Canopy cover influences food resources for stream grazers and filter feeders on a reach scale

Abstract

Food webs in ecosystems are mainly considered on a large spatial scale, but in forest streams, canopy cover in riparian forests might shape the food web structures on a reach scale. We hypothesized that the food resources for invertebrate consumers (e.g., grazers and filter-feeding caddisflies) shift along the river channel from open to closed canopy-covered reaches and vice versa on the riffle scale. In other words, the distance from a canopy-covered reach determines the food resources available to those consumers. To test the hypothesis, we analyzed the riffle scale variability in invertebrate consumers food resources in a canopy-covered stream using carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N, ‰) We measured the isotopes of grazer: Glossosoma spp. (ranged δ¹³C: − 13.7 to − 33.3, δ¹⁵N: 0.3 to 11.0) and filter feeder: Stenopsyche marmorata Navas, 1920 (δ¹³C: − 16.8 to − 30.7, δ¹⁵N: 0.7 to 11.0). The δ¹³C and δ¹⁵N of resources are varied: periphyton (δ¹³C: − 11.3 to − 32.5, δ¹⁵N: 0.1–6.6), SPOM (δ¹³C: − 13.8 to − 33.0, δ¹⁵N: 0.1 to 8.4), and BPOM (δ¹³C: − 14.0 to − 32.4, δ¹⁵N: 0.1 to 7.0). The results from the stable isotope-mixing models show that the periphytic contribution to the filter feeders was lower within and downstream of the canopy-covered riffle than in the open riffle. We found that canopy cover changed the basal food resources available to invertebrate consumers from periphyton to terrestrial matter on a riffle scale, particularly for the filter feeders. Thus, even within a reach, a loss of riparian forest could change the basal resource availability to the dominant consumers. Canopy cover affects the resources of the stream food web and can be significant even on a riffle scale. Thus, small-spatial level management of riparian forest can induce the resource heterogeneity to stream food webs.