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Eutrophication is a general term describing a process in which nutrients accumulate in a body of water, resulting in an increased growth of organisms that may deplete the oxygen in the water. [ 1 ] [ 2 ] Eutrophication may occur naturally or as a result of human actions.
In OMZs oxygen concentration drops to levels <10 nM at the base of the oxycline and can remain anoxic for over 700 m depth. [7] This lack of oxygen can be reinforced or increased due to physical processes changing oxygen supply such as eddy-driven advection, [7] sluggish ventilation, [8] increases in ocean stratification, and increases in ocean temperature which reduces oxygen solubility.
Oxygen depletion can result from a number of natural factors, but is most often a concern as a consequence of pollution and eutrophication in which plant nutrients enter a river, lake, or ocean, and phytoplankton blooms are encouraged.
This compounds the effects of eutrophication in coastal zones described above. Open ocean areas with no oxygen have grown more than 1.7 million square miles in the last 50 years, and coastal waters have seen a tenfold increase in low-oxygen areas in the same time. [34]
Dead zones are often accompanied by a decrease in biodiversity and collapse in benthic populations, lowering the diversity of yield in commercial fishing operations, but in cases of eutrophication-related dead zone formations, the increase in nutrient availability can lead to temporary rises in select yields among pelagic populations, such as ...
Oxygen is hard to produce without the continuous energy that comes from sunlight, but other scientists have also encountered unexpected oxygen molecules in remote, light-deprived places.
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).
However, most organisms are highly susceptible to slight changes in aquatic oxygen levels. When a respiring organism is presented with little to no oxygen, chances of survival decrease. Therefore, eutrophication and anoxic conditions in water lead to a reduction in biodiversity.