PRIMARY RESEARCH PAPER Stream regulation by small dams affects benthic macroinvertebrate communities: from structural changes to functional implications Aingeru Martı ´nez • Aitor Larran ˜aga • Ana Basaguren • Javier Pe ´rez • Clara Mendoza-Lera • Jesu ´s Pozo Received: 25 November 2011 / Revised: 11 January 2013 / Accepted: 19 January 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract We studied benthic macroinvertebrate communities upstream and downstream of five small reservoirs (surface release in autumn–winters) (north Spain) to assess the effect of flow regulation on structural and functional characteristics of stream ecosystems. We based our approach on the use of structural metrics (density, biomass, richness and diversity) in combination with two functional diver- sity indices based on biological and ecological traits: FD PG index, related to species richness, and FD Q , which incorporates evenness across taxa. Although water physicochemical parameters were unaffected by the reservoirs during the study period (autumn– winter), macroinvertebrate metrics were lower below the dams, with detritivores (shredders and collector- gatherers) being the most affected. The alder leaf breakdown rate estimated by the litter-bag technique was related to the density, biomass, richness, diversity and FD PG index of shredders, compromising the ecosystem functioning. The most plausible origin for the observed differences in macroinvertebrate metrics between upstream and downstream reaches was the change of the flow regime caused by the impound- ments at downstream sites, leading to droughts in summer in those naturally permanently flowing streams. The observed functional diversity loss might reduce the chances of the community to override natural or man-induced fluctuations in their environ- ment with possible repercussions on important eco- system functions and services. Keywords Headwater streams Á Small dams Á Benthic macroinvertebrates Á Functional diversity Á Ecosystem functioning Introduction Ecological effects of flow regulation have become an important subject in environmental research (Rosen- berg et al., 2000). All human societies have dammed rivers with different objectives: water supply, land irrigation, flood control, industrial use and energy generation. In Spain alone, about 1500 reservoirs are documented, of which 35% can be considered small, i.e. dams of less than 15 m of wall height (Spanish Handling editor: Sonja Stendera Electronic supplementary material The online version of this article (doi:10.1007/s10750-013-1459-z) contains supplementary material, which is available to authorized users. A. Martı ´nez (&) Á A. Larran ˜aga Á A. Basaguren Á J. Pe ´rez Á J. Pozo Laboratory of Stream Ecology, Department of Plant Biology and Ecology, University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain e-mail: aingeru.martinez@ehu.es C. Mendoza-Lera Department of Freshwater Conservation, Brandenburg University of Technology, Cottbus, Seestraße 45, 15526 Bad Saarow, Germany 123 Hydrobiologia DOI 10.1007/s10750-013-1459-z