High‐frequency measurements of productivity of planktonic algae using rugged nondispersive infrared carbon dioxide probes

We have constructed a system based on commercially available inexpensive NDIR CO 2 sensors to continuously monitor CO 2 concentrations in water bodies. Here we demonstrate its use in a boreal humic lake. With the system, we collect high frequency data with a 10‐min time resolution for determination of CO 2 consumption and production. We use and modify the computational methods commonly used in forest ecology to enable characterization of aquatic primary production. To demonstrate the advantages of the method, we present the results against the observations from the long‐term monitoring program applying traditional bottle methods. With the probes, we can detect a clear, photosynthesis‐driven daily pattern in the CO 2 concentration and determine the CO 2 consumption or production resulting in more than a 10‐fold improvement in the time resolution when compared with traditional incubation methods. The measuring frequency is high enough to determine the dependence of photosynthesis on irradiance. The high‐frequency data highlights the inherent problems of discrete bottle incubations and bring into question their reliability. Our free water approach yielded productivity and mineralization rates that were clearly higher than obtained with incubations.