PRIMARY RESEARCH PAPER Inter- and intraspecific variation of carbon and nitrogen stable isotope ratios in freshwater bivalves Adriana Novais . Ester Dias . Ronaldo Sousa Received: 8 January 2015 / Revised: 10 July 2015 / Accepted: 11 July 2015 / Published online: 26 July 2015 Ó Springer International Publishing Switzerland 2015 Abstract Freshwater bivalves provide important ecosystem functions and services, yet many of their ecological traits such as feeding mechanisms and resource use are largely ignored. In this study, we aimed to evaluate the potential overlap in resource use by bivalve species living in sympatry in European freshwater ecosystems. This was accomplished by analyzing the stable isotope ratios of carbon (C) and nitrogen (N) values of six bivalve species (five native species plus the invasive species Corbicula fluminea) in six distinct aquatic ecosystems. Results showed significant inter- and intraspecific differences in both stable isotope ratios. The interspecific variability suggests differences in the food sources consumed, which can be related to differences in feeding behav- ior. At the intraspecific level, there was a gradient in the stable isotope ratios from the oligotrophic River Paiva ( 15 N-depleted and 13 C-enriched) to the eutrophic Mira Lagoon ( 15 N-enriched and 13 C-de- pleted), suggesting a change in the resources used from benthic to pelagic food sources, respectively, and/or differences in the stable isotopic baseline in each ecosystem. Thus, flexible feeding strategies combined with size selectivity may decrease the possible competition for food sources by native and invasive species living in sympatry. Keywords Bivalves Á Corbicula fluminea Á Freshwater ecosystems Á Invasive and native species Á Stable isotopes Introduction Reports describing the decline of freshwater bivalves have increased in recent years, being these inverte- brates recognized today as one of the most threatened faunistic groups worldwide (Strayer et al., 2004; Re ´gnier et al., 2009). These declines may have important consequences at all levels of ecological organization, from individuals to ecosystems (Vaughn & Taylor, 1999; Vaughn et al., 2008; Sousa et al., 2011). At the ecosystem level, the decline or even disappearance of these species, along with possible replacements by invasive bivalves, may lead to changes in trophic relationships and food web dynam- ics, as these organisms are important primary con- sumers in many aquatic ecosystems (Strayer, 2010; Handling editor: Beat Oertli A. Novais (&) Á R. Sousa CBMA – Centre of Molecular and Environmental Biology, Department of Biology, University of Minho, Campos de Gualtar, 4710-057 Braga, Portugal e-mail: adriana.novais1990@gmail.com A. Novais Á E. Dias Á R. Sousa CIIMAR/CIMAR – Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Rua dos Bragas 289, 4050-123 Porto, Portugal 123 Hydrobiologia (2016) 765:149–158 DOI 10.1007/s10750-015-2408-9