Groundwater-fed fen wetlands are high-value ecosystems under threat of nutrient enrichment and diminished water inputs associated with upland agricultural activities. Wetlands are often hotspots for the formation of toxic and bioaccumulative methylmercury (MeHg). Here we describe experiments evaluating mercury, nutrient, and trace element biogeochemistry and MeHg trophic transfer within a Pleistocene terrace-fen complex along the Cedar River floodplain in Eastern Iowa. Ground and surface water were sampled eight times over the course of two years across a monitoring well transect spanning the terrace-fen boundary. Additionally, we measured Hg speciation in sediment cores and Hg levels in Gammarus, an abundant amphipod in the fen. Based on measurements of redox active solutes, a strong redox gradient exists across the terrace-fen boundary, creating the necessary reducing conditions for Hg methylation. Despite this, observed filterable MeHg concentrations in the fen were low (mean = 0.11±0.11 ng/L) and were only slightly higher than the upland wells (mean = 0.05±0.03 ng/L; p = 0.054 for one-way ANOVA). Elevated MeHg concentrations in the upper layers of fen sediment (up to 8 ng/g dry weight, and &gt;5% of total Hg) indicate active Hg methylation, however. Sediment MeHg concentrations were strongly correlated to sediment organic matter content (r2 = 0.93), suggesting that strong MeHg sorption to peat limits dissolved MeHg concentrations in groundwater. Gammarus collected in the fen showed approximately double the body burden of total Hg (30-50 ng Hg/g dry weight body mass) than organisms collected from near the spring seepage at the terrace base (15-25 ng/g dry weight). This finding further supports the idea that the fen is a hotspot for Hg methylation. This Hg methylation poses a significant threat to secondary consumers in the fen as MeHg bioaccumulates and magnifies up successive trophic levels. Overall, the fen appears to be a location of moderate MeHg production and a possible source of Hg to the Cedar River. Continuing to characterize Hg flux and trophic transfer in these unique wetland ecosystems should be made a priority in hopes of informing the development of environmental management strategies designed to limit the effects of Hg pollution.

Mercury methylation and bioaccumulation in an Iowa terrace-fen complex

255th National Meeting (2018)

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