Lack of knowledge about how the various drivers of global climate change will interact with multiple stressors already affecting ecosystems is the basis for great uncertainty in projections of future biological change. Despite concerns about the impacts of changes in land use, eutrophication and climate warming in running waters, the interactive effects of these stressors on stream periphyton are largely unknown. We manipulated nutrients (simulating agricultural runoff), deposited fine sediment (simulating agricultural erosion) (two levels each) and water temperature (eight levels, 0-6 °C above ambient) simultaneously in 128 streamside mesocosms. Our aim was to determine the individual and combined effects of the three stressors on the algal and bacterial constituents of the periphyton. All three stressors had pervasive individual effects, but in combination frequently produced synergisms at the population level and antagonisms at the community level. Depending on sediment and nutrient conditions, the effect of raised temperature frequently produced contrasting response patterns, with stronger or opposing effects when one or both stressors were augmented. Thus, warming tended to interact negatively with nutrients or sediment by weakening or reversing positive temperature effects or strengthening negative ones. Five classes of algal growth morphology were all affected in complex ways by raised temperature, suggesting that these measures may prove unreliable in biomonitoring programs in a warming climate. The evenness and diversity of the most abundant bacterial taxa increased with temperature at ambient but not with enriched nutrient levels, indicating that warming coupled with nutrient limitation may lead to a more evenly distributed bacterial community as temperatures rise. Freshwater management decisions that seek to avoid or mitigate the negative effects of agricultural land use on stream periphyton should be informed by knowledge of the interactive effects of multiple stressors in a warming climate.

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1. Agricultural land use commonly exerts multiple stressors on the functioning of stream ecosystems, including leaf litter decomposition and the utilization of this resource in stream food webs. If stressors interact, their cumulative effects on biotic responses cannot be predicted from knowledge of individual stressor effects, posing challenges for management and restoration of ecosystems. 2. We examined the individual and interactive effects of four common agricultural stressors and the role of litter quality on leaf litter decomposition and fungal decomposers. In 128 outdoor, flow-through mesocosms, we manipulated levels of nutrients, a nitrification inhibitor (dicyandiamide), deposited fine sediment and flow velocity. Interactions among these stressors can ensue because, for instance, they jointly affect physicochemical conditions around leaf litter colonized by fungi such as concentrations of dissolved oxygen and nutrients. 3. The two litter species used, deciduous birch and evergreen mahoe, showed contrasting decomposition dynamics, and these differences influenced their response to stressors. Fungi were important for birch litter decomposition but played a minor role for mahoe. Overall, flow velocity reduction and deposited fine sediment had the strongest, mainly negative effects on fungi and litter decomposition, probably as a consequence of reductions in dissolved oxygen available to fungi. However, fine sediment substantially increased mahoe litter mass loss, pointing at fungi-independent processes being relevant for its decomposition. 4. Although interactions among stressors were uncommon, they showed effects of the same magnitude as stressor main effects. Potential mechanisms underlying interactions included reductions in dissolved oxygen and changes in microbial community composition. 5. Knowledge of the effects of multiple agricultural stressors and of litter quality on litter decomposition and litter-associated fungi is crucial for management of forested riparian corridors, which have been shown to efficiently mitigate impacts of agricultural stressors on streams. The contrasting responses of the litter species used in our study warrant consideration of species composition of the riparian vegetation.