Species- and Age-Specific Differences in Bacterial Resource Utilization by Two Co-Occurring Cladocerans

The trophic interaction between zooplankton and bacteria was examined to determine how this food web linkage varied with resource abundance and consumer species and age. Life table experiments and feeding-rate measurements were conducted with two species of co-occurring cladocerans, Daphnia parvula and Ceriodaphnia lacustris. Cohorts of these cladocerans were followed from birth to death and fed either no food (particle-free lake water), natural bacteria only (I -m filtrate of lake water), whole lake water, or lake water enriched with 104 cells/mL of the green algae Chlamydomonas reinhardi. By comparing cohorts grown in lake water to those grown in enriched lake water, we also determined the extent of food limitation in the natural populations of these two potentially competitive species. As bacterial densities increased from winter through summer, the ability of the larger species, Daphnia parvula, to grow and reproduce on the bacterial fraction also increased. Bacteria, however, were never a primary resource for Daphnia. The rate of population increase (r) was always negative even at the highest in situ concentrations of bacteria (1-2 x 10' cells/mL). The smaller species, Ceriodaphnia lacustris, was able to increase (r = 0.070) on bacteria alone at lower bacterial concentrations (5 x 106 cells/mL). Ceriodaphnia juveniles had the highest age-specific survival rates (lx) on bacteria. The improved survivorship was not due to higher feeding rates on bacteria by Ceriodaphnia. Neither age nor species when adjusted for the effect of body mass accounted for a significant portion of the variance in filtering rates on bacteria. Natural field food concentrations were always suboptimal, based on comparisons of growth rates, reproduction, and r values for the cohorts grown in lake water vs. enriched lake water. Rates of increase (r), however, were always substantially greater than zero in the lake water treatment for both species, indicating that food is not the primary factor that causes seasonal declines of the cladoceran populations in this lake. These results demonstrate a differential utilization efficiency of the smallest resource size-classes by these two zooplankton species. The experiments also indicate the dynamic nature of the trophic interaction between zooplankton and bacteria. The strength of this food web pathway varies with both the seasonal abundance of bacteria and the age, size, and type of zooplankton consumer.