Effects of flow intermittence on ecosystem processes in mountain streams: are artificial and field experiments comparable?

Flow intermittence, and specifically riverbed drying, is becoming increasingly frequent due to the effects of climate change and human pressures, including in mid-elevation streams and rivers in the Alpine area. Due to the highly dynamic flow regimes of intermittent streams and interactions among multiple environmental factors, experiments in laboratory and field-based mesocosms can be used to simplify the set of abiotic factors that influence a biotic response. Results from such mesocosms require evaluation and, if necessary, calibration with data from field experiments. To assess the applicability of manipulative experiments to natural conditions, we compared the results of a field-based flume experiment simulating leaf litter processing with those obtained from natural field data. We compared an intermittent treatment (in which the surface sediments dried) with a control treatment with perennial flow in (a) an artificial flume system fed by a mountain stream and (b) two piedmont mountain streams. In both experiments, we sampled the macroinvertebrate community and measured leaf decomposition rates at comparable time intervals. We observed consistent patterns in both spatial and temporal total β-diversity and leaf mass loss in the two experiments, despite differences in shredder abundance. Where flowing water was present during the whole experiment, leaf litter decomposition was faster and macroinvertebrate communities were more stable and taxonomically richer. These results highlight drying events as a key influence on ecological communities, enabling their characterization in both controlled and natural conditions. Our results suggest that flume experiments can provide an effective proxy for naturally occurring processes in stream ecosystems.