Methylmercury Mass Budgets and Distribution Characteristics in the

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Methylmercury mass budgets and distribution characteristics in the Western Pacific Ocean Hyunji Kim, Anne L. Soerensen, Jin Hur, Lars-Eric Heimburger, Doshik Hahm, Tae Siek Rhee, Seam Noh, and Seunghee Han Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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Environmental Science & Technology

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Methylmercury mass budgets and distribution characteristics in the Western Pacific

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Ocean

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Hyunji Kima, Anne L. Soerensenb, Jin Hurc, Lars-Eric Heimbürgerd, Doshik Hahme, Tae Siek

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Rheef, Seam Noha, Seunghee Hana,*

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a

School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and

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Technology (GIST), Gwangju 500-712, Republic of Korea

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b

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Stockholm, Sweden

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c

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Korea

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d

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of Oceanography (MIO) UM 110, 13288, Marseille, France

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e

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f

Stockholm University, Department of Environmental Science and Analytical Chemistry,

Department of Environment and Energy, Sejong University, Seoul, 143-747, Republic of

Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute

Department of Oceanography, Pusan National University, Busan 46241, Republic of Korea

Korea Polar Research Institute, Incheon 406-840, Republic of Korea

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*Corresponding author (S. Han)

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Tel: 82-62-715-2438, Fax: 82-62-715-2434, E-mail address: [email protected]

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ACS Paragon Plus Environment


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Abstract

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Methylmercury (MeHg) accumulation in marine organisms poses serious ecosystem and

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human health risk, yet the sources of MeHg in the surface and subsurface ocean remain

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uncertain. Here, we report the first MeHg mass budget for the Western Pacific Ocean

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estimated based on cruise observations. We found the major net source of MeHg in surface

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water to be vertical diffusion from the subsurface layer (1.8 to 12 nmol m-2 yr-1). A higher

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upward diffusion in the North Pacific (12 nmol m-2 yr-1) than in the Equatorial Pacific (1.8–

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5.7 nmol m-2 yr-1) caused elevated surface MeHg concentrations observed in the North

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Pacific. We furthermore found that the slope of the linear regression line for MeHg versus

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apparent oxygen utilization was about twofold higher in the Equatorial Pacific than the North

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Pacific. We suggest this could be explained by redistribution of surface water in the tropical

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convergence-divergence zone, supporting active organic carbon decomposition in the

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Equatorial Pacific Ocean. Base on this study, we predict oceanic regions with high organic

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carbon remineralization to have enhanced MeHg concentrations in both surface and

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subsurface waters.

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Keywords: Methylmercury, Fluorescence dissolved organic matter, Apparent oxygen

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utilization, Mass budget, Western Pacific

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TOC/Abstract Art

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1. Introduction Mercury (Hg) reservoirs in active biogeochemical cycling have increased since the

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beginning of the industrial period due to increased anthropogenic emissions, such as those

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caused by fossil fuel–fired power plants, gold mining, and non-ferrous metal

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manufacturing.1,2 In 2008, approximately 65% of global anthropogenic Hg emissions were

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released in Asia and the Middle East, with the largest emissions from power generation,

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combustion for industrial and domestic purposes, and cement production.3 Although, global

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anthropogenic Hg emissions decreased 20% from 1990 to 2010, due to large decreases in

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North America and Europe, a 50% increase in anthropogenic Hg emissions was observed in

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Asia.4 An increase in Asian Hg emission could preferentially affect the Pacific Ocean. A

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recent modeling study suggested that ~20% of the Hg emitted in Asia was deposited during

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transport across the North Pacific.5

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The increases in Asian emissions are consistent with recent increases in Hg

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concentrations in North Pacific Intermediate Water (NPIW).6 Modeled Hg concentrations

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show a rapid increase in the NPIW since the 1980s and suggests that Hg concentrations in

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NPIW will double by 2050 if current atmospheric deposition rates are sustained.6 This trend

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is in contrast to the decrease in the surface water Hg concentration in the North Atlantic and

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the Mediterranean Sea since the 1980s-1990s.7 Increased Hg contamination in North Pacific

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water may lead to increased monomethylmercury (MMHg) accumulation in marine fish,8

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ultimately influencing human exposure.

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Maximum MeHg (MMHg + dimethylmercury [DMHg]) peaks have been found in

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the oxygen depletion zones of the North Pacific Ocean,6,9-11 and the Mediterranean Sea.9,12

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Several studies have observed positive associations between MeHg concentrations and either

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apparent oxygen utilization (AOU)9,11-14 or the organic carbon remineralization rate 4


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(OCRR).6 A few recent studies showed that the release of surface-formed MeHg from sinking

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particles during particulate organic carbon remineralization has little impact on the MeHg

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concentration in the oxygen depletion zone.10,12

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A significant source of MeHg in surface water could be the in situ methylation of

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inorganic Hg(II).13,15,16 The MeHg fraction produced in situ corresponded to about 80% of the

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MeHg present in the surface chlorophyll a maximum zone in the Arctic Ocean.16 Although in

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situ Hg(II) methylation has been observed in surface water, it is unclear whether in situ Hg(II)

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methylation is sufficient to maintain ambient MeHg values in surface water as MeHg is not

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only removed through biotic demethylation as in the deeper water but also through photolytic

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demethylation and evasion.

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To constrain major sources and sinks of MeHg in the surface and subsurface ocean,

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we collected samples for total Hg (THg) and MeHg analysis during two cruises (2012 and

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2014 SHIPPO) in the West Pacific Ocean. We used the observation to create MeHg mass and

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flux budgets for four distinctive ocean regions with significantly different hydrographic

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properties: the Western Subarctic Gyre, North Pacific Gyre, Western Pacific Warm Pool, and

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South Pacific Gyre (Fig. 1). The biogeochemical characteristics of each region, such as the

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concentrations of dissolved oxygen, nutrients, chlorophyll a, dissolved organic carbon (DOC),

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and fluorescent dissolved organic matter (FDOM), were determined along with the Hg

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species to examine the environmental controls on MeHg dynamics in the West Pacific Ocean.

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2. Materials and Methods

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2.1. Seawater sampling

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Seawater samples were collected from the upper 500 m of the North Pacific (40–

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51°N, 149–168°E) from July 13 to 28, 2012, and the Equatorial and South Pacific (31°S– 5



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22°N, 129–171°E) from April 2 to 23, 2014, onboard the RV Araon (Fig. 1). Samples were

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collected using 24 10-L standard Niskin bottles mounted on a rosette sampler.

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2.2. THg and MeHg analysis

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All Teflon bottles used for Hg measurements were pre-cleaned thoroughly in the

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laboratory via acid washing with 30% (v/v) HCl at 70°C for 24 h; then filled with 1% (v/v)

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trace metal–grade HCl solution and kept in double-ziploc bags.17 The unfiltered water

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samples were transferred from the Niskin bottles into 1 L Teflon bottles for THg (for the

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2014 SHIPPO survey only) and MeHg analysis using acid pre-cleaned silicon tubing. Total

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Hg samples were not collected from Site 10, because the rosette sampler was out of order.

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The THg and MeHg samples were acidified with 0.4 % (v/v) trace metal–grade HCl onboard

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and kept at 4°C until analysis. The details of THg and MeHg analysis are described in Text

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S1. The limit of detection (LOD) for MeHg determined with 360 mL sample volume was 5.5

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fM.

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2.3. MeHg flux estimation We estimated mass MeHg fluxes for surface and subsurface waters in the four

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selected regions. Data used for the flux calculations are given in Table S1. Deposition fluxes

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for the period of the cruises were estimated using precipitation data from NASA Earth

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Observations (http://neo.sci.gsfc.nasa.gov/) and MeHg concentrations (as 1% of THg) from

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Baeyens et al.18

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To estimate the air–sea exchange of DMHg (Equations 1 and 2), we followed

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Soerensen et al.19 for the determination of the gas transfer velocity (Kw) and the Henry’s law

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constant (H) for DMHg was from Lindqvist and Rodhe.20 6


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DMHgflux = Kw ([DMHg] - [DMHgair] / HDMHg)

(1)

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Kw = 0.25 × u2 × (ScDMHg/600)-0.5

(2)

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Here 600 is the Schmidt Number for CO2 at the reference temperature and salinity, and

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ScDMHg is the Schmidt's number for DMHg. We used wind speeds measured on the cruises

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while temperatures at the air–sea interface were obtained from the NASA Earth Observations.

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DMHg in the surface mixed layer was assumed to be 5% of the MeHg concentration (3-12 %,

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above thermocline of the North Pacific11) and DMHg in the marine boundary layer was set to

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4 pg m–3.21 A sensitivity test showed that the air concentration only had limited impact on the

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calculated evasion flux (a factor 10 change in air concentration resulted in 500 m).

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To calculate the settling flux of MeHg on suspended solids (Equation 3), we used

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Stoke’s law,24 with an average suspended solids density (dp) of 1.50 kg L–1,25 a density of

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seawater (dsw) of 1.03 kg L-1, a kinematic viscosity (v) of 0.017 × e-0.025×Temp,26 and an

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average particle radius (rpw) of 2 × 10–5 m.27

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vs (m s-1) = × [(dp - dsw) × 1000) / v] × g × rpw2

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The fraction of total MMHg found in the particulate form (fPMeHg) was estimated as

7


(3)


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fPMeHg = 1 - [1 / (1 + kd × SPM)]. In order to calculate the particle-water partition coefficient

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(kd), fractions of filtered (99%) and particulate (0.7%) MeHg from Bowman et al.10 were

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used. Here we assume that MMHg values reported in the study are similar to MeHg values.

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The suspended solids (SPM) concentration was estimated as 0.45 × 10-7 – 1.9 × 10-7 kg L–1

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for the surface water and 1.4 × 10-7 – 1.6 × 10-7 kg L–1 for subsurface water. These estimates

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were obtained using an empirical equation between chlorophyll a and suspended particle

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concentration described in Morel et al.28,29

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2.4. Analysis of dissolved oxygen, nutrients and chlorophyll a Dissolved oxygen concentrations were obtained from the CTD using a SBE43 sensor,

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and calibrated using a spectrophotometric method. For the calibration, water sampling and

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reagent preparation were carried out following the Joint Global Ocean Flux Study

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guidelines,30 and the concentrations were determined as described by Labasque et al.31 The

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precision of the measurement of the duplicated samples was better than 0.3%. AOU was

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calculated by subtracting the observed oxygen concentration from the saturated oxygen

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concentration at in-situ salinity and temperature. The saturated oxygen concentration was

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calculated according to Garcia and Gordon’s method.32

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Macronutrients (nitrate + nitrite, phosphate, and silicate) were analyzed on board

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with a gas segmented flow analysis system (QuAAtro, SEAL Analytical) in the 2012

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SHIPPO survey. The seawater samples (50 mL) were collected in sterilized conical tubes and

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stored in a refrigerator until the analysis. Before the measurements were conducted, the

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analytical system was calibrated with KANSO reference material (Lot. No. ‘BF’, KANSO

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Technos). In the SHIPPO 2014 survey, the nutrient samples were collected in the same way

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as 2012, and stored frozen until analysis in the laboratory. 8


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Two liters of seawater were sampled for each chlorophyll a measurement. The

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seawater samples were filtered through a GF/F filter (47 mm, Whatman), after which the

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filters were stored at –80 °C. Chlorophyll a was analyzed fluorometrically onboard the ship

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using a Trilogy Laboratory Fluorometer (Turner Designs), following the Parsons et al.’s

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method.33

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2.5. Analysis of DOC and FDOM components Samples for the DOC and FDOM component analysis were filtered through a

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Whatman GF/F filter (25 mm) immediately after collection. Samples were kept at –20°C in

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the pretreated glass vials until analysis. These vials were precleaned in the laboratory using

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10% (v/v) HCl for 24 h and then precombusted at 500°C for 3 h and kept in double-ziploc

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bags. The frozen samples were thawed at room temperature before analysis. The samples for

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the DOC analysis were acidified with HCl and purged with oxygen gas for 30 min to remove

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inorganic carbon, and then DOC was detected with a total organic carbon analyzer (Vario

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TOC, Elementar).

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A fluorescence excitation emission matrix (EEM) scan was conducted on a

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fluorescence spectrophotometer (F-7000, Hitachi) for the emission spectra from 280 to 550

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nm (1 nm increment) and the excitation wavelengths from 220 to 500 nm at 5 nm intervals.

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Excitation and emission slits were set to 10 nm and 5 nm, respectively, and the scanning

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speed was 12,000 nm min–1. To limit second-order Raleigh scattering, a 290 nm-cutoff filter

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was used for all the measurements. Samples with high DOC concentrations were

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appropriately diluted to avoid inner-filter correction. The background was subtracted using

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the EEM of a blank solution (Milli-Q water). The final EEMs were normalized by a Raman

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integrated area, following Lawaetz and Stedmon.34 Parallel Factor (PARAFAC) analysis was 9



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carried out using MATLAB 7.1 (MathWorks, Natick, MA, USA) with the DOMFluor

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Toolbox (http://www.models.life.ku.dk).35 A split-half analysis was applied to validate the

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identified components.

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3. Results

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3.1. Surface current and hydrographic characteristics

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The transect was divided into four regions based on water mass characteristics (Fig.

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S1): the Western Subarctic Gyre (N3–N8), the North Pacific Gyre (S12–S16), the Western

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Pacific Warm Pool (S5–S11), and the South Pacific Gyre (S2–S4).

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The Western Subarctic Gyre has a shallow halocline that limits the vertical exchange

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of seawater.36 Colder and fresher waters (temperature < 16°C and salinity < 33.5) are found

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in the surface of the Western Subarctic Gyre compared to southern sites due to the

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contribution of the Bering Sea and Okhotsk Sea waters (Fig. S1).37 The surface salinity and

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the temperature at the N3 and N4 sites located in the Subarctic Front were distinctively

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higher than those at northern sites, reflecting the mixing effect of warm and saline Kuroshio

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water (salinity of about 34 and temperature of about 25°C).38,39 NPIW is identified for the

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N3–N8 sites at the temperature-salinity diagram in Figure S1 (salinity 33.5–34.3). The

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subsurface of the N3 and N4 were reported to be a site where new NPIW was formed.40,41

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The surface current of the tropical Western Pacific is governed by two broad

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geostrophic westward flows: the North Equatorial Current and the South Equatorial Current

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(Fig. 1).42,43 Upon approaching the western boundaries, the North Equatorial Current and the

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South Equatorial Current split into two branches each. The tropical surface waters move

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poleward, away from the equator, in both hemispheres due to the Ekman divergence.

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Likewise, the westerly winds induce Ekman transports toward the equator, generating a 10


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tropical convergence between 10–30°N.42,44

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A distinctive colder (< 10°C) and fresher (salinity < 34.7) water mass, the Equatorial

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Pacific Intermediate Water (EPIW), was found at 500 m depth between 20°N and 20°S (Fig.

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S2). A recent geochemical tracer study showed that EPIW is primarily a combination of

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Antarctic Intermediate Water and Pacific Deep Water, which explains the low oxygen

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concentration (< 50 µmol kg–1) compared to the adjacent water masses.45

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3.2. THg THg was measured during the 2014 SHIPPO survey for the North Pacific Gyre

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(S12–S16), Western Pacific Warm Pool (S5–S11), and South Pacific Gyre (S2–S4; Table 1)

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but not in 2012. In the surface mixed water, THg ranged from 0.39 to 0.96 pM (0.68 ± 0.18

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pM; n = 10), with the highest values found in the Western Pacific Warm Pool (0.84 ± 0.12

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pM) (t-test, p < 0.005). Here, the surface mixed layer was defined as described by de Boyer

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Montegut et al.46 The surface mixed layer depth was mostly less than 20 m with the exception

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of the N3 and N4 sites where the surface mixed layer reached 50 m depth.

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The THg concentration typically ranged from 0.39 to 2.6 pM (n = 28) in the photic

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zone (0-100 m) and from 0.38 to 2.5 pM (n = 41) in the aphotic zone (100-500 m; Table 1).

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The range of THg in the photic zone is similar to or higher than those found in other regions

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of the Pacific Ocean (0.99 ± 0.32 pM, < 150 m, unfiltered;6 0.46 ± 0.21 pM, < 150 m,

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unfiltered;47 0,37 ± 0.23 pM, < 150 m, filtered;11 and 0.25-0.40 pM, < 130 m, filtered.9 Figure

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S3 provides the vertical structure of THg, which showed increasing concentrations with depth

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and peak concentrations in the EPIW. The mean THg concentration found in the EPIW was

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1.6 ± 0.37 pM (n=7) similar to the values found for North Pacific Deep Water (1.55 ± 0.01

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pM).9 This could be the effect of the North Pacific Deep Water contributing to the EPIW.45 11



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3.3. MeHg

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MeHg was measured during the 2012 and 2014 SHIPPO surveys for the overall sites

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(Table 1). In the surface mixed layer of the Western Subarctic Gyre, the MeHg concentration

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was determined to be < LOD to 34 fM (21 ± 12 fM, n = 12), and in the surface mixed waters

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of the North Pacific Gyre, Western Pacific Warm Pool, and South Pacific Gyre, the MeHg

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concentration was determined to be less than the LOD.

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MeHg concentrations typically ranged from < LOD to 178 fM (23 ± 30 fM, n = 57)

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in the photic zone and from < LOD to 909 fM (328 ± 271 fM, n = 66) in the aphotic zone

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with the highest values in the EPIW (Table 1). The MeHg concentrations in the photic zone

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were generally lower than what has been found in other parts of the Pacific Ocean. For

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example, 95 ± 52 fM of MeHg was observed at depths of < 150 m in the Northeastern

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Pacific6 and 56 ± 62 fM of MeHg was found at depths of 20–100 m in the tropical

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Southeastern Pacific Ocean.48 The observed subsurface maxima of MeHg coincided with the

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AOU and nutrients maxima (Figs. S2 and S3).

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3.4. DOC and FDOM

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The DOC and FDOM components were obtained from the 2014 SHIPPO survey for

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sites S2 to S16 (Table 1 and Fig. S3). The DOC concentration ranged from 68 to 88 µM (n =

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9) in the surface water, from 51 to 88 µM (n = 25) in the photic zone, and from 36 to 74 µM

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(n = 36) in the aphotic zone. DOC concentrations in surface water were found to be higher in

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the equatorial upwelling region than in the southern and northern sites (t-test, p < 0.001). We

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suggest this is due to the upward flux of subsurface water enriched with regenerated nutrients,

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inducing increased primary production. 12


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The EEM-PARAFAC results showed that a three-component model fully described

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the fluorescent fractions of DOM in the samples. The spectral features and the split-half

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validation are shown in Figures S4 and 2, respectively. The identified fluorescent components

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were compared to those previously reported in the literature.49,50 Component 1 (C1) was

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assigned to a marine and microbial humic component with a broad peak at the excitation

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wavelengths of 230 and 300 nm and the emission wavelength of > 400 nm.49 Components 2

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and 3 (C2 and C3) were characterized as tyrosine-like and tryptophan-like DOM,

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respectively.49,50

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Two components of terrestrial humic-like DOM, associated with the microbial

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mineralization process (C1, excitation 240 nm and emission 396 nm; C2, excitation 240 nm

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and emission 480 nm), and one component of marine humic-like DOM, associated with

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phytoplankton release (C5, excitation 300 nm and emission 408 nm), have been reported in

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samples from an Atlantic transect.49 In our data, the marine humic component seems to be

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mixed with a microbial humic component, as the excitation wavelength of C1 extends to

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cover 300 nm (Fig. S4).

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We found two distinct vertical distribution patterns for the FDOM components, as

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shown in Figure S3. The humic-like component (C1) typically exhibited low fluorescence in

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the surface water, and the intensity maxima was found at a depth of 300–500 m. In contrast,

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the protein-like components (C2 and C3) were elevated in the photic zone. In the surface

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mixed layer, elevated fluorescence of humic-type FDOM was observed in the equatorial

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upwelling zone (sites S9 and S11). Elevated fluorescence intensities of humic-type FDOM

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has also been reported for other oceanic upwelling regions, such as the Equatorial and South

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Atlantic, the Eastern South Pacific and the North Atlantic.51 This has been explained by a

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combination of high biological activity, upward flux of FDOM from subsurface water and 13



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decreased UV penetration. 51

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4. Discussion

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4.1. MeHg mass budgets

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Figure 3 presents a MeHg budget for each region and identifies the major sources

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and sinks in the surface and subsurface waters. The uncertainty levels of fluxes determined

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by the standard deviations of MeHg measurements are shown in Figure S5. For the surface

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water we find that upward diffusion from the subsurface water is the largest external MeHg

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input. This source largely exceeds atmospheric deposition. This result is supported by

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principal component analysis of our data (see section 4.3). The finding also agrees with

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recent results from Blum et al.15 that observe sharp decreases with depth in ∆199Hg and

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∆201Hg in fish, which indicate that most methylation occurs below the surface mixed layer

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and diffuses up to the surface layer. We furthermore find that upward diffusion is

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significantly larger for the Western Subarctic Gyre (12 nmol m–2 y–1) than for the southern

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sites (1.8–5.7 nmol m–2 y–1). The larger diffusion flux is caused by the shallower subsurface

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MeHg peak in the Western Subarctic Gyre.

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DMHg evasion to the atmosphere ranges from 0.85 to 2.9 nmol m–2 y–1 and exceeds

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photodemethylation and particle settling (Fig. 3). The ranges of DMHg/MeHg of 3 to 20%

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have been observed in the upper Pacific Ocean,11,48 but none of these are in the upper few

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meters of the surface mixed layer. Due to the loss of DMHg in the upper surface ocean both

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to photodemethylation and evasion, it is likely that the DMHg/MeHg fraction decreases

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below the 3-20% range close to the air-sea interface. We therefore tested how DMHg evasion

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responds to different %DMHg/MeHg. A %DMHg/MeHg of 1, 5, 10, and 20% resulted in

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DMHg evasion of 0.57, 2.9, 5.7, and 11 nmol m–2 y–1 in the Western Pacific Subarctic Gyre. 14


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This indicates that DMHg evasion flux is one of the least constrained fluxes in the surface

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ocean.

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The photodemethylation of MeHg ranges from 0.57 to 0.81 nmol m–2 y–1. MeHg is

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degraded mainly by ultraviolet (UV) radiation in the upper photic zone and by PAR deeper in

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the photic zone.22 Particle settling (0.039–0.16 nmol m–2 y–1) is small compared to other sinks,

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which is supported by observations in the equatorial Pacific.11

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We suggest that the unknown sink (0.080–8.7 nmol m–2 y–1), needed to balance the

333

MeHg budget, represents a net dark methylation flux (Fig. 3). Except for the South Pacific

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Gyre where the net methylation is close to zero (0.080 nmol m–2 y–1) we find that there is a

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general net loss of MeHg in the surface ocean. Microbial mediated demethylation processes

336

have been suggested from the difference in the demethylation rate between filtered and

337

unfiltered dark incubation in the Mediterranean Sea.52 One possible process is microbial

338

activity involving the merB gene that encodes to cleave MeHg to methane and Hg(II) as end-

339

products.53 Unfortunately, we did not measure methylation rate constants during the cruises.

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Previous studies in the Mediterranean Sea and the Canadian Archipelago have reported the in

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situ methylation rate of 0.3-6.3 % d-1and 0.65 ± 0.24 % d-1, respectively, for oxic surface

342

water.16,52 We hypothesize that the in situ Hg(II) methylation rate to be much smaller in the

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Pacific Ocean due to lower primary production than the reference sites. By assuming a

344

methylation rate constant of 50% of that measured in the Canadian Archipelgo,16 we estimate

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an in situ Hg(II) methylation flux of ~1 nmol m-2 yr-1 in the Western Subarctic Gyre. We

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therefore conclude that we do not lose important information on the sources of MeHg in the

347

surface ocean by ignoring in situ Hg(II) methylation. Nevertheless, measurements of in situ

348

Hg(II) methylation rates are necessary to confirm our hypothesis.

349

Our mass budget shows that for subsurface water, diffusion from the peak MeHg 15



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region (both upwards and downwards) results in the largest MeHg loss (1.8–12 nmol m–2 y–1),

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while particle settling is a minor loss term (0.0010–0.0035 nmol m–2 y–1; Fig. 3). A large

352

input is needed to balance these vertical diffusion losses. We suggest that net in situ Hg(II)

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methylation (3.5–15 nmol m–2 y–1) is accounting for the majority of the unknown source. The

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estimated net methylation flux is largest in the Western Subarctic Gyre (15 nmol m–2 y–1), in

355

agreement with our findings of the largest AOU values and macronutrient concentrations in

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the subsurface waters of the Western Subarctic Gyre (Fig. S2). Our calculated input of MeHg

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associated with settling particles from the surface mixed layer is in general two orders of

358

magnitude lower than the net in situ methylation. The small impact of settling particles in our

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budgets support recent studies on MeHg release from sinking particles during particulate

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organic carbon remineralization9,10 and direct observation of MeHg settling fluxes by

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Munson et al.11

362 363 364

4.2. Organic carbon remineralization and THg enrichment The difference in the THg-to-AOU ratio between water masses is shown in Figure

365

4A (the location of each water mass is seen on Figure S6). The positive correlation between

366

THg and AOU suggests that the vertical THg distribution is, to some extent, linked to the

367

remineralization of sinking organic matter.54 Mercury adsorbed on organic particles in the

368

photic zone is transported downwards via scavenging processes and remineralized at

369

intermediate depths.9,10 Nutrient-type THg distributions have been found in the eastern North

370

Pacific and North Atlantic Oceans.6,9,10 The weak correlation factor (r2 = 0.4) implies that the

371

role of in situ remineralization governing THg distribution is limited, and other processes,

372

such as lateral and vertical transport, are also important.

373 16


Page 16 of 33

Page 17 of 33

374 375


4.3. Organic carbon remineralization and MeHg enrichment We used principal component analysis to investigate which biogeochemical factors

376

correlate to MeHg in surface water (Table S2). Two principal components (PCs) explained 93%

377

of the overall variance in our data. The PC1, which shows a close correlation with nutrient

378

(i.e., nitrate + nitrite, silicate, and phosphate) variability, also shows a close similarity to the

379

MeHg variance with a correlation factor of 0.85. This suggests that MeHg formed in the

380

subsurface oxygen-depleted zone diffuses upward together with the regenerated nutrients9 as

381

also indicated by the mass flux budget.

382

Strong positive correlations between MeHg and AOU were found for the Western

383

Pacific Ocean (Fig. 4B). Such correlations have been reported in multiple oceanic water

384

columns, implying that MeHg formation in the oxygen depletion zone is linked to organic

385

carbon remineralization.6,12,13 The MeHg-to-AOU ratio was higher in the upper NPIW than in

386

the lower NPIW. The upper NPIW mass originates from the Okhotsk Sea, where dense shelf

387

water is formed in the coastal polynya entraining surface fresh water.55,56 Lower NPIW is, in

388

contrast, influenced by Pacific Deep Water.45 The EPIW (a combination of Antarctic

389

Intermediate Water and Pacific Deep Water)45,57 also showed a lower MeHg-to-AOU ratio

390

than that of the overlying Equatorial Pacific water. The younger upper water is characterized

391

by a higher MeHg-to-AOU ratio than the aged water (i.e., lower NPIW and EPIW), although

392

the aged water has experienced Hg(II) accumulation during its transport.

393

The slope of the linear regression line for MeHg versus AOU in the Equatorial

394

Pacific was 1.7 times higher than that of the North Pacific (ANCOVA, p < 0.001, Fig. 4B).

395

We suggest that this difference in the slope is attributable to the fact that the age of the water

396

in the Equatorial Pacific aphotic zone is younger than that in the North Pacific. In the

397

Equatorial Pacific, convergence and divergence zones are found associated with clockwise 17



398

North Pacific gyre and Ekman transport toward the center of gyre. The surface water is

399

continuously redistributed to the subsurface layer in these zones. The OCRR was estimated as

400

described by Sunderland et al.,6 using the water age found in the literatures58,59(Table S3, Fig.

401

S7). A single slope of MeHg versus OCRR was found for the Equatorial and North Pacific,

402

and furthermore, our calculated slope (54±6.4) was comparable to the slope seen in the

403

Northeastern Pacific (55±15).6 Figure S3 shows that DOC concentrations are higher in the

404

photic zone of the upwelling sites (S9 and S11; 80 ± 8.9 µmol L-1) than the photic zone of the

405

surrounding sites (71 ± 1.3 µmol L-1), implying that higher DOC at site S9 and S11 is

406

maintained by regenerated nutrients. The settling particle decomposition here would

407

stimulate Hg(II) methylation, by providing a substrate for microbial activity6. Indeed, the

408

highest MeHg concentrations were found at ~500 m depth of the sites E9 and E11 (Fig. S3),

409 410 411

4.4. Humic-type FDOM as a MeHg tracer The fluorescence of the humic-like FDOM was low in the surface water, but rapidly

412

increased in the subsurface layer (Fig. S3). This indicates the importance of microbial

413

transformation of labile DOM to humic-like matter as a source and the photochemical

414

decomposition as a sink of FDOM.29,50,60 The fluorescence of the humic-like FDOM also

415

showed a significant positive correlation with AOU (Fig. 4C), which agrees with the results

416

of multiple other studies.49,60,61 Previous reports on the vertical distribution of FDOM in the

417

global ocean have suggested that the production of humic-type FDOM is linked to the in situ

418

microbial activity in the aphotic ocean by the generation of precursor materials needed for

419

extracellular humification.60 The abiotic production of humic-like DOM was confirmed by

420

increases in humic-specific florescence intensities via extracellular cross linking of marine

421

lipids62. In addition, microbial degradation of phytoplankton (Microcystis sp.) showed that 18


Page 18 of 33

Page 19 of 33


422

humic-like fluorescence increased while protein-like fluorescence decreased63. The direct

423

release of humic matter by bacteria is unlikely because bacterial membranes are permeable to

424

molecules in the size range < 1 kD.60

425

A positive correlation was observed between fluorescence of humic-like FDOM and

426

the concentration of MeHg, as shown in Figure S8. This suggests that humic-like FDOM

427

fluorescence can be a surrogate for AOU, if terrestrial transport of humic-like FDOM can be

428

excluded. 60 This study is to our knowledge the first to present the empirical correlation

429

between FDOM composition and MeHg concentration in ocean water. When considering the

430

relationship between MeHg and humic-like FDOM it should be kept in mind that OCRR

431

associated with ocean circulation, such as Ekman convergence-divergence and thermohaline

432

overturning, cannot be traced by humic-like FDOM, like AOU. For instance, aged ocean

433

water typically shows large humic FDOM values and high AOU despite of relatively low

434

OCRR.6,60 It was indeed reported that only a minor portion (20 to 50%) of AOU in the

435

shallow aphotic ocean (< 400 m) is attributable to in situ remineralization of sinking organic

436

particles.64,65

437

Our results demonstrate that most MeHg in subsurface water is produced by in situ

438

reaction associated with organic carbon remineralization. The Ekman overturning process

439

causes active Hg(II) methylation in the Equatorial Pacific, resulting in higher ratios of the

440

MeHg-to-AOU than those of North Pacific. Using different approaches we show that MeHg

441

in the surface mixed layer is mainly transported from the subsurface layer by diffusion. We

442

highlight that ocean circulation should be considered in order to fully understand MeHg

443

dynamics and that alterations in circulation pattern in relation to climate change needs to be

444

investigated to predict future MeHg dynamics in the ocean.

445 19



446

Associated Content

447

Supporting information is available free of charge via the Internet at http://pubs.acs.org.

448 449

Acknowledgment

450

This study was supported by the National Research Foundation of Korea (NRF-

451

2015R1A2A2A01003774) and the Korea Ministry of Oceans and Fisheries (Long-term

452

change of structure and function in marine ecosystems of Korea, 20140507). Anne L.

453

Soerensen would like to acknowledge support from the Danish Council for Independent

454

Research.

455 456 457 458 459 460 461 462 463 464 465 466 467 468

20


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469 470

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Maas, R.; Dentener, F. Trend analysis from 1970 to 2008 and model evaluation of

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organic matter. Mar. Chem. 2011, 126 (1), 139-148.

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characteristics of dissolved organic matter in the deep waters of the Okhotsk Sea and the

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Field and experimental evidence. Water Res. 2009, 43, 4685-4697.

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638

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639 640

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641

642

643

644

645

646 27



Page 28 of 33

647

Table 1 The range and mean concentrations of the total mercury (THg), methylmercury

648

(MeHg), and dissolved organic carbon (DOC) found from the surface, photic, and aphotic

649

waters of Western Subarctic Gyre (WSG), North Pacific Gyre (NPG), Western Pacific Warm

650

Pool (WPWP), and South Pacific Gyre (SPG). The sampling sites are shown in Figure S1.

North Pacific WSG (N3-N8) THg (pM)

Surface (< 50 m)

MeHg (fM)

< LOD -34 (21±12)

DOC (µM) n

13

THg (pM)

Photic (0-100 m)

MeHg (fM)

< LOD-178 (35±32)

DOC (µM) n

30

THg (pM) Aphotic (100-500 m)

MeHg (fM)

64-750 (485±223)

DOC (µM) n LOD: limit of detection

17

Equatorial Pacific NPG (S12-S16)

WPWP (S5-S11)

SPG (S2-S4)

0.39-0.71 (0.58±0.14)

0.71-0.96 (0.84±0.12)

0.51-0.58 (0.54±0.049)

< LOD

< LOD

< LOD

68-71 (70±1.5)

70-88 (79±8.9)

69-74 (72±3.5)

4

4

2

0.39-1.1 (0.67±0.18)

0.67-2.6 (1.0±0.56)

0.51-1.4 (0.88±0.33)

< LOD-40 (7.1±11)

< LOD-62 (15±19)

< LOD-117 (39±54)

64-77 (70±3.9)

63-88 (75±8.5)

51-74 (60±6.8)

11

11

6

0.38-2.5 (1.1±0.60)

0.66-2.4 (1.3±0.48)

0.92-1.6 (1.2±0.25)

< LOD -653 (185±236)

25-909 (312±297)

29-245 (135±76)

44-70 (59±8.7)

36-71 (53±10)

45-74 (56±10)

14

15

6

651 28


Page 29 of 33


652

Figure 1 Sampling locations in the Western Subarctic Gyre (N3–N8) during the SHIPPO

653

2012 survey and the North Pacific Gyre (S12-S16), Western Pacific Warm Pool (S7–S11),

654

and South Pacific Gyre (S2–S4) during the SHIPPO 2014 survey. The surface currents are

655

shown in this figure according to the Hu et al. (2015). EKC: East Kamchatka Current; OC:

656

Oyashio Current; KC: Kuroshio Current; NEC: North Equatorial Current; SEC: South

657

Equatorial Current; GPC: Gulf of Papua Current; EAC: East Australian Current.

658

Figure 2 Spectral characteristics of three fluorescent components identified by the parallel

659

factor (PARAFAC) analysis. The line plots present split-half validation data. Excitation

660

(short-dashed lines) and emission (solid lines) loadings are presented for two independent

661

halves of the dataset (red and blue lines).

662

Figure 3 Simplified methylmercury mass budgets for Western Subarctic Gyre (N3-N8),

663

North Pacific Gyre (S12-S16), Western Pacific Warm Pool (S5–S11), and South Pacific Gyre

664

(S2–S4). The unit is nmol m-2 yr-1 for the fluxes and nmol m-2 for the masses. Blue arrow

665

represents calculated fluxes and black punctuated arrows represents fluxes used to balance

666

the surface and subsurface budgets.

667

Figure 4 Relationship between apparent oxygen utilization (AOU) and (A) total Hg (THg)

668

concentration, (B) methylmercury (MeHg) concentration, and (C) fluorescence intensity of

669

the humic-like fluorescent dissolved organic matter (FDOM), shown as a Raman unit (RU).

670

The photic group includes 0 to 100 m water depth and the aphotic group includes 100 to 500

671

m water depth. NP: North Pacific; EP: Equatorial Pacific; EPIW: Equatorial Pacific

672

Intermediate Water. The sampling locations are shown in Figure S6. The linear regression

673

model in (B) does not include lower NPIW and EPIW, and that in (C) does not include photic

674

water. The upwelling zone indicates S9 and S11.

675 29



676

Figure 1

677 678 679 680 681 682 683 684 685 686 687 688 30


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689


Figure 2 0.5

Component 1 (humic-like)

Loadings

0.4

0.3

0.2

0.1

0 200

250

690

300 350 400 Wavelength (nm)

450

500

0.5

Component 2 (tyrosine-like)

Loadings

0.4

0.3

0.2

0.1

0 200

250

691


450

500

0.5

Loadings

0.4

Component 3 (tryptophan-like)

0.3

0.2

0.1

0 200

692

250


450

500

693 694 695

Figure 3 31



696

697 698 699 700 701

Figure 4 32


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702

(A)

703 704

(B)

705 706

(C)

707 708 33


Methylmercury Mass Budgets and Distribution Characteristics in the

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