Is the amount of dissolved oxygen (DO) in a body of water important?

Yes, it is!  Just like humans, aquatic animals and plants need oxygen.  As an example, fish and some aquatic insects capture oxygen from the water using gills.  

You may be asking yourself, how is oxygen in water?  Consider the following: if you put salt in a glass of water and then stir this solution, the salt disappears.  However, the salt is still there.  If you taste the water, it is salty.  A similar thing happens with oxygen in water!  Molecules of water trap molecules of oxygen and keep it in a dissolved form.  Fish and some aquatic insects have gills; gills allow these organisms to remove some of the dissolved oxygen from the water.

Just like humans, aquatic organism need a certain amount of oxygen to survive.  Have you ever put a glass upside down over a burning candle?  After a few seconds, there is so little oxygen that the flame goes out.  Most organisms must receive a constant supply of oxygen, if these organisms do not receive oxygen they will soon die.

Is the amount of dissolved oxygen in each of these bodies of water different?

Yes, there is a huge difference!  The whitewater (left) probably contains as much oxygen as water can hold, while the bayou (right) probably contains very little.  There are several reasons that the amount of dissolved oxygen is different in these two markedly different bodies of water; these reasons will be explained in the presentation portion of this unit.

In summary, most aquatic organisms, just like land dwelling animals,  need to "breathe" oxygen to survive.  Organisms must have a minimum amount of oxygen to survive.  Unlike an increase in water temperature which can stress or even kill aquatic organisms, an increase in dissolved oxygen is not harmful.  Although, there is a minimum amount of dissolved oxygen required in water for organisms to survive, there is no maximum amount or upper limit for dissolved oxygen.  Water can have too many hydrogen ions, nitrates, or heavy metals dissolved in it, but, in terms of dissolved oxygen, "you can't have too much of a good thing".  A catfish that can survive in relatively low dissolved oxygen environments (e.g. a bayou), can also tolerate being in an environment having high levels of dissolved oxygen (e.g. a fast moving, whitewater stream).  Unfortunately, a trout living in fast moving, whitewater with high dissolved oxygen content cannot tolerate the low dissolved oxygen levels of a bayou.

In this lesson, you will explore the concept of dissolved oxygen and discover how it can affect water quality. 

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