Seasonal Succession and the Strength of Intra‐ and Interspecific Competition in a Daphnia Assemblage

Species of the genus Daphnia are generalized filter—feeders and potentially strong competitors, yet several species often coexist in the open waters of lakes. We investigated the population dynamics and competitive interactions of two Daphnia species in Gull Lake, Michigan. In each of 3 yr D. pulicaria was always common, while D. galeata mendotae became abundant only in late summer. The summer invasion of D. galeata mendotae was associated with a decline in abundance of D. pulicaria in some, but not all years. Decline of D. pulicaria was strongly predicted by the loss of oxygen from deep water during summer, which resulted in greater spatial overlap between the species, and by changes in the quality of the phytoplankton resources. We employed in situ enclosures to manipulate the density of each species and quantify the intensity of intra— and interspecific competition. Repeated competition experiments demonstrated that the relative exploitative ability of species reversed from early to late summer. Strong interspecific competition was often observed, but its strength varied with juvenile or adult stage, and with season. Daphnia pulicaria juveniles were always affected strongly by interspecific effects, while D. galeata mendotae juveniles were impacted largely by intraspecific effects. The relative strength of intra— and interspecific competition on adults reversed from early to late summer for each species. The magnitude of intra— and interspecific effects on D. pulicaria adults was similar during early summer, but by late summer interspecific effects were much stronger than intraspecific effects. The reverse was true for D. galeata mendotae adults; interspecific effects greatly decreased in late summer. This seasonal shift in the nature of competitive interactions provides an explanation for seasonal succession in this Daphnia assemblage.