Body size, depth of occurrence, and local oceanography shape trophic structure in a diverse deep-pelagic micronekton assemblage

Abstract

The Gulf of Mexico’s (GOM) deep-pelagic realm is one of the planet’s most speciose pelagic ecoregions, but detailed knowledge of ecosystem structure and function is lacking. Understanding trophic structure is critical to understanding ecosystem dynamics as trophic interactions regulate the flow of energy through ecosystems and influence the resilience of ecosystems to disturbance. Using novel stable isotope (SIA) data and historical stomach content (SCA) data, we examined deep-pelagic trophic structure using 58 species of micronekton from the GOM that encompassed a variety of migratory behaviors, depth distributions, and trophic strategies. We identified major trophic groupings, explored the extent that differences in diet, body size, vertical migration, and presence of a mesoscale feature (Loop Current) explained micronekton isotopic variation and estimated species-specific trophic positions. Cluster analysis of SIA data identified four trophic groups, although species were not strictly clustered by diet. Specifically, non-migratory zooplanktivores with elevated δ15N values were grouped with vertically migrating piscivores, suggesting some non-migratory species could be feeding within food chains with elevated isotopic baselines. The mean δ13C values of species encompassed a narrow range from −21.6 ‰ to −18.1 ‰, with variation in δ13C values of vertically migrating species explained by a positive relationship with body size and higher δ13C values in Loop Current water. In contrast, variation in δ13C values of non-migrators was primarily explained by elevated values in deeper-dwelling species and in larger species. The mean δ15N values of species ranged between 5.0 ‰ and 11.5 ‰, with variation in δ15N values of vertical migrators explained by a positive relationship with body size and lower δ15N values in Loop Current water. Variation in the δ15N values of non-migrators was largely explained by elevated δ15N values in deeper-dwelling species and in larger-bodied species. Trophic position (TP) estimations for the assemblage ranged between 2.6 and 4.9. The elevated δ15N values in non-migratory species led to higher TP estimates relative to estimates derived from stomach content data, but agreement between TP estimates using SIA and SCA was high for vertically migrating taxa. This discrepancy may be a factor of vertical migrating species having a more similar feeding depth than non-migrators. Our results provide important insight into the trophic organization of low-latitude oligotrophic ecosystems and demonstrate that trophic variation within micronekton assemblages is primarily driven by differences in body size, location in the water column and position relative to salient oceanographic features.