Spatial Variability in the Speciation and Bioaccumulation of Mercury in a Subtropical Reservoir System: Amistad International Reservoir, Texas, USA
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Mercury (Hg) is highly toxic and organic forms are known to bioaccumulate in aquatic organisms. Although Hg is a global pollutant there is a paucity of data on the behavior of Hg in subtropical reservoirs. I conducted a study examining spatial variation in concentration of Hg in Amistad International Reservoir, a large subtropical water body in the Rio Grande drainage. Sediments and muscle tissue of largemouth bass (Micropterus salmoides) were analyzed for concentrations of total Hg, methylmercury (MeHg), and environmental and biological factors known to influence the production (and bioaccumulation) of MeHg. The Rio Grande arm of the reservoir had the highest sediment concentrations of total mercury, but was below the TCEQ 100 ng/g sediment screening level. However, the concentration of MeHg was highest at sites in the Pecos River and Devils River arm (5.02 and 3.90 ng/g, respectively). Conditions in the sediments of the Pecos and Devils Rivers were likely more favorable to the production of MeHg, with higher sediment porewater dissolved organic carbon, porewater sulfate levels in the optimum range for methylation, and a higher number of detections for sulfate reducing bacteria, the microbial group believed to be associated with MeHg production. In 55 bass of legal sport fishing size 11% had concentrations over the TxDSHS screening value of 0.7 mg/kg, yet over 84% exceeded the 0.3 mg/kg US EPA screening value. Additionally, fish at a standardized length of 18.5 cm from the Devils River and San Pedro Canyon areas of the reservoir had higher muscle Hg concentrations than those collected in the Rio Grande arm, suggesting higher rates of bioaccumulation in the Devil’s River arm. This study adds to a growing body of evidence that spatial variation in Hg concentration of fish exists within lakes and reservoirs, and is potentially related to variation in Hg methylation.