In streams and rivers draining agricultural catchments, deposited fine sediment and elevated nutrient concentrations represent two key land-use related stressors commonly associated with degraded ecological conditions that can interact in complex ways to affect ecosystem structure and function. How warming induced via changes in riparian vegetation or global climate change predictions will interact with these stressors has not previously been investigated. I designed two ambitious stream mesocosm experiments to investigate the individual and interactive ecological responses of periphyton, macroinvertebrates and measures of decomposition to the agricultural stressors augmented nutrients and deposited fine sediment at water temperatures simulating warming induced via changes in upstream and adjacent riparian vegetation (Experiment 1; Chapter 2) or forecast climate change predictions (Experiment 2; Chapters 3-5). In Experiment 1, I investigated how a relatively small increase in water temperature of 1.4 °C during sunlight hours interacted with the effects of intermediate or high levels of sediment addition or nutrient enrichment. Sediment was found to be the most pervasive stressor and affected 93% of all biological response variables (either as an individual effect or via an interaction with another stressor) with generally negative effects on invertebrates but positive effects on algae and leaf decay, whereas nutrient enrichment affected 59% and raised temperature 59% of variables with mostly positive effects. Pair-wise stressor interactions affected 63% of all invertebrate, 48% of all algal and 50% of all decomposition response variables, with interactions between sediment and temperature most common. In Experiment 2, I studied the complex three-way interactive effects of both stressors with raised temperature and non-linear responses to sustained warming of up to 6 °C above ambient. Main stressor effects were broadly in agreement with those observed in Experiment 1. Sediment was consistently the most pervasive stressor, affecting 80% of all invertebrate response variables with mainly negative effects, and 95% of all periphyton and 71% of all decomposition variables with mainly positive effects. Nutrient enrichment affected 58% of all invertebrate, 83% of periphyton and 43% of decomposition variables, always with mainly positive effects. Raised temperature affected 67% of invertebrate response variables with mainly negative effects in the benthos, 83% of periphyton variables with mainly negative or non-linear effects, and 100% of decomposition variables with mainly positive or non-linear effects. Complex two-way or three-way interactions affected 52% of all invertebrate, 85% of periphyton and 42% of decomposition variables; interactions between sediment and nutrients were most common and typically resulted in either additive or antagonistic combined effects.A meta-analysis of the combined data of Experiment 2 suggests that rising temperatures may increase the frequency of non-additive interactions between augmented nutrients and deposited fine sediment, and the frequency of synergistic interactions at 6 °C above ambient temperature. The key management implication of my thesis is that freshwater management decisions that seek to avoid or mitigate the detrimental effects of intensive agricultural land use must be informed not only by knowledge of the interactive effects of the multiple agricultural stressors already in operation, but how such interactions may be further modified by climate change.
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Climate Change and Multiple Stressors in Agricultural Streams
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