Transportation And Transformation Of Mercury In Different Plant Habitats And Its Influencing Factors In Coastal Wetlands Of China

Mercury(Hg)is a volatile and persistent global pollutant that can be transported long distances with the atmosphere and deposited in terrestrial and marine ecosystems,causing global mercury pollution.After migration and deposition,Hg will participate in the internal mercury cycle in the ecosystem and transform into methyl mercury(MeHg),which is highly lipophilic and neurotoxic,and MeHg can be enriched through the food chain,posing a severe threat to human health and the ecological environment.The coastal wetlands located in estuaries are an important link between rivers,sea and land.The combined effect of rivers and tidal erosion leads to a more diversified source of mercury in the area,while the rich vegetation in the wetlands can fix the mercury in the atmosphere,making coastal wetlands an important sink for mercury.The coastal wetlands have both freshwater wetland and saline wetland characteristics,and the mercury migration and transformation are more complex.A large amount of terrestrial organic matter brought by rivers is deposited in coastal wetlands,resulting in a rich vegetation and microbial community in the area,making it a hot spot for MeHg production.Therefore,studying the mechanisms of the influence of river and seawater confluence and plant growth on the geochemical behavior of Hg in Chinese coastal wetlands will bring a new theoretical perspective to examine the altered biogeochemical cycle of Hg under climate and environmental change scenarios such as sea level rise.This paper first investigated the THg(Total mercury)and MeHg contents in the soils of Phragmites australis and Spartina alterniflora habitats in coastal wetlands,using five estuarine coastal wetlands,including Yellow River estuary wetlands,Linhong River estuary wetlands,Yangtze River estuary wetlands,Minjiang River estuary wetlands and Nanliu River estuary wetlands as the study areas,and initially discussed the current status of mercury pollution in Chinese coastal wetlands under different plant habitats and the main environmental factors controlling the distribution of THg and MeHg.Based on the above findings,this paper conducted focused sampling in the Yellow River estuary wetlands,aiming-to deeply reveal the influence of rivers,oceans and plants on Hg transport transformation in the Yellow River estuary wetlands.The main conclusions are as follows:(1)THg concentrations in Chinese coastal wetlands ranged from 17 to 168 ng/g,with higher THg concentrations in the soils of the Minjiang estuary and the Yangtze estuary wetlands,which may be significantly associated with higher local economic development.The concentration of MeHg in the coastal wetlands of the study area ranged from 0.01 to 0.81 ng/g.The distribution of THg and MeHg in Chinese coastal wetlands was mainly influenced by the distribution of TOC-in the soil.There were large differences in THg and MeHg concentrations between the two plant habitats,reed and intergrass,possibly due to the different degree of tidal or riverine influence on the different habitats.Most of the coastal wetlands in the study area had low to moderate ecological risk of Hg,while only the reed habitat in the Yellow River estuary wetlands had a high ecological risk.(2)The distribution of THg and MeHg in wetland soils of the Yellow River estuary is affected by the combined effect of river and tide,and the correlation between THg and its physicochemical properties in wetland soils has similar to the Yellow River’s suspended particulate matter,which is consistent with the understanding that wetland soils of the Yellow River estuary originate from the suspended sand of the Yellow River.Meanwhile,the different degree of influence of rivers and tides on wetland soils is the main reason for the obvious differences in the migration and transformation characteristics of THg and MeHg in soils of different habitats in the Yellow River estuary wetlands.(3)Phragmites australis,Spartina alterniflora and Suaeda Sulsa all have a strong enrichment capacity for THg and MeHg in soil,especially in reeds and intergrass,where soil pH and EC may affect the uptake and enrichment of THg and MeHg from soil by plants in the Yellow River estuary wetlands.The presence of plants did not increase the Hg load in the Yellow River estuary wetlands,in contrast,the extensive invasion and growth of intertidal areas of Spartina alterniflora may increase the local MeHg exposure risk by altering the soil physicochemical properties such as TOC content and increasing the MeHg content in the soil.