PRIMARY RESEARCH PAPER Fish shift the feeding behaviour and trophic niche diversification of their prey in subarctic Lake My ´vatn, Iceland Mireia Bartrons . Ignasi Arranz . Miguel Can ˜ edo-Argu ¨ elles . Serena Sgarzi . Torben L. Lauridsen . Frank Landkildehus . Xavier D. Quintana . Sandra Brucet . Erik Jeppesen Received: 3 December 2017 / Revised: 27 February 2018 / Accepted: 9 March 2018 Ó Springer International Publishing AG, part of Springer Nature 2018 Abstract Fish can alter food web structure through trophic cascades. While most studies conducted in oligotrophic subarctic lakes show strong top–down control on consumers in the presence of fish, several studies undertaken in eutrophic subarctic Lake My ´vatn, Iceland, suggest that it is consumer–resource interactions that drive the whole-lake community. Here, we used stable isotopes of carbon and nitrogen from the main food web compartments derived from a 3-month in situ-controlled mesocosm experiment involving two treatments (with and without fish) with three replicates each to determine the effects of fish on the trophic structure of the Lake My ´ vatn food web. We found that the whole food web trophic structure shifted towards the upper part of the water column (more planktonic habitat) in fishless enclosures. Addition- ally, the trophic niche of organisms occupying the base of the food web became more diversified when fish were absent, and the trophic redundancy of all taxa decreased (more dissimilar trophic niches). Stronger top–down effects may also result from global warm- ing, producing increased abundance of planktivorous fish in subarctic lakes. Our results indicate that this could lead to a shift in trophic niche and reduced trophic diversity of most food web organisms. Handling editor: Michael Power Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10750-018-3588-x) con- tains supplementary material, which is available to authorized users. M. Bartrons (&) Á I. Arranz Á M. Can ˜edo-Argu ¨elles Á S. Sgarzi Á S. Brucet Aquatic Ecology Group, University of Vic - Central University of Catalonia, Vic, Catalonia, Spain e-mail: mireia.bartrons@uvic.cat M. Can ˜edo-Argu ¨elles Freshwater Ecology and Management (FEM) Research Group, Departament de Biologia Evolutiva, Ecologia i Cie `ncies Ambientals, Institut de Recerca de l’Aigua (IdRA), Universitat de Barcelona (UB), Diagonal 643, 08028 Barcelona, Catalonia, Spain T. L. Lauridsen Á F. Landkildehus Á S. Brucet Á E. Jeppesen Department of Bioscience, Aarhus University, Silkeborg, Denmark T. L. Lauridsen Á E. Jeppesen Sino-Danish Centre for Education and Research (SDC), University of Chinese Academy of Sciences, Beijing, China T. L. Lauridsen Á E. Jeppesen Greenland Climate Research Centre (GCRC), Greenland Institute of Natural Resources, Nuuk, Greenland T. L. Lauridsen Á E. Jeppesen Arctic Research Centre, Aarhus University, Aarhus, Denmark X. D. Quintana GRECO, Institute of Aquatic Ecology, University of Girona, Girona, Spain 123 Hydrobiologia https://doi.org/10.1007/s10750-018-3588-x