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 pCO2 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 CO2 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, 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 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.