The coexistence of three species of daphnia in the Klostersee : I. Field studies on the dynamics of reproduction.

1. In the Klostersee in southern Bavaria three species of Daphnia have coexisted for at least several decades: Daphnia hyalina (H), D. galeata (G) and D. cucullata (C). These three species differ in size and compete for food. 2. All three species show marked seasonal fluctuation of abundance, with H and G dominating during spring and late autumn and C dominating during summer. The fluctuations of H and G are stronger correlated than H and C or G and C. 3. The three species are vertically segregated. C preferes the upper strata, H the lower and G the intermediate. This behaviour favours the smaller species C. On the average (from early spring to late autumn), C develops 1.2 times faster than G and 1.5 times faster than H. 4. H and G show higher clutch sizes than C throughout the year. This advantage however is compensated by the higher speed of development of C during thermally stratified seasons. 5. The egg ratio is influenced by both the age structure of the populations and the cluth size of the gravid females. 6. The clutch size depends mainly on the size of the females. The reaction of clutch size to environmental factors follows with a time lag of approximately D (duration of embryonic development). C and G depend more on meteorological influences than H. which is probably related to the vertical distribution. 7. The importance of the differential vertical distribution and the reaction to environmental stress as well as the impact on the size-efficiency hypothesis are discussed.