Longitudinal gradient of CPOM degradation in the Po river (NW Italy)

Alpine streams are increasingly suffering the effects of anthropogenic pressures because of water withdrawal, morpho-hydrological alterations and the effects of climate change. For these reasons, drought occurrence is becoming more frequent in Alpine stream ecosystems, which are known to be particularly fragile and vulnerable to the effects of this phenomenon. The aim of the study was to investigate CPOM degradation longitudinal patterns in the Po river, in four stations from upstream to downstream, where drought events are becoming more frequent. In particular, using leaf litter bags of chestnut and oak we investigated the CPOM degradation process above and below the treeline, in both perennial and recently intermittent reaches, by measuring the following: i) mass loss variations; ii) macroinvertebrate colonization patterns; iii) microbiota communities (fungi and bacteria) involved in the process. We hypothesized that the CPOM decomposition in the upstream and forested sites would be driven by a relevant contribution of the shredder functional feeding group, and that this trend will decrease moving downstream, according to the river continuum concept; moreover, we expect that in sites with permanent flow CPOM degradation process would be driven by a relevant contribution of both macro- and micro-consumers, while in the intermittent ones would be explained primarily by the role of micro-consumers. With continued drought conditions and changing CPOM subsidies, the variation of biodiversity, richness and density of colonizer communities could result in altered Alpine stream ecosystems.