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Complete understanding of the hydrological functioning of large-scale intertropical watersheds such as the Lake Chad basin is becoming a high priority in the context of climate change in the near future and increasing demographic pressure. This requires integrated studies of all surface water and groundwater bodies and of their quite-complex interconnections. We present here a simple method for estimating the annual mean water balance of sub-Sahelian lakes subject to high seasonal contrast and located in isolated regions with no road access during the rainy season, a situation which precludes continuous monitoring of in situ hydrological data. Our study focuses for the first time on two lakes, Iro and Fitri, located in the eastern basin of Lake Chad. We also test the approach on Lake Ihotry in Madagascar, used as a benchmark site that has previously been extensively studied by our group. We combine the δ18O and δ2H data that we measured during the dry season with altimetry data from the SARAL satellite mission in order to model the seasonal variation of lake volume and isotopic composition. The annual water budget is then estimated from mass balance equations using the Craig–Gordon model for evaporation. We first show that the closed-system behavior of Lake Ihotry (i.e., precipitation equal to evaporation) is well simulated by the model. For lakes Iro and Fitri, we calculate evaporation to influx ratios (E∕I) of 0.6±0.3 and 0.4±0.2, respectively. In the case of the endorheic Lake Fitri, the estimated output flux corresponds to the infiltration of surface water toward the surface aquifer that regulates the chemistry of the lake. These results constitute a first-order assessment of the water budget of these lakes, in regions where direct hydrological and meteorological observations are very scarce or altogether lacking. Finally, we discuss the implications of our data on the hydro-climatic budget at the scale of the catchment basins. We observe that the local evaporation lines (LELs) obtained on both lake and aquifer systems are slightly offset from the average rainfall isotopic composition monitored by IAEA at N'Djamena (Chad), and we show that this difference may reflect the impact of vegetation transpiration on the basin water budget. Based on the discussion of the mass balance budget we conclude that, while being broadly consistent with the idea that transpiration is on the same order of magnitude as evaporation in those basins, we cannot derive a more precise estimate of the partition between these two fluxes, owing to the large uncertainties of the different end-members in the budget equations.

In the Mediterranean area, the 4.2 ka BP event is recorded with contrasting expressions between regions. In the southern Alps, the high-altitude Lake Petit (Mercantour Massif, France; 2200 m a.s.l.) offers pollen and diatom-rich sediments covering the last 4800 years. A multi-proxy analysis recently revealed a detrital pulse around 4200 cal BP due to increasing erosion in the lake catchment. The involvement of a rapid climate change leading to increasing runoff and soil erosion was proposed. Here, in order to clarify this hypothesis, we measured the oxygen isotope composition of diatom silica frustules (δ18Odiatom) from the same sedimentary core. Diatoms were analysed by laser fluorination isotope ratio mass spectrometry after an inert gas flow dehydration. We additionally enhanced the accuracy of the age–depth model using the Bacon R package. The δ18Odiatom record allows us to identify a 500-year time lapse, from 4400 to 3900 cal BP, where δ18Odiatom reached its highest values (>31 ‰). δ18Odiatom was about 3 ‰ higher than the modern values and the shifts at 4400 and 3900 cal BP were of similar amplitude as the seasonal δ18Odiatom shifts occurring today. This period of high δ18Odiatom values can be explained by the intensification of 18O-enriched Mediterranean precipitation events feeding the lake during the ice-free season. This agrees with other records from the southern Alps suggesting runoff intensification around 4200 cal BP. Possible changes in other climatic parameters may have played a concomitant role, including a decrease in the contribution of 18O-depleted Atlantic winter precipitation to the lake water due to snow deficit. Data recording the 4.2 ka BP event in the north-western Mediterranean area are still sparse. In the Lake Petit watershed, the 4.2 ka BP event translated into a change in precipitation regime from 4400 to 3900 cal BP. This record contributes to the recent efforts to characterize and investigate the geographical extent of the 4.2 ka BP event in the Mediterranean area.

Located at the transition between the Saharian and Sahelian zones, at the center of one of the largest endoreic basins, the Lake Chad is ideally located to record regional environmental changes that occurred in the past. However, until now, no continuous archive from Lake Chad covering the Holocene has been studied. In this paper, we present pollen data from the first Holocene sedimentary sequence collected in Lake Chad (13° N; 14° E; Sahel region). Dated between ca. 6700 and ca. 5000 cal yr BP, this record encompasses the termination of the African Humid Period (AHP). Vegetational reconstructions are based on standard analyses of the pollen diagrams and are strengthened by quantitative approaches. Potential biomes that occurred at that time around Mega-Lake Chad are reconstructed using the biomization method and mean annual precipitation is estimated using the modern analogues technique. Results show that between ca. 6700 and ca. 6050 cal yr BP, a vegetation close to humid woodland or humid savanna, including elements currently found much further southward, thrived in the vicinity and/or the extra-local environment of the Mega-Lake Chad in place of the modern steppe, dry woodland and desert vegetation observed today. At the same time, montane forest populations extended further southward on the Adamawa plateau. This vegetation distribution is supported by biome reconstructions as well as by mean annual precipitation estimates of ca. 800 (−400/+700) mm for the period. The high abundance of lowland humid pollen taxa is interpreted as the result of a northward migration of the corresponding plants during the AHP driven by more favorable climatic conditions. Our interpretation in favor of a regional vegetation response to climatic changes is supported by other pollen data from several Northwestern African records. However, we cannot rule out that an increase of Chari-Logone inputs into the Mega-Lake Chad due to variations in hydrological regime might have contributed to the higher abundance of lowland humid pollen taxa observed in the mid-Holocene sedimentary sequence. Changes in the structure and floristic composition of the vegetation towards more open and drier formations occurred after ca. 6050 cal yr BP. This is also evidenced by a decrease in mean annual precipitation estimates to approximately 600 (−230/+600) mm. This change corresponds to the onset of the AHP termination. The constant presence of lowland humid taxa until ca. 5000 cal yr BP, even if displaying a slight decrease, contemporaneous with an increase in steppic taxa, demonstrates that at ca. 5000 cal yr BP the modern vegetation was not yet established in the vicinity of Lake Chad. Our data indicate that vegetational change during this period must have occurred progressively, at least over 1000 yr, and are inconsistent with an abrupt aridification and a collapse of the vegetation cover in the East-Central Saharan and Sahelian regions at the AHP termination.

Variations in the oxygen isotopic composition of precipitation (δ18Oprecipitation) in inter-tropical areas mainly record variations in water sources, amounts of precipitation, and atmospheric temperature and provide information regarding local climate and regional atmospheric circulation changes. On continents, fossil biogenic minerals and speleothems formed in isotopic equilibrium with water can produce continuous δ18O records and are becoming increasingly valuable for reconstructing past climate changes. Here, we explore the efficiency and limitations of using the oxygen isotopic composition of wood phytoliths (δ18Owood phytolith) from tropical rainforest soils as a suitable proxy for atmospheric temperature and δ18Oprecipitation values, under conditions that are assumed to be non-evaporative. Soil phytolith assemblages, that should contain 100s of years of phytolith production, were collected along four altitude, temperature, and precipitation gradients in the Queensland rainforests (Australia). Oxygen isotopic analyses were performed on 1.6 mg phytolith samples, after controlled isotopic exchange (CIE), using the IR Laser-Heating Fluorination Technique. Long-term mean annual precipitation (MAP) and mean annual temperature (MAT) values at the sampled sites were obtained using a regional GIS database. The δ18Oprecipitation values were estimated. The δ18Owood phytolith values from the leeward slopes were scattered but recorded the modern combination of weighted mean annual δ18Oprecipitation values and MAT. The empirical relationship was &Delta18Owood phytolith-precipitation (‰ vs. VSMOW) = −0.4 (±0.2) t (°C) + 46 (±3) (R2 = 0.4, p

Spatio-temporal changes in taxonomic composition and structure of an epipelic diatom assemblage from an intertidal mudflat on the French Atlantic coast was studied over an annual cycle along a cross-shore transect. The assemblage structure was described by estimating both relative abundance and contribution to biovolume of each species. Results showed that the assemblage was numerically dominated by small-sized species (mean relative abundance of 91%). Large species, however, significantly contributed to the total biovolume (mean contribution to biovolume of 49%). A factorial correspondence analysis indicated that the epipelic assemblage was relatively homogeneous along the cross-shore transect but emphasized the seasonal succession of diatom species. In relative abundance, the assemblage structure was characterized by the dominance of the small species Navicula phyllepta throughout the year with a seasonal succession of secondary species, including only one large diatom (Gyrosigma peisonis). In biovolume, small (N. phyllepta and Navicula gregaria) and large species (Pleurosigma angulatum and G. peisonis) alternatively dominated the assemblage. Describing the epipelic assemblage using cell volume emphasized the contribution of large species and revealed that the assemblage contained two diatom fractions, characterized by different biological and physiological behaviours, which may alternatively represent a large proportion of the biomass. (c) 2005 Elsevier Ltd. All rights reserved.

Cas groupes de leptospirose dans l’Ain durant l’ete et l’automne 2007. Introduction Nous rapportons une augmentation des cas de leptospirose dans un departement rural au cours de l’ete et l’automne 2007. Materiel et methode Un cas etait un patient habitant ou sejournant dans l’Ain de juillet a decembre 2007 avec des signes evocateurs de leptospirose (cas suspects) et une PCR ou une micro-agglutination positive (cas confirmes). Les cas etaient recherches dans les hopitaux de l’Ain et des departements limitrophes. Les expositions a risque etaient recherchees a l’aide d’un questionnaire standardise. Resultats 29 cas dont 26 certains ont ete identifies. Leur âge median etait 50 ans (16-74) et 27 etaient des hommes. Pour 83 %, une atteinte hepatique etait presente et pour 55 % une atteinte renale. Les serovars de leptospires connus pour 24 cas etaient : grippotyphosa (n = 8), icterohemorragiae (n = 5), australis n = 4), canicola (n = 3), sejroe (n = 3), pyrogenes (n = 1). Tous les patients ont gueri sous amoxicilline ou ceftriaxone. Aucune exposition commune ni aucun lieu d’exposition commun n’ont ete mis en evidence. Tous les cas ont en revanche eu une exposition a risque essentiellement lors d’activites de loisirs (jardinage : n = 9, peche : n = 2, canyoning : n = 6, baignade : n = 2). Conclusion L’exces de cas rapporte pourrait correspondre a une augmentation diffuse de la prevalence des leptospires dans ce departement rural ou l’eau est presente partout (etangs des Dombes, cours d’eau, lacs artificiels ou non). Les risques d’exposition (contact avec le sol ou l’eau contaminee) lors d’activites de loisirs justifient une reflexion sur le public concerne, sur les modes de communication et d’information et sur les mesures de prevention proposees.