Pollutants have been identified as one of the major environmental threats to humankind, wildlife and ecosystems, including the Arctic. Organo-halogenated chemicals (OHCs) and mercury (Hg) are transported from anthropogenic sources via long-range transport to the Arctic, where many of these pollutants biomagnify within Arctic food chains, leaving in particular apex predators at the top of food chains vulnerable to potential health effects due to their hazardous properties. From a risk assessment perspective, top predators are valuable sentinels because they integrate chemical exposure across large areas and long time spans and, owing to their position at the top of food chains, reflect pollutant trends and ecosystem fluctuations. Thus, monitoring chemical exposure and understanding its impacts on Arctic predators is crucial for developing science-based recommendations for targeted conservation and management initiatives. The overall objective of the thesis was to assess the exposure of key Arctic marine and terrestrial mammal, bird and fish species to OHCs and Hg as well as their biological effects at the individual and population level. Specifically, we investigated dietary drivers of mercury exposure in two sentinel predator species inhabiting the circumpolar Arctic, namely the Arctic subspecies of the grey wolf (Canis lupus arctos) and the Arctic fox (Vulpes lagopus). In Chapter I, total mercury levels were analysed in the fur of 30 Arctic grey wolves sampled between 1869 – 1998 in Greenland and High Arctic Canada. Diet was evaluated with the help of stable isotope analysis by determining dietary carbon sources (as inferred by δ13C) and trophic level (as inferred by δ15N). As expected, we observed high dietary variation of marine and terrestrial food sources at various trophic positions. Variability in Hg burden in the wolves was significantly driven by biomagnification (δ15N) rather than by carbon source (δ13C) or study site. In Chapter II, various pathogens and Hg levels were evaluated as potential drivers of the Arctic fox population decline during 1970 – 1980 on the distinct, small Mednyi Island belonging to the Russian Commander Islands. We chose a study design that allowed the comparison of Hg fur levels from historical specimens from the Commander Islands representative for the population pre-crash period on Mednyi Island, contemporary Mednyi foxes sampled in 2011, and two unrelated and geographically remote ecotypes represented by inland and coastal Icelandic Arctic fox populations. As expected, Hg levels were significantly higher in foxes inhabiting coastal habitats regardless of geographic location than in those from inland sites and were highest during the pre-crash period in the Mednyi population. This was most likely because the Mednyi population depends solely on marine vertebrates that have been shown to be highly contaminated. Our large-scale health assessment using serological and DNA based pathogen screening techniques suggested a low prevalence of pathogens in contemporary foxes, although it does not allow a proper assessment of the health status of the Mednyi Arctic foxes during or before the crash period. Most likely a complex interplay of stressors explains the high cub mortality observed on Mednyi Island, including high mercury body burden, which renders particularly young foxes vulnerable to infectious diseases. Chapter III evaluates whether fur can be used as a minimally-invasive sampling matrix to reliably determine total Hg levels in soft tissues, specifically in liver and kidney of Arctic foxes. Associations between Hg levels in the fur, liver and kidney of 35 Arctic foxes sampled in 2011-2012 on Iceland were investigated. Observed total Hg concentrations varied considerably among tissues, with liver generally showing higher levels than fur and kidneys, and significant linear and sex-independent relationships based on regressions allowed to reliably extrapolate mercury liver and kidney concentrations from fur levels. Measurements in ecotoxicological studies frequently use different tissues (usually liver), which hinders cross-study comparison. Thus, the derived regression equations improve direct comparison of Hg levels among fur and soft tissues reported for Arctic canids. Chapters I-III show that Arctic wolves from Greenland and High Arctic Canada tend to have relatively low Hg concentrations (< 5 µg g-1 dry weight), whereas coastal Arctic fox populations from Iceland and the Commander Islands had up to 7-fold higher Hg fur levels, while coastal populations had 3-fold higher Hg levels than inland ones. While the health status of the Arctic wolves was most likely not affected, the mercury levels in some of the Arctic foxes exceed putative thresholds for Hg-mediated toxic health effects. Apart from trophic magnification as major driver, the observed inter-population and intra-population variability in Hg levels likely results from a combination of varying ecosystem conditions, changes in emission patterns and biological factors. Although the results of Chapter I-III are limited in terms of conclusive evidence because historic study material and/or sample sizes were scarce and relatively small, respectively, they do provide novel tools and information on temporal and spatial variation in Hg pollution in these understudied Arctic canids. In Chapter IV, exposure levels and health impacts of mercury and OHCs were evaluated in key Arctic marine and terrestrial mammals, birds and fish, using population-specific exposure data published between 2010 and 2019. Various pollutant induced health effects were summarized by each endpoint ranging from molecular to individual and population level effects. We identified quantifiable effects on vitamin metabolism, immune functioning, thyroid and steroid hormone balances, oxidative stress, tissue pathology, and reproduction. On this basis we calculated risk quotients in order to estimate critical body burdens specifically as regard to Hg and polychlorinated biphenyls (PCBs) shown to impair reproduction functions in vertebrates. The outcome was that most Arctic marine mammal species are at no or low risk in terms of health effects or reproductive impairments mediated by Hg or PCB exposure. However, for some species at high marine trophic levels, such as polar bear, narwhal and hooded seal, a segment of the population had body burden indicating a high or severe risk of suffering health effects and reproduction impairments. While bird Hg and PCB concentrations were also above toxicity benchmarks in many areas of the marine environment, terrestrial mammals were not at risk – with the exception of the Arctic foxes from Iceland and Commander Islands analysed in Chapter I – III. Overall, the thesis demonstrates the usefulness of fur samples for monitoring Hg and carbon and nitrogen stable isotopes in Arctic canids. It provides a practical tool for cross-study comparisons of Hg across different tissue types and fur, which will help with interpreting exposure risks of Arctic canids in future studies. It further suggests that absolute exposure to pollutants may be less important than indirect contamination via the feeding ecology and feeding opportunities of canid predators which may in turn affect population health and stability. Although the release of numerous, hazardous OHCs and Hg into the environment have been limited and regulated for a long time, the thesis emphasises that certain Arctic predator species are still highly exposed, which may pose a potential threat to their populations and the integrity of their ecosystems. Since our understanding and assessment methods of the specific risks and chronic impacts of pollutants on wildlife populations are still limited, chemical risk assessment should in future be up-scaled to population level effects. The results of the thesis will complement the existing data that form the basis for science-based recommendations for conservation management and policy measures and were already considered by the Arctic Monitoring and Assessment Program. Finally, to improve the effectiveness of the regulation of the release of pollutants into the environment and to support conservation initiatives for Arctic predators, particularly in view of climate change as a future stressor, further investigations are needed to improve our understanding of the mechanisms and interplay of drivers of pollution and its effects on Arctic wildlife populations and ecosystems.
