Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean sea, France): a multi proxy study

Abstract

Abstract. The Gulf of Lions is a river-dominated ocean margin (RiOMar) that receives high loads of nutrients, sediments and particulate matter from the Rhône river. Most of the particulate materials from the Rhône settle rapidly on the seafloor, this raises the question of the fate of these large quantities of organic carbon delivered to the benthic boundary layer. Surface sediments (0–0.5 cm) were collected in the Rhône prodelta and its adjacent shelf during a period of low river discharge (April 2007, 16 stations). The sources, distribution and lability of sedimentary organic matter was examined using bulk (organic carbon, total nitrogen, grain size) and molecular-level (pigments, amino acids, fatty acids, δ13C of individual fatty acids) analyses. Our results confirmed previous observations of a southwestward Rhodanian imprint in the nearshore sediments. Fatty acid biomarkers and compound-specific δ13C signatures of most fatty acids clearly indicate that the Rhône inputs consist in a mixture of organic matter (OM) from different sources with a strong contribution from terrestrial plants, and a smaller input from freshwater microalgae, mostly diatoms. The influence of the Rhône River was prominent within the first ten kilometers, but may still be observed in the outer shelf (~21 km) as indicated by the occurrence of long chain fatty acids derived from vascular plants and their δ13C signatures. In the proximal prodelta, bacteria-specific fatty acids were abundant (1.65 mg g−1OC at the mouth site) and were relatively depleted in δ13C confirming that bacteria preferentially utilize terrestrial OM in this area. In the shelf area, the inputs of marine OM and its preferential utilization by the bacteria was confirmed, but the coupling between the pelagic and the benthic compartments appeared limited at this period of the year. Overall, degradation indexes based on amino acids (Dauwe's degradation index) and pigments (ratio of intact chlorophyll-a to the sum of chlorophyll-a + phaeopigment-a), as well as isotopic enrichment of source-specific fatty acids reveal an offshore gradient of OM decay reflecting the rapid deposition of the terrestrial material in the prodelta, the low mixing with OM deriving from marine sources and the efficient degradation of the OM once deposited. Terrestrial OM is usually considered as being refractory due to the presence of structural polymers and its advanced stage of degradation. However, the OM delivered by the Rhône is relatively labile as shown by the intermediary value of Dauwe's degradation index (DI = +0.1), the high proportion of bio-available nitrogen and the occurrence of polyunsaturated fatty acids. Deltaic sediments off the Rhône river should thus be of sufficiently high nutritional quality to sustain dense macrofaunal communities.