Carbon inputs of the Rhône River to the Mediterranean Sea: Biogeochemical implications

Since the damming of the Nile, the Rhône River is the main freshwater and sediment supplier to the Mediterranean Sea. We estimated for the period 1987–1996, the dissolved and particulate organic carbon (DOC and POC), dissolved inorganic carbon (DIC), and total suspended matter (TSM) fluxes of the Rhône River to the Mediterranean Sea to be 1.1 ± 0.2, 1.6 ± 0.5, 16.2 ± 0.3 × 10 10 moles C yr −1 , and 9.9 ± 6.4 × 10 6 t yr −1 , respectively. PIC flux was estimated to be 8.2 ± 5.4 × 10 9 moles C yr −1 . On the basis of literature data, we estimated that nearshore bacterial respiration of Rhône derived labile‐POC and ‐DOC (LPOC and LDOC) might produce in a few days ∼0.21 and 0.12 × 10 10 moles CO 2 yr −1 , respectively. Extended to the whole Mediterranean, this study suggests that bacterial respiration of labile organic carbon derived from Mediterranean rivers might rapidly (days) produce 2.6‐11 × 10 10 moles CO 2 yr −1 . On the continental shelf, up to 4.7 × 10 10 moles of organic carbon introduced by primary production and Rhône export would escape each year to sedimentation and bacterial mineralization and would be exported off the shelf. Moreover, as total carbon fixed by phytoplankton exceeds (+ 5.2 × 10 10 moles C yr −1 ) the CO 2 produced by bacterial respiration (on average), the biological system on the shelf, could be considered as an autotrophic system and then a sink for atmospheric CO 2 . However, these numbers need further examination because of the large uncertainties associated currently to the bacterial growth efficiency values (±100%).