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 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.

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

“…Later, the flood that occurred in early June brought silty material originating from the weathering of soils. This thick Organic matter quality shaping microbial communities SK Fagervold et al deposit was depleted in OM, more degraded and less labile than the POM delivered by the Rhô ne during normal discharge conditions (Bourgeois et al, 2011). Then, during the summer, the low discharge rates and the calm weather conditions were favorable to the compaction (that is, lower porosity) and further degradation of the material deposited in the prodelta.…”

“…Hydrolysable amino acids were analyzed by reversed-phase high-performance liquid chromatography as isoindole derivatives following precolumn derivatization with orthophtaldialdehyde (Lindroth and Mopper, 1979). Fatty acids were analyzed as methyl esters as previously described (Bourgeois et al, 2011) using a Saturn 2100T iontrap GC-MS instrument (VARIAN, Les Ulis, France) equipped with a fused-silica-capillary column (Factor Four, VF-Waxms, 30 m  0.25 mm ID, 0.25 mm film thickness, VARIAN). Porosity was calculated from natural moisture content assuming a density of 2.63 g cm À 3 for dry sediment and corrected for seawater salinity.…”

“…The fluorescence of the extracts was measured according to the method developed by Neveux and Lantoine (1993). Amino acids were released by acid hydrolysis (HCl 6 N, 100 1C for 24 h) and the hydrolysates were treated as described in Bourgeois et al (2011). Hydrolysable amino acids were analyzed by reversed-phase high-performance liquid chromatography as isoindole derivatives following precolumn derivatization with orthophtaldialdehyde (Lindroth and Mopper, 1979).…”

“…Stable carbon isotopes were measured on acidified samples according to the protocol described in Kerhervé et al (2001) and analyzed on an elemental analyzer (EA 3000 Eurovector, Eurovector, Milan, Italy) coupled with an isotopic ratio/mass spectrometer (GVI Isoprime, GVi, Cheadle, UK). Pigments were extracted as described in Bourgeois et al (2011). The fluorescence of the extracts was measured according to the method developed by Neveux and Lantoine (1993).…”

“…The highly dynamic prodelta of the Rhô ne River, with OM from contrasting origins (marine vs terrestrial) including modern vascular C3 plant detritus, soils and freshwater phytoplankton (Kim et al, 2010;Bourgeois et al, 2011;Cathalot et al, 2013) can thus be used as a model system to test the hypothesis that the OM quality and source are fundamental structuring factors for the microbial communities in benthic environments. To obtain samples reflecting the variability of sources, sediments from the prodelta were seasonally sampled at seven different layers for a coupled study of both OM and microbial communities.…”