Nitrogen transformations in Southeast Alaska streams are influenced by marine and terrestrial subsidies

Coastal streams in Southeast Alaska receive allochthonous inputs of nitrogen (N) from annual runs of Pacific salmon (Oncorhynchus spp.). In addition, terrestrial leaching through riparian zones dominated by N-fixing alder species (Alnus spp.), especially in harvested watersheds, can elevate inorganic N concentrations in streams. We observed increases in concentrations and export of dissolved nitrate (NO₃^-), during and after the salmon run in 2006 despite salmon not directly releasing NO₃^-. However, excretion of salmon-derived ammonium (NH₄⁺) may fuel nitrification, ultimately increasing in-stream NO₃^-. To determine the relative contribution of salmon- and alder-derived NO₃^-, we measured sediment nitrification rates in seven streams in Southeast Alaska, before and during the salmon run in 2007 and 2008. These streams spanned a timber harvest gradient (6-70% watershed area harvested) with varying areas of riparian alder. In 2007, sediment nitrification rates increased 3-fold in the presence of salmon (pre-salmon mean±SE=0.065±0.013, post-salmon=0.174±0.056 mgN gAFDM^-1 day^-1). Furthermore, post-salmon, stream NO₃^- concentrations were related to the area of riparian alder (r²=0.73), which likely influenced NO₃^- delivery via terrestrial leaching during high autumn rainfall. Our results suggest that in-stream transformations of varying allochthonous N-subsidies alter stream N availability, and thereby influence subsidy delivery to adjacent and downstream ecosystems.