Discrepancies between the O 2 and 14 C Methods for Measuring Phytoplankton Gross Photosynthesis at Low Light Levels

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Abstract
Primary production of phytoplankton in a eutrophic lake and of laboratory cultures of Selenastrum capricornutum was investigated using the O2 and 14C light and dark bottle techniques. The quotient O (gross O2 production divided by C fixation) ranged between 1.14 and 1.35 at light saturation, but at low light intensities it increased to values of 2-8. This pattern is ascribed to underestimation of gross photosynthesis by 14C fixation rates due to reassimilation of respired CO2, which is most important at low light levels. Gross photosynthesis was estimated from 14C fixation rates by adding the same value (e.g., 60% of dark respiration or 6% of maximal photosynthesis) at each light intensity down to the compensation point instead of using a constant multiplication factor.