Nutrients in Salmonid Ecosystems: Sustaining Production and Biodiversity

Trophic Level Implications When Using Natural Stable Isotope Abundance to Determine Effects of Salmon-Derived Nutrients on Juvenile Sockeye Salmon Ecology

Thomas C. Kline


Abstract.—The amount of nitrogen contributed by anadromous and semelparous Pacific salmon Oncorhynchus spp., marine-derived nitrogen (MDN), relative to other sources, was estimated for sockeye salmon O. nerka juveniles rearing in nursery lakes of the Karluk and Kvichak rivers (Alaska) from their nitrogen stable isotope abundance using an isotope mixing model (IMM). Because trophic level (TL) as well as MDN can lead to nitrogen-15 enrichment, it is critical to know, for the IMM, the TL of juvenile sockeye salmon (JSS) during their lacustrine life history phase. The initial a priori TL of 3.0 overestimated MDN. Regressing salmon escapement with stable isotope ratio and incorporating an updated herbivore isotope fractionation factor suggested that the TL of Kvichak JSS was 3.7. This TL value and the difference in carbon and nitrogen stable isotope ratios between JSS and net Zooplankton suggested that the TL of net Zooplankton was 2.6. Using TL = 2.6 for net Zooplankton and the difference in stable isotope ratios between Karluk JSS and net Zooplankton suggested that JSS TL was 4.3. These latter TL values suggested that the mean MDN for the Karluk system during the late 1980s and early 1990s was 67%, which was approximately half of that predicted using earlier fractionation and TL values. Sample isotopic variation and variation due to instrument error were minor in comparison to TL uncertainty involved in data modeling. Nonetheless, nitrogen stable isotope data provide a means for assessing MDN that can range significantly within and among systems.