Trophic position and periphyton carbon sources for macroinvertebrates in the litter patches of forested streams, with special reference to predatory species

Abstract Most research on lotic food webs has focused on reach-scale webs, whereas research on the structure and system openness of lotic food webs has not focused on the microhabitat scale. In forested streams, plant litter from riparian vegetation accumulates as discrete patches, forming a microhabitat for macroinvertebrates. We primarily aimed to determine the relative trophic position and contribution of periphyton to the assimilated diet of macroinvertebrate species in pool litter patches via stable isotope analysis. We specifically focused on predatory species in the context of their indirect effects on litter breakdown and estimated the range of their absolute trophic positions. The mean δ 15 N values of the 19 macroinvertebrate predator species considered ranged from 0.0 ‰ to 5.3 ‰. This range was greater than the overall mean enrichment factor of δ 15 N (Δ 15 N = 1.8 ‰) estimated from aquatic invertebrate predators based on five existing data sources, indicating that the absolute trophic position of predator species can extend across more than one level. The estimated periphyton contributions to the diets of predator species were almost half or more, indicating that the food webs of pool litter patches are closely connected with external periphyton-based webs. Our findings suggest that the top-down effect of predatory macroinvertebrates in litter patches is likely to be dissipated by the shift to the grazing food chains instead of cascading down to the litter.