Effects of a pulsed treatment with the herbicide afalon (active ingredientlinuron) on macrophyte-dominated mesocosms. II. Structural responses

Outdoor mesocosms (experimental drainage ditches) were treated repeatedly w ith linuron at nominal concentrations of 0, 0.5, 5, 15, and 50 mu g/L. Afte r each of three treatments (with 4-week intervals), the systems were kept s tatic for 1 week. Subsequently, they were flushed with nonpolluted surface water with a residence time of 5 d until the next treatment. Species compos ition and abundance of macrophytes, periphyton, phytoplankton, zooplankton, and macroinvertebrates were examined at prefixed time intervals. During th e treatment periods, significant decreases in oxygen concentration and pH w ere observed, whereas the systems recovered during the flushing periods. Th e effects of the pulsed treatments on the structural endpoints (biota) were rarely significant and consistently were difficult to interpret. We conclu de that under these experimental conditions, pulsed treatment with linuron causes only minor and negligible effects on the abundance of organisms in m esocosms, even when maximum permissible concentrations are exceeded by abou t 80 times. Risk assessment based on the more sensitive physiologic and fun ctional endpoints suggested that the safety factor of 0.1 times the acute E C50 of the most sensitive standard alga (Scenedesmus acutus), as adopted in the Uniform Principles of the European Union, is sufficient to protect the community of the mesocosms in an exposure regime that was realistic for Du tch drainage ditches, using a photosynthesis inhibitor as the benchmark com pound.