Qualité des eaux de trois bras morts de la Garonne variabilité spatio-temporelle) Water quality in three ancient alors of the river Garonne : spatio-temporal variability

Résumé

Pendant deux années, des mesures et analyses d'eau ont été faites mensuellement sur une station de la Garonne et sur trois bras morts différant par leur communication avec le fleuve. La variabilité spatio-temporelle a été dressée à partir de 14 variables physico-chimiques susceptibles d'influer les équilibres chimiques de l'eau et la vie aquatique. Les données ont fait l'objet d'une Analyse en Composantes principales précédée par une analyse de variance entre saisons et entre stations de mesures afin de déterminer l'importance des hétérogénéités spatiale et temporelle des données.

L'eau du fleuve est soumise à un cycle climatique annuel de température et de débit. De brèves fortes eaux de printemps alternent avec de longues périodes de débits stables et inférieurs aux moyennes établies sur plusieurs décennies. Par rapport aux charges de sulfates et de chlorures prises comme référence du drainage du bassin versant, les flux de nitrates ont un pic accentué au printemps, résultant des activités agricoles. Les phosphates présentent aussi un accroissement automnal qui pourrait traduire un cycle annuel de minéralisation-déminéralisation. Les matières organiques s'élèvent en rapport au taux de chlorophylle a. L'eau de Garonne est de bonne qualité et conforme à la typologie habituelle, hormis des taux déclassants d'ammoniaque provenant de l'agglomération toulousaine et dont l'autoépuration est souvent incomplète. Par rapport à la Garonne, les trois bras morts sont caractérisés par un cycle thermique accentué en été. Mise en évidence par l'ACP, la minéralisation des eaux de ces trois bras morts évolue selon un cycle saisonnier parallèlement aux fluctuations de débit de la Garonne. Elle indique un gradient de minéralisation croissante de l'hiver au printemps. La qualité de l'eau lors des fortes eaux printanières est homogénéisée et imposée par le fleuve. En phases de faibles débits, la qualité de l'eau évolue parallèlement à celle de la Garonne (concentrations des substances) pour un bras mort ventilé par une communication amont et aval. En revanche, les deux bras morts en simple communication aval présentent un retard à la concentration de l'eau d'autant plus évident que la communication est étroite. Les substances fertilisantes (nitrates, phosphates, ...) augmentant de l'amont vers l'aval dans ces deux derniers bras morts, sont en été en concentration inférieure par rapport au fleuve, en raison : 1) du remplissage printanier par des eaux diluées, puis du retard estival à l’équilibrage par simple communication aval et 2) de la consommation par les organismes végétaux aquatiques. L'élude des différences spatio-temporelles met ainsi en évidence un gradient saisonnier de minéralisation, un gradient aval-amont de productivité et un gradient aval-amont de réchauffement estival. La productivité apparaît donc liée au réchauffement estival plutôt qu'à la minéralisation ou aux teneurs en substances fertilisantes.

Mots-clés:

• Garonne,
• bras morts,
• qualité des eaux,
• variabilité spatio temporelle,
• analyse de variance,
• analyse en composantes principales

Abstract

For two years, water analysis and measurements were carried out monthly at one station on the Garonne and at three different ancient arms. Spatio-temporal variability was drawn up from 14 physical and chemical properties (water temperature, pH, electrical conductivity, oxygen, oxydability, total alcalinity, ammonia, nitrate, orthophosphate, sulphate, chloride, calcium, magnesium and chlorophyll a) which can have an effect upon the chemical balance of water and upon aquatic life. The data were studied by Principal Component Analysis preceded by Analysis of Variance between date and between station in order to determine the significance of spatio-temporal heterogeneity.

The river water is under the influence of an annual climatic cycle affecting temperature and flow. The flash floods in spring alternate with long periods of flow stability (these periods are lower than the established means for the flow over the last few decades). For each element specific water discharge-concentration relationships can be drawn, which may present time variations according to the hydrological regime. The particulate and dissolved soil-derived components increase with water discharge, whereas groundwater-derived elements are generally diluted during floods. In relation to the sulphate and chloride loads, which are used as a reference for the drainage of the basin, the nitrate levels have an accentuated peak in spring resulting from agricultural practices. Phosphates also display a high autumn level which could be due to an annual cycle of mineralisation-demineralisation. Organic matter increases correlatively with the chlorophyll a rate. The water in the Garonne is of a high quality conforming to the standards expected for this type, except for the ammonia levels which come from the Toulouse agglomeration, and mean that the self cleaning process in this case is offer incomplete. In relation to the Garonne, the three ancient, arms have thermal cycle which characteristically accentuated in the summer. Water temperature, electrical conductivity and pH value were not so varied from one to another ancient arms. This spatial homogeneity was coupled with the temporal homogeneity for the pH value. In the other hand, water temperature and electrical conductivity followed the different seasonal cycle. The temperature variations are parallel in all of the stations, but the amplitude of summer-winter cycle is important in three ancient arms. Within the ancient arms, the longitudinal profile of temperature was similar in summer, characterized by a little warming up according to the downstream-upstream gradient presented in Principal Component Analysis (PCP). This gradient was more pronounced in the ancient arms PV and GF than in ancient arm RB where subsists in summer a weak circulation of river flow.

The temperature presents a determinant role in the ancient arm productivity, reflected by the significant correlations between temperature on the one band and, the organic matter contents, chlorophyll a and pH values on the other hand. The increase of temperature in summer is accompanied by the production of organic matter, particularly in the upstream part of PV and GF. This phenomenon could be explained as a result of biological activity growth essentially phytoplankton because it was attached to an increase of chlorophyll a concentration. The biological activity stimulated by the increasing temperature contributes to reduce the pH value, accentuated by the diminution of oxygen saturation vigorously indicated in the morning, at the upstream part of the ancient arms. The percentage of oxygen saturation in the morning was essentially modified by the biological activity. It is stable and high during the spring period which corresponds to high water level of the river Garonne. Outwards this period where the river imposed the oxygen rate closely related to the air saturation, an upstream-downstream gradient occured in the weak water level periods (summer and winter). This is particularly indicated in the ancient arms which have not an upstream communication with the river. In this lentic ecosystem, it appears that the oxygen rate depends to the flora and fauna metabolic activity, confirmed by a highly significant correlation between the chlorophyll a concentration and the oxygen rate measured in the morning. The persistence of weak renewal water by the upstream part of ancient arm RB limited the summer warning up in this ancient arm compared to PV and GF. Thus, the productivity associated, evaluated by the chlorophyll a concentration, organic matter contents and the variations of pH and oxygen values are lower than the two last ancient arms where have only the downstream outlet.

The conductivity value in the ancient arms is closely allied to the river Garonne. It constitutes a first evidence to show that the water quality in the ancient arms was affected by the river. In spring, the regular re-covering of the ancient arms by the river water diluted induces an annual drop of conductivity.

Dressing by Principal Component Analysis, the mineral content in these three ancient arms shows a seasonal cycle which corresponds to the flow fluctuations of the river Garonne. The winter-spring gradient was indicated. This gradient was more evident for magnesium than for sulphate, chloride and calcium. The high spring water level reflected a low concentration of sulphate, chloride, calcium and magnesium due to water dilution. The water quality at this period is homogeneous because of the influence of the Garonne. When the water demands is weak, the quality of water in one of the ancient arms evolves parallel with that in the Garonne (concentration of matter) as it is alimented both up and downstream by the Garonne. On the other hand, the other two ancient arms have only the downstream outlet causing a delay in water concentration, especially as the connection with the main stream is narrow. The fertilising matter (nitrates, phosphates, etc.) which increases downstream, in summer, is at lower concentration in the ancient arms compared to the main stream for the following reasons :1) filling of the spring influx by diluted water, then the summer delay in equilibration of water via the single downstream inlet and 2) due to the absorption by the aquatic plants. Finally, the different spatiotemporal studies exhibit a seasonal mineralization gradient, a downstream-upstream productivity gradient and a downstream-upstream summer warning. The productivity therefore appears to be related to the summer warning rather than to the mineralization process or the presence of fertilising constituents.

Keywords:

• Garonne,
• ancient arms,
• water quality,
• spatio-temporal,
• variability analysis of variance,
• principal component analysis