Re: what are the major sources of oxygen in water

The following answer is provided by Dr Kai Bester:

There are two ways oxygen gets into the water. If it is a eutrophic
ecosystem, there will generally be an oxygen demand, especially in warm
summer nights, as the plants cannot produce oxygen without light, thus
oxygen has to be imported from the air, which consists of 23% oxygen.
Oligotrophic ecosystems and eutrophic systems which can absorb enough light
will produce oxygen (chemical reaction from water and bicarbonate) so the worlds oceans
deliver much of the oxygen which is in the air.

I hope this will answer your question.

Cheers

kai