Effects of exposure duration of herbicides on natural stream periphyton communities and recovery.

To improve risk estimates for herbicides in streams, the sensitivity of natural periphyton communities to four herbicides (metribuzin, hexazinone, isoproturon, and pendimethalin) was examined in experiments including varying exposure duration and a recovery phase. Effect parameters included assimilation of 14C and concentration of diagnostic pigments as proxies for photosynthetic activity and algal group composition, respectively. The results indicated that isoproturon, metribuzin, and hexazinone affected the photosynthetic activity of periphyton at distinctly lower concentrations than the effect concentrations published for standard single-species growth-tests with phytoplankton species. Pendimethalin did not show effects on the photosynthetic activity of periphyton at the concentrations tested. The effect concentration (EC50) of isoproturon and metribuzin decreased by one to two orders of magnitude when the duration of exposure increased from 1 h to 24 h, while hexazinone had a stimulating effect on the photosynthetic activity of periphyton after 1 h exposure and inhibiting effect after 24 h exposure. The photosynthetic activity after exposure to metribuzin for 1, 2, 6, 18, 23, or 48 h recovered almost completely after 48 h in herbicide-free water. However, different periphyton groups responded differently to metribuzin exposure: Chlorophytes were severely affected by exposure and did not recover, whereas diatoms and especially cyanobacteria recovered well. Overall the study showed that the effects of herbicides on periphyton are strongly affected by the duration of exposure, and even short-term exposure may have distinct effects on the periphyton community.