Pollutants in polar bears

Last updated 6 Decemeber 2022

The total stress of pollutants in polar bears (Ursus maritimus) in Svalbard is dominated by fat-soluble organic pollutants, their metabolites and perfluorinated compounds. Polar bears are exposed to high levels of persistent contaminants, increasing in concentration higher in the food chain. The monitoring shows a significant reduction in organic pollutants that are internationally regulated, but an increase in mercury levels.

Status and trend

Geographical trend

Pelagic polar bears from the Barents Sea, which follow the ice eastwards when the ice around Svalbard melts during the summer, have a higher intake of organic pollutants compared with coastal polar bears which remain around Svalbard throughout the year. There are several reasons for these differences. For example, pelagic polar bears eat a higher proportion of marine prey and at a higher trophic level than the coastal polar bears. They also have higher energy requirements and thus a higher intake of prey. Despite these higher energy requirements, pelagic polar bears were fatter compared with coastal bears. This is probably due to their high intake of seals throughout the year. In addition, pelagic polar bears eat a higher proportion of prey which they catch in the marginal ice zone and prey which are closer to polluting emission sources/transport routes. These are factors which could impact on the intake of pollutants. Although the intake of pollutants is higher in pelagic polar bears, concentrations of fat-soluble pollutants in the blood are similar if one compares levels in pelagic and coastal polar bears. Pelagic individuals have more fat on their body and the fat-soluble pollutants are mainly concentrated here. Levels of PFAS, on the other hand, which are not fat-soluble pollutants, occur in higher concentrations in the blood of pelagic polar bears.

Polar bears from Svalbard show approximately three times higher levels of perfluorinated substances (PFOS and perfluorinated carboxylic acids (∑PFCA)) than polar bears from eastern Greenland or Canada. Amongst the other pollutants which were measured, no unambiguous differences in trends have been found between polar bears from Svalbard and other polar bear populations. Mercury levels were significantly lower in polar bears from the Barents Sea compared with those in polar bears from Canada and Greenland.

Species trends

Polar bears are apex predators and thus have a high intake of pollutants which accumulate over time and become concentrated (biomagnified) in the food chain. Concentrations of fat-soluble pollutants such as PCBs, chlordanes and DDE in polar bears are significantly higher than in their main prey, ringed seal, but lower than those observed in orca. Levels of PCBs and chlorinated pesticides in polar bears from Svalbard are similar to those found in arctic foxes, while levels of perfluorinated compounds are significantly higher.

Time trends

Monitoring of polar bears in Svalbard indicates a mean decrease in concentrations of PCBs, β-HCH, oxyc-chlorodan and BDE-47 during the period between 1991 and 2017. Although there are wide variations in levels between years, the levels have on average decreased if one compares the concentrations in polar bears from the 1990s to the present day.

On average, DDE and HCB levels decreased through to 2010 (6% per year), and then concentrations increased until 2017 (21% per year for DDE and 8% per year for HCB).

Levels of PFOS in polar bears decreased by an average of 5 % per year between 2002 and 2022. Levels og PFCA increased during the first half of the period but is now decreasing. Like the other pollutants being measured, these pollutants also show wide variations between years.

Reported time trends were not affected by age, body condition or number of cubs. Mercury levels increased over time, particularly during the latter half of the study period.

Effect of climate-related changes on diet and access to food

In recent studies, levels and time trends of pollutants in polar bears from Svalbard have been investigated in relation to climate-related changes in body condition, diet and habitat use.

Sea ice is the most important hunting area for polar bears during the winter/spring. When there is little sea ice around, polar bears draw on their body fat, and pollutants stored in the fat become more concentrated and are released into the bloodstream when the animal becomes hungry. This leads to elevated concentrations of pollutants in tissue and blood, and the pollutants become available and are absorbed by vital organs such as the liver and brain. Results from a recent study from Svalbard show that polar bears had higher levels of fat-soluble pollutants in their blood and fat in the spring than in autumn, particularly after winters with little ice.

Concentrations of PFAS in polar bears were mainly affected by diet, and polar bears with a high intake of prey from marine sources generally had higher levels of PFAS compared with polar bears which eat prey from terrestrial sources. This may be because terrestrial food chains are shorter than marine food chains, and because PFAS therefore has less potential for enrichment in terrestrial food chains compared with marine food chains.  In addition, levels of many PFAS compounds were higher in polar bears that had recently fasted than in those which had eaten recently.

The diet of polar bears has changed over time; they eat less marine prey high up the food chain today than they tended to do in the past. These changes did not affect the observed time trends of organic pollutants in polar bears.

Mercury levels were lower in polar bears with a more terrestrial diet compared with polar bears with a more marine diet. When mercury levels were adjusted for changes in diet, the increase was somewhat faster, but the difference between the trends was not significant.

Causal factors

There is one main reason why the levels of the so-called old organic pollutants in polar bears are sinking. Their manufacture and use are nationally and internationally regulated. Efforts to regulate PCBs and chlorinated pesticides started in the late 1970s, and the international ban on the contaminants took effect in 2004, through the Stockholm Convention.  The main sources of emissions of these contaminants has therefore stalled. The reason for the contaminants still being present in the environment is because they are stable and that they can be recycled and concentrated in the food chain. Increases in DDE and HCB levels since 2010 are probably due to the melting of ice and permafrost. 

Production and use of the newer POPs BDE-47 and PFOS have been restricted in the past 15-20 years. Tetra-BDE, penta-BDE, hexa-BDE, hepta-BDE and PFOS were included in the Stockholm Convention in 2009, while deca-BDE was included in 2017. There are suggestions to include the long-chain perfluorocarboxyli acids (PFCAs) under the Stockholm convention and the production have decreased in many countries, but still many PFCA precursors are not regulated.

The rising mercury levels are probably driven by secondary emissions from melting sea ice and permafrost. Mercury emissions are regulated by international initiatives, the Minamata Convention on Mercury.

Consequences

Levels of organic pollutants remain high in polar bears, and polar bear cubs have been shown to have more than twice the level of fat-soluble pollutants compared with adult female polar bears.

Comparison of levels of contaminants in polar bears to threshold values for effects in other animals indicates that the health of polar bears from Svalbard may be affected by contaminants. Levels of PCBs in polar bears from Svalbard, on the other hand, are lower compared to conservative threshold values for harmful health effects of contaminants defined by AMAP (AMAP, 2018). The apparently contradicting results show that it is very challenging to define a limit value for the effects of environmental toxins in polar bears. Comparison of levels of mercury in polar bears to threshold values for effects in mink indicates that the health of polar bears from Svalbard is not affected by mercury.

Correlation and laboratory studies of polar bears indicates that the pollutant load can impact on the activity of important molecules in the brain, the immune system and hormones which are important for developmental processes and energy metabolism. Pollutants also have the ability to interfere with fat storage and fat-burning processes in polar bears.

The high levels of pollutants in polar bears, particularly in polar bear cubs, are disturbing, as these substances can impact on development, and make polar bears more susceptible to infection and disease. Changes in the functioning of genes have been linked to elevated levels of pollutants in polar bear cubs.

Like many other Arctic animals, polar bears have seasonal variations in their body condition. Female polar bears burn body fat when in their dens, and also lose a high proportion of their fat reserves when suckling their cubs. Polar bears also lose body fat when they are unable to find enough food and have to draw on their fat reserves.
Periods of hunger and drawing on body fat are natural for the polar bear, but can be critical because pollutants stored in their fatty tissue become more concentrated (in fat) and are released into the bloodstream when fat is burned. The pollutants then become available and are taken up by organs such as the liver and brain.

Polar bears also have high levels of PCB metabolites. This is because polar bears convert PCBs into water-soluble metabolites very effectively. The purpose of such conversion processes is to convert contaminants into other compounds that are more easily dissolved in water, and can thereby be excreted from the body. In this process, metabolites of PCBs are produced which remain in the body, and these compounds are often more toxic than the original PCB congeners.

About the monitoring

The polar bear is a predator at the top of the marine food chain in the Arctic, mainly eating seals hunted at the sea ice.

Current levels of pollutants could impact on polar bear health, which represents a threat to the polar bear population of Svalbard. Climate change, with reduced sea ice cover, could also indirectly affect levels of pollutants in polar bears, and further monitoring is therefore necessary.

Further reading