Why is METAALICUS Necessary?

Mercury is the most common contaminant in fish in the United States and Canada. Forty-two states have advisories against fish consumption due to high mercury levels, and unacceptable fish mercury concentrations exist in all Canadian provinces and the Northwest Territories, including remote "pristine" lakes. Ninety-seven percent of closures to fishing in Ontario are due to mercury contamination. Figure 1.  Lake 658 at the Experimental Lakes Area

Mercury is a neurotoxin. There is a particular concern for prenatal life and infants, for whom safe levels of mercury exposure are lower than in adults. Concern also exists regarding the toxic effects of mercury on reproduction, behavior and survival in fish-consuming birds and marine mammals.

Experts agree that the relationship between atmospheric mercury deposition and the mercury in fish can not be understood by examining historical or regional data. This is mainly because empirical attempts to isolate the effects of mercury loading on fish mercury levels are confounded by other environmental factors which cause variations in fish mercury concentrations in lakes. These factors include a wide range of local site conditions and annual weather variability, often combined with poorly documented mercury loads. By contrast, this experiment provides the control necessary to examine the effects of one critical factor: mercury loading. Furthermore, this experiment will also allow us to distinguish newly deposited mercury from background mercury that has accumulated over hundreds of years.

METAALICUS is a whole-ecosystem experiment in which mercury inputs to a headwater lake (example Lake 658, Figure 1) and its watershed will be increased experimentally. An ecosystem approach will be used because the complex pathway of mercury from the atmosphere to fish cannot be simulated in laboratory experiments.

The mercury will be added as stable, non-radioactive isotopes of inorganic mercury (Hg(II)). The power of using isotopes lies in the ability to follow the newly deposited mercury separately from the background mercury.

Different mercury isotopes will be added to the upland, wetland and the lake surface to determine if the route of entry affects how much is accumulated in the fish (Figure 2).

Figure 2.  Using stable mercury isotopes to evaluate the contributions of direct deposition, upland runoff and wetland outflow to fish mercury levels.

Comprehensive field studies will be carried out to establish site conditions and mercury concentrations in all compartments in the lake and watershed, and how they change with time. Detailed process studies will also be carried out. Movement of the mercury and transformations between mercury forms will be followed through the watershed and lake. Production of methylmercury (MeHg) will be studied in the lake sediments, upland and wetland, as will the bioaccumulation of MeHg into benthic organisms, plankton and fish. Methylmercury will be followed specifically because it is the dominant form in fish and the most toxic form of mercury in the environment.

This process-based approach will allow us not only to document what happens, but also to understand why. This is essential if we are to use the results of the study to make predictions for other locations. The approach will also provide critical information for an existing model that predicts fish mercury concentrations in lakes and the effects of remedial actions such as reductions in mercury loading.

The study will be carried out in two phases over a 5-year period. Phase 1 has started and involves a series of pilot studies, and background information on the study site. In Phase 2, the mercury loading to the whole ecosystem will be increased.

How Much Hg will be Added?

Figure 3.  Proposed addition of stable Hg(II) isotope during METAALICUS

Not much. ELA is a low deposition area for mercury, with approximately 3 ug/m2/yr of wet mercury deposition. The low mercury deposition rate at ELA means that adding the equivalent of about half a teaspoon of mercury over the entire 5 year study is enough to increase the atmospheric load to a 50 ha system by 4 to 5 times. This amount of mercury addition will result in a mercury deposition rate to the experimental system that is equivalent to rates currently observed in the Northeastern US (Figure 3).

A general research plan was drafted at two workshops in Toronto, Ontario in 1998 and in Washington, D.C. in 1999. It was decided that initial pilot scale studies are essential to refine the analytical methods and to ensure that the effects of the additions proposed will be measurable during the full-scale study. Pilot studies undertaken during 1999 include:

Isotopic 202Hg(II) additions to a small upland catchment (1100m2), a wetland plot (300 m2), and a pine tree stand. Refinements to methods for measuring isotopic mercury concentrations in various media. Preliminary studies of the experimental lake and its watershed (Figure 4).

Figure 4.  Fish sampling during baseline METAALICUS studies.

The pilot studies have confirmed that the analytical techniques are sufficiently sensitive to detect the small additions of added isotope, and that the general research design is viable.

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