Assessing δ15N values in the carbonate-bound organic matrix and periostracum of bivalve shells as environmental archives

Abstract

Though previous studies demonstrate the utility of nitrogen and carbon stable isotope ratios (δ15N and δ13C, respectively) in bivalve soft tissues as biogeochemical proxies, it is necessary to develop alternative proxies for environmental reconstructions when soft tissues are unavailable, such as with fossils or in museum-archived specimens. This study assesses the reliability of the δ15N values of carbonate-bound organic matrix (δ15NCBOM) and periostracum (δ15Nperiostracum) in bivalve shells as recorders of the δ15N values of particulate nitrogen (δ15NPN) by comparing the δ15NCBOM and δ15Nperiostracum values of live-collected freshwater mussels (Elliptio complanata) and estuarine clams (Rangia cuneata) to the δ15N values of particulate nitrogen (δ15NPN) in the water column. The δ15NCBOM and δ15Nperiostracum values in both species were within the range of the δ15NPN values that have been corrected for trophic-level enrichment. Thus, our findings illustrate that δ15NCBOM and δ15Nperiostracum values reliably record δ15NPN values in rivers and estuaries. The significant positive correlation between δ15NCBOM and δ15Nperiostracum values in both species indicates that they may be used in a similar manner to record δ15NPN values. The δ15N values in E. complanata muscle, mantle, and gill tissues were enriched by about +3.4‰ compared to δ15NPN from the water column, which suggests that they are primary consumers that reflect baseline trophic levels. On the other hand, δ15N values in the soft tissues of R. cuneata have trophic-level enrichment consistent with both primary and secondary consumption. Therefore, variations in the δ15N values of tissues in R. cuneata may be related to trophic-level shifts and/or changes in N sources. Differences between the δ15N values of soft tissue, CBOM, and periostracum in E. complanata and R. cuneata can be attributed to asynchronous growth, metabolic rate, and organic molecule composition. The δ15NCBOM values vary along a freshwater-estuarine gradient because of land-use change and differences in the trophic level of the compared species. The δ15NCBOM values between neighboring sites reflect influences from biosolid application and treated wastewater discharge. While δ15NCBOM values did not differentiate between sites dominated by urban and forested land-cover, δ15NCBOM values were highest at the site with the highest agricultural land-use. These results demonstrate the potential of δ15NCBOM values in bivalve shells to record long-term changes in watershed land use.