Impact of ocean acidification on the biogeochemistry and meiofaunal assemblage of carbonate-rich sediments: Results from core incubations (Bay of Villefranche, NW Mediterranean Sea)

Marine sediments are an important carbonate reservoir whose partial dissolution could buffer seawater pH decreases in the water column as a consequence of anthropogenic CO2 uptake by the ocean. This study investigates the impact of ocean acidification on the carbonate chemistry at the sediment-water interface (SWI) of shallow-water carbonate sediments. Twelve sediment cores were sampled at one station in the Bay of Villefranche (NW Mediterranean Sea). Four sediment cores were immediately analyzed in order to determine the initial distribution (T0) of dissolved inorganic carbon (DIC), total alkalinity (TA), pH and dissolved oxygen (O2) in the porewaters and to quantify sediment-water fluxes. Four other cores were kept submerged in the laboratory for 25 days with ambient seawater (pHT = 8.12) and the remaining four cores were incubated with acidified seawater (average pH offset of −0.68). This acidification experiment was carried out in an open-flow system, in the dark and at in-situ temperature (15 °C). Every three days, sediment-water fluxes (DIC, TA, pH, O2 and nutrients) were determined using a whole core 12-h incubation technique. Additionally, vertical O2 and pH microprofiles were regularly recorded in the first 2 cm of the sediment during the entire experiment. At the end of the experiment, TA, DIC and Ca2+ concentrations were analyzed in the porewaters and the abundance and taxonomic composition of meiofaunal organisms were assessed. The saturation states of the porewaters with respect to calcite and aragonite were over-saturated but under-saturated with respect to 12 mol% Mg-calcite, in both acidified and non-acidified treatments. The sediment-water fluxes of TA and DIC increased in the acidified treatment, likely as a consequence of enhanced carbonate dissolution. In contrast, the acidification of the overlying water did not significantly affect the O2 and nutrients fluxes at the SWI. Meiofaunal abundance decreased in both treatments over the duration of the experiment, but the organisms seemed unaffected by the acidification. Our results demonstrate that carbonate dissolution increased under acidified conditions but other parameters, such as microbial redox processes, were apparently not affected by the pH decrease, at least during the duration of our experiment. The dissolution of sedimentary carbonates and the associated release of TA may potentially buffer bottom water, depending on the intensity of the TA flux, the TA/DIC ratio, vertical mixing and, therefore, the residence time of bottom water. Under certain conditions, this process may mitigate the effect of ocean acidification on benthic ecosystems.