Temporal dynamics of stream algae under the combined impacts of climate and land-use stressors

Ecosystems are increasingly challenged by the multiple facets of climate change, coupled with intensifying land-use pressures. These co-occurring stressors can interact in complex ways, but we lack an empirical understanding of the impact of higher-order interactions on temporal patterns. Using 128 flow-through circular stream mesocosms, we investigated the individual and combined effects of two climatic stressors (CO2 enrichment, flow variability) and two land-use-related stressors (siltation, light-as lack of shading) on algal communities and net ecosystem productivity (NPP) in a full-factorial design. The sediment pulses coincided with high flow to mimic erosion events, whereas CO2 and light disturbances were applied continuously, as they would in nature. Sediment emerged as a key stressor for algal biomass. Land-use stressors had more pervasive individual effects, whereas climate-change-related stressors were more important in up to three-way interactions with other stressors. However, CO2 was a key driver of NPP, followed by sediment and light, whereas flow variability was only important in interactions with other stressors. The effects of sediment and flow on algal biomass depended strongly on their temporal patterns. The combined effect of an increasing number of stressors on algal biomass appeared to reach saturation with three active stressors, with no apparent additional effect caused by the fourth stressor. Synthesis. Sediment emerged as a key stressor in streams. Despite the mounting evidence and the multitude of available mitigation strategies, this problem persists worldwide. The complex interactions uncovered in our study illustrate context-dependency that can be masked in two-factor experiments. Our findings also highlighted that temporal patterns of individual and co-occurring stressors can determine the ecological outcome.