Contrasting Temporal Patterns of Mercury, Niche Dynamics, and Body Fat Indices of Polar Bears and Ringed Seals in a Melting Icescape

Abstract

Polar bears (Ursus maritimus) and ringed seals (Pusa hispida) have a strong predator–prey relationship and are facing climate-associated Arctic habitat loss and harmful dietary exposure to total mercury (THg) and other pollutants. However, little is known about whether both species inhabiting the same area exhibit similar temporal patterns in Hg concentration, niche dynamics, and body fat indices. We used THg, δ13C, and δ15N values of western Hudson Bay polar bear hair (2004–2016) and ringed seal muscle samples (2003–2015) to investigate temporal trends of these variables and multidimensional niche metrics, as well as body fat indices for both species. We found a decline in THg concentration (by 3.8% per year) and δ13C (by 1.5‰) in ringed seals suggesting a change in feeding habits and carbon source use over time, whereas no significant changes occurred in polar bears. In contrast, the polar bear 3-dimensional niche size decreased by nearly half with no change in ringed seal niche size. The δ13C spacing between both species increased by approximately 1.5× suggesting different responses to annual changes in sympagic-pelagic carbon source production. Ringed seal body fat index was higher in years of earlier sea ice breakup with no change occurring in polar bears. These findings indicate that both species are responding differently to a changing environment suggesting a possible weakening of their predator–prey relationship in western Hudson Bay.

Mercury in Ringed Seals (Pusa hispida) from the Canadian Arctic in Relation to Time and Climate Parameters

“…Increases in δ 15 N values were observed over time in seals from the Central Arctic and Hudson Bay, but trends were, however, not statistically significant; δ 15 N levels were also stable through time for the other regions (Figure 2). Constant piscivorous diet and trophic level position were also recently reported for ringed seals from Hudson Bay (Yurkowski et al 2020). The diet of ringed seals at higher latitudes has been shown to consist mainly of Arctic cod ( Boreogadus saida ) compared with capelin ( Mallotus villosus ) and sand lance ( Ammodytes sp.)…”

“…In addition to carbon availability, baseline δ 13 C values in Arctic waters may further be affected by other factors such as the phytoplankton physiology, community composition, and changing sea‐ice conditions (de la Vega et al 2019). The increasing use of carbon associated with phytoplankton was suggested to explain decreasing δ 13 C trends observed in Hudson Bay seals (Yurkowski et al 2020). Values for δ 13 C were, however, not found to influence THg accumulation in seals from that study.…”

“…Results for THg in liver and muscle samples from the 1970s were previously reported by Smith and Armstrong (1975) and for the mid‐1980s to mid‐1990s by Wagemann et al (1995, 2000). The present results for THg in seal tissues from selected locations have been previously used in spatial trend studies: 1998 to 2001 data from 5 sites in Rigét et al (2005), 2007 to 2010 data from 10 sites in Brown et al (2016), and 2003 to 2015 results for Arviat in Yurkowski et al (2020).…”

“…Links between metal accumulation and feeding ecology (i.e., trophic levels and carbon source), were reported by the Brown et al (2016) and Yurkowski et al (2020) for Canadian Arctic ringed seals. Age and trophic position have also been found to influence Hg accumulation in Greenland ringed seals (Rigét et al 2012).…”

“…For example, a shift in diet (inferred from δ 15 N values) has been found to influence the accumulation of Hg in eggs of thick‐billed murres ( Uria lomvia ) from Nunavut, Canada (Braune et al 2014). Moreover, correlations between total Hg (THg) concentrations and stable isotope values were reported in hair of polar bears from Beaufort Sea, Hudson Bay, and Barents Sea (Canada) and were associated with changing foraging patterns over time (McKinney et al 2017; Lippold et al 2020; Yurkowski et al 2020).…”