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Spatial Differences and Influencing Factors of Dissolved Gaseous Mercury in the Yellow Sea and Bohai Sea in Spring

Ruhai LIU Wen ZHENG Xixi CHONG Yan WANG Dan YI

LIU Ruhai, ZHENG Wen, CHONG Xixi, WANG Yan, YI Dan, 2021. Spatial Differences and Influencing Factors of Dissolved Gaseous Mercury in the Yellow Sea and Bohai Sea in Spring. Chinese Geographical Science, 31(1): 137−148 doi:  10.1007/s11769-021-1180-1
Citation: LIU Ruhai, ZHENG Wen, CHONG Xixi, WANG Yan, YI Dan, 2021. Spatial Differences and Influencing Factors of Dissolved Gaseous Mercury in the Yellow Sea and Bohai Sea in Spring. Chinese Geographical Science, 31(1): 137−148 doi:  10.1007/s11769-021-1180-1

doi: 10.1007/s11769-021-1180-1

Spatial Differences and Influencing Factors of Dissolved Gaseous Mercury in the Yellow Sea and Bohai Sea in Spring

Funds: Under the auspices of the National Natural Science Foundation of China (No. 41506128) , Shandong Natural Science Foundation (No. ZR2018MD004)
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  • Figure  1.  Location of the sampling stations in spring 2018, B01−B22 are in the Bohai Sea, N01−N20 are in the North Yellow Sea, and H01−H30 are in the South Yellow Sea

    Figure  2.  Spatial distributions dissolved gaseous mercury (DGM, pg/L) (A), reactive mercury(RHg, ng/L) (B), total mercury (THg, ng/L) (C), water temperature (℃) (D), chlorophyll-a (μg/L) (E) and relative humidity (F) in surface seawater of Yellow and Bohai Seas

    Figure  3.  Spatial distribution of dissolved gaseous mercury (DGM, pg/L) (A), reactive mercury (RHg, ng/L) (B) and dissolved oxygen (DO, mg/L) (C) for the bottom water of Yellow and Bohai Seas

    Figure  4.  Spatial distribution of gaseous mercury (DGM, pg/L) (A), reactive mercury (RHg, ng/L) (B). and water temperature (℃) (C). for the middle water of Yellow and Bohai Seas

    Figure  5.  Relationships between dissolved gaseous mercury (DGM) and reactive mercury (RHg) for each layer of the Yellow and Bohai Seas

    Table  1.   Comparison of dissolved gaseous mercury (DGM), active mercury (RHg) and their parameters at different depths in South Yellow Sea, North Yellow Sea, and Bohai Sea

    Sea areaWater layerDGM/(pg/L)RHg/(ng/L)Depth / mWater temperature /℃DO/(mg/L)Chlorophyll-a/(µg/L)
    South Yellow SeaSurface layer70.4 ± 53.8a0.63 ± 0.39b 4.68.910.71.78
    Middle layer61.8 ± 51.0a0.34 ± 0.29a18.08.210.41.69
    Bottom layer75.1 ± 66.6a0.63 ± 0.36b42.58.010.01.36
    North Yellow SeaSurface layer21.4 ± 7.4a0.12 ± 0.07a4.15.112.01.28
    Middle layer19.5 ± 11.1a0.13 ± 0.11a19.55.011.81.46
    Bottom layer44.6 ± 37.8b0.13 ± 0.07a43.94.811.51.70
    Bohai SeaSurface layer29.4 ± 26.5a0.16 ± 0.05a3.65.012.21.52
    Middle layer35.0 ± 19.9a0.13 ± 0.10a10.94.911.91.81
    Bottom layer40.2 ± 38.1a0.16 ± 0.09a22.84.711.42.33
    Notes: Significant differences among DGM, RHg at different depths in different sea areas are marked with different letters of a and b (P < 0.05)
    下载: 导出CSV

    Table  2.   Comparison of dissolved gaseous mercury (DGM), active mercury (RHg) and total mercury (THg) concentrations in surface water in the Yellow and Bohai Seas with other sea areas

    Sea areaPeriodDGM/(pg/L)RHg/(ng/L)THg/(ng/L)References
    South Yellow SeaSpring 201870.4 ± 53.80.63 ± 0.392.81 ± 1.67This study
    North Yellow SeaSpring 201821.4 ± 7.40.12 ± 0.072.35 ± 1.64This study
    Bohai SeaSpring 201829.4 ± 26.50.16 ± 0.053.18 ± 1.90This study
    Yellow Sea and Bohai SeaSpring 201844.3 ± 43.90.44 ± 0.632.65 ± 1.74This study
    South Yellow SeaFall 201390.9 ± 25.8Wang et al., 2017
    North Yellow Sea, BohaiFall 201363.2 ± 23.4Wang et al., 2017
    Yellow SeaSummer 201649 ± 190.80 ± 0.362.46 ± 0.36Gao, 2017
    Yellow SeaSpring 201227.0 ± 6.8Ci et al., 2015
    East China SeaFall 201349.6 ± 12.51.45 ± 0.62Wang et al., 2016
    South China SeaWinter 201553.7 ± 18.81.40 ± 0.48Ci et al., 2016
    Baltic SeaWinter 199817.4Wängberg et al., 2001
    Minamata Bay coast siteSpring 2014105 ± 462.68 ± 1.91Matsuyama et al., 2018
    Minamata Bay open seaSpring 201468 ± 291.10 ± 0.24Matsuyama et al., 2018
    Coastal waters of the Yellow Sea2008−20090.94 ± 0.282.69 ± 0.78Ci et al., 2011b
    Long Island Sound estuary1995−19978.0 −110.3Rolfhus and Fitzgerald, 2001
    Mediterranean SeaSummer 200030.1 ± 24.10.16 ± 0.060.29 ± 0.08Horvat et al., 2003
    Yellow Sea-North Pacific OceanSpring 201539.6 ± 22.90.33 ± 0.240.75 ± 0.51Liu et al., 2017
    Atlantic Ocean199948.1 ± 32.10.34 ± 0.240.58 ± 0.34Mason and Sullivan, 1999
    Note: − no data
    下载: 导出CSV

    Table  3.   Correlations between dissolved gaseous mercury (DGM), reactive mercury (RHg) and total mercury (THg) with other parameters for each layer in the Yellow and Bohai Seas (n = 72)

    Water layerSpeciesWater temperatureSalinityTurbidityChlorophyll-aDOpHWind speedRelative humidityAir temperature
    Surface waterDGM 0.278*0.238−0.075 0.170−0.247*−0.136−0.206 0.312* 0.013
    RHg0.506**0.2120.1040.167−0.455**−0.224−0.264*0.486**0.254*
    THg−0.1310.006−0.0750.279*0.0500.190−0.2180.377**−0.370**
    Middle waterDGM0.291*0.206−0.1270.281*−0.101−0.172
    RHg0.305*0.0530.130−0.006−0.23−0.246*
    Bottom waterDGM0.331**0.323**0.226−0.113−0.366**−0.167
    RHg0.515**0.266*0.192−0.276*−0.447**−0.251*
    Notes: **Significant correlations at the 0.01 level. * Significant correlations at the 0.05 level; DO, Dissolved oxygen; −, no data
    下载: 导出CSV
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  • 收稿日期:  2020-01-27
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Spatial Differences and Influencing Factors of Dissolved Gaseous Mercury in the Yellow Sea and Bohai Sea in Spring

doi: 10.1007/s11769-021-1180-1
    基金项目:  Under the auspices of the National Natural Science Foundation of China (No. 41506128) , Shandong Natural Science Foundation (No. ZR2018MD004)
    通讯作者: WANG Yan. E-mail: yanjane@ouc.edu.cn

English Abstract

LIU Ruhai, ZHENG Wen, CHONG Xixi, WANG Yan, YI Dan, 2021. Spatial Differences and Influencing Factors of Dissolved Gaseous Mercury in the Yellow Sea and Bohai Sea in Spring. Chinese Geographical Science, 31(1): 137−148 doi:  10.1007/s11769-021-1180-1
Citation: LIU Ruhai, ZHENG Wen, CHONG Xixi, WANG Yan, YI Dan, 2021. Spatial Differences and Influencing Factors of Dissolved Gaseous Mercury in the Yellow Sea and Bohai Sea in Spring. Chinese Geographical Science, 31(1): 137−148 doi:  10.1007/s11769-021-1180-1
    • Mercury (Hg) is a toxic heavy metal that can exist as gaseous element mercury (Hg0) in the atmosphere. It is chemically stable and is transported over long distances through atmospheric circulation, making it a global pollutant (Weiss-Penzias et al., 2003; Driscoll et al., 2007). The sea-air exchange process of mercury is important in the ocean, and approximately 60% of mercury comes from atmospheric deposition, and mercury released from the ocean is estimated to be 2880 t (Soerensen et al., 2014). Therefore, the oceans play an important role in the global mercury cycle. Mercury has many chemical forms in aquatic ecosystems, such as total mercury (THg), dissolved gaseous mercury (DGM), particulate mercury (PHg), reactive mercury (RHg), and methylmercury (CH3Hg). Various forms of mercury can be transformed into each other, and the reactions are complicated. DGM includes dimethylmercury and Hg0, and the low levels of dimethylmercury are easily decomposed, therefore, DGM exists mainly in the form of Hg0. RHg is a type of mercury that can be directly reduced by stannous chloride and is mainly present in the form of Hg2+. There are different factors affecting DGM concentrations in the ocean and previous studies have suggested that photoreduction of Hg2+ is the main method for Hg0 production (Ahn et al., 2010; Ma et al., 2015; Ci et al., 2016) in seawater. Solar radiation and water temperature promote the formation of DGM (Wang et al., 2016; 2017), and high concentration of dissolved organic carbon (DOC) and low pH can promote DGM production (Ahn et al., 2010). There was a negative correlation between DGM concentrations and salinities in the seawater of Minamata Bay (Marumoto and Imai, 2015), surface DGM distribution patterns in coastal seawater are higher than those in other oceans and the sea (Wang et al., 2016). It is also considered that the DGM concentrations in open sea areas are high (Wang et al., 2017).

      Because it is important to assess the function of oceans on the global mercury cycle, research has been done in the Pacific Ocean (Bowman et al., 2016; Liu et al., 2017), the Arctic Ocean (Andersson et al., 2008; Štrok et al., 2015), the Mediterranean (Ferrara et al., 2003; Horvat et al., 2003; Kotnik et al., 2007), and the Antarctic Ocean (Mason and Sullivan, 1999, Mastromonaco et al., 2016). DGM in surface water was studied in the Yellow and Bohai Seas in the summer (Ci et al., 2011a; Cheng et al., 2019), autumn (Ci et al., 2011b; Wang et al., 2017) and winter (Wang et al., 2017; Cheng et al., 2019), these studies were helpful to estimate mercury exchange and to recognize the regional mercury cycle. However, there are relatively few studies on the production of dissolved gaseous mercury and the transformation of mercury in spring. Research on the vertical variations of DGM and DGM sources at different water depths are scarce and are not helpful for studying the source of DGM in the marginal seas of China. Large amount of pollutants generated during the winter heating period (Chong et al., 2019) are transported to the sea by the northerly monsoon winds and settle into the marginal sea through dry and wet deposition. The low temperature and low solar radiation in winter are not conducive to the formation of DGM and the growth of algae in seawater (Wang et al., 2017), therefore, pollutants will accumulate in the sea. The increase in solar radiation, the growth of algae and the other parameters in spring may affect the production of seawater DGM (Xu et al., 2012; Gao et al., 2017). In addition, increase of wet deposition (Soerensen et al., 2014) and water masses (Zheng, 2020) also affect mercury transport in spring. Therefore, this study will further investigate the transformation of mercury in seawater in spring and will be helpful to understand the production mechanism and estimate the sources of atmospheric mercury in China’s marginal seas.

    • The study area includes the Bohai Sea, the southern Yellow Sea and the northern Yellow Sea, which is located in the east of China. The Yellow Sea is located between East China and the Korean peninsula and divided into the southern Yellow Sea (South Yellow Sea) and the northern Yellow Sea (North Yellow Sea) according to the shortest line, which is from Chengshantou of China to Changsangot of Korea. The Bohai Sea is China’s inland sea and is located at 37°07′N−40°56′N, 117°33′E−122°08′E. The Bohai Sea is a semiclosed sea area surrounded by the provinces of Liaoning, Hebei, Shandong and Tianjin of China.

      Seawater samples were collected in the Yellow and Bohai Seas from March 28, 2018 to April 17, 2018 during the spring cruise onboard ‘Dongfanghong 2’, an ocean research ship of Ocean University of China. According to the plan, the cruise was divided into two sections. The first section started from Qingdao and proceeded to the South Yellow Sea. The second stage started from Qingdao and proceeded to the North Yellow Sea and Bohai Sea. Surface (0−5 m), middle (10−20 m) and bottom (20 m below) layers water were collected using a CTD (Sea-Bird 911 plus CTD) and Niskin water sampler at 72 stations (Fig. 1).

      Figure 1.  Location of the sampling stations in spring 2018, B01−B22 are in the Bohai Sea, N01−N20 are in the North Yellow Sea, and H01−H30 are in the South Yellow Sea

    • After the water samples were collected, the seawater samples were bubbled immediately for 30 min with Hg-free nitrogen (Hg in nitrogen was removed by gold trap) at a flow of 0.35 L/min (Wang et al., 2017). DGM in water was trapped by a gold sand tube. 0.1 mL10% SnCl2 was added to another portion of the seawater samples and was then bubbled for 30 min to determine RHg. The gold sand tube was immediately measured on the ship using a cold steam atomic fluorescence spectrometer (CVAFS) (Model III, Brooks Rand Labs, USA).

      THg was measured according to Method 1631 (USEPA, 2002). The seawater samples were fixed with 1∶1 nitric acid (with excellent purity level) and brought back to the laboratory. Water samples were digested with BrCl for 24 h in the sample bottle, then 0.1 mL 20% hydroxylamine hydrochloride and 0.1 mL10% SnCl2 was added to reduce the Hg2+ to Hg0. The THg concentration was determined by method of CVAFS (Merx-T, Brooks Rand Labs, USA).

    • The water environmental parameters (salinity, water temperature, depth, chlorophyll-a and turbidity) were measured by a CTD (Sea-Bird 911 plus CTD) when the water samples were collected. Dissolved oxygen was measured by a portable water quality analyzer (HACH, HQ30), the water pH was measured by a pH meter (HACH HQ40d). The wind speed, relative humidity, and air temperature were provided by the ship’s meteorological observation instruments.

    • Hg concentrations in water were determined using ultra trace Hg techniques in natural water following USEPA Method 1631 (USEPA, 2002). The experimental samplingbottles and bubblers were soaked in hot 4 mol/L HCl aqueous solution then 1% HCl aqueous solution (trace-metal-grade, v/v), and were then thoroughly washed with deionized water to ensure removal of mercury from the experimental vessels. The cleaned bottles were filled with 1% HCl aqueous solution (trace-metal-grade, v/v) and double zip-locked using polyethylene bags until use. A low flow of N2 (99.99%) was maintained to avoid air contamination when the bubbler was not used between stations. The field blanks of DGM and RHg were measured using the first bubbled seawater at the same conditions for another 30 min. The method detection limits for DGM and RHg were 6 pg/L (3 pg) and 10 pg/L (5 pg), respectively. The method detection limit of THg was 0.030 ng/L.

      The Kriging interpolation method was used to draw the contour distribution map. Pearson correlation analysis and ANOVA were used to analyze the influence of environment parameters on the species of mercury.

    • Surface water may be affected by atmospheric parameters and solar radiation except water environment parameters, which is different with the middle and bottom water(Table 1). Therefore, mercury species of surface waterand the other water layer were analyzed respectively.

      Table 1.  Comparison of dissolved gaseous mercury (DGM), active mercury (RHg) and their parameters at different depths in South Yellow Sea, North Yellow Sea, and Bohai Sea

      Sea areaWater layerDGM/(pg/L)RHg/(ng/L)Depth / mWater temperature /℃DO/(mg/L)Chlorophyll-a/(µg/L)
      South Yellow SeaSurface layer70.4 ± 53.8a0.63 ± 0.39b 4.68.910.71.78
      Middle layer61.8 ± 51.0a0.34 ± 0.29a18.08.210.41.69
      Bottom layer75.1 ± 66.6a0.63 ± 0.36b42.58.010.01.36
      North Yellow SeaSurface layer21.4 ± 7.4a0.12 ± 0.07a4.15.112.01.28
      Middle layer19.5 ± 11.1a0.13 ± 0.11a19.55.011.81.46
      Bottom layer44.6 ± 37.8b0.13 ± 0.07a43.94.811.51.70
      Bohai SeaSurface layer29.4 ± 26.5a0.16 ± 0.05a3.65.012.21.52
      Middle layer35.0 ± 19.9a0.13 ± 0.10a10.94.911.91.81
      Bottom layer40.2 ± 38.1a0.16 ± 0.09a22.84.711.42.33
      Notes: Significant differences among DGM, RHg at different depths in different sea areas are marked with different letters of a and b (P < 0.05)
    • During the cruise, the mean DGM concentration in the surface water was (44.3 ± 43.9) pg/L, and ranged from 6.4 to 220.9 pg/L (Table 2). The mean DGM concentrations were (70.4 ± 53.8) pg/L, (21.4 ± 7.4) pg/L and (29.4 ± 26.5) pg/L in the South Yellow Sea, North Yellow Sea and Bohai Sea, respectively (Table 2). DGM concentrations in the South Yellow Sea were higher than those in the North Yellow Sea and Bohai Sea. The highest concentration was observed at station H10 (220.9 pg/L), to the south of the Shandong Peninsula, while the lowest concentration was detected at the N11 station (6.4 pg/L), in the middle of the Bohai Strait. Hg concentrations obtained by other researchers are listed in Table 1. The DGM concentrations in the South Yellow Sea were higher than those in the Mediterranean ((30.1 ± 24.1) pg/L) (Horvat et al., 2003). DGM in the Yellow Sea and Bohai Sea was lower than that in the South China Sea ((53.7 ± 18.8) pg/L, Ci et al., 2016), while it was higher than the concentrations in the Yellow Sea-Northwest Pacific ((39.6 ± 22.9) pg/L, Liu et al., 2017), and in the Baltic Sea (17.4 pg/L, Wängberg et al., 2001). In spring, DGM in the Yellow and Bohai Seas accounted for less than 2.2% of THg, far lower than for the Yellow Sea-Northwest Pacific surface seawater (8.06%, Liu et al., 2017) and for the Mediterranean Sea (20%, Kotnik et al., 2017) and for other open seas. This ratio was close to that of the nearshore waters of the Yellow Sea (3.6%, Ci et al., 2011a; 1.3%, Ci et al., 2011b). Research has also shown that higher levels of DGM/THg occur in deeper seas (Mastromonaco et al., 2017). Mercury concentrations in the oceans and deep seas are low and mercury mainly exists in a dissolved state (Horvat et al., 2003), which is beneficial for mercury reduction. DGM also comes from deeper waters in the oceans. Mercury concentrations in the marginal seas increase because of the input from land by runoff and atmospheric dry and wet deposition. Productivity in the coastal waters is higher (Muller-Karger et al., 2005; Xing et al., 2011), and more organic matter and particulate matter in seawater result in higher particulate mercury contents (Conaway et al., 2003; Ci et al., 2011c), resulting in lower DGM/THg values.

      Table 2.  Comparison of dissolved gaseous mercury (DGM), active mercury (RHg) and total mercury (THg) concentrations in surface water in the Yellow and Bohai Seas with other sea areas

      Sea areaPeriodDGM/(pg/L)RHg/(ng/L)THg/(ng/L)References
      South Yellow SeaSpring 201870.4 ± 53.80.63 ± 0.392.81 ± 1.67This study
      North Yellow SeaSpring 201821.4 ± 7.40.12 ± 0.072.35 ± 1.64This study
      Bohai SeaSpring 201829.4 ± 26.50.16 ± 0.053.18 ± 1.90This study
      Yellow Sea and Bohai SeaSpring 201844.3 ± 43.90.44 ± 0.632.65 ± 1.74This study
      South Yellow SeaFall 201390.9 ± 25.8Wang et al., 2017
      North Yellow Sea, BohaiFall 201363.2 ± 23.4Wang et al., 2017
      Yellow SeaSummer 201649 ± 190.80 ± 0.362.46 ± 0.36Gao, 2017
      Yellow SeaSpring 201227.0 ± 6.8Ci et al., 2015
      East China SeaFall 201349.6 ± 12.51.45 ± 0.62Wang et al., 2016
      South China SeaWinter 201553.7 ± 18.81.40 ± 0.48Ci et al., 2016
      Baltic SeaWinter 199817.4Wängberg et al., 2001
      Minamata Bay coast siteSpring 2014105 ± 462.68 ± 1.91Matsuyama et al., 2018
      Minamata Bay open seaSpring 201468 ± 291.10 ± 0.24Matsuyama et al., 2018
      Coastal waters of the Yellow Sea2008−20090.94 ± 0.282.69 ± 0.78Ci et al., 2011b
      Long Island Sound estuary1995−19978.0 −110.3Rolfhus and Fitzgerald, 2001
      Mediterranean SeaSummer 200030.1 ± 24.10.16 ± 0.060.29 ± 0.08Horvat et al., 2003
      Yellow Sea-North Pacific OceanSpring 201539.6 ± 22.90.33 ± 0.240.75 ± 0.51Liu et al., 2017
      Atlantic Ocean199948.1 ± 32.10.34 ± 0.240.58 ± 0.34Mason and Sullivan, 1999
      Note: − no data

      High DGM concentrations were observed in southeastern part of Qingdao, the central part of the southern Yellow Sea, northeastern Bohai Bay, and in the coastal waters of Dalian, while the lowest concentrations were present in the northern Yellow Sea and in the central Bohai Sea. In the southern Yellow Sea, DGM concentrations near the Yangtze River Estuary were lower than those in the central part of the South Yellow Sea and were different from what was observed in the fall of 2013. In the fall of 2013, DGM concentrations in the Yangtze River estuary and in the southern Yellow Sea were higher (Wang et al., 2017). During this period of this study, most of the southern Yellow Sea cruise had foggy conditions with high relative humidity (Fig. 2). The weakened solar radiation did not benefit the production of seawater DGM. The DGM concentrations on the surface of the Bohai Sea were higher in the offshore area and lower in the central area. As an enclosed sea, Bohai Sea is polluted by pollutants discharged from coastal human activities and rivers. Pollutants in coastal areas might benefit DGM formation.

      Figure 2.  Spatial distributions dissolved gaseous mercury (DGM, pg/L) (A), reactive mercury(RHg, ng/L) (B), total mercury (THg, ng/L) (C), water temperature (℃) (D), chlorophyll-a (μg/L) (E) and relative humidity (F) in surface seawater of Yellow and Bohai Seas

    • The average RHg concentration in the surface seawater of the Yellow and Bohai Seas is (0.44 ± 0.63) ng/L (Table 2), ranging from 0.03 to 4.40 ng/L. The highest RHg value in surface water appeared in the southern sea area of Shandong Province, and the RHg concentrations in the central of South Yellow Sea were also much higher. The average concentrations of RHg in the surface seawaters of the South Yellow Sea, the North Yellow Sea and the Bohai Sea were (0.63 ± 0.39) ng/L, (0.12 ± 0.07) ng/L, and (0.16 ± 0.05) ng/L, respectively (Table 2). The concentrations for the South Yellow Sea were higher than for the North Yellow Sea and the Bohai Sea. The RHg concentrations in seawater were lower in the north and higher in the south. The South Yellow Sea concentrations in spring were lower than those in summer ((1.08 ± 0.28) ng/L, Ci et al., 2011a), higher than for the Yellow Sea-Northwest Pacific Ocean (0.33 ng/L, Liu et al., 2016) and Atlantic Ocean (0.34 ng/L, Mason et al., 1999) (Table 2). Due to low seawater temperatures and foggy conditions in the South Yellow Sea, photooxidation weakens, resulting in lower RHg production in the seawater. The RHg concentrations in the surface waters of the Yellow Sea are similar to the DGM distributions, implying that DGM and RHg may be affected by the same factors. Moreover, the content of THg in the coastal areas of China is higher than that in open seas abroad, resulting in a lower RHg / THg ratio. The ratios of the percentages of RHg to THg in the surface layer of the Yellow Sea and Bohai Sea were between 1% and 59%, with an average of 17%, which was lower than in summer (29.9%, Gao, 2017). RHg / THg was also lower than that in the open sea or ocean. For example, RHg / THg ranged from 4% and 63%, with an average of 31% in the Mediterranean in spring (Kotnik et al., 2007). RHg / THg values ranged from 15% to 89%, with an average of 48% in the Yellow Sea-North Atlantic Ocean in spring (Liu et al., 2017) and were approximately 59% in the southern equatorial Atlantic (Mason and Sullivan, 1999).

      THg concentrations in the surface water of the Yellow and Bohai Seas were (2.65 ± 1.74) ng/L, ranging between 0.34 and 7.23 ng/L. Higher THg concentrations appeared in the center of the South Yellow Sea, the Bohai Strait, south of Dalian Yalu River estuary. The average concentrations were higher than the Yellow Sea in summer ((1.69 ± 0.35) ng/L, Ci et al., 2011a) and the South China Sea in winter ((1.40 ± 0.48) ng/L, Ci et al., 2016). The concentrations of THg in the Yellow Sea and Bohai Sea was much higher than for other oceans and deep seas, such as the Mediterranean Sea ((0.24 ± 0.06) ng/L, Kotnik et al., 2007) and the Northern Pacific Ocean ((0.23 ± 0.17) ng/L, Laurier et al., 2004), reflecting the influence of land on the marginal sea of China.

    • The DGM concentrations in the middle layer of the Yellow and Bohai Seas were 5.1–206.0 pg/L, with an average of (43.4 ± 39.9) pg/L. The South Yellow Sea had the highest average concentration ((61.8 ± 51.0) pg/L), followed by the Bohai Sea and the lowest concentration was in the North Yellow Sea at (19.5 ± 11.1) pg/L (Table 2). DGM concentrations in the bottom seawater were 9.8–284.0 pg/L, showing an average value of (56.5 ± 55.3) pg/L, with the following relationships being observed: South Yellow Sea > North Yellow Sea > Bohai Sea, which was different from the pattern of surface seawater. Higher DGM in bottom water appeared in the southeastern part of the South Yellow Sea, the southeastern part of the Shandong Peninsula and the central Bohai Sea (Fig. 3). The high value areas in the southeastern part of the South Yellow Sea and the central Bohai Sea all appeared in the deeper sea areas. With increases in depth for the southeastern region of the South Yellow Sea, the DGM concentrations in the bottom layer increased.

      Figure 3.  Spatial distribution of dissolved gaseous mercury (DGM, pg/L) (A), reactive mercury (RHg, ng/L) (B) and dissolved oxygen (DO, mg/L) (C) for the bottom water of Yellow and Bohai Seas

      The mean DGM in the bottom layer and the surface layer were generally greater than for the middle layer. The South Yellow Sea was characterized by bottom layer > surface layer > middle layer, while the North Yellow Sea and Bohai Sea were characterized by bottom layer > middle layer > surface layer (Table 1). The DGM concentrations in the bottom seawater were the highest. DGM concentrations in the Yellow Sea for the bottom layer were higher than for the surface water and were smallest for the middle layer. DGM concentrations in the Bohai Sea area increased gradually with increasing seawater depths. This property is different from that in summer (DGM decreased with depth) (Gao, 2017). This is mainly due to low water temperatures in the spring and weak solar radiation, especially during foggy weather in the South Yellow Sea. The amount of DGM produced by photoreduction in the surface seawater was small. The DGM released from the bottom seawater or sediment plays an important role. Gao et al. (2017) stated that the surface layer and middle layer seawater have low turbidity and high chlorophyll-a content, which promotes the photoreduction of DGM in summer. The increased turbidity due to high river runoff in summer is not conducive to photoreduction, which causes the seasonal difference. DGM released by the bottom sediment was larger than the surface seawater photoreduction in spring, due to the influence of fog, weak solar radiation and low water temperature.

    • The RHg concentrations in the North Yellow Sea and the Bohai Sea were much lower, and were close to those of the Mediterranean Sea ((0.16 ± 0.06) ng/L, Horvat et al., 2003). The high RHg values in middle water areas appeared mainly in the Bohai Strait, southeastern Shandong and on the eastern side of the Yangtze River estuary (Fig. 4). The highest values (0.99 ng/L) appeared in the southeastern Shandong sea area. The average concentration of RHg in the bottom layer was (0.35 ± 0.34) ng/L, ranging from 0.06 to 1.62 ng/L. The concentration distribution for RHg in bottom water was similar to the middle layer. The highest value appeared in southeastern Shandong and the east south of the Yellow Sea. The RHg concentrations in the sea near the mouth of the Yalu River, Huaihe River and the coast of the Bohai Sea were higher.

      Figure 4.  Spatial distribution of gaseous mercury (DGM, pg/L) (A), reactive mercury (RHg, ng/L) (B). and water temperature (℃) (C). for the middle water of Yellow and Bohai Seas

      RHg concentration differences in the vertical direction were not obvious. The RHg concentrations at the surface of the South Yellow Sea and Bohai Sea were larger than for the middle layer. Mercury in surface waters benefits the photooxidation of mercury. Higher RHg concentrations in bottom water might be due to releases from sediment. RHg concentrations in the surface and bottom layers of the South Yellow Sea were significantly higher than those in the middle layer (P < 0.05) (Table 1). The vertical variation difference of RHg concentrations between the North Yellow Sea and the Bohai Sea were small. The RHg concentrations in the South Yellow Sea were much higher than those in the North Yellow Sea and in the Bohai Sea. The differences in water temperature caused by latitude differences and the warm currents of the Yellow Sea affected the production and distribution of DGM and RHg in the South Yellow Sea (Gao et al., 2017). However, the temperature differences between the North Yellow Sea and the Bohai Sea were relatively small, and the vertical differences were small in these sea areas.

    • DGM in surface seawater is derived from the photoreduction of RHg in coastal seawaters (Wang et al., 2017). A significant positive relationship between surface DGM concentrations and RHg was seen in the Yellow Sea and Bohai Sea (r = 0.81, P < 0.01) (Fig. 5) and indicates that the generation of DGM and RHg may be controlled by the same factors. Mercury in seawater usually exists in the form of complex states, organic matter binding states, and particle states. Under solar radiation, mercury in these forms can be photooxidized and converted into RHg. RHg is more easily reduced by solar radiation and converted DGM. Net production of DGM depends on the RHg concentrations and redox equilibria in seawater (Whalin et al., 2007; Qureshi et al., 2010). DGM in the surface water of the Yellow Sea and Bohai Sea correlated positively with water temperatures (r = 0.278, P < 0.05) (Table 3). Water temperature has an important influence on the reduction rate of RHg. Some Hg2+ combined with organic or inorganic ligands is gradually released with increases in water temperature (Xu et al., 2012), which increases RHg concentrations in seawater. Affected by the Yellow Sea warm current (branches of the Kuroshio Current), the higher water temperatures in the central part of the South Yellow Sea correspond to higher RHg concentrations. These high concentrations were the result from the higher concentrations of RHg seawater being transported northward by the Yellow Sea warm current. The higher value areas of DGM and RHg appeared at the northern end of the warm current in the Yellow Sea, which may be related to the convergence of the Yellow Sea warm current and the cold water mass of the Yellow Sea. Other studies have also suggested that water temperature has an effect on DGM concentrations (Xu et al., 2012; Ci et al., 2015; Marumoto and Imai, 2015; Gao et al., 2017; Wang et al., 2017). Water temperature can reflect light intensity to a certain extent. Under the influence of the Yellow Sea warm current in the South Yellow Sea, the water temperature contours are curved. The surface water temperature contours of the northern Yellow Sea to the Bohai Sea are approximately parallel, reflecting the influence of solar radiation at different latitudes (Fig. 2d). The higher water temperatures were influenced by the light intensity, and higher DGM concentrations are produced by photoreduction in surface seawater. Studies have shown that photoreduction of divalent mercury is another important mechanism for Hg0 production in water environments (Amyot et al., 1997; Costa and Liss, 1999; Costa et al., 2000; Horvat et al., 2003). Solar radiation plays an important role in the generation of DGM (Ma et al., 2015; Liu et al., 2017). Some studies have found that ultraviolet light (Ma et al., 2015), chlorine or bromine (Horvat et al., 2003) and ·OH conditions can also oxidize Hg0 (Mason et al., 2001).

      Figure 5.  Relationships between dissolved gaseous mercury (DGM) and reactive mercury (RHg) for each layer of the Yellow and Bohai Seas

      Table 3.  Correlations between dissolved gaseous mercury (DGM), reactive mercury (RHg) and total mercury (THg) with other parameters for each layer in the Yellow and Bohai Seas (n = 72)

      Water layerSpeciesWater temperatureSalinityTurbidityChlorophyll-aDOpHWind speedRelative humidityAir temperature
      Surface waterDGM 0.278*0.238−0.075 0.170−0.247*−0.136−0.206 0.312* 0.013
      RHg0.506**0.2120.1040.167−0.455**−0.224−0.264*0.486**0.254*
      THg−0.1310.006−0.0750.279*0.0500.190−0.2180.377**−0.370**
      Middle waterDGM0.291*0.206−0.1270.281*−0.101−0.172
      RHg0.305*0.0530.130−0.006−0.23−0.246*
      Bottom waterDGM0.331**0.323**0.226−0.113−0.366**−0.167
      RHg0.515**0.266*0.192−0.276*−0.447**−0.251*
      Notes: **Significant correlations at the 0.01 level. * Significant correlations at the 0.05 level; DO, Dissolved oxygen; −, no data

      The DGM/RHg ratios ranged from 3.21% to 42.99%, with an average of 15.10%. The ratios were higher than observed during the summer research cruise (Gao et al., 2017). The low ratios of RHg to THg in the Yellow and Bohai Seas in spring indicated that the conversion efficiency between these is low. This indicates that the weak solar radiation in the spring, foggy conditions and lower water temperatures may affect RHg production in seawater, which is not conducive to the formation and release of DGM.

      DGM concentrations did not significantly correlate with THg during the spring cruise. THg was not the main factor controlling DGM concentrations. One of the sources of mercury in the ocean is the deposition of atmospheric particulate matter (Mason et al., 2012). Studies have shown that the deposition of gaseous mercury is higher in spring and that the deposition of mercury is related to particles in the air (Mastromonaco et al., 2016). The relative humidity during the spring cruise was higher (84.15%), and the majority of the stations in the southern Yellow Sea were cloudy and foggy conditions, with an average relative humidity of 98.20% according to the voyage record. The higher THg concentrations in spring may be caused by a high level of coal burning in winter, which led to an increase in the concentrations of particulate mercury in the atmosphere and to the deposition flux of particulate mercury in the spring period with atmospheric circulation (Mastromonaco et al., 2017). THg concentrations in the Yellow Sea and Bohai Sea area significantly correlated with the relative humidities (r = 0.377, P < 0.01) (Table 3). The surface water THg concentration was 3.23 ng/L for foggy days, and 2.45 ng/L for sunny days in the Yellow Sea. It has been found that THg concentrations in wet deposition were 50 times higher than those in seawater (Soerensen et al., 2014). This process may exist in foggy weather, as water vapor condenses with the particulate matter, and gaseous divalent mercury dissolves in the water drops, resulting in higher atmospheric wet deposition, increasing the THg concentrations in surface seawater. We also found that THg correlated positively with chlorophyll-a in surface seawater (r = 0.279, P < 0.05). In the horizontal distribution maps of THg and chlorophyll-a, their maximum locations were consistent. In the spring, algae blooms in the partial sea areas, and algae can absorb and accumulate mercury in the water, thus increasing THg concentrations in seawater.

    • There was also a significant correlation between DGM and RHg in the bottom seawater (r = 0.38, P < 0.01), but the correlation of the bottom seawater was weaker than that of the surface and middle layers of seawater (Fig. 5). The bottom seawater is close to the sediment and is influenced by the underlying sediments. Depths in the marginal sea are affected by the continental shelf, which caused different dissolved oxygen concentrations. Water temperature is also influenced by warm currents and latitude. The relatively higher water temperatures and the lower-oxygen environment at the bottom facilitate the anaerobic reactions of sediments and promote the production of DGM. The bottom seawater DGM concentrations correlated positively with the water temperatures (r = 0.331, P < 0.01). The higher the water temperature, the stronger the microbial activity of the bottom seawater and in the underlying sediment; therefore, the divalent mercury is more easily reduced anaerobically by microorganisms and DGM is released into the overlying seawater, resulting in the increase in DGM concentrations in the bottom seawater. The seawater temperatures are affected by the warm current and latitude of the Yellow Sea in spring, and higher water temperatures in the central-south of the South Yellow Sea resulted in the corresponding maximum DGM concentrations in the bottom seawater.

      DGM in the bottom seawater correlated negatively with DO (r = −0.366, P < 0.01) (Table 3), and the correlation of the bottom seawater was stronger than for the middle and surface waters. DGM concentrations decreased with increasing DO, which is consistent with the results of Bratkič (2016) and Kotnik (2017). DO decreases gradually with increasing depth, while the bottom DO decreases due to the respiration of heterotrophic microorganisms and due to oxygen consumption by organic matter decomposition (Zang et al., 2009).

      There were higher RHg concentrations in different water layers in the southeastern part of the Shandong Peninsula, where the Yellow Sea cold-water mass is located. Pollutants may be brought here and accumulate. High metal contents were found in this sea area and were caused by resuspension (He et al., 2008). The hydrological parameters of the middle seawater were affected by both the surface and the bottom seawater, resulting in weak correlations between DO and DGM.

    • Chlorophyll-a concentrations reflect primary productivity in the sea. Algae bloom at appropriate water temperatures, with adequate nutrients, high transparency and in the presence of spores. We found that the correlation coefficients between chlorophyll-a and DGM in each layer were 0.170, 0.281 (P < 0.05), −0.113, respectively (Table 3). They showed a positive correlation in the surface layer but this was not significant because the DGM produced in the surface seawater is released into the air, and DGM is also influenced by other factors such as solar radiation and water temperature. Therefore, the factors affecting the concentration of surface seawater DGM are complex. However, in the middle seawater, there was a significant correlation between DGM and chlorophyll-a, indicating that the influence of chlorophyll-a on the production of DGM is great, and this may be due to the organic carbon released by algae. Researchers found low levels of DGM in sea areas of the South Atlantic with low chlorophyll-a values (Bratkič et al., 2016). Some researchers believe that algae can convert Hg2+ into DGM, and the rate of Hg2+ reduction is proportional to the concentration of chlorophyll-a (Wu et al., 2014); Phytoplankton and cell secretions in seawater may also promote DGM production in the Yellow and Bohai Seas (Kim et al., 2016).

      DGM correlated positively with salinity in the seawater of different layers, and only significant correlations were seen for the bottom seawater. High-salinity seawater was present in open seas with high transparency, few suspended particles and organic matter. Low-salinity seawater is located in the estuaries of rivers and coastal sea areas with high levels of suspended particles, which is not beneficial for the photoreduction of mercury (Ma et al., 2015). A positive correlation between salinity and DGM appeared in coastal waters, suggesting that higher salinity may result in a decrease in Hg organic complex content and an increase in Hg0 concentrations (Rolfhus and Fitzgerald, 2001).

    • The mean concentration of DGM in the surface water of the Yellow Sea and Bohai Sea was 44.3 ± 43.9 pg/L, which was close to that of mid-latitude oceans and deep seas. The ratios of DGM to THg in this study were lower than for the deep sea and for the Yellow and Bohai Seas in summer and fall. The concentrations of surface DGM were the highest in the central portion of the South Yellow Sea and were higher than those in the Bohai Sea, and their spatial distributions were consistent with RHg.

      DGM ( r = 0.506, P < 0.01) and RHg ( r = 0.278, P < 0.05) showed significant positive correlations with water temperature in seawater. DGM and RHg were both controlled by solar radiation and by water temperature. Foggy days affected the production of DGM and RHg in spring. Chlorophyll-a influences the production of DGM in the various layers of seawater. Chlorophyll-a in surface seawater was also the main influencing factor of THg. DGM concentrations for the bottom seawater were mainly influenced by DO ( r = −0.366, P < 0.01) and water temperature (r = 0.331, P < 0.01), produced by the anaerobic reaction of the bottom seawater and by sediment microorganisms. The correlations between DGM and RHg in seawater gradually decreased from the surface layer to the bottom layer.

      DGM concentrations in the bottom seawater were greatest for different sea areas, and DGM in the sediment and the bottom water plays a more important role in spring. The concentrations of DGM and RHg in the surface and bottom layers of the South Yellow Sea were relatively greater than those in the middle layer. The vertical differences were small in the North Yellow Sea and the Bohai Sea. The production and distribution of DGM and RHg in the South Yellow Sea in spring were affected by the differences of latitude and by the Yellow Sea warm current.

    • This cruise was conducted onboard R/V ‘Dongfanghong 2’by Ocean University of China supported by the Oceanographic Research Vessel Sharing Plan.

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