In situ measurements of seawater pCO2

Abstract

A new instrument for studying seawater pCO2 dynamics is described. The Submersible Autonomous Moored Instrument for CO2 (SAMI-CO2) operates by equilibration of ambient seawater pCO2 with a colorimetric pH indicator contained in a gas-permeable membrane. The indicator is periodically renewed to improve the stability and sensitivity typically reported for similar indicator-based pCO2 sensors. The design combines off-the-shelf optical components, a miniature solenoid pump and valve, a low power data logger, and a fiber-optic flow cell to achieve low power consumption and easy assembly. SAMI-CO2 is designed to operate down to 100 m and contains sufficient batteries and reagent for deployment up to 6 months while making 48 measurements per day. Extensive in situ field tests have been performed, including deployment in Woods Hole Harbor for >30 d (May–June 1994). The field evaluation has confirmed that SAMI-CO2 is capable of measuring seawater pCO2 with exceptional long-term stability (no detectable drift in 1 month) and sensitivity comparable to ship-based equilibrator-infrared methods (± 1 µatm). The time-series data obtained from this study show that pCO2 can be highly variable in nearshore environments with up to 100-µatm changes detected over a 4-h period. The structurally rich data highlight the need for continuous mooring-based measurements of pCO2 for understanding carbon cycling in natural waters.