Deep cold‐water coral ecosystems in the Brittany submarine canyons (Northeast Atlantic): Hydrodynamics, particle supply, respiration, and carbon cycling

Abstract

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.