Julien Deloffre

Investigating possible links between the North Atlantic Oscillation and rainfall variability in northwestern France over the past 35 years

[1]xa0In this paper, the temporal dynamics of precipitation in northwestern France in relation to the dominant climatic pattern in Europe were investigated. The general trends and the nonstationary behavior of the North Atlantic Oscillation (NAO) were described using fractal analysis and Fourier spectral and continuous wavelet analysis of a NAO index time series over the 1865–2004 period. The 44-year and 8-year multidecadal components showed a clear increasing power during the second half of the last century. The possible link between rainfall variability and the NAO was then investigated. The links between the NAO and rainfall were not as obvious, as NAO-like components were not easily retrieved in the precipitation records: Relationships between the NAO and rainfall were very likely at certain timescales but were not systematically very obvious. For instance, the characteristic quasi-biennal oscillation (QBO) of the NAO was barely detected in precipitation, but a ≈6-year fluctuation beginning in the 90s was found to be statistically significant at a 95% confidence limit After investigating a possible link with the Southern Oscillation Index, the occurrence of this fluctuation in the beginning of the 90s could be related to the observed shift of the QBO toward slightly higher frequencies in the NAO time series. On the other hand, a modulation of the QBO by a ≈6-year interannual fluctuation would suggest the existence of a ≈6-year climate pattern that could affect precipitation and, to a lesser extent, the NAO. Cross-wavelet analysis between the NAO and precipitation revealed a loss in correlation across the 1970–2005 period, which seemed to be the fact of a QBO-like fluctuation. This loss of correlation was related to the above-mentioned shift of the QBO and 6-year rainfall interannual band since the 90s.

Tidally-induced shear stress variability above intertidal mudflats in the macrotidal seine estuary

Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge (<400 m3 s−1) in neap and spring tide conditions. Turbulent kinetic energy, covariance, and logarithmic profile methods were used and compared for the determination of shear stress. The cTKE coefficient value of 0.19 cited in the literature was confirmed. Shear stress values were shown to decrease above mudflats from the mouth to the fluvial part of the estuary due to dissipation of the tidal energy, from 1 to 0.2 N m−2 for spring tides and 0.8 to 0.05 N m−2 for neap tides. Flood currents dominate tidally-induced shear stress in the marine and lower fluvial estuary during neap and spring tides and in the upper fluvial part during spring tides. Ebb currents control tidally-induced shear stress in the upper fluvial part of the estuary during neap tides. These results revealed a linear relationship between friction velocities and current velocities. Bed roughness length values were calculated from the empirical relationship given by Mitchener and Torfs (1996) for each site; these values are in agreement with the modes of the sediment particle-size distribution. The influence of tidal currents on the mudflat dynamics of the Seine estuary was examined by comparing the tidally-induced bed shear stress and the critical erosion shear stress estimated from bed sediment properties. Bed sediment resuspension induced by tidal currents was shown to occur only in the lower part of the estuary.

Dynamics of Sulfate-Reducing Microorganisms (dsrAB genes) in Two Contrasting Mudflats of the Seine Estuary (France)

By combining molecular biology and biochemical approaches, the dynamics of sulfate-reducing microorganisms (SRM) was investigated in the sediments of the Seine estuary (France). Both intertidal mixing-zone and freshwater mudflats were sampled during a 1-year period; the quantification of SRM was realized by using competitive polymerase chain reaction (PCR) based on dsrAB gene amplification, previously described by Leloup et al. (2004), and sulfate reduction rate (SRR) was determined via the SO42 radiotracer method. Throughout the year, abundance of dsrAB genes and SRR were predominantly high in the top 15xa0cm of the sediment. A seasonal dynamic was observed; a predominance of activity was noted during the early summer, and seems to be mainly controlled by physical–chemical parameters (temperature and dissolved organic carbon concentration) and topographic evolution of the mudflat (erosion/deposit erosion).

Abundance, Diversity and Activity of Sulfate-Reducing Prokaryotes in Heavy Metal-Contaminated Sediment from a Salt Marsh in the Medway Estuary (UK)

We investigated the diversity and activity of sulfate-reducing prokaryotes (SRP) in a 3.5-m sediment core taken from a heavy metal-contaminated site in the Medway Estuary, UK. The abundance of SRPs was quantified by qPCR of the dissimilatory sulfite reductase gene β-subunit (dsrB) and taking into account DNA extraction efficiency. This showed that SRPs were abundant throughout the core with maximum values in the top 50xa0cm of the sediment core making up 22.4% of the total bacterial community and were 13.6% at 250xa0cm deep. Gene libraries for dsrA (dissimilatory sulfite reductase α-subunit) were constructed from the heavily contaminated (heavy metals) surface sediment (top 20xa0cm) and from the less contaminated and sulfate-depleted, deeper zone (250xa0cm). Certain cloned sequences were similar to dsrA found in members of the Syntrophobacteraceae, Desulfobacteraceae and Desulfovibrionaceae as well as a large fraction (60%) of novel sequences that formed a deep branching dsrA lineage. Phylogenetic analysis of metabolically active SRPs was performed by reverse transcription PCR and single strand conformational polymorphism analysis (RT-PCR–SSCP) of dsrA genes derived from extracted sediment RNA. Subsequent comparative sequence analysis of excised SSCP bands revealed a high transcriptional activity of dsrA belonging to Desulfovibrio species in the surface sediment. These results may suggest that members of the Desulfovibrionaceae are more active than other SRP groups in heavy metal-contaminated surface sediments.

Relationships between hydrosedimentary processes and occurrence of mercury-resistant bacteria (merA) in estuary mudflats (Seine, France)

The Seine estuary (France) is one of the worlds macrotidal systems that is most contaminated with heavy metals. To study the mercury-resistant bacterial community in such an environment, we have developed a molecular tool, based on competitive PCR, enabling the quantification of Gram-negative merA gene abundance. The occurrence of the Gram-negative merA gene in relation with the topology (erosion/deposit periods) and the mercury contamination of three contrasted mudflats was investigated through a multidisciplinary approach and compared with a non-anthropized site (Authie, France). The higher abundance of the Gram-negative merA gene in the Seine estuary mudflats indicates a relationship between the degree of anthropization and the abundance of the merA gene in the mudflat sediments. In the Seine mudflats, the maxima of abundance are always located in fresh sediment deposits. Therefore, the abundance is closely related with the hydrosedimentary processes, which thus seem to be determining factors in the occurrence of the Gram-negative merA gene in the surface sediments of the Seines mudflat.

Sediment transfer and accumulation in two contrasting salt marsh/mudflat systems: the Seine estuary (France) and the Medway estuary (UK)

Understanding the dynamics of fine sediment transport across the upper intertidal zone is critical in managing the erosion and accretion of intertidal areas, and in managed realignment/estuarine habitat recreation strategies. This paper examines the transfer of sediments between salt marsh and mudflat environments in two contrasting macrotidal estuaries: the Seine (France) and the Medway (UK), using data collected during two joint field seasons undertaken by the Anglo-French RIMEW project (Rives-Manche Estuary Watch). High-resolution ADCP, Altimeter, OBS and ASM measurements from mudflat and marsh surface environments have been combined with sediment trap data to examine short-term sediment transport processes under spring tide and storm flow conditions. In addition, the longer-term accumulation of sediment in each salt marsh system has been examined via radiometric dating of sediment cores. In the Seine, rapid sediment accumulation and expansion of salt marsh areas, and subsequent loss of open intertidal mudflats, is a major problem, and the data collected here indicate a distinct net landward flux of sediments into the marsh interior. Suspended sediment fluxes are much higher than in the Medway estuary (averaging 0.09 g/m3/s), and vertical accumulation rates at the salt marsh/mudflat boundary exceed 3 cm/y. Suspended sediment data collected during storm surge conditions indicate that significant in-wash of fine sediments into the marsh interior can occur during (and following) these high-magnitude events. In contrast to the Seine, the Medway is undergoing erosion and general loss of salt marsh areas. Suspended sediment fluxes are of the order of 0.03 g/m3/s, and the marsh system here has much lower rates of vertical accretion (sediment accumulation rates are ca. 4 mm/y). Current velocity data for the Medway site indicate higher velocities on the ebb tide than occur on the flood tide, which may be sufficient to remobilise sediments deposited on the previous tide and so force net removal of material from the marsh.

Potential of UAVs for Monitoring Mudflat Morphodynamics (Application to the Seine Estuary, France)

Intertidal mudflats play a critical role in estuarine exchange, connecting marine and continental supplies of nutrients and sediments. However, their complex morphodynamics, associated with a wide range of physical and biological processes, are still poorly understood and require further field investigation. In addition, mudflats are challenging areas for Structure-from-Motion (SfM) photogrammetric surveys. Indeed, the mudflats generally hold back residual tidal water, which can make stereo restitution particularly difficult because of poor correlations or sun-glint effects. This study aims to show the potential of light UAVs (Unmanned Aerial Vehicles) for monitoring sedimentary hydrodynamics at different spatial scales in a silty estuary. For each UAV mission an orthophotograph and a Digital Elevation Model (DEM) are computed. From repeated surveys the diachronic evolution of the area can be observed via DEM differencing. Considering the ground texture in such a context, the stereo restitution process is made possible because of the high spatial resolution of the UAV photographs. Providing a synoptic view as well as high spatial resolution (less than 4 cm), the UAV dataset enables multi-scale approaches from the study of large areas to the morphodynamics of smaller-scale sedimentary structures and the morphodynamics impact of plant ground cover.

Dating of sediment record at two contrasting sites of the Seine River using radioactivity data and hydrological time series

Sediment cores were collected at the outlet of the highly anthropogenized catchment of the Seine River at two contrasting sites: a flood plain of the lower Seine River and a quasi-permanently submerged harbour basin (or wet dock) in the upper tidal estuary. Analyses of artificial radionuclides ((137)Cs and plutonium isotopes), coupled with hydrological and bathymetric data, lead to a precise dating of the sediment cores collected at the two sites. (137)Cs signals originating from global fallout (early 1960s) and from the Chernobyl accident (1986) are identified, but at different levels due to the incomplete nature or variable continuity of the records. Anomalous (238)Pu concentrations found at both sites (1-2 Bq kg(-1)) are attributed to unknown industrial releases originating from upstream. Interpolating (137)Cs sediment activities under the assumption of a constant sediment rate, those releases were dated back to 1975 ± 1, thus providing a local but reliable time-marker. Age models have highlighted a very contrasting sediment filling dynamics in these two sites. This study presents the first sediment record of alpha- and gamma-emitting artificial radionuclides obtained at the outlet of the huge catchment area of the River Seine, over a period covering the last 50 years.

Fate of Antibiotics and Antibiotic-Resistant Fecal Bacteria in Water and Sediments from the Contamination Source to the Estuary: Impact and/or Resilience? Resilience to Contamination by Antibiotics

The aim of this study based on a multidisciplinary research program (FLASH) (FLASH (FLuxes of Antibiotic and Antibiotic-resistant bacteria and the corresponding genes in Surface Hydrosystems) a research project founded by CNRS EC2CO/GIP Seine-Aval), associating chemists, hydrologists, and clinical and environmental microbiologists, was to monitor the impact of antibiotic prescription in human and veterinary practices on water and sediment contamination by antibiotic and Escherichia coli (antibiotic resistance, integrons) and Enterococci (diversity, antibiotic resistance, and the corresponding genes) along a medical center–WWTP–river continuum. For this purpose, a multi-residue chemical methodology was developed in order to detect low levels of 34 antibiotics. In the medical center, the main prescribed antibiotic (amoxicillin) was weakly found in effluents. Along the continuum, contamination of water by antibiotics decreased from 160 μg.L−1 (cefotaxime) in hospital effluents to 1 ng.L−1 (ofloxacin) in the river. These concentrations were too low to exert a selective pressure (mg.L−1) on antibiotic-resistant bacteria. In the same samples, occurrence of antibiotic-resistant E. coli and those harboring a class 1 integrons was significantly (p-value < 0.001) decreased along the continuum. Among Enterococcus populations, E. faecium was mainly isolated (from 89 to 98 %). All E. faecium isolates from medical center effluents were multiple antibiotic resistant, containing erm(B) and mef(A) genes, and belonged to the hospital-adapted clonal complex 17 (CC17). The relative proportion of CC17 decreased in favor of other subpopulations, less resistant to antibiotics. In water, only persistent compounds were found (quinolones, macrolides, sulfonamides), but they did not correspond to the major resistances in E. coli and Enterococcus (penicillins, tetracyclines).