Avoidance by Daphnia magna of fish and macrophytes: chemical cues and predator-mediated use of macrophyte habitat

Recent biomanipulation studies suggest that macrophytes are an important refuge from fish predation for large pelagic zooplankton. We conducted two laboratory experiments that tested the behavioral responses of Daphnia magna to a macrophyte (Myriophyllum exalbescens L.) and a sunfish (Lepomis cyanellus Rafinesque) and whether responses were chemically (for fish) or structurally (for macrophytes) mediated. In the first experiment, we measured Daphnia response to four treatments in separate 38-liter tanks. In controls without macrophytes and fish, ∼15% of the daphnids were found in the central zone (∼50% of the tank area); the others were found around the tank perimeter (especially in the corners). With macrophytes present, 80% of the daphnids were found in the central zone (unvegetated in all treatments). When fish or fish odor alone were present 35% and 45%, respectively, of the Daphnia occupied the central zone. Thus, chemically mediated avoidance of Lepomis caused Daphnia to increase its occupation of macrophytes. In the second experiment, we tested whether the repellent effect of Myriophyllum resulted from structural characteristics of the macrophyte; the results suggest that both chemical and structural cues contributed to Daphnia avoidance of the macrophyte. Overall, our results are consistent with the suggestion that large pelagic zooplankton may use macrophytes as a refuge in shallow lakes where vertical migration is restricted.