Mercury Contamination of the Rio Grande Fish Community: Spatial Variation and Influence of Environmental Gradients




Smith, Alexandra

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Mercury (Hg) contamination of aquatic ecosystems is a global environmental problem and high levels of Hg can cause adverse health effects in humans and wildlife. Organisms at the base of the food web absorb methylmercury (MeHg) and this highly toxic and bioaccumulative form is passed onto fish and humans through their diets. While there is abundant data on Hg contamination and factors that affect Hg bioaccumulation in lake food webs, there is comparatively little data on large river systems. This thesis examines Hg concentrations of fish from the Lower Rio Grande drainage, Texas and several of its major tributaries in order to assess: (1) the overall level Hg contamination and potential risk to piscivorous organisms, (2) whether there is spatial variation in Hg concentrations in fishes of the lower Rio Grande drainage, and (3) if patterns of Hg contamination of Rio Grande fishes are related to abiotic and/or biotic factors that vary among sites (i.e., dissolved organic carbon (DOC), organic matter in sediments (%OM), and sulfates). We sampled fish at 15 sites from the Big Bend reach to the lower Rio Grande Valley and found that 52% of small-bodied trophic level 3+ fish had Hg concentrations exceeded ERA. Wildlife Criteria (>77 ppb). However, there was significant spatial variation in fish Hg concentrations with the highest concentrations found in the Big Bend reach. Principal Components Analysis revealed that fish Hg were positively related to river DOC, sediment total Hg, and sediment MeHg. Previous studies indicate that these factors are known to facilitate bacterial production of MeHg and its bioaccumulation. I hypothesize that high levels of inorganic Hg inputs to the Big Bend reach, such as runoff from abandoned cinnabar mines, Hg-rich rock formations, and atmospheric deposition from coal burning power plants in Mexico exacerbate Hg contamination of the fish community.



mercury, fish, spatial variations, Rio Grande, contamination, fish communities


Smith, A. (2009). Mercury contamination of the Rio Grande fish community: Spatial variation and influence of environmental gradients (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.


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