February 1st to 5th 2016
Olhão, Portugal
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A novel system to manipulate and control CO2 levels in mesocosms for ocean acidification experiments

Scientific Exhibition
Future Oceans
Wednesday, February 3, 2016 -
18:30 to 20:00

Sordo, L. 1 Santos, R. 2 Reis, J. 3 Silva, J. 4

1Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
2Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
3Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
4Centre of Marine Sciences (CCMAR), University of Algarve, Campus de Gambelas, 8005-139 Faro, Portugal

Contradictory results in Ocean Acidification (OA) experimental studies may derive from existing technical problems in the manipulation and control of desired CO2 levels. Most experimental OA systems rely on pH as an indirect way to control CO2. However, accurate pH measurements are difficult to obtain and shifts in temperature and/or salinity alter the relationship between pH and pCO2, leading to uncertainty in real pCO2 levels, even if pH is apparently stable. Trying to overcome this problem, we developed a system, in which the target pCO2 is controlled via direct pCO2 analysis of seawater. Water in a header tank is permanently re-circulated through an air-water equilibrator, in which a low-volume closed air circuit is equilibrated with the system water. The air is then routed to an infrared gas analyzer (IRGA) that conveys the pCO2 value to a PID (Proportional-Integral-Derivative) controller, which in turn commands a solenoid valve that opens and closes the CO2 flush to the header tank. The PID controller is pre-programmed and set to maintain a certain pCO2 and its embedded algorithm allows for a fine-tuning of the CO2 flush. The pre-mixed water of the header tank is then distributed to the experimental units in a low-flow open circuit. This direct type of control eliminates the problems associated with pH electrodes and accommodates potential temperature and salinity shifts, as the target variable is directly measured instead of calculated. A prototype unit using this control system is now running for two years in CCMAR field lab in southern Portugal, with good long-term stability. Because there is no direct bubbling into the experimental units, this system is also suitable for experiments with phytoplankton and most other marine organisms.
keywords: 
Ocean Acidification (OA), OA experimental design, gas bubbling, CO2

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