An assessment of the measurement of phytoplankton respiration rates from dark 14C incubations

Abstract

Marra and Barber (2004) have proposed a simple procedure for calculating phytoplankton respiration from measurements of the loss of 14C from the particulate fraction during dark post-incubations. We identify the necessary assumptions implicit in their approach, the primary ones being: (1) the respiration rates in the light must be the same as those in the dark, (2) the specific radioactivity (14C/12C) of the respiratory CO2 released in the dark must be known or assumed to be the same as the seawater carbonate, and (3) only phytoplankton respiration gives rise to the change in the 14C content of the particulate fraction in the dark. In most cases, with a combination of theory and observations, we have been able to make a first order quantitative assessment of the error associated with these assumptions and their consequences. When P/R ratios are ≥3, we conclude that uncertainties deriving from the necessary assumption that specific activity of the CO2 produced is the same as that of the labeled seawater bicarbonate, give rise to underestimates of respiration in the region of 5%-20%. The errors due to loss of 14C from the particulate fraction, other than phytoplankton respiration, will give overestimates as high as 20-40%. At P/R ratios >3, the non-photosynthetic assimilation of CO2 will give insignificant errors. The above generalizations will not hold when photosynthetic rates are low both in absolute terms and in relation to respiration.