High contributions of sea ice derived carbon in polar bear (Ursus maritimus) tissue

Abstract

Polar bears (Ursus maritimus) rely upon Arctic sea ice as a physical habitat. Consequently, conservation assessments of polar bears identify the ongoing reduction in sea ice to represent a significant threat to their survival. However, the additional role of sea ice as a potential, indirect, source of energy to bears has been overlooked. Here we used the highly branched isoprenoid lipid biomarker-based index (H-Print) approach in combination with quantitative fatty acid signature analysis to show that sympagic (sea ice-associated), rather than pelagic, carbon contributions dominated the marine component of polar bear diet (72-100%; 99% CI, n = 55), irrespective of differences in diet composition. The lowest mean estimates of sympagic carbon were found in Baffin Bay bears, which were also exposed to the most rapidly increasing open water season. Therefore, our data illustrate that for future Arctic ecosystems that are likely to be characterised by reduced sea ice cover, polar bears will not only be impacted by a change in their physical habitat, but also potentially in the supply of energy to the ecosystems upon which they depend. This data represents the first quantifiable baseline that is critical for the assessment of likely ongoing changes in energy supply to Arctic predators as we move into an increasingly uncertain future for polar ecosystems.

Fig 1. Structures of highly branched isoprenoid lipids.

Structures of C₂₅ highly branched isoprenoid lipids measured in polar bear liver for calculation of quantitative H-Prints.

Fig 2. Geographic setting.

Map of polar bear subpopulations and locations of harvest (red dots) in Baffin Bay (BB), western Hudson Bay (WH) and southern Hudson Bay (SH). Coastlines were created using the Global Self-consistent, Hierarchical, High-resolution Geography database distributed under the GNU Lesser General Public license [28].

Fig 3. Polar bear (Ursus maritimus) data.

a) QFASA estimates of marine mammal prey (Bearded seal (Erignathus barbatus), beluga whale (Delphinapterus leucas), harbour seal (Phoca vitulina), harp seal (Pagophilus groenlandicus), ringed seal (Pusa hispida) and walrus (Odobenus rosmarus)) consumed by individual polar bears (stacked coloured bars) and overlaid with H-Prints (black circles) of individual bears. Individual polar bears are grouped according to the geographical location of collection and the corresponding subpopulation designation: Baffin Bay, western Hudson Bay and southern Hudson Bay (see Fig 2). For each subpopulation, mean QFASA estimates of marine mammal prey and mean (black circles) and median (grey diamonds) H-Prints are summarised in the single plot adjacent to each subpopulation plot (for H-Print-derived estimates of sympagic carbon, refer to Table 1). b) δ¹⁵N of individual bears (grey squares). For each subpopulation, mean δ¹⁵N are summarised in the single plot (box and whiskers) adjacent to each subpopulation plot.