title: Effects of the organophosphate paraoxon-methyl on survival and reproduction of Daphnia magna: importance of exposure duration and recovery.
abstract: The aim of this study was to determine the influence of exposure duration (1 h, 24 h, continuous) to paraoxon-methyl on the magnitude of lethal and sublethal effects, the shape of the concentration-response relationships and the recovery processes in Daphnia magna. Survival was more severely reduced in the continuous than in the pulse exposure regimes. The lethal concentrations (3d median lethal concentration [LC50] values) were 233, 2.33, and 1.14 microg/L after 1-h, 24-h, and continuous exposure, respectively. The shapes of the concentration-response relationships for survival were significantly different after 1 h of exposure than after 24-h and continuous exposure. Indeed, the slopes of the curves defined by the ratios LC90/LC10 (ratio of 90 and 10% lethal concentrations) were 100, 1.74, and 1.97 for 1-h, 24-h, and continuous exposure, respectively. The large difference between 1 h and longer durations of exposure shows that the population is partially affected (10-90%) over a much broader range of concentrations when exposure is short. Negative effects on reproductive outputs occurred mostly at concentrations affecting partly the survival and therefore also over a broad range of concentrations after 1 h of exposure. However, these effects were only transient in the pulse exposure regimes as individual performances recovered. By contrast, reproductive outputs of survivors exposed continuously remained impaired. These results suggest that a refined risk assessment should consider exposure duration because it influences the magnitude of effects and recovery.
authors.affiliation: Umweltforschungszentrum Centre for Environmental Research, Department of Chemical Ecotoxicology, Leipzig, Germany. sabine.duquesne@ufz.de
authors.forename: Sabine
authors.initials: S
authors.lastname: Duquesne
authors.forename: Sebastián
authors.initials: S
authors.lastname: Reynaldi
authors.forename: Matthias
authors.initials: M
authors.lastname: Liess
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D003621:Daphnia / Q000187:drug effects* / Q000502:physiology*; D010261:Paraoxon / Q000031:analogs & derivatives* / Q000633:toxicity; D012098:Reproduction / Q000187:drug effects / Q000502:physiology; D015996:Survival Rate; D013997:Time Factors
chemical_list: D010261:Paraoxon; C008480:methylparaoxon