Submarine canyon heads are known as hotspots of biodiversity and productivity on continental margins. These incisions in the continental shelf support subtantial transfers of energy to deep-sea biological communities, through episodic transport of organic matter from the coastal zone to the canyon flanks and axial valley at several hundreds of meters deep. In the Gulf of Lion, these intermittent phenomena occur in winter under the influence of winds, by the formation of dense shelf water cascades or during storms. We hypothesize that these pulses of organic matter are quickly consumed by benthic communities and support their functionning over the year. To test this hypothesis, the MEDDESCENT cruise project proposed to monitor the ecological and biogeochemical responses in sediments of the canyon head during the period of water mass stratification in summer. This initial sampling program targeted two periods at the beginning and end of the stratification period. The operations were performed over 4 days in June and September to define reference states.

The first Leg  (MEDDESCENT1) took place from June 1 to 4, 2019, and allowed to sample sediments in the Lacaze-Duthiers canyon from the surface with a multitube corer and a VanVeen grab. 4 stations were sampled including 3 in the axial valley at depths of 500 to 700m (Figure 1). The program of the second Leg (MEDDESCENT2) aimed to sample sediments and macrofauna on the same stations. Core sampling was carried out at 500 and 600m depth and on the plateau, but the weather conditions did not allow grab samples to be collected. The sediments were conditioned on board for analyzes of biomarkers of the origin and degradation of organic matter in the sediments and of the composition of meiofauna communities, in addition to the O2 microprofiles carried out on board immediately after collection. Expected results should allow a first identification of the variables representative of the dynamics of benthic communities in the canyon head in connection with organic matter transfers and will contribute to a better assessment of the sensitivity of the ecosystem to climatic disturbances.