Many planktonic organisms in both freshwater and marine environments show a daily pattern of vertical migration (DVM).  Daphnia undergo such daily movements, migrating deeper in the water column by day and rising toward the surface at night.  The proximate and ultimate causes of DVM are incompletely understood.  In particular, how do fish chemical cues, representing predation risk, interact with different light levels to modify DVM behavior?  To investigate effects of the light environment, we compared DVM patterns during July 2007 at two sites, 72km apart on a steep longitudinal gradient in an Ozark reservoir (Bull Shoals Lake, Missouri–Arkansas).  

Results/Conclusions

The up-lake site, in the riverine-transition zone, had lower transparency (3.5m Secchi depth) than the down-lake, lacustrine site (6.9m Secchi depth).  Consistent with our expectation, Daphnia retrocurva showed a more pronounced DVM at the down-lake site (amplitude ≥8m) than at the uplake site (4m).  Nevertheless, multiple factors may operate simultaneously in the field (e.g., different fish assemblages).  To investigate the role of fish cues on Daphnia DVM, we are running experiments in laboratory cylinders.  The apparatus consists of four acrylic tubes (100cm x 4.5cm), housed in a dark cabinet, with controlled temperature and light.  By passing ice-water through condenser coils, we can develop a steep thermocline (6-20°C) in the tubes.  We are evaluating fish cues released by different feeding guilds: Lepomis macrochirus (bluegill, planktivore), fathead minnow (Pimephales promelas, planktivore), and Lepisosteus osseus (longnose gar, piscivore), all compared to control tubes filled with aged lake water.  We hypothesize that Daphnia will show the greatest DVM magnitude when exposed to plankivores, less to piscivores, and least to controls.