, may be more importantin low-UV lakes.