Global trophic position comparison of two dominant mesopelagic fish families (Myctophidae, Stomiidae) using amino acid nitrogen isotopic analyses

Abstract

The δ(15)N 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 δ(15)N values of individual amino acids are nearly uniform within both families of these fishes across five global regions despite great variability in bulk tissue δ(15)N values. Regional differences in the δ(15)N values of phenylalanine confirmed that bulk tissue δ(15)N values reflect region-specific water mass biogeochemistry controlling δ(15)N 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.

Figure 1. Map of sample collection locations.

Approximate capture locations for species of lanternfish (closed symbols) and dragonfish (open symbols) specimens analyzed in this study, from five distinct and globally distributed regions (Tasman Sea (TAS), California Current (CA), Gulf of Mexico (GOM), Hawaii (HI), and the Mid-Atlantic Ridge (MAR)).

Figure 2. Influence of regional biogeochemistry on consumer isotopic composition.

Relationship between δ¹⁵N values of phenylalanine (δ¹⁵N_phe) (‰) and bulk white muscle tissue (δ¹⁵N_bulk) (‰) in 33 specimens of mesopelagic lanternfishes (empty symbols) and dragonfishes (filled symbols) from offshore waters of the Tasman Sea (TAS), California Current (CA), Gulf of Mexico (GOM), Hawaii (HI), and the Mid-Atlantic Ridge (MAR). Error bars are standard deviations. δ¹⁵N_phe and δ¹⁵N_bulk values in fishes across all regions are significantly positively correlated (p<0.05, r² = 0.58; y = 0.75× –7.14).

Figure 3. Uniform amino acid based trophic positions for lanternfishes and dragonfishes.

Relationship between fish length (mm) and amino acid CSIA estimated trophic positions of a) individual lanternfishes, and b) individual dragonfishes from five regions. Also shown is the relationship between individual fish bulk tissue δ¹⁵N values (‰) and amino acid CSIA estimated trophic positions of c) lanternfishes and d) dragonfishes from five regions. Error bars indicate propagated error from trophic position calculation (see methods).