Environmental Project, 1041 – Pulse Effects of Herbicides on Periphyton in Streams and Recovery

Pulse Effects of Herbicides on Periphyton in Streams and Recovery

Summary and conclusions

During the last decade, it has been documented that pesticides frequently occur in measurable concentrations in Danish streams. Especially the possibly effects of insecticides have been in focus, since many invertebrates are very sensitive to insecticides. The effects of the insecticides are often visible, since they result in a reduction of the invertebrate population during runoff events. In monitoring programs of streams invertebrates are mostly included, which leaves it possible to detect effects by pesticides on crustaceans and insects. Increased drift and mortality of crustaceans and insects have been found in stream exposed to low concentrations of insecticides. Effects of herbicides on the microalgae in streams are less obvious and therefore even more difficult to document. In addition microalgae are only on rare occasions included in monitoring programs. Furthermore, effects of herbicides on periphyton in streams are difficult to predict from the traditional single-species toxicological tests, since these tests do not include influences, which occur on community level, for instance displacements in species composition, heterotrophic degradation of the pesticides, adhering of pesticides to surfaces, etc. The periphyton is attached to surfaces in a polysaccharide matrix along with heterotrophic microbes, fungi and detritus, which may serve as an effective barrier preventing the transfer of a contaminant. Generally, published data on effects of herbicides on periphyton communities are lacking.

In the present study, effects of four different herbicides on natural communities of periphyton from streams were investigated. The selected herbicides were metribuzin, hexazinone, isoproturon and pendimethalin. Effects on photosynthetic activity and composition of the periphyton communities were investigated.

In the present study, isoproturon, hexazinone and metribuzin affected the periphyton at low concentration, and at distinctly lower concentration than the effect concentrations published for standard single-species growth-test. Low concentrations of hexazinone stimulated the photosynthesis, while pendimethalin did not affect the photosynthetic activity of the periphyton in the present investigation. A significant result of this study was that even a short-term exposure to metribuzin affected the composition of the periphyton community.