Effects of temperature and ultraviolet radiation on diel vertical migration of freshwater crustacean zooplankton

In transparent, thermally stratified lakes, ultraviolet radiation (UV) and temperature are among the primary factors influencing diel vertical migration (DVM) of zooplankton. It is not well known how behavioral responses will vary across a wide range of depths with steep gradients in UV, photosynthetically active radiation (PAR), and temperature. Our objective was to determine the relative importance of UV and temperature on DVM of Daphnia catawba and Leptodiaptomus minutus in a high UV, thermally stratified lake. Using UV-transparent (+UV) and UV-opaque (-UV) columns suspended within the mixed layer and metalimnion, we found that both species were generally shallower in the -UV than in the +UV columns. Daphnia catawba responded negatively to UV, even below the 1% depth for 320 nm, whereas L. minutus responded to UV only within the mixed layer and upper metalimnion. Daphnia catawba did not migrate in the -UV in the mixed layer, but migrated upwards in the -UV in the deep part of the metalimnion, indicating a temperature-induced rather than phototactic response. At night, both species migrated upwards in the metalimnion and were evenly distributed in the mixed layer. These results indicate that in transparent lakes, UV may constrain some zooplankton to cooler, suboptimal temperatures, which may compromise fitness.