Growth inhibition and recovery of Lemna gibba after pulse exposure to sulfonylurea herbicides

The exposure of non-target aquatic organisms to pesticides often happens as short-term, high exposure events (pulses) and effects of these must be addressed in the current regulation in the EU. It is, however, questionable whether the effects of pulse exposures are adequately covered by the standardized ecotoxicological tests used in environmental effect assessments, since these aim at maintaining constant exposure concentrations during the incubation. Therefore, we investigated the effects of four sulfonylurea herbicides (flupyrsulfuron-methyl, metsulfuron-methyl, rimsulfuron, and thifensulfuron-methyl) on the growth of Lemna gibba over a 6-day period after 24 h of pulse exposure, and compared with effects observed in standard OECD tests with continuous exposure. It was observed that concentrations around the EyC50-values found in OECD tests did not affect the growth in the 6 days post-exposure period. Slightly higher concentrations initially resulted in lower growth in pulse exposure tests, but the growth rate of the plants reached the level of untreated plants during the 6 days post-exposure period. The 24 h pulse exposure tests gave 2–6 times higher EyC50-values than the OECD 7-d continuous exposure tests. The approach of this study enables experimentally based comparisons between observations of effects between the two exposure regimes. We propose that results obtained in this way be applied in effect assessments for intermittent releases.

► We studied pulse effects of four sulfonylurea herbicides on the growth of Lemna gibba.
► Exposure lasted 24 h and was followed by a 6-days post exposure period.
► We present experimentally based comparisons of effects between two exposure types.
► Pulse exposure lead to 2–6 times higher effect values than continuous exposure.
► Comparisons do not, however consider recovery and should thus be used with care.