Spatial variation in the trophic structure of micronekton assemblages from the eastern tropical North Atlantic in two regions of differing productivity and oxygen environments

• Published in: Deep Sea Research Part I: Oceanographic Research Papers
• Main Authors: Czudaj, Stephanie, Giesemann, Anette, Hoving, Henk-Jan T., Koppelmann, Rolf, Lüskow, Florian, Möllmann, Christian, Fock, Heino O.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2020
• Subjects: North Atlantic; Copepods
• Online Access: https://oceanrep.geomar.de/id/eprint/50166/; https://oceanrep.geomar.de/id/eprint/50166/1/Czudaj%20et%20al%202020.pdf; https://doi.org/10.1016/j.dsr.2020.103275

Highlights: • Micronekton ecology differed between equatorial (EQ) and low-oxygen waters (LO). • The δ15N enrichment per trophic level was lower in the productive LO than in the EQ. • Migrating fauna in the LO encounter increased competition for food resources. • The δ15N values of non-migrators increased with depth in the EQ, but not in the LO. • The δ15N values of mesopelagic micronektonivores were lower in the productive LO. Abstract: The ecology of vertically migrating mesopelagic micronekton is affected by physical properties of their environment. Increased light attenuation in particle-rich productive waters, as well as low oxygen conditions decrease the migration amplitude. This likely has implications on the trophic organisation of micronekton communities, which are predominantly governed by niche partitioning in the vertical dimension. We investigated trophic structures of pelagic communities in the eastern tropical North Atlantic by comparing micronekton species assemblages from the low-oxygen region influenced by Mauritanian upwelling between 8° and 11° N (LO) and the less productive and more oxygenated equatorial area between 0 and 4°N (EQ). We analysed stable isotopes of carbon (δ13C) and nitrogen (δ15N) in body tissues of 35 species of mesopelagic fishes, four species of cephalopods, two species of cnidarians, and two species of decapods and used these values as a proxy for their trophic niche and correlated them with the traits feeding guild, migration pattern, mean depth of occurrence and body size. Our results demonstrate significant regional differences in the food web structure and vertical trophic interactions of the investigated micronekton assemblages. Diurnally migrating fishes that predominantly feed on copepods exhibited higher δ15N values in the LO (9.6‰) than in the EQ (8.9‰), reflecting changes in baseline values of pelagic tunicates. Contrary, all other Feeding – Migrator guilds show lower or similar δ15N values in the LO compared to the EQ, indicating reduced isotopic enrichment ...