Deep-sea macrofaunal biodiversity of the Whittard Canyon (NE Atlantic)

Abstract

Submarine canyons are topographically complex features that incise continental and oceanic island margins. Due to their high environmental variability, faunal patterns and the drivers of these patterns within canyons remain poorly understood. With this in mind, benthic macrofaunal samples were collected from three branches (Western, Central and Eastern) of the Whittard Canyon, a major feature on the Celtic margin (NE Atlantic), and one site on the adjacent slope to the west of the canyon. A total of 136 core samples (each core 78.5 cm² surface area) was collected at depths of ~ 3500 m during 22 deployments of a hydraulically-dampened Megacorer at these four main sites. The samples yielded 4444 macrofaunal individuals (>300-µm sieve fraction) that were identified to higher taxon level; these included 2225 polychaetes identified to species level and 410 macrofaunal nematodes (>500-µm sieve fraction) identified to genus level. Macrofaunal abundance varied significantly among sites, and decreased from the Eastern branch (6249 ± standard deviation 1363 ind. m^-2) to the slope site (2744 ± SD 269 ind. m^-2). Ordination of macrofauna higher taxa and polychaete family and species level assemblages revealed that the Central and Eastern branches were substantially similar, while the Western branch and slope sites were relatively distinct. A similar pattern was evident in a corresponding ordination of environmental variables across the sites. In contrast, ordination of macrofaunal nematode assemblages grouped the three canyon sites together while the slope site was distinct. A total of 110 polychaete morphospecies was recorded, of which 46 species were found only in the canyon. Paramphinome jeffreysii was the most abundant species followed by Aurospio sp. B. Forty-eight nematode genera were identified, with Paramesacanthion and Metacylicolaimus being the most abundant. The canyon branches had broadly similar levels of polychaete species richness and diversity; these metrics and evenness values were higher on the slope than inside the canyon, while dominance was correspondingly lower on the slope. Polychaete feeding modes and nematode feeding groups shifted across the study site. Omnivorous polychaetes and predatory nematodes were more abundant inside the canyon compared with the slope site. Additional polychaetes were collected for molecular analysis (CO1 and 16S genes) from shallower parts ( ~ 1000 m) of the canyon and nearby Goban Spur. A total of 109 sequences was obtained from 139 specimens resulted in the identification of 33 polychaete species, of which 5 could be assigned Latin binomial names based on phylogenetic analyses. Only one of these species (P. jeffreysii) occurred at the deeper (3500 m) sites. Overall, this project highlighted the variability of macrofaunal assemblages within the soft-sediment habitat of a submarine canyon. This variation probably reflects the dynamic environmental conditions, which are influenced by the topographic profiles of individual canyon branches.

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