Littoral zone structures as Daphnia refugia against fish predators

The assumption that macrophytes can provide zooplankton a daytime refuge against fish predation is central to the diel horizontal migration hypothesis. However, previous observations and experiments have shown that largebodied zooplankton avoid macrophytes. To directly test these contrasting roles of macrophytes, we measured the reaction of Daphnia pulex to macrophytes (Elodea canadensis or plastic) in the presence and absence of chemical cues from two commonly occurring European fishes, roach (Rutilus rutilus) and perch (Perca fluvialitis). In the same series of laboratory experiments, we then tested the ability of different densities of E. canadensis and plastic macrophytes to reduce predation by roach or perch. In the absence of fish, only 27% of daphnids occurred in the macrophytes. However, 70–80% of daphnids occurred in the macrophytes when either roach or perch occurred. In addition, significantly more daphnids occurred in plastic versus real macrophytes, indicating that some chemically mediated avoidance of real macrophytes by Daphnia occurred. In the presence of fish chemical cues from either roach or perch, the differences between real and plastic macrophytes quickly disappeared as daphnids moved into macrophytes. However, this behavior decreased mortality only from roach predation, because perch spent significantly more time than roach foraging among macrophytes. Thus, although daphnids sought macrophyte refuge in the presence of both fishes, the effectiveness of the refuge depended on macrophyte density and predator identity. These results support the idea that macrophytes can increase survival of Daphnia populations in shallow lakes, depending on the fish species present. Daphnia, the large-bodied zooplankters whose herbivory on phytoplankton is so important in trophic cascades (Carpenter et al. 1985; Jeppesen 1998), are typically thought of as purely pelagic. Consistent with this notion, ‘‘shore avoidance’’ (sensu Hutchinson 1967) by Daphnia has been documented (Boikova 1986; Lauridsen et al. 1999). Also, Daphnia often use deep waters to migrate vertically and to hide from predators during the day in stratified lakes (Lampert 1993). However, this predator-avoidance strategy of diel vertical migration is likely less advantageous in shallow lakes, where light may penetrate to the lake bottom and where 1 Present address: The Ohio State University, School of Natural Resources, 2021 Coffey Road, Columbus, Ohio 43210. Acknowledgments We thank the staff of Freimann Animal Care Facility at UND and the Lake Group technicians at NERI for laboratory space. At UND, Trevor Oren and Abha Saddawi helped execute laboratory experiments. In Denmark, Lene Jacobsen and John Glargaard Rasmussen helped with fish collection and care. The Department of Biological Sciences at UND, U.S. NOAA Sea Grants (NA46RG0419-2 and 643-1532-04 to D.M.L.) and U.S. NSF Graduate Research Training Grant (945-2655) supported R.B. A Fulbright Fellowship for Study (to R.B.) in Denmark (98-99) provided for research in Denmark. We also acknowledge support from the Danish Natural Sciences Research Council (960-1711) and the Research Program (‘‘The Role of Fish in Aquatic Ecosystems’’ 1999– 2001) from the Danish Ministry of Agriculture, Fisheries and Foods. thermal stratification may not exist. In these systems, Daphnia sometimes use diel horizontal migration (DHM) to seek daytime refuge from predation among structure in littoral zones (Timms and Moss 1984; Lauridsen and Buenk 1996; Stansfield et al. 1997; Lauridsen et al. 1998; Moss et al. 1998). Understanding this interaction between macrophytes and Daphnia is critical to better management—including biomanipulation—in shallow (unstratified) lakes (Perrow et al. 1997; Jeppesen 1998), which are the most common lakes in the world (Wetzel 1990). Shallow lakes differ from the better-studied deep lakes in ways that may influence the Daphnia–littoral zone relationship (Moss et al. 1997). Biomass of fishes per unit volume is higher in shallow lakes than in deep lakes, and high densities of fishes can sustain predation pressure on zooplankton by relying on alternative benthic food sources (Blumenshine et al. 1997; Jeppesen et al. 1998). Thus, predation pressure on daphnids is potentially higher during the day in shallow versus deep lakes (Jeppesen 1998; Jeppesen et al. 1998), therefore increasing the importance of daytime refuge for Daphnia. However, the occurrence of DHM by large-bodied zooplankton apparently contradicts documented avoidance of macrophytes by Daphnia (Hasler and Jones 1949; Pennak 1973; Dorgelo and Heykoop 1985). This paradox has been only partially addressed in previous work. Daphnid responses to both fishes and macrophytes are partially chemically