The role of ultraviolet radiation and fish in regulating the vertical distribution of Daphnia

Previous studies have demonstrated independent effects of both solar ultraviolet radiation (UV; 280-400 nm) and planktivorous fish on the vertical distribution of Daphnia. We examined the behavioral response of adult and juvenile Daphnia to both UV and planktivorous fish simultaneously in a small temperate lake in eastern Pennsylvania by conducting a large-scale (15 m deep) in situ mesocosm experiment with full factorial treatments (+/- UV and +/- fish). UV induced an avoidance of the surface waters in both juvenile and adult Daphnia. In contrast, the response of Daphnia to fish depended on the presence of UV, with a clear interactive effect. In the presence of both UV and fish, Daphnia were deeper in the water column than they were in the absence of either UV or fish. Sampling of the lake also revealed a rapid upward shift in the depth distribution of both juveniles and adults following a rare and intense early-summer storm that reduced the lake's transparency to both UV (for example, 380 nm UV-A) and photosynthetically active radiation (400-700 nm) by 44% and 39%, respectively. Evidence of a novel benefit to UV avoidance behavior was also observed: surface avoidance of UV reduces the hazards of Daphnia getting caught in the surface air-water interface and perishing. These results highlight the interactive effects of fish and UV on Daphnia vertical distribution under near-natural conditions in situ.