Spatial variability in the speciation and bioaccumulation of mercury in an arid subtropical reservoir ecosystem

Abstract

Patterns of spatial variation of mercury and methylmercury (MeHg) were examined in sediments and muscle tissue of largemouth bass (Micropterus salmoides) from Amistad International Reservoir, a large and hydrologically complex subtropical water body in the Rio Grande drainage. The distributions of both Hg and MeHg were compared with environmental and biological factors known to influence production of MeHg. The highest concentrations of total Hg (THg) in sediment were found in the Rio Grande arm of the reservoir, whereas MeHg was highest at sites in the Devils River arm and inundated Pecos River (often more than 3.0 ng/g). Conditions in the sediments of the Devils River arm and Pecos River channel were likely more favorable to the production of MeHg, with higher sediment porewater dissolved organic carbon, and porewater sulfate levels in the optimal range for methylation. Although the detection of different groups of sulfate-reducing bacteria by polymerase chain reaction (PCR) was generally correlated with MeHg concentrations, bacterial counts via fluorescent in situ hybridization (FISH) did not correlate with MeHg. A sample of 156 largemouth bass (<30 cm) showed a spatial pattern similar to that of MeHg in sediments, where fish from the Devils River arm of the reservoir had higher muscle Hg concentrations than those collected in the Rio Grande arm. In 88 bass of legal sport fishing size (>35 cm), 77% exceeded the 0.3 mg/kg U.S. Environmental Protection Agency screening value. This study shows that significant variation in sediment MeHg and biotic Hg concentration can exist within lakes and reservoirs and that it can correspond to variation in environmental conditions and Hg methylation.

Modelling mercury concentrations in prey fish: Derivation of a national-scale common indicator of dietary mercury exposure for piscivorous fish and wildlife

“…It is well known that Hg in fish can vary widely among Simonin et al, 2008) and even within water-bodies (Becker et al, 2011). This variation is linked to variation in catchment land-cover and land use (Hurley et al, 1995), Hg inputs from catchments and the atmosphere (Grigal, 2002;Harris et al, 2007) and other environmental factors that control methylation and demethylation (Seller et al, 1996;Gilmour and Henry, 1991), MeHg availability (Driscoll et al, 1995), food-web structure and trophic transfer efficiency (Pickhardt et al, 2002;Cabana et al, 1994).…”

Key contributors to variations in fish mercury within and among freshwater reservoirs in Oklahoma, USA

“…Extended periods of summer bottom water oxygen depletion during stratification in the deeper sections of Grand Lake may have promoted anoxic conditions in sediments that in turn enhanced Hg methylation. The relatively high [THg] for some species in the Neosho River may reflect conditions in the river sediment, such as higher organic matter and porewater dissolved organic carbon, that can promote Hg methylation 42 . While measurements of Hg speciation in water samples were beyond the scope of this study, another study on Hg fate and transport in Grand Lake found elevated bottom water MeHg in the summer near the transition zone of the lake (beginning of mid Grand section), where the combination of deeper water stratification and particle settling from river inputs may have provided optimal conditions for MeHg enrichment 70 .…”

“…For instance, across many lakes covering a range of geological settings, pH may vary widely and be a key variable in explaining variations in Hg bioavailability and uptake, whereas within a lake, the range may be too small to observe variations in Hg concentrations as a function of pH. In addition, most studies on Hg biomagnification in lakes and reservoirs in North America were conducted in temperate zones 19, 24, 29, 41 , and relatively few 42–44 have focused on subtropical regions despite higher Hg emissions from power plants in these areas 45 .…”

“…For instance, across many lakes covering a range of geological settings, pH may vary widely and be a key variable in explaining variations in Hg bioavailability and uptake, whereas within a lake, the range may be too small to observe variations in Hg concentrations as a function of pH. In addition, most studies on Hg bio-magnication in lakes and reservoirs in North America were conducted in temperate zones, 19,24,29,41 and relatively few [42][43][44] have focused on subtropical regions despite higher Hg emissions from power plants in these areas. 45 To evaluate key factors that explain differences in sh Hg concentrations within and among reservoirs, we examined variations in Hg concentrations in sh occupying a range of habitats and trophic positions from two connected freshwater reservoirs in south central U.S., which are located within 100 km of six coal-red power plants (CFPPs).…”