Methylmercury Mass Budgets and Distribution Characteristics in the

Dec 26, 2016 - Methylmercury Mass Budgets and Distribution Characteristics in the Western ... For a more comprehensive list of citations to this artic...
0 downloads 0 Views 1MB Size
Subscriber access provided by Fudan University

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

Environmental Science & Technology

1 2

Methylmercury mass budgets and distribution characteristics in the Western Pacific

3

Ocean

4 5 6

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

7

Rheef, Seam Noha, Seunghee Hana,*

8 9

a

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

10

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

11

b

12

Stockholm, Sweden

13

c

14

Korea

15

d

16

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

17

e

18

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

19 20 21 22

*Corresponding author (S. Han)

23

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

24 1

ACS Paragon Plus Environment

Environmental Science & Technology

25

Abstract

26

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

27

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

28

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

29

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

30

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

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

carbon remineralization to have enhanced MeHg concentrations in both surface and

39

subsurface waters.

40 41 42 43 44 45

Keywords: Methylmercury, Fluorescence dissolved organic matter, Apparent oxygen

46

utilization, Mass budget, Western Pacific

47

2

ACS Paragon Plus Environment

Page 2 of 33

Page 3 of 33

48

Environmental Science & Technology

TOC/Abstract Art

49 50 51

52 53 54 55 56 57 58 59 60 61 3

ACS Paragon Plus Environment

Environmental Science & Technology

62 63

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

64

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

65

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

66

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

67

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

68

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

69

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

70

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

71

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

72

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

73

transport across the North Pacific.5

74

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

75

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

76

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

77

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

78

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

79

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

80

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

81

ultimately influencing human exposure.

82

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

83

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

84

Several studies have observed positive associations between MeHg concentrations and either

85

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

ACS Paragon Plus Environment

Page 4 of 33

Page 5 of 33

Environmental Science & Technology

86

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

87

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

88

concentration in the oxygen depletion zone.10,12

89

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

90

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

91

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

92

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

93

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

94

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

95

demethylation and evasion.

96

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

97

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

98

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

99

flux budgets for four distinctive ocean regions with significantly different hydrographic

100

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

101

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

102

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

103

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

104

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

105 106

2. Materials and Methods

107

2.1. Seawater sampling

108

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

109

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

ACS Paragon Plus Environment

Environmental Science & Technology

110

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

111

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

112 113

2.2. THg and MeHg analysis

114

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

115

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

116

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

117

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

118

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

119

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

120

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

121

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

122

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

123

fM.

124 125 126

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

127

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

128

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

129

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

130

Baeyens et al.18

131

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

132

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

133

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

ACS Paragon Plus Environment

Page 6 of 33

Page 7 of 33

Environmental Science & Technology

134

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

(1)

135

Kw = 0.25 × u2 × (ScDMHg/600)-0.5

(2)

136

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

137

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

138

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

139

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

140

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

141

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

142

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

152

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

153

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

154

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

155

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

156

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

157

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



7

ACS Paragon Plus Environment

(3)

Environmental Science & Technology

158

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

159

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

160

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

161

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

162

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

163

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

164

concentration described in Morel et al.28,29

165 166 167

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

168

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

169

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

170

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

171

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

172

calculated by subtracting the observed oxygen concentration from the saturated oxygen

173

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

174

calculated according to Garcia and Gordon’s method.32

175

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

176

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

177

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

178

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

179

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

180

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

181

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

ACS Paragon Plus Environment

Page 8 of 33

Page 9 of 33

182

Environmental Science & Technology

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

183

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

184

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

185

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

186

method.33

187 188 189

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

190

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

191

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

192

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

193

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

194

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

195

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

196

TOC, Elementar).

197

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

198

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

199

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

200

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

201

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

202

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

203

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

204

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

205

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

ACS Paragon Plus Environment

Environmental Science & Technology

206

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

207

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

208

identified components.

209 210

3. Results

211

3.1. Surface current and hydrographic characteristics

212

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

213

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

214

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

215

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

216

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

217

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

218

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

219

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

220

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

221

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

222

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

223

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

224

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

225

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

226

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

227

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

228

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

229

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

ACS Paragon Plus Environment

Page 10 of 33

Page 11 of 33

230

Environmental Science & Technology

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

231

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

232

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

233

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

234

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

235

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

236 237 238

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

239

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

240

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

241

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

242

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

243

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

244

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

245

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

246

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

247

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

248

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

249

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

250

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

251

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

252

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

253

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

ACS Paragon Plus Environment

Environmental Science & Technology

254 255

3.3. MeHg

256

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

257

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

258

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

259

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

260

concentration was determined to be less than the LOD.

261

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

262

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

263

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

264

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

265

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

266

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

267

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

268

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

269 270

3.4. DOC and FDOM

271

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

272

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

273

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

274

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

275

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

276

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

277

inducing increased primary production. 12

ACS Paragon Plus Environment

Page 12 of 33

Page 13 of 33

278

Environmental Science & Technology

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

279

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

280

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

281

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

282

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

283

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

284

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

285

respectively.49,50

286

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

287

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

288

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

289

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

290

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

291

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

292

cover 300 nm (Fig. S4).

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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

301

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

ACS Paragon Plus Environment

Environmental Science & Technology

302

decreased UV penetration. 51

303 304

4. Discussion

305

4.1. MeHg mass budgets

306

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

307

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

308

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

309

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

310

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

311

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

312

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

313

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

314

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

315

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

316

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

317

MeHg peak in the Western Subarctic Gyre.

318

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

319

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

320

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

321

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

322

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

323

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

324

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

325

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

ACS Paragon Plus Environment

Page 14 of 33

Page 15 of 33

Environmental Science & Technology

326

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

327

ocean.

328

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

329

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

330

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

331

which is supported by observations in the equatorial Pacific.11

332

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

334

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

335

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.

340

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

341

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

343

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

345

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

346

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

ACS Paragon Plus Environment

Environmental Science & Technology

350

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

351

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)

353

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

354

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

356

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

357

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

359

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

360

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

361

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

ACS Paragon Plus Environment

Page 16 of 33

Page 17 of 33

374 375

Environmental Science & Technology

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

ACS Paragon Plus Environment

Environmental Science & Technology

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

ACS Paragon Plus Environment

Page 18 of 33

Page 19 of 33

Environmental Science & Technology

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

ACS Paragon Plus Environment

Environmental Science & Technology

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

ACS Paragon Plus Environment

Page 20 of 33

Page 21 of 33

Environmental Science & Technology

469 470

References 1. Amos, H. M.; Jacob, D. J.; Streets, D. G.; Sunderland, E. M. Legacy impacts of all‐time

471

anthropogenic emissions on the global mercury cycle. Glob. Biogeochem. Cycles 2013, 27

472

(2), 410-421.

473 474 475

2. United Nations Environment Programme (UNEP). Sources, Emissions, Releases And Environmental Transport; UNEP: Geneva, Switzerland, 2013. 3. Muntean, M.; Janssens-Maenhout, G.; Song, S.; Selin,N. E.; Olivier J. G. J.; Guizzardi, D.;

476

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

477

EDGARv4 global gridded anthropogenic mercury emissions. Sci Total Environ 2014, 494-

478

495, 337-350.

479

4. Zhang, Y.; Jacob, D. J.; Horowitz, H. M.; Chen, L.; Amos, H. M.; Krabbenhoft, D. P.;

480

Slemr, F.; St. Louis, V. L.; Sunderland, E. M., Observed decrease in atmospheric mercury

481

explained by global decline in anthropogenic emissions. P. Natl. Acad. Sci. USA. 2016,

482

113, (3), 526-531.

483

5. Strode, S. A.; Jaeglé, L.; Jaffe, D. A.; Swartzendruber, P. C.; Selin, N. E.; Holmes, C.;

484

Yantosca, R. M. Trans‐Pacific transport of mercury. J. Geophys. Res. Atmos. 2008, 113

485

(D15), 1-12.

486

6. Sunderland, E. M.; Krabbenhoft, D. P.; Moreau, J. W.; Strode, S. A.; Landing, W. M.

487

Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights

488

from data and models. Glob. Biogeochem. Cycles 2009, 23 (2), 1-14.

489 490 491 492 493 494 495

7. Driscoll, C. T.; Mason, R. P.; Chan, H. M.; Jacob, D. J.; Pirrone, N. Mercury as a global pollutant: sources, pathways, and effects. Environ. Sci. Technol. 2013, 47 (10), 4967-4983. 8. Drevnick, P. E.; Lamborg, C. H.; Horgan, M. J. Increase in mercury in Pacific yellowfin tuna. Environ. Toxicol. Chem. 2015, 34 (4), 931-934. 9. Hammerschmidt, C. R.; Bowman, K. L. Vertical methylmercury distribution in the subtropical North Pacific Ocean. Mar. Chem. 2012, 132-133 (20), 77-82. 10. Bowman, K. L.; Hammerschmidt, C. R.; Lamborg, C. H.; Swarr, G. Mercury in the North 21

ACS Paragon Plus Environment

Environmental Science & Technology

496

Atlantic Ocean: The US GEOTRACES zonal and meridional sections. Deep Sea Res.,

497

Part II 2015, 116, 251-261.

498

11. Munson, K. M.; Lamborg, C. H.; Swarr, G. J.; Saito, M. A. Mercury species

499

concentrations and fluxes in the Central Tropical Pacific Ocean. Glob. Biogeochem.

500

Cycles 2015, 29 (5), 656-676.

501 502 503

12. Cossa, D.; Averty, B.; Pirrone, N. The origin of methylmercury in open Mediterranean waters. Limnol. Oceanogr. 2009, 54 (3), 837-844 13. Heimbürger, L.-E.; Cossa, D.; Marty, J.-C.; Migon, C.; Averty, B.; Dufour, A.; Ras, J.

504

Methyl mercury distributions in relation to the presence of nano-and picophytoplankton in

505

an oceanic water column (Ligurian Sea, North-western Mediterranean). Geochim.

506

Cosmochim. Ac. 2010, 74 (19), 5549-5559.

507

14. Cossa, D.; Heimbürger, L.-E.; Lannuzel, D.; Rintoul, S. R.; Butler, E. C.; Bowie, A. R.;

508

Averty, B.; Watson, R. J.; Remenyi, T. Mercury in the southern ocean. Geochim.

509

Cosmochim. Ac. 2011, 75 (14), 4037-4052.

510

15. Blum, J. D.; Popp, B. N.; Drazen, J. C.; Choy, C. A.; Johnson, M. W. Methylmercury

511

production below the mixed layer in the North Pacific Ocean. Nat. Geosci. 2013, 6 (10),

512

879-884.

513 514 515

16. Lehnherr, I.; Louis, V. L. S.; Hintelmann, H.; Kirk, J. L. Methylation of inorganic mercury in polar marine waters. Nat. Geosci. 2011, 4 (5), 298-302. 17. Kim, E.; Kim, H.; Shin, K.-H.; Kim, M.-S.; Kundu, S. R.; Lee, B.-G.; Han, S.

516

Biomagnification of mercury through the benthic food webs of a temperate estuary: Masan

517

Bay, Korea. Environ. Toxicol. 2012, 31 (6), 1254-1263.

518 519 520

18. Baeyens, W.; Ebinghaus, R.; Vasiliev, O., Eds. Global and Regional Mercury Cycles:

Sources, Fluxes and Mass Balances. 2nd, ed.; Springer: Netherlands, 2012. 19. Soerensen, A. L.; Jacob, D. J.; Schartup, A. T.; Fisher, J.; Lehnherr, I.; St. Louis, V.;

521

Heimbürger, L.-E.; Sonke, J. E.; Krabbenhoft, D. P.; Sunderland, E. M. A Mass Budget

522

for Mercury and Methylmercury in the Arctic Ocean. Glob. Biogeochem. Cycles 2016, 30

523

(4), 560-575. 22

ACS Paragon Plus Environment

Page 22 of 33

Page 23 of 33

Environmental Science & Technology

524

20. Lindqvist, O.; Rodhe, H. Atmospheric mercury-a review. Tellus 1985, 37B, 136-159.

525

21. Baya, P. A.; Gosselin, M.; Lehnherr, I.; St.Louis, V. L.; Hintelmann, H. Determination of

526

Monomethylmercury and Dimethylmercury in the Arctic Marine Boundary Layer.

527

Environ. Sci. Technol. 2015, 49 (1), 223-232.

528

22. Black, F. J.; Poulin, B. A.; Flegal, A. R. Factors controlling the abiotic photo-degradation

529

of monomethylmercury in surface waters. Geochim. Cosmochim. Ac. 2012, 84 (1), 492-

530

507.

531 532

23. Gargett, A. E.; Holloway, G. Dissipation and diffusion by internal wae breaking. J. Mar.

Res. 1984, 42 15-27.

533

24. Sunderland, E. M.; Knightes, C. D.; von Stackelberg, K.; Stiber, N. A. Environmental

534

Fate and Bioaccumulation Modelling at the US Environmental Protection Agency:

535

Applications to Inform Decision-Making; ILM publications: Dorset, U.K., 2010.

536

25. Rühlmann, J.; Kӧrschens, M.; Graefe, J. A new approach to calculate the particle density

537

of soils considering properties of the soil organic matter and the mineral matix. Geoderma

538

2006, 130, 272-283.

539

26. Poissant, L.; Amyot, M.; Pilote, M.; Lean, D. Mercury water-air exchange over the upper

540

St. Lawrence River and Lake Ontario. Environ. Sci. Technol. 2000, 34 (15), 3069-3078.

541 542 543 544

27. Sheldon, R.; Prakash, A.; Sutcliffe, W. The size distribution of particles in the ocean.

Limnol. Oceanogr. 1972, 17 (3), 327-340. 28. Morel, A. Optical modeling of the upper ocean in relation to its biogenous matter content(Case I waters). J. Geophys. Res. 1988, 93 (10), 749-10.

545

29. Nieto-Cid, M.; Álvarez-Salgado, X. A.; Gago, J.; Pérez, F. F. DOM fluorescence, a tracer

546

for biogeochemical processes in a coastal upwelling system (NW Iberian Peninsula). Mar.

547

Ecol. Prog. Ser. 2005, 297, 33-50.

548 549 550

30. Knap, A.; Michaels, A.; Close, A.; Ducklow, H.; Dickson, A., Eds. Protocols for the joint

global ocean flux study (JGOFS) core measurements; UNESCO, 1994 31. Labasque, T.; Chaumery, C.; Aminot, A.; Kergoat, G. Spectrophotometric Winkler 23

ACS Paragon Plus Environment

Environmental Science & Technology

551

determination of dissolved oxygen: re-examination of critical factors and reliability. Mar.

552

Chem. 2004, 88 (1), 53-60.

553 554 555 556 557 558 559

32. Garcia, H. E.; Gordon, L. I. Oxygen solubility in seawater: Better fitting equations.

Limnol. Oceanogr. 1992, 37 (6), 1307-1312. 33. Parsons, T.; Maita, Y.; Lalli, C., Eds. A Manual of Chemical and Biological Methods for

Seawater Analysis, 1st, ed.; Pergamon Press: New York, 1984. 34. Lawaetz, A. J.; Stedmon, C. A. Fluorescence intensity calibration using the Raman scatter peak of water. Appl. Spectrosc. 2009, 63 (8), 936-940. 35. Stedmon, C. A.; Bro, R. Characterizing dissolved organic matter fluorescence with

560

parallel factor analysis: a tutorial. Limnol. Oceanogr.: Methods 2008, 6 (11), 572-579.

561

36. Harrison, P. J.; Whitney, F. A.; Tsuda, A.; Saito, H.; Tadokoro, K. Nutrient and plankton

562

dynamics in the NE and NW gyres of the subarctic Pacific Ocean. J. Oceanogr. 2004, 60

563

(1), 93-117.

564

37. Kim, H.; Rhee, T.S.; Hahm, D.: Hwang, C.Y.; Yang, J.; Han, S. Contrasting distributions

565

of dissolved gaseous mercury concentration and evasion in the North Pacific Subarctic

566

Gyre and the Subarctic Front. Deep Sea Res., Part I 2016, 110, 90-98.

567 568 569

38. Yasuda, I. Hydrographic structure and variability in the Kuroshio-Oyashio transition area.

J. Oceanogr. 2003, 59 (4), 389-402. 39. Yasuda, I.; Okuda, K.; Shimizu, Y. Distribution and modification of North Pacific

570

Intermediate Water in the Kuroshio-Oyashio interfrontal zone. J. Phys. Oceanogr. 1996,

571

26 (4), 448-465.

572

40. Shimizu, Y.; Iwao, T.; Yasuda, I.; Ito, S.-I.; Watanabe, T.; Uehara, K.; Shikama, N.;

573

Nakano, T. Formation process of North Pacific Intermediate Water revealed by profiling

574

floats set to drift on 26.7 σθ isopycnal surface. J. Oceanogr. 2004, 60 (2), 453-462.

575

41. Talley, L. D. Distribution and formation of North Pacific intermediate water. J. Phys.

576 577

Oceanogr. 1993, 23 (3), 517-537. 42. Hu, D.; Wu, L.; Cai, W.; Gupta, A. S.; Ganachaud, A.; Qiu, B.; Gordon, A. L.; Lin, X.; 24

ACS Paragon Plus Environment

Page 24 of 33

Page 25 of 33

Environmental Science & Technology

578

Chen, Z.; Hu, S. Pacific western boundary currents and their roles in climate. Nature 2015,

579

522, (7556), 299-308.

580 581 582

43. Qu, T.; Lukas, R. The Bifurcation of the North Equatorial Current in the Pacific. J. Phys.

Oceanogr. 2003, 33 (1), 5-18. 44. England, M. H.; McGregor, S.; Spence, P.; Meehl, G. A.; Timmermann, A.; Cai, W.;

583

Gupta, A. S.; McPhaden, M. J.; Purich, A.; Santoso, A. Recent intensification of wind-

584

driven circulation in the Pacific and the ongoing warming hiatus. Nat. Clim. Change 2014,

585

4 (3), 222-227.

586

45. Bostock, H. C.; Opdyke, B. N.; Williams, M. J. Characterising the intermediate depth

587

waters of the Pacific Ocean using δ 13 C and other geochemical tracers. Deep Sea Res.,

588

Part I 2010, 57 (7), 847-859.

589

46. de Boyer Montégut, C.; Madec, G.; Fischer, A. S.; Lazar, A.; Iudicone, D. Mixed layer

590

depth over the global ocean: An examination of profile data and a profile‐based

591

climatology. J. Geophys. Res. Oceans 2004, 109 (C12), 1-20.

592 593 594

47. Laurier, F.; Mason, R.; Gill, G. a.; Whalin, L. Mercury distributions in the North Pacific Ocean—20 years of observations. Mar. Chem. 2004, 90 (1), 3-19. 48. Bowman, K. L. Mercury distributions and cycling in the North Atlantic and eastern

595

tropical Pacific Oceans. Ph.D. Dissertation, Wright State University, Dayton, OH, 2014.

596

49. Kowalczuk, P.; Tilstone, G. H.; Zabłocka, M.; Röttgers, R.; Thomas, R. Composition of

597

dissolved organic matter along an Atlantic Meridional Transect from fluorescence

598

spectroscopy and Parallel Factor Analysis. Mar. Chem. 2013, 157 (20), 170-184.

599

50. Stedmon, C. A.; Markager, S. Resolving the variability in dissolved organic matter

600

fluorescence in a temperate estuary and its catchment using PARAFAC analysis. Limnol.

601

Oceanogr. 2005, 50 (2), 686-697.

602

51. Jørgensen, L.; Stedmon, C. A.; Granskog, M. A.; Middelboe, M. Tracing the long‐term

603

microbial production of recalcitrant fluorescent dissolved organic matter in seawater.

604

Geophys. Res. Lett. 2014, 41 (7), 2481-2488. 25

ACS Paragon Plus Environment

Environmental Science & Technology

605

52. Monperrus, M.; Tessier, E.; Amouroux, D.; Leynaert, A.; Huonnic, P.; Donard, O.

606

Mercury methylation, demethylation and reduction rates in coastal and marine surface

607

waters of the Mediterranean Sea. Mar. Chem. 2007, 107 (1), 49-63.

608 609

53. Barkay, T.; Miller, S. M.; Summers, A. O. Bacterial mercury resistance from atoms to ecosystems. FEMS Microbiol. Rev. 2003, 27 (2-3), 355-384.

610

54. Lamborg, C. H.; Hammerschmidt, C. R.; Bowman, K. L.; Swarr, G. J.; Munson, K. M.;

611

Ohnemus, D. C.; Lam, P. J.; Heimbürger, L.-E.; Rijkenberg, M. J.; Saito, M. A. A global

612

ocean inventory of anthropogenic mercury based on water column measurements. Nature

613

2014, 512 (7512), 65-68.

614 615

55. Qu, T.; Mitsudera, H.; Qiu, B. A Climatological View of the Kuroshio/Oyashio System East of Japan. J. Phys. Oceanogr. 2001, 31 (9), 2575-2589.

616

56. You, Y.; Suginohara, N.; Fukasawa, M.; Yasuda, I.; Kaneko, I.; Yoritaka, H.; Kawamiya,

617

M. Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific intermediate

618

water. J. Geophys. Res. Oceans 2000, 105 (C2), 3253-3280.

619 620 621 622 623 624 625

57. Qu, T.; Lindstrom, E. J. Northward intrusion of antarctic intermediate water in the western pacific. J. Phys. Oceanogr. 2004, 34 (9), 2104-2118. 58. Watanabe, Y. W.; Watanabe, S.; Tsunogai, S. Tritium in the Northwestern North Pacific.

J. Oceanorg. Society Japan 1991, 47, 80-93. 59. McPhaden, M. J.; Zhang, D. Slowdown of the meridional overturning circulation in the upper Pacific Ocean. Nature 2002, 415, 603-608. 60. Jørgensen, L.; Stedmon, C. A.; Kragh, T.; Markager, S.; Middelboe, M.; Søndergaard, M.

626

Global trends in the fluorescence characteristics and distribution of marine dissolved

627

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

628

61. Yamashita, Y.; Cory, R. M.; Nishioka, J.; Kuma, K.; Tanoue, E.; Jaffé, R. Fluorescence

629

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

630

northwestern North Pacific Ocean. Deep Sea Res., Part II 2010, 57 (16), 1478-1485.

631

62. Harvey, H. R.; Fallon, R. D.; Patton, J. S. The effect of organic matter and oxygen on the 26

ACS Paragon Plus Environment

Page 26 of 33

Page 27 of 33

Environmental Science & Technology

632

detradation of bacterial membrane lipids in marine sediments. Geochim. et Cosmochim.

633

Acta 1986, 50 (5), 795-804.

634

63. Zhang, Y.; van Dijk, M. A.; Liu, M. Zhu, G.; Qin, B. The contribution of phytoplankton

635

degradation to chromophoric dissolved organic matter (DCOM) in eutrophic shallow lakes:

636

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

637

64 Doval, M. D.; Hansel, D. A. Organic carbon and apparent oxygen utilization in the

638

western South Pacific and central Indian Ocean. Mar. Chem. 2000, 68, 249-264.

639 640

65. Hansell, D. A.; Carlson, C. A.; Repeta, D. J.; Schlitzer, R. Dissolved organic matter in the Ocean. Oceanogr. 2009, 22 (4), 202-211.

641

642

643

644

645

646 27

ACS Paragon Plus Environment

Environmental Science & Technology

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

ACS Paragon Plus Environment

Page 29 of 33

Environmental Science & Technology

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

ACS Paragon Plus Environment

Environmental Science & Technology

676

Figure 1

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

ACS Paragon Plus Environment

Page 30 of 33

Page 31 of 33

689

Environmental Science & Technology

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

300 350 400 Wavelength (nm)

450

500

0.5

Loadings

0.4

Component 3 (tryptophan-like)

0.3

0.2

0.1

0 200

692

250

300 350 400 Wavelength (nm)

450

500

693 694 695

Figure 3 31

ACS Paragon Plus Environment

Environmental Science & Technology

696

697 698 699 700 701

Figure 4 32

ACS Paragon Plus Environment

Page 32 of 33

Page 33 of 33

Environmental Science & Technology

702

(A)

703 704

(B)

705 706

(C)

707 708 33

ACS Paragon Plus Environment