Sebastian Birk, Daniel Chapman, Laurence Carvalho, Bryan M. Spears, Hans Estrup Andersen, Christine Argillier, Stefan Auer, Annette Baattrup-Pedersen, Lindsay Banin, Meryem Beklioglu, Elisabeth Bondar-Kunze, Angel Borja, Paulo Branco, Tuba Bucak, Anthonie D. Buijse, Ana Cristina Cardoso, Raoul-Marie Couture, Fabien Cremona, Dick de Zwart, Christian K. Feld, Teresa Ferreira, Heidrun Feuchtmayr, Mark O. Gessner, Alexander Gieswein, Lidija Globevnik, Daniel Graeber, Wolfram Graf, Cayetano Gutiérrez-Cánovas, Jenica Hanganu, Ugur Iskin, Marko Järvinen, Erik Jeppesen, Niina Kotamäki, Marijn Kuijper, Jan U. Lemm, Shenglan Lu, Anne Lyche Solheim, Ute Mischke, Jannicke Moe, Peeter Noges, Tiina Noges, Steve J. Ormerod, Yiannis Panagopoulos, Geoff Phillips, Leo Posthuma, Sarai Pouso, Christel Prudhomme, Katri Rankinen, Jes Jessen Rasmussen, Jessica Richardson, Alban Sagouis, José Maria Santos, Ralf Bernhard Schäfer, Rafaela Schinegger, Stefan Schmutz, Susanne C. Schneider, Lisa Schülting, Pedro Segurado, Kostas Stefanidis, Bernd Sures, Stephen J. Thackeray, Jarno Turunen, María C. Uyarra, Markus Venohr, Peter Carsten von der Ohe, Nigel Willby, Daniel Hering

• Climate and land-use change drive a suite of stressors that shape ecosystems and interact to yield complex ecological responses (that is, additive, antagonistic and synergistic effects). We know little about the spatial scales relevant for the outcomes of such interactions and little about effect sizes. These knowledge gaps need to be filled to underpin future land management decisions or climate mitigation interventions for protecting and restoring freshwater ecosystems. This study combines data across scales from 33 mesocosm experiments with those from 14 river basins and 22 cross-basin studies in Europe, producing 174 combinations of paired-stressor effects on a biological response variable. Generalized linear models showed that only one of the two stressors had a significant effect in 39% of the analysed cases, 28% of the paired- stressor combinations resulted in additive effects and 33% resulted in interactive (antagonistic, synergistic, opposing or reversal) effects. For lakes, the frequencies of additive and interactive effects were similar for all spatial scales addressed, while for rivers these frequencies increased with scale. Nutrient enrichment was the overriding stressor for lakes, with effects generally exceeding those of secondary stressors. For rivers, the effects of nutrient enrichment were dependent on the specific stressor combination and biological response variable. These results vindicate the traditional focus of lake restoration and management on nutrient stress, while highlighting that river management requires more bespoke management solutions.