Nutrients in Salmonid Ecosystems: Sustaining Production and Biodiversity

<em>Abstract.</em>—Evidence for the importance of marine-derived nutrient (MDN) inputs from spawning salmon to terrestrial and freshwater ecosystems is rapidly accumulating, but the mechanisms by which MDN inputs are transferred and stored within spawning streams and their catchments are poorly understood. Presumed marine isotope signals have been found in riparian vegetation, suggesting that marine nutrients may impact terrestrial plant communities. Studies have suggested that MDN increases stream productivity both immediately after spawning and during the following spring. The peak of many spawning runs occurs at the end of the summer growing season, suggesting that overwinter storage of MDN must be occurring. A potential location for lateral nutrient transfers and overwinter MDN storage is the hyporheic zone within stream channels or in adjacent riparian floodplains. Within Lynx Creek, a sockeyespawning stream in the Wood River Lake drainage in southwestern Alaska, extensive floodplain hyporheic zones occur along spawning reaches. Surface water moves into the floodplain hyporheic zone and flows downstream 70–80 m before returning to the stream. Ambient nutrient concentrations within the surface and hyporheic zone indicate that marine-derived nitrogen and phosphorus entered hyporheic flow paths, where they were rapidly removed from the water during the spawning run. Some marine-derived ammonium was remobilized as nitrate and continued to move, but marine-derived phosphorus was stored for at least the duration of the spawning run. Hyporheic sediments are not scoured by winter floods and contain active heterotrophic biological communities that are capable of storing and transforming various forms of MDN. Thus, it seems likely that hyporheic storage and re-release of marinederived nutrients is an important mechanism by which salmon-derived nutrients are retained over winter within stream ecosystems and subsequently made available to primary producers the following growing season.