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conditions. Physiological biomarkers of glyphosate effects on the phytoplankton community, such as the pigment content, the shikimate content and oxidative stress markers (lipid peroxidation and antioxidant enzymes), were evaluated. In addition to these markers, species identification was determincd by microscopy to analyze the changes in the community composition and the Shannon’s diversity index.

3.5 Materiał and Methods

3.5.1 Sampling and test conditions

The phytoplankton community used in this study was collected from the Dumontier stream (45°36'41.38" N and 73°51 *38.55" O), located in southem Quebec (Canada) near the city of Boisbriand, a region of extensive agriculture under the pressure of urban sprawi. The agricultural site, adjacent to the sampled stream, is under soybean and maize rotation, both with genotypes genetically modified to resist to glyphosate. The water sampling was performed at the end of spring, 42 days after glyphosate-based herbicide application.

The water was filtered in the laboratory using nylon mesh (250 pm) to eliminate filamentous macroalgae and suspended macroparticles. To avoid nutrition constrain, soluble reactive phosphorus, inorganic nitrogen and silicates were added to the collected water, following Berard (1996). After agitation for several minutes, the pH was checked and corrected to its original value (7.4). Then, to remove zooplankton, water was filtered a second time using a nylon mesh (40 pin) before being transferred into 500 ml Erlenmeyer flasks.

The pesticide tested in this study was a commercial formulation of the herbicide glyphosate (Factor540®, IPCO, Winnipcg, Manitoba, Canada), which is normally used in the fields adjacent to the sampling site. Concentration values represent those of the pure active substance. A pesticide stock solution of 54 mg l'¹ was prepared in 0.22 pm filtered stream water, from the commercial formulation Factor 540®. After agitation, the solution was kept at 4 °C and this solution was added directly into the Erlenmeyer flasks containing filtered phytoplankton samples, in three replicate for each concentration. The glyphosate concentrations (0, 1,5, 10, 50, 100, 500 and 1000 pg I*¹) were chosen based on the rangę of environmental concentrations found in agricultural water streams in Canada and United