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In the bottom layer the Hg content controlled by both Seasonal variation and transportation source

Based on the investigation data of Jiaozhou Bay in 1992, the vertical distribution and seasonal variation of Hg in the surface and bottom layers from the center of the bay to the southeast of the bay were studied, and the seasonal distribution, variation range and horizontal distribution trend of Hg content in the surface and bottom layers were determined. The results showed that in May, August and October, in Jiaozhou Bay, from the central water area to the coastal water area in the southeast of the Bay, from the surface layer to the bottom layer, the range of Hg content in the bottom layer and the surface layer was: 0.007-0.040 μg/L, which conformed to the national water quality standard of class I seawater, and the water was not polluted by any Hg content. From May to October, in the central waters of Jiaozhou Bay, the seasonal changes of Hg content in the surface layer from low to high were spring, summer and autumn, and the seasonal changes of Hg content in the bottom layer from low to high were autumn, spring and summer. In the surface of the coastal waters in the southeast of Jiaozhou Bay, the seasonal changes of Hg content in the surface and bottom layers from low to high were autumn, spring and summer. The author put forward the law of Hg deposition: in summer, marine organisms propagated in large numbers, but in spring and autumn, there was no large number of marine organisms propagated. In this way, the Hg content sediment depended entirely on the combination of zooplankton, phytoplankton and floating particles in the sea water. Therefore, in summer, a large amount of Hg was taken to the seabed, while in spring and autumn, no large amount of Hg was taken to the seabed. In May, August and October, the change of Hg content in the surface and bottom layers fully confirmed the settlement law. Moreover, it was believed that a large number of marine organisms in the water were the main carriers to bring a large number of Hg to the seabed. Therefore, the variation of Hg content in the bottom was determined by the surface Hg content and the vehicles that transport Hg content vertically to the sea bottom, which resulted in the fact that the horizontal distribution trend of Hg content in the surface and bottom was same.


Introduction
Human discharge mercury (Hg) to the ocean, atmosphere and soil in activities. Through the land, sea and atmosphere, a large number of waste water, waste gas and particles containing Hg reach the surface of the ocean, and then migrate vertically to the seabed through the water body [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Therefore, based on the investigation data of Hg in Jiaozhou Bay in 1992, the vertical distribution and seasonal variation of Hg in the surface and bottom of Jiaozhou Bay from the center of the bay to the nearshore were studied, and the seasonal distribution, variation range and horizontal distribution trend of Hg in the surface and bottom waters were determined, showing the seasonal variation and vertical settlement process of Hg in Jiaozhou Bay, which provide scientific basis for the study of Hg vertical

Natural Environments of Jiaozhou
Bay. Jiaozhou Bay is located in the south of Shandong Peninsula. Its geographical location is 120 ° 04 ′ -120 ° 23 ′ E, 35 ° 58 ′ -36 ° 18 ′ N. It is bounded by the line between Tuan island and Xuejia island and connected with the Yellow Sea, covering an area of about 446km 2 , with an average water depth of about 7m. It is a typical semi closed Bay. There are more than ten rivers flowing into the sea in Jiaozhou Bay, among which Dagu River, Yang River and Haibo River, Licun River and Loushan River are the ones with large runoff and sediment concentration. These are all seasonal rivers. The hydrological characteristics of these rivers can alter in different seasons [15,16].

Materials and Methods.
The survey data of Hg in Jiaozhou Bay in May, August and October 1992 were provided by the North Sea Monitoring Center of The State Oceanic Administration. In May, August and October, 2 stations were set up in Jiaozhou Bay to take water samples: 55 and 60 stations ( Figure 1). The samples were taken in May, August and October of 1992 respectively. According to the water depth, water samples were taken (> 10m, surface layer and bottom layer, and < 10m, only surface layer). The Hg content of Jiaozhou Bay water body was investigated according to the national standard method, which was included in the National Marine Monitoring Code (1991) [17].

Surface and Bottom Waters.
In May, from the central water area to the coastal water area in the southeast of the bay, the Hg content in the surface layer was 0.011-0.022 μg/L, and that in the bottom layer was 0.013-0.019 μg/L. This showed that the Hg content in whole surface and bottom layers of the Jiaozhou Bay was less than 0.030 μg/L from the central water area to the coastal water area in the southeast of the Bay, which conformed to the water quality standard of the class I sea water of China. And the water was not polluted by any Hg content.
In August, the content of Hg in the surface layer was 0.021-0.035 μg/L, and that in the bottom layer was 0.021-0.025 μg/L. This showed that the Hg content of the whole surface and bottom layers in Jiaozhou Bay was less than 0.040 μg/L from the central water area to the coastal water area in the southeast of the Bay, which conformed to the water quality standard of class I sea water of China, and the water was not polluted by any Hg content. In October, the Hg content in the surface layer was 0.011-0.040 μg/L, and that in the bottom layer was 0.007-0.018 μg/L. This showed that the Hg content of the whole surface and bottom layers in Jiaozhou Bay was less than 0.050 μg/L from the central water area to the coastal water area in the southeast of the Bay, which conformed to the water quality standard of the class I sea water of China, and the water was not polluted by any Hg content.
Therefore, in May, August and October, in Jiaozhou Bay, from the central water area to the coastal water area in the southeast of the Bay, from the surface layer to the bottom layer, the range of Hg content in the surface and bottom layers was 0.007-0.040 μg/L, which conformed to the national water quality standard of class I seawater, and the water was not polluted by any Hg content.

Seasonal Change of Hg Content in Surface
Layer. From the central water area to the coastal water area in the southeast of the Bay, in May, the Hg content of surface in the water body was 0.011-0.022 μg/L; in August, the Hg content of surface in the water body was 0.021-0.035 μg/L; in October, the Hg content of surface in the water body was 0.011-0.040 μ g / L. This showed that in May, August and October, the range of Hg content in the surface layer was 0.011-0.040 μg/L, and the sequence of Hg content from low to high was May, August and October. Therefore, the seasonal variation of Hg content in water body from low to high was spring, summer and autumn.

Seasonal Change of Hg Content in Bottom Layer.
From the central water area to the coastal water area in the southeast of the Bay, in May, the Hg content of bottom layer in the water was 0.013-0.019 μg/L; in August, the Hg content of bottom layer in the water was 0.021-0.025 μg/L; in October, the Hg content of bottom layer in the water was 0.007-0.018 μg/L. This showed that in May, August and October, the range of Hg content in the bottom layer of the water body was 0.007-0.025 μg/L, and the sequence of Hg content from low to high was May, August and October. Therefore, the seasonal variation of Hg content in the bottom layer of water body from low to high was autumn, spring and summer.

3.4The Variation Range of Hg Content in Surface and Bottom Layers.
In May, when the Hg content in the surface layer was relatively low, between 0.011-0.022 μg/L, the Hg content in the corresponding bottom layer was 0.013-0.019 μg/L, a relatively level. In August, when the Hg content in the surface reached a relatively high level of 0.021-0.035 μg/L,the Hg content in the corresponding bottom layer was relatively high, 0.021-0.025 μg/L. In October, when the Hg content in the surface layer reached the highest level of 0.011-0.040 μg/L, the Hg content in corresponding bottom layer reached the lowest level of 0.007-0.018 μg/L. Moreover, the variation range of Hg content in the surface layer was 0.011-0.040 μg/L, which was larger than that in the bottom layer, 0.007~0.025μg/L, and the amplitudes of variation was basically the same. Therefore, in May, the Hg content of surface was relatively low, and the Hg content in corresponding bottom layer was relatively low; in August, when the Hg content in surface was relatively high, the Hg content in corresponding bottom layer was relatively high; however, in October, when the Hg content of surface was very high, that in the corresponding bottom layer reached the lowest level.

Horizontal Distribution Trend of Surface and Bottom Layers.
In May, from station 55 in the central waters of the bay to station 60 in the coastal waters in the southeast of the bay, in the surface layer, Hg content increased from 0.011 μg/L to 0.022 μg/L along the gradients. In the bottom layer, Hg content increased from 0.013 μg/L to 0.019 μg/L along the gradients. This showed that the horizontal distribution trend of Hg content in the surface and bottom layers was same.
In August, from station 55in the central waters of the bay to station60 in the coastal waters in the southeast of the bay. In the surface layer, Hg content increased from 0.021 μg/L to 0.035 μg/L along the gradients. In the bottom layer, Hg content increased from 0.021 μg/L to 0.025 μg/L along the gradients. This showed that the horizontal distribution trends of Hg content in the surface and bottom layers were same. In October, from station 55 in the central water area of the bay to station 60 in the coastal water area in the southeast of the bay, in the surface layer, Hg content decreased along the gradients from 0.040 μg/L to 0.011 μg/L. In the bottom layer, Hg content increased from 0.007 μg/L to 0.018 μg/L along the gradients. This showed that the horizontal distribution trends of Hg content in the surface and bottom layers were opposite.
In May and August, the horizontal distribution of Hg in the surface layer of Jiaozhou Bay was consistent with that in the bottom layer of the bay. In October, the horizontal distribution of Hg in the surface layer of Jiaozhou Bay was opposite to that in the bottom layer of the bay.

The Process of Deposition.
In water body, Hg appeared in the form of ions and had strong hydrophilicity. In summer, marine organisms multiplied in large numbers and increased rapidly [16]. It was easy for Hg to combine with zooplankton, phytoplankton and floating particles in the sea water. The colloid formed on the surface of floating particles and had the strongest adsorption capacity. In this way, when the Hg content passed through the water body, a large number of Hg ions would be adsorbed on the surface of the floating particles. When the floating particles settled, the Hg would continuously settle to the seabed [1][2][3][4][5][6][7][8][9][10][11]. Therefore, under the action of gravity and water flow, Hg was influenced by the effect of vertical water body [12][13][14], continuously settling from the surface to the seabed, presenting the process of Hg settlement and migration.

Seasonal Waters in Central Waters of the Bay.
In the surface of the central waters of the Jiaozhou Bay, in May, the Hg content began to rise from a low level of 0.011 μg/L, and gradually increased. In August, the Hg content reached the high level of 0.021 μg/L, then continued to rise, and gradually increased. In October, the Hg content reached the high level of 0.040 μg/L. Therefore, the seasonal variation of Hg content from low to high was spring, summer and autumn.
In the surface of the central waters of the Jiaozhou Bay, in spring, summer and autumn, there was no source of Hg content, and the settlement of Hg content only depended on the combination of plankton and floating particles in the sea water. In summer, marine organisms propagated in large numbers, and the number increased rapidly, while in spring and autumn, there was not a large number of marine organisms propagated. Therefore, in summer, a large amount of Hg was taken to the seabed, while in spring and autumn, there was no large amount of Hg taken to the seabed. Thus, in the bottom of the central waters of Jiaozhou Bay, in May, the Hg content began to change from a low level of 0.013 μg/L, then began to rise, gradually increased, and in August, the Hg content reached a high level of 0.021 μg/L. In October, the Hg content reached the lowest level of 0.007 μg/L. Therefore, theHg content in bottom layer changed from low to high in autumn, spring and summer.

Seasonal Variation of Coastal Waters in the Southeast of the Bay.
In the surface of the coastal waters in the southeast of the Jiaozhou Bay, in May, the Hg content began to change from a low level of 0.022 μg/L, then began to rise, gradually increased. And in August, the Hg content firstly reached a high level of 0.035 μg/L, then began to decline, gradually decreased. In October, the Hg content reached a lowest level of 0.011 μg/L. Therefore, the seasonal variation of Hg content from low to high was autumn, spring and summer.
In the surface of coastal waters in the southeast of Jiaozhou Bay, there was no source of Hg content in spring and autumn. In summer, the Hg content came from atmospheric subsidence, and the Hg content in transportation reached a very high level of 0.050 μg/L. In summer, marine organisms propagated in large numbers, and the number increased rapidly, while in spring and autumn, there was not a large number of marine organisms production. Therefore, the Hg content settlement not only depended on the combination of plankton and floating participle in the sea water, but also depended on a large amount of Hg content from atmospheric deposition. Therefore, in summer, a large amount of Hg was taken to the seabed, while in spring and autumn, there was no large amount of Hg taken to the seabed. Thus, in the bottom of the coastal waters in the southeast of Jiaozhou Bay, in May, the Hg content began to change from a low level of 0.019 μg/L, then began to rise, gradually increased, and in August, the Hg content reached a high level of 0.025 μg/L. By October, it began to decline and gradually decreased, and the Hg content reached the lowest level of 0.018 μg/L. Therefore, the Hg content in bottom layer changed from low to high in autumn, spring and summer.

Different Processes of Seasonal
Change. The seasonal variation in the central waters of Jiaozhou Bay was totally different from that in the coastal waters in the southeast of Jiaozhou Bay. In the central waters of Jiaozhou Bay, the seasonal variation of Hg content in the surface layer from low to high was spring, summer and autumn, and the seasonal variation of Hg content in the bottom layer from low to high was autumn, spring and summer. In the coastal waters in the southeast of Jiaozhou Bay, the seasonal changes of Hg content in the surface and bottom layers from low to high were autumn, spring and summer. Although the seasonal variation of Hg content in the surface layer from low to high was different between the Central Bay and the coastal waters in the southeast of Jiaozhou Bay, the seasonal variation of Hg content in the bottom layer from low to high was the same. This showed the different sedimentation process of Hg content in the two waters.
In the surface of the central waters of Jiaozhou Bay, in spring, summer and autumn, there was no source of Hg content. The Hg content settlement only depended on the combination of plankton and floating participle in the sea water. In summer, marine organisms propagated in large numbers, and the number increased rapidly, while in spring and autumn, there was not a large number of marine organisms propagated. Therefore, when there was no source of Hg content, the Hg content in the bottom layer was controlled by the seasonal variation.
In the surface of the coastal waters in the southeast of Jiaozhou Bay, in spring and autumn, there was no source of Hg content. In summer, there was the source of Hg content, the Hg content came from atmospheric subsidence, and the content in transportation reached a very high level of 0.050 μg/L. In summer, marine organisms propagated in large numbers, and the number increased rapidly, while in spring and autumn, there was not a large number of marine organisms propagated. Therefore, the Hg content settlement not only depended on the combination of plankton and floating participle in the sea water, but also depended on a large amount of Hg content from atmospheric deposition. Therefore, when there was the source of Hg content, the Hg content in the bottom layer was jointly controlled by the seasonal variation and source transport.

Deposition
Changes. On the scale of variation, from the central water area to the coastal water area in the southeast of Jiaozhou Bay, in May, August and October, the variation range of Hg contents in the surface and bottom layers was basically the same. In May, the Hg content in surface was relatively low, and that in the corresponding bottom layer was also relatively low; in August, when the Hg content in surface was relatively high, that in the corresponding bottom layer was relatively high; however, in October, when the Hg content of surface was very high, that in the corresponding bottom layer reached the lowest level, which revealed the law of Hg deposition: in summer, marine organisms propagate in large numbers, and the amount increases rapidly, while in spring and autumn, there is no large number of marine organisms propagated. Therefore, the Hg content settlement depends entirely on the combination of plankton and floating participle in the sea water. Therefore, in summer, a large amount of Hg was taken to the seabed, while in spring and autumn, there was no large amount of Hg taken to the seabed.
In May, August and October, the change of Hg content in the surface and bottom layers fully confirmed the settlement law. In May, the Hg content in surface was relatively low. At this time, without a large number of marine organisms breeding, a large amount of Hg will not be brought to the seabed, so the Hg content in corresponding bottom layer was relatively low. In August, when the Hg content of the surface was relatively high, there would be a large number of marine organisms breeding, which will bring a large amount of Hg to the bottom of the sea, so the Hg content in corresponding bottom was relatively high. In October, no matter the Hg content of surface was very