Growth and reproduction of migrating and non-migrating Daphnia species under simulated food and temperature conditions of diurnal vertical migration.

(1) The growth response of two Daphnia species coexisting in Lake Constance to constant and fluctuating conditions of temperature and food was tested in a flowthrough system. (2) In the lake D. hyalina exhibits a pronounced diurnal pattern of vertical migration, whereas D. galeata stays near the surface. The experiments were designed to measure growth and reproductive success of the species under the conditions as they are experienced by their counterparts in the field. (3) Both species grow better and produce more offspring under "non-migration" conditions (constant high temperature and high concentration of food). Thus there is no metabolic advantage by vertical migration. (4) D. hyalina is more successful than D. galeata under extreme "migration" conditions (high food level/high temperature at night and very low food/low temperature during day), but D. galeata grows slightly better under favourable conditions. (5) The results do not support the hypothesis that daphnids gain some metabolic advantage from vertical migration.