Comparison of radioactive and stable isotope tracer techniques for measuring photosynthesis: ^13C and ¹⁴C uptake by marine phytoplankton

Abstract

The stable carbon isotope 13 C has been used in the open ocean to estimate the inorganic carbon uptake by phytoplankton and this technique has been compared with the 13 C tracer method. An overall correlation coefficient of 0.806 and a regression slope of 1.29 were calculated from 50 sample pairs gathered during three cruises in widely different oceanic areas ranging in production rates from 0.01 to 6 mgC m −3 h −1 . However, significant differences between the two methods were apparent for cruises located in nutrient-depleted areas. Possible explanations lie either in a volume effect, the high silicate content of the 14 C solution which could stimulate the 14 C uptake or in errors associated with the particulate carbon measurements which are necessary to convert specific uptake rates to absolute uptake rates and to yield compatible units for the comparison, in laboratory cultures the 14 C technique overestimated the net particulate carbon increase by — 16%.