Comparative Population Regulation of Two Planktonic Cladocera (Holopedium Gibberum and Daphnia Catawba)

The abundance of two planktonic grazers (Holopedium gibberum and Daphnia catawba) fluctuated seasonally in a nonparallel manner in Lake Lacawac, Pennsylvania. Holopedium gibberum was abundant in early and midsummer, while D. catawba was common only in spring and late summer. I sought to explain this seasonal succession by identifying seasonal factors which regulate each population. I monitored population abundance, individual energy reserves (lipid content), egg production, and body size distributions in plankton samples from April to October in three years. Food resources and vertebrate predator abundance were also examined seasonally. Results of experimental manipulations of food availability and predators, conducted seasonally in both small (6.4—L) and large (6000—L) enclosures, supported interpretations drawn from descriptive data. Contrasting seasonal abundance patterns of the species are explained by differences in the intensity of food stress and predation affecting each species. Food supply restricted population growth of H. gibberum at most times. In the spring of each year, starvation was a major cause of mortality; during summer, individual energy reserves and fecundity were near zero for H. gibberum. Predation losses were negligible for adult H. gibberum, but neonate (smallest age class) mortality by fish was occasionally large. Daphnia catawba experienced a seasonal change in the intensity of regulating factors; food supply restricted population growth only in spring and late summer, but predation controlled growth during summer. During late summer, predation on D. catawba was reduced and its population abundance increased sharply. Experimental removal of fish caused a comparable increase of D. catawba during midsummer. At peak animal densities in late summer, both species were food stressed Life table experiments conducted at this time suggest that competition for food occurs between D. catawba and H. gibberum, and results in a reduction of H. gibberum density.