An integrated approach to studying the trophic ecology of a deep-sea faunal assemblage from the Northwest Atlantic

Despite being the largest ecosystem on Earth, the deep sea is still poorly known. Since the study of food webs allows a better understanding of ecosystems, the current research aimed to provide new insights into trophic relationships and element cycling within a deep-water faunal assemblage sampled in deep-sea areas of eastern Canada (Northwest Atlantic). The faunal assemblage consisted of a broad array of deep-sea taxa (143 species representing 8 phyla) collected within a tight window in space and time (100 km radius, 7 days), but across a large depth range (~1000 m) off insular Newfoundland. Functional diversity was studied along the bathymetric gradient. The integrated use of stable isotope, lipid, elemental, morphometric, and gut content analyses was crucial in obtaining an overall picture of the food web analyzed. Specifically, two major trophic pathways were recognized within the faunal assemblage: a pelagic pathway, relying on sinking organic matter (OM) as the primary food source; and a benthic pathway, in which settled OM constituted the base. A key role in energy and nutrient cycling was highlighted for pelagic vertical migrators and deep-water benthic communities. Vertical migrators actively provide inputs of food to benthic communities; benthic communities bioaccumulate certain energetic and nutritive compounds, and transfer them along the food web. Moreover, type and amount of lipids reflected not only dietary sources, but also environmental conditions typical of the deep sea. Large proportions of wax esters detected in certain species likely provide them with long-term energy reserves in a food-depleted environment. In addition, while the unsaturation level of phospholipid fatty acids increased, sterols diminished along the bathymetric gradient. This finding was interpreted to reflect adaptations of deep-water organisms to cope with increasing pressure and decreasing temperature with depth. Lastly, a preliminary analysis of the literature showed the existence of latitudinal trends in the isotopic and fatty acid composition of deep-sea benthic organisms, which exhibited lower C isotope ratios and higher proportions of ω3 fatty acids at temperate and polar latitudes than at tropical ones. This investigation raises concerns about potential effects of global climate change on deep-water communities, and about standardizing analytical methods to enable comparisons