COMPETITION BETWEEN A PLANKTIVORE, A BENTHIVORE, AND A SPECIES WITH ONTOGENIC DIET SHIFTS

Along a gradient of increasing productivity in lakes, the abundance of perch (Perca fluviatilis) first increases and then decreases, whereas the abundance of ruffe (Gymnocephalus cernuus) and that of roach (Rut ilus rutilus) continue to increase. Perch, unlike roach and ruffe, und ergoes dramatic ontogenetic diet shifts. The decrease in the abundance of perch along the productivity gradient may be due to competition fr om roach at its plankton-feeding stage and from ruffe at its benthic-m acroinvertebrate-feeding stage. Previous studies of these three specie s have focused on the effects of roach on the other species. To furthe r examine the competitive relationships among these species, we studie d the effect of increasing the density of ruffe, while maintaining a r elatively high, constant density of roach, on the diet and growth of p erch. Food resources were also monitored during this 2-mo experiment, which was conducted in 10 enclosures in a pond in southern Sweden. Bas ed on prior experiments we expected roach, an efficient planktivore, t o reduce the biomass of zooplankton in all enclosures. Moreover, we ex pected that the increased density of benthic-feeding ruffe would force perch to increase its consumption of zooplankton, resulting in reduce d growth. We found, as predicted, that the biomass of cladocerans and copepods decreased over time but was not affected by ruffe density. Th e biomass of the commonly consumed macroinvertebrates Sialis (a megalo pteran), trichopterans, and ephemeropterans was lower in enclosures wi th ruffe than in enclosures without ruffe. Moreover, the biomass of Si alis decreased and that of chironomids and the clam Pisidium increased over time. High ruffe densities were related to an increased proporti on of zooplankton in the diet of perch in August, but not in September . The diet of ruffe changed little along the ruffe gradient. Only the proportion of trichopterans (in August) and Pisidium (in September) in the diet of ruffe increased along the ruffe gradient. The growth of b oth perch and ruffe decreased with increasing ruffe density, whereas t he growth of roach was constant. These data suggest that one reason pe rch decrease in abundance along a productivity gradient may be because they are competitively sandwiched between planktivore and benthivore specialists.