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The exaerobic zone is found at the boundary of anoxic and hypoxic zones. Hypoxia can occur throughout the water column and also at high altitudes as well as near sediments on the bottom. It usually extends throughout 20–50% of the water column, but depends on the water depth and location of pycnoclines (rapid changes in water density with depth).
Methanogenesis is the process by which methanogen microbes form methane (CH 4). OMZs are known to contain the largest amount of methane in the open ocean. [25] Methanogens can also oxidize methane as they have the genes to do so, however this requires oxygen which they obtain from photosynthetic organisms in the upper anoxic zone. [25]
The area of temporary hypoxic bottom water that occurs most summers off the coast of Louisiana in the Gulf of Mexico [58] is the largest recurring hypoxic zone in the United States. [59] It occurs only during the summer months of the year due to summer warming, regional circulation, wind mixing and high freshwater discharge. [ 60 ]
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Many species actively avoid low oxygen zones, [32] [33] [34] while others take advantage of their predators’ low tolerance for hypoxia and use these areas as a refuge. [ 32 ] [ 33 ] [ 34 ] Zooplankton that exhibit daily vertical migrations to avoid predation and low oxygen conditions also excrete ammonium near the oxycline and contribute to ...
Redox gradients form in water columns and their sediments. Varying levels of oxygen (oxic, suboxic, hypoxic) within the water column alter redox chemistry and which redox reactions can occur. [10] Development of oxygen minimum zones also contributes to formation of redox gradients.
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Equatorward winds drive the upwelling of low dissolved oxygen (DO), high nutrient, and high dissolved inorganic carbon (DIC) water from above the oxygen minimum zone. Cross-shelf gradients in productivity and bottom water residence times drive the strength of DO (DIC) decrease (increase) as water transits across a productive continental shelf .