Effects of predation and food on the population dynamics of the raptorial cladoceran Leptodora kindtii

We assessed the trophic status of Leptodora kindtii in the food web of a shallow, eutrophic lake in which 0+ age group fish were the main predators. The mean biomass of 0+ fish during three successive years varied from 0.39 g dry wt m(-2) in the first year to 0.05 g dry wt m(-2)in the second year to 2.49 g dry wt m(-2) in the third year. In the years with high fish biomass, densities of small-bodied (<1-mm) cladocerans (e.g., Bosmina spp., Chydorus sphaericus) were relatively high, whereas in the year with low fish biomass, densities of large-bodied Daphnia galeata were high, and densities of small-bodied cladocerans were lower. During the three study years, the predation pressure of juvenile fish and biomass and production of Leptodora were negatively correlated. Despite the low 0+ fish biomass in the second year, the Leptodora population densities were high only during the first part of the growing season. The elevated Leptodora mortality in July coincided with the lowest observed densities of small-bodied cladocerans, preferred prey items of Leptodora, and with an abundance of large-bodied Daphnia, the preferred food for 0+ fish. We conclude that the population dynamics of Leptodora during the growing season is predominantly regulated by direct predation effects.