Lampert, W. Max Planck Institute of Limnology, +49 4522 763 310

Predator avoidance is considered to be the ultimate cause of diel vertical migration of zooplankton. In the absence of fish signal, however, zooplankton may select a depth were temperature and food conditions are optimal. In a lake with a metalimnetic or hypolimnetic chlorophyll maximum, zooplankton are faced with a dilemma: either they experience high temperature but low food availability or vice versa. We used the Ploen Plankton Tower system to test the hypothesis that the depth distribution of Daphnia reflects a trade-off between food availability and temperature. We stratified the towers thermally and created a hypolimnetic algal maximum. The epilimnion was always kept at 20oC, whereas the hypolimnion temperature varied between 10 and 18oC. We sampled Daphnia at 9 depths between the surface and 6 m to determine their depth distribution. We also measured individual sizes and counted eggs. The proportion of daphniids found below the thermocline is negatively related to the temperature difference between epi- and hypolimnion. Only about 15% of the population resided in the food-rich but cool hypolimnetic waters at a 10oC difference, but about 50% were found there at a 2oC difference. Large and egg-carrying Daphnia showed a stronger reaction to temperature differences than juveniles.
Day: Thursday, Feb. 4
Time: 11:00 - 11:15am
Location: Eldorado Hotel
Code: SS51TH1100E