| The quality of the world's freshwater resources continues to decline, despite reduced nutrient inputs. Shallow lakes are particularly vulnerable to eutrophication because they internally cycle nutrients more than deep lakes and can shift between turbid and clear states. Large, herbivorous zooplankton, such as Daphnia, are necessary to promote maintenance of clear water in shallow lakes. Due to high predation pressure, daphnids are believed to undergo horizontal migration to seek daytime refuge from pelagic predators among vegetation. While much is known about diel vertical migration (DVM) in deep lakes, relatively little information exists concerning horizontal migration in shallow lakes.; Using published literature, laboratory experiments and field experiments, this dissertation investigates the costs and benefits of diel horizontal migration (DHM) for daphnids. Primary literature suggested that predator-avoidance is the most likely cause for DHM and that abiotic factors are probably less influential for DHM relative to DVM. The role of resource availability is still unclear. However, macrophytes play a central role in DHM. Previous studies found that daphnids avoided macrophytes. In my experiments, chemical cues from macrophytes suppressed daphnid growth and resulted in longer maturation periods with less reproductive potential. However, when fish are present, daphnids actively seek refuge among macrophytes. Results from multiple experiments helped explain the apparent paradox between plant-avoidance and recent studies finding daphnids among macrophytes. In addition, daphnid mortality due to predation from some fishes significantly declined with increased macrophyte density. However, perch (Perca fluvialitis) foraged effectively on Daphnia despite complex structure.; Although macrophytes provide a refuge against fishes, aquatic macrophytes also harbor predacious invertebrates. In laboratory and field studies, larval odonates (dragonflies and damselflies) significantly reduced daphnid abundance, despite the presence of complex structure. Invertebrate predation may be one reason why DHM occurs in some lakes but not others. DHM is only likely to maintain daphnid populations when macrophyte density is high, enough piscivores, e.g. pike (Esox lucius), control planktivorous fishes, and littoral invertebrate predators are absent or occur at low densities. More studies will provide insight into ways of predicting when DHM should occur, and therefore serve as a possible tool in management. |
