VARIATION IN STABLE ISOTOPES OF FRESHWATER MUSSEL SHELLS IN THE LICKING RIVER OF KENTUCKY

Freshwater mussel shells are sensitive recorders of environmental change and impacts of human activity on the ecosystem. Mussels grow by forming new bands on their shell seasonally between warm and cold cycles, providing a seasonal record as old as the mussel shell. These seasonal records, up to 50 years long, can be reconstructed through the isotopic analysis of the regular banding of the shell. This study presents a comparative study of the isotopic composition of water and sediment with that of mussel shell organic and inorganic material to evaluate freshwater mussels as recorder of diet and environmental conditions in the Licking River of Kentucky.

Mussel shells representing 12 locations and seven species were collected from the two main branches of the Licking River. The organic δ¹³C and δ¹⁵N from the periostracum and δ¹³C, δ¹⁸O, and the δ¹⁵N of the prismatic layer were measured on the outermost growth band on all samples considered, along with multiple growth horizons on a subset of samples. To calibrate this data set, water and sediment samples were collected across a seasonal cycle, with sampling at February, May, and August 2018. Samples were analyzed for δ¹⁸O and δD of water, δ¹³C of DIC, and organic δ¹³C and δ¹⁵N of sediment.

Preliminary data of mussel shells from the Licking River in Kentucky show relationships linked to location, as well as differences between species. There is a general trend of isotope values for both inorganic carbon and oxygen that vary by stream location (upstream/downstream), as well as variability between the two main forks of the Licking River. Additionally, carbonate-bound δ¹⁵N was compared to the δ¹⁵N the outer periostracum at the same growth horizon. The results across 22 samples, including five species, were the same within analytical error.

These results will be placed in the context of the impact human activity has on freshwater systems, including multiple human constructed dams along the river as well as any changes in land use. Sampling sites are located on both sides of four dams, as well as spatially distributed to allow changes resulting from human activity to be visible. These results will also provide greater insight into the seasonal cycles of mussel growth and diet, and whether either has been affected by human activity within the length of the mussel’s seasonal records.