Does UV Play a Role in Changes in Predation and Zooplankton Community Structure in Acidified Lakes?

Changes in both zooplankton community structure and zooplanktivorous predators often accompany the anthropogenic acidification of lakes. While changes in pH can account for many of the observed changes, combined observations from laboratory bioassay and field experiments as well as surveys suggest that these patterns cannot be explained by changes in pH alone. Dissolved organic carbon (DOC) also declines with lake acidification. Because DOC is the primary factor regulating variation in the depth of penetration of ultraviolet radiation (UVR) in lakes, there is also likely to be an increase in UVR levels during acidification. This suggests that changes in UVR may play some role in changes in predator and prey communities during acidification. As a first step toward testing this hypothesis, we examined the UVR tolerance of larvae of two widespread and abundant zooplanktivorous predators. We performed a series of in situ incubation experiments with the sunfish Lepomis and the midge Chaoborus in a low DOC (high UVR) lake and in a moderate DOC (low UVR) lake. Substantial UVR-induced mortality of both predators was observed in the surface waters of the low DOC lake. The predators differed in their UVR tolerance levels: the sunfish survived for more than a day under high solar radiation conditions in the surface waters of a low DOC lake, while the midge perished in less than a day. These data and past literature are consistent with the hypothesis that UVR may play a role in changes in planktivorous predators and their prey during lake acidification and recovery.