223 - Importance of Lake Water Residence Time for Methylmercury Concentrations of Sediment and Fish in Artificial Reservoirs

Total Hg and methylmercury (MeHg) concentrations in water, sediment and fish, and methylation rate constant (km) in sediment have been monitored in the five artificial reservoirs from 2016 to 2020, as a pilot scale study to establish a long-term Hg monitoring plan for effectiveness evaluation of the Minamata Convention. The preliminary mass budgets were estimated to identify the primary input of MeHg in lake water. The monitoring results showed that sediment diffusion dominated over tributary and groundwater runoff in the long residence time reservoirs (440-490 days), while runoff dominated over sediment flux in the reservoirs of short residence time (10 days). Further, the water residence time was determined to be the primary factor correlated to the sediment MeHg concentration, while the km showed variations comparable with those of algal biomass. The residence time of reservoir water was also the most critical parameter to increase the Hg concentrations in bluegills, largemouth bass, and barbel steed. To summarize, the short-term Hg(II) methylation rate constant in sediment was promoted in the biologically productive reservoirs, while the long-term accumulation of MeHg in sediment and fish were higher in the lakes of the longer turnover period, which should be considered to meet the optimal efficiency of the Minamata Convention for the protection of human health from chronic Hg exposure.