The Effect Of Bioturbation On The Migration And Transformation Of Mercury And Other Heavy Metals In Sediments

Mercury（Hg）is a toxic heavy metal distributing ubiquitously in natural environments.It can be easily transformed to methylmercury（Me Hg）,which contains more toxicity and can be biomagnified and bioaccumulated along the aquatic food chain.Since the Minamata disease in the middle of the 20^th century,Hg pollution in aquatic environments has received wide attention.The sediment is a huge Hg sink in the aquatic environment,so the emission of Hg from sediments can be one of the major factors of the uncertainty of estimating the mass balance of Hg in the water body.There are large amounts of benthic fauna living in the sediments,exerting important influences not only on the exchange of materials,but also the migration and transformation of heavy metals on the sediment-water interface.As a result,bioturbation can be an important factor of the endogenous pollution caused by sediments in the natural aquatic system.It is of great importance to investigate Hg release from sediments under bioturbation for studying the globally biogeochemical cycling of Hg.Besides,sediments also store lots of heavy metals such as Cd,Cu,Zn and Pb,as well as other pollutants.These pollutants can be released into overlying waters through bioturbation and physical disturbance,increasing the ecological risk in the aquatic environments.Thus,this study chose Limnodrilus hoffmeisteri（SSY）,Cipangopaludina cathayensis（TL）and Corbicula fluminea（HX）which can be found in Chongde Lake in Southwest University to investigate the speciation distribution and migration of Hg and other heavy metals（Cu,Zn,Ni,As,Pb and Cr）in shallow lake sediments under the bioturbation of these three species of benthic fauna,and accordingly choosing the dominant benthic fauna.Based on this,we further studied the speciation distribution and migration of Hg and other heavy metals in sediments under bioturbation of different densities of benthic fauna（low density as LD,middle density as MD and high density as HD）.Meanwhile,we studied the migration and transformation of Hg and other heavy metals in sediments under different ways of bioturbation,in order to provide basic data for further research on Hg recycling and transformation in aquatic environments,as well as for evaluating the environmental effects and ecological risk of heavy metals in shallow lakes.The main results are as follows.1.Bioturbation of different species of benthic fauna can elevate Hg concentrations in the overlying water.Bioturbation significantly elevated THg,TMe Hg and PMe Hg concentrations in the overlying water（p<0.05）with no significant difference among different species of benthic fauna（p>0.05）.Bioturbation changed the speciation distribution of Hg in the overlying water,increasing TMe Hg/THg and the proportion of PMe Hg.Bioturbation of different species of benthic fauna can increase Me Hg contents and TMe Hg/THg in sediments,and SSY significantly promoted Hg methylation in sediments（p<0.05）.Bioturbation promoted the release of THg and TMe Hg from sediments,with their cumulative emissions of 13.57～19.49 and 0.72～1.63 ng,respectively.SSY was more beneficial to the release of THg,while HX was more beneficial to the release of TMe Hg.2.Bioturbation of different species of benthic fauna can affect concentrations and speciation distribution of other heavy metals in the overlying water.SSY can significantly increase the concentrations of total and particulate Zn,Ni and Cr（p<0.05）,while elevating the proportion of particulate Cu,Zn,Ni and Cr in the overlying water.HX can significantly increase the concentrations of total and dissolved As（p<0.05）,while elevating the proportion of particulate As in the overlying water.Bioturbation of different species of benthic fauna can promote the release of other heavy metals from the sediments.The cumulative emissions of Cu、Zn、Ni、As、Pb and Cr were 13.07～15.06、35.77～82.81、15.85～36.61、1.71～4.89、2.23～4.02 and 10.57～13.48μg,respectively.SSY was favorable for the release of Zn,Ni and Cr,TL was favorable for that of Cu,and HX was favorable for that of As and Pb.3.Bioturbation of different densities of benthic fauna can affect Hg concentrations and its speciation distribution in overlying water.HD can significantly increase THg,TMe Hg and PMe Hg concentrations（p<0.05）.It can also elevate TMe Hg/THg and the proportion of PMe Hg.Bioturbation of different densities of benthic fauna can promote the release of THg and TMe Hg from the sediments.The cumulative emissions of THg and TMe Hg were 20.40～34.15 and 0.67～1.62 ng,respectively,with the promoting effect of HD more significant than that of LD and MD（p<0.05）.4.Bioturbation of different densities of benthic fauna can affect concentrations and speciation distribution of other heavy metals in the overlying water.LD,MD and HD significantly increase the concentrations of total and particulate Zn（p<0.05）,with no significant difference among these three treatments;HD significantly increase the concentrations of total and dissolved As（p<0.05）,and significantly increase the concentrations of total and particulate Ni,Cr and Pb in a short term（p<0.05）;LD can significantly increase the concentrations of total and particulate Cu in a short term（p<0.05）.Bioturbation of different densities of benthic fauna can promote the release of other heavy metals from the sediments.The cumulative emissions of Cu、Zn、Ni、As、Pb and Cr were 6.67～18.97、17.25～34.24、2.52～7.81、16.57～28.93、5.55～9.44 and-0.18～7.94μg,respectively.HD was beneficial to the release of Ni,As,Pb and Cr,while HD was beneficial to that of Cu and Zn.5.Compared with bioturbation under natural reoxygenation（MD）,bioturbation under aeration（MDA）can increase the concentrations of TMe Hg and PMe Hg（p<0.05）,and elevate TMe Hg/THg and the proportion of PMe Hg.It can also increase the concentrations of total and particulate Cu and Zn（p<0.05）,and significantly increase that of total and dissolved As（p<0.05）.Bioturbation under aeration（MDA）can not only promote the release of THg and TMe Hg,but also promote that of Cu,Zn,As and Pb from the sediments.