Eutrophisation récente d'un lac de montagne sans occupation humaine (lac de Bastani, Corset : Conséquence d'agents atmosphériques? Recent eutrophisation of a smart pristine alpine lake (Bastani cake, Corsica - France) as a result of wind stirring and increased atmospheric input

Résumé

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.

Abstract

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⁺, SO^4--, 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.

Keywords:

• Alpine lake,
• eutrophication,
• atmospheric pollution,
• pristine watershe,
• Corsica