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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 Cantabrica, Cordillera Litoral Catalana, Sierra de Guadarrama, and Sierra Nevada). Regions showed a continentality gradient (assessed using the Gorczynski continentality index) from 7.7% (Cornisa Cantabrica) 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 Cantabrica, 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.

The damming of rivers and streams alters downstream habitat characteristics and biotic assemblages, and might thus alter stream functioning, although there is not much direct evidence of this impact. In this study we compared breakdown of alder leaves upstream and downstream from 4 small (, This study was funded by the Spanish Ministry of Science and Innovation project Imparios (CGL2007-66664-C04-03)

J. Jesús Casas et al., Terrestrial plant litter is important in sustaining stream food webs in forested headwaters. Leaf litter quality often decreases when native species are replaced by introduced species, and a lower quality of leaf litter inputs may alter litter decomposition at sites afforested with non-native species. However, since detritivore composition and resource use plasticity may depend on the prevalent litter inputs, the extent of the alteration in decomposition can vary between streams. We tested 2 hypotheses using 2 native and 3 introduced species of tree differing in quality in 4 Iberian regions with contrasting vegetational traits: 1) decomposition rates of all plant species would be higher in regions where streams normally receive litter inputs of lower rather than higher quality; 2) a higher resource-use plasticity of detritivores in regions vegetated with plants of lower litter quality will cause a greater evenness in decomposition rates among plant species compared to regions where streams normally receive higher-quality plant litter inputs. Results showed a highly consistent interspecific ranking of decomposition rates across regions driven by litter quality, and a significant regional effect. Hypothesis 1 was supported: decomposition rates of the five litter types were generally higher in streams from regions vegetated with species producing leaf litter of low quality, possibly due to the profusion of caddisfly shredders in their communities. Hypothesis 2 was not supported: the relative differences in decomposition rates among leaf litter species remained essentially unaltered across regions. Our results suggest that, even in regions where detritivores can be comparatively efficient using resources of low quality, caution is needed particularly when afforestation programs introduce plant species of lower litter quality than the native species. © 2013 Elsevier B.V., This research was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) projects IMPARIOS (CGL2007-66664-C04) and TEMPERIOS (CGL2010-22129-C04).JJC and MM contributed to this manuscript during tenure of the grants RIBARID (CGL2012-39635) and CARBONET (CGL2011-30474-C02-02) (MINECO), respectively.

The main purpose of our work was to elucidate factors responsible for the geographical differences in leaf-litter decomposition rates in Spanish oligotrophic headwater streams. Decomposition experiments with alder (Alnus glutinosa) leaf litter were carried out in 22 headwater streams in 4 different climatic regions across the Iberian Peninsula (Cornisa Cantbrica, Cordillera Litoral Catalana, Sierra de Guadarrama, and Sierra Nevada). Streams that were similar in size, flowed mainly over siliceous substrate in catchments with scarce human settlements and activities, and fell within a range of low nutrient concentrations were chosen in each region. Breakdown rates were regionally variable and were low (0.1090.198% ash-free dry mass [AFDM]/degree day [dd]) in the Cornisa Cantbrica, the most mesic and Atlantic region, and high (0.3020.639% AFDM/dd) in Sierra de Guadarrama, one of the coldest and most inland areas. Temperature was not the determining factor affecting differences in breakdown rates among regions, and breakdown rates were not related to concentrations of dissolved nutrients. However, microbial reproductive activity (sporulation rates) was significantly correlated with dissolved P concentration. Breakdown rates were explained better by presence and feeding activities of detritivores than by decomposer activity. Incorporation of breakdown rates in assessment schemes of stream ecological status will be difficult because leaf processing does not respond unequivocally to environmental factors when climatic regions are considered. Thus, regional adjustments of baseline standards in reference conditions will be required. © 2011 by The North American Benthological Society., This study was funded by the Spanish Ministry of Education and Science (project CGL2007-66664-C04), by the University of the Basque Country (Research grant GIU05/38), and by the Basque Government (Research grants IT-422-07 and IT-302-10)