Plastics, in the form of macro- or micro-debris (<5 mm), are an emerging threat for marine ecosystems. Despite large areas of accumulation identified, measurements in surface waters show a concentration deficit compared to the estimated annual flow of plastic waste arriving at sea, which has led to the 'missing plastics' paradigm. One of the suspected pathways is the export of these debris to the depths, notably via submarine canyon systems recognized as efficient particle transfer areas. The canyons are also home to rich biodiversity, supported by the presence of deep-sea coral reefs which have fundamental ecological and societal functions. However, observations in aquariums have shown that reef building species are subject to a fragile equilibrium. Particularly with regard to contamination by plastics, with responses that differ according to the size of the particles (macro- vs. microplastics). Moreover, we lack quantified in situ data to calibrate ecotoxicological experiments, making it hazardous to extrapolate the results acquired in aquariums to real habitat conditions. It is therefore urgent to estimate the real degree of exposure and the health status of plastic pollution on coral reefs in canyon ecosystems.

In this context, the objective of this campaign was twofold. First of all, it aimed at providing an estimate of macro- and micro-waste export flows at depth, in canyon ecosystems, to quantify the pollution status in these vulnerable marine ecosystems through a standardized approach, particularly in coral reefs, and collect data to establish a model of deposition and/or transfer to a lower level. A second objective concerned in situ measurement of the impact of plastics on coral communities and associated biodiversity, using integrative approaches at the holobiont (host and associated microbiome) and reef scales.

To meet these objectives, we used a 10-days work program at sea, including 7 days for HROV dives (observations, coral and sediment collection, deployment of biological and acoustic equipment) and 3 days for measurements from onboard operations (CTD, Tucker net). The chosen study area was the Lacaze-Duthiers canyon, located in the Gulf of Lion off Port-Vendres, which has already been the subject of numerous visits by the proposing team, allowing in situ experimental work on deep corals but also to observe the presence of numerous types of waste (plastic bags and bottles, fishing nets...), the nature of which would seem to vary according to the geographical area. We will thus aimed to:

1. Map the typology (type, nature, size) of macrodebris from HROV 100m transects at a scale that allowing us to propose a deposition model in relation to sediment dynamics and the anthropic pressure in the area.
2. Quantify and characterize microplastics in different compartments (deep water, sediment and coral tissue), with a view to proposing a model for the export and deposition of microplastics at depth and to estimate the concentrations (particles and associated contaminants) to which corals are exposed in their habitat.
3. Estimate the effect of plastics on the health of the coral holobiont (determination of host energy reserves and microbiome dynamics) in situ, based on the collection of fragments of colonies visually contaminated by plastics compared to visually 'healthy' colonies.
4. Collect coral fragments for an experimental aquaria study on the effect of microplastics on the health of colonies .
5. Studying the impact of plastics on the biodiversity associated with coral reefs using a visual (video imaging) and acoustic (deployment of hydrophones) approach.