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Reservoir stratification modulates the influence of impoundments on fish mercury concentrations along an arid land river system

James Willacker·Collin A. Eagles-Smith·Jim Chandler·Jesse Naymik·Ralph Myers·David P. Krabbenhoft
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Abstract

Impoundment is among the most common hydrologic alterations with impacts on aquatic ecosystems that can include effects on mercury (Hg) cycling. However, landscape-scale differences in Hg bioaccumulation between reservoirs and other habitats are not well characterized nor are the processes driving these differences. We examined total Hg (THg) concentrations of Smallmouth Bass ( Micropterus dolomieu ) collected from reservoir, tailrace, and free-flowing reaches along an 863 km segment of the Snake River, USA, a semiarid river with 22 impoundments along its course. Across three size-classes (putative 1-year-old, first reproductive, and harvestable sized fish), THg concentrations in reservoirs and tailraces averaged 76% higher than those in free-flowing segments. Among reservoirs, THg concentrations were highest in reservoirs with inconsistent stratification patterns, 47% higher than annually stratified, and 144% higher than unstratified reservoirs. Fish THg concentrations in tailraces immediately downstream of stratified reservoirs were higher than those below unstratified (38–130%) or inconsistently stratified (32–79%) reservoirs. Stratification regimes influenced the exceedance of fish and human health benchmarks, with 52–80% of fish from stratifying reservoirs and downstream tailraces exceeding a human consumption benchmark, compared to 6–17% where stratification did not occur. These findings suggest that impoundment and stratification play important roles in determining the patterns of Hg exposure risk across the landscape.

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James Willacker, Collin A. Eagles-Smith, Jim Chandler, Jesse Naymik, Ralph Myers, David P. Krabbenhoft. 2023-12-05. Reservoir stratification modulates the influence of impoundments on fish mercury concentrations along an arid land river system. https://doi.org/10.1021/acs.est.3c04646

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