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Marine Chemistry 114 (2009) 86­101

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Marine Chemistry

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r c h e m

Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin

T.A. Hollweg a,, C.C. Gilmour b, R.P. Mason a

a b

Department of Marine Sciences, University of Connecticut, Groton, Connecticut 06340, USA Smithsonian Environmental Research Center, 647 Contees Wharf Road, PO Box 28, Edgewaer, Maryland 21037, USA

a r t i c l e

i n f o

a b s t r a c t

Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential of these sediments as sources of MeHg to coastal and open waters. Estuarine, shelf and slope sediments contained on average 0.5 to 1.5% Hg as MeHg (% MeHg), which increased significantly with salinity across our study site, with weak seasonal trends. Methylation rate constants (kmeth), estimated using enriched stable mercury isotope spikes to intact cores, showed a similar, but weaker, salinity trend, but strong seasonality, and was highly correlated with % MeHg. Together, these patterns suggest that some fraction of MeHg is preserved thru seasons, as found by others [Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., Gilmour, C.C., Hintelmann, H., 2008. Temporal changes in the distribution, methylation, and bioaccumulation of newly deposited mercury in an aquatic ecosystem. Environmental Pollution 154, 77] Similar to other ecosystems, methylation was most favored in sediment depth horizons where sulfate was available, but sulfide concentrations were low (between 0.1 and 10 M). MeHg production was maximal at the sediment surface in the organic sediments of the upper and mid Bay where oxygen penetration was small, but was found at increasingly deeper depths, and across a wider vertical range, as salinity increased, where oxygen penetration was deeper. Vertical trends in MeHg production mirrored the deeper, vertically expanded redox boundary layers in these offshore sediments. The organic content of the sediments had a strong impact on the sediment:water partitioning of Hg, and therefore, on methylation rates. However, the HgT distribution coefficient (KD) normalized to organic matter varied by more than an order of magnitude across the study area, suggesting an important role of organic matter quality in Hg sequestration. We hypothesize that the lower sulfur content organic matter of shelf and slope sediments has a lower binding capacity for Hg resulting in higher MeHg production, relative to sediments in the estuary. Substantially higher MeHg concentrations in pore water relative to the water column indicate all sites are sources of MeHg to the water column throughout the seasons studied. Calculated diffusional fluxes for MeHg averaged 1 pmol m- 2 day- 1. It is likely that the total MeHg flux in sediments of the lower Bay and continental margin are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by biological and/or physical processes. Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf strongly suggest that the flux from coastal sediments is of the same order as other sources and contributes substantially to the coastal MeHg budget. © 2009 Elsevier B.V. All rights reserved.

Article history: Received 10 November 2008 Received in revised form 8 April 2009 Accepted 15 April 2009 Available online 22 April 2009 Keywords: Mercury Methylmercury Methylation Sediment Chesapeake Bay Continental shelf Continental slope Organic matter Sulfide Partitioning Redox Flux

1. Introduction To date U.S. states have issued methylmercury (MeHg)-based fish consumption advisories for more than three quarters of the eastern coastal waters of the United States (EPA, 2007). With the coastal and marine environments acting as the largest source of fish for human consumption (FAO, 2004), MeHg contamination in coastal fish has an impact on human health issues and the future of the fishing industry.

Corresponding author. Tel.: +1 860 405 9162; fax: +1 860 405 9153. E-mail address: [email protected] (T.A. Hollweg). 0304-4203/$ ­ see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2009.04.004

These concerns clearly mandate a better understanding of Hg cycling in the coastal zone to determine the main locations of MeHg production and their links to the coastal food web. The coastal zone, an important part of the Hg cycle, acts as a site of inorganic Hg entrapment and MeHg production (Cossa et al., 1996; Mason et al., 1999). Once formed, MeHg bioaccumulates and biomagnifies up the aquatic food chain, making the coastal food web an important vector between MeHg production and MeHg exposure to people and wildlife (Mergler et al., 2007; Scheulhammer et al., 2007). Since the start of industrialization, increases in Hg emissions have led to three-fold increases of Hg in the atmosphere,

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and consequent increases in loadings to coastal and oceanic environments (Mason et al., 1994; Mason and Sheu, 2002). It is predicted that these increases of Hg to the coastal zone have resulted in increases of both MeHg production and MeHg bioaccumulation into the coastal food web, as observed in freshwater systems (Hammerschmidt and Fitzgerald, 2006a; Harris et al., 2007). MeHg production in coastal zone sediments appears to be an important contributor to the MeHg budgets of these systems (Mason et al., 1999; Fitzgerald et al., 2007). Although there are a growing number of Hg and MeHg measurements in coastal systems (e.g. Kannan et al., 1998; Bloom et al., 1999; Hammerschmidt et al., 2004; Heyes et al., 2004; Sunderland et al., 2004; Canario et al., 2007; Heim et al., 2007; Monperrus et al., 2007b; Hammerschmidt et al., 2008; Liu et al., 2009), our understanding of the magnitude of MeHg production and the biogeochemical controls on net methylation in coastal systems remains incomplete. Similar to freshwater ecosystems, MeHg production in the coastal zone has been shown to be strongly related to organic matter (Hammerschmidt et al., 2004; Hammerschmidt and Fitzgerald, 2006b; Han et al., 2007; Ogrinc et al., 2007; Hammerschmidt et al., 2008) and dissolved Hg complexation with sulfide (Benoit et al., 1998; Conaway et al., 2003; Hammerschmidt et al., 2004; Heyes et al., 2006; Sunderland et al., 2006; Hammerschmidt et al., 2008), as both affect Hg bioavailability to methylating bacteria. However, knowledge gaps still persist in understanding Hg cycling in the coastal environment. For example, the relative importance of organic matter, microbial activity, redox conditions, and S and Fe cycling on Hg methylation is still unclear across a large spatial scale. Here we present the results of a study designed to understand the biogeochemical factors controlling Hg distribution and MeHg production in Chesapeake Bay, the largest estuary in the US, and the adjacent mid-Atlantic continental shelf and slope. Specifically, we examined in detail the seasonal, spatial and down-core patterns of net MeHg production across a full salinity gradient, and the biogeochemical controls on Hg methylation. This study allows us to address such questions as: 1) The importance of organic matter quantity/quality on Hg partitioning and bioavailability; 2) The role of sulfide on Hg methylation in a high sulfate system; and 3) The relative influence of microbial activity and chemistry on MeHg production. Since aquatic sediments are often the main site of MeHg production in aquatic ecosystems (Benoit et al., 2003), we focused our study of Hg speciation and biochemical processes in bottom sediments. A rough mass balance suggests that Chesapeake Bay is a potential sink for terrestrially derived Hg, with only 25% of the total Hg inputs to the Bay exported to the ocean, while 55% appears to be trapped in bottom sediments (Mason et al., 1999). Although riverine inputs of Hg are important to estuarine and coastal waters (Sunderland and Mason, 2007), it is believed that atmospheric deposition is the most important source of Hg to the open ocean (Cossa et al., 1996; Mason and Fitzgerald, 1996; Mason and Sheu, 2002), with rivers accounting for only 10% of the input (Mason and Fitzgerald, 1996; Mason and Sheu, 2002). The relative importance of these sources to the shelf environment depends on location and is not accurately known. A budget for the Chesapeake Bay estuary suggests that in situ production of MeHg, presumably in sediments, accounts for more than half of the total MeHg flux into the system (Mason et al., 1999). However, these budgets were constructed with limited information. While the Mason et al. (1999) draft budget suggests that Chesapeake Bay is a potential source of MeHg to the coastal ocean, there is insufficient information to assess the importance of the associated shelf and slope sediments as a source of MeHg to the coastal waters. Therefore, we also focused on MeHg production and flux to the water column in the coastal margin sediments of our study site. During 2005 and 2006, we conducted five cruises to Chesapeake Bay and the mid-Atlantic continental shelf and slope to investigate the magnitude and distribution of MeHg in the mid-Atlantic coastal region, and the biogeochemical controls on MeHg production. The

objectives of our study were to: 1) Observe spatial and temporal changes in solid-phase and pore water Hg and MeHg concentrations, and potential Hg methylation rates, in the mid-Atlantic coastal region; 2) Investigate biochemical factors that affect Hg speciation, bioavailability and methylation rates; and 3) Examine the potential of midAtlantic estuarine, shelf and slope sediments as sources of MeHg to the coastal and open waters. 2. Materials and methods 2.1. Study design Detailed biogeochemical measurements were made in surficial bottom sediments at seven stations in Chesapeake Bay and the adjacent mid-Atlantic continental shelf and slope. Sediment samples were collected at these stations during four cruises: May 1­8, 2005, July 9­15, 2005, August 30­September 5, 2005, and April 20­26, 2006. The study focused on bottom sediments although basic water column measurements were also made. We examined the top 12 cm of sediment because previous research suggests that this is typically the most active zone of microbial activity and net MeHg production (Benoit et al., 2003). Measurements were made during the spring, summer and fall since net MeHg production is related to temperature and microbial activity, and thus, known to vary seasonally (Benoit et al., 2003; Fitzgerald et al., 2007). For each site on each sampling date, multiple replicate sediment cores were collected using a box corer. Down-core measurements at 2 cm intervals were made for over 30 parameters, including microbial activity (CO2 and CH4 production, and sulfate reduction), Hg methylation rates using stable Hg isotope spikes, concentrations of Hg and MeHg in sediment and interstitial waters, and a large suite of ancillary chemical and physical parameters. Variables chosen for study were based on our understanding of the controls on MeHg production in other ecosystems, and focused on S, Fe and C cycling. Coupling down-core diagenetic processes with MeHg production is potentially useful to better understand the factors that affect Hg methylation in coastal sediments. 2.2. Site selection Sediments and overlying water were sampled along a seven station transect from the top of Chesapeake Bay to the mid-Atlantic continental slope (Fig. 1; Table 1). Three of the four stations occupied in Chesapeake Bay were in or near the main channel (STA 1, 2 and 4); a fourth (STA 3) was in shallow water at the same latitude as STA 2. These Bay stations have been repeatedly occupied for a variety of research and monitoring (Burdige and Homstead, 1994; Roden and Tuttle, 1996; MarvinDiPasquale and Capone, 1998; Zimmerman and Canuel, 2002), and represent the major regions and bottom types of Chesapeake Bay. Offshore samples were collected from two sites on the mid-Atlantic continental shelf and one on the slope, on the Currituck slide. The slope site has been repeatedly sampled (e.g. Ferdelman, 1994; Alperin et al., 1999; Thomas et al., 2002). However, the shelf stations were newly chosen for this study, since there has been little previous biogeochemical study of mid-Atlantic continental shelf sediments. 2.3. Sediment sampling methods Sediments were sampled using either a modified Ocean Instruments Mk III 50× 50 cm box corer, or a 25 × 25 cm Soutar box corer with a frame. The Soutar corer was only used in the organic Bay sediments. The Ocean Instruments box corer was modified for use in sandy sediments by adding foam pads to the spade, and by sub-sampling the box while it hung under tension from the sampling cable. These modifications minimized water and sediment loss from the box core, by creating and maintaining a seal between the box and the spade. Cores recovered with

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Fig. 1. Location of stations sampled in Chesapeake Bay and the mid-Atlantic continental shelf and slope during four cruises occurring in May, July and September 2005 and April 2006. Bathymetry contours are every 500 m.

substantially turbid overlying water were rejected. Box-core sediments were sub-sampled with clear polycarbonate tubes (4.8 cm inner diameter). Sub-sampled cores were corked on top, with 5 to 15 cm of overlying water present. Sediment cores for microbial rate measurements were stored at ambient bottom water temperature until further processing. Sediment cores for chemical bulk and pore water analyses were stored on ice until use. Intact sediment cores were processed on-board the ship inside an oxygen-free Coy glove bag, from minutes to 8 h after sub-sampling. In the glove bag, overlying water was removed and sediment was sectioned into 2 cm intervals down-core. For ancillary bulk-phase analyses, three cores were homogenized and sub-sampled for the different chemistries, including Fe(II), Fe(III), carbon, nitrogen, sulfur and pigments. For CO2 and CH4 production estimates, sediment was placed within serum vials with rubber caps and brought outside and stored at ambient temperature until the incubation began. Pore water was extracted from sediment subsections via vacuum filtration with acid-cleaned Nalgene polystyrene filter units with 0.2 m cellulose nitrate filters, fitted with a combusted 0.7 m glass fiber pre-filter. Filter units were flushed with deionized water, and held in the anaerobic chamber for several hours prior to use. Twelve to twenty cores were used for filtration in order to obtain enough volume

for all pore water chemistries. Aliquots of pore water for Hg and MeHg analyses were stored in Teflon bottles and acidified to 0.5% with trace-metal grade hydrochloric acid (HCl). Aliquots of pore water for sulfide analysis were preserved in sulfide anti-oxidant buffer (2 M NaOH. 0.2 M Na2EDTA, and 0.2 M ascorbic acid, made up in degassed deionized water) for shipboard analysis. Aliquots of pore water for Fe and Mn analyses were acidified to 0.2% with trace-metal grade HCl. 2.4. Water sampling methods Prior to sediment sampling, bottom water was sampled from all stations. Water sampling followed clean techniques outlined in Gill and Fitzgerald (1985). Bottom water was collected 1 m above the sediment surface using acid-washed General Oceanics GO-FLO sampling bottles with Teflon coated messengers deployed on a Kevlar line. GO-FLO bottles were quickly bagged and taken into a clean van on deck for all water processing. Water was filtered using a dome vacuum filtration set-up with acid-cleaned Savillex Teflon filter towers and collection bottles. Bottom water for HgT and MeHg analyses were filtered into separate Teflon bottles, acidified to 0.5% (trace-metal grade HCl) and stored in the dark. For May and June 2005 cruises, acid-cleaned 0.4 m polycarbonate filters were used for Hg filtration. For September 2005 and April 2006 cruises, combusted 0.7 m glass fiber filters were used. 2.5. Mercury methylation rate potentials Hg methylation rates were estimated by measuring the production of Me201Hg from 201Hg injected into replicate intact sediment cores (Gilmour and Riedel, 1995; Hintelmann and Evans, 1997; Hintelmann et al., 2000; Heyes et al., 2006). Stable 201Hg (98.11% purity) was obtained from Oak Ridge National Laboratory. In the field, a stock solution of 201HgCl was diluted with 0.22 µm filtered bottom water and equilibrated for an hour prior to use. Cores were incubated for 2 h at ambient bottom water temperature. At the end of the incubation

Table 1 Station location (latitude and longitude), depth, bottom water temperature and bottom water salinity. Station 1 2 3 4 6 7 9 Latitude 39 20.94 N 38 33.86 N 38 33.86 N 37 15.94 N 37 05.69 N 36 41.49 N 36 19.91 N Longitude 76 10.83 W 76 26.38 W 76 28.89 W 76 08.96 W 75 42.21 W 75 44.47 W 74 43.49 W Depth (m) 9.7 22 9.4 11 16 17 620 Bottom water temperature (°C) 12­15 11­27 11­28 10­26 8­18 8­17 5­7 Bottom water salinity (PSU) 4.3 18.3 11.6 24.5 30.9 31 34.9

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period sediment cores were sectioned and immediately frozen on dry ice. The concentration of 201Hg added to the sediment cores was different for each cruise, as shown in Table 2. The 201Hg spike increased the total Hg content of organic-rich sediments from 0.2 to 1.4× across the 4 cruises, while the average amount of increase in sandy sediments ranged from 0.9 to 9.6× across the 4 cruises. Since ambient concentrations were considerably increased, and since newly introduced Hg is potentially more bioavailable for Hg(II)-methylation than ambient Hg, estimated values represent potential methylation rates. Methylation rate constants (kmeth) were determined from the formation rate of Me201Hg (Eq. (1)), as discussed in Hintelmann et al. (2000), Martin-Doimeadios et al. (2004), and Heyes et al. (2006).

201

kmeth 201 HgðIIÞ Me Hg

ð1Þ

For calculation of the rate constant, it is assumed that methylation is a pseudo first-order kinetic reaction (Hintelmann et al., 2000), the inorganic mercury spike is not significantly depleted during the incubation, and demethylation of the methylated isotope is insignificant relative to new Me201Hg production. These assumptions are reasonable given the rate constants (b0.1 day- 1) and typical incubation times. Eq. (2) illustrates the calculation of the methylation rate constant (in day- 1), where t is the time of the incubation, [201Hg] is the total concentration of mercury added, and [Me201Hg] is the concentration of Me201Hg produced at the end of the incubation. i Me201 Hg = Â201 Ã Hg t h

kmeth

ð2Þ

In this study, the detection limits (DL) for kmeth were estimated to be 0.007 and 0.006 days- 1 for the organic-rich and sandy sediments, respectively, using similar calculations as described in Mitchell and Gilmour (2008). For laboratory analysis of ambient Hg standards over two years, the average relative standard deviation of the ratio of 201:202-Hg was 2.32%. 2.6. Total mercury and methylmercury analysis Total Hg (HgT) and MeHg analyses of sediment and pore water were done using Perkin Elmer ELAN DRC II Inductively Coupled Plasma Mass Spectrometers (ICP-MS); MeHg analysis of water column samples was preformed using a Tekran CVAFS Mercury Detector 2500. Total Hg analysis was conducted following digestion, reduction and goldtrapping techniques (Gill and Fitzgerald, 1987; Method 1631 EPA, 2002). MeHg analysis followed distillation, ethylation and gas chromatographic separation techniques (Method 1630 EPA, 2001). Sediment and surface water analyses were conducted at the Department of Marine Sciences at University of Connecticut. Pore water analyses were done at Smithsonian Environmental Research Center.

Table 2 Average ambient HgT bulk phase concentration in sediments sampled during four cruises, separated by sediment type (organic vs. sandy). Average ambient bulk-phase HgT concentration Organic sediment (pmol cm- 3) May 2005 July 2005 Sept. 2005 April 2006 205 165 155 160 Sandy sediment (pmol cm- 3) 75 85 85 48

201 201

Hg spike:ambient concentration Sandy sediment Avg 0.86 4.01 1.04 9.65 Std 0.50 3.41 0.78 7.10

Hg spike (pmol cm- 3) 37 185 55 185

Organic sediment Avg 0.20 1.35 0.40 1.37 Std 0.06 0.67 0.16 0.61

Concentrations of 201Hg spiked in sediment for methylation incubation experiments, and the proportion (average and standard deviation) of the 201Hg spike compared to the ambient concentration of HgT, separated by sediment type.

For the analysis of HgT in pore water, Hg in water samples was reduced to gaseous elemental mercury using tin chloride (SnCl2) in a bubbler set-up and purged onto gold-coated traps. Gold-coated traps were heated and elemental Hg was introduced directly into the ICP-MS torch in a stream of argon. Concentrations of ambient HgT were calculated using external standards. For the four cruises reported in this manuscript, detection limits for HgT in 10 to 30 ml pore water samples were 1­4 pM, based on 3× the standard error of filter blanks. Recovery of spikes (50 pg per 20 ml) averaged 112 ± 10%. The relative standard deviation (RSD) of field and laboratory replicates was 13 ± 12%. For the analysis of MeHg in pore waters, isotope dilution techniques were used as described in Hintlemann and Evans (1997) and Hintelmann and Ogrinc (2003). Enriched Me199Hg (96.4% purity) was added to each sample as an internal standard, prior to distillation. The concentration of Me199Hg was determined by reverse isotopedilution analysis, against certified standards. Methylmercury was synthesized from Hg199Cl2 using aqueous methylcobalamin (Hintelmann and Ogrinc, 2003). Pore water samples for MeHg analysis were brought up to 40 mL with DI water and distilled in 100 mL Teflon jars after the addition of 2 mL 0.2 M CuSO4, 0.2 mL 20% KCl and 1 mL 9 M H2SO4. Distillates were poured into bubblers, brought to approximately 150 mL with DI water, and buffered with acetate (500 L, 2 M acetate in 2 M acetic acid) prior to ethylation with sodium tetraethylborate (Strem Chemicals, 1 g per 100 mL 2% KOH). The MeHg derivatives were purged from bubblers and trapped on short columns packed with TENAX-TA® (60­80 mesh). The traps were subsequently heated onto a short GC column (50 cm long × 4 mm ID borosilicate glass U-tube packed with preconditioned 60/80 mesh 15% OV-3 on Chromasorb WAW-DMSC (Supelco)), held at 110 °C. The GC carrier gas (argon) effluent was introduced directly into the ICP-MS torch. For the four cruises reported in this manuscript, DLs for MeHg in 30 to 40 mL pore water samples were 0.08­0.4 pM, based on 3× the standard error of filter blanks. For the analysis of HgT in sediments, samples were homogenized with a hand-mixer and then digested with a 3:7 mixture of H2SO4: HNO3 on a hotplate for 2­6 h, followed by an addition of BrCl and water. Sediments were spiked with an enriched isotope internal standard (200HgCl) the night before analysis (Hintelmann and Ogrinc, 2003). Decanted samples were analyzed on a Perkin Elmer ELAN DRCII ICP-MS with an attached Flow Injection Auto Sampler (FIAS) system. In the FIAS system, the sample was mixed with 1.1% SnCl2, followed by gas­liquid separation. The reduced gaseous Hg was introduced into the ICP-MS in an argon stream. The concentration of ambient Hg, and the excess abundance of each isotope, were calculated using isotope dilution. The RSD of laboratory duplicate and replicate samples were 2.3% (n = 32) and 1.5% (n = 36), respectively. The average RSD between field duplicate cores was 8.5%. Certified Reference Materials (CRMs) used included Apple Leaves (NIST; SRM-1515; 44 ± 4 ng/g), Fucus sp. (IAEA; IAEA-140/TM; 38 ± 6 ng/g) and estuarine/marine sediment (NRC; MESS-3; 91 ± 9 ng/g). We obtained 43.6 ± 0.47 ng/g (99.1% recovery, n = 6) for Apple Leaves, 35.4 ± 2.2 ng/g (93% recovery, n = 8)) for Fucus, and 93.7 ± 4.1 ng/g (103% recovery, n = 50) for estuarine/marine sediment. The DLs for HgT analysis in 50 mL of digested sample was 10 pM (n = 34) for sediment analysis, based on 3× the standard error of laboratory blanks. Sediment and water column samples for MeHg analysis were distilled using KCl and H2SO4, and ethylated and separated as above. Concentrations of ambient MeHg and enriched MeHg isotopes were calculated using external standards. For MeHg analysis of sediments, the RSD for laboratory duplicates was 9% (n = 30), the RSD for field duplicates was 20%, and spike recoveries averaged 103% (n = 34). Estuarine sediment (IAEA; IAEA-405; 5.49± 0.53 ng/g) was used as a CRM for MeHg, and we obtained 5.27 ± 0.49 ng/g (96% recovery, n = 37). The DL for MeHg sediment analysis in 20 mL of distillate was 0.18 pM (n = 32), based on 3× the standard error of laboratory blanks. For water column MeHg analysis, spike recoveries averaged 88.2%

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(n = 17). The DL for MeHg water column analysis in 120 mL of distillate was 0.19 pM (n = 29), based on 3× the standard error of laboratory blanks. 2.7. Sulfate reduction, carbon dioxide production and methane production rates Dissimilatory sulfate-reduction rates (SRR) were measured in duplicate intact sediment cores using the 35SO4 radioisotracer technique of Fossing and Jorgensen (1989), as described in Mitchell and Gilmour (2008). Cores were incubated for 2 h at ambient bottom temperature, and then sectioned in 2 cm intervals. Sulfide was fixed by the addition of zinc acetate, and sections were immediately frozen on dry ice. Sulfate reduction into both acid-volatile sulfide (AVS) and chromium-reducible reduced sulfur (CRS) phases were measured using 6 N deoxygenated HCl for AVS, and hot, freshly prepared 1 M Cr (II) in 0.5 N HCl for CRS distillations. Sulfides were preserved in sulfide antioxidant buffer (SAOB: Brouwer and Murphy, 1994) and measured using an ion-selective electrode (Orion Sulfide Electrode). Counts of sulfur-35 trapped in SAOB were measured on a Packard Tri-Carb liquid scintillation counter. The SAOB buffer was prepared daily, using deoxygenated water. Rates of CO2 and CH4 production were measured using sediment incubations (Mitchell and Gilmour, 2008). Sediment samples (3­4 g wet weight) were incubated in sealed vials in the dark at ambient bottom temperature. Headspace CO2 and CH4 concentrations were measured for 48­72 h after sampling, using a gas chromatograph equipped with a flame ionization detector and methanizer. Inorganic carbon production rates were calculated by summing the measured concentrations of headspace CO2 and the sediment pore water concentrations of HCO- and CO2-, estimated from headspace CO2 3 3 concentration, soil porosity and pH. Rates of methanogenesis were calculated based on the increase in headspace CH4 through time. 2.8. Ancillary parameters 2.8.1. Bulk-phase analyses Ancillary bulk-phase analyses included AVS, CRS, Fe(II), Fe(III), pigment analysis, total carbon, total nitrogen, total sulfur, organic matter content, porosity and bulk-density. AVS and CRS were analyzed as described above. Bulk-phase extractable Fe(II) and Fe(III) were measured spectrophotometrically using ferrozine, after a 0.5 M HCl digestion (Stookey, 1970; Lovley and Phillips, 1986; Mitchell and Gilmour, 2008). Total C, N and S analyses were performed on freezedried sediments using a CE-440 Elemental Analyzer. Surface sediment pigments, including total chlorophyll a, active chlorophyll a and phaeophytin, were determined by fluorometry by the Nutrient Analytical Services at Chesapeake Biological Lab. 2.8.2. Pore water analyses Ancillary pore water analyses included pH and dissolved concentrations of sulfide, sulfate, chloride, phosphate, nitrate/nitrite, iron, and manganese. Sulfide was preserved in SAOB and measured (as discussed above) within 4 h, with a DL of 100 nM. Sulfate and chloride were analyzed using a Dionex ion chromatography system. Iron and manganese were analyzed by ICP-optical emission spectroscopy using a Perkin-Elmer Optima 3000DV. Nitrate/nitrite and phosphate were determined colorimetrically on a Technicon AAII by the Nutrient Analytical Services at Chesapeake Biological Lab. 2.9. Normalization of data and correlation matrix Statistical analyses were conducted using MATLAB 7.4.0 and SAS 9.0. Data were tested for normality using the Lilliefors goodness-of-fit test, the Shapiro­Wilk W test, and by assessing skewness and kurtosis. Non-normal data were successfully transformed using natural

logarithms. To examine relationships between Hg and different measured variables, correlation matrices were constructed. Relationships were considered statistically significant at p b 0.05. For stepwise multiple linear regression, residuals were examined for significant relationships with model variables. 2.10. Calculation of methylmercury diffusive flux The diffusive flux (F) for dissolved MeHg was calculated, similar to Gill et al. (1999) and Hammerschmidt et al. (2004), using a diffusive transport equation based on Fick's first law of diffusion, as follows: F= - /Dw AC 2 Ax ð3Þ

where is the porosity, is the tortuosity, Dw is the sediment diffusion coefficient, and C/z is the interfacial concentration gradient (Burdige, 2006). The interfacial concentration gradient is the concentration gradient along the sediment­water interface and was approximated by assuming: AC C Az z ð4Þ

where C is the difference in concentration between the filtered overlying water and the surficial pore water (0­2 cm), and z is the average depth of that pore water sample. The flux, in units of mol cm- 2 s- 1, is positive when the solute diffuses out of the sediment (i.e. the concentration in the overlying water is less than the concentration of the pore water, causing C to be negative). Filterable MeHg concentrations were measured in bottom water ( 1 m above sediment surface) at all sites on all cruises (0.4 m polycarbonate filters for May and July 2005 cruises, and 0.7 m glass fiber filters for September 2005 and April 2006 cruises). Tortuosity was assumed to be related to porosity as shown in Eq. (5), suggested by Boudreau (1996). 2 2 = 1 - ln / ð5Þ

Porosity in the fine-grained sediment of the upper and mid-Bay and 3 slope ranged between 0.7 and 0.9 cmwater/cm3 sediment, while the sandy sediment of the lower Bay and shelf had much lower values, ranging 3 between 0.3 and 0.5 cmwater/cm3 sediment (Table 3). Diffusive flux calculations were performed assuming MeHg was bound to either dissolved organic matter (DOM) or sulfide, with the diffusion coefficient at 25 °C (Dw) being 2 × 10- 6 cm2 s- 1 for MeHg­ DOM (Gill et al., 1999) and 1.2 × 10- 5 cm2 s- 1 for MeHg­SH0 (Hammerschmidt et al., 2004), providing maximal and minimal estimates of diffusive MeHg flux into the water column. Temperature corrections to DW were applied by the Stokes­Einstein equation, as discussed in Warken et al. (2000). 3. Results and discussion 3.1. Overview of study site characteristics Our study examined Hg cycling in sediments across a large range of biogeochemical characteristics. The study area transitions from a primarily freshwater system in the upper Chesapeake Bay, through a brackish system in the mid Chesapeake Bay, to an open marine system in the midAtlantic continental margin (Fig. 1). Bottom sediments at STA 1, 2 and 3, in the upper- and mid-Bay, are fine grained, characterized by high organic matter content and low bulk density, and associated high bulk-phase AVS, CRS and extractable Fe(II) concentrations (Table 3). Sulfide was detected in pore water of these sediments during all cruises, with highest values in the deep channel of the mid-Bay (Table 3). Organic matter is thought

T.A. Hollweg et al. / Marine Chemistry 114 (2009) 86­101 Table 3 Averaged data (± standard deviation) of sediment characteristics, ancillary bulk and pore water chemistries, and microbial rates of seven stations in our study. Bulk density (g wet wt cm- 3) Station Station Station Station Station Station Station 1 2 3 4 6 7 9 1.34 ± 0.11 1.06 ± 0.11 1.08 ± 0.09 1.69 ± 0.12 1.82 ± 0.10 1.77 ± 0.18 1.31 ± 0.08 Wet:dry ratio Porosity (g wet wt g- 1 dry wt) (mL cm- 3) 2.28 ± 0.18 4.51 ± 0.96 4.14 ± 0.53 1.44 ± 0.04 1.28 ± 0.01 1.27 ± 0.03 2.26 ± 0.27 0.746 ± 0.034 0.802 ± 0.052 0.808 ± 0.034 0.500 ± 0.012 0.391 ± 0.007 0.376 ± 0.022 0.714 ± 0.052 LOI (%) 6.38 ± 0.92 10.2 ± 1.5 10.3 ± 1.5 1.98 ± 0.32 1.16 ± 0.37 1.18 ± 0.74 8.36 ± 1.82

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C S N C:N ratio (mmol g- 1 dry wt) (mol g- 1 dry wt) (µmol g- 1 dry wt) (mol C mol- 1 N) 1.815 ± 0.11 2.36 ± 0.42 2.97 ± 0.77 0.52 ± 0.00 0.63 ± 0.31 0.66 ± 0.06 2.99 ± 0.27 258 ± 17 538 ± 89 627 ± 69 68.5 ± 5.8 22.2 ± 13.2 75.3 ± 6.0 63.1 ± 14.3 Active Chl a (mg g- 1 dry wt) 1.52 ± 0.52 14.2 ± 11.04 11.6 ± 9.7 3.13 ± 0.21 0.71 ± 0.37 0.52 3.22 Sulfate reduction (nmol cm- 3 h- 1) 56.3 ± 61.0 20.8 ± 20.6 9.09 ± 5.47 6.49 ± 5.13 6.33 ± 5.20 15.8 ± 11.0 169 ± 26.4 289 ± 98 352 ± 32 49.5 ± 5.15 34.1 ± 24.0 39.0 ± 21.5 230 ± 40 pH 8.36 ± 0.17 8.17 ± 0.17 7.83 ± 0.04 8.05 ± 0.08 8.03 ± 0.22 8.02 ± 0.27 8.03 ± 0.27 CO2 production (nmol cm- 3 h- 1) 289 ± 116 204 ± 156 58.4 ± 45.2 24.3 ± 6.2 28.2 ± 16.3 16.4 ± 4.22 9.50 ± 0.61 7.40 ± 0.77 7.44 ± 0.63 8.56 ± 0.30 18.0 ± 1.0 10.2 ± 0.8 11.2 ± 0.5 Nitrate + nitrite (M) 3.375 9.657 2.557 36.714 2.389 7.300 6.386 CH4 production (nmol cm- 3 h- 1) 0.170 ± 0.134 0.083 ± 0.123 Bd Bd Bd 0.033 ± 0.058

AVS CRS (mol g- 1 dry wt) (mol g- 1 dry wt) Station Station Station Station Station Station Station 1 22.4 ± 7.9 2 14.7 ± 12.4 3 7.04 ± 2.38 4 3.52 ± 0.27 6 2.57 ± 2.42 7 2.45 ± 2.12 9 1.52 ± 0.46 Sulfate (mM) Station Station Station Station Station Station 2 3 4 6 7 9 7.01 ± 2.65 7.38 ± 1.48 16.9 ± 5.9 22.1 ± 2.8 21.6 ± 1.78 24.8 ± 3.2 138 ± 30 147 ± 31 166 ± 31 65.4 ± 9.0 10.9 ± 1.4 44.9 ± 48.9 40.9 ± 10.3 Phosphate (µM) 180.3 ± 139.9 63.2 ± 9.7 25.8 ± 10.9 17.3 ± 1.9 23.8 ± 4.0 22.4 ± 0.2

Fe(II) Chl a Pheophytin (mol g- 1 dry wt) (mg g-1 dry wt) (mg g- 1 dry wt) 9.05 ± 3.67 9.17 ± 4.13 8.46 ± 0.67 4.24 ± 4.51 0.99 ± 0.18 1.07 ± 0.69 2.42 ± 0.46 Sulfide (µM) 557 ± 406 34.5 ± 65.9 1.03 ± 0.86 0.38 ± 0.17 0.78 ± 0.72 2.23 ± 1.11 6.95 ± 2.93 52.5 ± 40.5 62.4 ± 25.6 11.1 ± 4.8 1.62 ± 0.38 2.15 20.9 Fe (µM) 3.85 ± 4.52 32.9 ± 12.2 74.5 ± 65.3 34.7 ± 9.7 13.6 ± 13.8 20.5 ± 2.1 11.0 ± 4.9 77.4 ± 59.5 102 ± 32 16.1 ± 9.4 1.82 ± 0.01 3.28 35.8 Mn (µM) 18.7 ± 12.9 61.6 ± 4.8 23.9 ± 20.3 12.2 ± 3.5 3.14 ± 2.79 4.71 ± 1.22

Averages include data of the upper 12 cm from all cruises. Standard deviation is variation between cruises.

to be primarily terrestrial in origin at STA 1 and primarily marine derived at STA 2, as illustrated by variations in C/N (Table 3) and 13C values (Zimmerman and Canuel, 2001). As observed in prior studies, we measured extremely high rates of anaerobic bacterial activity in the midBay deep channel at STA 2 (Table 3), dominated by sulfate reduction in the top 12 cm (Roden and Tuttle, 1996; Marvin-DiPasquale and Capone, 1998; Burdige et al., 2000). Bioturbation is absent for the majority of the year in the deep channel sediments, as indicated by 201Pb activity profiles (Zimmerman and Canuel, 2002), except for early spring when polychaetes and bivalves are present (Kemp et al., 1990). The Chesapeake's deep central channel undergoes seasonal anoxia for many months each year (Cowan and Boynton, 1996; Kemp et al., 2005). Bottom sediments at STA 4, 6 and 7, in the lower Bay and on the continental shelf, are predominantly sand, low in organic matter with higher bulk density (Table 3). They have low AVS, CRS and extractable Fe(II) concentrations, which decrease offshore. Pore water sulfide was low but measurable on most occasions (Table 3). Sediments at STA 4 are heavily bioturbated by polychaetes and bivalves (Schaffner,1990), and the same is likely true for STA 6 and 7. Aerobic organic matter remineralization is more important at these locations than in upper Bay sediments (Marvin-DiPasquale and Capone, 1998). Organic matter C:N ratios at STA 6 and 7 were high, averaging 18.0 ±1.0 and 10.2±0.8, respectively (Table 3), with values measured at STA 7 similar to those observed in the South Atlantic Bight continental shelf sediments (Marinelli et al., 1998; Jahnke et al., 2005). Higher C:N ratios could be a result of grain size, as discussed in Bianchi (2007), or reflect an increase in refractory organic matter due to remineralization of more labile marine forms, discussed in Burdige (2006). Sulfate reduction rates and carbon dioxide production rates were lower in these sandy sediments, compared to the upper and mid-Bay, and methane production was below the DL in the upper 12 cm (Table 3). Sediments from the adjacent continental slope south of the mouth of Chesapeake Bay were also examined. STA 9 was located in 600 m of water, about half way down the slope, on the Currituck slide. Sediments in this area of the slope are organic-rich clays, heavily colonized by benthic fauna. Compared to the organic-rich sediments of the upper

and mid Bay, STA 9 had three-fold lower AVS, CRS and Fe(II) concentrations (Table 3). Sulfide was measurable in pore water on all cruises, but concentrations were low with little seasonal variation (Table 3). High sediment accumulation rates have been measured at this location (DeMaster et al., 1994; Thomas et al., 2002), with organic matter deposition primarily marine in origin and consisting of young and old particles (Thomas et al., 2002). Although the 13C is similar to that of the mid-Bay ( -21; Thomas et al., 2002), the C:N ratio is higher, at 11.2 ± 0.2 (Table 3), possibly indicating more refractory organic matter. Bioturbation (Ferdelman, 1994) and bioirrigation (Thomas et al., 2002) are prevalent. Sulfate reduction rates and carbon dioxide production rates were similar to those measured in the lower Bay and shelf sediments, and methane production was low (Table 3). 3.2. Distribution of mercury and methylmercury across the study area Total mercury concentrations in sediment (expressed on either a dry weight or volume basis) varied substantially across the study region (Fig. 2), with the highest concentrations in the organic-rich sediments of the upper- and mid-Bay (STA 1, 2 and 3). Our concentration data are similar to previously measured values for the organic-rich Chesapeake bottom sediments away from major ports -- for example in the Patuxent (Benoit et al., 1998; Heyes et al., 2006) and lower Susquehanna Rivers, and at Love Point (Mason et al., 1999) -- and for other moderately contaminated estuaries such as the Bay of Fundy (Sunderland et al., 2004). However, mid-Bay Hg concentrations were lower than those in sediments near big city ports in Chesapeake Bay, including Baltimore (Mason and Lawrence, 1999; Mason et al., 1999) and Annapolis (Mason et al., 1999), or in other contaminated estuaries (e.g. Mikac et al., 1999; Heyes et al., 2004; Han et al., 2007; Hammerschmidt et al., 2008). Total Hg concentrations were lowest in the sandy sediments of the lower-Bay and continental shelf (STA 4, 6 and 7), and similar to Hg concentrations in sandy sediments of the southern New England continental shelf (Hammerschmidt and Fitzgerald, 2006b), Tampa Bay, Florida Bay (Kannan et al., 1998) and northern Gulf of Mexico (Liu et al., 2009). In the organic-rich sediment

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T.A. Hollweg et al. / Marine Chemistry 114 (2009) 86­101

Fig. 2. HgT and MeHg concentrations in sediments of Chesapeake Bay and the mid-Atlantic continental shelf and slope. Left hand panels show average concentrations (± standard deviation of duplicate cores) on a dry weight basis for the top 12 cm of sediment, for each of 4 cruises in 2005 and 2006. Right hand panels show the total mass of HgT and MeHg to 12 cm on the same dates.

of the mid-Atlantic continental slope (STA 9), HgT concentrations were somewhat lower than found in the organic mid-Bay sediments, but an order of magnitude higher than the adjacent sandy shelf sediments. Because of the large differences in sediment bulk density across our study site, the range in Hg concentrations are somewhat muted when calculated on a per cubic centimeter basis. Sediment Hg concentrations were strongly correlated with organic matter content and bulk-density (Table 4). This strong interaction between Hg and organic matter has been observed in both waters and sediments of many aquatic ecosystems, including Chesapeake Bay (Benoit et al., 1998; Mason and Lawrence, 1999; Heyes et al., 2006), and other estuarine (Lindberg and Harriss, 1974; Coquery et al., 1997; Mikac et al., 1999; Conaway et al., 2003; Hammerschmidt and Fitzgerald, 2004; Lamborg et al., 2004; Canario et al., 2008; Hammerschmidt et al., 2008) and open marine (Hammerschmidt and Fitzgerald, 2006b; Liu et al., 2009) systems. Many researchers have attributed this relationship to the strong binding strength of Hg to reduced sulfur ligands (i.e. thiol groups) in organic matter, as outlined by Ravichandran (2004) and Skyllberg (2008). In addition, bulk-phase HgT was significantly related to bulk-phase AVS and CRS (Table 4). Applying a stepwise multiple linear regression for HgT, using normalized site-date averages for each site, and including all of the major chemical variables measured, organic matter content explained 48% of the variability in HgT (p b 0.0001, n = 26), and (AVS + CRS) explained an additional 8% (p b 0.05; n = 26). No other variables were significant in the model. This suggests that Hg interacts with inorganic sulfur ligands in solid Fe­S complexes in addition to interacting with organic sulfur ligands in organic matter. Point/nonpoint sources (i.e. Baltimore; Fig. 1), historical deposition, sediment mixing, and variability in binding strengths of different types and qualities of organic matter could explain the residual variability. Sediment MeHg concentrations were significantly related to HgT concentrations across sites (p b 0.01; Table 4), similar to many other moderately-contaminated aquatic ecosystems (Benoit et al., 1998; Kannan et al., 1998; Mason and Lawrence, 1999; Benoit et al., 2003; Conaway et al., 2003; Hammerschmidt and Fitzgerald, 2004; Hammerschmidt and Fitzgerald, 2006b). However, HgT explained only 25% of the variability in MeHg concentrations (Table 4),

highlighting the importance of other biogeochemical factors affecting MeHg production (discussed in Section 3.5). The highest MeHg concentrations were in the organic rich sediments of the upper and mid Bay, and the continental slope (Fig. 2). Normalized to sediment volume, and integrated to 12 cm, areal MeHg was highest in the slope sediments (Fig. 2), due to deeper and broader MeHg peaks in offshore sediments (Fig. 3). Because the sediment depth at which MeHg was maximal varied across the study area and were often found at N4 cm in offshore sediments, our cross-site comparisons use 0­12 cm averages where data are available. Seasonal variability in sediment MeHg concentrations were greater than for HgT. Within sites, variability in HgT across cruises generally averaged 15%, reflecting primarily spatial heterogeneity (Mason et al., 1998; Bloom et al., 1999). Temporal variations in MeHg concentration were typically double that of HgT for Bay and shelf sites, reflecting seasonality in net MeHg production. Similar to freshwater ecosystems, this seasonal pattern of MeHg production has been observed in different estuarine and marine systems (Bloom et al., 1999; Mikac et al., 1999; Stoichev et al., 2004; Lambertsson, 2006; Canario et al., 2007; Heim et al., 2007; Hammerschmidt et al., 2008; Mitchell and Gilmour, 2008). Seasonal variability in MeHg concentration at the slope site (STA 9) was much lower, reflective of its relatively constant bottom water temperature. Methylmercury as a percentage of total Hg (% MeHg) averaged about 1% in 0­12 cm surface sediments in the study area, and, interestingly, increased with salinity (Fig. 4; Table 4). Bottom water salinity was significantly correlated with the average % MeHg across all sites and dates (p b 0.01; Table 4). Although there have been relatively few studies of MeHg across large salinity gradients, Chesapeake Bay and its adjacent continental shelf is the only system that we know of where this trend has been observed (Heyes et al., 2006). A number of geochemical factors co-vary with salinity, many of which could affect net MeHg production. Sediments under the anoxic water column in the mid-Bay did not fit the salinity vs. % MeHg trend. Exceptionally high % MeHg at certain times of the year at this location may be due to MeHg production in anoxic mid-Bay bottom waters (Gilmour et al., in prep) and the resultant enhancement of sediment MeHg, as discussed below. As % MeHg is often a reasonable proxy for net MeHg production (Benoit et al., 2003; Heyes et al., 2006), it appears that the shelf and slope

T.A. Hollweg et al. / Marine Chemistry 114 (2009) 86­101 Table 4 Partial correlation matrix for Hg concentration, Hg partitioning, and potential methylation rates against important bulk-phase characteristics and ancillary chemistries.

93

HgT (pmol cm- 3) log(MeHg) (pmol cm- 3) log(%MeHg) (%) log(HgT) (pM) log(MeHg) (pM) Log(HgT KD) (L kg-1) Log(MeHg KD) (L kg- 1) log(kmeth) (day-1) Salinity - 0.496 0.0100 27 - 0.619 Bulk density (g wet wt cm- 3) 0.0006 27 LOI (g LOI cm- 3) 0.743 b0.0001 27 S (µmol cm- 3) 0.350 0.1304 22 log(AVS + CRS) 0.519 (µmol cm- 3) 0.0066 26 Organic S 0.293 -3 (µmol cm ) 0.1974 22 log(HS-) (M) 0.173 0.3993 26 log(SO4) (mM) - 0.356 0.0742 26 log(Sulfate Red.) 0.488 (nmol cm- 3 h- 1) 0.0114 26 log(CO2prod.) 0.288 (nmol cm- 3 h- 1) 0.1539 26 HgT (pmol cm- 3) 1.0 0.241 0.2352 27 - 0.233 0.2417 27 0.556 0.0026 27 - 0.400 0.0803 22 0.200 0.3268 26 - 0.390 0.0803 22 - 0.030 0.8838 26 0.251 0.2170 26 0.249 0.2202 26 - 0.097 0.6358 26 0.516 0.0059 27 1.0 0.814 b 0.0001 27 0.475 0.0122 27 - 0.273 0.1680 27 - 0.821 b 0.0001 22 - 0.577 0.0020 26 - 0.550 0.0099 22 - 0.405 0.0399 26 0.717 b 0.0001 26 - 0.306 0.1279 26 - 0.537 0.0047 26 - 0.540 0.0037 27 0.376 0.0531 27 1.0 0.594 0.0014 26 0.486 0.0117 26 0.002 0.9930 26 - 0.545 0.0130 22 - 0.050 0.8081 26 - 0.382 0.0880 22 - 0.227 0.2639 26 0.537 0.0047 26 0.136 0.5070 26 - 0.340 0.0893 26 - 0.251 0.2162 26 0.206 0.3126 26 0.420 0.0327 26 1.0 0.537 0.0047 26 0.389 0.0493 26 - 0.425 0.0305 26 - 0.501 0.0245 22 - 0.206 0.3138 26 - 0.364 0.1045 22 - 0.015 0.9409 26 0.451 0.0208 26 - 0.320 0.1112 26 - 0.312 0.1208 26 - 0.628 0.0006 26 - 0.034 0.8697 26 0.538 0.0046 26 0.348 0.0816 26 1.0 - 0.688 0.0001 26 - 0.881 b 0.0001 26 0.502 0.0091 26 - 0.519 0.0192 22 0.433 0.0273 26 - 0.563 0.0079 22 0.571 0.0023 26 - 0.628 0.0006 26 0.385 0.0519 26 0.624 0.0007 26 0.789 b 0.0001 26 0.241 0.2349 26 - 0.634 0.0050 26 - 0.657 0.0003 26 - 0.550 0.0036 26 1.0 - 0.550 0.0036 26 - 0.746 b 0.0001 26 0.615 0.0008 26 - 0.375 0.1033 22 0.321 0.1104 26 - 0.417 0.0603 22 0.334 0.0950 26 - 0.496 0.0100 26 0.465 0.0167 26 0.468 0.0160 26 0.802 b 0.0001 26 0.419 0.0332 26 - 0.442 0.0236 26 - 0.408 0.0385 26 - 0.781 b 0.0001 26 0.839 b 0.0001 26 1.0 0.484 0.0123 27 0.316 0.1081 27 - 0.005 0.9811 27 - 0.493 0.0271 22 0.021 0.9203 26 - 0.489 0.0245 22 - 0.360 0.0708 26 0.545 0.0040 26 - 0.096 0.6417 26 - 0.330 0.0993 26 - 0.069 0.7313 27 0.586 0.0013 27 0.610 0.0007 27 0.430 0.0283 26 0.266 0.1893 26 - 0.350 0.0793 26 - 0.167 0.4136 26

log(MeHg) (pmol cm- 3) log(% MeHg) (%)

log(HgT) (pM)

Log(MeHg) (pM)

Log(HgT KD) (L kg- 1) log(MeHg KD) (L kg- 1)

Data includes 0­12 cm averaged values from all stations and cruises. Listed from top to bottom: Pearson's correlation coefficient (r), p-value, and sample number (n).

sediments of our study site are more efficient in the net methylation of Hg than most of the organic-rich sediments in the estuary. The b1% MeHg values measured in the upper and mid Bay sediments are similar to values observed in other estuaries, including both organic-rich and sandy sediments (Kannan et al., 1998; Bloom et al., 1999; Mikac et al., 1999; Hammerschmidt and Fitzgerald, 2004; Sunderland et al., 2004; Heyes et al., 2006). The % MeHg values in offshore mid-Atlantic continental shelf sites were at the higher end of the range previously observed for coastal and shelf sediments (for summaries see Fitzgerald et al. (2007), Heyes et al. (2006) and Mitchell and Gilmour (2008)), suggesting relatively high net MeHg production in mid-Atlantic shelf and slope sediments. 3.3. Potential mercury methylation rate constants across the study area Like % MeHg, potential methylation rate constants (kmeth) generally increased with salinity (Fig. 4; Table 4). Although kmeth and % MeHg were significantly correlated (p b 0.01; Table 4), the seasonal variability

in kmeth was much larger than the seasonal change in % MeHg. This suggests that some fraction of the MeHg produced is preserved in sediments between seasons, as has been observed in freshwater sediments (Orihel et al., 2008). In addition, this confirms that the estimation of kmeth is an important tool to understand short-term variations in MeHg production. The average kmeth for the upper 12 cm of sediment varied by an order of magnitude across all sites and cruises (Fig. 4). Rates were generally highest in summer and early fall for Bay and shelf sites where there is a dramatic seasonal change in sediment temperature and redox status. Seasonal differences in the methylation rate constants were not as apparent on the slope, in concert with smaller temperature variability at this site (Table 1). Overall, kmeth values found in this study were in the same range as those measured previously with similar methods in estuarine sediments, including Chesapeake Bay (Kim et al., 2004; Heyes et al., 2006), Bay of Fundy (Sunderland et al., 2004; Heyes et al., 2006), Hudson River estuary

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Fig. 3. MeHg as a percentage of total Hg (% MeHg), with depth, in sediments of Chesapeake Bay and the mid-Atlantic continental shelf and slope. Bars represent the average % MeHg across all 4 cruise dates in 2005 and 2006.

(Heyes et al., 2004; Heyes et al., 2006), New York/New Jersey Harbor (Hammerschmidt et al., 2008), Long Island Sound (Hammerschmidt et al., 2004), Adour River estuary (Martin-Doimeadios et al., 2004) and Thau Lagoon (Monperrus et al., 2007b). To date, there have been very few offshore methylation rate estimates. The rates we measured for the mid-Atlantic shelf and slope sediments were similar to those measured in northern Gulf of Mexico (Liu et al., 2009) but at the lower end of those measured during September on the southern New England continental shelf (Hammerschmidt and Fitzgerald, 2006b). 3.4. Sediment­water partitioning of mercury and methylmercury Hg and MeHg concentrations in sediment interstitial water (depth-averaged) generally increased with salinity (Table 4; Fig. 5), with the exception of deep mid-Bay sediments, in which there was a dramatic increase in pore water HgT and MeHg concentrations under extremely sulfidic conditions with anoxic overlying water. The higher dissolved mercury concentrations in marine pore water may con-

Fig. 5. HgT and MeHg concentrations in sediment pore waters, for each of 4 cruises in 2005 and 2006. Each bar represents the average concentration in the top 12 cm for HgT and top 4 cm for MeHg.

Fig. 4. MeHg as a percentage of total Hg (% MeHg; top panel) and methylation rate constants (kmeth; bottom panel) in sediments of Chesapeake Bay and the mid-Atlantic continental shelf and slope, for each of 4 cruises in 2005 and 2006. Each bar represents the average value (± standard deviation of duplicate cores) over 12 cm depth.

tribute to the observed increase in methylation rates with salinity. With the exception of the deep mid-Bay site, pore water HgT concentrations varied by about one order of magnitude across all sites and dates. Pore water MeHg concentrations were more variable with space and time, as expected based on seasonal variations in net MeHg production. The ranges we observed are similar to what was previously measured in a tributary of Chesapeake Bay (Benoit et al., 1998) and in Hudson River estuary (Heyes et al., 2004), but lower than those measured in a number of other estuaries (Gagnon et al., 1996; Cossa and Gobeil, 2000; Hammerschmidt and Fitzgerald, 2004; Sunderland et al., 2004; Canario et al., 2008; Hammerschmidt et al., 2008). Offshore HgT pore water concentrations in our study were similar to those observed for three sites on the southern New England continental shelf (Hammerschmidt and Fitzgerald, 2006b) and six sites in northern Gulf of Mexico (Liu et al., 2009). However, MeHg pore water concentrations in our study were lower compared to those measured on the southern New England continental shelf (Hammerschmidt and Fitzgerald, 2006b). Given the differences in sediment Hg and MeHg concentrations across our study area, but the relative similarity in pore water concentrations, large spatial variations were observed in the partitioning of HgT and MeHg between the solid phase and pore water. The distribution coefficient (KD; L kg- 1) of both HgT and MeHg declined down-Bay and offshore, decreasing by almost 2 orders of magnitude, with the exception of the organic-rich slope sediments (Fig. 6). Overall, KD values for MeHg were typically an order of magnitude lower than those for HgT, driven by the lower binding strength of MeHg to organic ligands (Fitzgerald et al., 2007). Our distribution coefficients were calculated by assuming the aqueous phase as all filterable Hg complexes, including Hg bound colloids. Spatial differences in the KD for both HgT and MeHg were strongly associated with variations in the organic matter content and bulk density across sites (Table 4). This general relationship has been observed in other estuarine (Bloom et al., 1999; Hammerschmidt and Fitzgerald, 2004; Hammerschmidt et al., 2004; Sunderland et al., 2006; Hammerschmidt et al., 2008) and marine (Hammerschmidt and Fitzgerald, 2006b; Ogrinc et al., 2007; Liu et al., 2009) sediments.

T.A. Hollweg et al. / Marine Chemistry 114 (2009) 86­101

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Fig. 7. Relation between the HgT KD, normalized to organic matter content (% LOI), and bottom water salinity, for each station. Each point represents the average value over 12 cm depth.

Fig. 6. The distribution coefficient (KD) of HgT and MeHg, for each of 4 cruises in 2005 and 2006. Each bar represents the average KD in the top 12 cm for HgT and the top 4 cm for MeHg.

Using the data from all cruises, depths and stations, significant linear relationships were found between the sediment organic matter content and Hg or MeHg KD: Log KD ðHgTÞ =

ð3:30 F 0:06Þ + ð0:15 F 0:01Þ½kLOI ðr = 0:67; p b 0:01; n = 128Þ

2

ð6Þ

Log KD ðMeHgÞ = 2:44 F 0:11 + 0:14 F 0:01 2 r = 0:56; p b 0:01; n = 80

ð

Þ ð

Þ½kLOI

ð7Þ

The slopes and intercepts of these regressions are remarkably similar to those found by Hammerschmidt and Fitzgerald (2006b), but different than other ecosystems studied to date (Table 5). Differences may be due to variations in organic matter, as emphasized by the significant difference in slopes for the groups of sites with different organic matter content. It is apparent that the relationship between

organic matter and Hg partitioning behavior is non-linear, perhaps reflecting the differences in the sources and chemical composition of organic matter across large spatial scales. The relationships are probably ecosystem specific, reflecting the source and quality of the organic matter as well as the Hg inventory in the system. Within an ecosystem, the relationship between KD and % LOI is similar for both Hg and MeHg, such that the regression lines are parallel with only a shift in the y-intercept (Table 5). However, the degree of the shift between HgT KD and MeHg KD is different between ecosystems, ranging from b1 to N2 orders of magnitude. This is interesting, and supports the notion that additional factors besides the bulk organic content impact the partitioning of MeHg, and these influences vary across ecosystems. Possible factors include differences in MeHg interactions with the solid phase and pore water and/or production/decomposition rates. While the KD value is estimated based on the assumption of equilibrium partitioning, there is evidence to suggest that the rate of desorption is much slower than the rate of adsorption (Hintelmann, 2004) and thus a disequilibrium may persist under some conditions in sediments. This is likely to be more pronounced for MeHg given its relatively low concentration (range 0.1­14.0 pmol g- 1dry weight) and its overall higher decomposition rate constant compared to its rate of formation (Heyes et al., 2004; Kim et al., 2004; Sunderland et al., 2004; Heyes et al., 2006). In addition to the quantity of organic matter, the quality of organic matter appears to affect Hg partitioning in these sediments. For the Chesapeake/mid-Atlantic shelf data set, we found that the HgT KD, normalized to the organic content of sediments, declined significantly with salinity (Fig. 7). We interpret salinity as a proxy for changes in system biogeochemistry and organic matter quality. Specifically, we hypothesize that the binding strength/capacity of organic matter for Hg declines as organic matter ages, as illustrated by the general

Table 5 Regression equations of HgT and MeHg KD vs. % LOI (Log (KD) = y-intercept + slope [LOI]) for different coastal ecosystems, including Chesapeake Bay (CB) and the mid-Atlantic continental shelf (this study), Long Island Sound (LIS) and the Southern New England continental shelf (SNE), New York/New Jersey harbor (NY/NJ), northern Gulf of Mexico (GOM), Lavaca Bay Texas, Mediterranean Sea, and Bay of Fundy. Range of % LOI listed. LOI range % CB + mid-Atl. cont. shelf All data Low organic High organic US + SNE cont. shelf NY/NJ Harbor Northern GOM Lavaca Bay Mediterranean Sea Bay of Fundy 0.4­14 0.4­2.5 5­14 1.8­10 2­13 0.25­5b 0.25­2.25b 0.4­3.25b 2.5­6.5b Log (HgT KD) y-intercept 3.30 ± 0.06 2.79 ± 0.12 4.28 ± 0.18 3.13 ± 0.05a 4.61 ± 0.08a 3.2 4.2 2 2.9 Slope 0.15 ± 0.01 0.44 ± 0.07 0.049 ± 0.020 0.15 ± 0.01a 0.093 ± 0.010a 0.58b 0.44b 0.49b 0.14b Log (MeHg KD) y-intercept 2.44 ± 0.11 1.80 ± 0.14 ns 1.55 ± 0.05 2.59 ± 0.01 2.2 0.9 1.61 Slope 0.14 ± 0.01 0.41 ± 0.10 ns 0.13 ± 0.01 0.087 ± 0.008 0.48b 0.43b 0.22b This study This study This study Hammerschmidt and Fitzgerald (2006b) Hammerschmidt et al. (2008) Liu et al. (2009) Bloom et al. (1999) Ogrinc et al. (2007) Sunderland et al. (2006)

ns = not significant. a For Hg(II). b Assuming total organic matter is 40% organic carbon.

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increase in the C:N ratio with salinity (Tables 1 and 3). This trend is supported by the decrease in organic sulfur concentration with movement offshore (Fig. 8). Organic sulfur was estimated as the molar difference between total sulfur and inorganic sulfur (AVS and CRS). Using reducible Hg titrations, Lamborg et al. (2003, 2004) examined the concentration of Hg binding ligands in dissolved organic carbon (DOC) from different water sources and found that the ligandequivalent concentration normalized to DOC (equivalent/mole) was an order of magnitude higher for waters from Long Island Sound compared to the mid-Atlantic continental shelf, suggesting that coastal ocean organic matter has lower Hg-binding capacity. Reduced inorganic sulfur and other parameters that co-vary with salinity may also play a role in the observed relationship between Hg partitioning and salinity. 3.5. Biogeochemical controls on methylmercury production Methylmercury production in Chesapeake Bay and the midAtlantic continental shelf and slope is affected by the bioavailability of Hg(II) for methylation and by the activity of Hg-methylating microbes, both of which are significantly affected by biogeochemical changes across the salinity and productivity gradients of this study area. In sediments of all salinities, MeHg production was highest at sediment depths just below the oxic/anoxic transition; i.e. depths were microbial sulfate reduction was present, but where sulfide, which inhibits methylation, was relatively low (Benoit et al., 2003). Thus the vertical zonation of MeHg production was controlled by the depth of the redoxcline and the intensity of organic matter degradation. In the organic-rich sediments of the upper and mid Bay, only the top few millimeters of sediment contained oxygen, and Hg methylation was condensed into the upper few centimeters. The methylation rate profiles were broader and deeper in the sandy sediments of the lower Bay and shelf and the organic clays of the slope. The deeper and broader oxic­anoxic transition zones in offshore sediments resulted in a larger zone of Hg(II) methylation and higher depth-integrated MeHg inventories (Figs. 2 and 3). To illustrate the redox zonation of methylation in sediments across the Chesapeake system, we categorized the stations into four groups, based on salinity, organic matter content and redox conditions, as follows: 1) Eutrophic upper and mid Bay shallow water locations, characterized by fine-grained sediments high in organic matter content; 2) Mid-Bay deep channel sediments, often "ooze", under anoxic bottom waters; 3) Sandy, lower Bay and shelf sediments, in which oxygen generally penetrates a few cm into sediments; 4) Slope sediments, which are organic rich and heavily bioturbated clays. Example profiles from each of the four sediment types are shown in Fig. 9.

Fig. 8. Bulk-phase organic sulfur content and the molar ratio of organic sulfur to organic carbon (OS:OC) in sediment of Chesapeake Bay and the mid-Atlantic continental shelf and slope. Each point represents the average value over 12 cm depth of 4 cruises. Error bars represent standard deviation between cruises.

Shallow sediments from the upper and mid Bay sites (STA 1 and 3) are fine-grained and organic-rich (Table 3). Although overlying bottom waters were generally oxygenated, sediment microbial activity and oxygen demand were high, and sediments became rapidly anoxic with depth (Fig. 9A). Typical redox conditions at STA 1 and 3 favored Hg methylation in a narrow surface layer (b2 cm), which supports active sulfate reduction but accumulates relatively low sulfide (b10 M). At depths below the shallow surface layer, decreases in sulfate reduction rates and increases in sulfide concentration coincide with decreases in both kmeth and % MeHg. Interestingly, the highest and lowest methylation rate constants were measured in these productive sites, highlighting a delicate balance between microbial activity and Hg(II)-sulfide speciation, which reemphasizes the importance of Hg speciation in Hg bioavailability to methylating bacteria, and therefore, to net MeHg production in sediment. In Chesapeake Bay, two-layer estuarine flow generates strong salinity stratification. Along with significant system eutrophication, this leads to the development of permanent seasonal anoxia in the bottom waters of the deep central channel (Cowan and Boynton, 1996; Kemp et al., 2005). Sediments below this "dead zone" are very fined grained, organic rich, and fully anoxic and sulfidic for much of the year (Marvin-DiPasquale and Capone, 1998). Station 2 was located in the deep mid-Bay channel with anoxic overlying water present during our sampling. Pore water chemistry at this site was dramatically different from all the other stations, with anomalously high sulfide, HgT and MeHg concentrations (Figs. 5 and 9B). Sulfate reduction rates and potential Hg methylation rates were highest at the sediment surface. Although kmeth was generally somewhat lower at STA 2 than at the nearby shallow water station (STA 3), the % MeHg was often much higher (Figs. 4 and 9B). With sulfide concentrations greater than 100 M in surface pore waters, MeHg production should be inhibited (Benoit et al., 1999; Benoit et al., 2001). However, significant de novo water column MeHg production occurs during water column anoxia in Chesapeake Bay (Gilmour et al., in prep), which may contribute to high MeHg levels in deep channel sediments. Micro-molar water column sulfide levels are sufficient to result in precipitation of Fe­S solid phases (based on MINEQL+ simulations), which may contribute to MeHg removal from the water column to the surface sediment. It has been shown that MeHg binds strongly to the surface of Fe­S solid phases (Miller, 2005). Additionally, the high pore water Hg and MeHg in these highly sulfidic sediments may be enhanced by the formation of dissolved Hg­ and MeHg­S complexes. We hypothesize that particles sinking through sub-pycnocline waters may be the primary source of MeHg to these deep channel sediments during periods of water column anoxia, while in situ production within sediments is the major source during the rest of the year. The sandy, lower organic content sediments of the lower Bay and shelf (STA 4, 6, and 7) and organic-rich sediment of the slope (STA 9) typically had lower microbial activity, deeper oxygen penetration and a broader redox boundary (Fig. 9C and D). This deepening and expanding of the oxic­anoxic boundary is most likely due to sediment oxygenation from bioturbation and bioirrigation (Schaffner, 1990; Burdige and Homstead, 1994; Ferdelman, 1994; Thomas et al., 2002; Burdige et al., 2004), advection in permeable sediment (Huettel et al., 1996; Huettel et al., 1998) and/or different organic matter quality and supply rates than that of the Bay. The broad depth horizon where sulfate reduction occurred without build-up of dissolved sulfide resulted in a deeper and more diffuse region of MeHg production (Fig. 2). Peak MeHg production has been noted to occur at deeper depths in heavily bioturbated and/or sandy sediments (Hammerschmidt et al., 2004; Heyes et al., 2004; Sunderland et al., 2004; Benoit et al., 2006; Hammerschmidt and Fitzgerald, 2006b) and likely results in a larger inventory of MeHg (Sunderland et al., 2004; Benoit et al., 2006).

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Fig. 9. Sediment profiles of bulk-phase HgT and MeHg, % MeHg, potential methylation rate (kmeth), microbial activity, and ancillary bulk-phase and pore water geochemistries sampled at A) Station 3 during September 2005; B) Station 2 during July 2005; C) Station 6 during July 2005; D) Station 9 during April 2006. For bulk-phase concentrations, % MeHg, potential methylation rates and microbial activity each point represents the average value of duplicate cores. For pore water geochemistries each point represents a single sample.

3.6. Biogeochemical factors affecting Hg methylation rates The strong impact of sulfide on methylation has been repeatedly observed in estuarine and coastal systems (Benoit et al., 1998; Conaway et al., 2003; Hammerschmidt and Fitzgerald, 2006b; Heyes et al., 2006; Sunderland et al., 2006; Hammerschmidt et al., 2008), and has been characterized as a bell-shaped curve which represents the balance between the activity of Hg-methylating organisms and the production of sulfide, with its impact on Hg complexation (Benoit

et al., 2003). In the Chesapeake/Mid-Atlantic shelf system, kmeth was maximal at about 1 M sulfide (Fig. 10). However, substantial scatter is present in the data, with many points falling below the theoretical curve, indicating that other factors may also affect Hg methylation. In Chesapeake Bay, kmeth was significantly and positively correlated with pore water sulfate, but negatively correlated with pore water sulfide. In a multiple linear regression using all dates and depths in the Chesapeake sediment data set, pore water Hg, sulfate and sulfide concentrations accounted for more than half of the variability in

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T.A. Hollweg et al. / Marine Chemistry 114 (2009) 86­101 Table 6 Methylmercury concentration (± standard deviation) in filtered pore water (0­2 cm) and bottom water sampled during four cruises to Chesapeake Bay and the mid-Atlantic continental shelf and slope. Pore water MeHg (pM) Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station Station 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 6 6 6 6 7 7 7 7 9 9 9 May 2005 July 2005 Sept 2005 April 2006 May 2005 July 2005 Sept 2005 April 2006 May 2005 July 2005 Sept 2005 April 2006 May 2005 July 2005 Sept 2005 April 2006 May 2005 July 2005 Sept 2005 April 2006 May 2005 July 2005 Sept 2005 April 2006 July 2005 Sept 2005 April 2006 0.17 0.24 3.07 1.10 0.41 14.4 1.54 1.22 0.24 0.04 0.56 0.87 2.40 1.31 0.43 0.95 0.79 1.50 2.36 1.52 0.75 0.65 1.00 0.19 0.37 1.37 Bottom water MeHg (pM) 0.18 0.87 ± 0.78 0.09 0.02 0.057 ± 0.07 1.137 ± 0.89 1.38 0.18 0.867 ± 1.07 0.11 0.23 0.08 0.0767 ± 0.110 0.597 ± 0.25 0.12 0.41 0.068 ± 0.100 0.08 0.15 0.06 0.357 ± 0.54 0.247 ± 0.19 0.08 0.04 0.057 ± 0.03 0.147 ± 0.10 0.05

Fig. 10. Relation between methylation rate constant (kmeth) and pore water sulfide concentration, for each station. Figure includes data from four cruises and six 2-cm depth intervals.

kmeth. As observed in the Florida Everglades (Gilmour et al. 1998; Benoit et al. 2003), sulfate reduction rate alone was a poor predictor of kmeth. In addition to sulfide, organic matter quantity and quality clearly affect MeHg production, primarily by controlling Hg concentration in porewater. Methylation rate and % MeHg were negatively related (p b 0.05) to the organic S content in sediment of our study area (Table 4), supporting the idea that sediments low in organic S content have a decrease in Hg partitioning to the solid phase and an increase in dissolved Hg bioavailable to methylating bacteria. This relationship between MeHg production and Hg partitioning as a result of organic matter content has been observed in other estuarine and marine ecosystems (Hammerschmidt and Fitzgerald, 2004; Hammerschmidt and Fitzgerald, 2006b; Ogrinc et al., 2007; Hammerschmidt et al., 2008). In order to adequately model Hg methylation, the interactions between Hg and organic matter, especially under the anoxic, often mildly sulfidic conditions where methylation occurs, needs to be further defined. Initial research under these conditions shows unexpectedly strong interactions between Hg, dissolved sulfide, and organic matter (Miller et al., 2007), which can have a substantial impact on aqueous Hg concentrations and complexation. In soils of a Chesapeake salt marsh, Mitchell and Gilmour (2008) showed that methylation rates were positively related to molecular weight of DOM. Burdige and Gardner (1998) determined that pore waters of the mid-Atlantic continental shelf/ slope break contained a higher percentage of high molecular weight DOC compared to Chesapeake Bay. The authors suggested that different remineralization processes and rates result in different DOC concentrations and size classes, and that high molecular weight DOC is more labile and reactive (Burdige and Gardner,1998; Burdige et al., 2000). Overall, our results are consistent with these findings, and suggest that DOM size and source could be strong indicators of the impact of organic matter cycling in sediment on Hg bioavailability and methylation. 3.7. Sediments as a source of methylmercury to the water column Based on the MeHg concentration gradient between surficial pore waters and overlying surface waters (Table 6), sediments of Chesapeake Bay and the mid-Atlantic shelf and slope are almost always sources of MeHg to estuarine and coastal waters. It is important to quantitatively determine the sediment­water exchange of MeHg to better understand the relative importance of these sediment as a source of MeHg to the water column and, most importantly, to the aquatic food web. A calculation of MeHg diffusive flux provides a minimum estimate of mass exchange, since processes such as bioturbation/bioirrigation may increase MeHg fluxes significantly (Gill et al., 1999; Choe et al., 2004; Hammerschmidt and Fitzgerald, 2008; Benoit et al., in review), especially for the lower Bay, shelf and slope sediments. We calculated diffusive fluxes at our study site using the change in concentration between MeHg measured in filtered bottom water and

the MeHg concentration in interstitial waters of surficial sediments (upper 2 cm; Table 6; Fig. 11). Fluxes were calculated by assuming that MeHg in pore water is complexed with either dissolved sulfide or DOM. Calculated diffusional fluxes for the small MeHg­S complex averaged about 1 pmol m- 2 d- 1, while flux estimates were almost an

Fig. 11. The estimated diffusive flux of MeHg to the water column, assuming MeHg bound to S (MeHg­S; top panel) or dissolved organic matter (MeHg­DOM; bottom panel), for each of 4 cruises in 2005 and 2006. The diffusive flux was calculated by using the difference in MeHg concentration between the filtered overlying water and the surficial pore water. Positive flux signifies MeHg diffusing from sediment pore water to the overlying water.

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order of magnitude lower for the MeHg-DOM complex, driven by the large difference in diffusion coefficients related to differences in the molecular weight of the complexes. The highest MeHg diffusive flux rates were estimated for the highly sulfidic deep channel sediments (STA 2) in July 2005, when overlying bottom water was anoxic, and high dissolved HgT and MeHg concentrations were measured in the surface sediment (Fig. 5). Diffusional flux estimates for our shelf sites are similar to those estimated for the southern New England continental shelf (Hammerschmidt and Fitzgerald, 2006b) and Mediterranean Sea (Ogrinc et al., 2007), using similar assumptions. However, our estimates of MeHg flux from Chesapeake Bay sediments are lower than those for Long Island Sound (Hammerschmidt et al., 2004) and New York/New Jersey Harbor (Hammerschmidt and Fitzgerald, 2008), which are driven by higher pore water MeHg concentrations at these sites. It is likely that the total MeHg fluxes out of lower Bay and continental margin sediments are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by pore water pumping in permeable sediments (Huettel et al., 1996; Huettel et al., 1998) and/or bioturbation/bioirrigation. In support of this contention, we note that the sediments of the lower Bay and slope have been described to be heavily bioturbated (Schaffner, 1990; DeMaster et al., 1994; Alperin et al., 1999), with measured benthic fluxes for dissolved solutes larger than the estimated diffusional fluxes (Thomas et al., 2002; Burdige et al., 2004). In addition, similar to the mid-Atlantic shelf system, the estimated sediment­water flux for several constituents from sediments of the southern Atlantic Bight was, on average, 30 times greater than the calculated diffusional flux (Jahnke et al., 2005). Methylmercury fluxes from the sulfidic sediments of the upper and mid-Bay are most likely dominated by diffusion, as observed for DOC fluxes at STA 2 (Burdige and Homstead, 1994; Burdige et al., 2004), since bioturbation is absent for most of the year (Kemp et al., 1990; Zimmerman and Canuel, 2002). Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf significantly strengthen the argument that coastal sediments contribute substantially to coastal MeHg budgets. By combining our estimate of average MeHg­S diffusive flux (1.3 pmol m- 2 d- 1) with the area of the adjacent continental shelf off the Virginian coast (17,000 km2), the minimum contribution of MeHg from shelf sediments to the water column is 8 mol MeHg year- 1. This value is of the same order as other major inputs to the system, including the net flux of MeHg from Chesapeake Bay (27.8 mol year- 1; Mason et al., 1999) and from the atmosphere ( 3.5 mol year - 1 ; based on data in Mason et al. (2000)). Methylmercury production in the water column is another potential source. However, although in situ water column mercury methylation has been recently observed in oxic waters of the Mediterranean (Monperrus et al., 2007a,b), mercury methylation was not detected in mid-Atlantic continental shelf surface waters, using similar techniques (Whalin et al., 2007). Further examination of these fluxes requires the development of a detailed coastal MeHg model, which could provide more refined estimates of a total flux into the water column and help determine the relative importance of the coastal system to MeHg concentrations in coastal and open ocean food webs. 4. Conclusion Our study emphasizes the importance of estuarine, shelf and slope sediments in MeHg production in the coastal zone. High MeHg production potentials and inventories in estuarine and coastal sediments of the mid-Atlantic demonstrate that sediments must be a significant source to local benthic food webs. Furthermore, even our minimal, diffusion-based estimates of MeHg efflux suggest that sediments are a significant contributor to MeHg in the coastal water column. These data contribute to our evolving understanding of Hg and MeHg budgets for coastal systems.

Detailed down-core measurements across a full salinity gradient demonstrated that the dominant biogeochemical controls on MeHg production -- the interplay between sulfate-reduction, sulfide accumulation and organic matter -- are the same in estuarine and marine systems as they are in better-studied freshwater systems. Despite high sulfide concentrations at depth in marine and estuarine sediments, we observed zones of significant MeHg production above that depth at all salinities, in strata where microbial sulfate reduction was present, but sulfide concentrations were low. The vertical zonation of Hg methylation varied between sites, condensed into the upper few centimeters in the organic rich upper and mid Bay and broader and deeper in sediments of the lower Bay, shelf and slope. Increased vertical expanses of MeHg production resulted in higher MeHg inventories in offshore sediments. In addition, we found that the character of organic matter plays a significant role in Hg partitioning, bioavailability and methylation. Acknowledgements We thank the Captains and crews of the RV Henlopen, RV Hugh Sharp and RV Hatteras. We particularly thank our colleague Marcelino Suzuki and the crew of the RVs Henlopen and Hugh Sharp for their successful efforts to modify an Ocean Instruments box corer for use in sand. We also thank Georgia Riedel, Tyler Bell, Evan Malczyk, Liz Kerin and Genevieve Bernier for technical assistance in the field and lab. This work was supported by the National Science Foundation (OCE0351050 to R. Mason and C. Gilmour; and the NSF/SERC Research Experience for Undergraduates Program), by an EPA-STAR graduate fellowship to TH, and by the Smithsonian Marine Sciences Network. References

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Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin

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Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin