Dissolved oxygen
Oxygen is essential for marine life because all living species (fishes, invertebrates, corals and bacteria) require oxygen when they respire. The measurement of oxygen concentration provides information to oceanographers on the pathway and the age of the water masses in the Ocean.
How is oxygen measured with an optical sensor ?
In oceanography and especially onboard autonomous platforms (floats and gliders), the optode (« optical electrode ») is the preferred sensor to measure dissolved O2 concentration in seawater. At the sensor surface, there is a luminescent material that emits red light when it is stimulated by the light originating from a blue diode. When this material reacts with O2, it loses its capability to emit red light. The duration and the intensity of the red light thus decreases proportionally with the O2 content (Fig.1).
Fig.1 Quenching principle used by optode to measure dissolved O2 in seawater
How oxygen is distributed in the ocean ?
In cold surface waters near the poles, oxygen concentrations can reach up to 300-400 micromoles per kilogram of seawater (1 micromole of oxygen represents 0.032 mg/kg)(Fig. 2A). Dissolved oxygen concentration varies significantly according to the Oceans. Marine life is considered as constrained when oxygen concentrations descend to 120-60 micromoles per kilogram. Oceanographers have found the lowest oxygen concentrations around 10 micromoles per kilogram in the OMZs located in the Pacific and Indian Oceans (Fig.2B and 2C)
Fig. 2A - Oxygen concentrations in surface (A). The red areas represent the highest oxygen concentrations and the areas in blue the location of OMZs (from eWOCE data).
Fig. 2B - Oxygen concentrations at 200m. The red areas represent the highest oxygen concentrations and the areas in blue the location of OMZs
(from eWOCE data).
Fig. 2C - Oxygen concentrations at 2000m. The red areas represent the highest oxygen concentrations and the areas in blue the location of OMZs (from eWOCE data).
