Effects of sulfonylurea herbicides on non-target aquatic micro-organisms: Growth inhibition of micro-algae and short-term inhibition of adenine and thymidine incorporation in periphyton communities

Abstract

To study the differential sensitivity to sulfonylurea herbicides of aquatic micro-organisms we used the micro-plate technique as previously described by Blanck and Björnsäter [Blanck, H., Björnsäter, B., 1989. The algal microtest battery—a manual for routine tests of growth inhibition. Report to the Swedish National Chemicals Inspectorate, no 3/89]. This technique enables growth-inhibition tests to be carried out for a large number of species of micro-algae, and growth inhibition data for the two sulfonylureas chlorsulfuron and metsulfuron methyl were obtained for 20 marine species and 20 fresh-water species, respectively. The species-dependent variation in algal sensitivity was found to be very large, with strains of cyanobacteria and dinoflagellates being the most sensitive. The most sensitive species had EC 50 values for the inhibition of growth in the nanomolar range while EC 50 was higher than 1000 μM for the most tolerant species. Field experiments were also carried out to examine the sensitivity of periphyton communities in an agricultural area in southern Sweden. Periphyton communities were established on an artificial substratum in a creek affected by the runoff of agricultural pesticides and a creek with a watershed with no use of pesticides. Short-term toxicity of the sulfonylurea tribenuron methyl was assayed as inhibition of adenine and thymine incorporation weekly for 8 weeks at all sites. At the site with high exposure to pesticides and fertilisers the lowest observed effect concentration (LOEC) values for inhibition of adenine incorporation were always higher than environmentally expected concentrations indicating a high tolerance of periphyton to tribenuron methyl at this site. The toxicity of tribenuron methyl at the reference site was very variable over time. Adenine incorporation in periphyton from the reference site was in three out of seven experiments significantly inhibited at 3 nM tribenuron methyl, but in the remaining four experiments, LOEC values were much higher. The apparent high background variability in the short-term toxicity of tribenuron methyl makes an interpretation of the observed differences between the reference site and the site with higher exposure to pesticides very difficult. It is never the less interesting to compare the growth inhibition test data with some of the field toxicity tests at the reference site, showing that sulfonylurea herbicides can adversely affect both algal growth and periphyton activity at nanomolar concentrations.