Littoral zone structures as Daphnia refugia against fish predators

The assumption that macrophytes can provide zooplankton a daytime refuge ag ainst fish predation is central to the diel horizontal migration hypothesis . However, previous observations and experiments have shown that large-bodi ed zooplankton avoid macrophytes. To directly test these contrasting roles of macrophytes, we measured the reaction of Daphnia paler to macrophytes (E lodea canadensis or plastic) in the presence and absence of chemical cues f rom two commonly occurring European fishes, roach (Rutilus rutilus) and per ch (Perca fluvialitis). In the same series of laboratory experiments, we th en tested the ability of different densities of E. canadensis and plastic m acrophytes to reduce predation by roach or perch. In the absence of fish, o nly 27% of daphnids occurred in the macrophytes. However, 70-80% of daphnid s occurred in the macrophytes when either roach or perch occurred. In addit ion, significantly more daphnids occurred in plastic versus real macrophyte s, indicating that some chemically mediated avoidance of real macrophytes b y Daphnia occurred. In the presence of fish chemical cues from either roach or perch, the differences between real and plastic macrophytes quickly dis appeared as daphnids moved into macrophytes. However, this behavior decreas ed mortality only from roach predation, because perch spent significantly m ore time than roach foraging among macrophytes. Thus, although daphnids sou ght macrophyte refuge in the presence of both fishes, the effectiveness of the refuge depended on macrophyte density and predator identity. These resu lts support the idea that macrophytes can increase survival of Daphnia popu lations in shallow lakes, depending on the fish species present.