mon océan & moi

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.

 

What is oxygen and why it is important to measure it ?

Oxygen (O2) is a gas, which is essential for all marine organisms (as well as terrestrial ones) that use O2 for their respiration.
The presence of O2 in the ocean is due to two main processes :

1The dissolution of atmospheric O2 in surface waters

2The O2 production through phytoplankton photosynthesis.

O2-rich water masses are considered as “young”, meaning they have been recently in contact with the atmosphere at the ocean surface. When they leave the surface, these water masses become older as they progressively lose their O2 content as a consequence of respiration by living organisms, in particular bacteria.
Presently, oceanographers are focusing significant research efforts on regions with low O2 content (the so-called Oxygen Minimum Zones, OMZs) where marine life is in danger.

As a consequence of global warming, these zones are increasing in the ocean. Indeed, O2 dissolution in seawater is lower in warm than in cold waters, thus ocean warming induces a depletion of the O2 content.

It is also expected that O2 decrease due to ocean warming could augment sediment accumulation at the ocean bottom. Organic matter (dead organisms, fecal pellets) sinking from the surface would be partly preserved from bacterial consumption. These bacteria would actually be less efficient in degrading this sinking material under low or depleted oxygen conditions.

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).

optode-enFig.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)

 

oxygene-surfaceFig. 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).

 

oxygene-surfaceFig. 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).

 

oxygene-surfaceFig. 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).

 

 

 

 

 

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