Artificial water bodies like ditches, fish ponds, weirs, reservoirs, fish ladders, and irrigation channels are usually constructed and managed to optimize their intended purposes. However, human-made aquatic systems also have unintended consequences on ecosystem services and biogeochemical cycles. Knowledge about their functioning and possible additional ecosystem services is poor, especially compared to natural ecosystems. A GIS analysis indicates that currently only ~ 10% of European surface waters are covered by the European Water Framework directive, and that a considerable fraction of the excluded systems are likely human-made aquatic systems. There is a clear mismatch between the high possible significance of human-made water bodies and their low representation in scientific research and policy. We propose a research agenda to build an inventory of human-made aquatic ecosystems, support and advance research to further our understanding of the role of these systems in local and global biogeochemical cycles as well as to identify other benefits for society. We stress the need for studies that aim to optimize management of human-made aquatic systems considering all their functions and to support programs designed to overcome barriers of the adoption of optimized management strategies.

B WARE Research Centre Toernooiveld 1 6525 ED Nijmegen The Netherlands Department of Aquatic Ecology and Environmental Biology Institute for Water and Wetland Research Radboud University P O Box 9010 6500 GL Nijmegen The Netherlands

Catalan Institute for Water Research Emili Grahit 101 17003 Girona Spain

Chair of Hydrobiology and Fishery Institute of Agricultural and Environmental Sciences Estonian University of Life Sciences Kreutzwaldi 5D 51014 Tartu Estonia

Department Lake Research Helmholtz Centre for Environmental Research UFZ Brückstrasse 3a 39114 Magdeburg Germany

Department Lake Research Helmholtz Centre for Environmental Research UFZ Brückstrasse 3a 39114 Magdeburg Germany Eule GDI Steinbrede 4 48163 Münster Germany

Department of Aquatic Ecology and Environmental Biology Institute for Water and Wetland Research Radboud University P O Box 9010 6500 GL Nijmegen The Netherlands

Eule GDI Steinbrede 4 48163 Münster Germany Institute of Landscape Ecology University of Münster Heisenbergstraße 2 48149 Münster Germany

Institute of Hydrobiology Biology Centre of the Czech Academy of Sciences Na Sádkách 7 37003 Ceske Budejovice Czechia

Stockholm Resilience Centre Stockholm University 10691 Stockholm Sweden

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