High-frequency measurements of photosynthesis of planktonic algae using rugged nondispersive infrared carbon dioxide probes

Pertti Hari, Jukka Pumpanen, Jussi Huotari, Pasi Kolari, John Grace, Timo Vesala and Anne Ojala
Limnol. Oceanogr. Methods (XXXX) X:XXX-XXX

We have constructed a system based on commercially available inexpensive NDIR CO₂ sensors to continuosly monitor CO₂ 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 CO₂ consumption and production. We utilize and modify the computational methods commonly used in forest ecology to enable characterisation of aquatic photosynthesis. To demonstrate the advantages of the method we present the results against the observations from the long-term monitoring programme applying static bottle methods. With the probes we can detect a clear, photosynthesis-driven daily pattern in the CO₂ concentration and determine the CO₂ consumption or production resulting in over a tenfold improvement in the time resolution when compared with traditional incubation methods. The measuring frequency is high enough for determining the dependence of photosynthesis on light. The high-frequency data highlights the inherent problems of static incubations and bring into question their reliability. Our free water approach resulted in productivity and mineralization rates which were several times higher than obtained with incubations.