Design and performance evaluation of a mesocosm facility and techniques to simulate ocean acidification and warming

Forecasting the ecological consequences of a changing climate requires a range of approaches, including the use of mesocosms in which multiple physical and chemical parameters can be manipulated and the response of interacting organisms quantified. Here, we describe the design and evaluate the performance of a facility incorporating large mesocosms that can contain diverse biological assemblages within which CO 2 and temperature can be manipulated. The key infrastructure is: 15 covered mesocosms each with a maximum water volume of 2300 L, a gas mixer containing two mass flow controllers to produce CO 2 ‐enriched air and an individual heater/chiller unit for each treatment mesocosm to independently regulate temperature. Our results report an initial proof‐of‐concept experiment (total of 100 d duration) in which we constructed communities of biota that characterise temperate Australian kelp forests (i.e., kelp, their key competitors and herbivores) and then undertook procedures designed to reproduce CO 2 and temperature conditions forecasted for the year 2100 (target differences in midday means: 0.15 pH units, 2.5°C). The system achieved the intended environmental conditions, with CO 2 and temperature in treatment enclosures consistently different from the controls (mean midday treatment effect ± SE; 0.15 ± 0.01 pH units and 2.6 ± 0.1°C, respectively), yet still tracking their diurnal fluctuations. The reliability of the facility over the experimental period indicates it is a robust and accurate tool that can mimic intended scenarios of CO 2 and temperature change, facilitating the study of the influence of these factors on marine organisms and their interactions.