Méthodes d’évaluation de l’impact de pesticides sur le phytoplancton marin et le naissain d’huître creuse

The main goal of this work was the study of pesticide effects on natural communities of coastal phytoplankton and oyster spat, using in situ microcosms. Microbial communities (200 µm) were exposed to six pesticides (four herbicides, one insecticide and one fongicide) at four concentrations (0.1, 1, 10 and 100 µg/L of active substance) during ten in situ experiments. The microcosm content was analysed using three methods:
– Temporal Temperature Gradient gel Electrophoresis (TTGE) provides prokaryote (16S DNA) and eukaryote (18S DNA) community genetic fingerprints;
– flow cytometry allows the discrimination of photosynthetic populations according to their scatter and fluorescence signals;
– High Pressure Liquid Chromatography (HPLC) provides pigment fingerprints of the photosynthetic communities.
Microscopic observations were carried out for some experiments in order to provide further information. The main results suggest TTGE as the most sensitive method to detect pesticide effects on the whole prokaryote and eukaryote community structure, compared with the two others methods that only focus on photosynthetic communities.
No effect was detected when communities were exposed to the lowest concentration (0.1 µg/L) of each six pesticides. However, the three herbicides Basamaïs (bentazon), Roundup (glyphosate), Frontier (dimethenamid), and the insecticide Dursban 4 (chlorpyrifos) are likely to represent a danger for microbial communities from1 µg/L.
Young oyster spat was exposed in situ to the herbicide Basamaïs and the fongicide Opus separately at 10 and 70 µg/L, and to a mixture of Basamaïs and Opus at 10 µg/L each. The results show a synergistic effect of the mixture on spat growth, with treated spat growth being half the control value, when no effects were detected when both pesticides were tested separately at 10 µg/L.
This work shows that some pesticides can generate possible adverse effects in coastal areas. Roundup effects must be pointed out as they were detected at environmentally realistic concentrations. The results observed using oyster spat raise the issue of interactions between chemicals in the natural environment. Therefore, the relevance of reglementary toxicity thresholds, that do not include such possible substance interactions, should be re-examined.