B11G-01 INVITED
Hot Spots of Mercury Bioaccumulation in Amphibian Populations From the Conterminous United States
Mercury (Hg) contamination in the United States (U.S.) is well-documented and continues to be a public-
health issue of great concern. Fish consumption advisories have been issued throughout much of the U.S.
due to elevated levels of methylmercury (MeHg). Methylmercury contamination in the developing fetus and in
young children is a major public health issue for certain sectors of the global human population. Moreover,
identifying MeHg hot spots and the effects of MeHg pollution on environmental health and biodiversity are
also considered a high priority for land managers, risk assessors, and conservation scientists. Despite their
overall biomass and importance to aquatic and terrestrial ecosystems, Hg and MeHg bioaccumulation
dynamics and toxicity in amphibians are not well studied, especially when compared to other vertebrate taxa
such as birds, mammals, and fish species. Population declines in amphibians are well documented and likely
caused by synergistic and interacting, multiple stressors such as climate change, exposure to toxic
pollutants, fungal pathogens, and habitat loss and ecosystem degradation. Protecting quality of terrestrial
ecosystems in the U.S. has enormous ramifications for economic and public health of the nation's residents
and is fundamental to maintaining the biotic integrity of surface waters, riparian zones, and environmental
health of forested landscapes nationwide. Determining Hg concentration levels for terrestrial and surface
water ecosystems also has important implications for protecting the nation's fauna. Here I present an
overview of the National Amphibian Mercury Program and evaluate variation in MeHg hotspots, Hg
bioaccumulation and distribution in freshwater and terrestrial habitats across a broad gradient of physical,
climatic, biotic, and ecosystem settings to identify the environmental conditions and ecosystem types that are
most sensitive to Hg pollution. The role of geography, disturbance mechanisms, and abiotic and biotic
factors governing Hg distribution and bioaccumulation are also discussed.
http://www.hsph.harvard.edu/research/michael-bank/
B11G-02
Dissolved Organic Matter, Organic Matter Optical Properties and Mercury in Rivers and Streams
Interactions of mercury (Hg) with dissolved organic matter (DOM) play important roles in controlling concentrations, reactivity, bioavailability and transport of Hg in aquatic systems. Recent studies have shown that DOM influences Hg solubility through strong binding interactions and the stabilization of nanocolloidal mercuric sulfide. In this paper we present the results of watershed based studies associated with US Geological Survey NAWQA and WEBB Programs designed to better define the factors controlling the export of Hg in stream systems. We investigated the seasonal and spatial variability of dissolved organic matter quantity and quality, and the concentrations of dissolved Hg and methylmercury (MeHg) in 12 rivers and streams representing a range of watershed types that varied in climate, landscape, Hg deposition and water chemistry. DOM concentrations and composition, based on DOM fractionation and ultraviolet/visible absorption spectroscopic analyses, varied greatly both between sites, and seasonally within sites. Strong relationships were found between DOM and total dissolved Hg concentrations in almost all of the systems. The relationships between total dissolved Hg concentration and hydrophobic organic acid (HPOA) content (aquatic humic substances) were stronger than those observed between Hg and DOM, supporting the hypothesis that interactions between Hg and the HPOA fraction are important drivers for the transport of dissolved Hg in aquatic systems. The relationships between MeHg and DOM and HPOA content were not as strong as those observed with Hg. In all systems, UV absorbance measured at 254 nm correlated strongly with DOM, HPOA content and Hg concentrations. The relationships between DOM concentration and absorbance for the range of systems were quite variable because not all of the dissolved organic carbon in a given sample absorbs UV light to the same degree and each system exhibited a different relationship. However, the relationship between HPOA content and UV absorbance was stronger and more consistent because the HPOA fraction contains a greater percentage of UV absorbing compounds than other fractions of the DOM. These results demonstrate that optical properties, such as UV absorbance, are excellent proxies for DOM and HPOA concentrations within a given system. By extension, because of the strong relationships between Hg and DOM, these properties can also be used to derive relationships between DOM optical properties and Hg concentrations. Optical measurements are relatively inexpensive to obtain, can be designed into in situ monitoring devices and, when combined with discharge data, can be used to tighten estimates of both DOM and Hg flux in streams and rivers, especially at high flows when fluxes are greatest and manual sampling is difficult.
B11G-03 INVITED
Coupling water table fluctuation to mercury speciation and transport in wetland ecosystems
Hydrological processes exert a first-order control over both the conditions required for mercury methylation to occur, and the transport of methylmercury from sites of production. In the recent literature, evidence has been presented that a relationship exists between water level fluctuation and mercury levels in aquatic organisms. These observations have led to the conclusion that this fluctuation is stimulating mercury methylation in littoral sediments and wetland ecosystems through the creation of favourable biogeochemical conditions. Using data from a range of wetland ecosystems, and several experiments that subjected wetland soils to fluctuating water levels, a relationship between water table fluctuation frequency and methylmercury production will be presented. Experimental data show that longer frequency wetting and drying periods result in greater methylmercury production relative to a static or high frequency fluctuation. It was also found that mercury methylation processes in wetland soils are able to sustain elevated pore water concentrations over repeated wetting and draining events. These data suggest that methylmercury export from wetlands is likely limited by the degree of hydrological connectivity rather than biogeochemical processes, highlighting the need to better understand the nature of hydrological linkages among wetlands and adjacent ecosystems.
B11G-04 INVITED
Hydrologic Fluctuations Resulting From Climatic Variability Cause Methylation Events in Peatlands Impacted by Elevated Sulfate Deposition
A long-term sulfate addition experiment at the Marcell Experimental Forest of northern Minnesota has demonstrated the stimulatory effect of sulfate on mercury methylation at the ecosystem scale. Wetland margins have been shown to be principal zones of methylmercury (MeHg) production in sulfur-limited peatlands, but this research illustrates how the hydrologically isolated center of a small peatland effectively becomes a hot spot when exposed to elevated, atmospheric sulfate deposition. Furthermore, the chronic effects resulting from experimentally elevated sulfate deposition lead to the formation of a pool of reduced sulfur compounds highly sensitive to the changing redox conditions created by hydrologic and climatic variability. Our data reveal that water table rises following extended periods of drought cause natural "sulfate additions" and stimulate mercury methylation. This phenomenon was even observed in our control treatment following a severe drought in 2006. Hydrologic events that increase connectivity between the central bog and dominant wetland flowpaths, such as the infrequent, intense precipitation events predicted for this region by climate change models, could significantly increase MeHg flux from similar wetland systems.
B11G-05
Spatial Patterns of Peatland Pore Water Chemistry and Correlation With LIDAR Derived Geomorphic Gradients: Indicators of Methylmercury Hot Spots in Forested Landscapes
Geomorphology can impose strong environmental gradients within and between landscape elements, giving rise to spatial non-stationarity of key biogeochemical processes. Spatial non-stationarity implies that the underlying statistical properties of a distribution, such as the mean and variance, are spatially non-uniform. This typically indicates spatial variability of some underlying process of interest. For example, zones of high spatial variance often occur at the interface of adjacent ecosystem compartments and coincide with biogeochemical hot spots. In recent work, we have demonstrated that spatially discrete zones of elevated methylmercury to total mercury ratios (MeHg:THg) occur almost exclusively at the outer margins of forested peatlands in northern Minnesota and northwestern Ontario. Here, we report on subsequent findings of a correlation between LIDAR (Light Detection and Ranging) derived geomorphic gradients and non-stationarity of peatland pore-water chemistry, most notably zones of elevated MeHg:THg ratios and variability in near surface peat pore waters. Specifically, we used edge-detection to quantify the zone of upland influence, or lagg area, in the surveyed peatlands, first using high-resolution ground surface topography and then using spatially distributed measurements of peat pore water chemistry. The two independently-derived areas of enhanced upland-peatland interaction are coincident, demonstrating the enticing potential to use remotely sensed data products to investigate the nature and occurrence of biogeochemical hot spots in forested landscapes.
B11G-06
Cycling of Mercury in the Sediments of the Penobscot River (ME) and Great Bay (NH) Estuaries
Due to the significant sedimentation of river-borne particulate matter in estuarine, these zones act as repositories for particulate contaminants including Hg. The sediments of estuaries and salt marshes have a high degree of geochemical variability, especially with respect to sulfate and organic matter, and are subject to significant fluctuations in water level and salinity that result in redox transitions in the zone close to the sediment-water interface (SWI). We performed pore water and sediment chemical and molecular analyses to study Hg cycling in the Penobscot River estuary (ME) and Great Bay (NH). At the latter site, sampling was conducted along a transect from the salt marsh to the mudflat. We observed a correspondence between the abundance of sulfate-reducing bacteria and MeHg production, both of which reached a maximum close to the SWI. In some cases, we observed rapid MeHg demethylation close to the SWI that may be due to the activity of iron-reducing bacteria that are dominant close to the SWI, or to the presence bacteria carrying the mer-A gene that may be expressed in Hg contaminated sediments. Our findings suggest that induced shoaling of the redoxcline, such as that observed in salt pannes, may be correlated with a shoaling of the net MeHg production zone and an increase in net MeHg production rate. Based on this, environments such as salt pannes, where the shallow redoxcline leads to the shoaling of the MeHg front, are likely to be Hg methylation and release hotspots.
B11G-07
Hot Spots and Hot Moments of Methylmercury Production Associated With Agricultural and Non-agricultural Wetlands of the Yolo Bypass Wildlife Area, California
The Yolo Bypass Wildlife Area (YBWA) is part of the larger Yolo Bypass floodwater protection zone associated with the Sacramento River and the Sacramento–San Joaquin Delta, in California. While mercury contamination is widespread throughout the region due to historic mining practices, the Yolo Bypass is responsible for a high proportion of the aqueous methylmercury (MeHg) entering the Delta, and biota from the Yolo Bypass are particularly elevated in toxic MeHg. Land use in the YBWA includes seasonally flooded agricultural fields (white rice, wild rice, fallow fields), and permanently and seasonally flooded non-agricultural wetlands used for resident and migratory waterfowl. Mercury biogeochemistry was examined in 0-2 cm surface sediment, as a function of habitat type, wetland management, and agricultural practices during the 2007-08 crop year. In permanently flooded wetlands, MeHg concentrations varied within a narrow range (ca. 0.5-1.5 ng/g dry wt) throughout the study period. In contrast, the three types of agricultural fields had higher MeHg concentrations throughout the rice-growing season (June-Sept; ca. 1.5-3.5 ng/g), and exhibited the highest levels (ca. 3.3-6.3 ng/g) in the post-harvest winter period (Dec-Feb). Further, naturally dried sediment, sampled during July '08 from post-harvest drained fallow agricultural fields (prior to reflooding) had MeHg concentrations that were also quite elevated (3.1 +/- 1.5 ng/g). This suggests that the initial elevated concentrations of overlying water MeHg, sometimes measured soon after flooding previously dried fields, may be related to the release of MeHg formed during the previous wet season and trapped in dried sediment, as opposed to being MeHg newly produced by bacteria upon soil rewetting. These results indicate that the 'hot spots and hot moments' associated with MeHg production in this system are linked to hydrologic manipulations (wetting and drying) in the agricultural fields, and that the practice of post-harvest reflooding of rice fields, to promote rice straw decomposition during the fall and winter, may stimulate microbial activity associated with increased MeHg production during that period.
B11G-08
The Rhizosphere Zone: A Hot Spot of Microbial Activity and Methylmercury Production in Saltmarsh Sediments of San Francisco Bay, California
Tidal marshes of varying hydrology and salinity have been shown to have high rates of microbial methylmercury (MeHg) production, especially the periodically flooded, higher elevations which are densely vegetated with shallowly rooted plants. The specific influence of emergent wetland plants and their active rhizosphere (root zone) on mercury (Hg) biogeochemistry, however, is poorly understood. Seasonal and spatial patterns of Hg biogeochemistry were examined in 2005 and 2006 at three marshes along a salinity gradient of the Petaluma River, in Northern San Francisco Bay, California. In addition, to directly examine the influence of rhizosphere activity on MeHg production, a suite of devegetation experiments was conducted in 2006 within each marsh using paired vegetated and devegetated plots in two marsh subhabitats: poorly- drained interior sites and well-drained "edge" sites near slough channels. Surface sediment (0-2cm) was sampled in both April and August from these plots, as well as from 1st and 3rd order slough channels that were naturally free of vegetation. Vegetated marsh sites produced 3- to19-fold more MeHg than did slough sites, and MeHg production rates were greater in marsh interior sites compared to more oxic marsh "edge" sites. Microbial biomass (ng DNA gdrysed) was greater in vegetated marsh settings, compared to slough channels, and increased significantly between April and August at all marsh sites. Despite this seasonal increase in microbial biomass, MeHg concentrations and production rates decreased from April to August in vegetated surface sediments. Microbial indicators of methylation also decreased from April to August, including rates of microbial sulfate reduction and the abundance of iron- and sulfate- reducing bacterial DNA. Results from the devegetated plots suggest that root exudation of fermentative labile carbon to surface soils is responsible for the higher microbial biomass, and the higher relative abundance of iron- and sulfate-reducing bacterial DNA in vegetated sites. In contrast to microbial indicators of mercury methylation, no effects of devegetation or seasonal sampling were observed on sediment pools of reactive Hg (Hg(II)R), although the relative abundance of mercuric reductase functional genes (MerA) was reduced by devegetation and was correlated with live root density. These experiments, among others, demonstrate that the presence and activity of emergent wetland plants directly influence Hg cycling in densely rooted surface soils. Development of this shallow and dense rhizosphere is likely to be a primary reason as to why periodically flooded, high elevation marsh sites are a "hot spot" for mercury methylation.