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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]
The relationship between zooplankton and low oxygen zones is complex and varies by species and life stage. Some gelatinous zooplankton reduce their growth rates when exposed to hypoxia while others utilize this habitat to forage on high prey concentrations with their growth rates unaffected.
A 2007 study of the Atlantic croaker found a disproportioned sex ratio of 61% males to 39% females in hypoxic sites. This was compared with a 52% to 48% male-female ratio found in reference sites, showing impaired reproductive output for fish populations inhabiting hypoxic coastal zones. [51]
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 ...
Baltic Sea hypoxia refers to low levels of oxygen in bottom waters, also known as hypoxia, occurring regularly in the Baltic Sea. As of 2009 [update] the total area of bottom covered with hypoxic waters with oxygen concentrations less than 2 mg/L in the Baltic Sea has averaged 49,000 km 2 over the last 40 years.
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.