12 research outputs found
Mercury Cycling in Stream Ecosystems. 1. Water Column Chemistry and Transport
We studied total mercury (THg) and methylmercury (MeHg) in eight streams, located in Oregon, Wisconsin, and Florida, that span large ranges in climate, landscape characteristics, atmospheric Hg deposition, and water chemistry. While atmospheric deposition was the source of Hg at each site, basin characteristics appeared to mediate this source by providing controls on methylation and fluvial THg and MeHg transport. Instantaneous concentrations of filtered total mercury (FTHg) and filtered methylmercury (FMeHg) exhibited strong positive correlations with both dissolved organic carbon (DOC) concentrations and streamflow for most streams, whereas mean FTHg and FMeHg concentrations were correlated with wetland density of the basins. For all streams combined, whole water concentrations (sum of filtered and particulate forms) of THg and MeHg correlated strongly with DOC and suspended sediment concentrations in the water column
Lacustrine Responses to Decreasing Wet Mercury Deposition Ratesî—¸Results from a Case Study in Northern Minnesota
We present a case
study comparing metrics of methylmercury (MeHg)
contamination for four undeveloped lakes in Voyageurs National Park
to wet atmospheric deposition of mercury (Hg), sulfate (SO<sub>4</sub><sup>–2</sup>), and hydrogen ion (H<sup>+</sup>) in northern
Minnesota. Annual wet Hg, SO<sub>4</sub><sup>–2</sup>, and
H<sup>+</sup> deposition rates at two nearby precipitation monitoring
sites indicate considerable decreases from 1998 to 2012 (mean decreases
of 32, 48, and 66%, respectively). Consistent with decreases in the
atmospheric pollutants, epilimnetic aqueous methylmercury (MeHg<sub>aq</sub>) and mercury in small yellow perch (Hg<sub>fish</sub>) decreased
in two of four lakes (mean decreases of 46.5% and 34.5%, respectively,
between 2001 and 2012). Counter to decreases in the atmospheric pollutants,
MeHg<sub>aq</sub> increased by 85% in a third lake, whereas Hg<sub>fish</sub> increased by 80%. The fourth lake had two disturbances
in its watershed during the study period (forest fire; changes in
shoreline inundation due to beaver activity); this lake lacked overall
trends in MeHg<sub>aq</sub> and Hg<sub>fish</sub>. The diverging responses
among the study lakes exemplify the complexity of ecosystem responses
to decreased loads of atmospheric pollutants
Correction to Organic Carbon Burial in Lakes and Reservoirs of the Conterminous United States
Correction
to Organic Carbon Burial in Lakes and Reservoirs
of the Conterminous United State
Stable Mercury Isotopes in Polished Rice (<i>Oryza sativa</i> L.) and Hair from Rice Consumers
Mercury (Hg) isotopic signatures
were characterized in polished
rice samples from China, U.S., and Indonesia (<i>n</i> =
45). Hg isotopes were also analyzed in paired hair samples for participants
from China (<i>n</i> = 21). For the latter, we also quantified
the proportion of methylmercury intake through rice (range: 31–100%),
and the weekly servings of fish meals (range: 0–5.6 servings/weekly).
For these participants, 29% (<i>n</i> = 6) never ingested
fish, 52% (<i>n</i> = 11) ingested fish < twice/weekly,
and 19% (<i>n</i> = 4) ingested fish ≥ twice/weekly.
In rice and hair, both mass-dependent fractionation (MDF, reported
as δ<sup>202</sup>Hg) and mass-independent fractionation (MIF,
reported as Δ<sup>199</sup>Hg) of Hg isotopes were observed.
Compared to rice, hair δ<sup>202</sup>Hg values were enriched
on average (±1 standard deviation) by 1.9 ± 0.61‰,
although the range was wide (range: 0.45‰, 3.0‰). Hair
Δ<sup>199</sup>Hg was significantly inversely associated with
%methylmercury intake from rice (Spearman’s rho = −0.61, <i>p</i> < 0.01, <i>n</i> = 21), i.e., as the proportion
of methylmercury intake from rice increased, MIF decreased. Additionally,
hair Δ<sup>199</sup>Hg was significantly higher for participants
ingesting fish ≥ twice/weekly compared to those who did not
ingest fish or ingested fish < twice/weekly (ANOVA, <i>p</i> < 0.05, <i>n</i> = 21); Overall, results suggest that
Hg isotopes (especially MIF) in human hair can be used to distinguish
methylmercury intake from rice versus fish
Organic Carbon Burial in Lakes and Reservoirs of the Conterminous United States
Organic
carbon (OC) burial in lacustrine sediments represents an
important sink in the global carbon cycle; however, large-scale OC
burial rates are poorly constrained, primarily because of the sparseness
of available data sets. Here we present an analysis of OC burial rates
in water bodies of the conterminous U.S. (CONUS) that takes advantage
of recently developed national-scale data sets on reservoir sedimentation
rates, sediment OC concentrations, lake OC burial rates, and water
body distributions. We relate these data to basin characteristics
and land use in a geostatistical analysis to develop an empirical
model of OC burial in water bodies of the CONUS. Our results indicate
that CONUS water bodies sequester 20.8 (95% CI: 9.4–65.8) Tg
C yr<sup>–1</sup>, and spatial patterns in OC burial are strongly
influenced by water body type, size, and abundance; land use; and
soil and vegetation characteristics in surrounding areas. Carbon burial
is greatest in the central and southeastern regions of the CONUS,
where cultivation and an abundance of small water bodies enhance accumulation
of sediment and OC in aquatic environments
Use of Stable Isotope Signatures to Determine Mercury Sources in the Great Lakes
Sources
of mercury (Hg) in Great Lakes sediments were assessed
with stable Hg isotope ratios using multicollector inductively coupled
plasma mass spectrometry. An isotopic mixing model based on mass-dependent
(MDF) and mass-independent fractionation (MIF) (δ<sup>202</sup>Hg and Δ<sup>199</sup>Hg) identified three primary Hg sources
for sediments: atmospheric, industrial, and watershed-derived. Results
indicate atmospheric sources dominate in Lakes Huron, Superior, and
Michigan sediments while watershed-derived and industrial sources
dominate in Lakes Erie and Ontario sediments. Anomalous Δ<sup>200</sup>Hg signatures, also apparent in sediments, provided independent
validation of the model. Comparison of Δ<sup>200</sup>Hg signatures
in predatory fish from three lakes reveals that bioaccumulated Hg
is more isotopically similar to atmospherically derived Hg than a
lake’s sediment. Previous research suggests Δ<sup>200</sup>Hg is conserved during biogeochemical processing and odd mass-independent
fractionation (MIF) is conserved during metabolic processing, so it
is suspected even is similarly conserved. Given these assumptions,
our data suggest that in some cases, atmospherically derived Hg may
be a more important source of MeHg to higher trophic levels than legacy
sediments in the Great Lakes
Stream Mercury Export in Response to Contemporary Timber Harvesting Methods (Pacific Coastal Mountains, Oregon, USA)
Land-use activities can alter hydrological
and biogeochemical processes
that can affect the fate, transformation, and transport of mercury
(Hg). Previous studies in boreal forests have shown that forestry
operations can have profound but variable effects on Hg export and
methylmercury (MeHg) formation. The Pacific Northwest is an important
timber producing region that receives large atmospheric Hg loads,
but the impact of forest harvesting on Hg mobilization has not been
directly studied and was the focus of our investigation. Stream discharge
was measured continuously, and Hg and MeHg concentrations were measured
monthly for 1.5 years following logging in three paired harvested
and unharvested (control) catchments. There was no significant difference
in particulate-bound Hg concentrations or loads in the harvested and
unharvested catchments which may have resulted from forestry practices
aimed at minimizing erosion. However, the harvested catchments had
significantly higher discharge (32%), filtered Hg concentrations (28%),
filtered Hg loads (80%), and dissolved organic carbon (DOC) loads
(40%) compared to forested catchments. MeHg concentrations were low
(mostly <0.05 ng L<sup>–1</sup>) in harvested, unharvested,
and downstream samples due to well-drained/unsaturated soil conditions
and steep slopes with high energy eroding stream channels that were
not conducive to the development of anoxic conditions that support
methylation. These results have important implications for the role
forestry operations have in affecting catchment retention and export
of Hg pollution
Mercury Cycling in Stream Ecosystems. 2. Benthic Methylmercury Production and Bed Sediment−Pore Water Partitioning
Mercury speciation, controls on methylmercury (MeHg) production, and bed sediment−pore water partitioning of total Hg (THg) and MeHg were examined in bed sediment from eight geochemically diverse streams where atmospheric deposition was the predominant Hg input. Across all streams, sediment THg concentrations were best described as a combined function of sediment percent fines (%fines; particles < 63 μm) and organic content. MeHg concentrations were best described as a combined function of organic content and the activity of the Hg(II)-methylating microbial community and were comparable to MeHg concentrations in streams with Hg inputs from industrial and mining sources. Whole sediment tin-reducible inorganic reactive Hg (Hg(II)<sub>R</sub>) was used as a proxy measure for the Hg(II) pool available for microbial methylation. In conjunction with radiotracer-derived rate constants of <sup>203</sup>Hg(II) methylation, Hg(II)<sub>R</sub> was used to calculate MeHg production potential rates and to explain the spatial variability in MeHg concentration. The %Hg(II)<sub>R</sub> (of THg) was low (2.1 ± 5.7%) and was inversely related to both microbial sulfate reduction rates and sediment total reduced sulfur concentration. While sediment THg concentrations were higher in urban streams, %MeHg and %Hg(II)<sub>R</sub> were higher in nonurban streams. Sediment pore water distribution coefficients (log <i>K</i><sub>d</sub>’s) for both THg and MeHg were inversely related to the log-transformed ratio of pore water dissolved organic carbon (DOC) to bed sediment %fines. The stream with the highest drainage basin wetland density also had the highest pore water DOC concentration and the lowest log <i>K</i><sub>d</sub>’s for both THg and MeHg. No significant relationship existed between overlying water MeHg concentrations and those in bed sediment or pore water, suggesting upstream sources of MeHg production may be more important than local streambed production as a driver of water column MeHg concentration in drainage basins that receive Hg inputs primarily from atmospheric sources
Spatial Dependence of Reduced Sulfur in Everglades Dissolved Organic Matter Controlled by Sulfate Enrichment
Sulfate inputs to the Florida Everglades
stimulate sulfidic conditions
in freshwater wetland sediments that affect ecological and biogeochemical
processes. An unexplored implication of sulfate enrichment is alteration
of the content and speciation of sulfur in dissolved organic matter
(DOM), which influences the reactivity of DOM with trace metals. Here,
we describe the vertical and lateral spatial dependence of sulfur
chemistry in the hydrophobic organic acid fraction of DOM from unimpacted
and sulfate-impacted Everglades wetlands using X-ray absorption spectroscopy
and ultrahigh-resolution mass spectrometry. Spatial variation in DOM
sulfur content and speciation reflects the degree of sulfate enrichment
and resulting sulfide concentrations in sediment pore waters. Sulfur
is incorporated into DOM predominantly as highly reduced species in
sulfidic pore waters. Sulfur-enriched DOM in sediment pore waters
exchanges with overlying surface waters and the sulfur likely undergoes
oxidative transformations in the water column. Across all wetland
sites and depths, the total sulfur content of DOM correlated with
the relative abundance of highly reduced sulfur functionality. The
results identify sulfate input as a primary determinant on DOM sulfur
chemistry to be considered in the context of wetland restoration and
sulfur and trace metal cycling
Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes
Identifying
the sources of methylmercury (MeHg) and tracing the
transformations of mercury (Hg) in the aquatic food web are important
components of effective strategies for managing current and legacy
Hg sources. In our previous work, we measured stable isotopes of Hg
(δ<sup>202</sup>Hg, Δ<sup>199</sup>Hg, and Δ<sup>200</sup>Hg) in the Laurentian Great Lakes and estimated source contributions
of Hg to bottom sediment. Here, we identify isotopically distinct
Hg signatures for Great Lakes trout (<i>Salvelinus namaycush</i>) and walleye (<i>Sander vitreus</i>), driven by both food-web
and water-quality characteristics. Fish contain high values for odd-isotope
mass independent fractionation (MIF) with averages ranging from 2.50
(western Lake Erie) to 6.18‰ (Lake Superior) in Δ<sup>199</sup>Hg. The large range in odd-MIF reflects variability in the
depth of the euphotic zone, where Hg is most likely incorporated into
the food web. Even-isotope MIF (Δ<sup>200</sup>Hg), a potential
tracer for Hg from precipitation, appears both disconnected from lake
sedimentary sources and comparable in fish among the five lakes. We
suggest that similar to the open ocean, water-column methylation also
occurs in the Great Lakes, possibly transforming recently deposited
atmospheric Hg deposition. We conclude that the degree of photochemical
processing of Hg is controlled by phytoplankton uptake rather than
by dissolved organic carbon quantity among lakes