Measurement of daily primary production using 24 h incubations with the super(14)C method: A caveat

Computer simulations and experiments with cultured and natural phytoplankton were used to study super(14)C uptake kinetics and budgets over 24 h incubations. There was good agreement between experiments and simulations. After 24 h incubations and depending on the starting time of incubations, net super(14)C uptake varied by a factor of 2 in the laboratory and by a factor of 3 in the field. Both simulated and experimental data showed lowest super(14)C accumulation for incubations starting at sunrise (i.e. dawn-to-dawn incubation), while highest values corresponded to incubations beginning at sunset. Recommendations for field studies are as follows. For samples collected at night, incubations can be started at any time, but should be conducted for a full 24 h after dawn. Samples collected after dawn should be incubated for 24 h, and super(14)C accumulation should be corrected in order to obtain dawn-to-dawn values.