Caracterisation du phytoplancton de deux systemes limniques vis-a-vis d'un herbicide inhibiteur de la photosynthese la methode PICT (Pollution-Induced Community Tolerance): Application et signification

Abstract

Pollution-Induced Community Tolerance (PICT) was first proposed by Blanck et al. (1988) as an ecotoxicological tool for use in advanced hazard assessment of toxicants. This method can be used to retrospectively detect the impact of toxicants in polluted ecosystems. PICT is based on the idea that the toxicant exerts a selection pressure when exposure reaches a certain level for a sufficient period of time. Since organisms vary in their resistance to toxicants, this selection pressure will exclude the sensitive organisms which will be replaced by more resistant ones. The result is a community with an increased tolerance to these toxicants compared to one which had previously not been affected by them. This difference in resistance between the unselected and the selected communities can be detected by comparison of results from short-term physiological tests performed on the respective communities and comparison of each community structure (taxonomy). We used this method to compare the lake Geneva (Haute-Savoie), a mesotrophic lake with low herbicide pollution, and the Villaumur barrage (Ille-et-Vilaine) which is very eutrophic and highly polluted by herbicides inhibiting PS II. The structure of the phytoplanktonic community of these limnic systems is very different, and their response to short-term physiological tests seems to be different too. To confirm this selection result, we carried out an experimental study the purpose of which was to test the effect of herbicides present in the water at Villaumur on the structure of the phytoplankton community of lake Geneva. For this, one range of microcosms was contaminated with water filtered from the Villaumur barrage and another contaminated with a known concentration of atrazine (10 μg/L). Some tolerant algae species were selected in both ranges of microcosms after three weeks of treatment. We also compared the effects of atrazine on growth of strains isolated from each system. The results suggest that the tolerance increase in the population could also be due to selection of resistant genotypes. PICT a biological marker of undeniable interest in specific pollution studies in lakes. Our experimental approaches showed the effect of selection pressure on phytoplanktonic communities by toxicants, but the first in situ results indicate that the protocol needs to be improved to validate the PICT method on various and known aquatic systems.