From Eco Issues

Introduction

Mercury is a naturally occurring metal with unique properties that have been employed – and marveled at – since ancient times. At room temperature, it is an extremely dense liquid that also readily vaporizes. Because it combines easily with other metals to form amalgams, mercury has long been used in dentistry and to separate gold from other minerals. Because of its physical properties, it has been a preferred material in thermometers and barometers. Since mercury is an element, it does not degrade over time, nor can it be destroyed.

As a pollutant, mercury has particularly insidious characteristics. It is released from a very wide range of industrial sources (often as an inadvertent byproduct); in gaseous form, it has the capacity to circulate globally through ecosystems; and it can bioaccumulate to toxic concentrations in the top predatory species of food chains. Thus, fish-eating birds and mammals have a higher dietary exposure to mercury than to any other known component of aquatic ecosystems. Mercury toxicity for fish, birds and mammals can lead to reduced reproductive success, impaired growth and development, behavioral abnormalities, and death.

In northeastern North America, the wildlife species considered at greatest risk of mercury poisoning include common loons, bald eagles, osprey, mink and otter. Researchers have documented elevated mercury levels in common loons in the Maritime provinces and in New England, in bald eagles in Maine, and in osprey in northern Quebec. Since mercury occurs naturally and vaporizes readily, there has always been a certain amount of deposition from the atmosphere. But the deposition of mercury as a pollutant has increased substantially due to human activities such as the mining of mercury and the burning of fossil fuels, which releases previously bound mercury. Historic deposition rates can be measured in bog and lake sediment cores. Comparative studies show that average concentrations in sediments have increased two to three times since the onset of the industrial revolution.

Movement of mercury through ecosystems

Mercury has a complex global cycle, since it can be transported long-range as vapour, transformed to methyl and dimethyl forms in sediments and water, and also bioaccumulate in living organisms. Thousands of tonnes of mercury vapour are emitted each year into the atmosphere worldwide, from both natural and human processes. For example, it is estimated that global atmospheric emissions of mercury from major anthropogenic sources are over 2,200 metric tonnes a year. Mercury vapour can drift for a year or more, traveling great distances across the globe. Thus, mercury can be deposited with rain, snow and dust to regions without local mercury emission sources. Once deposited, mercury may vapourize again and drift back up into the atmosphere in a repeated cycle of deposition and evaporation. This cycle, called the “grasshopper effect,” tends to concentrate certain pollutants in the cold polar regions of the globe.

Mercury concentrations in the open waters of lakes, rivers and oceans are usually low, ranging from less than 1 to 20 parts per trillion. But the sediments of those waters have much higher levels of mercury, since mercury compounds are attracted to particles and decaying organic matter. In Canadian lakes and streams, mercury concentrations in sediments tend to be about a million times higher than in water, ranging from 0.005 to 99 parts per million (ppm). Certain kinds of bacteria in the top layers of lake sediments convert inorganic mercury to methylmercury, which is a more toxic and biologically active form.

This process is especially promoted in shallow acidic waters, in sediments rich in organic matter, and at warm temperatures. Since these conditions are often created in reservoirs formed after rivers are dammed, fish in reservoirs have relatively high concentrations of methylmercury.

The extent to which wildlife are exposed to mercury contamination depends on their place in the foodchain, their feeding habits and their size. Generally, wildlife that eat aquatic organisms are more exposed than consumers of terrestrial food chains. Top predators are most at risk, and within any given population, the largest individuals are likely to have accumulated the largest body burdens of mercury.

How high are mercury levels in wildlife?

It is clear that mercury levels in fish increase as fish get longer, heavier and older. A 1996 survey of Ontario sport fish by the Ministry of the Environment found that about 80 per cent of the population of large walleye in Ontario contained above 0.5 ppm mercury, and 10 per cent contained above 1.5 ppm. These concentrations are high enough to trigger Health Canada consumption restrictions for children and for women of childbearing age. Some researchers have estimated that fish containing more than 0.4 ppm mercury may cause reproductive problems in loons that eat them. Fish containing these levels of mercury, and that are small enough to be prey for loons, can be found in up to 30 per cent of Ontario lakes. Mercury levels in the muscle of common loons of northwest Ontario have been found, under normal conditions, to average 1.2 ppm, and in lakes near a mercury pollution source, up to 4.6 ppm. In a separate survey of chemical residues in Canadian waterfowl conducted by the Canadian Wildlife Service, muscle tissue of loons and mergansers was found to have the highest levels of mercury among the species studied, with levels in the range of 1.5 – 1.9 ppm. Loons in Kejimkujik National Park, Nova Scotia, have the highest level of blood mercury observed in this species in North America.

Essentially all of the mercury in large fish and in wildlife comes from their food. Mercury levels in wildlife can vary considerably, even among individuals of the same population of a species sharing similar habitats. Depending on the types of prey available, wildlife may develop higher or lower body burdens of mercury.

Are wildlife showing effects?

Though the levels of mercury observed in wildlife are rarely high enough to result in acute toxicity, they are within ranges where adverse effects are possible. Since chronic mercury toxicity shows up as neurological and brain damage, chronic symptoms can include subtle abnormal behaviour, impaired hunting ability, eating disorders, loss of balance, lack of coordination and paralysis of the legs. Such symptoms can be very hard to monitor directly in wild populations, since individuals suffering such symptoms tend to disappear quickly due to starvation and predation. Researchers may instead evaluate surrogate parameters, such as the ability of birds to raise chicks, or the overall reproductive success of a population.

Wildlife species vary considerably in their susceptibility to mercury poisoning. Mammals and birds tend to be more sensitive to mercury than fish. Fish contaminated with a certain level of mercury may not have noticeable problems, but the mink that eat those fish may develop health problems over their lifetime. In mink, methylmercury has been shown to be lethal at a dietary level of 5 ppm, and to cause sublethal effects at 1.8 ppm. In one study, a diet containing 1.0 ppm methylmercury, in combination with cold stress, was lethal to mink after two to three months’ treatment. In a Quebec study, 0.9 ppm mercury was lethal to half of a treated group of mink after about three months. A study of river otter found they died after about nine months at a controlled dietary exposure level of 2 ppm.

Measuring mercury’s effects

There are limitations in the ability of science to determine the ecological effects of mercury, particularly where relatively low-level exposures and chronic effects are concerned. While controlled laboratory experiments can be carried out on species such as mallard ducks, susceptible species such as loons cannot be kept for prolonged periods in labs, and it can be difficult to extrapolate lab results to them. Furthermore, wildlife may be exposed not just to a single contaminant such as mercury, but to additional multiple stresses such as lead toxicity, habitat disturbance and habitat loss. This makes it even more difficult for researchers to tease out the impacts caused by mercury. Nevertheless, researchers have found a clear exposure-response relationship between blood mercury concentrations in adult loons and their ability to nest and raise young. As well, high blood mercury levels in loon chicks appear to affect their behaviour adversely. A field study of common loons in Ontario found that all chicks died before fledging when fish containing between 0.3 and 0.4 ppm mercury were consumed. When fish contained higher levels of mercury, averaging 0.9 ppm, the adult loons became emaciated and also failed to raise young.

In Ontario, neither the Ministry of the Environment nor the Ministry of Natural Resources is monitoring mercury concentrations or impacts in vulnerable top predators such as loons or otters. MOE is, however, monitoring mercury levels in young-of-the-year yellow perch, which are widespread in headwaters and relatively easy to collect. Young-of-the-year reflect a single year’s exposure to mercury, and thus long-term monitoring is needed to identify any long-term trends. Collection of these fish at the needed times and sampling sites has sometimes been limited by budgets, but the project is ongoing.

Environment Canada has developed mercury tissue residue guidelines (TRGs) for a number of fish-eating mammals and birds, based on the observed sensitivity of each species. TRGs are estimated levels of mercury that should be safe food for wildlife. Prey-sized fish containing levels of mercury higher than the TRG indicate that mercury might be an environmental problem for the fish-eating predator species. Environment Canada surveyed a database of mercury concentrations in fish taken from over 3,000 locations across Canada, and found that mercury is a potential problem throughout Canada. For example, osprey can access prey-sized fish with unsafe levels of mercury in Saskatchewan, Manitoba, Ontario, Quebec and Newfoundland.

Practical implications

Our observations of wildlife so far present an early warning that ecosystem impacts of mercury may be subtle and pervasive, and that future ecosystem impacts from mercury contamination may increase. They also underscore the wisdom of taking aggressive steps to cut mercury emissions to the environment.

Humans are the only species with the ability to avoid food types known to contain elevated levels of mercury. In fact, advisories warning the public to avoid certain kinds of mercury- contaminated fish are already widespread in northeastern North America. For example, the Ontario Ministry of the Environment monitors contaminants in sport fish at over 1,500 locations at Ontario’s smaller inland lakes. At over 40 per cent of those locations, fish consumption restrictions apply, and in over 98 per cent of the cases, the restrictions are due to mercury contamination.

The U.S. Centers for Disease Control reported in 2001 that approximately 10 per cent of U.S. women of childbearing age have elevated mercury levels (within one-tenth of potentially hazardous levels) in their blood, primarily from eating fish. The U.S. Food and Drug Administration issued a consumer advisory at the same time, warning that women of childbearing age should not eat fish with high methylmercury levels, such as swordfish, shark, king mackerel or tilefish.

The elevated mercury levels in fish have become a difficult issue for public health officials, since fish has important nutritional value, and is also a key component of the diet of several cultural groups, such as First Nations.

In contrast to humans, fish-eating birds and mammals have no recourse to alternative food sources, and no way of recognizing mercury-contaminated food. They are embedded in their local ecosystems, and when these ecosystems become polluted, they suffer the full consequences.

Effectiveness of emission controls

Mercury contamination can be addressed and gradually reversed, with firm regulatory action. Ontario’s experience shows that control of local mercury sources can result in less contaminated wildlife over time. For example, in the early 1970s, controls were placed on major industrial discharges of mercury to certain Ontario waterways. MOE reports that over the next 30 years, mercury concentrations in large lake trout from Lake Superior steadily declined, from 0.5 ppm to approximately 0.2 ppm today.

This example shows that it would be worthwhile for Ontario to focus in a concerted way on reducing mercury emissions from Ontario sources, such as coal-fired power plants and sewage treatment plants. The following article, pages 122-126, describes an application submitted under the Environmental Bill of Rights regarding mercury emissions from Ontario’s coal-fired power plants. These facilities do not operate under mercury emission caps, despite being major sources of the pollutant. The ECO’s past annual reports have described the limited efforts Ontario has made thus far to manage mercury emissions from base metal smelting, dental amalgams and fluorescent lamps through the adoption of Canada-Wide Standards (CWSs). For example, although base metal smelting is a major emission source, the CWSs do not come into effect for existing smelters until the year 2008, at which time the smelters are expected to make a “determined effort” to achieve the new emission guideline. The ECO believes there is a need for a much more vigorous approach to controlling Ontario mercury emission sources.

At a minimum, the ECO sees a need for careful monitoring and clear public reporting of mercury’s impacts on Ontario ecosystems, including impacts on higher trophic levels, vulnerable species and sensitive ecosystem functions. An accurate, comprehensive inventory – or failing that, MOE’s best available estimate – of mercury loadings to Ontario’s environment should also become a regular component of the province’s annual air quality reports. Finally, MOE should identify and consult the public on further mercury reduction options, above and beyond the CWSs.

Recommendation 12: The ECO recommends that MOE establish a comprehensive program to develop an understanding of the pathways, movement and fate of mercury in Ontario ecosystems.

Ecosystem Impacts of Mercury

Contents

Introduction

Movement of mercury through ecosystems

How high are mercury levels in wildlife?

Are wildlife showing effects?

Measuring mercury’s effects

Practical implications

Effectiveness of emission controls

Further Reading

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