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Abstract Fluctuations in dissolved oxygen (DO) contents in natural waters can become intense during cyanobacteria blooms. In a reconnaissance study, we investigated DO concentrations and stable isotope dynamics during a laboratory experiment with the cyanobacterium Planktothrix rubescens in order to obtain insights into primary production under specific conditions. This observation was extended to sub‐daily timescales with alternating light and dark phases. Dissolved oxygen concentrations and its isotopes (δ 18 O DO ) ranged from 0.02 to 0.06 mmol · L −1 and from +9.6‰ to +23.4‰. The δ 18 O DO proved to be more sensitive than concentration measurements in response to metabolic variation and registered earlier shifts to dominance by respiration. Oxygen (O 2 ) contents in the headspace and its isotopes (δ 18 O O2 ) ranged from 2.62 to 3.20 mmol · L −1 and from +9.8‰ to +21.9‰. Headspace samples showed less fluctuations in concentration and isotope trends because aquatic processes were hardly able to alter signals once the gas had reached the headspace. Headspace δ 18 O O2 values were corrected for gas–water equilibration and were determined to be higher than the mean δ 18 O H2O of −8.7‰. This finding suggests that counteracting respiration was important even during the highest photosynthetic activity. Additionally, headspace analyses led to the definition of a fractionation factor for respiration ( α R ) of this cyanobacterium with a value of 0.980. This value confirms the one commonly used for cyanobacteria. Our findings may become important for the management of water bodies where decreases in DO are caused by cyanobacteria.
Dordoni et al. (Thu,) studied this question.