Calcium and strontium stable isotopes reveal similar behaviors of essential Ca and nonessential Sr in stream food webs

Recent studies showed the potential of stable isotopes of the macronutrient calcium (delta Ca-44/40) and nonessential strontium (delta Sr-88/86) as new trophic level indicators in terrestrial vertebrates and marine teleost fishes. In this study, we tested whether similar Ca and Sr isotopic fractionation trends existed in macroinvertebrate-dominated stream food webs compared to vertebrates despite their physiological differences. We have determined the delta Ca-44/40 and delta Sr-88/86 values as well as the Sr-87/Sr-86 ratios of stream macroinvertebrates and small gobies and their potential metal sources (stream water, periphyton, and terrestrial plant litter) in upper and lower reaches of two streams in the Lake Biwa catchment, central Japan. The Sr-87/Sr-86 ratios revealed that stonefly nymphs, crustacea, and gobies mostly relied on aquatic Sr sources. Higher Sr-87/Sr-86 ratios of some crane fly and caddisfly larvae, mayfly, dobsonfly, and dragonfly nymphs indicated greater terrestrial contributions via plant litter. Positive correlations between the delta Ca-44/40 and delta Sr-88/86 values implied that similar Ca and Sr sources existed, and that Ca and Sr stable isotopes underwent similar fractionation trends although Sr was not essential. The delta Ca-44/40 and partly the delta Sr-88/86 values were positively correlated with Sr/Ca ratios and negatively with delta N-15 values indicating trophic effects on Ca and Sr stable isotopes. The enrichment of Ca-44 and Sr-88 in large filter-feeding caddisfly larvae was a notable exception from these trophic trends. Our data confirm that the trophic Ca-44 and Sr-88 depletion observed for marine teleost fishes and terrestrial vertebrates also applied to macroinvertebrate-dominated stream food webs despite their different physiologies indicating that shared mechanisms of Ca and Sr isotopic fractionation may exist at the cellular or molecular level between these taxa.