NITROGEN ISOTOPE RECORDS IN UNIONID MUSSELS: ASSESSING FIDELITY TO STREAM CONDITIONS AND POST-MORTEM SHELL DEGRADATION

Several recent studies demonstrate that nitrogen isotope values (δ¹⁵N) measured in the organic fraction of mollusk shells can give insight into nutrient flux and some anthropogenic impacts to habitats. For example, Carmichael et. al. (2008) measured δ¹⁵N from Mercenaria spp. shell material and soft tissues to track sewage input to an estuary. Black et. al. (2017) demonstrated the potential utility of ancient shell δ¹⁵N proxies, suggesting that historical records of anthropogenic nitrogen can be created from archaeological shell midden deposits. The baseline data collected from the Black et al. study are not only valuable in addressing modern pollution concerns but also in assessing the extent of ancient human habitation, land use, agricultural practice, and related activities. However, key questions remain regarding quantifying the relationship between the environment, soft tissue, and the growing shell. Additionally, few shell δ¹⁵N studies have been conducted on freshwater taxa to assess for anthropogenic impacts to N loading in rivers.

To further examine these concerns, samples of freshwater Unionid mussels were collected from two separate locations in Alabama: the Sipsey River in western Alabama and Bogue Chitto Creek in the Black Belt Prairie. The Sipsey River collection location has suffered comparatively little anthropogenic impact, while Bogue Chitto Creek is in an area of intense agricultural activity. Comparisons in shell δ¹⁵N records between these two regions may yield insight into the sensitivity of this proxy method of assessing anthropogenic N inputs.

To address concerns of diagenesis, a subset of the collected shells from the Sipsey River was tagged, placed in mesh bags, and affixed to the river bottom near the collection site for over two years to simulate pre-depositional exposure. δ¹⁵N data were measured in local seston, benthic organic matter, and bulk soft tissue of these shells soon after collection. After retrieval, comparisons of shell and periostracum δ¹⁵N data will be made between those shells placed in the river and the others retained in a dry and stable laboratory climate. The resulting data permits an assessment of the fidelity of the shell isotope record to that of its habitat.