Influence of coastal Mediterranean rivers on the organic matter composition and reactivity of continental shelf sediments: The case of the Têt River (Gulf of Lions, France)

International audienceRiver-dominated ocean margins (RiOMars) are areas of high productivity, rapid sediment deposition, and in-tense recycling. The Gulf of Lions, located in the North occidental Mediterranean Sea, is no exception to thisgeneral rule and is under the prevalent influence of the Rhône River. One particularity of this continental shelf is that it is also fed by several small coastal rivers whose inputs actively contribute to the coastal dynamic and export to the deep-sea. Our objective was to gauge the influence of coastal rivers on the quality and reactivity of the organic matter delivered to the continental shelf as these properties control two important functions of coastal areas: benthic productivity and carbon sequestration. We chose for this study, the Têt River, which is typical of coastal Mediterranean rivers with a torrential regime. Sediment cores were sampled five times over a one-year period on four stations located along the dominant trajectory of the Têt River plume. The biogeochemical characteristics of the surface sediments were analysed to highlight spatio-temporal trends in the sedimentary organic matter composition and to identify the major environmental factors controlling its reactivity. Nutrient and dissolved oxygen fluxes at the water-sediment interface were furthermore measured at two stations using ex-situ whole core incubations.Results from this study highlight the seasonal influence of riverine versus autochthonous productions on thequality of sedimentary organic matter in the vicinity of the river mouth. Farther on the mid shelf, a terrestrial imprint is still visible, but the temporal signal is blurred as a consequence of strong mixing during frequent episodes of resuspension/deposition. Organic matter is efficiently remineralised in this system with total oxygen uptake (TOU) rates comparable to those measured off the Rhône River. Finally, organic matter quality and reactivity are also constrained in this system by physical forcings, which promote remineralisation and advection of sediments

Spatiotemporal changes in surface sediment characteristics and benthic macrofauna composition off the Rhône River in relation to its hydrological regime

International audienceThe consequences of changes in the water flow of the Rhône River on surface sediment characteristics and benthic macrofauna composition were assessed within 3 distinct areas: (1) the delta front, (2) the prodelta, and (3) the distal zone. Five stations were sampled during or closely after: (1) an oceanic flood (April 2007), (2) a generalized flood (May 2008), (3) a Cevenol flood (December 2008), and (4) a dry period (July 2011). Measurements of sediment characteristics included granulometry (D0.5), bulk descriptors of sedimentary organics (OC, TN and THAA), descriptors of labile components of sedimentary organics (chloropigments, EHAA), and both descriptors of origin (Chl-b/Chl-a, C/N) and lability (Chl-a/(Chl-a+Phaeo-a), EHAA/THAA) of sedimentary organics. Sediment Profile Images were collected during April 2007, May 2008 and July 2011. Temporal changes in both sedimentary organics and benthic macrofauna were more important in the delta front and the prodelta than in the distal zone. Bulk characteristics of sedimentary organics presented decreasing inshore/offshore gradients during both April 2007 and July 2011 but not during May and December 2008. There were significant temporal changes in EHAA/THAA at all stations. Changes in benthic macrofauna composition differed between: (1) the delta front and the prodelta, and (2) the distal zone. In the former area, the dry period was associated with establishing a mature community characterized by high abundances and species richness. The best description of spatiotemporal changes in benthic macrofauna composition by surface sediment characteristics was obtained using D0.5, Chl-b/Chl-a, Chl-a/(Chl-a+Phaeo-a) and EHAA, which supports the role of the quality of sedimentary organics in controlling benthic macrofauna composition

Environmental conditions, particle flux and sympagic microalgal succession in spring before the sea-ice break-up in Adélie Land, East Antarctica

Data pertaining to environmental conditions, sympagic (sea ice) microalgal dynamics and particle flux were collected before the spring ice break-up 2001 in Pierre Lejay Bay, adjacent to the Dumont d'Urville Station, Petrel Island, East Antarctica. An array of two multiple sediment traps and a current meter was deployed for five weeks, from 8 November to 6 December 2001. The sea-ice chlorophyll a and particulate organic carbon (POC) averaged 0.6 mg l−1 (30 mg m−2) and 20 mg l−1 (1 g m−2) near the coast. The POC export flux that reached a maximum of 79 mg m−2 d−1 during the study period was high compared to the one for the Weddell Sea. The flux was homogeneous from the surface to 47 m depth and increased sharply 33 days before the effective ice break-up. A north-western progressive vector of currents (i.e., Lagrangian drift) in the sub-ice surface waters was demonstrated. Bottom ice, platelet ice and under-ice water at 5 m were characterized by differences in colonization and short-term succession of microalgae

Biogeochemistry of fatty acids in a river-dominated Mediterranean ecosystem (Rhône River prodelta, Gulf of Lions, France): Origins and diagenesis

International audienceMajor rivers transfer high loads of continental particulate organic matter to deltaic environments, where the impact on the biogeochemistry and productivity of coastal sediments depends on the sources and lability of these inputs. Our aims are to provide new insights into the reactivity of riverine inputs in coastal environments and to delineate the parameters controlling their fate in these dynamic systems. Sediment cores were collected from a Mediterranean deltaic system (the Rhône prodelta and its adjacent shelf) during a period of moderate river discharge (Spring 2007) and analyzed for their fatty acid composition. Sediment properties were also described using biochemical bulk analyses (organic carbon and lipids), granulometry and profiles of redox potential. Based on the bulk compositional changes and hierarchical clustering of the fatty acid biomarkers, we determined the principal sources of organic matter and their spatial distribution. Fatty acids were primarily of continental origin in the prodelta, shifting to a higher contribution from marine sources in the shelf area. Fatty acids derived from both continental and marine sources were efficiently degraded in the sediments by microbial decay processes in the upper oxic layer. Degradation rate constants calculated from the down-core decreases in concentrations indicate that fatty acid degradation was enhanced in sediments influenced by the Rhône River. The most important parameters affecting fatty acid preservation were the inherent stability of individual molecular components and their physical association with mineral matrices, the source and freshness of the inputs, and the depositional environment (redox condition, accumulation rates)

Diatom valve distribution and sedimentary fatty acid composition in Larsen Bay, Eastern Antarctica Peninsula

8 pages, 6 figures, 1 tableDuring austral summer 2006–2007, five sediment cores were recovered from the Eastern Antarctic Peninsula (EAP) continental shelf. Microscopic observations and sediment fatty acid (FA) composition analyses were carried out to investigate whether the drastic changes at the sea surface in EAP may be reflected in the sedimentary record. A sharp decrease in the number of diatom valves was observed below 2 cm depth. This difference between the upper 2 cm of sediment and the deeper part of the sediment column was attributed to the drastic change in the upper water column conditions after the collapse of the ice shelves, which allowed the arrival of phytoplankton debris and fresh organic matter to the sea floor in EAP. The presence of bacterial-, zooplankton- and detrital-related FA throughout EAP cores suggests that there has been an input of older and more refractory organic matter into the region, presumably by lateral transport before the Larsen ice shelves disintegrationPeer reviewe

Nutrient status in coral reefs of the Îles Eparses (Scattered Islands): comparison to nearby reefs subject to higher anthropogenic influences (Mozambique Channel and Mascarenes, Indian Ocean)

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Fatty acid biomarkers as indicators of organic matter origin and processes in recent turbidites: the case of the terminal lobe complex of the Congo deep-sea fan

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Biochemical characteristics of settling particulate organic matter at two north-western Mediterranean sites: A seasonal comparison

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River organic matter shapes microbial communities in the sediment of the Rhône prodelta

International audienceMicrobial-driven organic matter (OM) degradation is a cornerstone of benthic community functioning, but little is known about the relation between OM and community composition. Here we use Rhône prodelta sediments to test the hypothesis that OM quality and source are fundamental structuring factors for bacterial communities in benthic environments. Sampling was performed on four occasions corresponding to contrasting river-flow regimes, and bacterial communities from seven different depths were analyzed by pyrosequencing of 16S rRNA gene amplicons. The sediment matrix was characterized using over 20 environmental variables including bulk parameters (for example, total nitrogen, carbon, OM, porosity and particle size), as well as parameters describing the OM quality and source (for example, pigments, total lipids and amino acids and δ13C), and molecular-level biomarkers like fatty acids. Our results show that the variance of the microbial community was best explained by δ13C values, indicative of the OM source, and the proportion of saturated or polyunsaturated fatty acids, describing OM lability. These parameters were traced back to seasonal differences in the river flow, delivering OM of different quality and origin, and were directly associated with several frequent bacterial operational taxonomic units. However, the contextual parameters, which explained at most 17% of the variance, were not always the key for understanding the community assembly. Co-occurrence and phylogenetic diversity analysis indicated that bacteria–bacteria interactions were also significant. In conclusion, the drivers structuring the microbial community changed with time but remain closely linked with the river OM input