Mercury Accumulation And Transformation Of Urban Plant Communities And Urban Waterscapes

Cities constitute the main areas of human activity.Human activities associated with mercury（Hg）releases make a major contribution to increased Hg levels in water,soil and air.Urban forests and urban waterscapes are important parts of the Hg cycle on an urban scale.To understand the complex interactions between human activities and Hg accumulation and transformation in a megacity,this study focused on the dry Hg deposition of litterfall in urban forest and the Hg in the surface sediments of urban waterscapes in Shanghai,China.The study selected 3 typical plant communities（Cinnamomum camphora community,Metasequoia glyptostroboides community and evergreen deciduous mixed community）and 32 urban waterscapes（lakes and rivers）in regions with different urbanization levels as the objects.The study collected field samples and conducted microcosm experiments to explore:（1）Annual Hg flux deposition in litterfall of typical plant communities;（2）Temporal and spatial distribution characteristics of THg and MeHg in sediments of urban waterscapes;（3）Hg accumulation,transformation and potential risks of urban waterscapes.The main conclusions of the study are as follows:（1）The average annual concentration of THg in litterfall of 3 plant communities in Shanghai urban forest was 37.9±14.1μg·kg^-1(n=96,16.5-79.7μg·m^-2·yr^-1).Compared to 2021,the total average THg concentration decreased by 23.3%,which may be related to the decrease in mercury emissions during the COVID-19 pandemic in 2022.And the concentration of THg in evergreen deciduous mixed community was significantly higher than that in Cinnamomum camphora community and Metasequoia glyptostroboides community（ANOVA test,p<0.05）.The annual THg concentration of the 3 typical plant communities showed a trend of:central urban area（Putuo District）>suburban area（Minhang District）>outer suburban area（Chongming District）,indicating that there was a good correspondence between the accumulation of Hg in the litterfall and the intensity of human activities.The average annual THg flux of litterfall from 3 communities was 21.4±11.4μg·m^-2·yr^-1(n=27,14.1-32.6μg·m^-2·yr^-1),and there was a significant positive linear correlation between the litterfall THg flux and the litterfall biomass flux（R²=0.825,p<0.001）.The results indicated that the main driving factor of litterfall THg deposition flux in typical plant communities in Shanghai was the change of litterfall biomass.The mean value and variation range of the annual litterfall THg flux in Shanghai are not significant different from those in remote areas of different regions.（2）The mean THg concentration in surface sediments at all sites throughout the year was 165.5±268.8μg·kg^-1(n=45,39.0-1345.5μg·kg^-1).The concentrations of THg in the surface sediments in the central urban area（CU）were significantly higher than those in the suburban area（SU）（Mann-Whitney U test,p<0.001）.Positive correlations between the THg levels and loss on ignition（LOI）in the sediments were observed for all SU samples and the majority of CU samples,indicating that THg in the surface sediments of most waterscapes was closely related to organic matter.Abnormal high THg concentration in surface sediments were approximately 2.6-13.2 times greater than95μg·kg^-1,the background value of soil in Shanghai,which were markedly deviated from the regression line.It was inferred that in addition to the input of natural atmospheric Hg depositions（litterfall and wet deposition）,direct anthropogenic mercury emission was the direct cause of abnormally high mercury accumulation in sediments in the CU.Thus,the linear fitting line between THg levels and LOI can be used as an effective"background line"to evaluate the intensity of anthropogenic Hg release.Hg concentration deviated from the background line can be considered to be significantly affected by direct anthropogenic Hg emissions.The annual mean concentration of MeHg in the surface sediments was 0.18±0.14μg·kg^-1(0.04-0.65μg·kg^-1,n=45).MeHg concentrations across the sites were generally higher in summer than in winter（p<0.05）,which could be attributed to low DO levels and high temperature observed near the bottom of the water bodies in summer,this is associated with anaerobic microbial activities coupled with the decomposition of organic matter.According to the index of geo-accumulation(I_geo),it was found that the Hg pollution of the sediments of urban waterscapes in Shanghai was relatively light.（3）In microcosm experiment 1,during the 21-day anaerobic incubation period,the addition of litterfall significantly enhanced net MeHg production in the sediments（p<0.001）,but MeHg levels decreased sharply following the addition of a specific sulfate-reducing bacteria（SRB）inhibitor（p<0.001）.These results indicated that input of fresh organic matter could significantly promote the activity of SRB,leading to an increase in the concentration of MeHg in sediments.In addition,THg in solution exhibited a gradually decreasing trend,and log K_d was increased,which indicated that dissolved THg was continuously transferred to the sediment solid phase with aging time.Compared to field experiments,the value of MeHg/THg in sediment（%MeHg）is less than 1%.These results indicated that new Hg inputs would be gradually mobilized in sediments with aging time,but relatively low Hg inputs might pose ecological risks to waterscapes within a short period of time.Therefore,direct mercury emissions from human activities should be strictly managed.