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

• Published in: Limnology and oceanography, methods Vol. 6; no. 1; pp. 1 - 11
• Main Authors: Williams, Peter J. le B., Lefèvre, Dominique
• Format: Journal Article
• Language: English
• Published: 01-01-2008
• ISSN: 1541-5856; 1541-5856
• DOI: 10.4319/lom.2008.6.1

Marra and Barber (2004) have proposed a simple procedure for calculating phytoplankton respiration from measurements of the loss of
14
C 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 (
14
C/
12
C) of the respiratory CO
2
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
14
C 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 CO
2
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
14
C 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 CO
2
will give insignificant errors. The above generalizations will not hold when photosynthetic rates are low both in absolute terms and in relation to respiration.