BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-11-4057-2014 Free-ocean CO<sub>2</sub> enrichment (FOCE) systems: present status and future developments Gattuso J.-P. https://orcid.org/0000-0002-4533-4114 2 1 Kirkwood W. 3 Barry J. P. 3 Cox E. 2 1 Gazeau F. 2 1 Hansson L. 4 Hendriks I. https://orcid.org/0000-0002-2238-6018 5 Kline D.I. 6 Mahacek P. 2 1 Martin S. 8 7 McElhany P. 9 Peltzer E. T. 3 Reeve J. 10 Roberts D. 11 Saderne V. https://orcid.org/0000-0003-3968-2718 12 Tait K. 13 Widdicombe S. 13 Brewer P. G. 3 Sorbonne Universités, UPMC Univ Paris 06, Observatoire Océanologique, 06230 Villefranche-sur-mer, France CNRS-INSU, Laboratoire d'Océanographie de Villefranche, 06230 Villefranche-sur-mer, France Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA Ocean Acidification International Coordination Centre, IAEA-Environment Laboratories, 4a Quai Antoine 1er, 98000 Monaco, Principality of Monaco Global Change department, IMEDEA (CSIC-UIB), Instituto Mediterráneo de Estudios Avanzados, C/ Miquel Marqués 21, 07190 Esporles (Mallorca), Spain Scripps Institution of Oceanography, Integrative Oceanography Division, University of California, San Diego, 9500 Gilman Drive, MC 0218, La Jolla, CA 92093–0218, USA Sorbonne Universités, UPMC Univ Paris 06, Station Biologique, Place Georges Teissier, 29688 Roscoff Cedex, France CNRS, Station Biologique, Place Georges Teissier, 29688 Roscoff Cedex, France Conservation Biology Division, Northwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2725 Montlake Blvd. E, Seattle, WA 98112, USA Department of the Environment, Channel Highway, Kingston, Tasmania, Australia Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart T 7001, Australia Benthic Ecology group, GEOMAR &ndash; Helmholtz Centre for Ocean Research in Kiel, Kiel, Schleswig-Holstein, Germany Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK 01 08 2014 11 15 4057 4075 Copyright: © 2014 J.-P. Gattuso et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/11/4057/2014/bg-11-4057-2014.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/11/4057/2014/bg-11-4057-2014.pdf

Free-ocean CO<sub>2</sub> enrichment (FOCE) systems are designed to assess the impact of ocean acidification on biological communities in situ for extended periods of time (weeks to months). They overcome some of the drawbacks of laboratory experiments and field observations by enabling (1) precise control of CO<sub>2</sub> enrichment by monitoring pH as an offset of ambient pH, (2) consideration of indirect effects such as those mediated through interspecific relationships and food webs, and (3) relatively long experiments with intact communities. Bringing perturbation experiments from the laboratory to the field is, however, extremely challenging. The main goal of this paper is to provide guidelines on the general design, engineering, and sensor options required to conduct FOCE experiments. Another goal is to introduce xFOCE, a community-led initiative to promote awareness, provide resources for in situ perturbation experiments, and build a user community. Present and existing FOCE systems are briefly described and examples of data collected presented. Future developments are also addressed as it is anticipated that the next generation of FOCE systems will include, in addition to pH, options for oxygen and/or temperature control. FOCE systems should become an important experimental approach for projecting the future response of marine ecosystems to environmental change.

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