Patterns of Cross-Ecosystem Food Subsidies to Rearing Salmonids within a Large Glacial Alaskan River

Riverine landscapes consist of a mosaic of habitats that receive food subsidies from terrestrial, riverine, and marine environments. The contributions of these resources to rearing salmon can shift over time and space, altering the energy pathways that limit fish production. Despite the need for understanding drivers of salmonid production, most riverine food web research has focused on small spatial scales and does not account for the broader heterogeneous nature of watersheds. Our study aimed to determine large-scale patterns in contributions of terrestrial, riverine, and marine food subsidies to juvenile salmonids in the large, glacially influenced Susitna River, Alaska. We quantified trophic patterns: (1) among macrohabitat types, (2) along an upstream to downstream continuum, and (3) across seasons, through stable isotope and gut content analysis. Stable isotope mixing models showed that the relative contribution of marine-derived nutrients to juvenile Chinook and Coho salmon increased from June to October, with availability of salmon eggs. Diet patterns differed by macrohabitat type, with aquatic and terrestrial invertebrates dominating in sloughs and side channels. Understanding broad patterns in the dynamics of foods used by juvenile salmonids can help inform management decisions about freshwater salmonids and the ecosystems from which their food supplies originate.