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

 
TRADING FOOD FOR TEMPERATURE: COMPROMISSES IN THE VERTICAL DISTRIBUTION OF DAPHNIA
 
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