Rivers bring large quantities of dissolved and particulate matter to the ocean (sediments, organic and inorganic matter, nutrients, contaminants) which will determine the productivity of coastal areas and, consequently, the distribution of pelagic ecosystems and benthic. Coastal marine ecosystems are particularly important since they produce nearly a third of goods and services of ecological origin, that is to say as much as terrestrial ecosystems or offshore marine ecosystems [Costanza et al., 1997]. In the coastal ocean, the biogeochemical cycles (C, N, P, Si) are mainly controlled by their sedimentary component and it is essential to quantify the dynamics of this reservoir. The spatial and temporal variability of fluxes at the water-sediment interface must therefore necessarily be taken into account to reduce uncertainties on the carbon balances at coastal interfaces [Borges et al., 2005; Krumins et al., 2013] and thus specify the net autotrophic or heterotrophic character of coastal marine ecosystems [Chen and Borges, 2009].

The continental shelves dominated by rivers (River-dominated Ocean Margins - RiOMar) are key environments for the coastal environment. In fact, the rates of primary production, sediment deposition, mineralization, and organic matter (OM) burial are among the highest of all marine systems [Dagg et al., 2004; McKee et al., 2004]. In terms of carbon, transfers by rivers represent an essential link in the global carbon cycle between the large terrestrial and marine biospheres. In fact, the contributions by rivers constitute the primary source of carbon for the coastal ocean and they are estimated at 1100 TgC / yr, of which 540 TgC in organic form and 560 TgC in inorganic form [Chen and Borges, 2009].

The effects of benthic recycling of organic matter on carbonate dissolution are generally poorly understood [Jahnke et al., 1997]. However, the degradation of M.O. has a direct influence on carbonate chemistry since bacterial respiration produces DIC and leads to a decrease in pH. However, the pH is the key parameter of the carbonate system and it is decisive with regard to the acidification of the oceans [Jahnke and Jahnke, 2004]. As a result, the role of sediment and early diagenesis on carbonate chemistry is largely underestimated. In this context, the scientific objectives of the DICASE campaign are:

1. Document the carbonate system (pH, TA, DIC) at the water-sediment interface (bottom water and pore water) and quantify the CO2 fluxes in the sediments of the Rhône prodelta,
2. Study the effects of benthic recycling on the chemistry of carbonates in sediments and determine the buffering role of carbonates vis-à-vis the bacterial production of CO2,
3. Complete the series of in situ measurements on the recycling of organic matter in the sediments of the Rhône prodelta,
4. Understanding the spatio-temporal variations of CO2 fluxes at the water-sediment interface in the coastal ocean will also make it possible to anticipate the impacts linked to ocean acidification.