The Impact of Impoundment on Mercury Bioaccumulation in Fish Downstream from a Newly Constructed Reservoir, Wujiang River, Southwest China

Abstract

Mercury concentrations in fish were investigated downstream from a newly impounded subtropical reservoir in August 2008. After 6-7 months of reservoir impoundment, mean mercury concentration in fish from downstream is significantly increased by 1.9 times. Not only carnivorous fish but also benthic fish had significantly higher total mercury concentrations than others. No significant correlation was found between total mercury concentrations and body length or weight of 13 fish species. Compared with the pre-impoundment, total mercury in fish from downstream is significantly increased by reservoir impoundment, but the increased rate is lower than those in subarctic and temperate areas. Fish samples surpassed the Chinese hygienic standard for tolerances of mercury in foods increased by 4.3%. More attention should be given to fish mercury levels from downstream sites to prevent possible adverse effects on the health of local people.

Reservoir Stratification Modulates the Influence of Impoundments on Fish Mercury Concentrations along an Arid Land River System

“…Numerous studies have documented increased fish THg concentrations within reservoirs compared to preimpoundment rivers or nearby reference sites in the years and decades following impoundment. − Even in older reservoirssuch as those in this study, which were ∼50–120 years old at the time of samplingfish THg concentrations often appear to remain elevated relative to natural waterbodies. , Of 11 species of fish compared in the northeastern United States, seven (64%; including all piscivorous species examined) had higher THg concentrations in reservoirs than in free-flowing rivers, with 15–309% (average = 88%) higher concentrations in reservoirs depending on species . In the northeastern United States, THg concentrations in Smallmouth Bass differed by 22% between reservoirs and free-flowing rivers, roughly half the difference observed between the same Snake River habitats in the current study (42%).…”

“…Prior studies have also noted elevated Hg concentrations in piscivorous fishes extending great distances downstream of reservoirs, over 300 km in some cases, whereas elevated concentrations in lower trophic-level species were minimal or extended shorter distances downstream. ,,, The mechanisms underlying these patterns are unclear but have been primarily attributed to differences in dietary exposure, with piscivorous fishes able to feed on stunned, injured, and killed fish prey passing through the dam. , This does not, however, explain why elevated concentrations would extend further downstream, unless it is assumed that larger biological material (i.e., fish passing through the dam) persists further downstream than the smaller biological material (e.g., plankton) that comprises the bulk of biological Hg export from most reservoirs. , Although this could be true at shorter distances in some cases, it is unlikely to explain the differences over the distances observed. Alternatively, these results may reflect the incorporation of reservoir-derived aqueous MeHg into the food webs utilized by the larger fish downstream, though previous studies have found that elevated MeHg concentrations in abiotic matrices typically do not extend far downstream of reservoirs. ,, Because the larger fish are presumably older, exposure to even modest increases in dietary MeHg exposure over their lifetime may also be sufficient to account for the patterns observed. The lack of clear insights into the drivers of these differences is a substantial gap in our understanding of Hg risk in these systems.…”

“…In the northeastern United States, THg concentrations in Smallmouth Bass differed by 22% between reservoirs and free-flowing rivers, roughly half the difference observed between the same Snake River habitats in the current study (42%). In contrast, Largemouth Bass THg was higher in unregulated rivers compared to reservoirs in the Southern Coastal Plain of the United States, in part, due to dietary differences among habitats but also because the rivers examined had intact floodplains that were seasonally important sites of MeHg production. − Elevated fish THg concentrations downstream of individual reservoirs have been linked to altered foraging ecology, , MeHg production in seasonally inundated sediments, ,, and biogeochemical conditions in the upstream reservoir. , By comparing THg concentrations in fishes among a diverse collection of reaches, the current study suggests that the differences observed in prior case studies extend beyond individual reservoirs and tailraces, instead reflecting a prevalent trend among these habitats . This commonality further suggests that the underlying drivers of these differences are likely tied to biogeochemical, ecological, or hydrologic conditions shared among a broad array of reservoirs.…”

“…In contrast, Largemouth Bass THg was higher in unregulated rivers compared to reservoirs in the Southern Coastal Plain of the United States, in part, due to dietary differences among habitats but also because the rivers examined had intact floodplains that were seasonally important sites of MeHg production. 104 − 106 Elevated fish THg concentrations downstream of individual reservoirs have been linked to altered foraging ecology, 55 , 107 MeHg production in seasonally inundated sediments, 100 , 101 , 108 and biogeochemical conditions in the upstream reservoir. 36 , 109 By comparing THg concentrations in fishes among a diverse collection of reaches, the current study suggests that the differences observed in prior case studies extend beyond individual reservoirs and tailraces, instead reflecting a prevalent trend among these habitats.…”

Mobilization and Transformation of Mercury Across a Dammed Boreal River Are Linked to Carbon Processing and Hydrology

“…The flooding of soil following dam construction is associated with environmental issues such as the transformation of mercury (Hg) into the neurotoxin methylmercury (MeHg) as well as an increase in carbon processing and subsequent greenhouse gas (GHG) emissions, which result from the remobilization and degradation of terrestrial organic matter (OM) (Mucci et al, 2008; Teodoru et al, 2012; Wang & Zhang, 2013). Globally, rivers export about 27 ± 13 Mmol a −1 of Hg (Amos et al, 2014), with recent increases observed in the Northern Pacific from Asian watersheds where a large percentage of rivers have been impounded (Lehner et al, 2011), and where enhanced Hg outputs from reservoirs also have a potential impact on downstream fish mercury levels (Anderson, 2011; Kasper et al, 2014; Li et al, 2017; Mason et al, 1994). Thus, increasing damming activity could enhance mercury and carbon fluxes globally, but the role of reservoirs as either sources of new mercury versus reactors for the methylation of mercury already present in the network, as well as potential links between the mercury and carbon cycles, remains unclear in these transformed systems.…”