| As a highly toxic metal element,the transport and geochemical behaviour of mercury(Hg)in the atmosphere,biosphere,hydrosphere and lithosphere is one of the hotspots of research in the geoscience.Especially in the production of MeHg,a highly toxic substance resulting from the methylation process of mercury.The estuarine coastal zone is constantly receiving mercury from riverine transport and atmospheric dry and wet deposition,making estuarine coastal wetlands a major sink for mercury.The Yangtze River basin and estuary are strongly influenced by anthropogenic activities,which pose a threat to estuarine ecosystems,yet studies on the cycling of mercury in the wetland(tidal flats)environment of the Yangtze River estuary are still not well studied,especially the effects of tidal flats vegetation growth,tidal action on the accumulation of mercury in sediments and mercury methylation.In this study,sediment samples were collected from the rhizosphere(0-36 cm)of the Dongtan wetland in Chongming from 2020 to 2021,and the seasonal variation of total mercury(THg)and MeHg in the rhizosphere sediments of three typical vegetation species(Spartina alterniflora,Phragmites communis and Scirpus mariqueter)in the Dongtan wetland was analysed.The organic carbon isotope δ13C was measured in the surface sediments and the proportion of the main organic matter sources in the sediments was estimated using the MixSIAR model.The key factor of mercury methylation,hgcA genes were measured in some sediment samples in summer.Incubation experiments were carried out to investigate the migration of mercury during oxidation-reduction processes.Based on the analysis of these studies,the following main conclusions were drawn:(1)The THg content of rhizosphere sediments in Chongming Dongtan wetland ranged from 12.3 to 114.9 ng/g,with a mean value of 75.9±18.4 ng/g(n=144).Compared with other estuaries in the world,the THg content of sediments from Chongming Dongtan wetland in the Yangtze estuary was medium-low.MeHg content ranged from 0.01 to 2.32 ng/g,with a mean value of 0.32±0.30 ng/g(n=144),also at a medium-low level compared to other estuaries around the world.(2)Sediment THg was highly variable in the depth profile with a smaller range of seasonal variation in the surface layer.Sediment THg content showed a significant positive correlation with the percentage of fine particles below 16 μm(R2=0.85,P<0.01)and also with loss on ignition(LOI)(R2=0.63,P<0.01),indicating that the spatial variation of total mercury in sediment is mainly influenced by fine particles below 16μm and organic carbon.The high THg content in the rhizosphere of Spartina alterniflora is closely related to its contribution to the siltation of fine particles,and thus Spartina alterniflora contributes to the accumulation of Hg in the sediment to some extent.(3)The results of the Hg thermal-desorption showed that the majority(>60%)of the THg was present as thermally desorbed mercury at 225℃ and 325℃(mainly organic bound mercury,mercury sulphide,etc.).There was a positive correlation between the sum of the two and the LOI(R2=0.50,P<0.01),indicating that organic carbon in the sediment was the main factor influencing the phase of inorganic mercury.In terms of MeHg,the MeHg content in the rhizosphere sediments of the three plants was generally similar.Overall,the MeHg content of the surface(0-12 cm)sediment was greater than that below 12 cm depth,especially at sampling sites C1 and C2 on the high tidal flat,where the depositional environment was stable.Correlation analysis showed that MeHg content was not significantly correlated with THg content,temperature,salinity and LOI,but was significantly and positively correlated with the abundance of the hgcA genes(R2=0.76,P<0.05),these results suggest that the organic matter input from litterfall of plants and its degradation process may be the main driver of the elevated MeHg content in the surface layer.In addition,the results of organic carbon sources estimated using MixSIAR showed that,the organic carbon sources in the rhizosphere surface sediments varied considerably between sampling sites and did not correlate with the MeHg content,and the organic carbon fraction from C3 plants(Phragmites communis and Scirpus mariqueter)was higher(>70%)in the sediments of sampling sites C1 and C2,further suggesting that under stable sedimentary conditions,plant apoplankton input is the main factor controlling Hg methylation in surface sediments.The influence of plant species is likely to be limited.(4)The results of the incubation experiments showed that the addition of litterfall of Spartina alterniflora and Phragmites communis significantly contributed to the methylation process and there were no significant differences between the two plants.These results further clarify that organic matter and its degradation processes are the main factors controlling the methylation of mercury in sediments.The group with extra Hg addition of 1.4 μg/g(an approximate 14-fold increase relative to the control),showed decreasing concentrations of total dissolved Hg over 50 days under changing oxidation-reduction conditions.But sediment MeHg levels increased with a maximum value reaching more than 10 times that of the control.These results indicate that if highly mercury-polluted waste were imported into the estuarine wetlands,the inorganic mercury in the sediment can be rapidly converted to MeHg by the input of organic matter such as vegetation and tidal action,and it would be difficult to be passivated within a short period of time(50 days).This means that the ecological risk to the Yangtze estuary wetlands is likely to persist for a long period of time after mercury inputs. |
