Attraction of Oxygen to Water-Oxygen is constantly entering and leaving water The amount of surface area between the water and the air will also determine the rate of oxygen exchange.

Attraction of Oxygen to Water

Oxygen Concentration

Oxygen is constantly entering and leaving water, but there is a certain amount of oxygen in water at all times.  This is because water has a natural attraction to oxygen.  When oxygen comes in contact with the surface of water, the oxygen tends to enter the water, becoming dissolved oxygen.  

The amount of attraction between oxygen and water depends on the amount of oxygen already in the water.  If there is very little oxygen in water, then the water is very attractive to oxygen.  But when water has a high concentration of DO, then the water is saturated, meaning that the water contains as much oxygen as it can hold.  Saturated water is not very attractive to oxygen.  

Water's attractiveness to oxygen also depends on the concentration of oxygen in the air coming in contact with the water.  The higher the concentration of oxygen in the air, the greater the attraction of the oxygen to the water.  

You can think of an oxygen molecule as a person who likes to live far away from other people.  If this person is looking for a place to live, they will move into the area with the lowest population.  Oxygen molecules do the same thing.  If the air is crowded with oxygen but the water is not, the oxygen will move into the water.  If the water is crowded with oxygen but the air is not, then the oxygen will move into the air.  

The greater the difference between the oxygen concentration in the air and the dissolved oxygen concentration in the water, the faster the oxygen will move into the water.  So if there is very little oxygen in the water, oxygen will dissolve into the water very quickly.   

This process of oxygen moving from an area with a high oxygen concentration to an area with a low oxygen concentration is known as diffusion.

Other Factors

Concentration of oxygen in the air and water are not the only factors which determine how much oxygen will be dissolved in water.  The amount of surface area between the water and the air will also determine the rate of oxygen exchange.  A bigger surface area, as when water runs over rocks in a rapids, will allow more oxygen to enter the water.  

Water temperature is also very important in determining the amount of oxygen which will become dissolved in water.  As you can see in the graph above, cold water is able to hold more oxygen than warm water.  

Foreign substances in water, such as salt or sugar, can also affect water's affinity for oxygen.

Microbiology

Microbiology is the study of microscopic forms of life, such as bacteria.  In microbiology, organisms can be divided up based on the type of oxygen they require for life.  The three categories are aerobic, anaerobic, and facultative.

Water, the home of most bacteria, contains oxygen in two forms.  The first form, free oxygen, is the most readily available form.  Free oxygen is basically the same as dissolved oxygen - oxygen from the atmosphere which has become dissolved in water.

Aerobic bacteria require free oxygen in order to survive.

Oxygen can also be found in the water in another form.  Food and even water itself contain oxygen, but this oxygen is tightly bound to the food and water.  As you can see in the picture above, each molecule of water contains one oxygen molecule (O) and two hydrogen molecules (H).  The oxygen can be ripped out of the water molecule by anaerobic bacteria, but it takes much more energy to break apart food and water in search of oxygen than it does to simply use free oxygen.  Since anaerobic bacteria use so much of their time and energy scrounging for oxygen, they take longer to digest organic matter in water.  Biochemical Oxygen Demand (BOD) is the amount of oxygen used by microorganisms.

The third type of microorganisms, those which are facultative, have properties of both aerobic and anaerobic organisms.  They can live with or without free oxygen. When the oxygen content of water is high, facultative bacteria consume food very quickly using the free oxygen in the water.  In low oxygen concentrations, facultative bacteria are still able to consume organic material, although they do so much more slowly.  

Surface Area and Mass

Microorganisms take in oxygen through their surfaces.  Just as oxygen from the air can diffuse into water through the water's surface, oxygen enters a bacteria through its outer surface.  The larger the surface, the more oxygen the bacteria is able to take in.  

A bacteria's oxygen requirement, how much oxygen it needs to survive, does not depend on the surface area though. The oxygen requirement depends upon the mass, or bulk, of the organism.  Bacteria of the same mass and metabolism have about the same oxygen requirement.  The bigger the bacteria, the more oxygen it needs to survive.  

The mass to surface area ratio is an important concept in a small microorganism:

Surface Area
Mass

You may remember that we discussed the relationship between surface area and volume in Lesson 4 when discussing aerators.  Small water droplets had a large surface area to volume ratio (which you can think of as a surface area to mass ratio.)  As a result, more air was able to enter these small water droplets.  Larger water droplets had a lower surface area to volume ratio, so less air was able to enter the larger droplets.

Oxygen in water is known as dissolved oxygen or DO.  In nature, oxygen enters water when water runs over rocks and creates tremendous amounts of surface area.  The high surface area allows oxygen to transfer from the air into the water very quickly.  

The same concept applies to the oxygen uptake by bacteria.  Small bacteria have a large surface area to mass ratio, so they are able to take up a lot of oxygen compared to how much oxygen they need to survive.  Large bacteria, in contrast, have a small surface area to mass ratio.  So these large bacteria take up a lot less oxygen compared to how much oxygen they use up in their daily life.  

Since large microorganisms have a harder time taking up enough oxygen to survive, they have to live in water with a high oxygen concentration.  If the DO content of water drops below a certain point, only the smaller microorganisms will be able to survive.

Humans

Of course, larger organisms also require oxygen to survive. In humans, we breathe in oxygen, which descends to our lungs, enters the blood, and then moves throughout the body. The purpose of our lungs is to move oxygen out of the air we breathe and into our blood.  In order to make oxygen flow quickly from air to blood, the lungs are divided up so that they have a very large surface area.  

Like oxygen moving into water, oxygen naturally flows into blood through the process of diffusion.  Blood is composed of plasma (which is a liquid base) and red blood cells.  The red blood cells contain a molecule known as hemoglobin which binds to oxygen and pulls it along as the blood flows from the lungs to the rest of the body.  

The amount of oxygen in the air is very important to us.  The normal oxygen content of the air is about 21% and as the oxygen content raises or lowers, our bodies begin to have problems.  When the oxygen concentration is less than 16.5%, humans blackout.  When concentrations rise above 40%, toxic oxygen radicals are formed in the body.  These radicals damage cell structure and function in a process known as oxygen toxicity.  Both oxygen toxicity and extremely low oxygen content in the air can lead to death. Thus, our lives are dependent on oxygen and are greatly affected by the concentration thereof.

Fish

Just as the oxygen content of the air in which we live is important to humans, the oxygen content of water is important to fish and other organisms living in water.  Fish take up oxygen from the water using their gills.  Fish with larger gills compared to their body size are like bacteria with a large surface area to mass ratio - these fish can take up enough oxygen to survive even in water with a low oxygen content. 

A fish's metabolism also helps determine the amount of oxygen which a fish needs to survive.  Fish with a high metabolism are fast-moving but also require a great deal of oxygen to survive.  Fish with a slow metabolism are more sluggish and require less oxygen.

Carp and trout are examples of the two extremes of fish oxygen requirements.  Trout have a small gill area and a high metabolism, so they only live in the ocean and in fast-moving streams where the oxygen levels are high.  Carp, in contrast, have a small gill area and a slower metabolism, so they can withstand low levels of oxygen and live in small lakes and ponds.

In order to maintain the diversity of life in streams, the oxygen content of water discharged from wastewater treatment plants is monitored.  Some fish can live with low DO levels of 3 to 4 milligrams per liter, but the National Pollution Elimination Discharge System (NPDES) requires that all discharge from sewage treatment plants must have a DO level of at least 5 milligrams per liter so that organisms with high oxygen requirements can also survive.  To meet this standard, most wastewater treatment plants must aerate the water before discharge.  Step aeration is the most common type of aeration used in wastewater treatment plants.

Wastewater treatment plants must also ensure that organic matter is removed from the water in the plant.  If water containing organic matter is released into streams and rivers, then bacteria will quickly begin to metabolize the organic matter, using up the water's oxygen in the process.  In this way, a wastewater treatment plant could indirectly cause the DO content of water to drop to a dangerous level.