Positive effects of UV radiation on a calanoid copepod in a transparent lake: do competition, predation or food availability play a role?

Zooplankton tolerant to ultraviolet radiation (UVR) could be indirectly affected by UVR through interactions with UV-sensitive species in the same ecosystem. In Lake Giles, Pennsylvania, USA, the calanoid copepod Leptodiaptomus minutus is more UVR tolerant than the cohabiting species Daphnia catawba and Cyclops scutifer. We asked whether L. minutus is affected by UV-induced mortality of a food competitor (D. catawba) or a predator of its nauplii (C. scutifer). We conducted two in situ enclosure experiments with six treatments: L. minutus alone, L. minutus + Daphnia and L. minutus + Cyclops in the presence and absence of UVR. There were few differences in survival among treatments in Experiment 1, which had enhanced food and a cumulative UVR (320 nm) dose of 9.3 kJ m-2. In Experiment 2, which had ambient food and a UVR (320 nm) dose of 20.0 kJ m-2, L. minutus survival and reproduction were higher in the +UVR compared to -UVR, regardless of competitors or predators. Chlorophyll a (Chl a) in Experiment 2 was higher in the +UVR than -UVR. While interactions between zooplankton species of differing UVR tolerances are potentially important, these results instead demonstrate that the beneficial UVR effect on L. minutus is independent of concurrent detrimental UVR effects on competitors and predators. Further research on the phytoplankton community is necessary to determine whether UVR alleviates bacterial competition, increases nutrient availability or affects phytoplankton by other mechanisms.