Distribution of total mercury and methylmercury and their controlling factors in the East China Sea

Journal Pre-proof Distribution of total mercury and methylmercury and their controlling factors in the East China Sea Chang Liu, Lufeng Chen, Shengkang Liang, Yanbin Li PII:

S0269-7491(19)31187-X

DOI:

https://doi.org/10.1016/j.envpol.2019.113667

Reference:

ENPO 113667

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Environmental Pollution

Received Date: 6 March 2019 Revised Date:

13 November 2019

Accepted Date: 21 November 2019

Please cite this article as: Liu, C., Chen, L., Liang, S., Li, Y., Distribution of total mercury and methylmercury and their controlling factors in the East China Sea, Environmental Pollution (2019), doi: https://doi.org/10.1016/j.envpol.2019.113667. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Effects of Water Mass Mixing on Mercury Distribution in the East China Sea (Green solid circle: CDW, Changjiang Diluted Water; Blue solid circle: KIC, Kuroshio Intermediate Current; Red solid circle: TWC, Taiwan Warm Current; Purple solid circle: KSSC, Kuroshio Subsurface Current)

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Distribution of total mercury and methylmercury and

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their controlling factors in the East China Sea

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Chang Liua, b, Lufeng Chenc, Shengkang Lianga, b, Yanbin Lia, b*

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a

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China), Ministry of Education, Qingdao 266100, China

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b

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Qingdao 266100, China

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c

Key Laboratory of Marine Chemistry Theory and Technology (Ocean University of

College of Chemistry and Chemical Engineering, Ocean University of China,

Institute of Environment and Health, Jianghan University, Wuhan 430056, China

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*Corresponding Author:

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Yanbin Li

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Tel.: +86-0532-66786355

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Fax: +86-0532-66782301

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Email: [email protected]

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Abstract

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Mercury (Hg) is among contaminants of public concern due to its prevalent existence,

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high toxicity, and bioaccumulation through food chains. Elevated Hg has been

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detected in seafood from the East China Sea (ECS), which is one of the largest

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marginal seas

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However, there is still a lack of knowledge on the distribution of Hg species and their

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controlling factors in the ECS water column, thus preventing the understanding of Hg

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cycling and the assessment of Hg risks in the ECS. In this study, two cruises were

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conducted in October 2014 and June 2015 in order to investigate the distribution of

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total Hg (THg) and methylmercury (MeHg) and their controlling factors in the ECS.

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The concentrations of THg and MeHg were determined to be 4.2±2.8 ng/L (THg) and

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0.25±0.13 ng/L (MeHg) in water from the ECS. The level of Hg in the ECS occupied

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the higher rank among the marginal seas, thus indicating significant Hg contamination

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in this system. Both the THg and MeHg presented complicated spatial distribution

37

patterns in the ECS, with high concentration areas located in both the nearshore and

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offshore areas. Statistical analyses suggest that temperature (T) and Hg in sediment

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may be the controlling factors for THg distribution, while, dissolved organic matter

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(DOM), T, and MeHg in the sediment may be the controlling factors for MeHg

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distribution in the seawater of the ECS. The relative importance of these

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environmental factors in Hg distribution depends on the water depth. T-salinity (S)

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diagram analyses showed that water mass mixing may also play an important role in

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controlling THg and MeHg distribution in the coastal ECS.

and an important fishing region in the northwestern Pacific Ocean.

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Keywords: Mercury, Methylmercury, East China Sea, Controlling factors·

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Capsule： ：Dissolved organic matter and water mass mixing play a critical role in

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controlling THg and MeHg distributions in the East China Sea.

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1. Introduction

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Mercury (Hg) is among contaminants of public concern due to its high toxicity,

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bioaccumulation through food chains, and capability of being distributed worldwide

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via atmospheric transportation (Liu et al., 2012). Mercury emitted into the

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environment comes from various natural sources (e.g., oceans, land and volcanoes)

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and anthropogenic sources (fossil fuel combustion, metal production, and waste

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incineration) (UNEP, 2013). Elevated Hg has been detected in fish from a large

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number of marine systems (Burger and Gochfeld, 2011; Canuel et al., 2006;

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Jeevanaraj et al., 2016; Luo et al., 2012), in particular in coastal areas (Agah et al.,

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2007; Bosnir et al., 1999; Gardner, 1978; Signa et al., 2017). Approximately 55, 000

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tons of Hg are annually discharged into the ocean via riverine input (Amos et al.,

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2014), most of which was retained in coastal areas due to the quick accumulation of

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particulate Hg in the sediment (Cossa et al., 1996). This results in the worldwide

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contamination of Hg in coastal areas (Aksentov, 2015; Mason and Lawson, 1999).

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Mercury in natural waters is present as both inorganic Hg and methylmercury

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(MeHg). Inorganic Hg (Hg2+ and Hg0) is the main form of Hg that is discharged into

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marine environments from both anthropogenic and natural sources (Clarkson and

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Magos, 2006). Hg0 in the seawater is the major form of Hg that is involved in the

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exchange of Hg at the air-sea interface (Mason et al., 1997). It is estimated that

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approximately 2,600 tons of Hg0 in the ocean are re-emitted into the atmosphere each

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year, accounting for 70% of the Hg that is emitted into the atmosphere annually

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(UNEP, 2013). MeHg is the most toxic Hg form in aquatic environments, and it is the 4

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major species of Hg that is bioconcentrated and biomagnified through the food chain

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(Mergler el al, 2007). The determination of the Hg species in seawater is necessary

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due to the fact that the toxicity of Hg depends not only on the gross Hg concentrations,

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but also on its chemical speciation (Figueiredo et al., 2016; Lang et al., 2017;

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Živković et al., 2017).

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The speciation and distribution of Hg in oceans are controlled by both

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microorganisms (Barkay and Poulain, 2007; Gionfriddo et al., 2016) and a variety of

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environmental factors including dissolved organic matter (Kim el al, 2014), redox

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conditions (Ci et al., 2016), salinity (Wang and Wang, 2010), temperature (Wang et al.,

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2015), salinity (Wang and Wang, 2010), DO (Lehnherr et al., 2011), pH (Watras et al.,

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1995), nitrate (Zhang et al., 2012), and sulfate (Acha et al., 2011). These

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environmental factors affect the cycling and fate of Hg by influencing a variety of

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significant

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oxidation/reduction, and adsorption/desorption (Ci et al., 2016; Daniela et al., 2016;

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Kucharzyk et al., 2015; Laurier et al., 2003) (Detailed correlations of these factors

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with Hg cycling processes were presented in Table S1). For instance, mercury can be

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quickly scavenged by particulate matter in coastal and estuarine systems (Graham et

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al., 2012), and this process highly depends on the characteristics of the suspended

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particles (Zhu et al., 2008). Mercury has a high affinity to DOM mainly through its

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chemical bonding to reduced sulphur groups (Ravichandran, 2004), and the bonding

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of Hg with DOM is reported to greatly affect the biotic and photochemical

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methylation/demethylation (Black et al., 2012; Jiang et al., 2017), reduction/oxidation

processes

in

Hg

cycling,

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e.g.,

methylation/demethylation,

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(Ci et al., 2016), and adsorption/desorption of Hg (Kim et al., 2014) in seawater.

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Redox conditions play an important role in affecting the reduction/oxidation of Hg (Ci

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et al., 2016) and the Hg microbial methylation process (which is dominated by

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anaerobic bacteria) (Figueiredo et al., 2016). High salinity was supposed to inhibit the

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complexation of Hg with DOM by forming HgClx complexes (Graham et al., 2012).

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The complexes of Hg with Cl- can subsequently inhibit the photodemethylation of

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MeHg (Wang and Wang, 2010), limit the bioavailable mercuric compounds for

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bacterial uptake, and promote the stability of mercury in the sediment (King et al.,

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2000). The water mass mixing is another important factor in the distribution of

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mercury, as well as other elements, in coastal seas (Daniela et al., 2016; Schroder and

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Fahrbach, 1999; Shi et al., 2014; Živković et al., 2017).

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The East China Sea (ECS) is one of the largest marginal seas (Fang et al., 2009)

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and an important fishing region (Shi et al., 2005) in the northwestern Pacific Ocean.

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The rapid urbanization and industrialization of the surrounding coastal areas of the

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ECS, coupled with intensively increasing anthropogenic activities, have accelerated

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the environmental deterioration, including Hg pollution. Rivers, in particular the

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Yangtze River, play an important role in controlling the cycling of Hg in the ECS. It

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has been estimated that 144 tons of Hg have been emitted into the ECS via the

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Yangtze River (Wang et al., 2016a). As a consequence, elevated mercury (higher than

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the safe value recommended by the US EPA) has been detected in seafood from the

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East China Sea (Huang et al., 2012; Xia et al., 2013), thus posing great risks to human

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health and highlighting the importance of understanding the cycling of Hg in this 6

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ecosystem.

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Some efforts have been made to investigate the pollution status and fate of Hg in

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the ECS (Duan et al., 2015; Fang and Chen, 2010; Kim et al., 2017). Most of these

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previous studies focused on the distribution of THg in the sediment of the ECS. For

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instance, Fang and Chen (2010) calculated the Hg accumulation rates in the ECS

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sediment. Duan et al. (2015) investigated the relationships of mercury with

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environmental factors, such as the sand–silt–clay ratio and iron content. Several

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studies were conducted to investigate the levels and fate of the THg in surface water

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(Wang et al., 2016a; Zheng et al., 2009). However, there is still lack of knowledge on

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the distribution of THg and MeHg in the ECS water column. In addition, the

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controlling factors for Hg distribution and speciation in the ECS are still unclear. The

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lack of this information limits the understanding of Hg cycling and the assessment of

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Hg risks in the ECS.

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The major objectives of this study were to investigate the distribution of THg

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and MeHg in the ECS and to identify the factors controlling the THg and MeHg

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levels in the ECS. To fulfil these aims, two cruises were conducted in October 2014

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and June 2015. The THg and MeHg levers at several water layers were analysed.

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Statistical analyses, i.e., correlation and multiple regression analyses, were performed

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to identify the major environmental factors affecting the distribution of THg and

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MeHg in the ECS. The influence of water masses on the distributions of the THg and

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MeHg was also investigated by using the T-S diagram method (Chen, 1996; Ren et al.,

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2015; Shi et al., 2014). 7

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2. Materials and methods

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2.1. Sample collection

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During the cruise sampling campaigns, 185 and 166 water samples were

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collected from 34 and 28 sites in October 2014 and June 2015, respectively (Fig. S1).

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Seawater at different depths (surface (3-4 m), 10 m, 20 m and bottom) was collected

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using a Niskin collector and stored in pre-cleaned borosilicate bottles at -20 °C with

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the addition of 0.5% (v/v) HCl for THg analysis and 0.4% (v/v) HCl for MeHg

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analysis.

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2.2. Sample analysis

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2.2.1. Analysis of total mercury in seawater

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The THg in seawater was determined following EPA method 1631. To 25 mL

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seawater sample 125 µL of BrCl (0.2N) was added and allowed to react for 12 h. After

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adding 62.5 µL of NH2OH·HCl (30% w/v) in order to remove the residual BrCl, 125

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µL of SnCl2 (20% w/v) was added to convert all Hg2+ to Hg0 and the generated Hg0

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was then detected using a MERX automated modular Hg system (Brooks Rand Labs,

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Seattle, WA, USA).

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2.2.2. Analysis of methylmercury in water

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The MeHg in seawater was analysed according to EPA method 1630. 45 mL of

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water samples was distilled at 125±3 °C under 90±10 mL/min of N2 for approximately

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3-4 h in order to get 35 mL of water. After adding 65 mL of de-ionized (DI) water, the

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mixture was reacted with 50 µL of NaBEt4 (1% w/v) for 15 min and purged at 200

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mL/min of N2 flow for 15 min in order to trap the generated methyl ethyl mercury on

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a Tenax-TA trap (35/60 mesh, Supelco, Bellefonte, PA, USA). The methyl ethyl 8

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mercury on the Tenax trap was then thermally desorbed at 200 °C, separated using an

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OV-3 column at 70 °C, decomposed to Hg0 at 800 °C, and finally detected using a

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Model III AFS (Brooks Rand Lab., Seattle, WA, USA).

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2.2.3. QA/QC

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Standard quality assurance and control procedures were followed during the

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analysis of THg and MeHg. Two method blanks, duplicate matrix spikes, and

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triplicate of one randomly chosen water sample were included for each batch analysis

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(20 samples). The recoveries (n=18 pairs) were 95-105% for THg and 88-105% for

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MeHg, which were within the acceptable range of the EPA method (70-130% for THg

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and 65-135% for MeHg). Acceptable relative standard deviations of the analysed

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triplicates (n=27) were also obtained (5.7% to 11.3% for THg and 4.3% to 13.7% for

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MeHg). For the Hg analyses, the limits of detection were 0.2 ng/L for THg and 0.02

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ng/L for MeHg, and the method blank ranged from 0.2 to 0.4 ng/L for THg (n=36)

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and 0.02 to 0.03 ng/L for MeHg (n=36), which were within the acceptable values of

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the EPA method (<0.5 ng/L for THg and <0.03 ng/L for MeHg). Three blanks (0.4%

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HCl) were prepared on board for each cruise in the pre-cleaned borosilicate bottles,

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brought back to the laboratory after the cruises and analysed in order to ensure the

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accuracy and reliability of the storage procedure. The concentrations of THg and

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MeHg in these blanks were 0.4 ng/L (0.3-0.5 ng/L) and 0.02 ng/L (0.02-0.03 ng/L),

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respectively, which were comparable to those in the method blanks during the

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analysis, thus indicating a negligible contamination of Hg during the transportation.

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The seawater samples were collected by regular CTD during the two cruises. 9

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Mercury concentrations analysed by the regular CTD sampler and clean CTD sampler

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(with a X-Niskin bottle attached to a Kevlar line) were compared during a cruise

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conducted in October 2015 in the ECS. As shown in the Fig.S2, the concentrations of

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THg and MeHg determined in seawaters of 3 stations collected by the CTD sampler

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and X-Niskin bottles were comparable, with an average RSD of 9.9±6.7% (ranged

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from 1.1 to 21.4%), and 7.2±5.7% (0.2-16.0%), respectively. The variation trends of

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THg and MeHg along the depth were also similar. These results suggested that the

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regular CTD sampler can also be utilized for collecting seawater samples for Hg

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analysis in China coastal seas.

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2.2.4. Analysis of ancillary parameters

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Sulphate (SO42-) was analysed using an ICS 2100 (Thermo Fisher, Waltham,

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USA). The nitrate (NO3-) concentrations were analysed using a Nutrient Automatic

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Analyzer 3 (Seal Analytical, Hanover, Germany). The routine parameters, including

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temperature (T), salinity (S), and dissolved oxygen (DO), were measured using a

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conductivity– temperature–depth (CTD) recorder (Seabird 911 Plus, Bellevue, US).

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The concentrations of dissolved organic carbon (DOC) were measured using a

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TOC-VCPH analyser (Shimadzu Corp., Tokyo, Japan), and the data have been

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previously reported by Li et al. (2018).

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2.3 Data Analysis

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The contour maps were drawn using ODV 4.6.10 (Alfred Wegener, Berlin,

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Germany). The correlation analysis and multiple linear regression analysis of the THg

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and MeHg against environmental parameters were performed using SPSS version 19 10

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for Windows (SPSS Inc, Chicago, USA), which was done in order to identify the

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main controlling factors for the distribution of Hg in the ECS. Mann-Whitney U tests

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were conducted using SPSS in order to test if the seasonal variations of THg and

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MeHg were significant. Outliers of THg and MeHg were identified using SPSS

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version 19.0 and these data were excluded in the subsequent analyses.

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Contributions of water masses to distribution of Hg in the ECS were investigated

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in this work. There are mainly six water masses (CDW (Changjiang Diluted Water),

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ZFCC (Zhejiang-Fujian Coastal Current), TWC (Taiwan Warm Current), KSC

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(Kuroshio Surface Current), KSSC (Kuroshio Sub Surface Current), and KIC

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(Kuroshio Intermediate Current)) taking part in the mixing processes in the East

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China Sea (Zhu et al., 2008) (Fig. S3). The seawater characteristics (temperature and

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salinity) of each water mass in autumn and summer were taken from the values that

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were reported in previous studies (Han et al., 2013; Kim et al., 2006) (detailed data

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can been found in Table S2). The T-S diagrams were drawn in the sections 1-X and

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7-X using ODV 4.6.10 (Fig. S4), and the samples were grouped into five major water

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masses (CDW, TWC, and KSSC in both seasons, and KSC in autumn and KIC in

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summer) according to their T and S characteristics.

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3. Results

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3.1. Distribution of the THg in the water of East China Sea

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The average concentrations of the THg in the ECS were determined to be

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3.9±3.3 ng/L (ND-13.2 ng/L) in October 2014 and 4.3±1.9 ng/L (ND-9.6 ng/L) in

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June 2015 (Table 1). There was no significant trend for THg with depth among the 11

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entire water column (Mann-Whitney U test, p> 0.05). The THg in June was slightly

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higher than that in October in the ECS (Mann-Whitney U test, p < 0.01). As for

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different water depths, THg in the surface water in June were significantly higher than

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that in October (Mann-Whitney U test, p < 0.05), while there were no significant

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seasonal variations for THg in the other layers (Mann-Whitney U test, p > 0.05). In

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addition, significant seasonal variations were only observed in offshore areas

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(Mann-Whitney U test, p < 0.05), but not the inshore areas (Mann-Whitney U test, p >

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0.05) (the inshore and offshore sites chosen can be found in Fig. S5). These results

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suggest that the seasonal variations in THg could be partially due to the seasonal

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change in the route of the Yangtze River. In June, larger amount of mercury were

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discharged into the ECS via the Yangtze River (Liu et al., 2016), which may result in

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the higher value of Hg in this season in comparison to the November. Compared to

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the other marine systems (Table 1), the THg in ECS seawater was approximately one

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order of magnitude higher than that in the open ocean and occupied the higher rank

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among the marginal seas (Table 1). The THg concentrations in the Yangtze River were

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approximate 18 times higher than that in ECS water, thus suggesting that terrestrial

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runoff may contribute greatly to the high concentrations of THg in this system.

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The horizontal distribution of the THg in the ECS seawater was presented in Fig.

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1. In autumn, there were two high concentration areas at the surface layer. One was

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located near the Hangzhou Bay, while the other was in offshore areas (Fig. 1A). At 10

246

m and 20 m, the THg generally exhibited a decreasing trend from the nearshore to

247

offshore, except for a ‘hot spot’ located at the northeastern corner of the sampling 12

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areas (Figs. 1B and C). However, for the bottom layer, high concentrations of THg

249

were observed at both the centre of the survey area and the northern offshore area (Fig.

250

1D). The THg exhibited a different distribution trend in summer compared to that in

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autumn (Figs. 1a-2d). It was distributed more evenly in comparison to autumn and

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high concentration areas mostly appeared in the south of the investigated area at the

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surface, 10 m, 20 m, and bottom layers. For the vertical distribution of the THg in the

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ECS, high concentrations of THg were observed in the nearshore shallow water of

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sections 1-X and 7-X in both summer and autumn and the subsurface water of

256

sections 2-X and 4-X in autumn (Figs. 2A-2D). High concentrations of THg also

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existed in the offshore deep water of sections 1-X and 7-X in autumn and sections

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4-X and 7-X in summer. In addition, THg ‘hot spots’ were also found in the offshore

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surface water of sections 1-X, 2-X and 4-X in autumn.

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In summary, high concentrations of THg were frequently detected in the northern

261

nearshore areas, thereby indicating the importance of Yangtze River discharge in

262

controlling the THg in the ECS. THg ‘hot spots’ also existed in offshore deep water in

263

both seasons, which may be highly affected by the Taiwan Warm Current or Kuroshio

264

Current, thus suggesting the significant contributions of water mass mixing to the

265

THg in the ECS. In addition to the riverine input and water mass mixing,

266

environmental factors, e.g., DO, T, and S (Ci et al., 2016; Kucharzyk et al., 2015;

267

Laurier et al., 2003), may also play important roles in controlling the THg in ECS

268

water. The relative importance of these factors on the THg in the ECS will be

269

discussed in a later section. 13

270

3.2. Distribution of methylmercury in the water of the East China Sea

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The average concentrations of MeHg in the ECS were determined to be

272

0.30±0.14 ng/L (ND-0.70 ng/L) in October 2014 and 0.22±0.11 ng/L (ND-0.61 ng/L)

273

in June 2015. No significant vertical trend of MeHg was found among the water

274

column (Mann-Whitney U test, p> 0.05). The MeHg in October 2014 was

275

significantly higher than that in June 2015 in the ECS (Mann-Whitney U test, p <

276

0.01). There were no significant seasonal variations for MeHg in various water layers

277

(Mann-Whitney U test, p > 0.05), and no significant seasonal variation for MeHg at

278

inshore or offshore areas was found (Mann-Whitney U test, p > 0.05). Previous

279

studies have suggested that photodemethylation may be the dominant MeHg

280

removing process in water, and thus the seasonal variation of MeHg in seawater may

281

be caused by the quick photodemethylation of MeHg due to higher light intensity in

282

summer. Similar to the THg (Table 1), the MeHg concentrations in ECS seawater

283

were approximately one order of magnitude higher than that in the open ocean and

284

were at the last quartile of values reported in marginal seas (Table 1). In addition,

285

ratios of MeHg/THg in the ECS were much larger than that in the Yangtze River,

286

suggesting that the high concentrations of MeHg in the ECS may be from the in situ

287

production rather than the anthropogenic discharge.

288

The horizontal distribution of MeHg was presented in Fig. 3. In comparison to

289

the THg, the distribution patterns of MeHg were more complicated. High

290

concentration areas were observed in both inshore and offshore areas, and in middle

291

of the investigation areas. In the surface layer, high concentrations of MeHg were

14

292

observed in both the inshore and offshore areas in both seasons (Figs. 3A and 3a).

293

MeHg presented two high concentration areas in the 10 m layer: one was in the

294

northern offshore areas, while the other was located at the centre of the sampling area

295

in autumn (Fig. 3B). A generally decreasing trend from the nearshore to offshore was

296

observed in the 20 m layer in both seasons (Figs. 3C, 3c). The MeHg in the 10 m and

297

bottom layers followed a decreasing trend from the north to the south in summer (Figs.

298

3b and 3d). For the vertical distribution patterns, high concentrations of MeHg were

299

observed in the nearshore shallow water of sections 1-X and 7-X in autumn (Figs. 4A,

300

4B). “Hot spots” of MeHg were also observed in the deep water of sections 1-X, 4-X

301

and 7-X in autumn (Figs. 4A and 4C), indicating the importance of MeHg production

302

in deep water or sediment releasing processes in controlling MeHg levels. In summer,

303

high concentration areas were observed in the nearshore bottom water of sections 1-X,

304

2-X, and 4-X (Figs. 4a-4c) and in the offshore deep water of sections 4-X and 7-X

305

(Figs. 4c and 4d).

306

As aforementioned, THg and MeHg in the seawater can be affected by a variety

307

of environmental factors and input/output processes. The importance of these factors

308

on the MeHg in the ECS will be discussed in the later section.

309

4. Discussion

310

4.1. Environmental factors controlling the distribution of the THg and MeHg in ECS

311

seawater

312

Spearman’s correlation analysis was conducted to test the correlations of the

313

THg, MeHg, and their ratio in surface, 10 m, 20 m, and bottom water with various

314

environmental factors including T, S, DO, NO3-, SO42-, DOM, and concentrations of 15

315

THg and MeHg in sediment (Liu et al., 2018)(seen in SI, Figure S6). THg was also

316

included when identifying the factors controlling the MeHg distribution. The

317

Spearman’s analysis showed that THg in surface water has a significantly negative

318

relation with temperature (Tab. S3), which was supported by the opposite distribution

319

patterns between THg and temperature (Fig. S6A, 6a and S6B, 6b). Previous studies

320

reported that higher temperature could facilitate the formation of Hg0 and subsequent

321

the emission of Hg0 from the surface seawater to the air (Wang et al., 2015). At the

322

bottom layer, significant relationships between THg in water and that in sediment

323

were observed, while none of the other parameters showed a significant relationship

324

with the THg. As shown in Figs. S6F, 6f and S6G, 6g, similar distribution patterns

325

were observed for THg in the bottom seawater and sediment. These results suggest

326

that the diffusion of Hg from sediment to water may be an important source of THg in

327

the water body and play an important role in controlling the Hg in bottom water of the

328

ECS.

329

MeHg levels were found to be significantly correlated with DOM and NO3- in

330

the entire water column (Table 2). By conducting multiple regression analysis, DOM

331

was identified to be the most important factor (as represented by its high β)

332

influencing the MeHg in the ECS. This factor can explain approximately 40% of the

333

variation

334

methylation/demethylation are the processes controlling the in situ production of

335

MeHg (Cossa et al., 2017; Heimbürger et al., 2015). The complexation of DOM with

336

MeHg may facilitate the photodemethylation of MeHg in water (Kim and Zoh, 2013),

of

the

MeHg

in

the

ECS.

16

Previous

studies

suggested

that

337

which could result in the negative correlation of MeHg with the DOM in shallow

338

water. This assumption was further supported by the negative correlation of

339

MeHg/THg ratio (could serve as an indicator of net production potential of MeHg

340

(Beldowski et al., 2015)) with DOM in surface water (R=-0.30, p < 0.05) (Tab. S3).

341

Significant negative relation was also observed in bottom water (R=-0.30, p < 0.05),

342

where biotic methylation process may dominate the source of MeHg (Cossa et al.,

343

2017; Lehnherr et al., 2011). This suggest that DOM may also have influence on

344

MeHg in the water by inhibiting Hg methylation (Figueiredo et al., 2016). In addition,

345

MeHg in bottom water was significantly correlated with MeHg in the sediment (Table

346

S3), and similar distribution patterns were observed for MeHg in bottom water and

347

sediment (Figs. S6H, 6h and S6I, 6i). This suggests that MeHg in the sediment may

348

be an important source for MeHg in the bottom water. Significant negative

349

correlations were also observed between MeHg/THg and T in surface water and

350

MeHg and T at 10 m layer, which may be due to its influence on MeHg demethylation

351

process (Lehnherr, et al., 2011; DiMento et al., 2017).

352

4.2. Importance of water mass mixing in controlling the distributions of THg and

353

MeHg in ECS seawater

354

Regional ocean circulation patterns have been reported to be an important factor

355

in controlling the distribution of Hg in some marine systems (Cossa et al., 2017;

356

Mastromonaco et al., 2017; Schroder and Fahrbach, 1999). There are mainly six water

357

masses (CDW, ZFFC, KIC, TWC, KSC, and KSSC) taking part in the mixing

358

processes in the East China Sea (Zhu et al., 2008) (Fig. S2). By using the T-S diagram 17

359

method (Chen, 1996; Chen, 2008; Shi et al., 2014), the composition of the water

360

masses in the investigated area was identified first. The seawater characteristics

361

(temperature and salinity) of each water mass in autumn and summer were taken from

362

the values that were reported in previous studies (Han et al., 2013; Kim et al., 2006).

363

As shown in Fig. 5, CDW, TWC, KSC and KSSC were identified to be the major

364

water masses contributing to the ocean circulation in the ECS in autumn (Figs. 5A and

365

5B), while CDW, TWC, KSSC and KIC were the major water masses in summer

366

(Figs. 5C and 5D). This was consistent with the findings of previous studies (Shi et al.,

367

2014; Wang and Chen, 1998). As shown in Table 3, the CDW and KSSC dominated

368

water masses had relatively higher concentrations of THg (4.4 ng/L in autumn and 5.2

369

ng/L in summer for CDW, and 5.1 ng/L in autumn and 4.8 ng/L in summer for KSSC)

370

in comparison to the TWC (3.3 ng/L in autumn and 4.1 ng/L in summer) and KSC

371

(3.3 ng/L in autumn). The distribution patterns of the THg presented a good

372

correlation with the water mass composition. High THg values were mainly observed

373

in areas that were highly affected by the CDW and KSSC. For instance, the high THg

374

concentration areas in the nearshore shallow water of sections 1-X and 7-X in both

375

seasons were identified to be caused by the CDW (Figs. 5A-5C). The KSSC resulted

376

in high concentrations of Hg in the deep water of section 1-X in autumn (Figs. 5A).

377

These results suggest that water mass mixing may play a dominant role in controlling

378

the THg distribution in the ECS. The Yangtze River discharges approximate 144 tons

379

of Hg into the ECS (Wang et al., 2016), and the concentration of Hg in the Yangtze

380

River could be as high as 123.1 ng/L (this study) or 133 ng/L (Zheng et al., 2009). 18

381

This could result in high concentrations of THg in areas that are highly affected by the

382

Yangtze River. The bottom water of the ECS can be disturbed by the intrusion of the

383

KSSC, which led to the resuspension process in sediment. The resuspension process

384

might release certain amounts of labile mercury into the upper water (Li et al., 2014;

385

Liu et al., 2017). This may be the reason for the high Hg concentration in the deep

386

water that is affected by the KSSC.

387

For MeHg (Table 3), the KSC in autumn (0.28 ng/L) and the CDW in the

388

summer (0.27 ng/L) had the highest concentrations of MeHg, while the TWC in

389

autumn had the lowest MeHg concentration (0.19 ng/L). The distribution of MeHg

390

presented similar patterns with the water mass composition. The major areas with

391

high values of MeHg were greatly affected by the CDW and KSC. For instance, high

392

MeHg concentrations were observed in the nearshore shallow water of sections 1-X in

393

both seasons (Figs. 6A and 6C), which were mainly caused by the CDW (Figs.

394

6A-6C). The KSC resulted in a high MeHg concentration in the offshore shallow

395

water of section 7-X in autumn (Fig. 6B). The KSSC and KIC were observed to have

396

high concentration in deeper water of sections 1-X and 7-X in summer (Figs. 6C and

397

6D). These results suggest that water mass mixing may also play an important role in

398

controlling the MeHg distribution in the ECS. The Yangtze River discharged

399

approximately 0.9 t/a of MeHg into the ECS (as estimated by Liu et al., 2016). This

400

would lead to high concentrations of MeHg in the northern nearshore water.

401

5. Conclusion

402

In this study, two cruises were conducted in October 2014 and June 2015 in order 19

403

to investigate the distribution and speciation of Hg in the ECS and their controlling

404

factors. Correlation analysis and multiple regression analysis were performed to

405

identify the major environmental factors affecting the distribution of THg and MeHg

406

in the ECS. The influence of water masses was also investigated by using the T-S

407

diagram method.

408

The THg and MeHg concentrations in ECS water were found to occupy the

409

higher ranks among the marginal seas (at the last quartile of reported concentrations),

410

thus indicating severe Hg contamination in the ECS system. Spearman’s correlation

411

analysis showed that T and Hg in sediment may be the controlling factors for THg

412

distribution, while dissolved organic matter (DOM), temperature and MeHg in the

413

sediment may be the controlling factor for MeHg distribution in the seawater of the

414

ECS. The relative importance of these environmental factors in Hg distribution

415

depends on the water depth. Multiple regression analysis further suggests that DOM

416

may be the most important environmental factor controlling MeHg in the ECS. DOM

417

can significantly affect Hg methylation and demethylation in the water column. These

418

results indicate that in situ production/reduction processes may control the levels of

419

MeHg in the ECS. This finding highlights the importance of investigating Hg in-situ

420

methylation and demethylation in the ECS in future studies.

421

By using the T-S diagram method, the CDW, TWC, KSC and KSSC were

422

identified to be the major water masses contributing to the ocean circulation in the

423

ECS in autumn, while the CDW, TWC, KIC and KSSC were identified to be the

424

major water masses in summer. The CDW, KSC, and KSSC played much more 20

425

important roles in controlling distribution of Hg in autumn, while the CDW and

426

KSSC were more important for Hg distribution in summer. The CDW had the highest

427

THg and MeHg concentrations in summer, while the THg in KSSC and MeHg in

428

KSC also were relatively higher in comparison to the TWC and KIC. The distribution

429

patterns of the THg and MeHg had a good correlation with the water mass

430

composition, thus indicating that water mass mixing may play a dominant role in

431

controlling the THg and MeHg distributions in the ECS.

432

However, due to the limited information provided by T-S diagram method,

433

uncertainty still existed with respect to the identified dominant water mass. Multiple

434

chemical tracing methods, e.g., Nd isotopic proportion and rare earth elements and

435

isotopes, should be adopted in future studies in order to improve the accuracy of the

436

water mass identification (Che and Zhang, 2018; Zhang et al, 2018). In situ Hg

437

methylation and demethylation incubation experiments should also be conducted in

438

future studies in order to estimate the in situ production flux of MeHg in ECS water.

439

ACKNOWLEDGEMENT

440

This research was partially supported by the National Natural Science Foundation of China

441

(21577134) and the Fundamental Research Funds for the Central Universities (201762031,

442

201841008).

21

443

References

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657

26

658

Figure Captions

659

Fig. 1. The horizontal distribution of THg (ng/L) in the East China Sea in autumn

660

2014 (A-D) and summer 2015 (a-d).

661

Fig. 2. The vertical distribution of THg (ng/L) in the East China Sea in autumn 2014

662

(A-D) and summer 2015 (a-d).

663

Fig. 3. The horizontal distribution of MeHg (ng/L) in the East China Sea in autumn

664

2014 (A-D) and summer 2015 (a-d).

665

Fig. 4. The vertical distribution of MeHg (ng/L) in the East China Sea in autumn 2014

666

(A-D) and summer 2015 (a-d).

667

Fig. 5. The vertical distribution of the water masses and the THg in the East China

668

Sea in autumn 2014 (A and B) and summer 2015 (C and D). The red solid circle

669

represents the TWC (Taiwan Warm Current). The green solid circle represents the

670

CDW (Changjiang Diluted River). The yellow solid circle represents the KSC

671

(Kuroshio Surface Current). The purple solid circle represents the KSSC (Kuroshio

672

Sub Surface Current). The blue solid circle represents the KIC (Kuroshio Intermediate

673

Current). The black solid circle represents the mixing water.

674

Fig. 6. The vertical distribution of the water masses and MeHg in the East China Sea

675

in autumn 2014 (A and B) and summer 2015 (C and D). The red solid circle

676

represents the TWC (Taiwan Warm Current). The green solid circle represents the

677

CDW (Changjiang Diluted River). The yellow solid circle represents the KSC

678

(Kuroshio Surface Current). The purple solid circle represents the KSSC (Kuroshio

679

Sub Surface Current). The blue solid circle represents the KIC (Kuroshio Intermediate 27

680

Current). The black solid circle represents the mixing water.

681

Fig. 1.

682

28

683

Fig. 2.

684

29

685

Fig. 3.

686

30

687

Fig. 4.

31

688

Fig. 5.

689 690

32

691

Fig. 6.

692 693

33

694

Table 1. The THg and MeHg concentrations and the MeHg/THg ratio in the ECS and

695

other marine systems. The THg and MeHg concentrations in the Yangtze River water

696

samples that were collected at Xuliujing station were measured monthly from

697

November 2017 to March 2018.

Area (water)

Open Ocean

Marginal Sea

THg

MeHg

Ratio

(ng/L)

(ng/L)

(%)

Canadian Arctic

0.42

0.016

3.81

(Heimbürger et al., 2015)

Amundsen and Ross Seas

0.6

0.04

6.6

(Mastromonaco et al., 2017)

North Atlantic Ocean

0.17

0.020

8.9

(Lang et al., 2017)

Pacific Ocean

0.2

0.01

5.00

(Laurier et al., 2003)

N. Atlantic Ocean

0.2

0.01

5.00

(Aspmo et al., 2006)

Arctic

0.4

0.02

5.00

(Biswas et al., 2017)

Kagoshima Bay, Japan

1.54

0.36

23.3

(Ando et al., 2010)

San Francisco Bay

8.1

0.11

1.35

(Cloern and Jassby, 2012)

Ionian Sea

7.5

0.14

1.87

(Ferrara et al., 1990)

Mediterranean Sea

0.3

0.06

20

(Cossa et al., 2017)

Adriatic Sea

0.7

0.11

16

(Živković et al., 2017)

Alboran Sea

5.3

0.05

0.94

(Cossa et al., 1994)

East China Sea (2014.10)

3.9±3.3

0.27±0.14

6.7±4.4

Surface

2.9±2.3

0.27±0.12

9.8±6.7

10 m

4.3±3.8

0.26±0.12

6.7±4.1

20 m

4.0±2.6

0.25±0.09

6.0±2.5

Bottom

4.9±3.6

0.31±0.17

5.6±3.8

East China Sea (2015.6)

4.3±1.9

0.22±0.11

6.6±5.6

Surface

4.6±2.3

0.18±0.09

5.5±4.6

10 m

4.3±1.9

0.22±0.13

6.4±4.2

20 m

4.7±2.1

0.23±0.13

6.8±6.7

Bottom

4.1±1.6

0.23±0.13

6.9±5.3

Yangtze River

123.1±23.3

0.94±0.17

0.7±0.2

34

Reference

This study

This study

This study

698

Table 2. Correlation analyses and multiple regression analyses of the THg, MeHg and

699

the MeHg/THg ratio in water against the environmental parameters. The

700

environmental factors that had significant correlations with the THg or MeHg were

701

labelled in bold. The β values can be used to evaluate the relative importance of the

702

environmental factors. The data of the THg, MeHg and the MeHg/THg ratio were log

703

transformed in order to follow a normal distribution.

704 Parameters

R of correlation analysis logTHgw

logMeHgw

Multiple regression analysis

Multiple regression analysis

logMeHgw

log(ratio)w

log(ratio)w β

r

2

p

β

T

-0.03

-0.09

-0.00

-

-

S

0.09

-0.05

-

-0.09

DO

-0.03

-0.01

-0.14﹡ 0.09

-

-

DOM

0.11

-0.18﹡

-0.21

0.05 0.02

0.13﹡ -0.10

-0.21﹡﹡ 0.00

-0.24

-

-0.07

-

-

logTHgw

/

0.09

-0.08

-

-

logTHgs

/

/

/

-

-

logMeHgs

/

/

/

-

-

NO3 SO4

2-

0.03

35

0.16

<0.001

-

r2

p

0.05

<0.01

705

Table 3. The THg and MeHg concentrations in the different water masses of the East

706

China Sea. CDW represents the Changjiang Diluted Water. TWC represents the

707

Taiwan Warm Current. KSC represents the Kuroshio Surface Current. KSSC

708

represents the Kuroshio Sub Surface Current. KIC represents the Kuroshio

709

Intermediate Current. THg (ng/L)

Water Mass

MeHg (ng/L)

autumn

summer

autumn

summer

CDW

4.4 ±4.1 (0.6-11.8)

5.2 ±2.2 (2.7-7.1)

0.23 ±0.11 (0.09-0.43)

0.27 ±0.10 (0.09-0.43)

TWC

3.3 ±3.0 (0.5-11.8)

4.1 ±2.2 (1.5-9.4)

0.19 ±0.09(0.09-0.38)

0.25 ±0.05 (0.18-0.32)

KSC

3.3 ±3.3 (0.5-11.9)

KSSC

5.1 ±2.8 (ND-8.0)

KIC

0.28 ±0.11 (ND-0.43) 4.8 ±2.3 (ND-9.6) 4.3 ±2.6 (1.3-6.0)

710

36

0.24 ±0.09 (0.09-0.38)

0.22 ±0.13 (ND-0.61) 0.22 ±0.12 (0.09-0.33)

Highlights 1.

The level of Hg in the East China Sea (ECS) was among the highest among the marginal seas.

2.

High concentrations of THg and MeHg were located in both the nearshore and offshore areas of the ECS.

3.

Dissolved organic matter (DOM) may be the controlling environmental factor for MeHg in ECS seawater.

4.

Water mass mixing may play an important role in controlling THg and MeHg in the ECS.

Liu Chang: Investigation, Writing-Original draft. Chen Lufeng: Methodology, Writing – review & editing. Liang Shengkang: Investigation, Writing- Reviewing and Editing. Li Yanbin: Conceptualization, Methodology, Writing-Original draft, Writing – review & editing.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: