Pollution history documented through shell remains provides a tool to study ecosystem changes without destroying endangered freshwater mussels
March 27, 2003
In the early 1900s, there were 42 species of freshwater mussels in the North Fork of the Holston River in Southwest Virginia. There were 33 downstream of Saltville. Now there are only nine species of mussels downstream of Saltville, and none directly below Saltville.
When Virginia Tech geosciences student Megan Brown of Colonial Heights, Va., decided to study the local extinctions of these creatures, some of which have been known to live 200 years and many of which are endangered species, she didn't want to have to use the traditional means of pulverizing them to measure chemical uptake.
At the joint meeting of the Northeastern and Southeastern Sections of the Geological Society of America, Brown will report on her non-invasive means to determine whether pollution or environmental stresses are threatening freshwater mussels. The GSA meeting is March 25-27 in Tysons Corner, Va.
Brown measured the mercury content in freshwater mussel shells in the river near Saltville, Va., where industry had polluted the river from 1950 until 1972, measured the damage done to the shells, and observed stages of recovery.
She looked at two sites upstream, unaffected by the pollution, at a site at Saltville, the point of the contamination, and at two sites downstream. "There was a very low level of mercury in shells upstream. I had to go 30 miles downstream to find a site with mercury levels at the background levels of the upstream sites," Brown says.
Dead mussel shells reflected the levels of mercury with high levels directly below Saltville and decreasing levels with increasing distance from Saltville. Brown examined shells to see if those from areas with no living populations looked different from shells in areas with living populations. She observed such characteristics as whether shells were still hinged together, external luster, edge preservation, and how broken they were.
"Wear could have been due to a change in stream gradient, but we found the most destruction was at the site of heaviest contamination and determined the heavy destruction was because there was no input of fresh-dead material," Brown says. "And wear was present whether the shell was thin or thick."
By documenting what happened to the mussels near Saltville, Brown has developed a strategy for study of other areas. "We can look at geochemical characteristics of the shell to determine what kind of pollution has impacted a system - to determine whether the local extinction is from pollution or an environmental stress such as heavy sedimentation," explains Brown. "And we can observe the kinds of destruction, such as the kinds of damage to shells, to help determine how long ago populations were still alive."
Brown will present the paper, "Using geochemical and taphonomic signatures of freshwater mussel shells to explore industry-related extirpations in the North Fork Holston River, Va. (60-8)," at 10:40 a.m. Saturday, March 27, in the Lord Thomas Fairfax Room of the Hilton McLean Tysons Corner. Co-authors are Virginia Tech geological sciences professor Michal Kowalewski, biology professor Donald Cherry, fisheries and wildlife professor Richard Neves, and geosciences professor Madeline Schreiber.
Brown, who received her undergraduate degree in biology from the University of Virginia, says she undertook the mussel study because "I'm interested in learning about the problems we've created and how we can remedy them." She expects to receive her master's degree from Virginia Tech in May and would like to work with the Fish and Wildlife Service.
The focus of this research is to develop non-invasive techniques to obtain new, independent insights into extirpations (loss of populations) induced by anthropogenicchanges. We use freshwater ecosystems of a mercury-polluted river to test the approach. ... The effluent containing elemental mercury and chloride salts had devastated the freshwater mussel fauna downstream of Saltville by the early 1970's. In a two-pronged approach, we targeted (1) geochemical mercury signatures of empty mussel shells and (2)taphonomic signatures based on quantifying preservational qualities of shells. The ICPMS analysis shows that shells collected upstream from Saltville (above the pollution source) have very low Hg concentrations (<10 ppb). In contrast, the shells collected directly below the pollution source contain significantly higher Hg concentrations (exceeding 160 ppb for some specimens). Shells collected further downstream have notable Hg concentrations, which decrease with distance from Saltville. Taphonomic signatures of shells also confirm that extirpation patterns relate to the pollution source. Shells are most heavily altered and fragmented in areas directly downstream of the contamination point, which have been devoid of extant populations for at least 30 years, and thus, contain shells that are at least 30 years old. In contrast, upstream sites, unaffected directly by the mercury contamination, contain many fresh-dead shells indicating the presence of reproducing populations, which continuously contribute recent mortalities to the death assemblage. Areas further downstream, which have recently been recolonized by mussel populations, represent an assemblage with an intermediate taphonomic signature. The study demonstrates that non-invasive techniques focused on dead mussel shells may offer a powerful, independent source of information about freshwater communities with different extirpation and contamination histories. Geochemical and taphonomic signatures of shell remains should become especially useful tools in documenting the pollution history and ecosystem changes in regions with unknown extirpation histories.