Lydia Kifner, MS Researcher, University of Maine; Department of Civil and Environmental Engineering

Carly Eakin, PhD Researcher, University of Maine; Department of Wildlife, Fisheries, and Conservation Biology

Many people know mercury (Hg) as the toxic metal that was used to make thermometers, but you might not know that it can also be found in vernal pools. It is a naturally occurring metal that in certain forms and exposures is a toxin that can be damaging to humans and other animals. Although pollution from some human activities (e.g. coal combustion, improper waste disposal) deposit mercury in the environment, there are also several natural sources of mercury.

Methyl mercury (MeHg) is a toxic form of mercury that is formed in the absence of oxygen (anaerobic conditions), which are common in the waterlogged sediments of wetlands (Benoit et al. 2013). Vernal pools are wetlands that can produce methyl mercury. Does this mean that these pools contain dangerous levels of Mercury? In some cases it does, but it’s not the pool’s fault. Here’s the deal:

Pools receive mercury inputs from the atmosphere. When the bottom waters and sediment become anaerobic during the wet season, methyl mercury can form. Vernal pools could be sites of significant MeHg production because of the high organic material found in the pools, which reduces the pH of pool water and provides the carbon molecules that are needed to facilitate the conversion of mercury to methyl mercury (known as mercury methylation). Methylation of mercury can be a function of mercury concentration and environmental factors, such as temperature, pH, and carbon concentration (Benoit et al. 2003, Brooks et al. 2012). In addition, the water levels in vernal pools fluctuate throughout the summer and can lead to alternating anaerobic conditions when mercury can be converted to methyl mercury and aerobic conditions when the presence of oxygen prevents methylation. This flip-flop in oxygen conditions in vernal pools ultimately produces more MeHg overall than if the pools remain anaerobic the entire time (Brooks et al. 2012).

Although MeHg is naturally produced in vernal pools, it is toxic and the artificially high levels of mercury in the atmosphere (i.e., mercury pollution) mean that there may be high levels of MeHg produced even in the most pristine vernal pools. One problem associated with high methyl mercury concentrations is bioaccumulation. Animals exposed to MeHg can experience detrimental effects, and the MeHg can also have magnified effects up the food chain as larger aquatic organisms and mammals consume the smaller organisms. As predatory animals eat other organisms, MeHg moves from organism to organism and the concentration increases (Brooks et al. 2012). In vernal pools, it is hypothesized that mercury can enter nearby terrestrial food webs from amphibian and invertebrate metamorphs emigrating from the pools (Brooks et al. 2012, and Loftin et al. 2012). While bioaccumulation of MeHg from vernal pools may pose health risks for wildlife and possibly even humans, one should remember that some level of MeHg production is naturally occurring and likely doesn’t harm wildlife and humans. As of yet, we have only a cursory understanding of vernal pool chemical/biogeochemical processes and the role of these processes in the ecosystem. More research about how methalyation fits into ecosystem processes, how human activities are affecting it, and the effects of mercury bioaccumulation on animals and ecosystems is needed to gain a better understanding of vernal pools importance.


Benoit, J.M., Cato, D.A., Denison, K.C., and Moreira, A.E. (2013). Seasonal mercury dynamics in a New England vernal pool. Wetlands:33, 887-894.

Benoit, J.M, Gilmour, C.C., Heyes, A., Mason, R.P., and Miller, C.L. (2003). Geochemical and biological controlds over methylmercury production and degradation in aquatic ecosystems. In: Cai Y Braids OC (eds) Biochemistry of Environmentally Important Trace Elements. ACS Symposium Series 835, 262-297.

Brooks, R.T., Eggert, S.L., Nislow, K.H., Kolka, R.K., Chen, C.Y., and Ward, D.M. (2012). Preliminary assessment of mercury accumulation in Massachusetts and Minnesota seasonal forest pools. Wetlands:32, 653-663.

Loftin, C.S., Calhoun, A.J.K., Nelson, S.J., Elskus, A.A., and Simon, K. (2012). Mercury bioaccumulation in wood frogs developing in seasonal pools. Northeastern Naturalist:9(4), 579-600.