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 tolerant than cladocerans, regardless of lake UV transparency. On the basis of these differences, we hypothesized that UV avoidance behavior would be greatest in zooplankton with low UV tolerance, particularly in high-UV systems. Both downward and upward movements of zooplankton were examined in cylindrical acrylic columns that either transmitted full sunlight or blocked UV-B and short wavelength UV-A radiation. Columns were suspended vertically in the surface waters of a high- and low-UV lake at varying times of day and cloud cover. In the high-UV lake, the cladoceran Daphnia catawba displayed a distinct avoidance of the surface waters in the UV+ treatment but often showed a preference for the surface in the UV- treatment. Copepods were less responsive to UVR with calanoids displaying a somewhat greater UV avoidance than cyclopoids. In the low-UV lake, no behavioral differences were observed among UV treatments. These results suggest that UVR may influence the vertical distribution and habitat partitioning of certain zooplankton taxa in high-UV lakes while other factors, such as predation and food limitation, may be more important in low-UV lakes.