Limited depth zonation among bathyal epibenthic megafauna of the Gully submarine canyon, northwest Atlantic E.L. Kenchington n , A.T. Cogswell, K.G. MacIsaac, L. Beazley, B.A. Law, T.J. Kenchington Ecosystem Research Division, Department of Fisheries and Oceans, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada B2Y 4A2 article info Keywords: Submarine canyons Bathyal zone Epibenthos Video transects Species composition Species associations Soft bottom communities abstract The Gully is a large submarine canyon incised into the Scotian Shelf, in the northwest Atlantic. A submersible-mounted camera was used to collect 17 km of high resolution video imagery of the soft sediment flanks and floor of the canyon between 1000 and 2500 m depth. A depth-stratified random survey design with two transects per stratum was followed. Depth strata were drawn at 1000–1500 m, 1500–2000 m and 2000–2500 m reflecting global and regional faunal boundaries. The 47,614 individuals or colonies observed on the transects were identified into 49 unique taxa drawn from 7 phyla, while 21 additional taxa were observed between the transect lines. Cnidaria was the most diverse phylum (22 taxa) on the transects, followed by the Echinodermata (15 taxa). Most fauna were sessile or sedentary suspension feeders. A species of Xenophyophorida, likely Syringammina sp., was the most abundant taxon. Cluster analysis of transects based on a Bray–Curtis (BC) matrix of species abundance identified one cluster of five transects and one independent transect – the deepest transect (2406 m average depth). A similarity profile test indicated that this structure is not random. There was a significant change in the megafaunal assemblage with depth over scales of 10s of kilometers, accounting for 65% of the variation in the BC matrix. Both total organic carbon and labile carbon were higher in cores from the 1000–1500 m stratum than at greater depth, consistent with transport of food and sediments into the canyon from shallower waters. The first principal component calculated from species abundances separated the two transects from the 1000–1500 m depth stratum from the others indicating that sediment food supply influences community structure and composition. Over small spatial scales ( r10 m), eight groups of species associations were identified. However, the associations between the species within each group were not strong with the highest correlation between the Xenophyophorida and the Pennatulacea (ρ ¼0.66). At this spatial scale substrate type was an important factor, with hard and soft bottom species associations being identified. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. 1. Introduction Submarine canyons are common features along the continental margins of the world's oceans (Allen and Durrieu de Madron, 2009; De Leo et al., 2010; Harris and Whiteway, 2011). Many are character- ized by V-shaped sections with steep gradients, incised into the bedrock and sediments of continental shelves and slopes (Normark and Carlson, 2003). Geologically active canyons can be important conduits of sediment and associated nutrients from the continents to the deep ocean (Shepard and Dill, 1966). They generate complex water flows, channel occasional turbidity currents and focus internal tides that resuspend sediment and organic matter, which is advected to deeper water (Gardner, 1989; Puig et al., 2003; Tesi et al., 2010; Vetter and Dayton, 1999). Consequently, they can be of considerable importance to the community structure and functioning of marine ecosystems. Canyon geomorphology and local hydrodynamic conditions shape the distribution, abundance and composition of resident epibenthic megafauna by determining the depositional and erosional processes which affect surficial substrate type and heterogeneity, both of which then influence the diversity and abundance of benthic organisms (e.g., Cunha et al., 2011; Hunter et al., 2012; McClain and Barry, 2010; Paterson et al., 2011; Vetter and Dayton, 1998). Since each canyon has unique physical characteristics, their benthic faunas can differ mark- edly (Levin and Gooday, 2003), while being equally different from those on adjacent continental slopes (Levin et al., 2010). Only a fraction of the world's submarine canyons have yet been studied (Harris and Whiteway, 2011; Levin and Gooday, 2003) and information from additional examples increases our ability to draw conclusions on the regional and global roles of canyons in marine ecosystems. The eastern continental margin of North America has a higher concentration of shelf-break canyons than many other parts of the world (Allen and Durrieu de Madron, 2009). One of the largest of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr2.2013.08.016 n Corresponding author. Tel.: þ1 902 426 2030; fax: þ1 902 426 6695. E-mail address: Ellen.Kenchington@dfo-mpo.gc.ca (E.L. Kenchington). Please cite this article as: Kenchington, E.L., et al., Limited depth zonation among bathyal epibenthic megafauna of the Gully submarine canyon, northwest Atlantic. Deep-Sea Res. II (2013), http://dx.doi.org/10.1016/j.dsr2.2013.08.016i Deep-Sea Research II ∎ (∎∎∎∎) ∎∎∎–∎∎∎