Input of pesticides into the aquatic environments often occurs in pulsesrather than continuous influx, resulting in fluctuating and time-variableexposure of aquatic life to these pollutants. For example, herbicidesreach surface water through runoff and drainage mostly during and afterrain events, which results in river concentrations that vary greatly withtime. Traditional ecotoxicological effect assessment however, does notreflect the time varying character of pesticide exposure, since toxicityexperiments are based on continuous exposure of an organism to a singlepollutant. The present study aims to examine how we can account forfluctuating exposure to pollutants in risk assessment and how short-termrisk limits (also called acute quality standards) can be defined insurface waters. The project will be focused on pesticides with aparticular emphasis on herbicides, which are commonly detected in surfacewaters and whose concentrations are often above long-term water qualitystandards during rain events. Algal species are usually the most sensitivespecies to herbicides. Our goal is hence to understand, describe and modeltoxic effects on algae from fluctuating herbicide exposure. The herbicidesinvestigated include different specific modes of toxic action and we willexplore the role of the mechanism for the time dependence of toxicity. Aparticular focus is on the role of the recovery phase between the pulses.We will use two reference scenarios to simulate realistic exposure. Longerpulses of lower concentration with only short recovery phases mimicconditions that are typically encountered in larger rivers in Switzerland.Short pulses of high concentrations and longer recovery phases are typicalfor smaller brooks. In the second part of the project, the goal is torelate algal toxicity to internal concentrations as a potential unifyingparameter for describing time dependent exposure. We will determinebioconcentration, metabolism and internal/ target site concentrations as afunction of time and effect. These measurements should serve thedevelopment of toxicokinetic and toxicodynamic models, which will relateexternal exposure concentrations to internal and target siteconcentrations in algae. The results of this project should serve as thescientific basis for proposing short-term aquatic risk limits or acutewater quality standard for non-continuous exposure and to proposestrategies for monitoring of herbicides in surface waters.