Multiple stressor effects of fine sediment and herbicide additions on structural and functional components of experimental stream ecosystems

Expansion and intensification of agricultural production has resulted in biodiversity losses and changes in ecosystem functioning in streams worldwide. However, the interplay between the multiple stressors causing these biological impairments is still poorly understood. We investigated whether (i) increased levels of fine sediment and a glyphosate-based herbicide have additive effects on stream biota, or (ii) increased sediment reduces the bioavailability of glyphosate and thus its toxicity, or (iii) adsorbed glyphosate prolongs the exposure period for benthic organisms. To test these hypotheses, we applied four levels each of glyphosate (0, 50, 200, 370µg·l^-1) and fine sediment (0-100% cover) to 96 outdoor circular stream channels using a randomized block design. Benthic algal cover, leaf decomposition and invertebrate drift were determined before, during and after pesticide addition. Algal cover increased with time only in the highest sediment treatment, possibly indicating a reduced invertebrate grazing pressure relative to lower sediment levels. Leaves decayed faster as sediment levels increased, and also in channels with the highest glyphosate addition. Invertebrate drift within the first four hours after glyphosate addition was highest in the highest pesticide treatment. These results imply that sediment and glyphosate addition mainly affected the benthic community independently, rather than in an interactive manner.