Disturbance and predation risk mediate trophic interactions in an aquatic food web

Background/Question/Methods

The ability of organisms to affect their environment is mediated by both biotic and abiotic conditions. In many streams, predation and stream drying may interact to influence the effects of consumers on benthic communities. Using 190-liter stream mesocosms, we determined whether drying (full/reduced water levels) and predation risk (smallmouth bass present/absent) interacted to mediate the effects of the omnivorous crayfish Orconectes eupunctus and O. neglectus on algal chlorophyll and Chironomidae density. Each mesocosm had shallow and deep areas and received 4 of each crayfish species for a total density of 12 crayfish*m^-2. We hypothesized that in the absence of bass a trophic cascade would result from crayfish indirectly facilitating algal growth by reducing chironomid densities and that crayfish effects would be greater in reduced tanks because of decreased surface area. In tanks with bass, crayfish were expected to reduce foraging to minimize predation risk, and chironomids would depress algal biomass. 
Results/Conclusions

We found an interaction between bass and drying for chironomid density. Bass presence increased chironomid density in full tanks but not in reduced tanks. However, chlorophyll abundance was not affected by drying, bass presence, or their interaction. In the absence of bass crayfish did not cause a trophic cascade despite a 35x reduction in chironomid densities. Crayfish are omnivorous and likely diluted the effects of the trophic cascade by feeding on algae as well as chironomids. Crayfish had strong effects on chironomids in both full and reduced tanks in the absence of bass, but drying and predation risk strongly interacted, probably because drying reduced the ability of bass to prey on crayfish. This experiment demonstrates that disturbance and predation may interact to modify the effects of organisms on stream structure and function.