Changes in Sr/Ca, Ba/Ca and 87Sr/86Sr ratios between trophic levels in two forest ecosystems in the northeastern USA

The variability and biological fractionation of Sr/Ca, Ba/Ca and Sr-87/Sr-86 ratios were studied in a soil-plant-invertebrate-bird food chain in two forested ecosystems with contrasting calcium availability in the northeastern U.S.A. Chemical measurements were made of the soil exchange pool, leaves, caterpillars, snails, and both the femurs and eggshells of breeding insectivorous migratory songbirds. Sr-87/Sr-86 values were transferred up the food chain from the soil exchange pool to leaves, caterpillars, snails and eggshells without modification. Adult birds were the one exception; their Sr-87/Sr-86 values generally reflected those of lower trophic levels at each site, but were lower and more variable, probably because their strontium was derived in part from foods in tropical winter habitats where lower Sr-87/Sr-86 ratios are likely to predominate. Sr/Ca and Ba/Ca ratios decreased at each successive trophic level, supporting previous suggestions that Sr/Ca and Ba/Ca ratios can be used to identify the trophic level at which an organism is primarily feeding. The changes in Sr/Ca and Ba/Ca ratios we measured for vegetation and insects were comparable to similar measurements made previously (but based on single samples of each organism) in an alpine ecosystem. Changes in Sr/Ca and Ba/Ca ratios between birds and their food have not previously been measured, but the values we obtained were similar to those for herbivorous and carnivorous mammals at similar trophic levels. Our results provide evidence that supports the use of Sr/Ca ratios in the determination of human paleodiets and suggests that Sr/Ca ratios may also provide a useful tool in studies of modern food webs. Furthermore, our findings suggest that Sr-90 from nuclear fallout will not bioaccumulate in forests and that changes in Sr/Ca ratios between trophic levels will need to be considered in some cases when using Sr-87/Sr-86 as a tracer of calcium biogeochemistry.