Compound specific isotope analyses of harp seal teeth: tools for trophic ecology reconstruction

As sentinels of ecosystem health, high trophic level predators integrate information through all levels of the food web. Their tissues can be used to investigate spatiotemporal variability in foraging behaviour, and with the appropriate analytical methods and tools, archived samples can be used to reconstruct past trophic interactions. Harp seal (Pagophilus groenlandicus) teeth collected in the 1990s from the Northwest Atlantic were analysed for bulk stable carbon and nitrogen isotopes (δ¹³C_bulk and δ¹⁵N_bulk), and compound specific stable nitrogen isotopes of amino acids (δ¹⁵N_AA) for the first time. We developed a fine-scale, annual growth layer group (GLG) dentine sub-sampling method corresponding to their second and third year of life. In accordance with previous diet studies, while there was individual variability in δ¹⁵N_bulk, δ¹³C_bulk, and δ¹⁵N_AA measurements, we did not detect significant differences in isotopic niche widths between males and females, or between GLGs. Relative trophic position was calculated as the baseline corrected δ¹⁵N_AA values using trophic (glutamic acid) and source (phenylalanine and glycine) amino acids. Variability was measured between individuals in their relative trophic position, but within individual variability was low, suggesting that they fed at the same trophic level over these 2 years of life. These novel δ¹⁵N_AA data may therefore suggest individual, specialist harp seal foraging behaviour in sub-adults. Our findings show that compound specific stable isotope signatures of archived, inert predator tissues can be used as tools for the retrospective reconstruction of trophic interactions on broad spatiotemporal scales.