Document: RIC-3-73-10

Nutrient dynamics in SW Alaskan Sockeye streams: What role do fish play?

EDWARDS, R.* and T.O'KEEFE

University of Washington, Seattle, WA 98195 ¹

Abstract:
Spawning salmon bring large amounts of nutrients to natal streams, fertilizing them and increasing the production of algae, invertebrates and fish. Research using stable isotopes has suggested that even riparian forest growth is enhanced by salmon carcasses, although the nutrient transfer mechanisms are poorly understood. We describe a study in the Wood River Lakes region in southwest Alaska designed to determine the extent of hyporheic transfer of marine-derived nutrients to the riparian zone of Lynx Creek, a sockeye spawning stream. A piezometer grid was installed within a floodplain meander bend. Conservative tracer injections and constituent maps were used to determine the extent of hyporheic flows and detect salmon-derived nutrients within the subsurface. Hyporheic flows, with 2 day residence times, were detected beneath the entire floodplain meander. Neither nitrogen nor phosphorus increased within surface or subsurface waters during or after spawning. Within nearby watersheds, pre and post-fish run nitrogen concentrations were related to proximity to upland alder stands, not to presence or absence of a sockeye spawning run. Flux budgets revealed that salmon provide less than 4% of annual stream nitrogen flux, making it unlikely that marine nitrogen within the stream is a significant influence on stream or riparian primary production. However, salmon phosphorus inputs were about 123% of the phosphorus flux. Because nitrogen and phosphorus are decoupled during inorganic nutrient transformations, stable nitrogen isotopes are a poor tracer of marine phosphorus pathways at this site, and may significantly underestimate the importance of marine phosphorus in these rivers.

Keywords: hyporheic, marine nutrients

This abstract is being presented at: 10:15 AM in session:
Oral Session #70: Aquatic Ecology.