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Submarine canyons that incise continental slopes are considered an important conduit for transport of suspended matter between shelf seas and deep-ocean. However, the exact mechanisms influencing matter transport and deposition are still largely unknown. We present moored observations of current flows, acoustic echo intensity and, in high-resolution, temperature variation from two contrasting mooring sites 6.5 km apart horizontally in 790 and 1130 m water depth in one of the tributaries incising the flank of the easternmost branch of Whittard Canyon, North-East Atlantic Ocean. The shallow site is situated on a steep thalweg slope which is supercritical for semidiurnal internal tides that have >100 m amplitudes and turbulent overturning evenly distributed over the 240 m range of observations, with no particular intensification near the seafloor. In contrast, the deep site is located on an along-canyon slope that is approximately critical for semidiurnal internal tides, with steeper slopes up-canyon and to the sidewalls. The internal tide amplitude at the deep site is about 75 m. In the lower 100 m, flow intensifies and becomes more rectilinear along the canyon axis, together with intensifying mean turbulence dissipation rate and tidally-averaged down-canyon flow. Over a semidiurnal tidal period three episodes of relatively large turbulence are observed. Turbulence is largest when an upslope moving bore passes. Smaller but still relatively large turbulence is associated with a secondary upslope bore and with strong convective overturning during the downslope tidal phase. The convection is clearly distinguished in temperature variance spectra between the buoyancy and Ozmidov frequencies, the range in which anisotropic stratified turbulence is expected. Averaged over the entire depth-time series, mean turbulent kinetic energy dissipation amounts to 4.4 ± 2 × 10-7 m2 s-3, equal for both sites. In the tributary, the first upslope moving bore and the convective overturning at the deep site will affect sediment resuspension most.

Cold-water coral (CWC) reefs are distributed globally and form complex three-dimensional structures on the deep seafloor, providing habitat for numerous species. Biogeochemical cycling and community metabolism of CWC reefs have mostly been studied by targeting sub-sampled organisms (e.g. corals, sponges). Here, we measured the community O2 and dissolved inorganic nitrogen (DIN) flux of CWC reef habitats with different coral cover and bare sediment (acting as reference site) in the Logachev Mound area (NE Atlantic). Two methodologies were applied: the non-invasive in situ aquatic eddy co-variance (AEC) technique, and ex situ whole box core (BC) incubations. The AEC system was deployed twice per coral mound (69 h in total) for a total of 69 h, providing an integral estimate of the O2 flux from a total reef area of up to 500 m2, with mean O2 consumption rates ranging from 11.6 ± 3.9 to 45.3 ± 11.7 mmol O2 m-2 d-1 (mean ± SE). Six box cores from two coral mounds were incubated onboard for ~24h to quantify O2 and DIN fluxes. The CWC reef community O2 fluxes obtained from the BC incubations ranged from 5.7 ± 0.3 to 28.4 ± 2.4 mmol O2 m-2 d-1 (mean ± SD) while the O2 flux measured by BC incubations on the bare sediment reference site reported 1.9 ± 1.3 mmol O2 m-2 d-1 (mean ± SD). Overall, O2 fluxes measured with AEC and BC showed reasonable agreement, except for one station with high habitat heterogeneity. Our results suggest O2 fluxes of CWC reef communities in the North East Atlantic are around five times higher than of sediments from comparable depths and living CWCs are driving the increased metabolism. DIN flux measurements by the BC incubations also revealed around two times higher DIN fluxes at the CWC reef (1.17 ± 0.87 mmol DIN m-2 d-1), compared to the bare sediment reference site (0.49 ± 0.32 mmol DIN m-2 d-1), due to intensified benthic release of NH4+.Parallel measurements of the DIN flux in the BC incubations also revealed intensified benthic release of NH4+, which gets partly nitrified, at the CWC reef (1.17 ± 0.87 mmol DIN m-2 d-1), compared to the bare sediment reference site (0.49 ± 0.32 mmol DIN m-2 d-1). Our data indicate that the amount of living corals and dead coral framework largely contributes to the observed variability in O2 fluxes on CWC reefs. A conservative estimate, based on the measured O2 and DIN fluxes, indicates that CWC reefs process 20% to 35% of the total benthic respiration on the southeasterly Rockall Bank area, which demonstrates that CWC reefs are important to carbon and nitrogen mineralization at the habitat scale.