Prey and detritus subsidies in riverine ecosystems: a watershed perspective of marine, terrestrial, and headwater inputs for fish-bearing food webs in coastal Alaska

M.S. Wipfli. USGS Cooperative Research Unit, Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska, 99709

Prey and detritus subsidies can have dramatic effects on receiving food webs, influencing community structure, population dynamics, and productivity. In this paper I summarize data from invertebrate and detritus trap catches, and fatty acid, stable isotope, and diet analyses to assess the inputs and effects of prey (invertebrate) and detritus subsidies from marine, terrestrial, and headwater sources on salmonid-bearing food webs in coastal Alaska. Quantity and nutritional quality of subsidies varied widely depending on source and season. Aquatic habitats lower in watersheds received more marine subsidies and less headwater subsidies than those farther upstream, and vice versa. Terrestrial prey inputs from riparian habitats were generally the most abundant and seasonally consistent subsidies in streams. Biota in habitats receiving marine inputs from returning salmon showed strong marine isotope and marine fatty acid signals, and contained higher concentrations of whole-body lipids. Salmonid diet analyses indicated that fish consumed about equal amounts of freshwater and terrestrially-derived prey during most of the growing season, but ingested substantial amounts of marine resources (salmon eggs and decomposing salmon tissue) when these subsidies were present. Quantity, quality, and timing of resource subsidies all appear to be important driving forces in riverine food web dynamics and community nutrition.