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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).
Dead zones are hypoxic (low-oxygen) areas in the world's oceans and large lakes. Hypoxia occurs when dissolved oxygen (DO) concentration falls to or below 2 ml of O 2 /liter. [2] When a body of water experiences hypoxic conditions, aquatic flora and fauna begin to change behavior in order to reach sections of water with higher oxygen levels.
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]
Global map of low and declining oxygen levels in coastal waters (mainly due to eutrophication) and in the open ocean (due to climate change).The map indicates coastal sites where oxygen levels have declined to less than 2 mg/L (red dots), as well as expanding ocean oxygen minimum zones at 300 metres (blue shaded regions).
Scientists prepare to collect near-bottom water aboard the R/V Pelican to verify oxygen measurements used to determine the size of the Gulf of Mexico hypoxic zone. (NOAA/LUMCON/LSU) A "dead zone ...
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.
When dissolved oxygen levels decline to hypoxic levels, fish and other marine animals suffocate. As a result, creatures such as fish, shrimp, and especially immobile bottom dwellers die off. [36] In extreme cases, anaerobic conditions ensue, promoting growth of bacteria. Zones where this occurs are known as dead zones.