Search

Ce travail est une contribution au débat biodiversité/stabilité. L'utilisation de paramètres pris comme indicateurs de la diversité biologique et de la stabilité du chimisme de l'eau : le nombre de taxons de macro-invertébrés benthiques (selon IBGN) et la variabilité (écart-type exprimé en pourcents) du rapport rubidium/strontium dissous, nous a permis de montrer une corrélation négative significative entre les deux indices. Cette relation, établie sur 36 stations de 22 cours d'eaux du genevois, est interprétée comme mettant en évidence un effet négatif de la variabilité temporelle de la qualité de l'eau sur le nombre d'espèces de macro-invertébrés. Bien que reposant sur des aspects réducteurs de la biodiversité et de la stabilité des écosystèmes, elle est un exemple de lien inverse entre l'instabilité et la diversité biologique. Elle tend, en outre, à souligner l'efficacité d'une utilisation dynamique des données physicochimiques par rapport à l'interprétation de valeurs absolues., The aim of this work is to make a contribution to the biodiversity/stability debate. The use of given parameters as indicators of biologic diversity and stability of the chemical composition of water, more specifically, the average number of macroinvertebrate taxons (according to the IGBN) and the variability (standard deviation expressed as a percentage) of the Rb/Sr dissolved ratio, allowed the determination of a significant negative correlation between these two indices. This relationship, based upon the results obtained from 36 sampling sites located on 22 rivers in the canton of Geneva (Switzerland), is interpreted as evidence of the negative impact of temporal variations in water quality on the number of macroinvertebrates species. Though relying upon reductive aspects of biodiversity and stability of ecosystems, it represents an example of the inverse relationship between instability and biologic diversity. Moreover, it tends to emphasize the advantage of using dynamic variations of chemical parameters rather than their absolute values in order to appreciate the loops and relationships occurring in natural ecosystems.

This work aims to establish a physico-chemical indicator of freshwater quality with respect to biological quality in order to facilitate the modeling of aquatic systems with a management perspective. In the context of the diversity-stability debate and relying on previous work, we proposed the hypothesis that unsteady systems which are unstable in terms of water composition are less favorable to biological diversity. We tested this hypothesis by considering the distribution of the conductivity/calcium ratio as a function of the flow with respect to the Global Biological Index based upon the determination of macroinvertebrates. We used the results obtained in rivers from the Geneva region (19 rivers, 30 sampling sites, sampled monthly for one year for physico-chemical parameters and four times a year for mean biological index determination). In rivers (or sampling stations) with biological indexes superior to 13 (considered as not or only moderately polluted) the conductivity/calcium ratio remains stable independently of the flow, whereas in rivers (or sampling stations) presenting poor biological diversity (biological index less than 13), we observed not only changes in the conductivity/calcium ratio but also the deterioration of the stability for the given flow ranges. The conductivity/calcium index relies on two easily measured parameters, its relationship with the flow can easily be modeled, thus allowing not only the modeling of the water quality response to watershed management, but also the determination of the critical flow ranges where inputs will have the worst impact on the biology of the receiving system. We are now in the process of applying the index to numerical modeling of the impact of urbanization on water quality in rivers from the Geneva region.

Les etudes realisees sur le plancton du Leman de 1854 a 1981, ont permis de recenser 500 especes d'algues; en cas de synonymie, certaines especes ont ete reclassees sous leur denomination actuelle, dans la mesure du possible.