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.     

Spatial and Temporal Assessment of Mercury and Organic Matter in Thermokarst Affected Lakes of the Mackenzie Delta Uplands, NT, Canada

We examined dated sediment cores from 14 thermokarst affected lakes in the Mackenzie Delta uplands, NT, Arctic Canada, using a case-control analysis to determine how retrogressive thaw slump development from degrading permafrost affected the delivery of mercury (Hg) and organic carbon (OC) to lakes. We show that sediments from the lakes with retrogressive thaw slump development on their shorelines (slump-affected lakes) had higher sedimentation rates and lower total Hg (THg), methyl mercury (MeHg), and lower organic carbon concentrations compared to lakes where thaw slumps were absent (reference lakes). There was no difference in focus-corrected Hg flux to sediments between reference lakes and slump-affected lakes, indicating that the lower sediment Hg concentration in slump-affected lakes was due to dilution by rapid inorganic sedimentation in the slump-affected lakes. Sedimentation rates were inversely correlated with THg concentrations in sediments among the 14 lakes considered, and explained 68% of the variance in THg concentration in surface sediment, further supporting the dilution hypothesis. We observed higher S2 (algal-derived carbon) and particulate organic carbon (POC) concentrations in sediment profiles from reference lakes than in slump lakes, likely because of dilution by inorganic siliciclastic matter in cores from slump-affected lakes. We conclude that retrogressive thaw slump development increases inorganic sedimentation in lakes, and decreases concentrations of organic carbon and associated Hg and MeHg in sediments.     

Importance of the forest canopy to fluxes of methyl mercury and total mercury to boreal ecosystems

The forest canopy was an important contributor to fluxes of methyl mercury (MeHg) and total mercury (THg) to the forest floor of boreal uplands and wetlands and potentially to downstream lakes, at the Experimental Lakes Area (ELA), northwestern Ontario. The estimated fluxes of MeHg and THg in throughfall plus litterfall below the forest canopy were 2 and 3 times greater than annual fluxes by direct wet deposition of MeHg (0.9 mg of MeHg ha(-1)) and THg (71 mg of THg ha(-1)). Almost all of the increased flux of MeHg and THg under the forest canopy occurred as litterfall (0.14-1.3 mg of MeHg ha(-1) yr(-1) and 110-220 mg of THg ha(-1) yr(-1)). Throughfall added no MeHg and approximately 9 mg of THg ha(-1) yr(-1) to wet deposition at ELA, unlike in other regions of the world where atmospheric deposition was more heavily contaminated. These data suggest that dry deposition of Hg on foliage as an aerosol or reactive gaseous Hg (RGM) species is low at ELA, a finding supported by preliminary measurements of RGM there. Annual total deposition from throughfall and litterfall under a fire-regenerated 19-yr-old jack pine/birch forest was 1.7 mg of MeHg ha(-1) and 200 mg of THg ha(-1). We found that average annual accumulation of MeHg and THg in the surficial litter/fungal layer of soils since the last forest fire varied between 0.6 and 1.6 mg of MeHg ha(-1) and between 130 and 590 mg of THg ha(-1) among sites differing in drainage and soil moisture. When soil Hg accumulation sites were matched with similar sites where litterfall and throughfall were collected, measured fluxes of THg to the forest floor (sources) were similar to our estimates of longterm soil accumulation rates (sinks), suggesting that the Hg in litterfall and throughfall is a new and not a recycled input of Hg to forested ecosystems. However, further research is required to determine the proportion of Hg in litterfall that is being biogeochemically recycled within forest and wetland ecosystems and, thus, does not represent new inputs to the forest ecosystem.     

Methylmercury and total mercury in plant litter decomposing in upland forests and flooded landscapes

The overall objectives of this study were to examine the effects of flooding on the decomposition and mercury (Hg) content of tissues from plants common to boreal upland forests at the Experimental Lakes Area in northwestern Ontario. We used litterbags to study changes in total Hg (THg), methyl Hg (MeHg), carbon (C), and nitrogen (N) in 12 different plant tissues (birch, alder, blueberry, and Labrador tea leaves, bunchberry plants, jack pine needles, Sphagnum spp., Polytrichum spp., and Pleurozium spp. bryophytes, lichen, and fresh and extensively decomposed wood) placed on unflooded boreal forest soils and in experimentally created reservoirs over an approximately 800 day period. Rates of decomposition (as indicated by differences in the percentage of C and N mass left in the tissues over time) were slower in plant tissues placed on unflooded soils compared to the same tissues that were inundated in reservoirs. Depending on tissue type and initial THg concentrations, decomposing litter on both unflooded and flooded soils was either a source or a sink for THg. Tissues where initial THg concentrations were greater than 30 ng g(-1) represented a source of THg to the surrounding environment, whereas tissues that had initial concentrations of less than 30 ng g(-1) gained THg mass. Initial rates of change in THg were more rapid in plant tissues placed in reservoirs compared to the same plant tissue placed on unflooded soils, but there were no differences in final THg masses after approximately 800 days. Plant tissues placed in reservoirs exhibited large increases in MeHg mass, whereas MeHg mass decreased in the same plants placed on unflooded soils. This is the first study examining THg and MeHg cycling in decomposing plants in upland boreal forests and reservoirs.     

Hair mercury levels and food consumption in residents from the Pearl River Delta: South China

The Pearl River Delta (PRD) is located in the Southern part of China and is the main region for fish culture in Guangdong Province. In order to assess the potential health risks associated with dietary consumption of mercury, hair samples from 91 urban, town and fishing village residents, 37 species of fish, cereal, vegetables, and meat samples were collected. The average total mercury (THg) and methylmercury (MeHg) concentrations in hair were 1.08 ± 0.94 and 0.58 ± 0.59 μg/g, respectively. Daily Hg intake via fish consumption is significantly correlated with THg and MeHg accumulated in human hair (r = 0.48, p < 0.01; r = 0.43, p < 0.01). The estimated daily intake of Hg via different food types showed that both fish and cereal consumption were the two main routes of Hg exposure for residents in the sampling areas. Besides food intake, smoking was also an important source for daily THg intake in the smoke group, contributing 11–18% to EDI of THg.     

Mercury distribution across 14 U.S. forests. Part II: Patterns of methyl mercury concentrations and areal mass of total and methyl mercury

This study characterized distribution patterns of monomethyl mercury (MeHg) and areal mass of total mercury (THg) and MeHg across U.S. upland forests. MeHg concentrations increased from surface litter (average: 0.14 μg kg(-1)) to intermediate (0.47 μg kg(-1)) and deeper, decomposed litter (1.43 μg kg(-1)). MeHg concentrations were lower in soils (0.10 μg kg(-1) at 0-20 cm depth; 0.06 μg kg(-1) at >20 cm depth). Ratios of MeHg to THg were higher in litter compared to soils. In soils, MeHg concentrations positively correlated with THg across all sites, and MeHg concentrations also increased with C content and latitude. THg areal mass ranged from 41.6 g ha(-1) to 268.8 g ha(-1). Largest THg mass at all sites was sequestered in soils (average of 91%), followed by litter (8%) and aboveground biomass (<1%). MeHg mass (litter plus soils only) ranged from 75 to 443 mg ha(-1), of which 88% was found in soils. Both THg and MeHg mass correlated with latitude, with average mass increases of 10.6 g ha(-1) (THg) and 20 μg ha(-1) (MeHg) per degree latitude, indicating that highest THg and MeHg accumulation in upland forests are expected in northern sites.     

Mercury mass budget estimates and cycling seasonality in the Florida Everglades

We estimated the mass budget for mercury (Hg) seasonally deposited into the Florida Everglades and investigated seasonality of Hg cycling by analyzing data obtained for water, soil, flocculent detrital material (floc), periphyton, and mosquitofish collected throughout the Everglades freshwater marshes in the 2005 dry and wet seasons. Higher wet season total Hg (THg) in soil, floc, and periphyton agreed with greater Hg amounts entering these compartments during the wet season, probably owing to substantially greater Hg deposition in the wet season than in the dry season. Seasonal differences were absent for THg in surface water. Methylmercury (MeHg) showed mixed seasonal patterns, with higher water and soil MeHg and lower periphyton MeHg in the dry season but no seasonality for floc MeHg. Seasonal variations in Hg deposition, MeHg production and transport, and mass of ecosystem compartments could be responsible for the seasonality of MeHg cycling. Higher mosquitofish THg, higher bioaccumulation factors, and higher biomagnification factors from periphyton to mosquitofish were observed in the wet season than in the dry season, indicating that the wet season is more favorable for Hg bioaccumulation. The mass budget estimation agreed with this result.     

Organic material: the primary control on mercury methylation and ambient methyl mercury concentrations in estuarine sediments

Estuarine environments that have no direct sources of mercury (Hg) pollution may have sediment concentrations of methylmercury (MeHg) as high as those of polluted marine environments. In this study we examined the biogeochemical factors affecting net methylation and sediment MeHg concentrations in an unpolluted estuarine environment, the Ore River estuary, which discharges into the Bothnian Bay (20-120 ng total Hg g(-1) dry sediment, salinity 3-5% per hundred). We analyzed the spatial and temporal differences in surface sediment profiles of MeHg concentration, Hg methylation, MeHg demethylation, and concentrations of sulfide and oxygen between accumulation and erosion type bottoms. The main difference between the bottoms studied was in the proportion of organic material (OM) in the sediment, ranging between 0.8% and 10.8%. The pore water sulfide concentration profiles also differed considerably between sites and seasons, from 0 to 20 microM, with 100 microM as the extreme maximum. The sediment MeHg concentration profiles (0-10 cm) mostly varied between 0.1 and 7 ng g(-1) dry weight (dw, as Hg). The MeHg demethylation rates were relatively low and the depth profiles of the rates were relatively constant over season, site, and depth. In contrast, both rates and depths of maximum Hg methylation differed between the bottoms. The results indicate that the amount of OM accumulated at the bottoms was the main factor affecting net MeHg production, while the total amount of Hg had little or no influence on the amount of MeHg in the sediment.     

Tidally driven export of dissolved organic carbon, total mercury, and methylmercury from a mangrove-dominated estuary

The flux of dissolved organic carbon (DOC) from mangrove swamps accounts for 10% of the global terrestrial flux of DOC to coastal oceans. Recent findings of high concentrations of mercury (Hg) and methylmercury (MeHg) in mangroves, in conjunction with the common co-occurrence of DOC and Hg species, have raised concerns that mercury fluxes may also be large. We used a novel approach to estimate export of DOC, Hg, and MeHg to coastal waters from a mangrove-dominated estuary in Everglades National Park (Florida, USA). Using in situ measurements of fluorescent dissolved organic matter as a proxy for DOC, filtered total Hg, and filtered MeHg, we estimated the DOC yield to be 180 (±12.6) g C m(-2) yr(-1), which is in the range of previously reported values. Although Hg and MeHg yields from tidal mangrove swamps have not been previously measured, our estimated yields of Hg species (28 ± 4.5 μg total Hg m(-2) yr(-1) and 3.1 ± 0.4 μg methyl Hg m(-2) yr(-1)) were five times greater than is typically reported for terrestrial wetlands. These results indicate that in addition to the well documented contributions of DOC, tidally driven export from mangroves represents a significant potential source of Hg and MeHg to nearby coastal waters.     

Microbial reduction and oxidation of mercury in freshwater lakes

The evasion of elemental mercury represents a significant pathway for reducing the level of this potentially toxic material in aquatic ecosystems. The evasion rate is controlled by the concentration of dissolved gaseous mercury (DGM) across the air-water interface, water, and air temperature as well as wind speed. Here we investigate the role of microbial mercury oxidation and reduction in regulating DGM diel patterns in two freshwater lakes, Jack's Lake and Lake Ontario. Three replicate diurnal cycles of DGM in Brookes Bay, Jack's Lake peaked at 313 fM between 9:00 to 10:30 and decreased to 79.6 fM by 16:00. Microbial mercury reductase activity (converts Hg2+ to Hg0) increased with DGM concentrations and mercury oxidase activity (converts Hg0 to Hg2+) increased as DGM concentrations decreased in the mid-afternoon. This illustrates that mercury oxidase activity was linked to hydrogen peroxide (H2O2) diurnal patterns. Thirty minutes after spiking Lake Ontario water with H2O2, mercury oxidase activity increased by 250% and by 60 min, DGM decreased to 28% of its initial value. Two hours after the H2O2 spike, mercury oxidase activity had declined, but mercury reductase activity and DGM both increased. Four hours after the spike, mercury reductase and DGM levels had returned to original levels. Our results are consistent with the following sequence of events. In the morning, microbial activity produces DGM (in addition to any DGM formed through photoreduction of Hg2+). As photochemically produced H2O2 increases in concentration it induces the biologically mediated decrease in DGM concentrations throughout the afternoon. To predict concentration of DGM in surface waters and flux rates to the atmosphere, the contribution of photoreduction and photooxidation must be placed in context with reduction and oxidation rates due to microbial activity.     

Phytoremediation of mercury and organomercurials in chloroplast transgenic plants: enhanced root uptake, translocation to shoots, and volatilization

Transgenic tobacco plants engineered with bacterial merA and merB genes via the chloroplast genome were investigated to study the uptake, translocation of different forms of mercury (Hg) from roots to shoots, and their volatilization. Untransformed plants, regardless of the form of Hg supplied, reached a saturation point at 200 microM of phenylmercuric acetate (PMA) or HgCl2, accumulating Hg concentrations up to 500 microg g(-1) with significant reduction in growth. In contrast, chloroplast transgenic lines continued to grow well with Hg concentrations in root tissues up to 2000 microg g(-1). Chloroplasttransgenic lines accumulated both the organic and inorganic Hg forms to levels surpassing the concentrations found in the soil. The organic-Hg form was absorbed and translocated more efficiently than the inorganic-Hg form in transgenic lines, whereas no such difference was observed in untransformed plants. Chloroplast-transgenic lines showed about 100-fold increase in the efficiency of Hg accumulation in shoots compared to untransformed plants. This is the first report of such high levels of Hg accumulation in green leaves or tissues. Transgenic plants attained a maximum rate of elemental-Hg volatilization in two days when supplied with PMA and in three days when supplied with inorganic-Hg, attaining complete volatilization within a week. The combined expression of merAB via the chloroplast genome enhanced conversion of Hg2+ into Hg,0 conferred tolerance by rapid volatilization and increased uptake of different forms of mercury, surpassing the concentrations found in the soil. These investigations provide novel insights for improvement of plant tolerance and detoxification of mercury.     

Deposition and emission of gaseous mercury to and from Lake Michigan during the Lake Michigan Mass Balance Study (July, 1994-October, 1995)

This paper presents measurements of dissolved gaseous mercury (DGM) concentrations in Lake Michigan and the application of a mechanistic approach to estimate deposition and emission fluxes of gaseous mercury (Hg2+ and Hg0) to and from Lake Michigan. Measurements of DGM concentrations made during May and July, 1994 and January, 1995 indicate that Lake Michigan was supersaturated with DGM suggesting that transfer of Hg0 occurs from the water to the atmosphere. Over-water concentrations of gaseous Hg2+ were estimated from total gaseous Hg (TGM) concentrations measured at five sites in the basin and used to model dry deposition fluxes of Hg2+. The modeling approach combines estimates of dry deposited Hg2+ with known photochemical and biotic reduction rates to form Hg0, which is available for re-emission. The model accounts for temporal and spatial variations in the deposition velocity of gaseous Hg2+ and the transfer velocity of Hg0 using high temporal and spatial resolution meteorological data. The modeled DGM concentrations agree well with the observed DGM concentrations in Lake Michigan. The modeled dry deposition fluxes of Hg2+ (286-797 kg yr(-1)) are very similar to the emission fluxes of Hg0 (320-959 kg yr(-1)), depending on the gaseous Hg2+ concentration used in the model.     

Episodes of elevated methylmercury concentrations in prairie streams

Methylmercury (MeHg) and total mercury (THg) concentrations were determined weekly from late March through mid-December 2000 in unfiltered water samples from two prairie streams in south-central Minnesota. The Little Cobb River and County Ditch 86 drain predominantly agricultural watersheds, but riparian areas along much of the Little Cobb River are forested, whereas County Ditch 86 is a utilitarian drainage waterway with grassed riparian areas. Episodes of elevated MeHg concentration (1.3 ng/L) were observed in both streams in early May during a period of algal bloom. Between late May and late September, MeHg concentrations varied between 0.05 and 0.39 ng/L in the Little Cobb River and between 0.03 and 0.29 ng/L in County Ditch 86. Precipitation was well below average in August and September, and discharge in both streams ceased by late September. MeHg concentrations in the Little Cobb River increased dramatically in early October after autumnal leaf fall, increasing from 0.44 to 4.90 ng/L over a 5-week period. In contrast, MeHg concentrations in County Ditch 86 varied between 0.22 and 0.48 ng/L over the same time frame. The observed differences are thoughtto reflect differences in the amounts of litterfall entering the two streams. These results show that algal bloom and leaf fall events can result in elevated MeHg concentrations in surface waters, potentially leading to increased MeHg accumulation in fish.     

Estimate of mercury and methyl mercury intake associated with fish consumption from Sagua la Grande River,Cuba

This paper provides an estimate of the weekly intake of total mercury (THg) and methyl mercury (MeHg) from consumption of fish from the Sagua la Grande River, Villa Clara, Cuba, by determining the THg levels in different fish species. The levels varied between 0.143 and 0.484 μg g^?1 on a fresh weight basis. None of the analysed fish was found to have levels above the national and international regulatory levels, although THg levels over 0.2 μg g^?1, the threshold concentration established by the World Health Organization (WHO) for the consumption by vulnerable population groups, were found in 75% of samples. The MeHg level was found to be 84% of the THg content. A Food Frequency Survey was given to 127 townspeople to estimate river fish consumption. The weekly intake of MeHg was found to be greater than the value established by the provisional tolerable weekly intake (PTWI) in 50% of children, in 80% of pregnant women, and in 75% of women in childbearing age. These weekly intakes can represent an important risk to the population's health, especially for vulnerable groups.     

Accumulation of inorganic and methylmercury by freshwater phytoplankton in two contrasting water bodies

Phytoplankton concentrate mercury from their aqueous surroundings and represent the primary entry point for Hg in aquatic food webs. We used 203Hg to compare the uptake of inorganic mercury, Hg(II), and methylmercury, MeHg, in four phytoplankton species (a diatom, a chlorophyte, a cryptophyte, and a cyanobacterium) in two waters containing different concentrations of dissolved organic carbon (DOC). At steady state, volume concentration factors (VCFs) for Hg(II) in the four species were similar and ranged from 0.5 to 5 x 10(4) for both water types, whereas VCFs for MeHg exceeded those for Hg(II) and ranged from 1.3 to 14.6 x 10(5). The VCFs for MeHg in the three eukaryotic cells in the high DOC water were 2-2.6 times greater than those in the low DOC water, but the VCFs for the prokaryote were similar in both waters. Higher cell surface area to volume ratios correlated with increased MeHg concentrations but not with Hg(II). In both water types, VCFs of Hg(II) were similar for living and heat-killed cells, but the VCFs of MeHg were 1.5-5.0 times greater in living cells, suggesting an active uptake component for MeHg. Hg(II) and MeHg were entirely bound to cell surfaces of the dead cells, whereas 59-64% of the MeHg and 9-16% of the Hg(II) in living cells entered the cytoplasm.     

Mercury,selenium, PCBs and fatty acids in fresh and canned fish available on the Slovenian market

In order to contribute to the general understanding of the risks and benefits of consuming fish and fish products, total mercury (THg), monomethylmercury (MeHg), polychlorinated biphenyls (PCBs), selenium (Se), n-3 and n-6 fatty acids were determined in fresh and canned fish on the Slovenian market. Furthermore, the mercury exposure of pregnant women in Slovenia was assessed by determining total mercury (THg) in hair, and through fresh and canned fish consumption obtained by a food frequency questionnaire. Based on the frequency of fish consumption and levels of MeHg, THg and PCBs determined in the present study, it can be concluded that fish available on the Slovenian market do not represent a health risk. It was also confirmed that fish are important sources of selenium and n-3 fatty acids.     

Importance of dissolved neutral mercury sulfides for methyl mercury production in contaminated sediments

Biotic transformation of inorganic mercury, Hg(II), to mono methyl mercury (MeHg) is proposed to be largely controlled by passive uptake of neutral Hg complexes by sulfate reducing bacteria (SRB). In this study, the chemical speciation of Hg(II) in seven locally contaminated sediments covering environments such as (i) brackish water, (ii) low-productivity freshwater, and, (iii) high-productivity freshwater was related to potential Hg methylation rates, determined by incubation at 23 degrees C for 48 h under N2(g), and to total MeHg concentrations in sediments. Pore water speciation was modeled considering Hg complexes with halides, organic thiols [Hg(SR)2(aq), associated to dissolved organic matter], monosulfides, and bisulfides. The sum of neutral mercury sulfides [Hg(SH)20(aq)] and [HgS0(aq)] was significantly, positively (p < 0.001, n = 20) correlated to the specific methylation rate constant (Km, day(-1)) at depths of 5-100 cm in two brackish water sediments. Total Hg, total mercury sulfides or Hg(SR)2(aq) in pore water gave no significant relationships with Km. In two subsets of freshwater sediments, neutral mercury sulfides were positively correlated to total Hg in pore water, and therefore, total Hg also gave significant relationships with Km. The sum of [Hg(SH)20(aq)] and [HgS0(aq)] was significantly, positively correlated to total sediment MeHg (microg kg-1) in brackish waters (p < 0.001, n = 23), in southern, high-productivity freshwaters (p < 0.001, n = 20), as well as in northern, low-productivity freshwater (p = 0.048, n = 6). The slopes (b, b') of the relationships Km (day-1) = a + b([Hg(SH)20(aq)] + [HgS0(aq)]) and MeHg (microg kg-1) = a' + b'([Hg(SH)20(aq)] + [HgS0(aq)]) showed an inverse relationship with the C/N ratio, supposedly reflecting differences in primary production and energy-rich organic matter availability among sites. We conclude that concentrations of neutral inorganic mercury sulfide species, together with the availability of energy-rich organic matter, largely control Hg methylation rates in contaminated sediments. Furthermore, Hg(SH)20(aq) is suggested to be the dominant species taken up by MeHg producing bacteria in organic-rich sediments without formation of HgS(s).     

Experimental evidence of a linear relationship between inorganic mercury loading and methylmercury accumulation by aquatic biota

Developing effective regulations on mercury (Hg) emissions requires a better understanding of how atmospheric Hg deposition affects methylmercury (MeHg) levels in aquatic biota. This study tested the hypothesis that MeHg accumulation in aquatic food webs is related to atmospheric Hg deposition. We simulated a range of inorganic Hg deposition rates by adding isotopically enriched Hg(II) (90.9% 202Hg) to 10-m diameter mesocosms in a boreal lake. Concentrations of experimentally added ("spike") Hg were monitored in zooplankton, benthic invertebrates, and fish. Some Hg(II) added to the mesocosms was methylated and incorporated into the food web within weeks, demonstrating that Hg(II) deposited directly to aquatic ecosystems can become quickly available to biota. Relationships between Hg(II) loading rates and spike MeHg concentrations in zooplankton, benthic invertebrates, and fish were linear and significant. Furthermore, spike MeHg concentrations in the food web were directly proportional to Hg(II) loading rates (i.e., a percent change in Hg(II) loading rate resulted in, statistically, the same percent change in MeHg concentration). This is the first experimental determination of the relationship between Hg(II) loading and MeHg bioaccumulation in aquatic biota. We conclude that changes in atmospheric Hg deposition caused by increases or decreases in Hg emissions will ultimately affect MeHg levels in aquatic food webs.     

Elimination of mercury by yellow perch in the wild

The rate of methylmercury (MeHg) elimination by fish is important in determining the extent of bioaccumulation and for predicting recovery times of MeHg-contaminated fisheries. Rates of MeHg elimination remain uncertain in existing bioaccumulation models due to a lack of field studies. We addressed this problem by monitoring fish that had naturally accumulated isotopically enriched MeHg (spike MeHg) during a whole-ecosystem experiment. We transported yellow perch (Perca flavescens) from the experimental lake to an untreated lake and monitored spike total mercury (THg, most of which was MeHg) losses over 440 d. Spike THg was distributed among fish tissues in a similar way as ambient THg (background non-spike THg). We observed rapid loss of spike THg from liver and other visceral tissues (approximately 90 d) followed by a plateau. Subsequently, there was prolonged redistribution of spike THg into muscle (180 d). Loss of spike THg from the whole fish occurred > 5 times slower (half-life of 489 d) than in past laboratory studies using this species. We determined that MeHg bioaccumulation models with laboratory-based elimination rates produced faster losses than those observed in wild fish. The present findings provide support for refining elimination rates in MeHg models and show the importance of examining biological processes under natural conditions.