Bilan du phosphore dans un bassin versant du lac Léman : Conséquences pour la détermination de l'origine des flux exportés

Cet article s'intéresse à la dynamique du transfert du phosphore dans un bassin rural de la région lémanique, le bassin versant du Foron (51,5 km2). Nous montrons qu'en établissant, pour les périodes de tarissement, la relation concentration moyenne en phosphore - débit moyen à l'exutoire, il est possible d'évaluer globalement:1) la rétention du P dans le réseau hydrographique lors de ces périodes; 2) la contribution moyenne des sources ponctuelles de P aux exportations du bassin.La rétention est un phénomène temporaire, mais c'est une étape clé de la dynamique du phosphore. Elle crée d'importants stocks de P dans le réseau hydrographique; ces stocks se situent pour l'essentiel au niveau des sédiments comme l'ont montré diverses études antérieures et sont exportés lors des crues. Les quantités stockées à l'issue de longs étiages dépassent souvent largement les exportations enregistrées pendant les plus fortes crues. La rétention s'accompagne d'une transformation partiellement irréversible de la spéciation du phosphore transféré.Ces phénomènes sont pris en compte pour établir des bilans annuels de pollution, et notamment la balance diffus - ponctuel.The effective management of phosphorus in watersheds requires knowledge of the origin and magnitude of phosphorus inputs. However, for large and complex watersheds, it is impossible to determine the phosphorus mass balances by measuring the flow from each source and the rate of transfer of phosphorus originating from upstream. Because of these difficulties, we have developed a strategy based on the measurement of phosphorus mass balances during base flow periods (with no storm-flow, which means with a constant or decreasing flow). This approach was applied to the Foron River watershed (drainage area 51.5 km2, annual average water discharge 0.5 m3 /s).The Foron River, a hillside stream with a torrential flow regime, is a tributary of Lac Léman (a large meso-eutrophic lake also known as Lake Geneva). Land use within the Foron watershed is diverse, with 50% of the watershed area being in a naturally forested area (on steep upland slopes), 36% in agricultural land lying on a gentle relief of moraine deposits (70% permanent pasture and hay meadows, 30% cereal grain crops) and 3% in marshes. The population is scattered amongst four small villages, with an overall density of about 100 inhabitants per km2. Stream flow in the river was continuously monitored and automatically sampled at the outlet from 1990 to 1993. Additional grab samples were taken at various locations, springs, agricultural sub-watersheds and brooks in the naturally forested area. Total suspended solids, soluble total phosphorus, orthophosphate and total phosphorus were determined according to standard methods. In addition, sampling was carried out to determine phosphorus stored in river sediments in places that are known to be temporary sinks for sediments.To validate the efficiency of the approach, we carried out a detailed study of the point sources and the sewage systems in the watershed. Only two-thirds of domestic wastewaters were processed in treatment plants. The minimum load of point source inputs was established to be about 60 kg total P per week and many point sources (that is to say discharged at discrete locations) remained unidentified (i.e., not all households were connected to sewers, farmsteads contributed for unknown quantities of total P).During base flow periods, phosphorus was predominantly soluble and orthophosphate was found throughout the hydrographic network. Particulate phosphorus and suspended matter concentrations were negligible (respectively 0.8 m3 /s, a constant P export regime was reached as all the inputs were transported to the outlet. Consequently, output at the outlet was equal to input into the river. This approach permits the calculation of the total point sources. Knowing the inputs from point sources and the total P export at the outlet, we can calculate the diffuse sources.Significant relationships between weekly mean total-P, soluble-P or orthophosphate concentrations at the outlet and weekly mean discharge were established to validate these observations. The best fit of these measured values didn't follow a standard law of dilution because the dilution effect was attenuated by the antagonist effect due to the increase in P transport inputs from sewage with increasing discharge. ([soluble P]=0.02+ 0.148 Q-1/2 ; n=82; r2 =0.8).During base flow when Q < 0.8 m3 /s, a portion of phosphorus inputs accumulated in the river biota and sediments through deposition, biological uptake and sorption. This accumulation may explain the higher levels of total-P and the easily exchangeable P in the sediments downstream of point source discharges. The efficiency of these storage processes depends not only on water discharge, but also (secondarily) on some characteristics of the antecedent hydrological conditions. We found that the amount stored increased significantly in the weeks following large storm-flows. We assumed that this indicated the effect of an intense exchange of the P loaded sediments with unsaturated sediments that were generated by erosion of natural upstream areas. As in-stream processes alter phosphorus speciation, a part of soluble-P becomes particulate. During storm-flows following base flow periods with a discharge lower than the limit discharge, the stored phosphorus was released from both particulate and soluble forms and exported. We calculated an annual mass balance of soluble phosphorus to evaluate the amount of particulate phosphorus exported at the outlet due to the transformation of phosphorus from soluble to particulate forms. The result obtained suggested that ¼ of particulate-P (1 t per year) resulted from this speciation change.To summarize, analyses of phosphorus mass balances during base flow could lead to a new procedure to evaluate the mean total mass of P originating from point sources, without referring to field surveys. In addition, this procedure allowed the quantification of both total-P storage as well as changes in speciation that occur during transfer of phosphorus in the hydrographic network

JMM Profile: Actinobacillus pleuropneumoniae: a major cause of lung disease in pigs but difficult to control and eradicate

The Gram-negative bacterium Actinobacillus pleuropneumoniae is the causative agent of pleuropneumonia in pigs, its only known natural host. Typical symptoms of peracute disease include fever, apathy and anorexia, and time from infection to death may only be 6 h. Severe lung lesions result from presence of one or two of the ApxI-III toxins. Control is through good husbandry practice, vaccines and antibiotic use. Culture and presence of the species-specific apxIV gene by PCR confirms diagnosis, and identification of serovar, of which 19 are known, informs on appropriate vaccine use and epidemiology

Prélèvement et échantillonnage dans les petits bassins-versants ruraux

Cet article traite de la représentativité des prélèvements d'eau effectués dans le cadre de suivis de bassins versants ou de rivières.Les réflexions proposées sont en premier lieu basées sur des expérimentations de laboratoire et de terrain destinées à comparer divers systèmes et modalités de prélèvements. Les résultats obtenus montrent en particulier que les prélèvements de matières en suspension de dimensions > 100 µm sont peu fiables alors que ce type de particules présente, dans certain cas, une contribution non négligeable aux flux de phosphore total.Les suivis réalisés en divers points représentatifs de petits bassins expérimentaux fournissent des éléments de discussion complémentaires sur le choix des sites des prélèvements et sur les pas de temps de ceux-ci. Nous montrons entre autre, toute l'importance de saisir le début des crues dans les bassins à forte pollution ponctuelle et de considérer des suivis des périodes suffisamment longues pour intégrer les divers fonctionnements hydrochimiques du bassin versant étudié.Water and suspended matter sampling in hydrographic networks is studied through various sampling systems and procedures, and through devices used in fields and in laboratories (fig. 1).Large size particles tend to be underestimated whatever the sampling procedure (manual or automatic) (fig. 2, 3). On the other hand, the sampling for soluble elements and small site particles (less man 50 µm) is very reliable. This is observed in suspended matter concentrations, eg < 2 g.l-1.The largest particles of suspended matter sometimes contribute to nutrient transfer, e.g. phosphorous. This however depends upon the presence of soil microaggregates and nutrient geochemistry. Moreover, natural hydrological variations give rise to another problem for sampling, especially for suspended matter (fig. 7, 9, 10, table 2).Comparatively the variation due to preservation of water is of secondary importance (fig. 11).However, thanks to results from monitoring in experimental watersheds, it is possible to find the best sampling station and the hast time to carry out sampling (fig. 12, 13; fable 3).Duration of survey and intervals between each sample depend, of course, upon the objectives of the study, but also upon watershed characteristics. Considering these characteristics, we propose an example of sampling stategy in the text (table 6, fig. 5)

Is there a reentrant glass in binary mixtures?

By employing computer simulations for a model binary mixture, we show that a reentrant glass transition upon adding a second component only occurs if the ratio $\alpha$ of the short-time mobilities between the glass-forming component and the additive is sufficiently small. For $\alpha \approx 1$, there is no reentrant glass, even if the size asymmetry between the two components is large, in accordance with two-component mode coupling theory. For $\alpha \ll 1$, on the other hand, the reentrant glass is observed and reproduced only by an effective one-component mode coupling theory.Comment: 4 pages, 3 figure

Model for Glass Transition in a Binary fluid from a Mode Coupling approach

We consider the Mode Coupling Theory (MCT) of Glass transition for a Binary fluid. The Equations of Nonlinear Fluctuating Hydrodynamics are obtained with a proper choice of the slow variables corresponding to the conservation laws. The resulting model equations are solved in the long time limit to locate the dynamic transition. The transition point from our model is considerably higher than predicted in existing MCT models for binary systems. This is in agreement with what is seen in Computer Simulation of binary fluids. fluids.Comment: 9 Pages, 3 Figure

Sound-propagation gap in fluid mixtures

We discuss the behavior of the extended sound modes of a dense binary hard-sphere mixture. In a dense simple hard-sphere fluid the Enskog theory predicts a gap in the sound propagation at large wave vectors. In a binary mixture the gap is only present for low concentrations of one of the two species. At intermediate concentrations sound modes are always propagating. This behavior is not affected by the mass difference of the two species, but it only depends on the packing fractions. The gap is absent when the packing fractions are comparable and the mixture structurally resembles a metallic glass.Comment: Published; withdrawn since ordering in archive gives misleading impression of new publicatio

Rapid detection of A. pleuropneumoniae from clinical samples using recombinase polymerase amplification

Actinobacillus pleuropneumoniae (APP) is the causative agent of porcine pleuropneumonia, resulting in high economic impact worldwide. There are currently 19 known serovars of APP, with different ones being predominant in specific geographic regions. Outbreaks of pleuropneumonia, characterized by sudden respiratory difficulties and high mortality, can occur when infected pigs are brought into naïve herds, or by those carrying different serovars. Good biosecurity measures include regular diagnostic testing for surveillance purposes. Current gold standard diagnostic techniques lack sensitivity (bacterial culture), require expensive thermocycling machinery (PCR) and are time consuming (culture and PCR). Here we describe the development of an isothermal point-of-care diagnostic test - utilizing recombinase polymerase amplification (RPA) for the detection of APP, targeting the species-specific apxIVA gene. Our APP-RPA diagnostic test achieved a sensitivity of 10 copies/µL using a strain of APP serovar 8, which is the most prevalent serovar in the UK. Additionally, our APP-RPA assay achieved a clinical sensitivity and specificity of 84.3% and 100%, respectively, across 61 extracted clinical samples obtained from farms located in England and Portugal. Using a small subset (n = 14) of the lung tissue samples, we achieved a clinical sensitivity and specificity of 76.9% and 100%, respectively) using lung imprints made on FTA cards tested directly in the APP- RPA reaction. Our results demonstrate that our APP-RPA assay enables a suitable rapid and sensitive screening tool for this important veterinary pathogen