PARENT SESSION
Oral Session #7: Aquatic Ecology: Stream ecosystems. Presiding: T. Ehlinger.
Monday, August 6, 2001. 8:00 AM to 12:00 PM. Hall of Ideas G.
Complex trophic interactions in a coldwater stream: fish, benthic macroinvertebrates, and periphyton.
Ruetz, Carl1,2, Newman, Raymond2, Vondracek, Bruce3,1,2, 1 2 3
ABSTRACT- We conducted an enclosure/exclosure experiment during summer 1998 and 1999 to assess the top-down effects of slimy sculpin (Cottus cognatus) on benthic macroinvertebrates and periphyton in Valley Creek, Minnesota. We used 1-m2 cages (6-mm mesh) to manipulate slimy sculpin density within six riffles. Each riffle contained an enclosure (stocked with three slimy sculpin), exclosure (fishless), and control (open on downstream sides of cage). In 1998, total invertebrate and grazer densities (Glossosoma and Baetis) on clay tiles were highest in exclosures, whereas densities of individual grazer taxa did not differ among fish treatments. In 1999, Glossosoma densities on tiles and in Hess samples were highest in enclosures, whereas densities of total invertebrates in Hess samples did not differ among fish treatments. Regardless of grazer responses in either year, periphyton biomass did not differ among fish treatments even though there was a negative correlation between Glossosoma density and periphyton biomass. Periphyton biomass was higher on tiles with a barrier that excluded Glossosoma from their surface. The effects of slimy sculpin on invertebrates were inconsistent between years and varied among taxa. Moreover, fish effects did not cascade down to primary producers even when Glossosoma regulated periphyton biomass and responded to predators.
KEY WORDS: Cottus cognatus, Glossosoma sp., streams, trophic cascade