Does predation by the introduced rainbow trout cascade down to detritus and algae in a forested small stream in Patagonia?

Cascade effects of an exotic predator, the rainbow trout (Oncorhynchus mykiss), on periphyton and leaf litter were analysed in a headwater, forested stream of Andean Patagonia (Argentina). We conducted seasonal field sampling and two field experiments measuring leaf litter mass, periphyton biomass and macroinvertebrate biomass in relation to the presence and absence of rainbow trout. In the field survey, the presence of trout influenced resource mass: leaf litter (60% decrease in summer, P = 0.024) and periphyton (tenfold increase in chlorophyll a, P < 0.001) were affected, which were mediated by a decrease in the biomass of shredders (95% decrease in summer, P < 0.001) and scrapers (90% decrease, P < 0.001). There was an effect on leaf litter biomass only in the summer, whereas fish presence reduced periphyton biomass all year except in the winter. In the field experiments, we observed that leaf litter breakdown and periphyton development were effectively controlled by consumers in the absence of fish. In contrast, the presence of fish caused a release of herbivory and detritivory resulting in a significant increase in periphyton biomass (100% increase, P < 0.001) and a decrease in leaf litter decay (40% decrease, P < 0.001). Our results suggest that in low order streams and in the presence of visual predators, trophic cascades may operate both on detritus and algae, but with different timing.