| Summary: | Quantifying the amount of carbon dioxide (CO₂) in seawater is an essential component of ocean acidification research; however, equipment for measuring CO₂ directly can be costly and involve complex, bulky apparatus. Consequently, other parameters of the carbonate system, such as pH and total alkalinity (AT), are often measured and used to calculate the partial pressure of CO₂ (pCO₂) in seawater, especially in biological CO₂-manipulation studies, including large ecological experiments and those conducted at field sites. Here we compare four methods of pCO₂ determination that have been used in biological ocean acidification experiments: 1) Versatile INstrument for the Determination of Total inorganic carbon and titration Alkalinity (VINDTA) measurement of dissolved inorganic carbon (CT) and AT, 2) spectrophotometric measurement of pHT and AT, 3) electrode measurement of pHNBS and AT, and 4) the direct measurement of CO₂ using a portable CO₂ equilibrator with a non-dispersive infrared (NDIR) gas analyser. In this study, we found these four methods can produce very similar pCO₂ estimates, and the three methods often suited to field-based application (spectrophotometric pHT, electrode pHNBS and CO₂ equilibrator) produced estimated measurement uncertainties of 3.5–4.6% for pCO₂. Importantly, we are not advocating the replacement of established methods to measure seawater carbonate chemistry, particularly for high-accuracy quantification of carbonate parameters in seawater such as open ocean chemistry, for real-time measures of ocean change, nor for the measurement of small changes in seawater pCO₂. However, for biological CO₂-manipulation experiments measuring differences of over 100 μatm pCO₂ among treatments, we find the four methods described here can produce similar results with careful use.
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