_applied sciences_  
_Editorial_  
**Mercury and Methylmercury Contamination of Terrestrial and Aquatic Ecosystems**  
**[PERSON] 1,*\({}^{\text{(c)}}\)** and [PERSON] 2,*\({}^{\text{(c)}}\)**  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
In 2017, 128 countries signed the Minamata Convention on Mercury (Hg) to protect human health and the environment from the adverse effects of mercury. Although Hg naturally occurs at a global scale, anthropogenic releases of Hg affect its natural biogeochemical cycle. Since the industrial revolution, anthropogenic pressure and Hg contamination have risen, affecting natural ecosystems (e.g., lake eutrophication, deforestation, mining, and climate change) and urban areas (wastewaters, atmospheric emissions, and industrial discharges). Consequently, it has resulted in a general increase in Hg levels in all components of ecosystems, particularly in Hg bioaccumulation in trophic chains, mostly as methylmercury (MeHg). However, Hg and MeHg impacts are dependent on a large variety of abiotic and biotic factors with complex interactions among them, which often results in locally specific responses that we are still trying to understand. Identifying sources, magnitude, pathways, and mechanisms of Hg and MeHg contamination in terrestrial and aquatic environments is crucial to determine and predict future impacts of this contaminant in the environment. It is also difficult to predict the risks and the Hg toxicity for wildlife and humans. The identification of processes driving Hg release from current and historical sources, key transformations (i.e., methylation/demethylation), and Hg accumulation in terrestrial and aquatic ecosystem is still challenging to assess. These interconnected processes can affect both natural (temperate and extreme) and anthropogenic (urban, agricultural, and mining) areas at various spatial and temporal scales. Nine articles were accepted following the October 2019 call for a Special Issue (SI) on \"Mercury and Methylmercury Contamination of Terrestrial and Aquatic Ecosystems\", announced at the 14 th International Conference on Mercury as a Global Pollutant--Krakow, Poland (8-13 September 2019). They present a diverse but coherent collection that endeavors to identify the historical evolution of Hg and MeHg levels in aquatic environments and to evaluate the impact of human activities, in particular mining, on receptor ecosystems and food chains. The need to evaluate the evolution of mercury and methylmercury contamination from historical to modern times is a key to assessing the impact of global regulations and to predicting future trends. Various approaches are presented in this SI, including fossils or modern organisms as bioindicators of trophic contamination. Using Hg levels in the hair of prehistorical animals, [PERSON] et al. [1] showed the first evidence of natural changes in environmental Hg level at the paleoscale (45 to 10 ka yr BP) suggested to be driven by high dust loads in cold periods and thawing permafrost in warm climatic periods. [PERSON] and [PERSON] [2] used marine mussels (_Mytilus_ spp.) as sentinel organisms to monitor Hg contamination along the Canadian marine coast for the past 40 years. They show that, despite a decrease in Hg concentrations in the Atlantic Ocean's marine waters during the last decades, the difference in two mussels watch surveys between 2016-2019 and 1977-1979 were not significant. Consequently, they suggest rivers as the main Hgsource in such estuarine systems. In contrast, in the United States and Canadian Lakes, [PERSON] et al. [3] showed that regional- to continental-scale decreases in both mercury and sulfate emissions have benefitted aquatic resources, even in the face of global increases in mercury emissions. In long-term biomonitoring of three lakes, the authors found a decline in aqueous MeHg, whereas biotic THg concentrations declined significantly in only one lake.