Carcass Additions Influence Food Webs Through Bottom-Up and Direct Consumption Pathways Along a Fish Species Assemblage Gradient

The loss of subsidies delivered by anadromous fish to inland stream ecosystems may have profound influences on stream food webs. However, studies have focused on food web responses in ecosystems where the fish assemblage is dominated by salmonids. We evaluated food web responses to carcass additions in three locations of an interior Columbia River Basin stream with varying native fish assemblages. Periphyton biomass responses were mixed with increases in treatment compared to control reaches of the middle and downstream pairs, but not in the upstream pair where scavenging by bears removed the majority of carcasses. Stable isotope analysis revealed marine-derived nutrient (MDN) enrichment of periphyton and invertebrate functional feeding groups in the middle and downstream pairs (up to 12% MDN derived), but not in the upstream pair. Non-salmonid fish exhibited limited MDN assimilation (~ 5 and 10% MDN derived), with isotopic enrichment patterns similar to lower trophic levels and little evidence of eggs and carcass material consumption, suggesting bottom-up transfer of MDN to non-salmonid fishes. In contrast, across the three study pairs, juvenile Chinook Salmon ( Oncorhynchus tshawytscha ) and steelhead trout ( O. mykiss ) assimilated MDN rapidly, obtaining up to 25% and 57% of their nitrogen from carcasses, respectively. Diet analysis and isotopic enrichment patterns indicated that juvenile salmonid assimilation occurred primarily through direct consumption of eggs and carcass tissue. Our findings suggest that salmonids in this region may experience more benefit from carcass additions containing eggs, while non-salmonids are likely to experience minimal MDN incorporation whether or not eggs are present.