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The river continuum concept represents the most general framework addressing the spatial variation of both structure and function inriver ecosystems.In the Mediterranean ecoregion, summer drought events and dams constitute the main sources of local disturbance to thestructure and functioning of river ecosystems occurring in the river basin.In this study, we analysed patterns of spatial variation of detritusprocessing in a 7th order river of the Mediterranean ecoregion( River Tirso, Sardinia-Italy) and in three 4th order sub-basins which wereexposed to different summer drought pressures.The study was carried out on Phragmites australis and Alnas glutinosa leaf detritus at 31 fieldsites in seasonal field experiment Detritus processing rates were higher for Alnus glutinosa than for Phragmites australis plant detritus.Processing rates of Alnus glutinosa leaves varied among seasons and study sites from 0.006d-1 to 0.189 d-1 and those of Phragmites australis leaves ranged from 0.0008 d-1 to 0.102 d-1, with the lowest values occurring at sites exposed to summer drought.Seasons and sites accountedfor a significant proportion of such variability.Alder detritus decay rates generally decreased with increasing stream order, while reed detritusdecay rates generally increased on the same spatial gradient.Summer drought events affected these spatial patterns of variation by influencingsignificantly the decay rates of both plant detritus.The comparisons among and within sub-basins showed strong negative influence of summerdrought on detritus processing rates.Similarly, in the entire River Tirso basin decay rates were always lower at disturbed than at undisturbedsites for each stream order; decay rates of reed detritus remained lower at those sites even after the end of the disturbance events, while alderdecay rates recovered rapidly from the summer drought perturbations.The different recovery of the processing rates of the two leaves could alsoexplain the different patterns of spatial variation observed between the two leaves.
