VARIATIONS IN THE HG AND N CONTENT OF SHELLS FROM FRESHWATER MUSSELS EXPOSED TO HG CONTAMINATION

The extirpation of freshwater mussels in the North Fork Holston River of Virginia has been linked spatially and temporally to industrial mercury pollution. Chemical analysis of freshwater mussel shells revealed that Hg levels in shells were relatively high close to the source of pollution and decreased downstream, while upstream Hg content was near or below the Hg detection limit (ppb). Further, taphonomic analysis showed that past mussel populations were greater prior to the release of Hg (Brown et al. 2005, ES&T 39, 1455). These relationships can be explored in greater detail by conducting sclerochronological analyses of mussel shells of Brown's collection from the North Fork Holston River.

Sequential samples were collected from three shells of Pleurobema oviforme by milling away material parallel to growth lines using a Dremel drill. These samples were then analyzed for: 1) Hg content by direct mercury analysis and 2) N content by elemental analysis. Because Hg and N are primarily associated with the organic fraction of the shell (< 5% by wt), sequential samples were combined to generate sufficient mass (~ 200 mg for Hg and ~30 mg for N) for meaningful measurements that were multi-annual in resolution.

Mercury concentrations averaged 95 ppb and displayed significant variations throughout ontogeny (3 ppb min., 830 ppb max). Patterns in Hg content across the shell were not consistent among individual mussels but this could easily be the result of the analysis of non-contemporaneous mussels that were exposed to differential contamination histories during their lifetimes. On the other hand, nitrogen concentrations averaged 500 ppm and displayed little variation throughout ontogeny (below detection limit to 800 ppm max.). This lack of variation is interpreted as an even distribution of proteinaceous material throughout shells at the present sampling resolution. No significant correlation was observed between N and Hg, although both of these elements were expected to be related to organic matter content. The relationship between Hg and N will be explored further by targeting organic-rich (dark) growth bands in additional samples to determine the relationship between Hg and N in shells exposed to Hg contamination.