PARENT SESSION
Poster Session #12: Modeling.
Tuesday, August 7, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall
6
Incorporating concentration dependence in stable isotope mixing models.
Phillips, Donald 1, Koch, Paul2, 1 2
ABSTRACT- Stable isotopes are frequently used to quantify contributions of multiple sources to mixtures; e.g., food sources in a consumer¿s diet. Standard dual-isotope mixing models assume that sources¿ proportional contributions are equal for both elements (e.g., C, N). This is reasonable if concentrations are similar among sources, but one source may be particularly rich or poor in one element, leading to a proportionate increase or decrease in its contribution for that element relative to the other element. We developed a concentration-weighted mixing model, which assumes that for each element, a source[:apos]s contribution is proportional to the contributed mass times its elemental concentration. The model is outlined for two elements and three sources, but generalizes to n elements and n+1 sources. Sensitivity analyses for C and N in three sources indicated that varying just the N concentration of one source had substantial and differing effects on the estimated source contributions of mass, C, and N. Similarly in a study of bears feeding on salmon, moose, and N-poor plants, the estimated salmon biomass contribution was markedly less than the standard model estimated. Model application to a captive feeding study of mink fed on salmon, lean beef, and C-rich, N-poor beef fat closely reproduced the known dietary proportions, whereas the standard model could not determine a set of positive source proportions. Concentration-weighted model use is recommended whenever the elemental concentrations vary substantially among sources.
KEY WORDS: stable isotopes, mixing model, C:N ratios, dietary analysis