Factors Influence Mercury Methylation In Sediments From The Eutrophication Reservoir In Guizhou Province

MeHg accumulating in the aquatic food chain, especially amplification andtransformation in predatory fish, is the main way of human exposure to MeHg.Therefore, the risk of mercury methylation in sediments has a potential threat toaquatic organisms and human health.Methylation influenced by many factors, such as pH, Temperature,microbial species and organic matter, etc. OM is one of the most importantfactors, because not only the content, but also the structure of OM affectsmethylation in sediments. Cage culture and evolution can lead to eutrophicationin reservoir, and settlement of organic matter to the surface layer of sediments,as fish and human activities input many pollutants. At the same time, under thesynergy of reservoir seasonal anaerobic results in MeHg methylation activies insurface sediments.This thesis selected two reservoirs(Hongfeng Reservoir and WujiangduReservoir) and a natural lake(Caohai Lake) with different sources andcomposition of organic matter as the study object, Sediment samples werecollected from the surface layer of sediments at0-9cm depth, where methylationwas active. The study included measurements of mercury species in porewaterand interface water, with composition of organic matter and other parameters,and analyzed the main factors influencing sediment methylation. Main resultsand conclusions were as follows: 1) DTHg in Caohai was1.7-6.4ng/L, DTHg in porewater had littlecontribution to the overlying water. DTHg in Hongfeng Reservoir was1.96-14.67ng/L, significantly higher in Daba than Jiangjunwan and Houwu, andthe lowest concentration appeared in Houwu. DTHg in Wujiangdu Reservoirwas1.88-17.29ng/L, significantly higher in surface layer.2) THg in Caohai was105.3±18ng/g, and still had a trend of increase insurface sediments. THg was373.7±67.6ng/g in Hongfeng Reservoir, anddeclined slightly, compared with previous studies. THg contents in WujiangduDaba was459.7±181.2ng/g, higher than Hongfeng Reservoir and Caohai. THgincreased obviously in surface sediments, indicating new mercury source inputin recent years.3) MeHg in Caohai was significantly lower than the other two reservoirs,but MeHg/THg was0.96%at7-9cm depth, suggesting Caohai had a high riskof mercury methylation. MeHg in Hongfeng Reservoir had peaks in the surfacelayer, but MeHg was lower than previous studies, should be related to reservoirmanagement. MeHg enriched significantly in surface sediments fromWujiangdu Daba, but no obvious rise was observed compared with early study.4) DOC promoted the production rate of MeHg obviously in Caohai, butMeHg contents were low. Maybe depths were shallow, and MeHg-DOMphotodegradation increased inorganic mercury in Caohai. DOC acceleratedmercury methylation in Hongfeng Reservoir. MeHg in Daba and Jiangjunwanwas more consistent with DOC, but reduction of DOC in Houwu increased theratio of DTHg. No statistical correlation between mercury and DOC inWujiangdu Reservoir, which may be related to reservoir drainage.5) OM was36.01%, but did not inhibit mercury methylation in Caohai. OMdropped down in Hongfeng Reservoir compared to previous research, but nosignificant effect on production rate of methylation. OM was7.15%inWujiangdu Reservoir, and was still rising, which promoting mercurymethylation. Continuous highly active OM input caused by eutrophication was one of the important factors to maintain high levels of mercury methylation.Mercury methylation in Hongfeng Reservoir Daba and Wujiangdu Daba washigher than other sites; but OM activity was restricted in center of Caohai, for itsOM was less interference.6) MPA was absolutely dominant population in Caohai, which improvedthe MeHg demethylation. SO2-4was higher in surface sediments from HongfengReservoir, and promoted SRB activity. Strong mercury methylation observed inHongfeng Reservoir Daba, possibly caused by combined activity of SRB andFeRB. Concentrations of SO2-4and Ca2+were high at surface layer in WujiangduDaba, increasing activities and abundance of SRB, which could promotemercury methylation.7) F3was the main substrate of mercury methylation in WujiangduReservoir, and mercury combined to organics was the main fraction, whichpromoted mercury methylation. The different methylation productions, withsimilar concentration of OM in Wujiangdu Reservoir and Jiangjunwan wasobserved, which suggested higher F4in porewater limited F3in Jiangjunwan.OM changed mercury species in sediments, and accelerated the transformationof F5to F3and F4, thus improved the proportion of mercury combined toorganics, increasing the ability of mercury methylation.