The significance of marine-derived biogenic nitrogen in anadromous Pacific salmon freshwater food webs

Abstract

The natural abundance of the stable isotope ratios $\sp{15}$N/$\sp{14}$N and $\sp{13}$C/$\sp{12}$C expressed as $\delta\sp{15}$N and $\delta\sp{13}$C was used to trace biogenic nutrients delivered by returning adult anadromous Pacific salmon into freshwater systems. These systems were Sashin Creek, a rapidly flushing stream located on Baranof Island, southeastern Alaska and Iliamna Lake, the major sockeye salmon, Oncorhynchus nerka, nursery lake in the Kvichak River watershed, Bristol Bay, southwestern Alaska. Marine-derived nitrogen (MDN) was quantifiable by use of an isotope mixing model based on comparison of biota $\delta\sp{15}$N in areas used for spawning by anadromous salmon with salmon-free controls within the same watershed. Control periphyton (benthic primary producers) $\delta\sp{15}$N values $\sim$0 suggested that the control N pool was derived from N$\sb2$ fixation without significant recycling. In contrast, periphyton abundant in areas of intense spawning activity or carcass aggregation had $\delta\sp{15}$N $\sim$ +7. These two values were the basis for comparison of $\delta\sp{15}$N values of higher trophic level biota. A mixing model relating $\delta\sp{15}$N to MDN with trophic level was used to estimate consumer MDN through incorporation of a priori isotopic trophic enrichment factors established in the literature. Distinctive $\delta\sp{13}$C signatures along the Sashin Creek stream gradient and between Iliamna Lake littoral and limnetic production were used in concert with $\delta\sp{15}$N. Sashin Creek fishes reflected isotopic signatures of periphyton and thus production within the same stream section. Isotopic data suggested an overall importance of limnetic production in Iliamna Lake resident fish and juvenile sockeye salmon diets. Salmon eggs and emergent fry retaining the parental marine isotopic signature were distinguishable from autochthonous production derived from marine N, and appear to be a minor dietary component in both Sashin Creek or Iliamna Lake fishes. The proportion of MDN in resident fish N, including juvenile salmon after turnover of the natal N pool, was proportional to the escapement of spawners. Thus there is now direct evidence for a significant natural fertilization process: the flow of remineralized marine-derived biogenic nutrients from returning anadromous Pacific salmon into freshwater food webs.