High levels of pollutants in polar bears from the Barents Sea – what are the reasons behind?

Persistent organic pollutants (POPs) are man-made chemicals. They have been used intensively for numerous agricultural, industrial, and commercial applications and it takes many decades for them to break down in the environment. Atmospheric and ocean currents, as well as river outflows, bring these pollutants to the Arctic, where they biomagnify in food webs and bioaccumulate in individual animals over a lifetime.

Recent research by the Norwegian Polar Institute and collaborators from other Fram Centre, national and international institutes has provided knowledge about drivers of pollutant exposure in Barents Sea polar bears.

The use and production of the so-called “legacy” POPs has long been banned or restricted by international regulations such as the Stockholm Convention. In response, Barents Sea polar bears’ exposure to legacy POPs has generally decreased over the past 20 years.

Nonetheless, levels of some compounds have been increasing during the last decade in Barents Sea polar bears. This can probably be attributed to re-emission of pollutants from melting sea ice, glaciers and permafrost.

Polar bears are top predators of the Arctic ecosystem and can live for up to 30 years. This means they are exposed to relatively high levels of pollutants over a long period of time, which may cause a wide range of adverse health effects.

Polar bears from the Barents Sea have considerably higher concentrations of some pollutants than other subpopulations.

Living offshore or along the coast

Arctic sea ice, which is the main habitat where polar bears travel, hunt and mate, is declining very fast in the Barents Sea. The loss of sea ice is the greatest threat to polar bears. It is therefore crucial to understand how the combined impacts of habitat loss, pollutant exposure and reduced food availability might affect the bears.

Barents Sea polar bears have two distinct ways of using their environment to cope with seasonal variation of sea ice extent.

“Offshore bears” undertake long annual migrations to follow the sea ice as it retreats into the north-eastern Barents Sea.

“Coastal bears” stay close to the Svalbard archipelago in the western part of the Barents Sea year-round using sea ice close to shore and glacier fronts as preferred hunting areas.

Sea ice loss due to climate change means that the migration routes of offshore bears are getting longer. Around Svalbard, longer periods with reduced sea ice force coastal bears to feed more on land-based prey. Consequently, in the Barents Sea, offshore and coastal polar bears must cope with very different ecological challenges.

Food, distribution and energy

New research has identified three different factors that drive pollutant exposure in offshore and coastal bears.

What the bears eat:

Offshore polar bears are exposed to higher concentrations of pollutants than coastal bears because they feed primarily on marine prey high up in the food web, for instance seals. Coastal bears rely on a mixed diet including land-based prey such as seabird eggs and reindeer.

Where they spend their time:

Offshore polar bears are distributed further north and east than coastal polar bears, preferentially in the transition zone between open ocean and sea ice. Farther north, the uptake of pollutants released from melting ice and snow during peak spring plankton blooms leads to high concentrations in the food web. Farther east, the bears’ prey is more polluted, probably owing to proximity to pollutant emission sources and transport pathways.

How much energy they use:

Offshore polar bears have higher energy expenditure because they spend more time reaching their foraging habitat and because they hunt for seals over larger areas. Coastal bears live in more restricted areas and feed opportunistically on whatever is available locally. Consequently, offshore bears need more energy, eat more food, and hence assimilate more pollutants than coastal bears.

Current levels and consequences

Despite these differences in exposure, only one type of studied pollutants is currently higher in the blood plasma of offshore bears: perfluoroalkyl substances (PFASs). PFASs bind to proteins in blood and liver, whereas other persistent organic pollutants, such as polychlorinated biphenyls (PCBs) and chlorinated pesticides, are stored in fatty tissues.

Concentrations of pollutants that accumulate in fatty tissues are similar in offshore and coastal bears. This is because offshore bears are fatter than coastal bears, and pollutants that bind to fat are more diluted in fat bears than in thin coastal bears.

Pollutant exposure has been related to various adverse health effects in polar bears. A recent circumpolar review led by the Norwegian Polar Institute concludes that polar bears’ immune and hormone systems and their ability to store and burn fat are likely affected by pollutant exposure. There is also evidence that polar bear’s neurochemistry may be altered by pollutant exposure. However, current understanding of population level risks and effects of contaminants in polar bears is still very limited.

References:

Blévin P, Aars J, Andersen M, Blanchet MA, Hanssen L, Herzke D, Jeffreys R, Nordøy ES, Pinzone M, de la Vega C, Routti H (2020) Pelagic vs coastal – Key drivers of pollutant levels in Barents Sea polar bears with contrasted space-use strategies. Environmental science & technology, 54(2), 985-995. DOI: 10.1021/acs.est.9b04626

Lippold A, Bourgeon S, Aars J, Andersen M, Polder A, Lyche JL, Bytingsvik J, Jenssen BM, Derocher AE, Welker JM, Routti H (2018) Temporal Trends of Persistent Organic Pollutants in Barents Sea Polar Bears (Ursus maritimus) in Relation to Changes in Feeding Habits and Body Condition. Environmental science & technology, 53(2), 984-995. DOI: 10.1021/acs.est.8b05416

Tartu S, Aars J, Andersen M, Polder A, Bourgeon S, Merkel B, Lowther AD, Bytingsvik J, Welker JM, Derocher AE, Jenssen BM, Routti H (2018) Choose your poison—Space-use strategy influences pollutant exposure in Barents Sea polar bears. Environmental science & technology, 52(5), 3211-3221. doi: 10.1021/acs.est.7b06137

Blanchet MA, Aars J, Andersen M, Routti H (2020) Space-use strategy affects energy requirements in Barents Sea polar bears. Marine Ecology Progress Series 639, 1-19. https://doi.org/10.3354/meps13290

Routti H, Atwood TC, Bechshoft T, Boltunov A, Ciesielski TM, Desforges JP, Dietz R, Gabrielsen GW, Jenssen BM, Letcher RJ, McKinney MA, Morris AD, Rigét FF, Sonne C, Styrishave B, Tartu S (2019) State of knowledge on current exposure, fate and potential health effects of contaminants in polar bears from the circumpolar Arctic. Science of the Total Environment 664, 1063-1083. https://doi.org/10.1016/j.scitotenv.2019.02.030