Global Trophic Position Comparison of Two Dominant Mesopelagic Fish Families (Myctophidae, Stomiidae) Using Amino Acid Nitrogen Isotopic Analyses

The delta N-15 values of organisms are commonly used across diverse ecosystems to estimate trophic position and infer trophic connectivity. We undertook a novel cross-basin comparison of trophic position in two ecologically well-characterized and different groups of dominant mid-water fish consumers using amino acid nitrogen isotope compositions. We found that trophic positions estimated from the delta N-15 values of individual amino acids are nearly uniform within both families of these fishes across five global regions despite great variability in bulk tissue delta N-15 values. Regional differences in the delta N-15 values of phenylalanine confirmed that bulk tissue delta N-15 values reflect region-specific water mass biogeochemistry controlling delta N-15 values at the base of the food web. Trophic positions calculated from amino acid isotopic analyses (AA-TP) for lanternfishes (family Myctophidae) (AA-TP similar to 2.9) largely align with expectations from stomach content studies (TP similar to 3.2), while AA-TPs for dragonfishes (family Stomiidae) (AA-TP similar to 3.2) were lower than TPs derived from stomach content studies (TP similar to 4.1). We demonstrate that amino acid nitrogen isotope analysis can overcome shortcomings of bulk tissue isotope analysis across biogeochemically distinct systems to provide globally comparative information regarding marine food web structure.