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.

Cold-Water Coral Habitats in Submarine Canyons of the Bay of Biscay

“…This sediment was not being remobilized due to the low internal wave energy and lower current speeds in the canyons of the Aquitaine margin (Koutsikopoulos and Le Cann, 1996;Mulder et al, 2012), preventing erosion and, therefore, the exposure of hard substrate. This erosion is observed in Blackmud, Audierne and Guilvinec Canyons on the northern and central Armorican margin (Mulder et al, 2012;Khripounoff et al, 2014). Such a regime of higher sedimentation toward the southern Bay of Biscay likely explains both the quasi absence of scleractinian habitats and the dominance of soft substrate habitats on the southern Armorican margin and the Aquitaine margin.…”

“…The Bay of Biscay is characterized by peculiar hydrodynamics mainly due to the numerous canyons and the steep continental slope. Tidal currents strengthened along the canyon seafloor and internal waves on the upper part of the slope favor exchanges between deep and superficial water masses and are reported in several canyons in this basin (Pingree and Mardell, 1985;de Madron et al, 1999;Mulder et al, 2012;Pichon et al, 2013;Khripounoff et al, 2014). Measurements from Guilvinec canyon, for example, show internal waves with vertical speeds enhancing the renewal of water with considerable daily variations in temperature, salinity and oxygen (Khripounoff et al, 2014).…”

“…Tidal currents strengthened along the canyon seafloor and internal waves on the upper part of the slope favor exchanges between deep and superficial water masses and are reported in several canyons in this basin (Pingree and Mardell, 1985;de Madron et al, 1999;Mulder et al, 2012;Pichon et al, 2013;Khripounoff et al, 2014). Measurements from Guilvinec canyon, for example, show internal waves with vertical speeds enhancing the renewal of water with considerable daily variations in temperature, salinity and oxygen (Khripounoff et al, 2014). Thus, it increases the amount of suspended material, a potential food for CWCs, that could move to water depths up to 2,800 m (Pichon et al, 2013), corresponding to the observed maximum water depth of antipatharians/gorgonians on hard substrate habitat in this study.…”

“…Tidal currents can have speeds up to 1 m/s (de Madron et al, 1999;Mulder et al, 2012;Khripounoff et al, 2014) and are weaker in the southern region of the Bay of Biscay (lower than 45 • N) (Koutsikopoulos and Le Cann, 1996). These currents follow a semi-diurnal tidal frequency (e.g., Pingree and Le Cann, 1989;Koutsikopoulos and Le Cann, 1996;de Madron et al, 1999;Mulder et al, 2012) and can extend deep into the canyons (Pichon et al, 2013).…”

“…These processes have an influence on the sediment accumulation within canyons and are thought to transport organic matter from the continental shelf to the deep sea and to increase the suspended particulate matter concentration Harris and Whiteway, 2011). Internal waves enhance the mixing of water masses and the release of nutrients which in turn favor the development of plankton (Pingree and Mardell, 1985;Huthnance, 1995;Khripounoff et al, 2014). This primary production will be transported into the canyons, increasing the amount and quality of food (Huthnance, 1995;Amaro et al, 2015Amaro et al, , 2016.…”