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Ocean deoxygenation is the reduction of the oxygen content in different parts of the ocean due to human activities. [ 2 ] [ 3 ] There are two areas where this occurs. Firstly, it occurs in coastal zones where eutrophication has driven some quite rapid (in a few decades) declines in oxygen to very low levels. [ 2 ]
The rise in water temperature leads to an increase in oxygen demand and the increase for ocean deoxygenation which causes these large coral reef dead zones. For many coral reefs , the response to this hypoxia is very dependent on the magnitude and duration of the deoxygenation.
This type of ocean deoxygenation is also called dead zones. Secondly, ocean deoxygenation occurs also in the open ocean. In that part of the ocean, there is nowadays an ongoing reduction in oxygen levels. As a result, the naturally occurring low oxygen areas (so called oxygen minimum zones (OMZs)) are now expanding slowly. [30]
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.
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] Measurement of dissolved oxygen in coastal and open ocean waters for the past 50 years has revealed a marked decline in oxygen content.
The upper ocean (above 700 m) is warming the fastest. At an ocean depth of a thousand metres the warming occurs at a rate of nearly 0.4 °C per century (data from 1981 to 2019). [39]: Figure 5.4 In deeper zones of the ocean (globally speaking), at 2000 metres depth, the warming has been around 0.1 °C per century. [39]:
The waves reach their maximum height when the rate at which they are travelling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in the Southern Hemisphere in the Roaring Forties, long, organised masses of water called swell roll across the ocean.
Oceanography (from Ancient Greek ὠκεανός (ōkeanós) ' ocean ' and γραφή (graphḗ) ' writing '), also known as oceanology, sea science, ocean science, and marine science, is the scientific study of the ocean, including its physics, chemistry, biology, and geology.