Water's the Matter-- Lesson Presentation: Dissolved Oxygen

How does oxygen get in the water?

Basically, oxygen gets into water by three different ways. It is by these means that most lakes, rivers, streams, and oceans receive the oxygen necessary to support aquatic life.

Diffusion from surrounding air. If you remember the definition of diffusion, it is the movement of a substance from an area of higher concentration to an area of lower concentration. In this case of oxygen, if the air in the atmosphere has a higher concentration of oxygen than water - oxygen diffuses or is "pushed" from the air into the water. The speed of this movement of oxygen is related to the difference in the concentration of oxygen in the air and in the water and the atmospheric pressure. To see a current measure of atmospheric pressure, click here. Atmospheric or barometric pressure is measured in units of "inches of mercury" and on a normal day, at sea level, the pressure is 29.92 inches of mercury.

This relatively calm water in the lake (shown above) is an example of a body of water that receives its oxygen by natural diffusion.

Aeration of water. A river that flows rapidly will have a turbulent surface, with much more surface area for oxygen to diffuse across than a flat, slow moving river. Thus the atmospheric pressure can drive more oxygen into the water. Also, the turbulence created by churning waters causes air to hit the water at a high pressure, allowing more oxygen to become dissolved.

This white water creek is an example of how turbulence creates more surface area, allowing more oxygen to diffuse into the water.

Waste products of plants. Rooted aquatic plants and algae "breathe" in the same way that trees and other land dwelling plants do. They use carbon dioxide as fuel and generate oxygen as a waste product. This oxygen is immediately dissolved into the water. There is only one problem with this source of aquatic oxygen: the process is reversed at night! In darkness, plants will consume oxygen as fuel. Thus, a body of water with a high plant density will have high dissolved oxygen levels during the day and low levels of dissolved oxygen at night.

Aquatic Vegetation

Aquatic vegetation can supply dissolved oxygen provided that there is sunlight. When there is no light for photosynthesis, plants consume some of the oxygen, leaving less oxygen for the fish.

Does the amount of dissolved oxygen (DO) differ for every body of water?

Absolutely! The conditions stated above have a large impact on the amount of dissolved oxygen. Barometric pressure will influence how much dissolved oxygen will diffuse into the water. Thus, altitude of the body of water and prevailing weather conditions can be factors that affect the amount of dissolved oxygen in water.

Do you remember the pictures on the introduction page to this unit? A bayou swamp will usually have much less dissolved oxygen than a white water river. The water in the bayou is calm and, therefore, has less surface area for oxygen diffusion.

The presence of algae and rooted aquatic plants will also influence the amount of dissolved oxygen in water.

There are also other factors that can affect the amount of oxygen dissolved in water. Some of these factors are associated with the impact of humans on the environment.

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