Origin, quality and reactivity of organic matter in the sediments of the Rhône prodelta and adjacent shelf (Gulf of Lions, France).
The Rhône River is the major source of freshwater and terrigenous material to the Mediterranean basin. The Rhône prodelta is characterised by high deposition rates and efficient transformation of sedimentary organics, which is typical for deltaic environments. This raised the question of the fate of these large quantities of organic carbon, delivered by the river to the coastal marine environment, and particularly the impact on benthic microbial community and organic matter recycling efficiency. The multi-disciplinary program " Climate and Human-induced Alterations in Carbon Cycling at the River seA connection " (CHACCRA) was designed to study the fate of nutrients and organic matter delivered by the Rhône River in the prodelta and its adjacent shelf. Sediment cores were collected in the Rhône river mouth and the distal area during four field campaigns (April 2007, May 2008, December 2008 and June 2009). Bulk parameters (grain size, total nitrogen and organic carbon) were analysed to investigate the distribution gradient and location of the flood deposit. In addition, molecular-level proxies (pigments, fatty acids, total and bioavailable amino acids) were used to assess the origin, quality and state of degradation of the organic matter in the sediments. Bacterial community structure was characterised using capillary electrophoresis-single strand conformation polymorphism (CE-SSCP) procedure. This work revealed gradients of origin and quality of organic matter from the river mouth to the open sea. Moreover, the influence of flood events is clearly visible in the nearshore sediments and the biochemical signatures of the deposits are specific to the type of flooding. Finally, the oxic degradation of sedimentary organic matter was assessed experimentally on sediments from the river mouth incubated 40 days at the laboratory. This experiment underlines the high biochemical reactivity of the organic matter labile fraction.