Geochemical and Taphonomic Signatures of Freshwater Mussel Shells as Evidence of Mercury-Related Extirpations in the North Fork Holston River, Virginia

Abstract

This study utilized freshwater mussel shells to assess the role of mercury contamination in the North Fork Holston River, an aquatic habitat affected by extensive extirpations of mussel populations starting in the early 1970's. Mussel shells (n=366) collected from 5 sites, upstream and downstream of Saltville (where mercury was used from 1950-1972) were analyzed to test if: (1) geochemical signatures of shells record variation in mercury levels relative to the contamination source; and (2) shell taphonomy could be used to differentiated affected and unaffected sites.

Analysis of 40 shells for geochemical signatures using atomic absorption spectroscopy indicated a strong longitudinal pattern. Mercury content was as follows: upstream sites had low Hg concentrations (<5 to 31ppb), shells directly below Saltville had high concentrations (23-4,637ppb), shells 18km downstream of Saltville displayed intermediate values (7-115ppb), and those 38.4km downstream were comparable to upstream sites (<10ppb). Two pre-industrial shells collected from Saltville in 1917 also yielded Hg estimates (5-6ppb) comparable with upstream estimates. The Hg content was not correlated with shell length (r=-0.3; p=0.2) or degree of taphonomic alteration (r=0.18; p=0.28). Analysis of 366 shells for taphonomic signatures indicated that shells are most heavily altered and fragmented directly downstream of Saltville. In contrast, upstream sites, inhabited by reproducing mussel populations, contain many fresh-dead shells. Taphonomic signatures can thus be used to differentiate sites with different extirpation histories. Relic mussel shells can provide useful spatial and temporal data on Hg concentrations in polluted ecosystems and offer a tool for delineating areas with unknown extirpation histories.