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Methylmercury Production in Tidal Salt Marsh Sediments and Potential Control Using Iron Amendments


Tidal wetlands can be important sources of methylmercury (MeHg) in aquatic ecosystems, such as the San Francisco Bay-Delta estuary. As a result of the tendency of bacteria in wetland sediments to methylate mercury, the restoration of wetland habitat may cause an increase of MeHg concentrations. To balance the need for tidal wetland habitat with concerns over increased MeHg exposure, landscape-scale techniques for minimizing the production and export of MeHg from wetland sediments are needed. One potential approach is to use an iron sediment amendment to reduce net MeHg production. The addition of Fe[II] decreases MeHg production by lowering the concentration of the inorganic Hg[II] species that are methylated by bacteria. In this research, the potential for reducing MeHg production and export via an iron amendment was evaluated in laboratory microcosm experiments and a field study in a tidal salt marsh in the San Francisco Bay estuary. Additionally, sediment incubation experiments were conducted in anaerobic containers and in in situ cores to evaluate the effect of iron and sulfur redox cycling on MeHg production.

Two laboratory microcosm experiments (Chapter 2) were conducted to test the iron amendment hypothesis under simulated tidal wetland conditions: one with devegetated sediments and one with live wetland vegetation. The microcosms consisted of intact sediment cores collected from Gambinini Marsh, a tidal salt marsh in the San Francisco Bay estuary dominated by pickleweed (Sarcocornia pacifica). The microcosms were maintained under simulated tidal conditions and amended at four iron doses (0, 180, 360, and 720 g-Fe/m2). Following iron addition to the devegetated sediments, porewater S[-II] concentrations decreased for each dose relative to the control. The average weekly export of MeHg in the surface water decreased by 82% and 89% for the two highest iron doses, respectively. Despite substantial variability within treatment groups, similar trends were observed in the vegetated microcosms. The results suggest that iron addition has the potential to provide a landscape-scale control on MeHg export from restored tidal wetlands under certain conditions.

The cycling of iron, sulfur, and mercury in tidal wetlands is a complex process, with the combination of daily tides, changes in the growth state of wetland plants, and a highly productive microbial community resulting in temporal and spatial variations in MeHg production and export. Sediment incubation experiments (Chapter 3) were used to evaluate the effect of these processes on MeHg concentrations in the sediments of Gambinini Marsh. Sediments were incubated for 7-days in sealed jars under the following conditions: untreated sediments, addition of sodium molybdate to suppress sulfate reduction, and the addition of formaldehyde as an abiotic control. Similar rates of Fe[II] production were observed in both the untreated and Mo-amended incubations, suggesting that both iron-reducing and sulfate-reducing bacteria co-existed in the same sediment layers. Additionally, MeHg production was not observed when sulfate reduction was suppressed, suggesting that mercury methylation was mediated by sulfate- reducing bacteria. The in situ incubations, which were conducted with open and closed cores, demonstrated that during the summer months when plants were active, separation of sediments from live plant roots and gas exchange with the atmosphere resulted in more reduced sediment conditions. Additionally, sediments at the surficial layers (0-1 cm and 3-4 cm depths) exhibited more reducing conditions during the winter than in the summer, suggesting that the oxidation of reduced iron species occurs more rapidly during the summer.

To better understand the effect of iron amendments on in situ tidal marsh biogeochemistry, a 17-month field study was conducted in the Gambinini Marsh (Chapter 4). Before and after amending the sediments with 77 g-Fe/m2, porewater from pickleweed-dominated sediments in the high marsh plain were analyzed for iron, sulfur, organic carbon, and methylmercury. Sulfide was not detected in the sediment porewater, and the iron amendment had no observable effect on net MeHg production. However, porewater iron concentrations were elevated for at least 6 weeks following the amendment. Porewater concentrations of MeHg and dissolved organic carbon were lower throughout 2010 than during the summer of 2009 when the experiment was initiated. However, these concentrations increased during the period of pickleweed flowering in 2010, further demonstrating the strong effect that wetland vegetation can have on sediment biogeochemical processes.

This research demonstrated that an iron sediment amendment has the potential to be an effective control of MeHg production and export in tidal wetland sediments under certain conditions. While the in situ amendment showed no effect in the high marsh plain of Gambinini Marsh, the microcosm experiments demonstrated that a strong effect may be possible in sulfide- rich sediments. Additional research is necessary to evaluate the efficacy of the iron amendment in sulfide-rich field sediments, such as those found in low marsh environments.

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