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SummaryThe effect of spray-free zones acting as an agent for the protection of streams and lakes is documented by experimental studies performed in close collaboration with the local farmers at six selected small (< 2 m) streams in the areas around Bjerringbro and Silkeborg in Denmark. The streams were representative of Danish streams as a whole, as more than 80% of Danish are less than 2 m wide. The field experiments included a vegetation analysis and measurements of the effect on the flora in transepts reaching from the field and across the stream. A recently developed method for effect studies in terrestrial plants was used. By using this method, accumulated biochemical effects were detected in plants growing up to 14 m away from the field. The strongest effect was seen within the nearest 2 m of the field edge. Three out of eight field studies showed effects in plants in the spray-free zones. Wind direction and wind speed during crop spraying was decisive for the effect on the plants. The vegetation analysis showed that the plant community in the tested spray-free zones was poor in species. The most abundant species were common nettle, couch grass, smooth meadow grass, codlings-and-cream, and cleavers. More species were observed in spray-free zones where the distance to the field was 11 - 21 m.
The field experiments also included an investigation of in-stream macro-invertebrate communities and survival of the amphipods Gammarus pulex. The in-stream concentration of pesticides was furthermore analysed during the sampling period. In addition to the six streams with agricultural activities within their riparian zones, six spray-free reference streams were sampled. A total of 36 different pesticides were found, out of which only a few were insecticides.
Concentrations were generally low and there was no significant difference between the two types of streams (with and without spraying). There was no use of insecticides in the sprayed zones during the sampling period. No impacts on the macro-invertebrate communities, or on the survival of Gammarus, were detected that could be related to pesticide exposure during the sampling period. In order to assess specific impacts of pesticides in low concentrations
on the biota, a series of controlled experiments with lambdacyhalothrin were performed using both plants and macroinvertebrates. These experiments showed that the macro-invertebrate
community was affected by lambda-cyhalothrin in concentrations as low as 0.05 µg l-1.
The present investigation showed effects of spraying at a distance of 14 m from the source. Pesticide drift in the riparian zone was related to several site-specific features such as vegetation height, wind speed, wind direction and the use of pesticides on adjacent fields. As the
investigation was undertaken under unconstrained field conditions, it was not possible to predict, or influence, the use of pesticides. Overall, only low concentrations of pesticides were found in the streams investigated. If guidelines are followed, pesticides are applied at low wind speeds. The low number of pesticides found indicates that the involved farmers followed guidelines for application practice. Furthermore, the direct impact of insecticides in the riparian zone could not be evaluated, as interviews with the farmers showed that there was no recorded use of insecticides. However, results from the controlled experiments showed that very low concentrations of insecticides could affect stream macro-invertebrates. Therefore, if results on herbicide wind drift are representative for pesticides as a whole (including insecticides), it can be predicted that both adult aquatic insects in the riparian zone and in-stream macroinvertebrates could be impacted. Moreover, the ecological status of the in-stream macro-invertebrate community was poorer in agricultural streams than in the reference streams. In addition, vegetation in the riparian zone was more diverse in the two streams with the widest buffer strips between the stream and the agricultural fields. The poorer ecological status of the agricultural streams is probably due to several inter-related factors such as pesticides, nutrients, dredging and erosion, which are all agricultural impacts. It is not possible to elucidate the causality between the biota and these factors separately, or in concert, in the present investigation. However, the results demonstrate that an un-impacted riparian zone could improve the ecological quality, both in the riparian zone itself and in the stream. This improvement could probably be achieved even if a large part of the catchment is agricultural land, as there were no major differences between the number of pesticides found in the two types of streams (reference and agricultural). This finding indicates that the land-use in stream catchments was comparable, despite the differences in ecological quality at the reach (sampling) scale. The present investigation was of limited scope, but the results suggest that a buffer strip without any agricultural activities would improve the ecological quality. The buffer strip should be wider than the current stipulated 2 m. The optimum width of the buffer would be between the interval 10 to 20 m.
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