Title: Effects of Sediment Oxygenation on Methylmercury Bioaccumulation in Benthic Biota
Project Type: Research
Start Date: 03/01/2006
End Date: 02/28/2007
Congressional District: Washington, Fifth
Focus Categories: Toxic Substances, Sediments, Ecology
Keywords: methylmercury, bioaccumulation, sediments
Principal Investigator: Beutel, Marc
Federal Funds: $22,965
Non-Federal Matching Funds: $46,359
Abstract: Mercury (Hg) contamination of lake biota is a widespread problem that affects the health of wildlife and humans that consume contaminated fish, including potentially vulnerable segments of society (eg, subsistence fishers, tribes, pregnant women, infants). Hg discharged to the atmosphere from industrial processes travels for miles and enters aquatic ecosystems via atmospheric deposition. Under anaerobic conditions common in lake sediments, Hg is transformed to methylmercury (MeHg) by anaerobic bacteria. MeHg then accumulates in the base of the food web (eg, worms, algae), and concentrates up the food web as biota consume contaminated prey (eg, insects, fish, waterfowl, humans). Because source control of Hg emissions will have little impact on near-term rates of MeHg bioaccumulation, it is critical to evaluate potential in-lake management strategies, such as lake oxygenation, which may limit MeHg production and inhibit Hg uptake into lake biota. A key unknown related to lake oxygenation, which should limit MeHg production and release from lake sediments, is its impact on bioaccumulation in benthic biota. Reoxygenation of previously ?dead? anaerobic sediments could exacerbate bioaccumulation by opening up contaminated sediments to respiring benthic organisms, which could then bioaccumulate Hg themselves and pass it up the food web.
We hypothesize that higher DO levels at the sediment-water interface will result in lower bioaccumulation in benthic biota by three potential mechanisms: (1) deeper DO penetration into the sediments will push the site of methylation by SRB downwards and away from the sediment-water interface where benthic biota primarily reside, (2) deeper DO penetration will enhance oxidative demethylation, the microbiological conversion of MeHg to inorganic Hg and CO2, and (3) bioturbation will enhance oxygen penetration into sediments, thereby reinforcing mechanisms (1) and (2).
To test this hypothesis, we will perform laboratory bioassay experiments to examine rates of Hg bioaccumulation in oligochaetes, worms that commonly live in lake sediments, as a function of DO concentration in overlaying water. Organisms will be grown in replicate experimental chambers containing sediment and overlaying water from three study sites in WA (Fazon, Deer and Samish) with a gradient of Hg contamination in sediments (25 to 100 µg/kg). Treatments will include 0, 2, 5, 10 and 15 mg/L DO in water overlaying sediments. After ~15 days of exposure, worms will be tested for Hg. All Hg analyses will be carried out at WSU using a state-of-the-art Tekran 2600 Hg auto analyzer that measures Hg using cold vapor atomic fluorescence spectrometry. The project will fund a PhD student for the summer and fall of 2006 as well as an undergraduate assistant.
Results from this study will provide natural resource and lake managers with critical information needed to evaluate the feasibility of using lake oxygenation to actively managing conditions at the sediment-water interface to minimize MeHg bioaccumulation in lake biota. If oxygenation is proven to be effective in repressing sediment release of Hg while also not resulting in significant bioaccumulation in benthic biota, it will be possible to design and implement relatively simple and cost effective engineered systems for practical use in lakes and reservoirs. This will give resource managers a new management strategy to combat Hg bioaccumulation in lakes.
Progress/Completion Report, PDF