Décomposition chimique et isotopique d'un hydrogramme de crue d'un torrent méditerranéen - Réflexions méthodologiques

La décomposition chimique et isotopique d'un hydrogramme de crue d'un petit torrent méditerranéen (Cannone, Corse, France), a été tentée dans un cas simple, en confrontant les résultats des deux méthodes et en discutant leurs limites. L'outil isotopique permet de bien différencier l'eau préexistante et l'eau nouvelle. L'analyse de la variation des éléments chimiques met en évidence la participation d'eau du soi, sans qu'il soit possible de quantifier sa contribution; elle démontre que la composition de la pluie n'est pas conservative; le rôle du pluviolessivage est mis en évidence, conjointement à celui du transfert dans la zone non-saturée. Ainsi les décompositions chimiques des hydrogrammes (en particulier celles réalisées avec la conductivité) reposent le plus souvent sur des approximations théoriquement contestables et conduisent à des erreurs notables, à l'exception toutefois de celles réalisées avec la silice.Problems of environmental water quality such as transfer of pollutants and ecosystem acidification call for a new insight to the path and the contact time of water in the different subsurface reservoirs. An isotopic and chemical hydrograph separation is carried out for a flood in a small upland watershed in the Mediterranean region (Cannone, Corsica Island, France) with special attention to a comparison between the two methods and discussion of their limits. Here, old and new water are clearly distinguished by isotopic composition. The pattern of the dissolved constituents of stream water shows the contribution of ground water, often assumed to be negligible. The non-conservative behaviour of rain-water chemistry during its path to the stream channel due to enrichment by throughfall and leaching of soils, is the main obstacle to chemical separation. Chemical separation, especially through specific conductance seems generally to be hazardous, except perhaps for dissolved silica. Checking the variation of all dissolved constituents would prevent questionable approximations

Eutrophisation récente d'un lac de montagne sans occupation humaine (lac de Bastani, Corset : Conséquence d'agents atmosphériques?

Le Lac de Bastani (Corse - France), petit lac d'altitude (2089 m) à bassin versant rocheux, est dépourvu de toute implantation ou activité humaine. Ce lac est mésotrophe à eutrophe par certains de ses caractères alors que 4 autres lacs étudiés dans le même contexte sont oligotrophes. De plus, il était décrit au début du siècle comme oligotrophe.Dans ce type de lacs, les apports atmosphériques constituent la majeure partie de la source externe de nutriments. L'apport d'azote inorganique dissous atmosphérique au bassin versant est important, de l'ordre de 0,8 g.N.m-2 par an, soit l'équivalent du rejet d'une trentaine d'habitants permanents. Les trois-quarts sont relargués à la fonte, pendant environ un mois; cela représente l'équivalent des rejets d'environ 300 habitants sur le bassin qui, du fait de la faible activité biologique dans les sols à cette température, arrive pratiquement en entier au lac. Ces apports de nutriments à la fonte sont responsables du démarrage très précoce de la production primaire.En l'absence d'un bilan complet des nutriments la comparaison du lac de Bastani avec les autres lacs oligotrophes et en particulier avec celui de Capitello, aux caractéristiques voisines, permet de cerner les facteurs responsables de la différence de leur niveau de trophie. Bastani et Capitello recevant une charge externe semblable, la source interne est probablement responsable de la situation de Bastani. Les concentrations estivales de N-NO3 notables (0,2-0,3 mg.l-1) des lacs oligotrophes sont un indice de la limitation de leur production primaire par le phosphore. Par contre à Bastani, l'azote étant pratiquement entièrement consommé en été le lac ne semble pas limité par le phosphore. Le vent violent et fréquent qui règne à Bastani semble le facteur déterminant de son niveau trophique supérieur : il assure le brassage de l'eau du lac et la remontée du phosphore (et aussi de l'azote) libéré des sédiments jusque dans la zone photique. Le phytoplancton consomme presque entièrement l'azote, alors que dans les lacs oligotrophes abrités du vent où le phosphore est limitant, les nitrates restent à des teneurs notables.L'eutrophisation récente du lac de Bastani peut s'expliquer par l'accroissement des apports atmosphériques, les seuls qui aient varié. Le phosphore étant disponible la production primaire devait être limitée auparavant par l'azote. Les apports d'azote inorganique dissous ont été dans cette région multipliés par 2 au moins par la pollution à longue distance. Cette dernière pourrait entraîner à elle seule l'évolution trophique d'un lac de haute montagne, dans un site vierge de toute occupation humaine, lorsque le brassage par le vent assure la mobilisation du phosphore des sédiments.Lake Bastani (Corsica, France), a small alpine lake (2089 m), is located in a pristine rocky watershed. It presents some characteristics of eutrophic lakes, whereas four other similar lakes are oligotrophic or ultraoligotrophic. Moreover, at the beginning of the century it was described as being oligotrophic.The lake is ice-covered from November to June. Intense phytoplankton activity begins before the ice melts completely and goes on until fall freezing. Consequently Secchi disk transparency is between 3.2 m in June and 1.8 m in August. The algal biomass, as expressed by chlorophyll-a content, is maximum at the bottom. Chlorophyll-a is about 4-5 mg-m-3 at ice melting and reaches a maximum of 46 mg.m-3 in August. Primary production is highest in the upper levels of water with 41 mg.C.m-3.d-1 at - 5 m. These values are 20 to 80 times that of the other Corsican lakes. Regarding the different trophic classifications this lake presents some features of eutrophic lakes.In 1923 a survey of phytoplankton by Pestalozzi showed that the lake was oligotrophic. This trophic evolution is therefore recent; it is shown by the complete disappearance of Desmidiaceae, the near disappearance of Diatoms, the appearance of green algae with Oedogonium and Spirogyra dominant in summer and autumn, and the development of blue-green algae with Oscillatoria.The seasonal pattern of phytoplankton composition displays diatom development at ice- and snow-melting with the green algae Oedogonium. During the summer warmth, green algae are dominant with the blue-green Oscillatoria. Green algae Oedogonium and Spirogyra increase until the fall turnover.The lake is 2nd-order dimictic with two turnovers in spring and autumn, and two periods of thermal stratification. A strong and frequent wind stirs the lake water inducing homogeneisation of the epilimnion and sometimes the disappearance of the hypolimnion. Dissolved oxygen is always present even at the bottom where O2 saturation is about 50% in winter. Nitrogen is very low - 0.01-0.04 mg.l-1 N-NO3 - and phosphorus (PO4) is below the detection level in the epilimnion during summer, due to intense phytoplankton activity, The hypolimnion is richer in NH4, NO2 and PO4. In spring when the snow begins to melt the surficial waters are enriched with N (NO3 + NH4) and P-PO4, showing the contribution of concentrated snow water.In fact atmospheric input is the dominant external loading in such a watershed. Nitrogen atmospheric input is extrapolated from data obtained at Lake Bavella 30 km southwards for 3 years - 1984 to 1986. The dissolved inorganic nitrogen input is about 0.8 g.m-2 .y-1 , equivalent to wastes from some 30 permanent inhabitants for the whole watershed, About 80 % of annual rainfall is stored in the snowpack, as are ¾ of the dissolved inorganic nitrogen. This nitrogen returns to the lake during a period of about one month which represents 100 kg and a loading of 80 mg.m-2.d-1 for 30 days; it is equivalent to wastes from some three hundred capita during one month. Moreover it is known that the first melwaters are considerably enriched with several ions, especially those of ecological concern, H+, SO4--, NO3¯, NH4+. Using findings from literature we can estimate that in the first two weeks of melting, up to half the annual atmospheric input of dissolved inorganic nitrogen reaches the lake with meltwater concentrations of more than 1 mg.N.l-1. The total dissolved nitrogen input is estimated at 1.2 g.m-2.y-1 and the total nitrogen input at 1.3 g.m-2 .y-1.No data are available for the phosphorus atmospheric input but estimations from literature are about 10 mg.m-2 .y-1 for orthophosphate and 60 mg.m-2 .y-1 for total phosphorus. The release of P-PO4 from the melting snowcover and from ice, added to the tremendous release of inorganic nitrogen, explains the start of vigorous phytoplankton production before the ice melts completely. By this early start primary production may avoid the flush of nutriments resulting from the flow of melting water.Trophic conditions in Lake Bastani are a matter of question as the other four lakes studied with the same edaphic conditions are oligo- or ultraoligotrophic. A comparison between Lake Bastani and the ultraoligotrophic Capitello Lake, which has the same geomorphological and ecological environment, reveals the determining factors of this paradoxical situation, though a nutrient budget is still not available.A low flushing rate (17 months) cannot be the explanation, as that of Lake Capitello is similar (12 months).The external loading (from watershed and atmosphere)being the same, the difference between the two lakes is to be found in the internal loading.The fact that Lake Capitelto and the other oligotrophic lakes present notable N concentrations in summer (0.2-0.3 mg.l-1 ), when Bastani displays N- concentrations 10 times lower, is a sign that they are limited in phosphorus. Conversely Lake Bastani is not phosphorus-limited.The determining factor in this situation seems to be windstirring. Lake Bastani is downwind of a pass in the axial range of Corsica and is submitted to strong frequent winds. Windstirring induces an epilimnion homogeneisation, the downward migration of the thermo- and chemoclines. Thus phosphorus from the sediments can be transferred to the photic zone and is not limiting for phytoplankton. Phytoplankton exhausts nitrogen which is very low at the end of summer. In Lake Capitello as in the other lakes, shielded from the wind by rockwall screens, no mixing occurs, phosphorus cannot be supplied from the sediments and is limiting : nitrogen is not used up and remains at relatively high levels of 0.2-0.3 mg.l -1.Recent eutrophication cannot be explained by an increase of nutrient availability from the sediments due to windstirring, as no climatic change has occurred in the fast century. The probability that internal loading had reached a threshold by progressive accumulation of nutrients in the sediment is very slight if there is no change in the external loading (the unique source of "new" nutrients) and/or the trophic status of the lake. Thus an increase in the external loading seems to be responsible for the eutrophication of Lake Bastani. The atmospheric inupt is the only external reason for a change in the nutrient rate. An increase in the phosphorus atmospheric input by local (forest fires) and regional (long range pollution) human activities is suspected, but does not seem sufficient to change the trophic status of the lake as it has no effect on other lakes, which remain oligotrophic. Moreover, atmospheric P loading is well below the level of phosphorus release from sediments, as estimated from literature. Conversely, an atmospheric inorganic nitrogen input has increased at least twice due to long range pollution (agricultural, industrial and domestic activities). This was estimated from a typologic classification of rain events in Corsica. This increase in atmospheric nitrogen loading might explain the change in the trophic status of the lake, N having been limiting. In the other P-limited lakes the increased atmospheric N input has only increased the N-NO3 concentration in lake water. This explanation being the most realistic, it is of great interest to note that long range atmospheric pollution could induce the eutrophication of a pristine oligotrophic lake

22. -Les pluies et crues exceptionnelles de la Toussaint 1993 en Corse

Des pluies et des crues dévastatrices ont touché la Corse le 31 octobre et le 1er novembre 1993. L'intensité des précipitations et leur répartition spatiale ont été étudiées particulièrement dans le bassin-versant de la Solenzara (Bassin Versant Représentatif Expérimental). Des pluies de 923 mm en 39 h ont été enregistrées et un pluviomètre totalisateur indique 1150 mm+/-30 mm pour l'épisode. La station de jaugeage ayant été détruite, la crue corrélative a été estimée par diverses méthodes, qui convergent vers un débit de pointe de l'ordre de 1500 m3/s, soit 14 m3/s/km². La période de retour probable se situe, selon les méthodes d'estimation, entre 100 et 200 ans. Des débits spécifiques de crue supérieurs à 10 m3/s/km² et de période de retour probables supérieures à 100 ans ont été observés pour 4 bassins versants du Sud de la Corse.Loye-Pilot M. D., Pasquier C. 22. -Les pluies et crues exceptionnelles de la Toussaint 1993 en Corse. In: Crues et inondations. 23emes journées de l'hydraulique. Congrès de la Société Hydrotechnique de France. Nîmes, 14-15-16 septembre 1994. Tome 1, 1994

The impact of Saharan dust on the particulate export in the water column of the North Western Mediterranean Sea

Simultaneous measurements of atmospheric deposition and of sinking particles at 200 and 1000 m depth, were performed in the Ligurian Sea (North-Western Mediterranean) between 2003 and 2007, along with phytoplanktonic activity derived from satellite images. Atmospheric deposition of Saharan dust particles was very irregular and confirmed the importance of sporadic high magnitude events over the annual average (11.4 g m(-2) yr(-1) for the 4 years). The average marine total mass flux was 31 g m(-2) yr(-1), the larger fraction being the lithogenic one (similar to 37%). The marine total mass flux displayed a seasonal pattern with a maximum in winter, occurring before the onset of the spring bloom. The highest POC fluxes did not occur during the spring bloom nor could they be directly related to any noticeable increase in the surface phytoplanktonic biomass. Over the 4 years of the study, the strongest POC fluxes were concomitant with large increases of the lithogenic marine flux, which had originated from either recent Saharan fallout events (February 2004 and August 2005), from 'old' Saharan dust 'stored' in the upper water column layer (March 2003 and February 2005), or alternatively from lithogenic material originating from Ligurian riverine flooding (December 2003, Arno, Roya and Var rivers). Those associated export fluxes defined as 'lithogenic events', are believed to result from a combination of forcing (winter mixing or Saharan events, in particular extreme ones), biological (zooplankton) activity, and also organic-mineral aggregation inducing a ballast effect. By fertilising the surface layer, mixed Saharan dust events were shown to be able to induce 'lithogenic events' during the stratification period. These events would be more efficient in transferring POC to the deeper layers than the spring bloom itself. The extreme Saharan event of February 2004 exported similar to 45% of the total annual POC, compared to an average of similar to 25% for the bloom period. This emphasises the role played by these 'lithogenic events', and in particular those that are induced by the more extreme Saharan events, in the carbon export efficiency in the North-western Mediterranean Sea

Traçage chimique et isotopique d'une crue d'automne sur un petit bassin versant méditerranéen incendié

After the forest fire that swept accross the Massif des Maures (Southern France) in August 1990, a drastic change to the hydrological cycle was observed on the little burnt catchment of Rimbaud (Real Collobrier ERB). Along with usual hydrological and hydrochemical measurements, stable isotopes were measured as regards rainfall and runoff. Vegetation disappearance and consequently, influence on throughfall encouraged us to compare 18 oxygen content with chloride content. During the flood of the 9 th of december 1990, these two elements showed a very similar behaviour, and it was interesting to notice that the hydrographs separation obtained by isotopic methods are comparable. When usual factors of Chloride and 18 oxygen evolutions were examined, forty five percent of runoff came from old water and in this case, unsatured area did not play a significant role. More over, as regards the evolution of water temperature indicating the end of runoff, we could estimate new water amount going to underground table at less than thirty percent. / A la suite des incendies qui ont ravagé le massif des Maures au cours de l'été 1990, le petit bassin du Rimbaud (bassin versant du Réal Collobrier) a vu son comportement hydrologique fortement modifié. Parallèlement au suivi hydrologique et hydrochimique habituel, les isotopes stables ont été mesurés sur les précipitations et l'écoulement. La disparition de la végétation et donc de son influence sur les pluviolessivats nous a incité à comparer l'évolution respective des teneurs en oxygène 18 et en chlore. Au cours de la crue du 9 décembre 1990, ces deux éléments présentent un comportement très similaire et, fait original, les décompositions d'hydrogramme qu'ils permettent d'établir sont comparables. En considérant les facteurs habituels d'évolution des chlorures et de l'oxygène 18, il apparaît qu'une proportion de l'ordre de 45% "d'eau ancienne" participe aux écoulements de crue et que la zone non saturée ne joue dans ce cas précis aucun rôle significatif. En outre, en s'appuyant sur l'évolution des températures de l'eau qui révèle l'arrêt du ruissellement, la quantité d'eau qui rejoint le réservoir souterrain peut être estimée à au moins 30%