CO2 exchange with Dissolved Oxygen in Water

ScottBurton

I hate to come across as a complete novice, but I am struggling with understanding this.

I have found resources for the dissolved oxygen capacity of fresh and salt water based on pressure and temperature. I've also found many resources that infer how this process works, but I am not sure I'm 'getting it'

What determines whether air passing through a body of water gives up CO2 or O2 to the water and vice versa. Is it simply a matter of the DO capacity of the water versus the amount of DO in the water?

I know if there an animal breathing in the water, they are turning DO to CO2 in the water (in simple terms), and likewise if there is a plant photosynthesizing in the water, it is (generally) putting more dissolved oxygen into the water than it is producing CO2.

(Yes, I realize that technically plants both make and consume oxygen and CO2, but they typically produce more oxygen than CO2, and consume more CO2 than Oxygen)

Thank you everyone.