Trophic relationships of deep-sea benthic invertebrates on a continental margin in the NW Atlantic inferred by stable isotope, elemental, and fatty acid composition

Abstract

As deep-sea benthic ecosystems of continental margins provide numerous functions and services to humans, a better understanding of these key habitats and their communities is needed to help predict climate-driven shifts and support conservation efforts. Here stable isotope (δ13C and δ15N), elemental (%C, %N, and molar C:N), and fatty acid (FA) composition of 50 different deep-sea species, belonging to 7 major taxa, were analyzed in order to characterize their diet and trophic position, and to study the fate of energy and essential nutrients in the food web. In addition, relationships between depth and biochemical signatures (δ13C, δ15N, %C, %N, C:Nmol, and FAs) were also investigated. In this regard, %C, oleic acid (18:1ω9), and arachidonic acid (ARA, 20:4ω6) increased with depth. While the increase of %C was likely due to the preferential assimilation of the more nutritious N along the gradient, that of 18:1ω9 was presumed to reflect the need for longer-term energy reserves in deeper organisms, and that of ARA to indicate a higher reliance on the benthic trophic pathway at greater depth. Analyses also revealed that the focal deep-sea invertebrates occupied three trophic levels, whereas the weak correlation between δ13C and δ15N indicated that two or more trophic pathways were represented. Several feeding modes were also recognized within the assemblage. The lowest trophic positions were occupied by sponges most likely feeding on bacteria. Intermediate positions were mainly occupied by suspension feeders (e.g. sea anemones, corals), detritivores (e.g. the sea urchin Phormosoma placenta), and predators on small infaunal animals (e.g. the sea star Leptychaster arcticus). Conversely, predator/scavengers (e.g. various sea stars, gastropods, polychaete worms) occupied the highest trophic positions, together with sponges that were determined to be either carnivorous (e.g. Iophon piceum), or to feed on 15N- and 13C-enriched organic matter. Energetic compounds (i.e. 20:1ω11(13), 20:1ω9, and 22:1ω7) and essential nutrients (i.e. ARA) increased in proportion across the trophic levels of this food web, emphasizing the importance of certain dietary FAs for optimal organism health, and the key role of benthic communities in carbon cycling.