In this article, we evaluate the performance of a commercially available lifetime-based optode and compare it with data obtained by other methods. We performed a set of 10 different tests, including targeted laboratory evaluations and field studies, covering a wide range of situations from shallow coastal waters and wastewater treatment plants to abyssal depths. Our principal conclusion is that, owing to high accuracy (± 2 µM), long-term stability (more than 20 months), lack of pressure hysteresis, and limited cross-sensitivity, this method is overall more suitable for oxygen monitoring than other methods.

Environmental data were obtained on the nodule fields of the Clipperton-Clarion Fracture Zone at 5,000-m depth in June 2004 during the Nodinaut cruise. The long-term effects of physical disturbance made by a dredge 26 yr ago on the sediment were investigated. We focused our study on its track, which is still visible on the bottom. The first major consequence of the passage of this equipment was to remove 4 cm 6 0.5 cm of superficial sediment. The physical and chemical properties of this disturbed sediment sampled in the track did not change significantly over time and has not shown any recovery since the disturbance. However, an exception was observed for depth variations of manganese concentration in the sediment, which were similar in the track to those in the surrounding undisturbed sediment and can be explained by the diffusion over time of oxygen from the sea water. On the other hand, the biological activity measured in the track with the respirometer RAP2 carried by the submersible Nautile did not differ from the unperturbed site, which suggests that the benthic fauna has completely recovered, as have nutrient fluxes at the water-sediment interface.

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.

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