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

Abstract

Major 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).3

Energy transfer in the Congo deep-sea fan: From terrestrially-derived organic matter to chemosynthetic food webs

“…We did not focus on identifying new compounds as our final goal was to study feeding strategies, which could be achieved using known biomarkers. Fatty acids used as trophic biomarkers, in this study, are listed in Table 3. Repeatability of the analysis, determined by comparing relative standard deviations of 41 fatty acids in six different preparations of our standard solution, ranged from 1.0-5.4% with a mean value of 2.4% (Pruski et al, 2015). Results are expressed in µg.g -1 DW tissues or %.…”

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)

“…Fatty acids were extracted and converted into fatty acid methyl esters (FAMEs) by direct acid transmethylation and analysed by GC/MS with a Saturn 2100T ion trap detector (Varian). Details regarding the extraction protocol and analytic procedure are given in Bourgeois et al (2011) and Pruski et al (2015). Repeatability of the analysis ranged from 3.2% to 13% for individual fatty acids with a mean value of 5.9%.…”

“…Fatty acid concentrations (initially in μg.g −1 dry weight) were normalised to total organic content (mg.g TOC −1 ) to remove the dilution effect of the mineral matrix. Compounds were grouped as follows: the saturated fatty acids (SAFAs) of mixed origin, the terrestrial long-chain saturated fatty acids (LC-FAs, ≥ 24), the terrestrial C 18 polyunsaturated fatty acids (C 18 -PUFAs, 18:2ω6 and 18:3ω3), the remaining PUFAs attributed to phytoplankton, the monounsaturated fatty acid series (C 16 and C 18 MUFAs), and the straight and branched odd-numbered fatty acids of bacterial origin (BAFAs; Pruski et al, 2015). Fatty acids account for one of the most labile fractions of the organic matter; fatty acid yields (OC-normalised concentrations) are thus indicative of inputs or preservation of labile organic matter.…”

“…This assertion must be modulated to take into account biotic (fauna abundance and diversity) and abiotic parameters (accumulation rates, hydrodynamism, redox conditions, interactions with minerals), which are key drivers of organic matter degradation. Organic matter recycling in the Rhône prodelta has been the focus on many studies, which provided deep insights on the degradation rates, microbial processes and environmental drivers that constrained benthic mineralisation (Cathalot et al, 2010;Lansard et al, 2009Lansard et al, , 2008Pastor et al, 2011;Pruski et al, 2015). So far, the influence of smaller coastal rivers on organic matter cycling is poorly constrained.…”

“…At the scale of the Mediterranean Sea, coastal rivers occupy 38% of the surface drainage area, but contribute to about 58% of the natural suspended particulate matter flux (Sadaoui et al, 2018). The Gulf of Lions is one of the largest and most productive shelves in the Mediterranean Sea, and compounds (Bourgeois et al, 2011;Pruski et al, 2015), which promote microbial degradation processes (Cathalot et al, 2010;Pastor et al, 2011) and sustain the benthic food web (Bonifácio et al, 2014;Darnaude et al, 2004;Goineau et al, 2012;Hermand et al, 2008). The suspended particles delivered by smaller coastal rivers are enriched in organic matter by a 2.5-3 factor in comparison to the Rhône River (Higueras et al, 2014) and may thus strongly impact the biogeochemical dynamic on the adjacent marine prodeltas.…”