Mercury cycling and human health concerns in remote ecosystems in the Americas

Abstract

Fish constitutes a praised—and sometimes irreplaceable—component of the daily diet for numerous communities throughout the Americas. These populations can be exposed to mercury (Hg)—ubiquitous in fish flesh—to levels potentially harmful to their health. Mercury is subjected to various chemical transformations once released to the environment. From diffuse loading on land and waters to accumulation in the aquatic food web, this paper draws a general picture of the main processes influencing the Hg cycle in different ecozones. Contrarily to other pollutants, the amount of Hg found in fish tissues is not clearly related to the extent of Hg loading in a given ecosystem. In the Arctic, the sudden stripping of gaseous elemental Hg from the atmosphere—known as the Hg depletion events—and its subsequent transformation into methylmercury (MeHg) seems to be one of the driving force behind Hg accumulation in freshwater aquatic organisms and marine mammals. In boreal environments, numerous watershed-based processes influence the transport and methylation of Hg deposited from atmospheric source and selectively control the amount of MeHg transferred from source to fish. In the tropics, small-scale gold mining operations have induced the release of considerable amount of Hg to the environment. However, the extensive deforestation of the tropical forest and the subsequent erosion and lixiviation of land-deposited Hg down to tropical rivers appears to have a greater impact on the observed wide-scale fish Hg contamination. Mercury accumulation in remote ecosystems do represent a significant and complex environmental issue, considering: (a) the high levels of Hg monitored in fish and marine mammals flesh; (b) the large-scale occurrence of such worrying Hg bioaccumulation patterns; (c) the extensive use of aquatic resources by numerous remote communities; (d) the fact that this issue shall persist at least on the mid-term, even if strong measures are taken to lessen Hg anthropogenic loadings to the global atmosphere. Although there are no simple answers to the potential threats to human health caused by Hg accumulation, several political actions to lessen human exposure to Hg are possible and are discussed here.