what is the loss of dissolved oxygen from water in water bodies called? how it causes

The phenomenon is called as eutrophication.
Kindly refer to the answer posted by Yerram Varun and the following link also:

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this is caused by the excessive quantities of chemicals which get washed from the fields these act as nutrients for algae to flourish.once these algae die they serve as food for decomposers like bacteria .a lot of oxygen in the water body gets used up this results in a decrease in the oxygen level which may kill the aquatic organisms.
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