What is pH?

The pH measurement shows how acidic or basic a water body is. The amount of hydrogen ion activity in water determines the level of pH on a scale from 0–14. The lower the pH value, the more acidic the water is. The pH range for natural bodies of water in the United States is around 6.5–9.

Shallow wetland with submerged aquatic plants and a forested border.
Photo credit: Dave Brenner, Michigan Sea Grant

The pH of a waterbody is important because aquatic organisms depend on a specific pH range to survive.

Why do we measure pH?

The pH is an important water quality parameter. Aquatic animals and plants are adapted to a certain pH range with most preferring between 6.5−8.0. An increase or decrease in pH outside a water body’s normal range can be detrimental to organisms, depending on their sensitivity.

A bar graph that illustrates the pH ranges that support aquatic life. Bacteria can live in the largest range from 1-13 on the scale. The pH range that supports plants is 6-12. Carp, suckers catfish and some insects can survive within a pH range of 5.5 to 9.9. Bass, bluegill and crappie prefer a narrower range of 6 to 8.5. While 7 to 10 is the pH range that supports snails, clams and mussels. Finally the largest variety of aquatic animals require the smallest range of pH from 7 to 9.
Adapted from: Michigan Sea Grant

What affects pH measurements?

A variety of natural and human factors can influence the pH level of a body of water.

  • Natural changes in pH could result from the surrounding landscapes like bogs, marshes, or pine forests. These habitats contain vegetation that produces organic acids during decomposition (e.g. sphagnum moss or pine needles).
  • Carbon dioxide can be added to a lake or the ocean through gas exchange or photosynthesis and respiration. This impacts the carbonate system and can contribute to changes in pH in waterbodies. On a global scale, this process leads to ocean acidification. 
  • Regional geology also impacts the carbonate system and therefore pH levels. For example, lakes with a limestone bed have a natural buffering ability to neutralize acid inputs.     
  • Water can also become more acidic through acid rain or from mine drainage, which both contain acidifying compounds.

pH Factsheet

Ask Dr. Fish: How might pH levels affect Great Lakes fishes?