The respiration of photosynthetic carbon by phytoplankton populations from upwelling regions was calculated from the difference in 6- and 24-h productivity incubations after the light intensity received in the 6-h incubations had been normalized to the light intensity received in 24 h. The results indicate that the phytoplankton off Northwest Africa respired 13.4% of the previously fixed photosynthetic carbon. A direct experimental test of the results confirms the calculations....