Benthic remineralization at the land-ocean interface: A case study of the Rhone River (NW Mediterranean Sea)

Abstract Biogeochemical processes in sediments under the influence of the Rhone River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC ( δ 13 C OC ) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhone delta system. Large spatial variations were observed in both the total O 2 uptake (5.2 to 29.3 mmol m −2  d −1 ) and NH 4 + release (−0.1 to −3.5 mmol m −2  d −1 ) rates at the sediment–water interface. The highest fluxes were measured near the Rhone River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of δ 13 C OC ranged from −26.83‰ to −23.88‰ with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf. The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhone mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhone POC input is either buried (∼52%) or remineralized (∼8%), mostly on the prodelta area. The remaining fraction (∼40%) is either mineralized in the water or exported outside the Rhone delta system in dissolved or particulate forms.