Trophy-Balanced Turnover Velocities in a Two-Species System of Competing Daphnids: A Test of the “Energy Residence Time” Concept

The primary and secondary productivity of the pelagic zone of Lake Constance has been investigated since 1980. During the growing seasons (May to autumn) two Daphnia species were the dominant herbivores. The productivity of the two Daphnia populations was estimated from the dynamics of their population densities, their individual body weights, and their development times. The seasonal course of the population densities can be generalized as a logistic growth with a log-phase in spring and a subsequent food-limited equilibrium density throughout the summer. Body weights varied according to nutritional state (as measured by the average clutch size) /1/. Development times were estimated as function of body size and environmental temperature /2/, the latter of which was monitored in a detailed investigation of the specific vertical migration of the Zooplankton populations and simultaneous recording of the lake temperatures in 10-min-intervals within the upper 40 meters of the water column /3/. Daphnia galeata showed no migratory behaviour and, living in the warm epilimnic layers, had short development times. D. hyalina began to migrate in June when the population density reached equilibrium. Its development was prolonged, therefore, by the low hypolimnetic temperatures as experienced during the days.