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Saint Paul, MN, United States

Tsui M.T.K.,University of Michigan | Blum J.D.,University of Michigan | Finlay J.C.,University of Minnesota | Balogh S.J.,Metropolitan Council Environmental Services | And 2 more authors.
Limnology and Oceanography | Year: 2013

Photodegradation is an important sink for highly toxic methylmercury (MeHg) in aquatic ecosystems. Lakes have been extensively studied for MeHg photodegradation, but much less is known about streams, mainly because of the heterogeneity in sunlight availability along stream reaches and because there has been a lack of tools with which to integrate this longitudinal variability. We utilize odd-mass anomalies of stable Hg isotopes (i.e., Δ199Hg) as a proxy for estimating the relative extent of MeHg photodegradation in streams. In a northern California stream network, levels of MeHg in water and biota increased with increasing stream size in headwater and intermediate streams (drainage areas ranging from 0.6 to 150 km2), but MeHg levels decreased substantially in larger streams (drainage areas up to 1212 km2). In smaller streams, the increase of MeHg levels is attributed to increasing in situ MeHg production and is accompanied by only a small increase in Δ199Hg, indicating that the rate of MeHg photodegradation is low relative to the rate of in situ MeHg production. In larger streams, decreasing MeHg levels coincided with significant increases in Δ199Hg of MeHg (an average increase of 1.5% 6 0.5%, n 5 4), indicating that increasing MeHg photodegradation reduced MeHg levels in these wider, more open channels. Our findings clearly indicate that increasing sunlight availability in stream channels substantially increases MeHg photodegradation, which can reduce MeHg contamination in stream food webs. © 2013, by the Association for the Sciences of Limnology and Oceanography, Inc.

Blumentritt D.J.,University of Minnesota | Engstrom D.R.,St. Croix Research | Balogh S.J.,Metropolitan Council Environmental Services
Journal of Paleolimnology | Year: 2013

It can be advantageous to revisit coring locations in lakes years after an initial paleolimnological study is completed, to assess environmental changes in the intervening time interval. We revisited sediment core sites in Lake Pepin (Minnesota, Wisconsin) more than a decade after an original set of 10 cores was collected, dated radiometrically, and studied in 1996. Prominent magnetic susceptibility features were used to align the new core set with the older set, such that traditional radiometric dating was not necessary to obtain a chronology for the new cores. The procedure used to align the two core sets accounted for compaction of former surface sediments by burial with new sediment. The amount of new sediment, mercury, and phosphorus accumulated at each core site was determined and extrapolated to the depositional area of the lake to estimate recent (1996-2008) whole-basin loads. Recent sediment accumulation in Lake Pepin compared well (within 3%) with monitored inflow data from a gauging station on the upper Mississippi River just before it enters the lake. Bulk sediment accumulation rate remained very high (772,000 t/year) for the recent period (1996-2008), down slightly from the peak in 1990-1996 (876,000 t/year), and almost an order of magnitude above pre-settlement rates. Total phosphorus deposition remained constant since a peak in the 1960s, but was also well above pre-settlement rates. Mercury continued its precipitous decline since peaking in the 1960s. © 2013 Springer Science+Business Media Dordrecht.

Coleman Wasik J.K.,University of Wisconsin - River Falls | Engstrom D.R.,St. Croix Research | Mitchell C.P.J.,University of Toronto | Swain E.B.,Minnesota Pollution Control Agency | And 6 more authors.
Journal of Geophysical Research G: Biogeosciences | Year: 2015

A series of severe droughts during the course of a long-term, atmospheric sulfate-deposition experiment in a boreal peatland in northern Minnesota created a unique opportunity to study how methylmercury (MeHg) production responds to drying and rewetting events in peatlands under variable levels of sulfate loading. Peat oxidation during extended dry periods mobilized sulfate, MeHg, and total mercury (HgT) to peatland pore waters during rewetting events. Pore water sulfate concentrations were inversely related to antecedent moisture conditions and proportional to past and current levels of atmospheric sulfate deposition. Severe drying events caused oxidative release of MeHg to pore waters and resulted in increased net MeHg production likely because available sulfate stimulated the activity of sulfate-reducing bacteria, an important group of Hg-methylating bacteria in peatlands. Rewetting events led to increased MeHg concentrations across the peatland, but concentrations were highest in peat receiving elevated atmospheric sulfate deposition. Dissolved HgT concentrations also increased in peatland pore waters following drought but were not affected by sulfate loading and did not appear to be directly controlled by dissolved organic carbon mobilization to peatland pore waters. Peatlands are often considered to be sinks for sulfate and HgT in the landscape and sources of MeHg. Hydrologic fluctuations not only serve to release previously sequestered sulfate and HgT from peatlands but may also increase the strength of peatlands as sources of MeHg to downstream aquatic systems, particularly in regions that have experienced elevated levels of atmospheric sulfate deposition. ©2015. American Geophysical Union. All Rights Reserved.

Tsui M.T.-K.,University of North Carolina at Greensboro | Blum J.D.,University of Michigan | Finlay J.C.,University of Minnesota | Balogh S.J.,Metropolitan Council Environmental Services | And 3 more authors.
Environmental Science and Technology | Year: 2014

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 δ202HgMeHg; and mass-independent fractionation, as Δ199HgMeHg) 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, δ202HgMeHg (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 Δ199HgMeHg. 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. © 2014 American Chemical Society.

Tsui M.T.K.,University of Michigan | Blum J.D.,University of Michigan | Kwon S.Y.,University of Michigan | Finlay J.C.,University of Minnesota | And 2 more authors.
Environmental Science and Technology | Year: 2012

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 δ202Hg (mass-dependent fractionation) and Δ199Hg (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 δ202Hg 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 δ202Hg 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 δ202Hg 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. © 2012 American Chemical Society.

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