Niveau de contamination par le mercure des sédiments de surface et des crevettes du fjord du Saguenay

De 1947 à 1976, plusieurs dizaines de tonnes de mercure d'origine industrielle ont été déversées dans les eaux de la rivière Saguenay et une grande partie de ce métal toxique se retrouve aujourd'hui dans les sédiments du Fjord. Les sédiments de surface (0-2 cm) de 17 stations réparties sur tout le bassin intérieur du fjord ont été prélevés et analysés pour le mercure total. De plus, environ 150 crevettes (Pandalus borealis) ont été capturées à Sainte-Rose-du-Nord pour fins d'analyse du mercure et pour une expérience de bioaccumulation à partir d'une diète contaminée. L'analyse des données disponibles pour les sédiments de surface montre que les teneurs moyennes (0,93 ± 0,11 µg.g-1 poids sec) en mercure n'ont pas varié de façon significative au cours des dix dernières années dans la région de Sainte-Rose-du-Nord, située dans la moitié amont du bassin intérieur. Les concentrations de mercure dans le muscle comestible des crevettes varient de 0,13 à 0,58 µg.g-1 (poids humide) selon la taille avec une valeur moyenne de 0,36 ± 0,11 µg.g-1. On trouve une concentration moyenne de 0,26 ± 0,09 µg.g-1 dans le céphalothorax et la cuticule. Un accroissement rapide et important du mercure dans, l'appareil digestif a été observé chez tes crevettes adultes soumises à une diète de chair de moules préalablement contaminée (6,0 ± 1,0 µg.g-1). Un taux de bioaccumulation de 0,09 µg.g-1 par jour dans le muscle comestible a été estimé pour les 14 premiers jours de diète contaminée.From 1947 to 1976, many tons of industrial mercury were tipped into the Saguenay River and a large amount of this toxic heavy metal is now in the sediments of the Saguenay Fjord. Surface sediments (0-2 cm) were collected at seventeen stations along the inner basin of the Saguenay Fjord and analysed for total mercury content. About 150 shrimps (Pandalus borealis) fished in the Sainte-Rose-du-Nord area were also used for mercury analyses and the determination of mercury uptake rate from contaminated food. The mercury concentrations in surface sediments ranged from 0.18 to 0.20 µg.g-1 (dry weight) with a mean value of 0.63 µg.g-1. This mean level is about one order of magnitude higher than the background level found in deep sediments. The examination of available data for surface sediments in the Sainte-Rose-du-Nord vicinity, located in the first half of the inner basin, shows the "steady state" of the mercury contamination over the last 10 years. Indeed, the mercury concentrations observed in surface sediments ranged from 0.75 to 1.20 µg.g-1, with a mean value of 0.93 µg.g-1 since 1976. The steady state of mercury contamination can be explained by two hypothesis : (1) an unexpected highly active bioturbation mechanism contributes to the mercury remobilisation from lower sediment layers (10-20 cm) and its vertical transportation up to the surface, or (2) the anthropogenic upstream discharge of mercury was not really stopped in 1976 and one or many unidentified sources are still active along the Saguenay River. The mercury concentrations in the edible part of shrimps (fished in November 1985) ranged from 0.13 to 0.58 µg.g-l (wet weight) with an average value of 0.36 ± 0.11 µg.g-1. A positive and significative linear relationship (r = 0.786) is observed between the Hg concentration in the edible part and the total wet weight of the shrimp. The mean Hg in the edible part found in 1985 is not significantly different from the mean value reported in 1932. The mean concentration found in the cephalothorax and the cuticle (taken together) of shrimps was 0.26 ± 0.09 µg.g-1. In order to estimate the mercury uptake rate by shrimps from contaminated food, a number of adult shrimps were fed with pre-contaminated mussel tissues (0.6 ± 1.0 µg.g-1) for three weeks. A high and rapid increase of mercury concentration was observed in the digestive organs after only 24 hours. The uptake rate in the edible part was estimated at 0.09 µg.g-1 per day during the first fourteen days of the bioassay. These findings clearly indicate the fragility of the balance between the biota and the physical environment and how fast major changes can occur when the level of contamination of the diet is modified

Organic carbon abundance, distribution and metabolism at the Oyster, Virginia study site

This report describes a pilot study conducted at the DOE Subsurface Science Program\u27s study site in Oyster, VA. The objective of this study was to examine whether 2 organic matter associated with the solid and dissolved phases was labile enough to support microbial activity. Organic matter availability was assessed in two ways: (1) by quantifying the amount and distribution of total organic carbon (TOC) associated with the solid phase and (2) laboratory experiments to examine the utilization of dissolved organic matter by measuring total microbial respiration. In addition to assessing total respiration, we specifically addressed organic matter respiration via denitrification. The focus on denitrification was due to the environmental field conditions at the study site (low concentrations of dissolved oxygen and high nitrate concentrations) suggesting that nitrate respiration would be a likely process for organic matter utilization

Characterization and limits of a cold atom Sagnac interferometer

We present the full evaluation of a cold atom gyroscope based on atom interferometry. We have performed extensive studies to determine the systematic errors, scale factor and sensitivity. We demonstrate that the acceleration noise can be efficiently removed from the rotation signal allowing to reach the fundamental limit of the quantum projection noise for short term measurements. The technical limits to the long term sensitivity and accuracy have been identified, clearing the way for the next generations of ultra-sensitive atom gyroscopes

Nitrogen cycling through a fringing marsh-aquifer ecotone

Fringing wetlands are critical components of estuarine systems, and subject to water fluxes from both watersheds and estuaries. To assess the effect of groundwater discharge on marsh nitrogen cycling, we measured N-cycling in sediments from a fringing mesohaline marsh in Virginia which receives a seasonal groundwater input. Mineralization, nitrification, potential denitrification (DNF), and potential dissimilatory nitrate reduction to ammonium (DNRA) rates were estimated along with porewater concentrations of oxygen, sulfide, and conductivity during high (May 1997) and low (October 1997) groundwater discharge. All N-cycling processes were confined to the upper 1 to 1.5 m of marsh, where organic matter and ammonium were most abundant. Depth-integrated rates for mineralization, nitrification, DNRA, and DNF ranged between 1.0-11.2, 0.0-2.2, 0.9-6.1, and 1.8-17.6 mmol N m(-2) h(-1), respectively. During spring discharge (May), porewater conductivity, and dissolved sulfide decreased by approximately 50%, and a groundwater-driven O-2 flux of 27 mu mol m(-2) h(-1) into the marsh subsurface was estimated, Although mineralization, nitrification, and DNRA rates were up to 12x, 6x, and 7.5x greater in May. respectively, than during low discharge (October), DNF was 10x higher in October. The largest difference in seasonal rates was observed nearest the upland border, where groundwater discharge had the greatest effect on sediment geochemistry. We suggest that a synergy between an increased flux of electron accepters, porewater mixing, and flushing of salt and sulfide was responsible for the elevated mineralization and nitrification rates in May. Natural-abundance delta N-15 measurements of the NH4+, NO3-, and N-2 pools showed that nitrification is important in mediating N export by linking mineralization and denitrification in this marsh. However, despite accelerated mineralization and nitrification in May, there was not an equivalently large export of N via coupled nitrification-denitrification. The DNF:DNRA ratio in May (0.6) was 25-fold lower than that seen at low discharge, indicating that during spring discharge, a greater proportion of nitrified N was recycled internally rather than exported via denitrification

Nutrient Enrichment and Food Web Composition Affect Ecosystem Metabolism in an Experimental Seagrass Habitat

Food web composition and resource levels can influence ecosystem properties such as productivity and elemental cycles. In particular, herbivores occupy a central place in food webs as the species richness and composition of this trophic level may simultaneously influence the transmission of resource and predator effects to higher and lower trophic levels, respectively. Yet, these interactions are poorly understood

Time-resolved photoelectron and photoion fragmentation spectroscopy study of 9-methyladenine and its hydrates: a contribution to the understanding of the ultrafast radiationless decay of excited DNA bases.

The excited state dynamics of the purine base 9-methyladenine (9Me-Ade) has been investigated by time- and energy-resolved photoelectron imaging spectroscopy and mass-selected ion spectroscopy, in both vacuum and water-cluster environments. The specific probe processes used, namely a careful monitoring of time-resolved photoelectron energy distributions and of photoion fragmentation, together with the excellent temporal resolution achieved, enable us to derive additional information on the nature of the excited states (pp*, np*, ps*, triplet) involved in the electronic relaxation of adenine. The two-step pathway we propose to account for the double exponential decay observed agrees well with recent theoretical calculations. The near-UV photophysics of 9Me-Ade is dominated by the direct excitation of the pp* (1Lb) state (lifetime of 100 fs), followed by internal conversion to the np* state (lifetime in the ps range) via conical intersection. No evidence for the involvement of a ps* or a triplet state was found. 9Me- Ade–(H2O)n clusters have been studied, focusing on the fragmentation of these species after the probe process. A careful analysis of the fragments allowed us to provide evidence for a double exponential decay profile for the hydrates. The very weak second component observed, however, led us to conclude that the photophysics were very different compared with the isolated base, assigned to a competition between (i) a direct one-step decay of the initially excited state (pp* La and/or Lb, stabilised by hydration) to the ground state and (ii) a modified two-step decay scheme, qualitatively comparable to that occurring in the isolated molecule

Tracking the fate of a high concentration groundwater nitrate plume through a fringing marsh: A combined groundwater tracer and in situ isotope enrichment study

A groundwater plume enriched in (15)NO(3)(-) was created upgradient of a mesohaline salt marsh. By measuring the changes in concentration and isotopic enrichment of NO(3)(-), N(2)O, N(2), NH(4)(+), and particulate organic nitrogen (PON) during plume transport through the marsh, in situ rates of dissimilatory nitrate reduction to ammonium (DNRA) and denitrification (DNF) were estimated, as well as N storage in the reduced N pools. For groundwater discharge within the top 10 cm of marsh, NO(3)(-) removal was 90% complete within the 50 cm of marsh nearest the upland border. The peak NO(3)(-) loss rate from the plume ranged from 208 to 645 muM d(-1). Rates of DNRA (180 muM d(-1)) and DNF (387-465 muM d(-1)) processed 30% and 70% of the NO(3)(-) load, respectively. Terminal N(2)O production was approximately equal to N(2) production rates during DNE Comparison of (15)N lost from the (15)O(3)(-) pool and (15)N gained in each of the reduced products accounted for only 22% of the reduced (15)N, thus indicating N export from the system. Despite high rates of DNRA, the NH(+) produced was not a long-term repository for the groundwater-derived N but was instead rapidly immobilized into marsh PON and retained on longer timescales. The small inventory of (15)N in the N(2)O and N(2) pools relative to DNF rates, coincident with an undersaturation of dissolved argon, indicated that denitrified N was exported to the atmosphere on short timescales. The relative magnitudes of DNF and DNRA in conjunction with the immobilization of NH(4)(+) and evasion of N gases dictated the extent of export versus retention of the groundwater NO(3)(-) load

Matter-wave laser Interferometric Gravitation Antenna (MIGA): New perspectives for fundamental physics and geosciences

The MIGA project aims at demonstrating precision measurements of gravity with cold atom sensors in a large scale instrument and at studying the associated applications in geosciences and fundamental physics. The first stage of the project (2013-2018) will consist in building a 300-meter long optical cavity to interrogate atom interferometers and will be based at the low noise underground laboratory LSBB in Rustrel, France. The second stage of the project (2018-2023) will be dedicated to science runs and data analyses in order to probe the spatio-temporal structure of the local gravity field of the LSBB region, a site of high hydrological interest. MIGA will also assess future potential applications of atom interferometry to gravitational wave detection in the frequency band $\sim 0.1-10$ Hz hardly covered by future long baseline optical interferometers. This paper presents the main objectives of the project, the status of the construction of the instrument and the motivation for the applications of MIGA in geosciences. Important results on new atom interferometry techniques developed at SYRTE in the context of MIGA and paving the way to precision gravity measurements are also reported.Comment: Proceedings of the 50th Rencontres de Moriond "100 years after GR", La Thuile (Italy), 21-28 March 2015 - 10 pages, 5 figures, 23 references version2: added references, corrected typo

Low noise amplication of an optically carried microwave signal: application to atom interferometry

In this paper, we report a new scheme to amplify a microwave signal carried on a laser light at $\lambda$=852nm. The amplification is done via a semiconductor tapered amplifier and this scheme is used to drive stimulated Raman transitions in an atom interferometer. Sideband generation in the amplifier, due to self-phase and amplitude modulation, is investigated and characterized. We also demonstrate that the amplifier does not induce any significant phase-noise on the beating signal. Finally, the degradation of the performances of the interferometer due to the amplification process is shown to be negligible

Effluent Organic Nitrogen (EON): Bioavailability and Photochemical and Salinity-Mediated Release

The goal of this study was to investigate three potential ways that the soluble organic nitrogen (N) fraction of wastewater treatment plant (WWTP) effluents, termed effluent organic N (EON), could contribute to coastal eutrophication - direct biological removal, photochemical release of labile compounds, and salinity-mediated release of ammonium (NH4+). Effluents from two WWTPs were used in the experiments. For the bioassays, EON was added to water from four salinities (∼0 to 30) collected from the James River (VA) in August 2008, and then concentrations of N and phosphorus compounds were measured periodically over 48 h. Bioassay results, based on changes in DON concentrations, indicate that some fraction of the EON was removed and that the degree of EON removal varied between effluents and with salinity. Further, we caution that bioassay results should be interpreted within a broad context of detailed information on chemical characterization. EON from both WWTPs was also photoreactive, with labile NH4+ and dissolved primary amines released during exposure to sunlight. We also present the first data that demonstrate that when EON is exposed to higher salinities, increasing amounts of NH4+ are released, further facilitating EON use as effluent transits from freshwater through estuaries to the coast