Mercury accumulation and transformation in the surface sediments of urban waterscapes in the megacity of Shanghai, China

Purpose: The effects of urbanization on the distribution and methylation of mercury (Hg) in local-scale aquatic ecosystems have been poorly characterized. Here, we aim to characterize and compare total Hg (THg) concentrations and methylmercury (MeHg) production in sediments in waterscapes across the megacity of Shanghai, China, and evaluate the impacts of organic matter and Hg inputs on changes in sediment MeHg production during reduction and reoxidation. Materials and methods: Surface sediments were collected in summer and winter from 15 lakes in parks and 17 rivers located in central urban or suburban areas, and THg, MeHg and environmental parameters were measured. In addition, two different incubation series were conducted under different redox conditions over a short period (21 or 56 days). Results and discussion: Concentrations of THg and MeHg generally increased in sediments from rural to urban areas, with abnormally high levels at several urban sites. The historical point-source anthropogenic Hg releases could be important contributors to the increases, as reflected by the marked deviations from the regression line between Hg and loss on ignition. Furthermore, both litterfall organic matter and Hg inputs strongly promoted net MeHg production in sediments over a short period, resulting in potential MeHg bioaccumulation in waterscapes. Conclusions: Our study suggested that urbanization in the Shanghai area resulted in Hg loading in central urban areas. However, the low %MeHg but abnormally elevated Hg levels observed at some sites in the field indicated that the potential ecological risks of Hg could be limited, even with new Hg inputs.