By Roberta Attanasio, IEAM Blog Editor
Moving in an endless cycle, mercury goes from the atmosphere to soil, water and sediment, and then back to the atmosphere. While moving around, mercury changes to its various forms, often becoming the highly toxic and bioaccumulative methylmercury, the type that builds up in living tissue and increases in concentration up the food chain – including the food we consume.
It may seem simple, but it’s not. If we look at the cycle in more detail, we can see extreme complexity – and much of this complexity is still a puzzle in search of solutions. As I said in a previous post, “Although the global cycle of mercury has been characterized to a certain extent, and the mechanisms leading to the transformation of inorganic mercury to the highly toxic form of methylmercury is well understood, there are still a lot of unknowns related to the regional and local characteristics of mercury deposition and its effects on specific ecosystems.”
Mercury is released into the environment from both natural and anthropogenic sources. Not only are these sources continuously shifting and blending as we speak – they have been doing so for millennia. Ancient human practices, such as biomass burning for agricultural activities and extensive mining in the Peruvian Andes, have been influencing the movement and distribution of mercury well before the industrial revolution.
To find mercury levels representative of natural emissions – not significantly influenced by human activity – we have to go back thousands of years, to a period of time that Bruce Hope and Jeff Louch call the pre-Anthropocene (≤2000 BCE). But why do we want to know what the mercury levels were in the pre-Anthropocene? Hope and Louch provide a clear answer in the recently published paper “Pre-anthropocene mercury residues in North American freshwater fish” (Integrated Environmental Assessment and Management, published online January 23, 2014).
The neurotoxic form of mercury – methylmercury – accumulates in fish and shellfish and may cause significant adverse effects in people that consume it. Therefore, Federal Agencies along with State and local authorities regulate limits on the maximum concentrations of mercury allowed in seafood. As more information on the adverse health effects of methylmercury becomes available, the allowed concentrations become smaller and smaller. But how small is too small? Are we, in some cases, trying to bring these limits down to levels that are impossible to achieve – levels that are below those deriving from natural emissions?
Hope and Louch set to find out. Natural emission levels are those present in the pre-Anthropocene. Because fish specimens and data on surface water from that period are not available, Hope and Louch estimated the pre-Anthropocene concentrations of methylmercury in the tissues of prey and predatory fish using an integrated model. This model takes into account the different forms of mercury, its transport and fate, as well as the food web – and is based on estimated concentrations of mercury in soil, sediment and atmospheric deposition.
Results from the model indicate that, in some watersheds, natural levels of mercury in fish may be higher than those allowed by current regulations. On the basis of this finding, the authors suggest that alternative regulatory strategies should be developed for these watersheds.
The mercury cycle is indeed complex, and our understanding of it is still incomplete, but as the Hope and Louch study shows, correct use of models may help in putting together some of the puzzle pieces, including those related to the presence of mercury in fish.