Vertical Distribution of the Activity of the Respiratory Electron Transport System in Marine Plankton1

The activity of the respiratory electron transport system in plankton is measured by tetrazolium reduction and used to estimate oxygen utilization rates in the waters of the eastern tropical Pacific Ocean. Vertical profiles of the electron transport system from the ocean surface to 5,000 m suggest a rapid decrease in the oxygen utilization rate from 300 µ l O 2 liter −1 yr −1 (0.8 µ l O 2 liter −1 day −1 ) in the cuphotic zone to 3 µ l O 2 lite −1 yr −1 at 500 m and a more gradual decrease to 0.5 µ l O 2 liter −1 yr −1 at 5,000 m.