Quantifying pCO2 in biological ocean acidification experiments: A comparison of four methods

Area of Science:

• Oceanography
• Marine Chemistry
• Environmental Science

Background:

• Accurate quantification of seawater carbon dioxide (CO2) is vital for ocean acidification research.
• Direct CO2 measurement equipment can be expensive and complex.
• Indirect methods using pH and total alkalinity (AT) are common for calculating CO2 partial pressure (pCO2), especially in field and biological studies.

Purpose of the Study:

• To compare the accuracy and applicability of four different methods for determining seawater pCO2.
• To evaluate methods suitable for biological CO2-manipulation experiments, including field applications.

Main Methods:

• Comparison of four pCO2 determination methods: VINDTA for dissolved inorganic carbon (CT) and AT, spectrophotometric pHT and AT, electrode pHNBS and AT, and direct CO2 measurement using a non-dispersive infrared (NDIR) gas analyzer.
• Evaluation of measurement uncertainties for field-applicable methods.

Main Results:

• All four methods produced comparable pCO2 estimates.
• Field-suited methods (spectrophotometric pHT, electrode pHNBS, and CO2 equilibrator) showed measurement uncertainties of 3.5-4.6% for pCO2.
• The methods are suitable for detecting differences greater than 100 μatm pCO2 in experimental treatments.

Conclusions:

• The four compared methods provide similar pCO2 estimates for biological CO2-manipulation experiments.
• Established methods remain preferred for high-accuracy open ocean measurements or detecting small pCO2 changes.
• These methods offer viable alternatives for specific experimental designs, particularly in field settings.