Oxygen and Carbon Dioxide

What Makes Water Healthy?
If you’ve ever played on a see saw, you know that it’s important to have the two sides equally balanced. In a lake, a good balance of oxygen and carbon dioxide is just as important.

Who Needs Oxygen
Dissolved oxygen, also called DO, is vital to the health of aquatic habitats. Plants and animals need oxygen to survive. A low level of oxygen in the water is a sign that the habitat is stressed or polluted.

Where does oxygen come from? Oxygen from the air is mixed into water with the help of rain, wind, waves and currents. Fast-moving water contains more DO than still water because it has more contact with the air, allowing more oxygen to mix into the water. Underwater plants and algae also contribute oxygen during the process of photosynthesis.

Photosynthesis is the process green plants use to make sugars, part of the food they need to grow.

Fishy Business
Dissolved oxygen in the Great Lakes is affected by weather and temperature. Cold water holds more oxygen than warm water. Because trout or salmon need a high level of oxygen to survive, they live in fast-moving, cold streams and rivers, or in deep, cold lakes. Warm-water fish such as bluegills, crappie, perch, walleye, catfish and carp can tolerate lower levels of dissolved oxygen in the water.

It is important to monitor DO, since it can be used as an indicator of water quality. Anoxia occurs when oxygen levels are low and often results when dry, hot weather causes water to warm and evaporation increases. If these conditions are severe, large “fish kills” (floating dead fish) may result due to lack of oxygen.

Anoxia can also result from the runoff of fertilizer and organic wastes (including pet droppings) from streets, lawns and farms. When excess nutrients, including nitrogen and phosphorus, enter the water, algae grows quickly—creating an algae bloom.

Algae produce oxygen during the day through photosynthesis but also quickly consume oxygen at night during respiration. Bacteria decompose the algae after the bloom dies, using a significant amount of oxygen in the process. The result is a lack of available oxygen for other plants and animals that need it, possibly causing a fish kill.

Carbon Dioxide in the Water
Carbon dioxide, also called CO2, is found in water as a dissolved gas. It can dissolve in water 200 times more easily than oxygen. Aquatic plants depend on carbon dioxide for life and growth, just as fish depend on oxygen. Plants use carbon dioxide during the process of photosynthesis.

Sometimes carbon dioxide levels in water become too high. Pollution can produce too much carbon dioxide. In these conditions, fish have a hard time getting the oxygen they need from the water. They can even suffocate and die.

Keeping a good balance of carbon dioxide and oxygen is one reason why plants and animals are both valuable in a lake. Each makes what the other uses.

More Clues to Water Quality

Life on the Bottom
Many different bottom-dwelling animals, called benthic organisms, live in or on the sediment at the bottom of lakes and rivers. These animals are often tiny, even microscopic and may include worms, insect larvae, crustaceans, amphipods, mussels and clams.


Each benthic species requires certain habitat and water quality conditions to survive. Some organisms can survive in a wide range of conditions and are more “tolerant” of pollution. Others are very sensitive to changes in habitat condition and are “intolerant” of pollution.

We can determine the relative quality of the water by studying the species that live there. Scientists use a bottom dredge to scoop sediment samples from a lake bottom and examine the benthic life.
Adult Mayfly

The presence of some of these organisms in a lake may mean that water quality is relatively good:

  • Mayflies
  • Stoneflies
  • Some caddisflies
  • Riffle beetles
  • Hellgrammites

The presence of some of these pollution-tolerant organisms may mean that water quality is relatively poor:

  • Sludge worms
  • Leeches
  • Certain midge larvae