Technical note: A simple and cost-efficient automated floating chamber for continuous measurements of carbon dioxide gas flux on lakes

Abstract

Freshwaters emit significant amounts of CO2 on a global scale. However, emissions remain poorly constrained from the diverse range of aquatic systems. The drivers and regulators of CO2 gas flux from standing waters require further investigation to improve knowledge on both global-scale estimates and system-scale carbon balances. Often, lake-atmosphere gas fluxes are estimated from empirical models of gas transfer velocity and air-water concentration gradient. Direct quantification of the gas flux circumvents the uncertainty associated with the use of empirical models from contrasting systems. Existing methods to measure CO2 gas flux are often expensive (e.g. eddy covariance) or require a high workload in order to overcome the limitations of single point measurements using floating chambers. We added a small air pump, a timer and an exterior tube to ventilate the floating chamber headspace and passively regulate excess air pressure. By automating evacuation of the chamber headspace, continuous measurements of lake CO2 gas flux can be obtained with minimal effort. We present the chamber modifications and an example of operation from a small forest lake. The modified floating chamber performed well in the field and enabled continuous measurements of CO2 gas flux with 40 min intervals. Combining the direct measurements of gas flux with measurements of air and waterside CO2 partial pressure also enabled calculation of gas exchange velocity. Building and using the floating chamber is straightforward. However, because an air pump is used to restart measurements by thinning the chamber headspace with atmospheric air, the duration of the air pump pause-pulse cycle is critical and should be adjusted depending on system characteristics. This may result in shorter deployment duration, but this restriction can be circumvented by providing a stronger power source. The simple design makes modifications of the chamber dimensions and technical additions for particular applications and systems easy. This should make this approach to measuring gas flux flexible and appropriate in a wide range of different systems.