.......... Many aquatic environments naturally exhibit carbon dioxide partial pressures (PCO2) that greatly exceed the average global freshwater PCO2 value of ca. 0.3 kPa (ca. 3100 μatm; 1 kPa ≈ 10,000 μatm ≈ 1% ≈ 7.5 Torr ≈ 7.5 mmHg) (Cole et al., 1994; Hasler et al., 2016; Park et al., 1969; Raymond et al., 2013) due to a number of factors including microbial respiration, high biomass, presence of carbonate deposits, and the physicochemical properties of CO2 (Brauner et al., 2019). For example, PCO2 may reach values as high as 2–8 kPa in tropical systems (Cole et al., 1994; Li et al., 2013; de Rasera et al., 2013), 2–4 kPa in aquaculture ponds (Damsgaard et al., 2015), 0.5–2 kPa (10–40 mg L−1) in recirculating aquaculture systems (Brauner et al., 2019), 4.7 kPa (55 mg L−1) during fish transport (Allred et al., 2020), and 5–11 kPa (100–200 mg L−1) where CO2 is used as a chemical barrier against invasive species (Donaldson et al., 2016; Kates et al., 2012).......