DENSITY DEPENDENT INTERACTIONS IN LAKE ECOSYSTEMS - WHOLE LAKE PERTURBATION EXPERIMENTS

Density dependent interactions between higher and lower trophic levels were studied in two consecutive whole lake experiments in a highly productive lake. In the first experiment, the zooplanktivorous fish species, roach (Rutilus rutilus) (primary carnivore), was reduced to 27% of its previous population size (53%) of biomass) by a selective rotenone treatment. In the second experiment, zander (Stizostedion lucioperca) (secondary carnivore), were stocked in the lake resulting in a reduction in the population size of roach to 51% of its previous population size (31% of biomass). In both experiments, seasonal average biomass of Daphnia cucullata) increased in the two years with the lowest roach biomasses. but no shift to larger zooplankton forms took place. In the years with increased Daphnia biomass, phytoplankton biomass and transparency showed an increased seasonal variation (measured as coefficient of variation). In contrast to manipulations in which planktivorous fish were totally removed, no effect on seasonal average phytoplankton biomass was observed. Due to the high reproductive capacity of roach, the system returned to previous conditions within a year or two after the perturbations. Under unperturbed conditions, the lake showed few indications of instability as suggested by the paradox of enrichment hypothesis. This lack of instability can. among other things, be related to high zooplanktivore predation pressure present in highly productive lakes preventing overexploitation of primary producers by grazers. In addition, the omnivorous feeding characteristics of roach (feeding on both zooplankton and algae/detritus) and interactions between the open water and the detritus nutrient pool are likely to increase the stability of the system. The two perturbation experiments plus a previous natural experiment (winter fish kill), which all were pulsed perturbations, provide no evidence for the presence of alternative stable states in highly productive lakes. Possibly, a sustained perturbation over several years may shift highly productive lakes from a dominance of phytoplankton production to a dominance of macrophyte production leading to an alternative state.