An extremely inefficient food web in the tailwater of a large dam

Large dams drive substantial discontinuities in river structure and function. In the arid Southwestern US, dams release clear, cold water that supports elevated primary and secondary production of largely non-native assemblages. Although these assemblages can be extremely productive, mismatches between consumers and resources may lead to unprecedented ecological inefficiencies. We quantified gut contents and secondary production of invertebrate and fish consumers to construct seasonal flow food webs for the tailwater reach of Glen Canyon, Colorado River. We found that a relatively small proportion of total invertebrate production was consumed by top predators (i.e., Rainbow Trout), and an extremely low proportion of primary production was consumed by primary consumers. Much of this inefficiency may be explained by taxa that dominate production but provide little nutrition for consumers – namely filamentous green algae (Cladophora glomerata) and New Zealand mud snails (Potomopyrgus antipodarum). A large amount of surplus productivity is exported to downstream segments, where its fate is yet unknown. Large dams, such as Glen Canyon, drastically alter river structure and function and may lead to disassembled communities, food web inefficiencies, and large serial subsidies of excess production.