Study On Heavy Metal And Stable Mercury,Carbon,Nitrogen Isotope Composition Of The Flathead And Surface Sediments In The Leizhou Peninsula Waters

In this paper,fish（Platycephalus indicus,flathead）and surface sediment samples from the east,south and west of Leizhou Peninsula were collected.The heavy metal contents in gills,livers,muscles of the flathead and sediments were determined,and their ecological risks were evaluated.The mercury,carbon,and nitrogen isotopic compositions of the flathead and sedements were determined,and the source of mercury in fish and the fractionation of mercury isotopes in the process of nutrient transfer were studied.By comparing with the mercury isotopic composition characteristics in sediments,the degree of mercury pollution and source characteristics were judged.The carbon and nitrogen isotopic compositions of the organism were used to analyze their food sources and trophic levels,and to study their relationship with heavy metal content and mercury isotopes.The main understandings and results obtained are as follows:1.The content of six heavy metals in the flathead in the Leizhou Peninsula waters is not much different.Except for As exceeding the limit,Cu,Pb,Zn,Cd and Hg are all lower than the limit in the relevant standards.The pollution levels of the six heavy metals were As>Zn>Hg>Pb>Cu and Cd.2.The order of heavy metal content in surface sediments is Zn>Pb>Cu>As>Cd>Hg.Most of the heavy metals in the sediments of Zhanjiang Port in the east of the peninsula were higher than those in the Xuwen and Jianghong waters in the west of the peninsula,and the content of heavy metals in winter,spring and summer was higher than that in autumn.The pollution levels of the six heavy metal elements is As>Pb>Cu>Zn>Cd>Hg,and the ecological risk of heavy metal pollution in sediments is mainly As,which is consistent with the status of As pollution in fish.3.Mass dependent fractionation（MDF）and mass independent fractionation（MIF）of mercury isotope occurred in all fish samples.The variation in mercury isotopic composition in fish samples was relatively small.The MDF isotopes are mostly negative,while MIF isotopes are mostly positive.Except for the relatively big difference inδ²⁰²Hg,the mercury isotope values of fish in the three sea areas are relatively similar.The mercury isotope values of muscles and gills are not significantly different,while the liver is relatively depleted.The MIF of mercury isotopes in the flathead may be mainly affected by the photodegradation process of methylmercury,and also by the photochemical reduction process of inorganic mercury.The carbon and nitrogen content of different fish tissues are very different.Theδ¹⁵N values of different tissues in the three sea areas were relatively similar;theδ¹³C values of muscles and gills were relatively similar,and the liver was much depleted,and the differences among the three sea areas were also big.From the trophic levels calculated from theδ¹⁵N value,it can be seen that the vast majority of fish in the Jianghong and Xuwen sea areas are in the middle trophic level（low-grade carnivorous fish and intermediate carnivorous fish）.The trophic level of the flathead is 2.78 to 3.41,with an average of 3,which is higher than that of omnivorous and low-grade carnivorous fish such as shrimp,crab and eel,which is consistent with the feeding habit of the flathead.4.MDF and MIF occurred in all surface sediment samples,and the variation range of mercury isotopic composition was relatively small.Among the seven mercury isotopes,except forΔ²⁰⁰Hg which is close to 0,the other six isotopes have negative values.From a geographical point of view,the mercury isotope values in Xuwen and Jianghong sea areas are very similar,while that in Zhanjiang Port is significantly different from the two.In terms of seasons,the mercury isotope values of surface sediments in summer and autumn are not much different.The distribution characteristics of the mercury isotope values of the flathead and the surface sediments are different,indicating that the mercury in the sediments may not be directly related to the mercury in the flathead.There is little difference between the values of carbon and nitrogen stable isotopes in the sediments of the two quarters.The analysis showed that the collected sediment organic matter mainly came from marine authigenic organic matter.5.The results of correlation analysis between mercury isotopes andδ¹³C value andδ¹⁵N value of the flathead and surface sediments show that there is no obvious correlation betweenδ¹³C value andδ¹⁵N value and all mercury isotopes,and theδ¹⁵N value of the flathead has no obvious correlation with MDF and MIF,which indicating that the mercury isotopes of the flathead may be mainly derived from food.The isotope and heavy metal analysis of the flathead show that there is no obvious correlation between all isotopes and heavy metals.The analysis of isotopes and heavy metals in surface sediments shows that,except for the significant correlation between As andδ²⁰¹Hg andδ²⁰²Hg,there is no obvious correlation betweenδ¹³C value,δ¹⁵N value,other mercury isotopes and heavy metals.The analysis ofδ¹³C,δ¹⁵N,δ²⁰²Hg,andΔ¹⁹⁹Hg of the flathead and surface sediments shows that the carbon sources of the surface sediments of the three sea areas are relatively similar.The analysis of Cu,Zn,As,and Hg in the flathead and surface sediments showed that the heavy metals in the flathead are unlikely to be derived from the heavy metals in the surface sediments.