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Flux of total and methyl mercury to the northern Gulf of Mexico from U.S. estuaries Clifton Buck, Chad R. Hammerschmidt, Katlin Bowman, Gary A. Gill, and William M Landing Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03538 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on October 29, 2015
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Flux of total and methyl mercury to the northern Gulf of Mexico
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from U.S. estuaries
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Clifton S. Buck,†* Chad R. Hammerschmidt,‡ Katlin L. Bowman,‡ Gary A. Gill, § and William M.
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Landingǁ
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†Department of Marine Science, Skidaway Institute of Oceanography, University of Georgia, 10 Ocean
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Science Circle, Savannah, Georgia 31411, United States
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‡
Department of Earth & Environmental Sciences, Wright State University, 3640 Colonel Glenn Hwy.,
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Dayton, Ohio 45435, United States
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§Marine Science Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 98382, United
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States
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ǁDepartment of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, Florida
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32306, United States
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*
Corresponding Author: email:
[email protected]; phone: 912-598-2418; fax: 912-598-2310
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Abstract: To better understand the source of elevated methylmercury (MeHg) concentrations in Gulf of
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Mexico (GOM) fish, we quantified fluxes of total Hg and MeHg from 11 rivers in the southeastern United
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States, including the 10 largest rivers discharging to the GOM. Filtered water and suspended particles
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were collected across estuarine salinity gradients in Spring and Fall 2012 to estimate fluxes from rivers to
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estuaries and from estuaries to coastal waters. Fluxes of total Hg and MeHg from rivers to estuaries varied
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as much as 100-fold among rivers. The Mississippi River accounted for 59% of the total Hg flux and 49%
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of the fluvial MeHg flux into GOM estuaries. While some estuaries were sources of Hg, the combined
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estimated fluxes of total Hg (5200 mol y-1) and MeHg (120 mol y-1) from the estuaries to the GOM were
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less than those from rivers to estuaries, suggesting an overall estuarine sink. Fluxes of total Hg from the 1 ACS Paragon Plus Environment
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estuaries to coastal waters of the northern GOM are approximately an order of magnitude less than
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atmospheric deposition rates. However, fluxes from rivers are significant sources of MeHg to estuaries
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and coastal regions of the northern GOM.
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INTRODUCTION
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Mercury (Hg) contamination of coastal marine ecosystems poses a potential health risk to humans.
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Inorganic forms of Hg can be transformed to methylmercury (MeHg) in coastal waters and sediments1,2 as
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well as in estuaries.3,4,5,6 Mercury contamination of the coastal ocean is concerning because consumption
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of marine fish is the primary vector for human exposure to MeHg,7 and the majority of commercial
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fisheries are in coastal waters.8 Methylmercury bioaccumulates in marine organisms and biomagnifies in
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food webs often resulting in high concentrations in commercially valuable predatory fish (e.g., grouper,
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mackerel, swordfish).9 Consumption of contaminated fish has resulted in as much as 30% of the U.S. Gulf
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coast population having blood MeHg concentrations that increase risk of neurodevelopmental problems in
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children.10
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Fluvial inputs are often the principal source of Hg to estuaries,11,12,13,14 with much of the riverine Hg
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derived from atmospheric deposition to the watershed.13 The relative significance of river discharge
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sources versus direct atmospheric deposition decreases as the surface area of the receiving waters
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increases, such that rivers are estimated to account for less than 10% of Hg delivered to the remote
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continental margin and open ocean.2,15,16,17 Riverine inputs of Hg to the global ocean are estimated to be
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20–60% that of atmospheric deposition with 90% of the river-derived Hg confined to sediments on
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continental margins.18,19,20 However, very limited data exist for riverine Hg fluxes to the global ocean,18,21
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especially the net flux through estuaries to the coastal zone including around the Gulf of Mexico
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(GOM).22
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We investigated the flux of total Hg (HgT) and MeHg from 11 U.S. rivers and estuaries to the
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northern Gulf of Mexico (nGOM). The nGOM is an ideal study region because the estuaries vary with
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regard to flow and morphology and include the Mississippi and Atchafalaya River systems, which are
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estimated to deliver as much as 75% of the HgT load to the nGOM.23 While the nGOM coast receives the
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highest rates of atmospheric Hg wet deposition in the U.S. (≥18 µg m-2 HgT),24 the riverine inputs are
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hypothesized to dominate relative to atmospheric inputs in this area because of high rates of freshwater
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discharge. The GOM is also an important seafood fishery, and by extension a potential source of MeHg 3 ACS Paragon Plus Environment
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exposure, accounting for 15% of U.S. marine commercial and about 40% of the marine recreational fish
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catch in 2008.10
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We sought to (1) quantify the riverine fluxes of HgT and MeHg to estuaries around the nGOM, (2)
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determine whether, and to what extent, estuarine processes (e.g., scavenging, Hg methylation) alter
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riverine fluxes to the Gulf, and (3) quantify net fluxes of HgT and MeHg from estuaries to coastal waters
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of the nGOM. The results of this synoptic survey present a comprehensive set of Hg concentrations and
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estimated fluxes from the largest estuarine systems in the nGOM. Our sampling approach was intended to
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maximize spatial coverage, in lieu of temporal resolution, during the expected wet and dry seasons of
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2012.
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METHODS AND MATERIALS
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Water sampling. Filtered water and suspended particles were sampled from 11 estuaries in the nGOM in
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spring (May–June) and fall (September–October) of 2012 (Figure 1). Excluding the Escambia River,
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these are the ten largest American rivers discharging to the Gulf. Surface water was sampled by peristaltic
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pump from the freshwater end-member through eight additional locations along the salinity gradient
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down the main channel to the seawater end-member of each estuary. Sampling was focused on the oligo-
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and mesohaline reaches of each estuary to observe mixing behavior in the areas expected to be most
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influenced by changes in salinity. Target salinities for sampling were about 0, 3, 6, 9, 12, 15, 20, 25 and
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35. Duplicate samples were collected at salinities of about 0 and 12. Salinity was measured with a YSI
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multi-parameter water quality sonde that was calibrated with standards traceable to the U.S. National
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Institute of Standards and Technology (NIST). While this sampling approach disregards shorter time scale
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fluctuations in Hg concentrations, the long-term water discharge records allow for the estimation of flux
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to the coastal ocean with reasonable certainty.
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An aluminum pole was used to extend acid-cleaned Teflon tubing into surface water (~1 m depth)
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upstream of the boat, which was either anchored or motored slowly into the current. The Teflon tubing
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was connected to acid-cleaned C-Flex tubing (Masterflex) in the pump roller, and water was pushed 4 ACS Paragon Plus Environment
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through Teflon tubing to a series of Teflon “tee” fittings that divided the flow between in-line glass fiber
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filters (GF/F, for suspended particles) and 0.4 µm capsule filters (for filtered water). The GF/F filters (47
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mm, 0.7 µm, Whatman 1825-047) were cleaned prior to use by heating for four hours at 550 ºC, and the
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capsule filters (AcroPak-200, polyethersulfone membrane, Pall 12941) were cleaned with 0.1 M HCl.25
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Samples for analysis of filtered HgT and MeHg were collected in pre-cleaned glass and polycarbonate
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bottles, respectively.26 Samples were stored in the dark on ice and shipped within 24 hours of sampling to
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Pacific Northwest National Laboratory (PNNL) and Wright State University (WSU), where they were
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either analyzed immediately (filtered MeHg) or preserved by freezing (filters) or acidifying (filtered HgT).
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Prior to and between sampling expeditions, all components of the pumping and filtering system were
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cleaned in a class-1000 clean laboratory by first rinsing with ultrapure water (UHP; >18.2 MΩ-cm),
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followed by immersion in 0.5 M HCl (sub-boiling quartz distilled) and rinsing with UHP water. All
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sampling equipment was double bagged in plastic before leaving the clean lab.
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Mercury Analyses. Filtered (< 0.4 µm) HgT in 0.5 L samples of water was measured at PNNL following
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U.S. EPA Method 1631E. Briefly, this method uses cold-vapor atomic fluorescence spectrometry
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(CVAFS) to measure Hg that has been oxidized to Hg(II) with BrCl and then reduced to Hg0. Elemental
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Hg is purged from the sample and concentrated on Au, from which it is desorbed and quantified. The
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detection limit for filtered HgT was 0.5 pM and the relative percent difference (RPD) of duplicate samples
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ranged between 5 and 7%.
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Filtered MeHg and particulate MeHg and HgT were extracted and quantified at WSU. Methylmercury
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in filtered water was determined from an acidified 2 L sample (0.2 M H2SO4; >12 h), then neutralized
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with KOH, buffered with acetate, and derivatized with sodium tetraethyl borate before purging,
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concentrating on Tenax, and quantification by gas-chromatographic CVAFS.27,28 We suspect that nearly
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all of the measured MeHg was as monomethyl mercury, as opposed to dimethyl mercury, although the
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two species were not determined independently. Glass fiber filters containing particles filtered from 0.5 to
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3.8 L of water were digested with 4.57 M HNO3 for 12 h in a water bath (60°C) prior to MeHg analysis 5 ACS Paragon Plus Environment
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by gas-chromatographic CVAFS).29 An aliquot of each digestate was oxidized with BrCl solution for >12
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h prior to determination of HgT by dual-Au CVAFS.30,31,32 Method detection limits were 0.030 pM for
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filtered MeHg, 0.002 pM for particulate MeHg and 0.02 pM for particulate HgT. Reproducibility between
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duplicate samples averaged 15 ± 9 RPD for filtered MeHg, 13 ± 9 RPD for particulate MeHg, and 9 ± 8
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RPD for particulate HgT. The following discussion will refer to the sum concentrations of the filtered plus
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particulate fractions for both MeHg (ΣMeHg) and HgT (ΣHgT); however, an additional manuscript will
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describe mercury partitioning as well as the role of other chemical parameters including total suspended
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solids. The cumulative analytical uncertainty for the summed dissolved and particulate fractions was
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calculated by standard propagation of errors techniques.
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Flux Model. We estimated the flux of ΣMeHg and ΣHgT from rivers to estuaries (Friv) and from estuaries
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to the coastal ocean (Fsea) based on measured Hg concentrations and river discharges. Flux estimates were
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calculated for each season as well as for the year 2012. Seasonal Friv were estimated as the product of the
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observed concentration (C0) at the zero salinity river end-member (or lowest salinity sample) and river
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discharge (Driv; equation 1). In each river, the mean daily discharge (m3 d−1), over a 90 day period
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bracketing each sampling event, was used to estimate daily Hg flux (mol d-1) from each system during the
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spring and fall. River discharges were obtained from USGS gauges closest to the river/estuary confluence
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(www.waterdata.usgs.gov/nwis) and the water flux was assumed to be constant downstream of the gauge
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(Supplemental Table S1). The average daily discharge for 2012 was calculated from USGS discharge data
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and used to calculate the estimated annual flux (mol yr-1) by multiplication with the mean of the two
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seasonal C0 values. Additionally, long-term average discharge was determined from at least 10 years of
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USGS flow measurements (Supplemental Table S1).
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The Hg flux from the estuary to the coastal ocean (Fsea) was estimated from the product of Driv and
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the modeled effective riverine end-member concentration (C*; equation 2). Seasonal and annual Hg Fsea
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estimates were produced following the same procedure as for Friv. The C* concentration was estimated
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from the observed concentration distribution through the estuary following an idealized conceptual model 6 ACS Paragon Plus Environment
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of non-conservative behavior (Figure S1).33 The model assumes that the Hg concentration in the river is
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greater than that in the coastal ocean (Csea). Accordingly, a linear decrease in concentration between the
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riverine and marine end-members is interpreted to demonstrate conservative mixing within the estuary.
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Deviations in the distribution above the conservative mixing line are indicative of inputs within the
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estuary (e.g. in situ production, benthic mobilization, sediment resuspension). Likewise, deviations below
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the mixing line suggest removal of Hg (e.g. scavenging). Subtraction of Friv from Fsea yields Festuary
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(Equation 3), which describes either net addition (positive flux) or removal (negative flux) within the
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estuary.
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Friv = C0 × Driv
Equation 1
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Fsea = C* × Driv
Equation 2
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Festuary = Fsea – Friv
Equation 3
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Our field observations of ΣMeHg and ΣHgT rarely followed the idealized patterns of the conceptual
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model, often with both addition and removal occurring in different reaches of the estuaries. Therefore,
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three different effective river concentrations were estimated by plotting Hg concentration versus salinity
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and extrapolating upstream from the high salinity end-member to the y-intercept (S = 0) to estimate
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effective riverine concentrations identified as either C*max, C*mid, or C*min (Figure 2). The value for C*max
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was estimated from a line drawn from the marine end-member concentration through the highest observed
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concentration to the y-intercept. Estimates of C*mid and C*min were derived with the line passing through
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either the majority of the data points (C*mid) or the sample with the lowest Hg concentration (C*min). In
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cases of quasi-conservative behavior, C*mid was comparable to the observed C0 in the zero-salinity
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sample. Both Friv and Fsea were calculated for each of the two sampling periods with the corresponding C0
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and C*mid, respectively. Annual Friv and Fsea in each estuarine system were estimated as the product of
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either the mean C0 or C*mid between the two sampling events and the annual river discharge in 2012.
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RESULTS AND DISCUSSION
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Discharge. River discharge to the nGOM is influenced by natural variations in precipitation and
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evaporation as well as flow control from dams. Springtime river discharge was significantly (15–90%)
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greater than during the fall sampling (Wilcoxon Signed Rank Test, p = 0.02) with the exception of the
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Suwannee River, because fall sampling occurred soon after Hurricane Isaac had passed through the area
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(Supplemental Table 1). Compared to the long-term average river discharge from USGS records, 2012
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was a year of relatively low discharge from nGOM rivers.
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Riverine Concentration (C0) and Flux from Rivers to Estuaries. Riverine end-member concentrations
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(C0) of ΣMeHg varied 20-fold among rivers and were greatest in the Escambia and Pascagoula Rivers
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(Table 1). Measured concentrations of ΣMeHg were within the range reported in other North American
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streams.12,34,35,36,37 In four nGOM rivers, C0 was greater during the fall sampling event than in the spring
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(Suwannee, Mobile, Mississippi and Lower Atchafalaya), while three other systems (Apalachicola,
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Sabine and Brazos) showed no large difference between sampling periods. The other three rivers
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(Escambia, Pearl and Trinity) were either sampled only once or the ΣMeHg concentration was greater in
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the spring (Pascagoula).
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While concentrations of ΣMeHg differed between sampling campaigns, estimated river fluxes (Friv)
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were remarkably consistent between spring and fall, in most cases (Table 2). The greatest temporal
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difference of Friv was estimated for the Brazos River, which had a 10-fold higher flux in spring as a result
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of greater spring discharge. However, ΣMeHg flux from the Brazos, and the two other Texas rivers
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(Sabine and Trinity), was insignificant relative to the other rivers. The greatest Friv (~0.1 mol d−1) was
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delivered from the Mississippi River because of its high flow. The flux of ΣMeHg from the Mobile River
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(0.012 mol d−1) was about an order of magnitude less than the Mississippi and about 10-fold greater than
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the nine other rivers. As with the river-specific Friv values, the cumulative flux of ΣMeHg to nGOM
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estuaries from all rivers was similar between sampling events with input of 0.17 mol d−1 during spring
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and 0.16 mol d−1 during fall (Table 2). In sum, the flux of river-sourced ΣMeHg to the estuaries was
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estimated to be about 142 mol y−1, with 59% from the Mississippi River. The Mobile and Pascagoula 8 ACS Paragon Plus Environment
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Rivers were also substantial sources to their respective estuaries. The other rivers each discharged less
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than 5 mol y−1.
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Measured ΣHgT C0 were less variable between sampling campaigns than those of ΣMeHg (Table 3)
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and were also within the range observed in other North American streams.12,34,35,36,37 Concentrations in the
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Suwannee, Apalachicola, and Brazos Rivers were higher in the fall than during the spring and lower in
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the Pascagoula and Mobile Rivers. The two highest C0 were in the Escambia (32.8 ± 0.9 pM) and Pearl
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(30.8 ± 0.04 pM) Rivers, which were not sampled in the spring. The largest temporal difference was
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observed in the Suwannee River, where the mean fall C0 (22.1 ± 1.7 pM) was approximately fivefold
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greater than the single measurement taken in the spring (4.8 pM). Rainfall and watershed flushing
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associated with Hurricane Isaac may have caused the concentration to be much higher in the fall. Mercury
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concentrations in Hurricane Isaac rainfall were 36 pM at Chassahowitzka (28.7486° N, 82.555° W),
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located south of the Suwannee watershed, and 25 pM at the Suwannee’s headwaters at Okefenokee
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(30.7404° N, 82.1283° W; NADP, 2014). Alternatively, increased precipitation related to the hurricane
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(12.4 cm of rain at Chassahowitzka) may have resulted in increased Hg mobilization from the Suwanee
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watershed.38,39 The Friv values were greatest in the Mississippi and Lower Atchafalaya Rivers due to their
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high flow compared to the other rivers (Table 4; Table S1). Differences in ΣHgT flux between sampling
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periods from the Mississippi and Lower Atchafalaya were driven mostly by variability in freshwater
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discharge, because their respective spring and fall C0 were similar. The annual Friv for the Mississippi was
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3100 mol y-1 of ΣHgT, which was about half of the total estimated 6300 mol y−1 cumulative flux from the
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rivers studied.
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Effective Riverine Concentrations (C*) and Hg Fluxes to the Coastal nGOM. Mercury input and
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removal processes within estuaries cause non-conservative behavior and complicate estimates of flux to
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the coastal ocean. We observed non-conservative behavior of ΣMeHg and ΣHgT in most estuaries
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(Figures S2 and S3), which resulted in large differences among the C*min, C*mid and C*max estimates
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within some estuaries. Differences among C* estimates were about two to fivefold for ΣMeHg with 9 ACS Paragon Plus Environment
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comparatively less variability among ΣHgT C* estimates (Tables 1 and 3). However, the estuaries were
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often characterized by complex non-conservative behavior with both input and removal processes
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indicated in different reaches along the salinity gradient. Therefore, the Hg flux from each estuary to the
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coastal ocean (Fsea) was estimated as the product of Driv and C*mid (Tables 2 and 4).
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Despite differences in ΣMeHg C*mid, spring and fall Fsea showed differences on the order of only a
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few mmol d−1 for most of the estuaries (Table 2). The Mississippi River was the most notable exception
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exporting nearly 0.4 mol d−1 in the spring and about 0.1 mol d−1 in the fall. This fourfold difference was a
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function of substantially greater discharge in the spring rather than a difference in C*mid (0.3 pM in spring
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and 0.2 pM in fall). On an annual basis, the estuaries were a source of 116 mol ΣMeHg y−1 to the Gulf,
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with the Mississippi accounting for 76% of the flux. The Lower Atchafalaya (12 mol y−1) delivered 10%
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of the total riverine input, twice that of the Pascagoula at 5.6 mol y−1, the third largest source.
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Most of the Fsea values for ΣHgT were less than 0.5 mol d−1 (Table 4). The Mississippi (12.1 and 5.4
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mol d−1 in spring and fall, respectively) and Lower Atchafalaya (1.9 and 1.2 mol d−1) were exceptions.
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The larger flux from the Mississippi during the spring was due to higher discharge. The sum of ΣHgT flux
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was estimated to be about 5200 mol y−1, with 70% of the total from the Mississippi and 14% from the
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Lower Atchafalaya River.
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Estuarine Input and Removal. Estuaries are often assumed to be net sinks for ΣHgT because of internal
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scavenging on particles while their role in the flux of ΣMeHg to the coastal ocean is more
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ambiguous.5,12,13,40,41,42 Differences in ΣMeHg fluxes within the estuaries (Festuary) were relatively small
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and often Friv and Fsea were nearly equivalent resulting in Festuary that were near zero. However, springtime
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Festuary values in the Mississippi and Lower Atchafalaya Rivers were sufficiently large to suggest estuarine
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inputs of ΣMeHg (Table 2). Mercury methylation within sediments has been observed in other GOM
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estuaries with differing morphologies, including the mangroves of Florida Bay42 and the Shark Bay
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estuary.43 By comparison, the Mississippi River is characterized by a swift current and deep channel. The
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Lower Atchafalaya, created through diversion of 30% of the Mississippi River’s flow to join the Red 10 ACS Paragon Plus Environment
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River, passes through the largest contiguous swamp in the U.S. before becoming an estuary.44 Except for
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the Mississippi and Lower Atchafalaya, the studied estuaries were net sinks of ΣMeHg. Overall, nGOM
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estuaries removed 26 mol of ΣMeHg annually, or about 20% of the riverine input.
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Values of Festuary for ΣHgT were more variable between sampling periods than for ΣMeHg, and in the
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Pascagoula and Lower Atchafalaya River estuaries the result was a transition between net input and
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removal (Table 4). Despite a net input of 500 mol y−1 in the Mississippi River estuary, the nGOM
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estuaries collectively removed over 1000 mol y−1, which is about 20% of the total annual riverine flux.
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The most significant sinks for ΣHgT were the Lower Atchafalaya and Mobile River estuaries. Both of
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these systems are relatively wide estuaries with slow current, allowing for significant sedimentation and
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Hg scavenging. However, these observations should be interpreted with caution because each system was
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sampled only once during each season. Higher frequency sampling is needed to better describe the impact
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of estuarine processes.
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Significance of Fluvial Sources. Mercury in the GOM is derived from three primary sources: the North
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Atlantic Ocean, atmospheric deposition, and rivers. Atlantic Ocean water arriving through the Yucatan
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Channel via the Loop Current is the largest estimated source delivering an estimated 78–88% of HgT
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loadings assuming a ΣHgT concentration of 2.2 pM;16 however, recent measurements suggest that ΣHgT
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concentrations in North Atlantic surface waters are
