MEPS prepress abstract   -  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 subsequent studies indicate 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 four feeding guilds spanning three 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) 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.