Benthic epifauna assemblages, biomass and respiration in The Gully region on the Scotian Shelf, NW Atlantic Ocean

Abstract

Epifauna community assemblages were identified by analysis of 35 mm still pictures taken at 92 stations in The Gully, a submarine canyon on the edge of the Scotian Shelf off Canada's east coast. Numbers of epifauna taxa, seabed type and area of hard (gravel) substrate were measured in images from underwater camera systems. A substrate hardness index, and epifauna community biomass and respiration rates were calculated at 31 stations within The Gully and on adjacent banks using image analysis, volume conversion factors and an empirical allometric regression between wet weight and measured respiration rates for major epifauna taxa. Average turnover time (TT) in days for each location was calculated as energy stored in biomass to calories respired. Non-parametric correlation analysis on dissimilitarity matrices and cluster analysis based on water mass, substrate type and bathymetry distinguished 6 types of benthic habitats and 7 assemblages of co-occurring assemblages for 175 epifauna taxa associated with specific habitat types. Sandy areas on adjacent banks (50 to 300 m) were dominated by echinoderms. Stations with glaciomarine sediments near the head of The Gully (100 to 500 m), dominated by anemones, sponges and soft corals, had the greatest numbers of taxa. Epifauna biomass and respiration rates were highest where gravel cover was >50%. There were fewer taxa with lower biomass and respiration in deeper (>500 m) water. Deep-water brittle stars, hard and soft coral and anemone species predominated where the proportion of hard substrate decreased (<50%). Average TT was low (388 and 453 d) at stations 200 to 500 m deep and in the mid-central areas of The Gully, where the percentage of rock cover was >50%, with a trend to higher values (>600 d) in deeper water. Stations with low TT values were dominated by molluscs. The observations suggest that organically rich particulate matter used by suspension- and deposit-feeding epifauna is transported from the head of the canyon and surrounding banks down the axis of The Gully.