Effects of trout and sculpins on benthic macroinvertebrates, periphyton, and leaf litter decay in Valley Creek, Minnesota

I used three approaches to investigate top-down control by fish in stream food webs. First, I excluded fish from leaf packs in a stream to ascertain whether fish can cause a detritus-based trophic cascade. Fish reduced the densities of total invertebrates and shredders inhabiting leaf packs and that in turn affected leaf breakdown rates. However, fish effects on leaf breakdown were not always consistent with trophic cascade predictions due to differential responses of shredder taxa. I also manipulated the density of slimy sculpin (Cottus cognatus) to determine whether benthic fish can cause an algal-based trophic cascade. Slimy sculpin had inconsistent effects on invertebrates. I did not detect atrophic cascade because Glossosoma larvae, which regulated periphyton biomass independently of other grazers, were not susceptible to predation and did not alter their behavior in response to fish. Therefore, top-down control by fish was stronger in detritus-based than algal-based food webs. Second, I developed a mathematical model to predict optimal drift strategies for prey faced with the conflicting demands of feeding efficiently and avoiding both benthic and drift predators. This model provided a framework to evaluate the effects of food availability and predation risks on the drift propensity of stream invertebrates. The model indicated that prey should drift from a site when food availability was low, benthic predation risk was high, and drift predation risk was low. Drifting should be affected by the likelihood of encountering an acceptable site and the rate of settling from the drift. Prey with high settling rates spent less time drifting and were less susceptible to drift predators. Finally, I tested the strength of interspecific competition and predator facilitation between benthic-feeding slimy sculpin and drift-feeding brown trout (Salmo trutta) in stream enclosures. Brown trout growth was unaffected by slimy sculpin, whereas slimy sculpin growth was reduced when brown trout were present. Gammarus pseudolimnaeus densities were lowest when brown trout were present, indicating that exploitative competition may govern this interaction. In conclusion, top-down control by fish can be an important factor regulating stream food webs; however, the strength of top-down control appeared to be context dependent.