Abstract
The δ15N 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 δ15N values of individual amino acids are nearly uniform within both families of these fishes across five global regions despite great variability in bulk tissue δ15N values. Regional differences in the δ15N values of phenylalanine confirmed that bulk tissue δ15N values reflect region-specific water mass biogeochemistry controlling δ15N values at the base of the food web. Trophic positions calculated from amino acid isotopic analyses (AA-TP) for lanternfishes (family Myctophidae) (AA-TP ∼2.9) largely align with expectations from stomach content studies (TP ∼3.2), while AA-TPs for dragonfishes (family Stomiidae) (AA-TP ∼3.2) were lower than TPs derived from stomach content studies (TP∼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.
Highlights
Deep oceanic waters constitute the largest habitat on the planet
Estimates of TP is commonly estimated using d15Nbulk values (TPbulk) were variable across the five regions for both lanternfishes and dragonfishes and did not align with trophic positions (TPs) estimated from stomach content (SC) studies (Table S1, Table 2, Table S3)
Despite regional oceanographic influences on bulk stable isotope (SI) and CSIA data, strongly uniform amino acids (AAs)-TPs were observed across the five global regions for both fish groups
Summary
Deep oceanic waters (offshore depths .,200 m) constitute the largest habitat on the planet. There is growing evidence that overharvesting of these top trophic level animals may affect the stability and resilience of marine food webs through changes in system structure and function (e.g., [2,3]). Improved understanding of trophic structure and food web interactions at a time of changing climate dynamics is critical for anticipating future changes in exploited marine populations. We utilize a promising and emergent tool, compound-specific nitrogen isotope analysis of individual amino acids (CSIA), to compare the trophic positions (TPs) of widespread pelagic micronekton fishes from five biogeochemically distinct global ecosystems: Tasman Sea, California (CA) Current, Gulf of Mexico (GOM), northern MidAtlantic Ridge (MAR), and the North Pacific Subtropical Gyre (NPSG) near Hawaii
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