Impacts of River Damming on Sediment Methylmercury Dynamics

Reservoirs are reported to stimulate mercury (Hg) methylation and thereby affect methylmercury (MeHg) dynamics in rivers by trapping sediment and increasing the hydraulic residence time (HRT). However, whether there are similarities in sediment %MeHg (MeHg/Hg) in relation to reservoir properties for different rivers and their associated mechanisms remain unknown. Here, we investigated the sediment MeHg in two heavily dammed rivers and one naturally flowing river in the same region. We found that river damming results in the accumulation of sediment Hg and OC and a higher abundance of hgcAB, thus elevating the sediment %MeHg. Sediment %MeHg showed a positive correlation with reservoir HRT, and the methylation time of Hg acts as the threshold value to determine the impact of river damming on sediment %MeHg. However, sediment %MeHg had either a positive or a negative correlation with reservoir age in different rivers, and the effect of reservoir age on sediment %MeHg depends on whether the stock of substances in the reservoir has a dominance of internal consumption or is one of external supply. These findings provide an important reference for further research on Hg biogeochemical cycling and MeHg risk management in dammed rivers.