Direct and continuous measurement of dissolved carbon dioxide in freshwater aquatic systems—method and applications

Understanding of the processes that control CO 2 concentrations in the aquatic environment has been hampered by the absence of a direct method to make continuous measurements over both short‐ and long‐term time intervals. We describe an in situ method in which a non‐dispersive infrared (NDIR) sensor is enclosed in a water impermeable, gas permeable polytetrafluoroethylene (PTFE) membrane and deployed in a freshwater environment. This allows measurements of CO 2 concentration to be made directly at a specific depth in the water column without the need for pumps or reagents. We demonstrate the potential of the method using examples from different aquatic environments characterized by a range of CO 2 concentrations (0·5–8·0 mg CO 2 ‐C l −1 , equivalent to ca 40–650 µmol CO 2 l −1 ). These comprise streams and ponds from tropical, temperate and boreal regions. Data derived from the sensor was compared with direct measurements of CO 2 concentrations using headspace analysis. Sensor performance following long‐term (>6 months) field deployment conformed to manufacturers' specifications, with no drift detected. We conclude that the sensor‐based method is a robust, accurate and responsive method, with a wide range of potential applications, particularly when combined with other in situ sensor‐based measurements of related variables.