Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in the water. An adequate DO saturation is required to sustain aquatic life and ensure that the aquatic ecosystem remains functional. There are numerous factors controlling dissolved oxygen including instream vegetation type and abundance, water temperature, salinity, atmospheric pressure and organic waste among others.

The South African target water quality range is between 80% - 120% (within 24 hrs) and the lethal being < 40%(lowest value in one day). Super saturated waters (> 100%) may be indicative of eutrophic conditions (high nutrient pollution), leading to increased primary productivity subsequently producing high dissolved oxygen during the day (photosynthesis) and very low levels during the evening (respiration), often resulting in the death of oxygen sensitive aquatic biota.

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Sources:

(1) https://www.usgs.gov/special-topic/water-science-school/science/dissolved-oxygen-and-waterqtscience_center_objects=0#qt-science_center_object

(2) http://www.dwa.gov.za/iwqs/wq_guide/Pol_saWQguideFRESHAquaticecosystemsvol7.pdf