Monitoring Changes in Trace Element Concentrations in Amblema plicata in the Guadalupe River basin using a Caged Transplant Experiment
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Exposure to contaminants, including trace elements, can be one of the reasons for unionid mussel population declines throughout North America. Texas has over 50 species of native mussels, including 16 species that are state threatened; however, little is known about the impact of contaminants on Texas mussels. This study investigated the accumulation of seven essential (Co, Cu, Fe, Mn, Ni, and Zn) and eight non-essential (Ag, As, Bi, Cd, Cr, Hg, Pb, and U) trace elements in gill tissue of Threeridge (Amblema plicata) at six sites in the Guadalupe River Basin, Texas, using a caged transplant experiment. Mussels were collected from a reference site (Lake Wood) and transplanted to five different sites. After 3 and 12 weeks, mussels were collected and the concentration of trace elements in gill tissue was determined using microwave acid digestion and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis. The concentration of trace elements in gill tissue and calculated Biota Sediment Accumulation Factors (BSAF) were compared within and among sites and between week 0, 3 and 12. Overall, mean gill tissue concentrations for essential trace elements were greater than non-essential trace elements. For all sites and time points combined, mean gill concentrations (µg/g dry weight) were greatest for Mn (4641), Fe (838), Zn (433), and As (8.81), and lowest for Cd (0.248), U (0.107), Hg (0.075), and Ag (0.051). Bismuth (Bi) was the only element that was below the detection limit (0.0280 μg/g) at all sites and time points. Changes in trace element concentrations varied by element and location, and there was no clear accumulation patterns over time. This could be due to heavy rain events that occurred during the experiment which resulted in either trace elements being added to the river due to urban and agricultural runoff or the dilution of trace elements at a given site. The mean Se:Hg molar ratio in gill tissue was > 1:1 at each site, indicating that Se may have a protective effect against Hg toxicity in freshwater mussels. A. plicata at each site had a mean BSAF > 1 for all trace elements except Co, Cr, Fe, Ni, Pb and U indicating that mussels are generally more enriched in trace elements than sediment at each location. Previous caged transplant experiments have been successful in identifying accumulation patterns in mussels placed in smaller riverine systems; however, the findings of this study identified the difficulties associated with conducting the same experiment in a large river system with a flashy regime.