Variation in δ¹⁵N from shell-associated organic matter in bivalves: Implications for studies of modern and fossil ecosystems

Nitrogen stable isotopes (δ¹⁵N) of organic matter can aid in the reconstruction of food webs and, when applied to fossil systems, may help resolve uncertainty about trophic position and provide tests for hypotheses developed using traditional paleoecological techniques. However, in order to use those values to reconstruct trophic relationships in ecosystems and assess changes in their dynamics, an understanding of the variability of isotopic signatures within individuals and across populations is critical. The purpose of this study is to develop a methodology by which to characterize the variability in nitrogen isotope values within bivalve shells and to propose an approach for effective paleoecological and paleoceanographic studies using such data. Here we present high-resolution sclerochronological and whole-shell δ¹⁵N values of shell-associated organic matter derived from modern Spisula solidissima bivalves from three different localities. High-resolution δ¹⁵N values vary by about 4‰ over the sampled life history, which reflects the interplay of factors such as diet, physiological change over ontogeny, and background nitrogen isotopic signatures of dissolved nutrient nitrogen. A significant correlation of temporally aligned δ¹⁵N with δ¹⁸O at different lags suggests that the δ¹⁵N has a seasonal control. Furthermore, an enrichment in δ¹⁵N following peak primary productivity in the region suggests that seasonal turnover of nutrients is the primary driver of δ¹⁵N variability. Unfortunately, the fossil record does not always yield well-preserved whole shells, and working with limited shell fragments is unlikely to capture the full range of variation exhibited within a shell. To this end, we also analyzed a field-collected sub-fossil set of Spisula for whole-shell δ¹⁵N. The sample set demonstrates 5‰ of variation in δ¹⁵N among individual shells, similar to the range observed within serially-sampled shells. This likely reflects a variation that we might expect to see in a population collected from a single horizon in the fossil record. Studies involving shell-associated nitrogen isotopes in a paleo setting should analyze numerous specimens in a stratigraphic unit to capture the whole range of δ¹⁵N values within a population. Moreover, for understanding trophic relationships it is important to analyze and compare the δ¹⁵N values of multiple coeval taxa each taxa.