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Near-Decadal Oscillation of Water Levels and Mercury Bioaccumulation in the Laurentian Great Lakes Region

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journal contribution
posted on 15.01.2020, 14:07 by C. J. Watras, H.-Y. Teng, A. W. Latzka, M. W. Meyer, Z. Zhang
Mercury (Hg) contamination in remote lakes stems from atmospheric Hg transport to surface waters and subsequent conversion by microbes to neurotoxic methylmercury (MeHg) that biomagnifies in pelagic food webs. Despite declines in anthropogenic Hg emissions and downward trends for Hg in precipitation, many fisheries remain contaminated in otherwise pristine regions. Here, we report that trends in bioaccumulation are confounded by a near-decadal oscillation of the water cycle that is connected to large-scale atmospheric circulation patterns. In hundreds of lakes within the upper Laurentian Great Lakes region, mercury levels in pelagic fish and piscivorous birds oscillate in phase with near-decadal changes in water level, independent of atmospheric Hg loading. Geochemical data suggest that the cause is variation in the microbial production of MeHg as littoral sediments dry out and reflood. A recently amplified oscillation of the water cycle points to more intense episodes of fish contamination as the climate warms, further obscuring the benefit of reduced Hg emissions.