We describe a mesocosm facility that can be used for in situ CO2 perturbation experiments. The facility consists of a floating raft, nine impermeable cylindrical enclosures (each approximately 2400 L in volume), pCO2 regulation units, and bubble-mediated seawater mixers. Each enclosure is two-thirds filled with the seawater, and the headspace above is filled with air at a target pCO2 concentration. Each enclosure is capped with a transparent dome that transmits incoming radiation. To produce pCO2 levels higher than the ambient concentration, the mass flow controller in the pCO2 regulation unit delivers varying amounts (10-740 mL min-1) of ultra-pure CO2 into the gas mixer where it is rapidly mixed with ambient air (approximately 50 L min-1). To produce pCO2 levels lower than the ambient concentration, CO2-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 pCO2 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 pCO2 air stream continues to flow into the enclosure headspace. A performance evaluation of the mesocosm facility assessed attainment of target pCO2 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 pCO2 perturbation experiments.
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