Growth and survival of Daphnia in epilimnetic and metalimnetic water from oligotrophic lakes:: the effects of food and temperature

1. In oligotrophic lakes, phytoplankton and bacteria growing in the deep chlorophyll maximum in the cool metalimnion of lakes often dominate biomass and production, but the importance of this source of food for zooplankton is unknown. 2. During much of the day, Daphnia rosea in two mountain lakes inhabited deep chlorophyll layers where food availability was at least equal to that in the epilimnion. 3. To determine the importance of the two strata (epilimnion and metalimnion) for Daphnia, we used a cross-classified factorial experiment to measure how epilimnetic and metalimnetic food and temperature (10 and 16 degreesC) influenced survival, growth and reproduction. 4. Daphnia survived and grew better when fed seston from the epilimnion of one lake, although chlorophyll, particulate nitrogen and particulate carbon were 2-2.5 times greater in the metalimnion. 5. Temperature had no significant influence on Daphnia survival or growth. Similar results were obtained with food from the second lake, with Daphnia surviving and reproducing better when provided with epilimnetic, rather than metalimnetic food, although the quantities of chlorophyll and carbon in the two strata were similar. 6. Food quality, rather than quantity or temperature, appeared to be the most important determinant influencing survival, growth and reproduction, and the greater food quantity in the metalimnia was not used effectively by the Daphnia.