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Seasonal Succession in a Natural Daphnia Assemblage

DeMott, William R.
Ecological monographs 1983 v.53 no.3 pp. 321-340
Chlamydomonas, Daphnia longispina, algae, autumn, birth rate, digestive system, eggs, field experimentation, fish, grazing trials, intraspecific competition, lakes, males, mortality, parthenogenesis, phytoplankton, population growth, population structure, predation, reproductive success, spring, summer, Vermont
This study uses descriptive data and field experiments to examine a seasonal succession between two planktonic grazers (Daphnia rosea and Daphnia pulicaria) in a small Vermont lake. Each year Daphnia populations hatched from resting eggs during May and both species produced numerous parthenogenetic generations before the appearance of males and ephippia in late September. During the 2 yr of study, D. rosea had an apparent advantage in spring and early summer, but was rapidly replaced by D. pulicaria during late summer and autumn. These shifts in the species balance were associated with reversals in reproductive success. Moreover, the species with the higher birth rate also tended to have a lower mortality rate. An analysis of population structure, reproduction, and mortality suggests that inter— and intraspecific competition, not fish predation, limited population growth. Field experiments in both small (3.8—L) and large (12 000—L) enclosures demonstrate that the demise of D. rosea during late summer was a direct consequence of competition with D. pulicaria. However, the outcome of field experiments was sensitive to seasonally changing conditions. During spring and early summer, when D. rosea had a reproductive advantage at low densities in the lake, it was also the stronger competitor at high densities within experimental enclosures. The shift in species advantage during summer seems associated with a changing resource base. In spring the food for grazers was composed mostly of highly edible flagellates, while "resistant" algae (mostly gelatinous greens) were predominant during late summer and autumn. Grazing experiments and observations on gut contents suggest how D. pulicaria was favored by this change in food conditions. Grazing experiments with a radioactively labeled flagellate (Chlamydomonas) show that the two Daphnia species had very similar feeding rates on this alga. In addition, direct observations on the composition of gut contents demonstrate that they ingested virtually identical proportions of common phytoplankton species. However, experiments with labeled natural algae and observations on the quantity of ingested algae indicate that D. pulicaria was more effective in utilizing "resistant"algae.