Changes in Sr/Ca, Ba/Ca and 87Sr/86Sr ratios between trophic levels in two forest ecosystems in the northeastern U.S.A.

摘要

The variability and biological fractionation of Sr:Ca, Ba:Ca and 87Sr:86Sr ratios were studied in a soil-plant-invertebrate-bird food chain in 2 forested ecosystems with contrasting Ca availability in the USA (Hubbard Brook Experimental Forest in New Hampshire, and Charles Downer State Forest in Vermont). Chemical measurements were made of the soil exchange pool, leaves, caterpillars, snails, and both the femurs and eggshells of breeding insectivorous migratory songbirds. 87Sr:86Sr 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 87Sr:86Sr values generally reflected those of lower trophic levels at each site, but werelower and more variable, probably because their Sr was derived in part from foods in tropical winter habitats where lower 87Sr:86Sr 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 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 obtained here were similar to those for herbivorous and carnivorous mammals at similar trophic levels. The results provide evidence that supports the use of Sr:Ca ratios in the determination of human palaeodiets and suggests that Sr:Ca ratios may be useful in studies of modern food webs. Furthermore, the findings suggest that 90Sr 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 87Sr:86Sr as a tracer of calcium biogeochemistry.