The aim of this study was to evaluate changes in leaf-litter breakdown rates due to flow regulation, through weirs and water diversions, in headwater mountain streams. In order to find common patterns and reveal singularities that might occur in different regions, we studied 17 streams located in 4 different climatic regions of the peninsular Spain (Cornisa Cantábrica, Cordillera Litoral Catalana, Sierra de Guadarrama, and Sierra Nevada). Regions showed a continentality gradient (assessed using the Gorczynski continentality index) from 7.7% (Cornisa Cantábrica) to 32% (Sierra Nevada). We hypothesized that stream regulation would impair habitat conditions and guilds associated with leaf litter breakdown, leading to a slowdown of litter breakdown rates. We further hypothesized that the extent to which breakdown rates changed downstream of fluvial regulation would depend on the combination of the nature and scale of the fluvial regulation and the environmental setting of each region. In all cases we found that leaf litter breakdown rates were lower downstream of the regulation. This change was most pronounced in the Sierra Nevada region, where the breakdown rate decreased by 50.7%, and least pronounced in the streams of Cornisa Cantábrica, where breakdown rates decreased by 20.8% downstream of regulation. In terms of the Gorczynski index, the extent to which fluvial regulation led to slow down litter breakdown decreased from continental to oceanic regions. Richness and sporulation rates of aquatic hyphomycetes did not show significant differences either between upstream and downstream sites, or between regions. In all regions we detected a consistent trend of lower abundance and biomass of shredders colonizing litter bags in sites downstream of flow regulation. Also, there were significant differences in the composition of macroinvertebrate communities between regions, which could be the cause of the differences in the leaf litter breakdown rates observed from one region to another. We conclude that changes of streamflow rates in headwaters slow down leaf litter breakdown rates downstream of the disturbance, but the magnitude of this slowdown is substantially influenced by the continentality of the region in which the stream is located. The differences in breakdown rates between upstream and downstream sites can be mainly ascribed to a reduction in shredder abundance in the latter, which is likely to be caused by altered in-stream habitat and riparian vegetation downstream of the point of flow regulation.
Keywords: Stream regulation, Climatic regions, Leaf litter breakdown, Spain, Aquatic hyphomycetes, Macroinvertebrates