SEASONAL SUCCESSION AND THE STRENGTH OF INTRASPECIFIC AND INTERSPECIFIC COMPETITION IN A DAPHNIA ASSEMBLAGE

Species of the genus Daphnia are generalized filter-feeders and potent ially strong competitors, yet several species often coexist in the ope n waters of lakes. We investigated the population dynamics and competi tive interactions of two Daphnia species in Gull Lake, Michigan. In ea ch of 3 yr D. pulicaria was always common, while D. galeata mendotae b ecame 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 predict ed 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 enclosure s to manipulate the density of each species and quantify the intensity of intra- and interspecific competition. Repeated competition experim ents demonstrated that the relative exploitative ability of the specie s reversed from early to late summer. Strong interspecific competition was often observed, but its strength varied with juvenile or adult st age, and with season. Daphnia pulicaria juveniles were always affected strongly by interspecific effects: while D. galeata mendotae juvenile s were impacted largely by intraspecific effects. The relative strengt h of intra- and interspecific competition on adults reversed from earl y to late summer for each species. The magnitude of intra- and intersp ecific effects on D. pulicaria adults was similar during early summer, but by late summer interspecific effects were much stronger than intr aspecific effects. The reverse was true for D. galeata mendotae adults ; interspecific effects greatly decreased in late summer. This seasona l shift in the nature of competitive interactions provides an explanat ion for seasonal succession in this Daphnia assemblage.