Submarine canyon systems have been described as biodiversity hotspots that enhance benthic communities, especially when compared to nearby open slope habitats at similar depths. These complex systems act as major conduits of organic matter and sediment transport within continental shelves and promote gradients in food resources, habitat heterogeneity, sediment resuspension and sediment deposition. However, only few studies have investigated
macrobenthic community metrics in relation to the multiple environmental factors that are altered by canyons. The research within this thesis was conducted as part of a 5-year multidisciplinary program funded by BOEM, USGS and NOAA, which focussed on two of the largest canyons on the shelf of the Mid-Atlantic Bight (MAB) (western North Atlantic shelf, USA), Baltimore and Norfolk canyons. The overarching aim of this study was to describe the physical characteristics of these canyons and relate these to different aspects of
Firstly, the main oceanographic drivers acting within the two canyons and their adjacent slopes were described. Canyons and slopes exhibited similar hydrographic features in terms of the presence of characteristic water masses, namely the shelf-slope front, Western North Atlantic Central Water (WNACW) and Western Atlantic Sub-Arctic Intermediate Water (WASIW) were observed. However, near-bottom measurements from a year of data revealed several clear differences between the two canyons. Current speed and direction in both
canyons were driven by semidiurnal tides (M2) that were strongest near the heads of both canyons and exhibited a persistent up-canyon directionality. In both canyons, benthic nepheloid layers were present that were not observed in adjacent slope transects. It is likely that differences observed, are related to a difference in canyon morphology, the orientation of the canyons to the shelf and differing interactions with internal waves.
Secondly, the benthic macrofaunal ecology of Baltimore and Norfolk canyons and respective adjacent slopes were described from replicated sediment cores that were collected along their main axes (~180-1200 m) and at comparable depths on the adjacent slopes. Cores were sorted and whole community macrofaunal (>300 μm) abundance, diversity and standing stocks were
assessed. Coupling family-level community data, with sediment grain-size and
biogeochemistry data yielded insight into community dynamics across depth and biogeochemical gradients. Canyon communities were significantly different from slope communities with differences in diversity, abundance patterns and community assemblages, 12 which were attributed to high levels of organic matter enrichment within the canyons. The two canyons hosted different communities that were indicative of environmental disturbance, with bivalves dominating mid-canyon depths in Baltimore Canyon, and deposit-feeding
polychaetes in Norfolk Canyon. Abundance-biomass curve comparisons confirmed that lower canyon communities (800-1180 m) were disturbed, as well as upper slope communities (180-555m) on the adjacent slope.
Thirdly, for the first time in a canyon system, a biological traits approach was used to define functional differences between the two MAB canyons and their adjacent slope communities. A total of 49 trait modalities across 10 biological traits were used to characterise the community and showed that higher functional richness was present within upper and middle canyon communities compared to slope communities across the studied depth gradient. Lower canyon communities (800-900 m) were less functionally rich, a feature attributed to substantial biomass contribution of opportunistic and dominant taxa that benefited from organically enriched sediment in the canyons. Bioturbation potential was higher in the canyons than adjacent slope, especially within Baltimore Canyon, and was attributed to the high affinities for surface and subsurface sediment modifiers and sediment ingestion or filterfeeding
bioturbators. The trait affinities within canyons showed propensity for sediment
reworking to greater depths, suggesting that canyon communities may enhance nutrient fluxes and burial of accumulated organic matter. The findings confirmed that enhanced macrofaunal community ecosystem function and higher bioturbation occurred within the canyons compared to the adjacent slopes and provided new insight into the distinct functional roles found within canyon and slope macrofauna.
This study was the first to characterise the physical setting and macrofaunal communities of the Baltimore and Norfolk canyons in detail. Overall, the canyons were notably more dynamic than the adjacent slopes, acting as true disruptors to established shelf/slope patterns in biodiversity, abundance and functioning of macrofauna.