Alexis Khripounoff

The submarine canyons of the Brittany continental slope (Bay of Biscay—Northeast Atlantic) were studied to describe the conditions in which cold‐water corals occur in these areas and their consequences for coral metabolism. Near‐bottom current, oxygen, temperature, and particle flux, simultaneously measured for 2 yr at 850 m depth, revealed by spectral analysis the presence of several frequencies with a dominant semidiurnal tidal cycle and a current direction determined exclusively by the canyon topography. Sediment trap data also showed a seasonal input of material with a large peak of particles and carbon fluxes at the end of winter. Daily particle and carbon fluxes at 20 m above the bottom were 2.49 g m⁻² d⁻¹ and 20 mg C m⁻² d⁻¹, respectively. At the same area, in situ incubation of two cold‐water coral species, Lophelia pertusa and Madrepora oculata, using the Calmar benthic chamber indicated a coral oxygen consumption rate of 7.3 µmol O₂ dry weight g⁻¹ d⁻¹, and a production rate of ΣCO₂ and NH⁺₄ equal to 7.6 µmol CO₂ g⁻¹ d⁻¹ and 0.18 µmol NH⁺₄ g⁻¹ d⁻¹, respectively. The respiratory coefficient was equal to 1, indicating an omnivore food source. This conclusion was also confirmed by the O : N index. The coral energy requirement was equal to 2.1 mmol C m⁻² d⁻¹. Comparison between carbon input in terms of particle flux and carbon oxidation rate by corals indicates that the energy supply to the bottom seems not sufficient to sustain the development and growth of cold‐water coral all year in the Brittany canyons. However, coral necessarily require another source of energy such as zooplankton, particularly in summer and autumn when particle flux is reduced. Overall, the presence of abundant cold‐water coral in the Brittany submarine canyons indicates that this location is ideal for their development given appropriate hydrological condition, favorable hard substrate, and also food availability.

The deep outer margin of the Gulf of Lions and the adjacent basin, in the western Mediterranean Sea, are regularly impacted by open-ocean convection, a major hydrodynamic event responsible for the ventilation of the deep water in the western Mediterranean Basin. However, the impact of open-ocean convection on the flux and transport of particulate matter remains poorly understood. The variability of water mass properties (i.e., temperature and salinity), currents, and particle fluxes were monitored between September 2007 and April 2009 at five instrumented mooring lines deployed between 2050 and 2350-m depth in the deepest continental margin and adjacent basin. Four of the lines followed a NW–SE transect, while the fifth one was located on a sediment wave field to the west. The results of the main, central line SC2350 ("LION") located at 42°02.5′ N, 4°41′ E, at 2350-m depth, show that open-ocean convection reached mid-water depth (≈ 1000-m depth) during winter 2007–2008, and reached the seabed (≈ 2350-m depth) during winter 2008–2009. Horizontal currents were unusually strong with speeds up to 39 cm s⁻¹ during winter 2008–2009. The measurements at all 5 different locations indicate that mid-depth and near-bottom currents and particle fluxes gave relatively consistent values of similar magnitude across the study area except during winter 2008–2009, when near-bottom fluxes abruptly increased by one to two orders of magnitude. Particulate organic carbon contents, which generally vary between 3 and 5%, were abnormally low (≤ 1%) during winter 2008–2009 and approached those observed in surface sediments (≈ 0.6%). Turbidity profiles made in the region demonstrated the existence of a bottom nepheloid layer, several hundred meters thick, and related to the resuspension of bottom sediments. These observations support the view that open-ocean deep convection events in the Gulf of Lions can cause significant remobilization of sediments in the deep outer margin and the basin, with a subsequent alteration of the seabed likely impacting the functioning of the deep-sea ecosystem.