| Summary: | Agriculture is the primary cause of sedimentation, nutrient enrichment, and insecticide contamination of freshwater ecosystems. Despite the widespread co-occurrence of these ecological stressors, little is known about their potential interactive effects. I conducted three experiments manipulating combinations of these stressors in order to evaluate their cumulative effects on freshwater ecosystems at different scales of biological organization (community, ecosystem, meta-ecosystem). First, I evaluated stream invertebrate community responses to sedimentation, nutrient enrichment, and the insecticide chlorpyrifos using laboratory microcosms with distinct microhabitats. I demonstrated that chlorpyrifos can interact non-additively with fine sediment (reversal) and nutrients (antagonism), with potentially deleterious impacts on small-sized invertebrates. Furthermore, invertebrates in gravel microhabitats were more severely affected than those in leaf packs. Second, I manipulated levels of nutrients, sediment, and the insecticide imidacloprid in experimental pond ecosystems. I demonstrated these stressors had antagonistic effects on pelagic and benthic invertebrate diversity. Moreover, the results suggested imidacloprid increased ecosystem metabolism indirectly, through negative effects on invertebrate consumers. Finally, I explored processes at the scale of the river network meta-ecosystem. Using a network of experimental channels, I investigated how multiple-stressor interactions within tributaries affected downstream ecosystems. My results indicated that complex nutrient-sediment interactions within tributaries could strongly alter the flux of organisms from tributaries to downstream ecosystems. Furthermore, I observed that at small spatial scales, these alterations of within-network migration patterns could be more influential than the transport of the stressors from headwaters to recipient ecosystems. My research contributes novel evidence suggesting that complex interactions among nutrient enrichment, sedimentation, and insecticide contamination are frequent in freshwater ecosystems, and have distinct mechanisms operating at different scales. In particular, these findings underscore the importance of considering multiple-stressor interactions in insecticide environmental risk assessments; even at low concentrations, interactions with other stressors may result in unexpected negative effects for aquatic biota and ecosystem processes. === Forestry, Faculty of === Graduate
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