“Impoundments, or dams, are one of the most common man-made changes to river systems,” said James Willacker, USGS ecologist and lead author of the study. “The reservoirs resulting from these dams along the Snake River can affect mercury cycling. We wanted to find out the degree to which fish mercury levels differed across reservoirs and their upstream and downstream environments, and how that translated into potential exposure and health risk to fish, wildlife, and humans.” 

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The Snake River, the largest tributary of the Columbia River and a massive river system in its own right, includes 22 federal and private dams built between 1901 and 1982. These dams, and the reservoirs they create, provide hydropower, flood control, agricultural irrigation, recreation, and municipal water supplies. However, they can also influence the hydrology, biogeochemistry, and ecology of the Snake River.    

Reservoir waters can stratify – or become layered. In systems like the Snake River, stratification is primarily driven by temperature, and this has implications for oxygen levels. As surface water warms, it stops mixing with colder, bottom water. Algae growing in surface waters, fueled by nutrient inputs from surrounding lands, eventually settle in deeper water and decompose, which uses up available oxygen and creates anoxic conditions. These anoxic conditions in reservoirs often create ideal conditions for the creation of methylmercury. 

Methylmercury is naturally formed by microbes living under certain conditions in water, sediments, and soils. Methylmercury is easily incorporated into, but not easily removed from, living tissue and therefore accumulates in animals’ bodies, reaching higher levels in animals at the top of the food web. Mercury is therefore not only a potential risk to the smallmouth bass, but also in species with similar feeding habits or that may consume smallmouth bass – including humans. Mercury contamination is not unique to the Snake River. In fact, a bass consumption advisory is in place for the entire state of Idaho. 

To understand the role of reservoir stratification on fish mercury concentrations, scientists compared concentrations in several Snake River habitats, including stratifying and non-stratifying reservoirs, and different sections of free-flowing water along the river. The study also looked at methylmercury exposure risk to smallmouth bass in each habitat and potential risk to other fish, wildlife, and humans.    

Researchers found that mercury concentrations in smallmouth bass were higher in reservoirs than in the free-flowing river. Twice as many harvestable bass from reservoir-influenced habitats had levels of mercury higher than Environmental Protection Agency suggested level for protecting humans compared to harvestable bass in free-flowing areas. The scientists also found that bass mercury concentrations were influenced by reservoir stratification, with the highest concentrations seen in, and downstream of stratifying reservoirs.  

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Smallmouth bass fisheries are one of the many culturally and economically important fisheries along the Snake River system. Research on the smallmouth bass may give insight into the potential for mercury exposure in other species who share these habitats. The area has also provided critical spawning and rearing habitat for numerous other fish species of cultural significance since time immemorial, many of which are of conservation concern. These species include bull trout, chinook salmon, pacific lamprey, sockeye salmon, steelhead, and white sturgeon.  

“The results underscore the influence of reservoirs and their biogeochemical conditions as potential drivers of mercury exposure risk,” said co-author Collin Eagles-Smith, USGS Supervisory Research Ecologist. “Resource managers can take these factors into account as they evaluate current advisories in a manner that balances harvest opportunities with consumption risk.”  

The study “Reservoir Stratification Modulates the Influence of Impoundments on Fish Mercury Concentrations along an Arid Land River,” was published on December 5, 2023 in Environmental Science & Technology.