Because CO2 is the dominant product of organic carbon degradation across nearly all aquatic environments, its fluctuations are a key indicator of net ecosystem metabolism (Hopkinson, 1985; Smith and Hollibaugh, 1993). As such, it is essential to measure parameters that define the CO2 system. Today, there are four readily measurable parameters—all of which can be determined using Apollo SciTech aquatic instruments, which were acquired by LI-COR Environmental in 2023. • The Dissolved Inorganic Carbon Analyzer measures dissolved inorganic carbon (DIC), or the concentration of CO2 and other inorganic carbon species dissolved in water. • The Underway pCO2 System measures partial pressure of CO2 (pCO2), or the concentration of CO2 gas in water. • The Spectrophotometric Seawater pH Analyzer measures pH, or the acidity (concentration of H+) or alkalinity (concentration of OH-) of water. • The Total Alkalinity Titrator measures total alkalinity, or water’s capacity to neutralize acids.