Analysis of narwhal tusks reveals lifelong feeding ecology and mercury exposure

Rune Dietz,Jean-Pierre Desforges,Frank F Rigét,Aurore Aubail,Eva Garde,Per Ambus,Robert Drimmie,Mads Peter Heide-Jørgensen,Christian Sonne

doi:10.1016/j.cub.2021.02.018

Abstract

The ability of animals to respond to changes in their environment is critical to their persistence. In the Arctic, climate change and mercury exposure are two of the most important environmental threats for top predators.1-3 Rapid warming is causing precipitous sea-ice loss, with consequences on the distribution, composition, and dietary ecology of species4-7 and, thus, exposure to food-borne mercury.8 Current understanding of global change and pollution impacts on Arctic wildlife relies on single-time-point individual data representing a snapshot in time. These data often lack comprehensive temporal resolution and overlook the cumulative lifelong nature of stressors as well as individual variation. To overcome these challenges, we explore the unique capacity of narwhal tusks to characterize chronological lifetime biogeochemical profiles, allowing for investigations of climate-induced dietary changes and contaminant trends. Using temporal patterns of stable isotopes (δ13C and δ15N) and mercury concentrations in annually deposited dentine growth layer groups in 10 tusks from Northwest Greenland (1962-2010), we show surprising plasticity in narwhal feeding ecology likely resulting from climate-induced changes in sea-ice cover, biological communities, and narwhal migration. Dietary changes consequently impacted mercury exposure primarily through trophic magnification effects. Mercury increased log-linearly over the study period, albeit with an unexpected rise in recent years, likely caused by increased emissions and/or greater bioavailability in a warmer, ice-free Arctic. Our findings are consistent with an emerging pattern in the Arctic of reduced sea-ice leading to changes in the migration, habitat use, food web, and contaminant exposure in Arctic top predators.

Highlights

We explore the lifetime profiles of feeding ecology and mercury exposure in response to pronounced environmental change in the high Arctic narwhal, a strongly ice-associated species expected to be highly vulnerable to climate change
Lifetime changes in feeding ecology of high Arctic narwhal Using the reconstructed stable isotope profiles from ten narwhal tusks, we found marked intra-population differences in isotopic niches (Figure 2A) as well as large intra-individual variability in lifetime profiles of d15N and d13C (Figure 2B; Figure S1)
While we show how the narwhal tusk can be used to explore temporal feeding patterns, similar temporal analyses in other species have been carried out using regular teeth growth layers[30,31,32] or gradients along hair/fur,[33,34] feathers,[35] whiskers,[36,37] baleen plates,[38] or layers of cerumen in earplugs.[39]

Summary

Introduction

We explore the lifetime profiles of feeding ecology and mercury exposure in response to pronounced environmental change in the high Arctic narwhal, a strongly ice-associated species expected to be highly vulnerable to climate change. We found no significant linear temporal trend in mean annual d13C values (R2 = 0.04, p = 0.17; n = 48 years), likely due to non-linear increases and decreases within an individual lifetime over the study period (1962–2010)

Results

Conclusion