Abstract. The Gulf of Lions is a river-dominated ocean margin that receives high loads of nutrients and particulate matter from the Rhône River but most particulate materials settle rapidly on the nearshore seafloor. One question is raised on the fate of these large quantities of organic carbon delivered by the river to the coastal marine environment. 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 were examined using bulk (organic carbon, total nitrogen, stable carbon isotope ratios, and grain size) and molecular-level (pigments, amino acids, fatty acids, and δ 13 C of individual fatty acids) analyses. Our results confirmed previous observations of a southwestward Rhodanian imprint in the nearshore sediments, with 97 % of terrigenous inputs of organic matter near the river mouth. Isotopic values of bulk organic carbon, as well as fatty acid biomarkers and compound-specific δ 13 C signatures of most fatty acids clearly indicate that the Rhône inputs consist of a mixture of organic matter (OM) from different origins with a strong contribution from terrestrial sources (soil and plant debris), 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 on the outer shelf (∼21 km) as indicated by the occurrence of long chain fatty acids, which are derived from vascular plants, and their δ 13 C signatures. In the proximal prodelta, Correspondence to: A. M. Pruski (audrey.pruski@obs-banyuls.fr) bacteria-specific fatty acids were abundant (1.65 mg g −1 OC at the mouth site) and were relatively depleted in δ 13 C confirming that bacteria mostly utilize land-derived OM. In the shelf area, the inputs of marine OM and its predominant 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 + phaeopigmenta), 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. The OM delivered by the Rhône is relatively labile based on the intermediary value of Dauwe's degradation index, the high proportion of bioavailable 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.

Climate change in the Arctic is ongoing and causes drastic modification on the ecosystem functioning. In soft-bottom environments, organic matter remineralization is considered an important ecosystem function. Here we provide a large-scale assessment of the current knowledge on the benthic organic matter remineralization and its potential response to climate change. Sediment oxygen demand (SOD) values (n = 1154), measured throughout the Arctic, were gathered from 30 publications and 16 databases, and nutrient flux values, available in a far lesser extent (n < 80), were also compiled. Generalized additive models were used to estimate the influence of explanatory variables on benthic oxygen fluxes and for interpolating SOD to the whole Arctic region. This first Pan-Arctic review of the distributions of SOD showed that oxygen fluxes strongly depended on water depth, i.e., followed the general trend observed for other regions, and also on the availability of labile organic matter. The continental shelves (representing~50% of Arctic Ocean's total area) were characterized by the highest SOD values (10.5 ± 7.9 mmol O 2 m À2 d À1 ), and differences among shelves were observed; SOD values in inflow, interior, and outflow shelves were 11.8 ± 8.0, 6.2 ± 5.6, and 3.9 ± 3.5 mmol O 2 m À2 d À1 , respectively. Moreover, seasonal variation in SOD changed significantly among areas. The interpolation based on the best fitted model showed high respiration in the inflow and interior shelves. In the inflow shelves, characterized by productive waters, benthic activities replenish bottom water with nutrients which may augment primary productivity, whereas sediments from the interior shelves, e.g., under the direct influence of the Mackenzie River, consume nutrients.