Spatial and seasonal dynamics of total suspended sediment and organic carbon species in the Congo River
The Congo (Zaire) River, the world's second largest river in terms both of water discharges and of drainage area after the Amazon River, has remained to date in a near-pristine state. For a period between 2 and 6 years, the mainstream near the river mouth (Brazzaville/Kinshasa station) and some of the major and minor tributaries (the Oubangui, Mpoko, and Ngoko-Sangha) were monitored every month for total suspended sediment (TSS), particulate organic carbon (POC), and dissolved organic carbon (DOC). In this large but relatively flat equatorial basin, TSS levels are very low and organic carbon is essentially exported as DOC: from 74% of TOC for the tributaries flowing in savannah regions and 86% for those flowing in the rain forest. The seasonal patterns of TSS, POC, and DOC show clockwise hysteresis in relation to river discharges, with maximum levels recorded 2 to 4 months before peak flows. At the Kinshasa/Brazzaville station, the DOC distribution is largely influenced by the input from the tributaries draining the large marshy forest area located in the center of the basin. There is a marked difference between specific fluxes, threefold higher in the forest basins than in the savannah basins. The computation of inputs to the Atlantic Ocean demonstrates that the Congo is responsible for 14.4 × 106 t/yr of TOC of which 12.4 × 106 t/yr is DOC and 2 × 106 t/yr is POC. The three biggest tropical rivers (the Amazon, the Congo, and the Orinoco), with only 10% of the exoreic world area drained to world oceans, contribute ~4% of its TSS inputs but 15-18% of its organic carbon inputs. These proportions may double when considering only world rivers discharging into the open ocean.
(Global Biogeochemical Cycles. vol. 19, n° 0886-6236, pp. 4019, 01/12/2005)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LMTG, UT3, Comue de Toulouse, CNRS, SISYPHE, UPMC, EPHE, PSL, PSL, CNRS, HSM, IRD, UM2, INSU - CNRS, CNRS
Comparison between shell morphology and genetic diversity in two sympatric lymnaeid snails, vectors of fasciolosis
Species determination of freshwater snails is particularly important in the case of disease vectors. In central France, for example, it has been shown that Galba truncatula (Müller, 1774) and Omphiscola glabra (Müller, 1774), which can be found in sympatry, are intermediate hosts for fasciolosis. Although these two species are distinguishable based on their shell morphology, some snails present in sympatric zones possess shell characteristics that are common to both species, suggesting possible gene flow. To test this hypothesis, we carried out multilocus enzyme electrophoreses on individuals sampled in one zone of sympatry. Our results clearly show that all snails with shell characteristics common to both species are O. glabra. We also note an absence of hybridization between G. truncatula and O. glabra in the study area. Remarkably, we observe a total monomorphism at the six studied loci among individuals of G. truncatula. Similarly, we find a total lack of heterozygotes in the O. glabra sample (with classic or intermediate shell characteristics). These results suggest a predominance of selfing in the reproductive mode of both species. Our results suggest that conchological characteristics can provide information about the taxonomic position of lymnaeids. However, they are insufficient to confidently differentiate species.
(Canadian Journal of Zoology. vol. 83, n° 0008-4301, pp. 1643–1648, 01/12/2005)
GEMI, UM1, CNRS, IRD [Occitanie], IRD, UNILIM, UMR ISEM, Cirad, IRD, EPHE, PSL, UM, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Palaeoclimatology and palaeohydrography of the glacial stages on Celtic and Armorican margins over the last 360000 yrs
Core MD03-2692 was retrieved in a water-depth of 4064 m on the Celtic margin (Bay of Biscay) during the SEDICAR cruise onboard the RV Marion Dufresne II. It covers the last 360 ka in a total length of 39 m. Multidisciplinary analyses have been applied to this sequence with the aim of studying the palaeoclimatic and palaeoenvironmental signals of the last few climatic cycles. The analyses undertaken include: (1) non-destructive logging with: physical properties (magnetic susceptibility, sediment colour), X-ray radiography and measurement of the major elements by X-ray-fluorescence, (2) analyses of planktonic and benthic foraminifera, lithic grains and stable isotopic measurements (oxygen and carbon). We have focused on the longterm evolution of glacial stages (with special attention to terminations and Heinrich events). The results obtained confirm the close correlation between deep-sea sedimentation recorded on the Celtic margin and changes in the terrestrial environment of the adjacent continent. Heinrich layers have been identified in MIS 2, 3, 6 and 8. We note the occurrence of laminated facies within deglacial sequences deposited during Termination I and MIS 6. These facies are closely linked to disintegration phases of the British-Irish Ice Sheet (BIS). The laminations contain lower ice-rafted detritus (IRD) concentrations than the equivalent Heinrich layers and are linked to abrupt changes in sea-surface palaeotemperatures. We suggest that the laminations are formed by an annual cycle of meltwater and iceberg release from the disintegrating BIS generating cascading plumes of dense turbid meltwater coeval with IRD release.
(Marine Geology. vol. 224, n° 0025-3227, pp. 57-82, 01/11/2005)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, IFREMER, SHOM
Structuring factors and recent changes in subtidal macrozoobenthic communities of a coastal lagoon, Arcachon Bay (France)
Fourteen years after a previous investigation in Arcachon Bay (SW France), the quantitative distribution of subtidal macrozoobenthic communities was assessed in 2002 through a stratified sampling strategy involving a larger number of stations (89 vs. 18) than in 1988. A total of 226 taxa were recorded. Cluster Analysis and Correspondence Analysis identified nine station groups corresponding to benthic faunal assemblages and their characteristic species. Multiple Discriminant Analysis showed that the main environmental factors influencing the distribution of faunal assemblages were sediment parameters and distance from the ocean. Depth was a minor structuring factor. At the scale of the lagoon, biogenic structures such as Zostera marina beds, Crepidula fornicata-dominated bottoms or dead oyster shell bottoms did not display any particular assemblage of infauna. Comparison with previous quantitative data from the 1988 survey provided more precision on the distribution of benthic assemblages and revealed community changes at a 14-year scale. These modifications reflected a general increase of silt and clay content in the sediment in the internal parts of channels, inducing community change. These changes can be correlated to the recent first signs of a moderate eutrophication process which have appeared, since 1988, through the development of green macroalgae in some parts of the lagoon. This trend was enhanced in transverse channels with reduced hydrodynamics and led to muddy areas where green macroalgae tended to accumulate. Locally, the dredging of sandbanks induced stronger currents and allowed the marine influence to occur, and also induced community change. These observations confirm that surveys of macrobenthic communities are useful tools to assess coastal ecosystem change even in moderately disturbed environments.
(Estuarine, Coastal and Shelf Science. vol. 64, n° 0272-7714, pp. 561-576, 01/09/2005)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Sources and transfers of particulate organic matter in a tropical reservoir (Petit Saut, French Guiana): a multi-tracers analysis using d13C, C/N ratio and pigments
Carbon cycling and organic matter transfers in the tropical Sinnamary river system (French Guiana), including a mid-stream reservoir (Petit Saut) and its estuary on the Atlantic coast, were studied during the dry season by analyzing the organic carbon isotopic ratios (d13C-OC), C/N ratios and pigments contents of suspended matter, sediments, sediments traps and epiphytic and epilithic biofilms. In the River upstream as well as in surface sediments at the entrance of the reservoir and at the littoral zone of the reservoir, particulate organic matter (POM) was in majority of terrestrial origin, with a d13C-C/N signature close to the one of soil and litter collected in the surrounding forest and with high OC/total pigments ratios. High concentrations of Pheopigments a and b in these surface sediments showed that this terrestrial POM, either carried by the river and eolian transport or present in the soil before flooding, undergoes intense degradation. Deeper in the sediment, the d13C profile showed a decreasing trend with depth typical of what is found in soils, showing that the flooded soil still remains present at the reservoir bottom 10 years after flooding. At the center of the reservoir, POM in the water column, in sediment traps and in surface sediments was in majority of aquatic origin with low C/N and OC/total pigments ratios. In the oxic epilimnion at 3 m depth, Chl a, Chl b and Lutein showed the predominance of Chlorophyceae to the phytoplankton community. At this depth, a C/N ratio of 21 suggests a large contribution of transparent exopolymeric particles to the bulk POM, which, in addition, was 13C-depleted due to a significant contribution of methanotrophic bacteria. At 7 m depth, below the oxicline, high concentrations of BChl d and occasionally BChl c revealed the presence of anoxygenic phototrophic bacteria, namely Chlorobiaceae. In the sediment traps, Chl a, Chl b, Lutein and BChl c and BChl d confirmed the contribution of plankton to the sedimentary POM. This material was undergoing intense degradation as revealed by high concentration of pheopigments and by an increase in C/N ratio and an increase in d13C-OC with trap depth. Scytonemin was found in a biofilm developed on tree trunks at the reservoir surface and in all sediment traps. Other tracers showed however that the contribution of the biofilm to the sedimentary POM was minor compared to the planktonic source. In the Sinnamary downstream of the dam, POM became more 13C-depleted showing a larger contribution of methanotrophic bacteria. Chl b, Lutein and BChl c + BChl d originating from the reservoir progressively decreased downstream as the result of mineralization. At the estuarine mouth, fucoxanthin showed the presence of diatoms and the d13C-C/N signature matched the one of POM carried by the Amazonian coastal mobile mud belt.
(Biogeosciences Discussions. vol. 2, n° 1810-6277, pp. 1159-1196, 30/08/2005)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Contamination by heavy metals (Cd, Zn, Cu, Hg) of eigth fish species in the Gironde estuary (France)
(Estuaries. vol. 28, n° 0160-8347, pp. 581-591, 01/08/2005)
LMBA, UBS, UBO EPE, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Heinrich events: hydrological impact
The hydrological impact of the massive iceberg discharges during Heinrich events has been quantified for Heinrich 4 about 37 kyr ago. This event took place in a context of relatively stable orbital forcing and global ice volume. In this article, we propose to quantify the hydrological impact of the Heinrich event 1 which occurred at about 16 kyr BP. This time period is particularly interesting because orbital forcing was strong and it marks the onset of the last deglaciation. The results obtained here show that Heinrich event 1 has a larger isotopic amplitude than Heinrich event 4 when considering the cores located near the melting source. Concerning surface circulation, the northward progression of the North Atlantic Drift is affected only by Heinrich event 4, possibly because of the latitudinal extension of the melting area during this event. To cite this article: E. Cortijo et al., C. R. Geoscience 337 (2005). 2005 Académie des sciences. Published by Elsevier SAS. All rights reserved. Résumé Les événements de Heinrich : impact hydrologique. L'impact hydrologique des décharges massives d'icebergs liées aux événements de Heinrich a été quantifié pour l'événement de Heinrich 4, datant d'environ 37 ka BP, dans un contexte de relative stabilité des paramètres orbitaux et du volume global de glace. Dans cette note, nous proposons de quantifier l'impact hydrologique de l'événement de Heinrich 1, il y a 16 ka BP, qui s'est déroulé au début de la dernière déglaciation, sous des conditions de forçage orbital très différent. Les résultats montrent que l'amplitude isotopique de l'événement de Heinrich 1 est plus grande que celle de l'événement de Heinrich 4, lorsqu'on se place à proximité des sources de fonte. En revanche, la dérive nord-atlantique n'est affectée que pendant l'événement de Heinrich 4, vraisemblablement à cause de la disposition latitudinale de la langue de fonte pendant cet événement. Pour citer cet article : E. Cortijo et al., C. R. Geoscience 337 (2005).
(Comptes Rendus. Géoscience. vol. 337, n° 1631-0713, pp. 897-907, 01/08/2005)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CLIM, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA
Architecture and depositional pattern of the Rhône Neofan and recent gravity activity in the Gulf of Lions (western Mediterranean)
The Rhône Deep Sea Fan is the largest sedimentary body in the western Mediterranean Sea. We present here the results of a detailed geophysical and sedimentological analysis of the most recent gravity deposits situated on the western flank of this large sedimentary body. These results are based on the interpretation of seismic profiles as well as acoustic and coring data acquired during the MARION cruise conducted in 2000 onboard R/V Le Suroît and Images V cruise on board of the R/V Marion Dufresne (1999). We found that (1) a new channel-levee unit developed during the last stage of the Rhône Neofan evolution and (2) the last phase of up-building of this channel/levee system dates back to 15.1 ka BP (14C). This implies that the last lobe of the Rhône deep-sea fan was built during the last low sea-level and the ensuing sea-level rise. We also identified (3) 10 recent sandy deposits, previously interpreted as related to the Neofan activity. In fact, their origin is probably linked to the Sète canyon that collect sandy shelf-edge deposits remobilised at the head of the Pyreneo-Languedocian canyons. We also characterised (4) the morphology and discuss the origin of the scours previously identified in the study area. These results contrast with previous interpretation which assigned a much younger age to the Neofan (Méar and Gensous, 1993 and Torres et al., 1997).
(Marine and Petroleum Geology. vol. 22 (6-7)gust 2005, Pages, n° 0264-8172, pp. 827-843, 01/08/2005)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LDO, INSU - CNRS, UBO EPE, CNRS
Challenges to identify phytoplankton species in coastal waters by remote sensing
During spring and summer 2004, intensive field campaigns were conducted in the Eastern English Channel. This region is characterized by relatively intense phytoplankton blooms, low bathymetry, strong tide ranges and great river inputs. The sampling period accounts for episodic blooms of prymnesiophyceae Phaeocystis globosa and diatoms. Hyperspectral radiometric measurements (TRIOS; 350-950 nm, with a 3 nm spectral resolution) were concurrently performed with water sampling for biogeochemical and optical characterization. The remote sensing reflectance, Rrs, is analyzed in conjunction with variation of the water composition. We particularly focus on the capability to identify some phytoplankton species from Rrs in this very variable environment. Different methods, based on multispectral and hyperspectral data are tested and compared for that purpose. We show that no Rrs ratio allows to discriminate between diatoms and Phaeocystis. In contrast, the derivative analysis applied to hyperspectral data stresses large differences in some part of the Rrs spectra collected in diatoms or Phaeocystis dominated waters.
(. vol. 58885, 31/07/2005)
UC San Diego, UC, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, ELICO, ULCO, CNRS, ULCO, LOV, OOVM, UPMC, INSU - CNRS, CNRS, UPMC, INSU - CNRS, CNRS
Effect of intermediate host size (Cerastoderma edule) on infectivity of cercariae of Himasthla quissetensis (Echinostomatidae: Trematoda)
The edible cockle (Cerastoderma edule), a common marine bivalve in semi-sheltered sandflats, is a natural host for many parasite species of the genus Himasthla (Echinostomatidae: Trematoda). In a previous paper, Wegeberg et al. (1999) showed segregation of cockle infection by three Himasthla species (H. interrupta, H. continua, H. elongata) in relation to host tissue (foot, mantle, siphon) and host size (1·5 to 14 mm shell length). Following the same experimental procedure, a fourth species, H. quissetensis, an introduced dominant parasite of cockles in Arcachon Bay (south-west France) was investigated. The infection pattern was very similar to the patterns shown by H. elongata and H. continua. Cercariae (the free-living stage shed from prosobranch snails and encysting as metacercariae in bivalves) were most successful in the shell-length range of 6–14 mm, where 74% of the added cercariae were recovered as metacercariae. The comparison with the other Himasthla species supports the previous conclusion that the efficiency of cercariae to infect cockles depends on host size.
(Journal of the Marine Biological Association of the United Kingdom. vol. 85, n° 0025-3154, pp. 809-812, 27/06/2005)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS