Ecosystem Responses to Changed Atmospheric Mercury Load: Results from Seven Years of Mercury Loading to Lake 658
The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. To test directly the response of fish contamination to changing mercury deposition, we are conducting the METAALICUS study, a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed us to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed. Loading began in 2001 and ended in 2007. In this paper we will present mercury and methylmercury budgets for the study lake for the entire 7 year loading period. Overall, we increased the total Hg load to L658 and its watershed by roughly a factor of 3. However, we only increased the Hg load the lake itself by about 2X, since, during the seven years of addition, almost none of the Hg spike deposited to the watershed was transported all the way to the lake. Spike Hg concentrations in lake water rose each year during the open-water loading period and declined rapidly each winter. Methylmercury production in the lake responded rapidly to changes in mercury load during the first year of addition. After about 3 years, the increase in MeHg in lake water and in surface sediments slowed, suggesting that MeHg production was approaching a new level, or different rate, in response to the increased Hg load. We will discuss major input and loss terms for newly deposited Hg, the timing and proportionality of response, the timing and locations of MeHg production within the lake.
Multi Year Total and Methyl Mercury Exports from Two Major Sub Arctic Rivers Draining into Hudson Bay, Canada
From 2003 to 2007, concentrations of total and methyl mercury (THg and MeHg), were continuously measured in two Canadian sub Arctic rivers (the Nelson and the Churchill) that drain into western Hudson Bay. THg and MeHg concentrations were low in the Nelson River (mean ± standard deviation; 0.88±0.33 and 0.05±0.03 ng/L, respectively). The Churchill River, however, had high concentrations of Hg, particularly MeHg (1.96±0.8 and 0.18±0.09 ng/L, respectively), and hence may be an important source of MeHg to organisms feeding in the Churchill River estuary. A large portion of THg in the Nelson River was particulate- bound (39±23%), while in the Churchill River, most was in the dissolved form (78±15%) and is likely DOC-bound Hg originating in surrounding wetlands. In fact, both the Nelson and Churchill Rivers had high DOC concentrations and were therefore large exporters of DOC to Hudson Bay (1480±723 and 392±309 x 103 tonnes/year, respectively) compared to rivers to the south and east. Despite high Churchill River Hg concentrations, due to large Nelson River flows, average THg and MeHg exports to Hudson Bay from the Churchill River (37±28 and 4±4 kg/year, respectively) were ∼ one third and half Nelson River exports (113±52 and 9±4 kg/year). Interestingly, combined Hg exports to Hudson Bay from Nelson and Churchill River discharge are comparable to THg inputs from Hudson Bay spring-time snowmelt (177±140 kg/year) but are approximately 13 times greater than MeHg snowmelt inputs (1±1 kg/year). Although Hg inputs from rivers and snowmelt together may account for a large portion of the THg pool in Hudson Bay, these inputs account for a lesser portion of the MeHg pool, thus highlighting the importance of water column Hg(II) methylation as a large source of MeHg to Hudson Bay marine foodwebs.
Total mercury and methylmercury export from terrestrial uplands in a changing climate
Terrestrial uplands hold a large reservoir of legacy mercury (Hg) accumulated from decades of elevated anthropogenic deposition. The fate of this legacy Hg and new inputs of atmospheric Hg will affect the supply of Hg ultimately available for uptake in aquatic food webs, the primary exposure pathway to humans and wildlife through contaminated fish consumption. We have been investigating the dynamics of total mercury (THg) and methylmercury (MeHg) stream export in relation to organic matter quantity and quality in diverse forested landscapes over a range of hydrologic conditions in the northeastern USA. Stream export of THg and MeHg is highly episodic during high flows and is tightly linked to the transport of dissolved and particulate organic carbon. Here, we summarize our state of knowledge of Hg in these systems, and based on this understanding project how Hg and MeHg may respond to future climate scenarios. Regardless of future Hg deposition patterns, a changing climate may increase future THg and/or MeHg export through a) increases in the frequency and magnitude of high flows; b) intensification of organic carbon cycling due to warmer temperatures, increased productivity and decomposition rates, and/or changes in atmospheric deposition chemistry; c) increased net Hg methylation rates at higher temperatures; and d) shifts in DOC quality.
Methylmercury Production Across San Francisco Bay Regional Habitats: Balancing Benthic Microbial Activity and Inorganic Mercury Availability
The specific controls on the production of highly toxic methylmercury (MeHg) in sediments are many, but can largely be grouped into those that control either the activity of bacteria that methylate inorganic divalent mercury (Hg(II)) or the availability of Hg(II) to those bacteria. The San Francisco Bay estuary and its watershed is a region contaminated with mercury from both historic mining sources and contemporary anthropogenic inputs. For more than a decade, the USGS has conducted research into the factors that control benthic MeHg production in a diverse suite of habitats throughout this system, across a range of both salinity and hydrologic gradients. Consistent approaches were used throughout multiple individual regional studies to assess both rates of microbial activity and available Hg(II) concentration, as well as the geochemical constituents that influence each of these two major factors. This presentation will briefly summarize this body of research, the results of which are widely applicable to other diverse freshwater, estuarine, and coastal environments. Key findings include: a) total mercury concentrations are a poor predictor of MeHg concentrations; b) only a small fraction (ca. < 0.1% to 5%) of total Hg(II) is available for microbial methylation, and this fraction decreases as sediment conditions become more chemically reducing; c) the activity of the Hg(II)-methylating microbial community varies more widely across different habitat types than does the availability of Hg(II) to those communities; d) iron-reducing bacteria may play a central role in mediating Hg(II)-methylation in freshwater habitats; and e) seasonal wetlands and other periodically inundated habitats are more important zones of MeHg production than permanently submerged habitats.
Mercury Deposition and Emission under the Forest Canopy in the Adirondacks of New York
This study investigated mercury input, output, cycling, and interactions between deposition and emission at the Huntington Wildlife Forest of Newcomb, New York during 2005 and 2006. Total Hg wet deposition and deciduous throughfall were collected using modified MIC-B precipitation collectors. The volume-weighted mean (VWM) total Hg concentration in throughfall (6.6 ng L-1) was higher than in precipitation (4.9 ng L-1), while the total cumulative Hg flux in throughfall (12.0 ug m-2) was very similar to precipitation (11.6 ug m-2) due to relatively lower precipitation depths in throughfall. The total deposited cumulative flux in precipitation and deciduous throughfall were very similar (11.6 ug m-2 and 12.0 ug m-2, respectively) because the higher concentrations in throughfall were offset by smaller throughfall depths. The emission flux of gaseous elemental mercury (GEM) from the forest floor was measured using a polycarbonate dynamic flux chamber (DFC). The Hg emission flux ranged between -2.5 ng m-2 hr-1 and 27.2 ng m-2 hr-1. The measured Hg emission flux was highest in spring, and summer, and lowest in winter. During leaf-off periods, the Hg emission flux was highly dependent on solar radiation and less dependent on temperature. During leaf-on periods, the Hg emission flux was fairly constant because the forest canopy was shading the forest floor. The empirical models estimated the cumulative emission flux was 7.0 ug Hg0 m-2 year-1.
Mercury dynamics in an urban watershed ecosystem
Mercury (Hg) ecosystem cycling and the associated biogeochemical relationships are affected by land use. However, studies concerning developed ecosystems are limited with previous research focused on non-urban, remote ecosystems. In our study, we investigated the effects of land use on watershed Hg cycling in a series of nested, increasingly-urbanized watersheds (Baltimore, MD) with samples collected to target baseflow and elevated discharge conditions with subsequent sampling to examine Hg-organic carbon relationships. Under base flow conditions, total Hg (THg) and methyl Hg (MeHg) concentrations varied from 0.17-207 ng/L (mean ± SD, 4.5 ± 1.2) and 0.01-5.5 ng/L (0.10 ± 0.03), respectively. While increasing with urbanization, Hg concentrations and fluxes were lower in urbanized watersheds relative to those in a forested reference watershed with the percentage of THg as MeHg not correlated with dissolved organic carbon (DOC). During events, THg concentrations at the urban outlet increased to 100 ng/L and flux increased dramatically with discharge. THg was correlated with TSS, without a similar correlation with DOC or MeHg, signifying particulate- dominated flux. The decrease in particulate POC content with discharge suggests in-stream mobilization, raising the importance of Hg-OC solid interactions. In general, our results suggest that changes in carbon cycling associated with urbanization appear to have altered the tight coupling among DOC, THg, and MeHg dynamics that strongly influences watershed Hg exports from forested ecosystems. Also, urbanization supplies additional sources of Hg and DOC in the watershed with anthropogenic organics, rather than natural humic material, as the source of the additional base flow DOC.