Weak top-down control of grazers and periphyton by slimy sculpins in a coldwater stream

We conducted an enclosure/exclosure experiment during summer 1998 and 1999 to test the strength of top-down control by slimy sculpins (Cottus cognatus) on benthic macroinvertebrates and periphyton. We used 1-m(2) cages to manipulate slimy sculpin density within 6 riffles in Valley Creek, Minnesota. Each riffle contained an enclosure stocked with 3 slimy sculpins, a fishless exclosure, and an open control cage. In 1998, densities of total invertebrates and grazers on clay tiles were significantly higher in exclosures than enclosures and control cages; however, densities of individual grazer taxa (Glossosoma and Baetis) on tiles did not differ significantly among treatments. Fish had no significant effect on invertebrates inhabiting gravel substrate. In 1999, densities of total invertebrates and grazers on tiles and inhabiting gravel substrate did not differ significantly among treatments. Slimy sculpins did not have a detectable effect on the drift propensity of invertebrates during either year. Regardless of the effect of fish on grazers in either year, periphyton biomass did not differ significantly among the fish treatments even though there was a strong, negative correlation between Glossosoma density and periphyton biomass. Periphyton biomass was significantly higher on tiles with a barrier that excluded Glossosoma larvae from the surface, indicating that Glossosoma limited periphyton biomass. We conclude that the strength of top-down control by slimy sculpins on invertebrates was weak. We did not detect an algae-based trophic cascade during either year because Glossosoma larvae were relatively invulnerable to fish predation and did not alter their behavior in the presence of fish.