- Previous research on the combined action of various paired agricultural stressors in stream ecosystems has shown that non-additive outcomes (i.e. synergisms or antagonisms) are more common than simple additive effects on communities, but the interplay between deposited fine sediment and widely used herbicides such as glyphosate as stressors is not well understood.
- A 28-day experiment in outdoor stream mesocosms simulated effects of varying levels of fine sediment and a glyphosate-based herbicide on macroinvertebrates and ecosystem function (leaf breakdown) in a fully factorial repeated-measures design. Our aim was to determine whether (i) increased levels of sediment and glyphosate had individual and/or additive combined effects on invertebrates and leaf breakdown, (ii) increased sediment reduced the toxicity of glyphosate (antagonistic multiple stressor interaction) or (iii) sediment-adsorbed glyphosate prolonged the effects of exposure (synergistic interaction).
- Sediment addition positively or negatively affected eight of the 10 most common invertebrate taxa colonising the mesocosms and increased or decreased the representation of 12 of 15 biological trait categories possessed by the invertebrates, while glyphosate addition negatively affected only two taxa and two invertebrate trait categories, indicating that glyphosate entering streams as a result of herbicide addition to farmland is less likely to be problematic for invertebrates than fine sediment from catchment erosion.
- No significant overall sediment-by-glyphosate interaction was detected for invertebrate taxonomic or trait compositions or for any common individual taxon, trait or community metric, indicating that the two stressors were acting additively (i.e. without interaction), rather than synergistically or antagonistically.
- Leaf breakdown rates (measured as mass and strength loss) were accelerated by sediment and glyphosate as individual stressors. Further, the two stressors interacted for leaf strength loss, with their combined effect being less than additive when both stressors were at their highest levels.
- Population, community and ecosystem variables responded differently to multiple stressors, highlighting the value of combining structural, functional taxonomic and trait data to assess the effects of anthropogenic stressors.
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