Pelagic prey and benthic predators: impact of odonate predation on Daphnia

Interactions between benthic predators and pelagic prey, such as larval odo nates and Daphnia, are often used to describe classic predator-prey relatio nships in laboratory-studies. However, few field studies explore the potent ial impact of benthic predators on pelagic prey. Recent studies of cladocer ans document diel horizontal migration (DHM), where large-bodied zooplankto n (i.e., Daphnia) decrease their exposure to pelagic predators by seeking r efuge among macrophytes. However, daphnids undergoing DHM may simultaneousl y increase their likelihood of encountering benthic predators that commonly occur in littoral zones. In laboratory experiments, we showed that dragonf ly nymphs (Epitheca cynosura) effectively eliminated all Daphnia within 24 h, regardless of macrophyte presence or architecture. We also tested whethe r additions of larval damselflies (Ischnura elegans, Coenagrion puella, C. pulchellum) and dragonflies (Somatochlora flavomaculata) (total odonate den sity of 35-55/m(2)) significantly reduced total zooplankton or benthic inve rtebrate abundance in field enclosures with different macrophyte densities (20, 40, 80% volume infested [PVI]). Odonates significantly reduced Daphnia abundance at 20 PVI However, the magnitude of the influence of odonates on daphnids, as well as Ceriodaphnia and Polyphemus, decreased with increasin g macrophyte density. Odonate predation did not significantly affect benthi c taxa abundance. Thus, daphnids undergoing DHM may lower predation from pe lagic predators, but our results suggest that mortality from littoral preda tors may be significant. The net benefit of DHM may, therefore, differ amon g lakes as a function of the relative threats posed by pelagic and littoral . predators.