As a result of climate change, coastal seas are subject to increasingly variable river inputs, with periods of increased drought and an increase in episodes of intense flooding. Coastal marine ecosystems are also subject to more intense storms and periods of unprecedented heat waves, particularly in the Mediterranean (2022, 2023). In order to gain a better understanding of the influence of these phenomena on the coastal environment, the OCEANIS team has organised, as part of the Golfe du Lion-Rhône worksite of the PPR-RiOMar, a major one-year monitoring programme of the mouth of the Rhône, with a three-day outing at sea every 15 days in 2023-2024.
The aim was to gain a better understanding of the biogeochemical mechanisms involved in high-intensity events (floods, storms) in the water column and sediments. Flooding of the Rhône is responsible for a significant proportion of dissolved (50%) and particulate (75%) inputs to the coastal zone over very short periods (1 to 2 weeks per year). These inputs have a major influence on the fate of particulate organic matter, whether of fluvial or marine origin, between degradation, export or burial, with the largest proportion of these inputs being deposited in the sediments of the prodelta. In addition, the marine heat waves that are intensifying in the Mediterranean are having negative and lasting effects on the habitats of fixed species (corals, gorgonians), but there is little information on the impact on the fauna and the biogeochemistry of sediments. This is why the OCEANIS team, in collaboration with the MIO in Marseille, the Observatoire Océanologique in Banyuls-sur-Mer, the CEFREM in Perpignan, the LOPS in Brest and the University of Nîmes, has set up a system for monitoring biogeochemical inputs and transformations in the water column and sediments, using a series of cores and measurements in the water column from October 2023 to September 2024 to provide answers about the impact of these events on pelagic and benthic ecosystems.
A total of 12 sea trips were carried out out of a planned total of 15: some trips had to be cancelled due to bad weather. These campaigns were carried out in conjunction with day trips on the N/O Antédon II from Marseille.
Coring and hydrology (CTD-O2 probe and Niskin bottle) were carried out at two stations in the Rhône prodelta: Station Mesurho (20 m deep) and AK (45 m deep).
During these missions, sediments and their pore waters were sampled to measure DIC, total alkalinity, ammonium, sulphates, methane and porosity, enabling material balances to be drawn up on the dynamics of recycling during and following flooding and heat waves. Core samples were taken to measure exchange fluxes at the water-sediment interface, based on core incubations in the MIO. Van Veen skips were also sampled in order to estimate changes in macrofaunal biodiversity linked to summer heat waves: these did not occur in 2024 and this part of the project is to be continued in 2025. The results of these campaigns will be analysed as part of Ocea Van Loenen’s thesis and the PPR-RiOMar programme.
The water column was also sampled at the surface and at the bottom using the SOMLIT protocol for physicobiogeochemical monitoring: temperature, salinity, dissolved O2, DIC, alkalinity, pH, nutrients, metals, chlorophyll, SS, POC, DOC. In addition, a mini-AUV was deployed during the November flood to carry out physico-chemical radials (T, S) between the coast and the Mesurho station.
All these measurements and samples of water and sediment are currently being analysed. They will be used to quantify the inputs from the Rhône and the associated biogeochemical dynamics in the water column and sediments of the Rhône prodelta as a function of extreme events. NB: Well aware of the need to limit our carbon footprint as much as possible, these missions were carried out alternately by the LSCE and the MIO (Univ Aix-Marseille and CNRS): those at the beginning of the month by the LSCE (using the TGV to get to Marseille) and those in the middle of the month by the MIO.
Photos : Lars-Eric Heimbürger and Google Earth