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Carbon dioxide (CO₂) is very dynamic in water as it is highly soluble, produced by animal and microbial respiration and decay processes, and used during photosynthesis. CO₂ reacts with water to form carbonic acid (H₂CO₃) which dissociates to bicarbonate (HCO₃⁻), then carbonate (CO₃²⁻) and hydrogen ions (H⁺), depending on alkalinity, pH, salinity, and (to a lesser extent) temperature.

Frequency of testing: Testing is not common and is usually done in high‐risk situations (e.g. intensive aquaculture or incoming well water).

Sampling: CO₂ should be measured on site and within about 30 minutes of sampling.

Testing: Commercial colorimetric tests are available. CO₂ can also be estimated based on pH and alkalinity using standard curves available on‐line and in Hargreaves and Brunson (1996). Because of the narrow range of pH in salt water (e.g. 7.5–8.5), a low pH in salt water may indicate high levels of CO₂. A crude estimate of CO₂ can be made by collecting water from the system in an open‐topped container, measuring the pH, vigorously aerating the water for about 60 minutes, and measuring the pH again. If the pH increases by more than one unit due to off‐gassing of CO₂, the CO₂ may be too high.

Units: CO₂ is reported in milligrams per liter (mg/L).

Target values: Less than 20 mg/L is a common target.

Practical considerations:

 High CO2 is problematic for fish because it affects pH and oxygen availability.

 High values are common in intensive aquaculture systems with limited off‐gassing, closed transport containers, well water, and following an algal or phytoplankton die‐off.

 Low levels are not a health concern for fish, but vascular plants, algae, and phytoplankton need CO2 for photosynthesis.

 CO2 is inversely related to pH and is a component of alkalinity, which affects pH stability.

 CO2 can vary significantly across the day because of diurnal changes in respiration and photosynthesis; values are highest at dawn (Figure A2.2).