Water-level fluctuations influence sediment porewater chemistry and methylmercury production in a flood-control reservoir

Abstract

Reservoirs typically have elevated fish mercury (Hg) levels compared to natural lakes and rivers. A unique feature of reservoirs is water-level management which can result in sediment exposure to the air. The objective of this study is to identify how reservoir water-level fluctuations impact Hg cycling, particularly the formation of the more toxic and bioaccumulative methylmercury (MeHg). Total-Hg (THg), MeHg, stable isotope methylation rates and several ancillary parameters were measured in reservoir sediments (including some in porewater and overlying water) that are seasonally and permanently inundated. The results showed that sediment and porewater MeHg concentrations were over 3-times higher in areas experiencing water-level fluctuations compared to permanently inundated sediments. Analysis of the data suggest that the enhanced breakdown of organic matter in sediments experiencing water-level fluctuations has a two-fold effect on stimulating Hg methylation: 1) it increases the partitioning of inorganic Hg from the solid phase into the porewater phase (lower log K values) where it is more bioavailable for methylation; and 2) it increases dissolved organic carbon (DOC) in the porewater which can stimulate the microbial community that can methylate Hg. Sulfate concentrations and cycling were enhanced in the seasonally inundated sediments and may have also contributed to increased MeHg production. Overall, our results suggest that reservoir management actions can have an impact on the sediment-porewater characteristics that affect MeHg production. Such findings are also relevant to natural water systems that experience wetting and drying cycles, such as floodplains and ombrotrophic wetlands.

Challenges and opportunities for managing aquatic mercury pollution in altered landscapes

“…Additionally, sulfide production during wet periods modifies dissolved NOM with reduced sulfur moieties that enhance Hg availability for methylation (Graham et al 2012 ; Poulin et al 2017 ). Sediment wetting and drying cycles can increase the breakdown of organic matter, which can result in increased partitioning of sediment-bound Hg into the porewater phase and increased dissolved organic carbon production, both of which have been shown to enhance Hg methylation in reservoirs (Eckley et al 2017 ). Finally, reservoir water-level fluctuations have also been shown to increase sediment erosion and resuspension of Hg in the water column, which may make it more available for methylation (Mucci et al 1995 ).…”

“…In addition to direct emission of Hg to the air, mine operations can also impact Hg methylation in downstream aquatic systems by providing a substantial source of sulfate to receiving waterbodies (Berndt et al 2016 ; Bailey et al 2017 ) and by altering the surrounding hydrology as a result of dewatering around the mine pit/tunnels during operations and subsequent rewetting after closure (Willacker et al 2016 ; Eckley et al 2017 ). The drawdown and wetting cycles yield similar impacts on Hg methylation as that noted for reservoirs.…”

Mercury and water level management in lakes of northern Minnesota

“…The management of water level fluctuations to maximize economic and ecological benefits simultaneously continues to be a major concern for many natural resource managers. Several studies have found evidence that periodically inundated sediments can increase methylmercury production by sulfate‐reducing bacteria (Eckley et al 2017, Xiang et al 2018). Moreover, ecosystem‐scale studies have found statistical associations between fish methylmercury content and water level fluctuations (Sorensen et al 2005, Selch et al 2007, Watras et al 2019).…”

“…Nearshore sediments that have been recently inundated or have been dry for a long period before re‐inundation are often favorable to sulfate‐reducing bacteria because they are carbon‐rich and can easily become anoxic (Benoit et al 2003, Gilmour et al 2004). A few studies have demonstrated that sediments that are periodically inundated produce more methylmercury than sediments that are permanently exposed or permanently inundated (Eckley et al 2017, Xiang et al 2018). In principle, larger WL fluctuations result in more periodically inundated sediments, which then translates into increased methylmercury entering the food web.…”

Insights into the factors influencing mercury concentrations in tropical reservoir sediments

“…During the operation of the reservoir, water level fluctuations may continue to promote Hg methylation due to changing redox conditions 16,18,19 . Such redox shifts result in, e.g., the oxidation of sulfur, which enhances the activity of sulfate-reducing bacteria, including Hg methylators 20,21 .…”