The Loire is the longest river in France and its catchment area corresponds to one fifth of the metropolitan area of France. The Loire estuary, which connects the river to the Atlantic Ocean, is an ideal object of study to understand the functioning of the land-ocean continuum under temperate latitudes under various anthropogenic pressures. This macrotidal estuary has a maximum turbidity zone that is the site of intense biogeochemical processes. During the summer, these processes result in severe hypoxic conditions in the water column. Despite regulatory constraints on the watershed, which have resulted in a significant decrease in phosphorus concentrations in the river, a nutrient limiting primary productivity, and phytoplankton biomass, the frequency and severity of the hypoxias remain. The main objective of the REBELRED project is to examine the role of the sedimentary compartment and, in particular, of phosphorus inherited from decades of unsubstantiated agricultural practices, in the dynamics of primary production under the conditions of oxygenation of the bottom waters. The REBELRED project also aims to bring together experts in estuarine biogeochemistry working on the Gironde, the Seine and the Rhône to compare methods and knowledge around a common object: the Loire estuary. This should make possible to propose a methodology which could be applied in the various major French and European rivers and thus give environmental agencies the knowledge and tools needed to develop management and restoration policies for these complex and fragile ecosystems.

The series of REBELRED campaigns was divided into 3 phases: the RR1 campaign documented 6 stations in the muddy sediment outside the navigation channel between Nantes and Saint-Brévin-les-Pins during the decadal flood of February 2021. About fifteen interface cores were used to characterize the sedimentary environment and its exchanges with the water column. This campaign was characterized by a strong erosion of the sediments upstream of Paimboeuf. The remobilized particles fed the plume of the Loire that extended beyond the Plateau du Four to the open sea. The abraded sediments showed signs of intense methane degassing probably induced by the decompression generated by the erosion of several decimeters of sediment. The same methodological approach was adopted in August 2021 during the RR2 cruise, with the addition of a frankly marine station southeast of the Four Plateau. Low water conditions were moderate and no significant deoxygenation was noted. The RRK campaign, conducted in June 2021, allowed the sampling of couples of longer cores (~2m), thanks to the implementation of a Küllenberg corer on 3 of the 6 previously sampled stations. On these stations, a multitube corer also allowed to carry out a work similar to the previous campaigns. The June period was characterized by an intermediate flow with a severe deoxygenation at Paimboeuf. Methane degassing was again noted, this time moderate, nearly 4 months after the RR1 campaign.