KEY FINDING 11.Concentrations of legacy contaminants in terrestrial, freshwater, and marine systems have generally declined over the past 10 to 40 years. Concentrations of many emerging contaminants are increasing in wildlife; mercury is increasing in some wildlife in some areas.
Contaminants are substances that are introduced into the environment through human activity. Some, like mercury, are naturally occurring but are increased in concentration through human activity to levels that could harm ecosystems and humans. Contaminants may travel great distances through the atmosphere and oceans and end up in ecosystems distant from their sources. This key finding considers only contaminants that persist in the environment and accumulate in the tissues of plants and animals. Legacy contaminants have been banned or restricted but are still widespread in the environment. Emerging contaminants are newer chemicals, or substances that have been in use for some time and have recently been detected in the environment – usually emerging contaminants are still in use or only partially regulated.
Contaminants can harm species and ecosystems and impair ecosystem services. They can directly affect animals when present in their diets, such as by impairing reproduction, and can also become a problem for humans who rely on them for food – particularly for Aboriginal people with diets heavily reliant on marine mammals and fish.1 The widespread presence of contaminants in wildlife has been a concern in Canada since the 1970s and concentrations of selected contaminants have been monitored in some species and locations over various periods since then. There are long-term, ongoing datasets adequate for trend analysis, but these are restricted to a few areas, such as the Great Lakes and parts of the Arctic.
Several persistent organic pollutants, including the pesticide DDT and the industrial chemicals PCBs, are considered legacy contaminants. Despite being banned or restricted, some of these substances persist at levels that may impair animal health in some populations of long-lived top predators (including killer whales2 and polar bears3) and in areas where there is a history of heavy use of some of these substances (such as the Great Lakes4).
Brominated flame retardants, for example PBDEs, are one class of emerging contaminants that have been detected in the environment, even in remote locations, at increasing levels since the mid-1980s. Concentrations of some brominated flame retardants show signs of stabilizing or declining in the last few years in response to new regulation and reductions in their use.1 Other emerging contaminants include some pesticides and herbicides in current use.
Mercury is a third example of a contaminant that can accumulate in wildlife. While mercury is a naturally occurring element, much of the mercury in marine and freshwater systems is from industrial sources such as coal burning – and mercury releases are increasing in parts of the world.5 Mercury levels in animals are highly variable and trends are mixed.1
The story of the peregrine falcon shows that contaminants can have major effects on biodiversity and that banning and restricting contaminants works. Peregrines in Canada declined dramatically from the 1950s to 1970s, mainly from egg-shell thinning caused by DDT and its breakdown products.6 With the banning of DDT in Canada in 1970, 1972 in the U.S., and 2000 in Mexico, DDT slowly declined in the environment. Conservation actions and reintroductions helped populations to increase once DDT levels were low enough for eggs to hatch successfully. Some parts of Canada such as the Okanagan Valley of British Columbia may still have too much legacy DDT for peregrine falcons to nest successfully.7
Sources: burbot: Stern, 2009;8 murres: Braune, 20079 updated by author; beluga: Stern, 200910 and Tomy, 2009;11 cormorants and gulls: Environment Canada, 2009;12 lake trout: Carlson et al., 201013 and Ismail et al., 200914
The charts show a range of trends and levels of two legacy contaminants (PCBs and DDT), mercury, and an emerging contaminant (PBDEs) in wildlife. Amounts and trends are partly related to proximity to contaminant sources and partly to otherfactors that influence an animal's exposure to and uptake of contaminants, including position in the food web. Magnitudes ofcontaminant levels should be compared from chart to chart only in general terms - datasets are not all comparable in the types of tissues sampled and in analysis and data reporting methods.
Trends in contaminants in the Great Lakes Legacy contaminants and mercury are generally decreasing in the Great Lakes in response to clean-up of contaminated sites and improved pollution control.4,13However, the large volumes of water and sediment in the system act as a storehouse – contaminants continue to be released from sediments and to recycle through the water, sediment, and food webs.19, 20 Contaminants also continue to be deposited into the lakes through long-range atmospheric transport21 and, in the case of mercury, from industrial emissions in the Great Lakes Basin.4 The net result is that rates of decline of some legacy contaminants and mercury have slowed in areas of the Great Lakes, leaving some contaminants at levels that are of concern and likely to remain so for some time to come.13,20
Brominated flame retardants (PBDEs) increased rapidly in fish and birds starting in the early 1980s,22-24 but levels have now stabilized or are declining in response to action taken to curtail the use and release of these substances.24, 25 Many other emerging contaminants have been detected more recently in environmental samples, often in trace amounts, but little is known about the risk to ecosystems from most of them.26 Chemicals of concern include PFOS, originating in water-repellent coatings and fire-suppression foam, detected in fish samples throughout the Great Lakes, and known to build up in food webs.27 Emerging contaminants also include endocrine disrupting substances, which come from a range of sources, including pharmaceuticals. Potential effects include abnormal gonad development in fish.28 Many emerging contaminants do not originate in industrial emissions, but rather from use and disposal of health and personal-care products and consumer goods, leading to a need for new risk management approaches for contaminants in the Great Lakes.26
Interactions between contaminants and environmental change
Changes in environmental conditions caused by stressors, including climate change and invasive non-native species, may, in some cases, make wildlife more vulnerable to contaminants. Environmental change can increase the exposure of some aquatic species to contaminants through changes in water flow and chemistry and through changes in food webs.15, 16 Interactions may also make animals more vulnerable to the effects of contaminants. For example, salmonids in the Great Lakes have switched to a diet that includes alewife, an invasive non-native fish, leading to thiamine (vitamin B1) deficiencies that may interact with the effects of contaminants like PCBs to increase mortality rates in young fish.17
Impact of less sea ice on contaminants in seals and polar bears
With changes in sea-ice conditions, western Hudson Bay polar bears are feeding less on
ice-associated bearded seals (which eat invertebrates) and more on open-water seals (which eat fish).18 Because fish-eating seals have higher levels of contaminants, some legacy contaminants in polar bears may not be declining as much as would be expected if their diet had not changed and levels of emerging contaminants may be increasing at a faster rate. Concentrations of brominated flame retardants (PBDEs) in western Hudson Bay polar bears are estimated to have increased 28% faster from 1991 to 2007 than would have occurred if the bears had not changed their diet.18
Impact of changes in fire regimes on mercury in fish
Changes in fire regimes can increase algae in lakes and contaminants in fish. A study in Jasper National Park16 found that fire in the catchment area of a lake in 2000 increased the input of nutrients to the lake over a period of several years. This led to an increase in production of algae, which led to an increase in the abundance of invertebrates, making the lake's food web more complex. The outcome was an increase in mercury accumulating in lake trout and rainbow trout.
Persistent organic pollutants, as well as mercury, tend to accumulate in aquatic ecosystems more than in terrestrial ones. These levels are magnified as they move up the food web. This means that the highest levels of these contaminants are found in top predators – especially marine mammals and fish-eating birds.
There is no evidence of current widespread effects of contaminants on Canadian Arctic wildlife, though polar bears of southern and western Hudson Bay, as well as some high Arctic seabirds, have contaminant levels that may be placing them at risk.3 However, what is known is based only on studies of a few species and is usually based on the effects of a single contaminant. Little is known about impacts of the contaminant mixtures that wildlife are exposed to, or about interactions of contaminants with other changes in ecosystems.3
Contaminant levels are much higher in some areas of southern Canada than they are in the Arctic (see graphs of contaminant trends earlier in this section). Levels of contaminants measured in wildlife often exceed thresholds beyond which biological effects are known to occur from laboratory studies (usually based on species other than those of concern in the wild). While direct evidence of impacts on wildlife populations is difficult to obtain, associations between high contaminant levels and observations of effects – like tumours, abnormal gonads, or poor reproductive success17,28 – underscore conservation-level concerns for some populations. The clearest example of known impacts is that of DDT-associated egg-shell thinning in birds29 – but high levels of contaminants are suspected to contribute to declines in several wildlife populations, for example, herring gulls in the Great Lakes30 and beluga whales in the St. Lawrence Estuary.31, 32
Contaminants in killer whales off the Pacific Coast
Average levels in killer whale biopsy samples, mid-1990s, parts per million
PCBs and PBDEs are known to adversely affect neurological development, reproductive development, and immune system function of marine mammals.33 Because they are long-lived top predators, killer whales accumulate high concentrations of persistent organic pollutants, including PCBs and PBDEs.29,34, 35 The concentrations of PCBs in the three killer whale populations along the B.C. coast exceed levels known to affect the health of harbour seals,33 and the PCB levels of two populations are among the highest in marine mammals in the world.35
The large variation in contaminant concentrations among the populations is related to their feeding habits. Transient whales feed on marine mammals, placing them higher in the food web, while both resident populations feed largely on salmon that acquire contaminants from global sources in the North Pacific Ocean.29 Southern resident whales also feed on prey that pick up contaminants from the industrial coastal waters of southern B.C. and northwest Washington, leading to higher PCB and PBDE accumulation.29 These or other contaminants may be a factor in the decline of this endangered population of killer whales (see Marine Biome).36