Individual and Combined Effects of Fine Sediment and Glyphosate Herbicide on Invertebrate Drift and Insect Emergence: A Stream Mesocosm Experiment.

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Freshwater Science

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University of Chicago Press





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Pesticides and deposited fine sediment have been associated independently with changes in relative abundance and number of sensitive species in aquatic ecosystems, but the interplay between these stressors in agricultural streams is poorly understood. We used a 28-d experiment in outdoor streamside mesocosms to examine the effects of varying levels of fine sediment and a glyphosate-based herbicide on 2 important aspects of macroinvertebrate community dynamics, invertebrate drift and adult insect emergence. We applied 4 levels of each stressor in a full factorial, repeated-measures design (4 glyphosate × 4 sediment levels × 2 replicates of each treatment combination). We focused on the propensities of the invertebrates to drift or emerge (number drifting per mesocosm/benthic individuals per m2). Sediment addition affected drift propensities of 4 of the 10 most common invertebrate taxa, community-level measures of drift (total taxon and Ephemeroptera, Plecoptera, Trichoptera [EPT] richness) and adult emergence (total propensity and taxon richness), whereas glyphosate addition affected drift propensity of only 1 taxon. These results suggest that glyphosate entering streams during terrestrial herbicide operations may be less problematic for aquatic invertebrate community dynamics than fine sediment from catchment erosion. No significant sediment × glyphosate interaction was detected for any individual taxon drift propensity or community-level drift or emergence measure, indicating that the 2 stressors acted additively, rather than synergistically or antagonistically. The dynamic measures (invertebrate drift and adult insect emergence propensities) often showed the opposite pattern compared to benthic invertebrate densities in the same experiment, a result highlighting the value of using dynamic aspects of stream invertebrate communities to identify mechanisms underlying stressor effects on benthic standing stocks.

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