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

Pulse Effects of Herbicides on Periphyton in Streams and Recovery

1 Introduction

During the last decade it has been documented that pesticides frequently occur in measurable concentrations in Danish streams (County of Fyn, 1999; County of Aarhus, 1999; NERI, in prep). A significant transport of pesticides from agricultural areas to streams has also been documented throughout Europe and North America (Kreuger, 1999; Baker & Richard, 1989; Lundbergh et al. 1995). Pesticides are transported to the streams by drift, drainage and runoff, and the highest concentrations of pesticides have been found during the spraying season and during periods of high precipitation (Kreuger, 1998; County of Fyn, 1988, Liess & Schulz; 1999). Frequency and concentration of pesticides in stream water can be very variable. In Danish streams pesticides have occurred in 7-60% of the samples analysed and the concentrations have ranged between 0.001-3 µgl^-1 (NERI, in prep). When sampling for pesticides in stream water in an agricultural area in southern Sweden 1990-1996, selected herbicides were found in 73% of the water samples with concentrations up to 45 µgl^-1 (Krueger, 1998).

Especially the possibly effects of insecticides have been in focus, since many invertebrates are very sensitive to insecticides. While the effects of insecticides on macroinvertebrates in the streams have been documented repeatedly (e.g. Pusey et al. 1994, Liess and Schultz 1999, Sibley et al. 1991, Aanes and Bække 1994), the effects of herbicides on the autothrophic organisms in streams are less obvious and therefore even more difficult to establish. Lotic environments are dynamic systems, highly influenced by seasonal changes which affects the influence of the herbicide on periphyton (Guasch et al. 1997). 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 (Wang et al. 1999). Hence it is important to include this complex matrix in the experiments, as well as approaching natural physical condition in experimental test designs, when investigating effects of contaminants on the periphyton. Furthermore, in the single-species tests the possibility of recovery is not investigated, although this is highly relevant, since algal populations are opportunistic with short generation times and might be able to recover fast.

In this project effects of selected herbicides on natural communities of periphyton from streams were investigated. The selected pesticides were metribuzin, hexazinon, isoproturon and pendimethalin. Effects on photosynthesis and species composition of the epiphytic communities were investigated. Furthermore the recovery was studied by transferring the periphyton to clean water (only for metribuzin experiment).