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

L’intelligence de la pratique

Si chacun a le pouvoir de vivre selon la raison, comment se fait-il que si peu la suivent, alors même qu’un grand nombre s’en réclament ? Certains voient le meilleur, mais font le pire. D’autres font le pire en croyant qu’il est le meilleur. Tous font tout ce qu’ils peuvent, et se réjouissent finalement de ce qu’ils sont. La philosophie de Spinoza rend compte de ces paradoxes : toute puissance est en acte. Qui peut le plus s’efforce nécessairement de faire le plus et ne peut faire moins. Qui peut le moins fait le moins volontiers, sans pouvoir faire plus. Chacun est aussi parfait qu’il peut l’être, et agit de la façon dont il y est disposé, malgré lui mais de gré, si ce n’est de bon gré. Le concept de disposition, tel qu’il s’élabore dans l’Éthique, permet de saisir la pratique commune des hommes dans un cadre nécessitariste et actualiste, de l’inconstance affective à la régularité des coutumes, des obsessions passionnelles à l’éducation et à l’affranchissement de la servitude. L’existence humaine n’est pas une comédie, encore moins une tragédie. Avec Spinoza, il s’agit d’en produire l’intelligence.If everyone has the power to live according to the guidance of reason, how is it that so few follow it, even though so many claim they do? Some see the best and do the worst, whereas others do the worst believing it is the best. All do everything they can and finally rejoice over what they are. The philosophy of Spinoza explains this human condition. All power is actual. He who can do the most cannot do less, and he who can do the least does so willingly, but cannot do more. Everyone is as perfect as they can be and acts the way they are disposed to, despite themselves, but readily. The concept of disposition, as it is developed in the Ethics, enables us to grasp the common practice of humans in a necessitarist and actualist context, from affective inconsistency to the regularity of habits, from passionate obsessions to education and emancipation. Human existence is not a comedy, even less a tragedy. With Spinoza, it has to be understood

Dosage rapide de la chlorophylle a et des phéopigments a par fluorimétrie après extraction au méthanol. Comparaison avec la méthode classique d'extraction à l'acétone

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Effects of ultraviolet-B stressed diatom food on the reproductive output in Mediterranean Calanus helgolandicus (Crustacea ; Copepoda)

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Nearshore dynamics of nutrients and chlorophyll during Mediterranean-type flash-floods

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It’s what’s inside that counts: computer-aided tomography for evaluating the rate and extent of wood consumption by shipworms

International audienceExperiments were done to investigate in situ colonization of pine wood blocks by marine wood borers at the mouth of a small mountain river in the foothills of the Eastern Pyrenees. Standardized blocks were recovered after remaining underwater for increasingly long durations, until the available resource was exhausted by the shipworms assemblage that developed. Computer-aided tomography (CT) was used for visualizing and quantifying biogenic structures into the wooden blocks. The biodiversity survey of the wood pieces colonized indicated that up to three species of shipworms shared the resource at the same time. The specific wood consumption rate of Nototeredo norvagica was estimated 185 mm 3 ind −1 day −1. The quantification of voids created by shipworm crowding indicated that total tunnelling represents, on average, 60% of the initial volume of a wood block, revising upward earlier estimates of wood destruction by 28%. CT analysis provides the quantitative measurements necessary to parameterize individual-based growth models linking wood consumption with the species diversity of shipworm assemblages

Ecogeochemical fate of coarse organic particles in sediments of the Rhône River prodelta

Coarse particulate organic matter (CPOM) represents a small portion of the inner shelf sediments but occurs across all river outlets. To consider the ecogeochemical fate of CPOM in such an environment, we examined both the infauna community and secondary evidence of geochemical reactions preserved in the surface sediments of the Rhône prodelta. ICP-AES, scanning electron microscopy and energy dispersive X-ray spectrometry of the CPOM showed that the fate of organic matter in this environment is driven by sulphate reduction and geochemical reactions resulting from the precipitation of sulfide due to the presence of large amounts of iron-bearing minerals. Leaf litter debris contained such high quantities of iron that after dry ashing the remaining material is easily attracted by a magnet. The observed geochemical trade-off was proposed as a mechanism that helps to maintain a bioturbating animal community that in turn contributes to the mineralization of organic matter within this suboxic environment. This study showed that the accumulation of refractory organic carbon in sediments was intimately associated with the sequestering of iron and sulphur by providing a nucleation point for mineral deposition and also that the extent of decomposition of the organic materials did not necessarily increase progressively from coarser to finer particles

Ecogeochemical fate of coarse organic particles in sediments of the Rhône River prodelta

Coarse particulate organic matter (CPOM) represents a small portion of the inner shelf sediments but occurs across all river outlets. To consider the ecogeochemical fate of CPOM in such an environment, we examined both the infauna community and secondary evidence of geochemical reactions preserved in the surface sediments of the Rhône prodelta. ICP-AES, scanning electron microscopy and energy dispersive X-ray spectrometry of the CPOM showed that the fate of organic matter in this environment is driven by sulphate reduction and geochemical reactions resulting from the precipitation of sulfide due to the presence of large amounts of iron-bearing minerals. Leaf litter debris contained such high quantities of iron that after dry ashing the remaining material is easily attracted by a magnet. The observed geochemical trade-off was proposed as a mechanism that helps to maintain a bioturbating animal community that in turn contributes to the mineralization of organic matter within this suboxic environment. This study showed that the accumulation of refractory organic carbon in sediments was intimately associated with the sequestering of iron and sulphur by providing a nucleation point for mineral deposition and also that the extent of decomposition of the organic materials did not necessarily increase progressively from coarser to finer particles

Sources partitioning in the diet of the shipworm <i>Bankia carinata</i> (J.E. Gray, 1827): An experimental study based on stable isotopes

International audienceAdaptations that allow teredinids to maintain and thrive on wood, a nutritionally unbalanced food, make these marine bivalves remarkable. Capable of filter-feeding, shipworms house endosymbiotic bacteria synthesizing cellulolytic enzymes for digestion of wood carbohydrates and providing nitrogen to their host through nitrogen fixation. To what extent each of these nutrition modes contributes to the shipworm's metabolism remains an open question. In this experimental study, we estimated source partitioning through the determination of δ13C and δ15N values in original biological samples. For this purpose, pieces of common alder (Alnus glutinosa) were immersed at a coastal station of the north-western Mediterranean Sea. The shipworm Bankia carinata infected wood logs and stable isotope mixing models suggested it got most of the carbon and nitrogen it needs from separate sources. From 71 to 77% of the carbon was derived from the digestion of wood carbohydrates, whereas between 42 and 82% of the nitrogen originated from N2 fixation. These first semi-quantitative estimations suggest that the contribution of N2 fixers to nitrogen requirements of this shipworm species is far from incidental

Characterization and sources of colored dissolved organic matter in a coral reef ecosystem subject to ultramafic erosion pressure (New Caledonia, Southwest Pacific).

International audienceThe eastern lagoon of New Caledonia (NC, Southwest Pacific), listed as a UNESCO World Heritage site, hosts the world's second longest double-barrier coral reef. This lagoon receives river inputs, oceanic water arrivals, and erosion pressure fromultramafic rocks, enriched in nickel (Ni) and cobalt (Co). The aimof this study was to characterize colored dissolved organic matter (CDOM), aswell as to determine its main sources and its possible relationships (through the use of Pearson correlation coefficients, r) with biogeochemical parameters, plankton communities and trace metals in the NC eastern lagoon. Water sampleswere collected inMarch 2016 along a series of river/lagoon/open-ocean transects. The absorption coefficient at 350 nm (a350) revealed the influence of river inputs on the CDOMdistribution. The high values of spectral slope (S275–295, N0.03m−1) and the lowvaluesof specific ultraviolet absorbance (SUVA254, b4 L mg-C−1 m−1) highlighted the photodegradation of CDOM in surface waters. The application of parallel factor analysis (PARAFAC) on excitation-emission matrices (EEMs) allowed the identification of four CDOM components: (1) one humic- and one tyrosine-like fluorophores. They had terrestrial origin, exported through rivers and undergoing photo- and bio-degradation in the lagoon. Thesetwo fluorophores were linked to manganese (Mn) in southern rivers (r = 0.46–0.50, n = 21, p b 0.05). (2) A tryptophan-like fluorophore, which exhibited higher levels offshore. It would be potentially released from the coral reef. (3) A second tyrosine-like (“tyrosine 2-like”) fluorophore. Linked to Prochlorococcus cyanobacteria (r= 0.39, n =47, p b 0.05), this fluorophore would have an oceanic origin and enter in the lagoon through its southern and northern extremities. It also displayed relationshipswith Ni and Co content (r=0.53–0.54, n=21, p b 0.05). This work underlines the diversity of CDOM sources in the NC eastern lagoon