Freshwater Mussel Shells as Environmental Chronicles: Geochemical and Taphonomic Signatures of Mercury-Related Extirpations in the North Fork Holston River, Virginia
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- 作者：Megan E. Brown ; Michal Kowalewski ; Richard J. Neves ; Donald S. Cherry ; and Madeline E. Schreiber
- 刊名：Environmental Science & Technology
- 出版年：2005
- 出版时间：March 15, 2005
- 年：2005
- 卷：39
- 期：6
- 页码：1455 - 1462
- 全文大小：239K
- 年卷期：v.39,no.6(March 15, 2005)
- ISSN：1520-5851
文摘
This study utilized freshwater mussel shells to assessmercury (Hg) contamination in the North Fork HolstonRiver that extirpated (caused local extinctions of) a diversemussel fauna. Shells (n = 366) were collected from fivesites situated upstream (two sites), just below (one site), anddownstream (two sites) of the town of Saltville, Virginia,where Hg was used to produce chlorine and caustic sodafrom 1950 to 1972. Shell samples were used to test the(1) utility of geochemical signatures of shells for assessingthe spatial variation in Hg levels in the river relative tothe contamination source and (2) value of taphonomy (post-mortem shell alteration) for distinguishing sites thatdiffer in extirpation histories. Geochemical signatures of40 shells, analyzed using atomic absorption spectroscopy,indicated a strong longitudinal pattern. All shells fromthe two upstream sites had low Hg concentrations(<5-31 
g/kg), shells directly below Saltville had variable,but dramatically higher concentrations (23-4637 g/kg),and shells from the two downstream sites displayedintermediate Hg levels (<5-115 g/kg) that declined withdistance from Saltville. Two pre-industrial shells, collectedat Saltville in 1917, yielded very low Hg estimates (5-6g/kg). Hg signatures were consistent among mussel species,suggesting that Hg concentrations were invariant tospecies type; most likely, highly variable Hg levels, bothacross sites and through time, overwhelmed any interspecificdifferences in Hg acquisition. Also, a notable post-mortem incorporation of Hg in mussel shells seemedunlikely, as the Hg content was not correlated with shelltaphonomy (r = 0.18; p = 0.28). The taphonomic analysis (n= 366) showed that the degree of shell alteration reliablydistinguished sites with different extirpation histories.At Saltville, where live mussels have been absent for atleast 30 years, shells were most heavily altered andfragmented. Conversely, fresh-looking shells aboundedupstream, where reproducing mussel populations are stillpresent. In summary, relic shells offered valuable spatio-temporal data on Hg concentrations in a polluted ecosystem,and shell taphonomic signatures discriminated sites withdifferent extirpation histories. The shell-based strategiesexemplified here do not require sampling live specimens andmay augment more standard strategies applied toenvironmental monitoring. The approach should proveespecially useful in areas with unknown extirpation andpollution histories.
g/kg), shells directly below Saltville had variable,but dramatically higher concentrations (23-4637 g/kg),and shells from the two downstream sites displayedintermediate Hg levels (<5-115 g/kg) that declined withdistance from Saltville. Two pre-industrial shells, collectedat Saltville in 1917, yielded very low Hg estimates (5-6g/kg). Hg signatures were consistent among mussel species,suggesting that Hg concentrations were invariant tospecies type; most likely, highly variable Hg levels, bothacross sites and through time, overwhelmed any interspecificdifferences in Hg acquisition. Also, a notable post-mortem incorporation of Hg in mussel shells seemedunlikely, as the Hg content was not correlated with shelltaphonomy (r = 0.18; p = 0.28). The taphonomic analysis (n= 366) showed that the degree of shell alteration reliablydistinguished sites with different extirpation histories.At Saltville, where live mussels have been absent for atleast 30 years, shells were most heavily altered andfragmented. Conversely, fresh-looking shells aboundedupstream, where reproducing mussel populations are stillpresent. In summary, relic shells offered valuable spatio-temporal data on Hg concentrations in a polluted ecosystem,and shell taphonomic signatures discriminated sites withdifferent extirpation histories. The shell-based strategiesexemplified here do not require sampling live specimens andmay augment more standard strategies applied toenvironmental monitoring. The approach should proveespecially useful in areas with unknown extirpation andpollution histories.