GREEN OCEAN OCEAN OXYGEN (O 2 ) Oxygen (O2) is a vital element for life and is critical to the health of the planet. The oceans provide nearly half of the world’s oxygen. As a result of mainly human activities, deoxygenation of the ocean has been observed over the last decades. It is thus essential to monitor the changes and distribution of oxygen in the ocean. World’s Ocean Minimum Oxygen Distribution...
…and the Depths at which these Minimums Occur
The figure shows the minimum amount of oxygen found in the water column as expressed in micromoles per kilogram (µ mol/kg). The data is averaged over about 2 decades.
The figure shows the depth (in meters) where the oxygen minimum is found in the water column (shown in the figure to the left) as expressed in micromoles per kilogram (µ mol/kg). Shown are only the depth values where the oxygen minimum is < 90 μ mol/kg). The rest is not shown (seen in white in the figure).
OXYGEN MINIMUM ZONES (OMZ) Oxygen Minimum Zone (OMZ) distribution is controlled by ocean circulation as well as local biogeochemical and physical processes. Oxygen Minimum Zones with O2 concentrations reaching the suboxic threshold (low levels of O2) are predominantly found in the Pacific and Indian Oceans. In the Atlantic Ocean, areas with intense O2 minimums are found close to eastern coastal regions.
EUTROPHICATION
Large areas of low oxygen levels are formed when there is a large O2 demand such as in coastal upwelling systems where there is a high level of primary productivity. Low O2 levels can have a devastating impact on the ecosystem. This can include nitrogen loss, pH imbalances, carbon outgassing and even the production of toxic gases, resulting in the mortality of marine life. Over the recent decades, OMZs have expanded due to a warming induced decrease in O2 solubility and changes in ocean ventilation and circulation.
Eutrophication
Coastal Hypoxia
Agricultural run-off and other pollutants (which contain nutrients) are flushed into the ocean by rains or drainage.
3 This bloom can block sunlight from penetrating into the water
1
1
3 2
Phosphorus Nitrogen
Block sunlight
Algal bloom + Phytoplankton
4
6 5
2 These nutrients can cause phytoplankton and algae to grow rapidly on the surface and reduce water quality.
4 This inhibits photosynthesis of the phytoplankton and other plant life below, killing them. 5 The algae bloom dies and sinks to the bottom of the shallow ocean. Bacteria decompose this dead organic matter, a process that consumes oxygen. 6 This process leaves the shallow water layer with very little oxygen (a state of hypoxia). This is devastating to the ecosystem and in some cases the zones can become nearly lifeless.