Net and gross productivity: weighing in with 14C

Data from the 14 C method, C assimilation time courses and single endpoint comparisons to O 2 fluxes, imply 2 different things. Time course (linear) experiments suggest that C assimilation measures gross production while the single endpoint comparisons with O 2 fluxes suggest that the 14 C method approximates net community production. Considering only phytoplankton metabolism, a way out of the paradox is to assume that all CO 2 respired by mitochondria is refixed in photosynthesis, meaning that photosynthesis uses more H 2 O than CO 2 from the ambient environment since the latter has an internal source. The 14 C method, therefore, would estimate net primary production relative to O 2 fluxes in the absence of heterotrophic interactions. The method can also be applied to the estimation of phytoplankton respiration from the nighttime loss of labeled organic matter. A summary of available data shows that C assimilation corrected for organic C lost is statistically equivalent to gross production measured independently using the 18 O method.