Les effets de l'oxygénation dans un lac eutrophe étudiés par la méthode de l'«enceinte»

Le lac de Comabhio (Lombardie, Italie du Nord) est un lac peu profond (z- = 4,4 m) en comparaison à sa surface (3,44 km2). L'eutrophisation naturelle du lac a été accélérée par l'apport d'effluents domestiques pas traités. Pour améliorer la situation, les effluents conduits par un collecteur circumlacustre, ont été recueillis à l'extérieur du bassin versant.Les effets seront visibles seulement dans plusieurs années et ces interventions ne seront pas suffisantes pour ramener le lac à l'état de mésotrophie. Afin d'accélérer l'assainissement du lac, mais surtout pour éviter la forte mortalité du poisson périodiquement à la fin de l'été, on a pris la décision d'oxygéner artificiellement l'hypolimnion. Pour prévoir les effets de l'oxygénation on a fait une expérience dans le lac avec deux enceintes en PVC (diamètre : 40 m; profondeur : 6 m; volume : 7000 m3). On a gardé une enceinte comme témoin; dans l'autre a été insufflé de l'oxygène pur. On a recueilli, à certains intervalles de temps, des échantillons d'eau et de plancton du lac et des deux enceintes. Les éléments nutritifs ont été analysés; la température, la transparence, la conductivité et le pH ont été mesurés sur place. L'effet le plus visible du traitement consistait dans une augmentation de la concentration de l'oxygène visant à produire des conditions normales pour la vie des poissons. Les effets de l'oxygénation sur les caractéristiques chimiques, physiques et biologiques du milieu sont discutés.Lake Comabbio (Lombardy, Northern Itaty) is a shallow Lake (zmax = 8.0 m; z-= 4.4 m; surface = 3.44 km2). Its naturally high trophic level has been increased by the huge nutrient loading from domestic effluents. Each year (except in winter, when commonly the lake is covered with ice) more phytoplankton blooms are observed and in the late summer oxygen depletion, with a consequent mass mortality of fish, occurs.At present, to reduce the nutrient charge, the effluents are collected in a channel and diverted from the lake watershed. Because of the significant nutrient release from the sediment ("internal eutrophication") the reduction of the external loading may slow the eutrophication rate, but it is not sufficient to restore the lake to an acceptable mesotrophic state within a reasonable time.To prevent the periodic mass mortality of fish and possibly to accelerate the restoration process of the ecosystem, the Lake Comabbio Protection Committee decided to oxygenate the hypolimnetic layer artificially. In order to avoid undesirable effects of aeration (e.g. nitrogen enrichment of lake water), dissolved oxygen injection was preferred. As there is no general agreement on the ecological effects of the oxygen (or air) addition it was decided, before applying oxygenation to the whole lake, to carry out experiments using the "enclosure" method. One experiment was carried out from 26th September to 9th December 1986 and this was replicated in 1987 from 22nd April to 30th November. This paper is concerned with the first experiment, as data treatment of the second is still in progress. Two cylindrical "enclosures" of PVC (diameter = 40.0 m; height = 6.0 m; volume = 7000 m3) were settled in Lake Comabbio to isolate a water column with its sediments. One enclosure was treated with oxygen from 15th November to 6th December, the other was kept as control. The water aspirated from the enclosure was oxygenated (20.3 mg O2/l) and injected in the same enclosure at 4 m depth at a rate of 4 m3/h. During the enclosure settlement the fish escaped from the enclosures. To simulate a complete ecosystem, the plan was to add to each enclosure 60 kg of the most abundant fish species (Scardinius erythrophtalmus). By mistake, fish were added to the enclosure that was to be oxygenated but not to the control. Water and zooplankton were sampled simultaneously from both the enclosures as well as from the Lake. Zooplankton was collected by vertical hauls from the bottom to the water surface. The following parameters were measured : temperature, transparency, pH, electrical conductivity, dissolved oxygen, alkalinity, phosphorus and nitrogen compounds, chlorophyll-a and pheopigments. The injection of water rich in oxygen did not resuspend the sediments and the absence of bubbles showed that the oxygen had completely dissolved.Soon after oxygenation the difference in oxygen concentration in the treated enclosure and in the control was progressively reduced until 26th November, when both the enclosures had the same oxygen concentration. At the end of the experiment (9th December) the oxygen concentration in the control was about 4 mg/l, whereas in the treated enclosure it attained a concentration of 6 mg/l, although in the latter the oxygen consumption was greater than in the control, owing to fish respiration and the increased activity of aerobic microorganisms. The higher concentrations of total phosphorus (TP), soluble reactive phosphorus (SRP) and ammonium (N-NH4) measured in the lake were probably due to the external loading of nutrients. The nutrient concentrations in the treated enclosure were similar to those of the control. The greater concentration of nutrients was probably the cause of more abundant phytoplankton in the lake, which was also demonstrated by the lower values of water transparency, when compared with those measured in both the enclosures. The pH values of the control were similar to those of the treated enclosure. The pattern of abundance and species composition of the zooplankton in the treated enclosure did not differ from that of the control. The Entomostraca populations decreased in the lake as well as in both the enclosures from the beginning to the end of the experiment.From this experiment, carried out when oxygen depletion affected the whole water column, we may conclude that oxygenation can prevent fish mortality, without significantly influencing the structure and biomass of the planktonic community. The variations in the plankton and chemical and physical characteristics of the water (except oxygen concentration) were essentially due to the season and mot to the artificial oxygenation. In addition, oxygenation, by abolishing the periodical mortality of fish, may also prevent the release of a huge amount of organic substances from the dead fish into the water

N = 2 AdS4 supergravity, holography and Ward identities

We develop in detail the holographic framework for an N = 2 pure AdS supergravity model in four dimensions, including all the contributions from the fermionic fields and adopting the Fefferman-Graham parametrization. We work in the first order formalism, where the full superconformal structure can be kept manifest in principle, even if only a part of it is realized as a symmetry on the boundary, while the remainder has a non-linear realization. Our study generalizes the results presented in antecedent literature and includes a general discussion of the gauge-fixing conditions on the bulk fields which yield the asymptotic symmetries at the boundary. We construct the corresponding super- conformal currents and show that they satisfy the related Ward identities when the bulk equations of motion are imposed. Consistency of the holographic setup requires the super- AdS curvatures to vanish at the boundary. This determines, in particular, the expression of the super-Schouten tensor of the boundary theory, which generalizes the purely bosonic Schouten tensor of standard gravity by including gravitini bilinears. The same applies to the superpartner of the super-Schouten tensor, the conformino. Furthermore, the vanishing of the supertorsion poses general constraints on the sources of the three-dimensional boundary conformal field theory and requires that the super-Schouten tensor is endowed with an antisymmetric part proportional to a gravitino-squared term

First detection of ND in the solar-mass protostar IRAS16293-2422

In the past decade, much progress has been made in characterising the processes leading to the enhanced deuterium fractionation observed in the ISM and in particular in the cold, dense parts of star forming regions such as protostellar envelopes. Very high molecular D/H ratios have been found for saturated molecules and ions. However, little is known about the deuterium fractionation in radicals, even though simple radicals often represent an intermediate stage in the formation of more complex, saturated molecules. The imidogen radical NH is such an intermediate species for the ammonia synthesis in the gas phase. Herschel/HIFI represents a unique opportunity to study the deuteration and formation mechanisms of such species, which are not observable from the ground. We searched here for the deuterated radical ND in order to determine the deuterium fractionation of imidogen and constrain the deuteration mechanism of this species. We observed the solar-mass Class 0 protostar IRAS16293-2422 with the heterodyne instrument HIFI as part of the Herschel key programme CHESS (Chemical HErschel Surveys of Star forming regions). The deuterated form of the imidogen radical ND was detected and securely identified with 2 hyperfine component groups of its fundamental transition in absorption against the continuum background emitted from the nascent protostar. The 3 groups of hyperfine components of its hydrogenated counterpart NH were also detected in absorption. We derive a very high deuterium fractionation with an [ND]/[NH] ratio of between 30 and 100%. The deuterium fractionation of imidogen is of the same order of magnitude as that in other molecules, which suggests that an efficient deuterium fractionation mechanism is at play. We discuss two possible formation pathways for ND, by means of either the reaction of N+ with HD, or deuteron/proton exchange with NH.Comment: Accepted; To appear in A&A Herschel/HIFI Special Issu

Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum