Controls of Methylmercury Bioaccumulation in Forest Floor Food Webs.

Compared to the extensive research on aquatic ecosystems, very little is known about the sources and trophic transfer of methylmercury (MeHg) in terrestrial ecosystems. In this study, we examine energy flow and trophic structure using stable carbon (δ13C) and nitrogen (δ15N) isotope ratios, respectively, and MeHg levels in basal resources and terrestrial invertebrates from four temperate forest ecosystems. We show that MeHg levels in biota increased significantly ( p < 0.01) with δ13C and δ15N at all sites, implying the importance of both microbially processed diets (with increased δ13C) and trophic level (with increased δ15N) at which organisms feed, on MeHg levels in forest floor biota. The trophic magnification slopes of MeHg (defined as the slope of log10MeHg vs δ15N) for these forest floor food webs (0.20-0.28) were not significantly different ( p > 0.05) from those observed for diverse temperate freshwater systems (0.24 ± 0.07; n = 78), demonstrating for the first time the nearly equivalent efficiencies with which MeHg moves up the food chain in these contrasting ecosystem types. Our results suggest that in situ production of MeHg within the forest floor and efficient biomagnification both elevate MeHg levels in carnivorous invertebrates in temperate forests, which can contribute to significant bioaccumulation of this neurotoxin in terrestrial apex predators.

The influence of sample drying and storage conditions on methylmercury determination in soils and sediments.

The separate influences of drying and storage conditions on methylmercury (MeHg) concentrations in soil and sediment samples were investigated. Concentrations of MeHg and total Hg were determined in various soil and sediment samples that had been stored or dried under differing conditions. The influence of drying conditions (oven-drying (40 °C) versus freeze-drying) on MeHg concentrations in marine sediments, river sediments, soils, and paddy field soils was investigated (n = 43). The ratio of the MeHg concentration in oven-dried sub-samples divided by the concentration in freeze-dried sub-samples ranged from 0 to 336%. In order to confirm the production of MeHg during storage in some samples, Hg2+ was added at 15 mg kg-1 to a paddy soil, and the sample was then stored at 30 °C. The concentrations of MeHg at 1-h, 1-day, 4-days and 7-days after Hg2+ spiking were 2.0 ± 0.1, 13.8 ± 1.0, 36.0 ± 5.0, and 24.9 ± 1.6 µg kg-1 (n = 3), respectively. The concentration of MeHg at 4-days after Hg spiking and sterilizing (121 °C, 30 min) was 1.8 µg kg-1, similar to the original value. These results indicate that bacterial Hg methylation and MeHg demethylation occurred within days in the soil. In addition, tests of the stability of MeHg in wet and dry samples during storage were also performed. Overall, our results indicate that the best way to preserve MeHg in soil and sediment samples is to freeze the samples immediately after collection, followed subsequently by freeze-drying, grinding, homogenization, and storage of the dry material in cool, dark conditions until analysis.

An inter-laboratory comparison of different analytical methods for the determination of monomethylmercury in various soil and sediment samples: A platform for method improvement.

An inter-laboratory study was conducted to compare results from different analytical methods for monomethylmercury (MeHg) concentrations in 17 soil and sediment samples. The samples were collected from mercury-contaminated areas, including Minamata Bay and Kagoshima Bay in Japan, the Idrija mercury mine in Slovenia, and an artisanal small-scale gold mining area in Indonesia. The Hg in these samples comes from several different sources: industrial waste from an acetaldehyde production facility, volcanic activity, Hg mining activity, and artisanal and small-scale gold mining activity (ASGM). MeHg concentrations in all the samples were measured in four separate laboratories, using three different determination methods: Kagoshima University (Japan), using high-performance liquid chromatography-chemiluminescence detection (HPLC-CL); National Institute for Minamata Disease (Japan), using gas chromatography-electron capture detection; and Metropolitan Council Environmental Services (USA) and Jozef Stefan Institute (Slovenia), both using alkylation-gas chromatography-atomic fluorescence spectrometry detection. The methods gave comparable MeHg results for most of the samples tested, but for some samples, the results exhibited significant variability depending on the method used. The HPLC-CL method performed poorly when applied to samples with elevated sulfur concentrations, producing MeHg concentrations that were much lower than those from the other methods. Additional analytical work demonstrated the elimination of this sulfur interference when the method was modified to bind sulfur prior to the analytical step by using Hg2+ as a masking agent. These results demonstrate the value of laboratory intercomparison exercises in contributing to the improvement of analytical methods.

Variation in terrestrial and aquatic sources of methylmercury in stream predators as revealed by stable mercury isotopes.

Mercury (Hg) is widely distributed in the environment, and its organic form, methylmercury (MeHg), can extensively bioaccumulate and biomagnify in aquatic and terrestrial food webs. Concentrations of MeHg in organisms are highly variable, and the sources in natural food webs are often not well understood. This study examined stable isotope ratios of MeHg (mass-dependent fractionation, as δ(202)HgMeHg; and mass-independent fractionation, as Δ(199)HgMeHg) in benthic invertebrates, juvenile steelhead trout (Oncorhynchus mykiss), and water striders (Gerris remigis) along a stream productivity gradient, as well as carnivorous terrestrial invertebrates, in a forested watershed at the headwater of South Fork Eel River in northern California. Throughout the sampling sites, δ(202)HgMeHg (after correction due to the effect of MeHg photodegradation) was significantly different between benthic (median = -1.40‰; range, -2.34 to -0.78‰; total number of samples = 29) and terrestrial invertebrates (median = +0.51‰; range, -0.37 to +1.40‰; total number of samples = 9), but no major difference between these two groups was found for Δ(199)HgMeHg. Steelhead trout (52 individual fishes) have MeHg of predominantly aquatic origins, with a few exceptions at the upstream locations (e.g., 1 fish collected in a tributary had a purely terrestrial MeHg source and 4 fishes had mixed aquatic and terrestrial MeHg sources). Water striders (seven pooled samples) derive MeHg largely from terrestrial sources throughout headwater sections. These data suggest that direct terrestrial subsidy (e.g., terrestrial invertebrates falling into water) can be important for some stream predators in headwater streams and could represent an important means of transfer of terrestrially derived MeHg (e.g., in situ methylation within forests, atmospheric sources) to aquatic ecosystems. Moreover, these findings show that terrestrial subsidies can enhance MeHg bioaccumulation of consumers in headwater streams where aqueous MeHg levels are very low.

Sources and transfers of methylmercury in adjacent river and forest food webs.

Nearly all ecosystems are contaminated with highly toxic methylmercury (MeHg), but the specific sources and pathways leading to the uptake of MeHg within and among food webs are not well understood. In this study, we report stable mercury (Hg) isotope compositions in food webs in a river and an adjacent forest in northern California and demonstrate the utility of Hg isotopes for studying MeHg sources and cross-habitat transfers. We observed large differences in both δ(202)Hg (mass-dependent fractionation) and Δ(199)Hg (mass-independent fractionation) within both food webs. The majority of isotopic variation within each food web could be accounted for by differing proportions of inorganic Hg [Hg(II)] and MeHg along food chains. We estimated mean isotope values of Hg(II) and MeHg in each habitat and found a large difference in δ(202)Hg between Hg(II) and MeHg (∼2.7‰) in the forest but not in the river (∼0.25‰). This is consistent with in situ Hg(II) methylation in the study river but suggests Hg(II) methylation may not be important in the forest. In fact, the similarity in δ(202)Hg between MeHg in forest food webs and Hg(II) in precipitation suggests that MeHg in forest food webs may be derived from atmospheric sources (e.g., rainfall, fog). Utilizing contrasting δ(202)Hg values between MeHg in river food webs (-1.0‰) and MeHg in forest food webs (+0.7‰), we estimate with a two-source mixing model that ∼55% of MeHg in two riparian spiders is derived from riverine sources while ∼45% of MeHg originates from terrestrial sources. Thus, stable Hg isotopes can provide new information on subtle differences in sources of MeHg and trace MeHg transfers within and among food webs in natural ecosystems.

In situ production of methylmercury within a stream channel in northern California.

Natural stream ecosystems throughout the world are contaminated by methylmercury, a highly toxic compound that bioaccumulates and biomagnifies in aquatic food webs. Wetlands are widely recognized as hotspots for the production of methylmercury and are often assumed to be the main sources of this neurotoxin in downstream ecosystems. However, many streams lacking wetlands in their drainage basins (e.g., montane and semiarid regions in the western United States) have significant methylmercury contamination, and the sources of methylmercury in these streams remain largely unknown. In this study, we observed substantial production of methylmercury within a highly productive stream channel in northern California (South Fork Eel River) within a drainage basin lacking wetlands. We found that in situ methylmercury production is positively related to phosphorus removal and water temperature within the stream channel, supporting hypothesized biological mediation of in situ mercury transformation. Moreover, our data suggest that epiphytic microbial communities on a dominant filamentous alga (Cladophora glomerata) could play a role in in situ methylmercury production. Because peak in situ methylmercury production coincides with the period of the highest biological productivity during summer baseflow, methylmercury produced internally may be efficiently routed into local stream food webs. Our study provides strong evidence that stream channels, especially those associated with high primary productivity, can be important for regulating the bioavailability and toxicity of this global contaminant.

Methylmercury input to the Mississippi River from a large metropolitan wastewater treatment plant.

Methylmercury (MeHg) and total mercury (THg) inputs to the Mississippi River from a large metropolitan wastewater treatment plant were measured to characterize the relative contribution of the treatment plant to in-stream loads of these contaminants. Concentrations of MeHg and THg were determined in filtered and unfiltered whole water samples collected weekly from the treatment plant effluent stream and from the river upstream of the plant discharge. Unfiltered MeHg concentrations in the plant effluent ranged from 0.034 to 0.062 ng L(-1) and were always less than those in the river (range: 0.083-0.227 ng L(-1)). The MeHg loading to the river from the treatment plant ranged from 0.026 to 0.051 g d(-1) and averaged 0.037 g d(-1) over the 13-week sampling period. The in-stream MeHg load in the river upstream varied widely depending on hydrologic conditions, ranging from 0.91 to 18.8 g d(-1) and averaging 4.79 g d(-1). The treatment plant discharge represented 1.6%, on average, of the in-stream MeHg load, ranging from 0.2 to 3.5% depending on flow conditions in the river. MeHg in treatment plant effluent was primarily in the filtered phase (mean: 57%, <0.2 microm), but in the river the filtered/unfiltered ratio (F/UF) was typically less than 30% except during a major precipitation runoff event, when F/UF increased to 78%. The MeHg/THg ratio in unfiltered treatment plant effluent varied little (range: 1.6-1.9%), suggesting that THg concentration can serve as a relatively accurate proxy for MeHg concentration in this effluent stream. Supplemental sampling of the treatment plant influent stream showed that removals of MeHg and THg across the treatment process averaged 97% and 99%, respectively. These results show the treatment plant to be effective in removing MeHg and THg from wastewater and in minimizing its impact on Hg levels in the receiving water.

Characteristics of mercury speciation in Minnesota rivers and streams.

Patterns of mercury (Hg) speciation were examined in four Minnesota streams ranging from the main-stem Mississippi River to small tributaries in the basin. Filtered phase concentrations of methylmercury (MeHg), inorganic Hg (IHg), and dissolved organic carbon (DOC) were higher in all streams during a major summertime runoff event, and DOC was enriched with MeHg but not with IHg. Particulate-phase MeHg and IHg concentrations generally increased with total suspended solids (TSS) concentrations but the event data did not diverge greatly from the non-event data, suggesting that sources of suspended sediments in these streams did not vary significantly between event and non-event samplings. The dissolved fractions (filtered concentration/unfiltered concentration) of both MeHg and IHg increased with increasing DOC concentrations, but varied inversely with TSS concentrations. While MeHg typically constitutes only a minor portion of the total Hg (THg) in these streams, this contribution is not constant and can vary greatly over time in response to watershed inputs.

Mercury mass balance at a wastewater treatment plant employing sludge incineration with offgas mercury control.

Efforts to reduce the deliberate use of mercury (Hg) in modern industrialized societies have been largely successful, but the minimization and control of Hg in waste streams are of continuing importance. Municipal wastewater treatment plants are collection points for domestic, commercial, and industrial wastewaters, and Hg removal during wastewater treatment is essential for protecting receiving waters. Subsequent control of the Hg removed is also necessary to preclude environmental impacts. We present here a mass balance for Hg at a large metropolitan wastewater treatment plant that has recently been upgraded to provide for greater control of the Hg entering the plant. The upgrade included a new fluidized bed sludge incineration facility equipped with activated carbon addition and baghouse carbon capture for the removal of Hg from the incinerator offgas. Our results show that Hg discharges to air and water from the plant represented less than 5% of the mass of Hg entering the plant, while the remaining Hg was captured in the ash/carbon residual stream exiting the new incineration process. Sub-optimum baghouse operation resulted in some of the Hg escaping collection there and accumulating with the ash/carbon particulate matter in the secondary treatment tanks. Overall, the treatment process is effective in removing Hg from wastewater and sequestering it in a controllable stream for secure disposal.

Elevated methylmercury concentrations and loadings during flooding in Minnesota rivers.

Previous studies have identified flooded landscapes (e.g., wetlands, impoundments) as sites of elevated methylmercury (MeHg) production. Here we report MeHg and total Hg (THg) concentrations and mass loadings in rivers in Minnesota during major flooding episodes in the summer of 2002. Frequent intense precipitation events throughout the summer resulted in extraordinarily wet conditions in east-central and northwestern Minnesota. Streamflow remained at record-setting high levels in many rivers and streams in these regions for several weeks. We observed high concentrations of MeHg (>1.4 ng/L) accompanied by high MeHg/THg ratios (0.39 to 0.50) in the Roseau River in northwestern Minnesota and in the Elk and Rum Rivers in east-central Minnesota. Very high MeHg mass loadings were observed in the Mississippi River just upstream of Minneapolis on July 17 (51 g MeHg/day) and July 23 (42 g MeHg/day), when MeHg concentrations at this site were 0.89 and 0.99 ng/L, respectively. The elevated MeHg concentrations in the Roseau River were associated with low dissolved oxygen and high dissolved reactive phosphorus concentrations, both of which are characteristic of anoxic waters. These rivers drain landscapes containing varying amounts of wetlands, and some of the MeHg discharged is thought to have been flushed from anoxic wetland soils. In addition, the flooding of vast areas of normally dry land surfaces probably also resulted in increased MeHg production, adding to the quantities of MeHg exported from these watersheds. Changing climate patterns are expected to result in more frequent heavy precipitation and flooding events in Minnesota. Our results suggest that as flooding and wet conditions in this region increase, the production of MeHg and its export from terrestrial areas to surface waters will increase also.

A comparison of total mercury and methylmercury export from various Minnesota watersheds.

Methylmercury (MeHg) bioaccumulates in aquatic food webs and can pose health risks to animals at higher trophic levels. Characterization of MeHg production in and export from watersheds can help clarify exposure scenarios for aquatic life downstream. A number of studies have demonstrated that anoxic conditions in the saturated soils of wetlands can promote the production of MeHg, and these wetlands may be major sources of MeHg to connected water bodies. Here, we report in-stream loadings of total mercury (THg) and MeHg for five rivers in Minnesota (USA). The watersheds of these rivers differ widely in the proportion of land area made up by wetlands and in other land use, drainage, and soil characteristics. Export of THg from these rivers varied widely, with much higher loadings and annual average concentrations of THg in streams of the Minnesota River basin compared to streams in the headwater Mississippi River basin. In contrast and despite the apparent differences in the makeup of these watersheds, yields and annual average concentrations of MeHg were remarkably similar for the rivers studied here. Differences in land use/land cover, drainage, soils, and other characteristics of these watersheds influence the export of both THg and MeHg in these rivers, but overall MeHg yields vary less than THg yields.

Redox chemistry in Minnesota streams during episodes of increased methylmercury discharge.

Mercury (Hg) and methylmercury (MeHg) are flushed from watersheds during hydrological events, contaminating downstream surface waters and resident fish populations. We monitored total mercury (THg), MeHg, and ancillary water chemistry parameters in two streams (Cedar Creek and Trott Brook) in east-central Minnesota on a weekly or semiweekly basis from April through October 2003. Heavy precipitation in late June resulted in discrete episodes of high concentrations (>1.2 ng/L) of MeHg in both streams in early July. The MeHg/THg ratio increased from 0.15 to 0.36 in Cedar Creek and from 0.13 to 0.46 in Trott Brook during the event. The high MeHg concentrations were accompanied by low dissolved oxygen concentrations and increased concentrations of dissolved organic carbon, Mn, Fe, and orthophosphate. A prolonged absence of precipitation during August and early September brought stream levels back to baseflow values, and MeHg concentrations decreased to less than 0.1 ng/L. These results suggest that warm-weather, high-discharge events are the primary route of export of MeHg from these watersheds, and baseflow contributes much less MeHg to downstream waters. The redox water chemistry during the,events sampled here suggests that MeHg in these streams is discharged from wetland areas where anoxic/anaerobic conditions prevail.