Dissolved oxygen (DO) is the amount of oxygen that is present in water.
All aquatic animals need DO to breathe. Low levels of oxygen (hypoxia) or no oxygen levels
(anoxia) can occur when excess organic materials, such as large algal blooms, are
decomposed by microorganisms. During this decomposition process, DO in the water is
consumed. Low oxygen levels often occur in the bottom of the water column and affect
organisms that live in the sediments.
DO is considered an important measure of water quality as it is a direct indicator of an
aquatic resource’s ability to support aquatic life.
Each organism has its own DO tolerance range, generally, DO levels below 3 milligrams per
liter (mg/L) are of concern and waters with levels below 1 mg/L are considered hypoxic and
usually devoid of life.