In situ ecosystem‐based carbon dioxide perturbation experiments: Design and performance evaluation of a mesocosm facility

We describe a mesocosm facility that can be used for in situ CO 2 perturbation experiments. The facility consists of a floating raft, nine impermeable cylindrical enclosures (each approximately 2400 L in volume), pCO 2 regulation units, and bubble‐mediated seawater mixers. Each enclosure is two‐thirds filled with seawater, and the headspace above is filled with air at a target pCO 2 concentration. Each enclosure is capped with a transparent dome that transmits incoming radiation. To produce pCO 2 levels higher than the ambient concentration, the mass flow controller in the pCO 2 regulation unit delivers varying amounts (10–320 mL min −1 ) of ultra‐pure CO 2 into the gas mixer where it is rapidly mixed with ambient air (approximately 50 L min −1 ). To produce pCO 2 levels lower than the ambient concentration, CO 2 ‐free air and ambient air are mixed in the gas mixer. Prior to daily seawater sampling, approximately 0.5 L min −1 of the target concentration pCO 2 air stream is diverted to the seawater mixer for thorough mixing with the seawater in the enclosure, while the major fraction of the target concentration pCO 2 air stream continues to flow into the enclosure headspace. A performance evaluation of the mesocosm facility assessed attainment of target pCO 2 concentrations in the headspace and enclosure seawater, and the mixing efficiency of the seawater mixer. The results indicate that the facility is suitable for carrying out in situ pCO 2 perturbation experiments.