MEPS 747:19-34 (2024)  -  DOI: https://doi.org/10.3354/meps14697

ABSTRACT: Quantifying food web structure and interactions is important for understanding food web ecology, conservation biology and management. Trophic position (TP) is a useful metric for these purposes, describing an organism’s hierarchical role within the food web. TP can be estimated using the ratio of nitrogen isotopes (δ¹⁵N) of specific amino acids within animal tissue. An important component of the TP calculation is the trophic discrimination factor (TDF), which represents the enrichment of ¹⁵N with each trophic step. Early work suggested a fixed TDF throughout the food chain, but more recent studies indicate that this may not be appropriate. This study focuses on a shelf marine food web in the Northwest Atlantic, where we used bulk and amino acid-specific δ¹⁵N to determine TDFs for 4 feeding guilds spanning 3 trophic levels. Additionally, we compared TDFs between different tissues: muscle, liver and homogenised whole organism. TDFs calculated from amino acid δ¹⁵N decreased with increasing TP: 6.9 ± 2.3‰ (mean ± SD) for zooplankton, 5.1 ± 1.5‰ for pelagic fish, 4.5 ± 1.0‰ for Atlantic cod and 3.1 ± 1.0‰ for harp seals. This resulted from a combined effect of reduced ¹⁵N enrichment of trophic amino acids and ¹⁵N enrichment in the source amino acid phenylalanine among higher TP consumers. Finally, there was no evidence for differences in TDF between tissue types. By providing trophic guild-specific TDF values, our study enhances the accuracy of TP estimates and supports the development of amino acid isotope analysis as a tool for assessing food web structure across diverse taxa in the Northwest Atlantic.

KEY WORDS: Food web · Trophic position · Trophic discrimination factor · Stable nitrogen isotopes · Amino acids · Tissue comparison