Dynamics of a stable, large volume, laboratory ecosystem containing Daphnia and phytoplankton

Daphnia magna and phytoplankton were maintained in stable, large-volume (3400 l), indoor, planktonic model ecosystems for up to 20 weeks. Ecosystem stability was demonstrated by damped oscillations in population abundance, and by a return to steady-state conditions if the system was perturbed. The steady-state biomass of daphnia and algae responded to variations in nutrient loading rates and daphnia harvesting rates in accordance with predictions from a mathematical model. Increased nutrient addition resulted in an increase in biomass of daphnia, but not phytoplankton, if both were present. Increased harvesting caused an increase in chlorophyll and decreased daphnia biomass. Phosphorus additions were balanced by sedimentation losses which were proportional to daphnia biomass. Increased harvesting rates resulted in decreased phosphorus loss rates and increased efficiency of phosphorus conversion into harvested daphnia. Damped oscillations had a periodicity of 3.5 weeks and could be successfully modelled assuming a half-saturation constant for phosphorus control of algal growth of 1 μg l−1. The ecosystem became unstable if nutrient loading rates exceeded a defined limit which was a function of harvesting rates. Steady-state chlorophyll and daphnia biomass levels were similar to levels approached in Lake Michigan in summer in 1983 and 1984, when the zooplankton biomass was dominated by Daphnia pulicaria.