Experimentation in mesocosms is arguably the single most powerful approach to obtain a mechanistic quantitative understanding of ecosystem-level impacts of stressors in complex systems, especially when embedded in long-term observations, theoretical models and experiments conducted at other scales. AQUACOSM builds on an established European network of mesocosm research infrastructures (RI), the FP7 Infra project MESOAQUA (2009-2012), where 167 users successfully conducted 74 projects. AQUACOSM greatly enhances that network on pelagic marine systems in at least 3 ways: first by expanding it to 10 freshwater (rivers and lakes), 2 brackish and 2 benthic marine facilities, and by involving 2 SMEs and reaching out to more, thereby granting effective transnational access to world-leading mesocosm facilities to >340 users on >11500 days; second, by integrating scattered know-how between freshwater and marine RI; and third, by uniting aquatic mesocosm science in an open network beyond the core consortium, with indust