The extraction of marine aggregates is one of the main activities that have been carried out for many years in the coastal area. France is currently the 5th European country in terms of the annual volumes of extracted aggregates (ICES WGEXT annual report 2018). However, the extraction projects are very often controversial, particularly because of its effects (destruction of benthos, creation of a turbid plume, etc.) and potential impacts on environments (habitat modifications, disturbance of the sediment dynamics and effect on coastline migration...). Although the direct effects of extractions have been well described (Dalfsen et al., 2000; Birchenough et al., 2010; Degrendele et al. , 2010; Froján et al., 2011; Hussin et al., 2012; de Backer et al., 2014; de Jong et al., 2014; de Jong et al., 2015; Waye-Barker et al., 2015; Cooper and Barry, 2017; Desprez, 2000; Desprez et al., 2009; Le Bot et al., 2010; Duclos et al., 2013), only a few studies have focused on the resilience of extraction sites after cessation of the operations (Kenny et al., 1994; Boyd et al., 2004; Cooper et al., 200 ; Foden et al., 2009; Frojàn et al., 2011; Desprez, 2000; Le Bot et al., 2010).

The extraction site of Le Pilier (located off the Loire estuary) has been operated between 1986 and 2017. During this period, more than 40 Mm3 of sand and gravel were collected. The extractions led to the creation of a pit of almost 2.5 km² reaching a maximum of 7-8 m depth. The environmental monitoring carried out in accordance with regulations has shown changes in the nature of the superficial sediments. In some places, deeper coarser sediments are now exposed due to the removal of the finer superficial sediments, while in contrast other areas now evidence finer sediments, contributing to a change in benthic community. This site is one of the first large-scale extraction site closed to extraction, and therefore no further extraction will be carried out. Therefore, the site seems particularly well suited to start a scientific research program focusing on the resilience of a site under anthropogenic pressure. During the scientific cruise, biological and physical proxies of the system will be monitored in order to investigate the interaction between ecosystem functioning and resilience.  

The aim of this project was therefore to acquire the necessary data to monitor and observe the physical and biological resilience (or lack thereof) of a site subject to anthropic pressure.

The work carried out corresponded to an identical reproduction (or with some adaptation according to the constraints encountered) of the operations carried out during the campaigns in autumn 2020, 2021 and 2022 and in spring 2021 and 2022 in order to obtain repetitive monitoring data for the observation of the resilience of the site.

The aim was to acquire bathymetric (SMF EM2040) and backscatter (Klein 3000 side scan sonar) data with fullcoverage of the former extraction site and a field of dunes located to the east of the perimeter, as well as disjointed profiles towards the open sea, in order to monitor the upstream sediment inputs to the pit and towards the coast in order to assess the evolution of the dynamics under the influence or not of the pit as well as any inputs from the Loire river.
Shipeck sampling was also carried out to calibrate the acoustic images (95 stations per mission). 56 grab points were carried out at the same positions (in 2020, 2021 and 2022) in order to monitor changes in the nature of the seabed at these stations.

In order to obtain an estimate of the types of deposits accumulated and their thickness, we used a Kullenberg corer (in gravity and piston mode) and a Fantacore interface corer (no Ronanberg during the 2022 missions) at stations defined as priorities for the characterisation of the benthic fauna or the objectives of the project (zone of strong disturbance, accumulation zone).

The evolution of the benthic fauna was monitored at 7 stations (with CTD and video) collected with a Day grab (with 7 replicates) and selected according to the following characteristics:

• In the license area within the main pit reaching 8 metres in depth (x2);
•  In the license area and within the very first perimeter that was authorised for extraction in 1986 (x1);
• Within the license area and in the vicinity of the last sectors that have been extracted (x1);
• Outside the license area, in a sector subject to the influence of the turbid plume (x1);
• Outside the license area, at the peripheral limit in a zone where slope adjustment phenomena can be expected (x1);
• Outside the license area, at a so-called reference station located upstream of the sedimentary inputs from the open sea (x1).

In order to characterise the hydro-sedimentary processes, we wanted to be able to combine high-frequency measurements of the sedimentary processes (suspended matter fluxes and bed load flux) with low-frequency measurements of the sedimentary dynamics captured by successive bathymetric surveys.

In the autumn of 2020, we were unable to deploy the mooring in the area, due to the loss of the equipment during the SOLIBOB mission, which was to be deployed on site after reconditioning. In addition, only two days of work in the area (instead of four) could be carried out due to poor sea conditions. However, an acquisition strategy was implemented (in a hurry) and allowed the collection of :

• Measurements of temperature, salinity, turbidity, TSS and particle size parameters at two fixed points in and out of the license area at various times of the tide.
• Current measurements using an ADCP towed through the deepest part of the pit from west to east at different times of the tide.
• Experimental measurements of sand drift were made using a hydrophone deployed on the bottom to "listen to the sand grains". The structure, also equipped with a turbidimeter, was deployed just outside the trench during the 2 days of work.

In the autumn of 2021 one mooring comprising an ADCP, a turbidimeter and an altimeter had been deployed in the centre of the pit (feedback from the Local Fisheries Committee had informed us of the risk of hooking for a mooring deployed outside the concession, linked to the practice of trawling on the study site). CTD profiles at the same position for 1 tidal cycle (13 hours) and during 2 consecutive days had been carried out. This mooring was recovered in spring 2022.