CALIBRATION OF BIVALVE SHELL δ15N VALUES AS A PALEOENVIRONMENTAL PROXY IN FRESHWATER AND ESTUARINE ENVIRONMENTS

Nitrogen isotope ratios (δ¹⁵N) of bivalve tissues have been used to trace N variability, pinpoint pollutants, and reconstruct food web dynamics in freshwater and estuarine environments. Considering that bivalve tissues are not preserved in archaeological and fossil records, we examine whether shell carbonate bound organic matrix (CBOM) δ¹⁵N values can be used to understand past biogeochemistry. Previous suggest that there are species-specific discrepancies related to shell mineralogy and tissue turnover time that must be well-constrained before CBOM δ¹⁵N values can be applied as a reliable proxy. This study calibrates the CBOM δ¹⁵N values in the aragonitic shells of: (1) estuarine clam Rangia cuneata, and (2) freshwater mussel Elliptio complanata. Preliminary data suggests that muscle tissues of R. cuneata have higher δ¹⁵N values (12.6±0.6‰) compared to mantle (11.4±0.3‰), gill (11.2±0.2‰), and stomach (10.9±0.6‰) tissues. The CBOM δ¹⁵N values (10.0‰), which presumably have a negligible turnover time, most closely reflect stomach values. Thus, stomach tissues likely have the fastest turnover time among the tissues sampled. Given that coeval particulate N (PN) δ¹⁵N values in the estuary are 4.7±1.8‰, and the expected trophic level enrichment is ~3.4‰, the relationship between δ¹⁵N_PN, tissue δ¹⁵N, and CBOM δ¹⁵N values is not straightforward. This is in agreement with studies that suggest R. cuneata are non-selective filter feeders, consuming primarily sand, silt, clay, and detritus as opposed to phytoplankton from the water column.  Freshwater mussel E. complanata tissues have similar muscle (9.7‰), mantle (9.1‰), and gill (9.2‰) δ¹⁵N values, and lower stomach values (7.8‰). Again, shell δ¹⁵N (7.0±0.4‰) most closely resembles stomach values. The average river δ¹⁵N_PN value (6.4±1.4‰) is approximately one trophic level below the muscle, mantle, and gill tissues, in accordance with previous studies that suggest E. complanata are primary consumers. In conclusion, stomach tissues and CBOM values have a rapid turnover time indicating that CBOM may be capable of capturing short-term variation in estuarine and riverine N cycling.