Once precipitation hits the ground, a variety of organic and inorganic chemical reactions may take place to alter the pH of water. The geology of a region exerts a strong control on the pH of natural waters. This ability to buffer, or resist the changes in pH, is called alkalinity. See the alkalinity information sheet for more background. Under normal stream conditions, pH levels are usually highest at the end of a day of photosynthesis, lowest after a night of plant respiration.
All aquatic life has a specific pH range that it can tolerate and to which it is adapted.
If the pH changes even slightly, it will stress the creatures and may even kill them. Immature stages of aquatic insects and young fish are extremely sensitive to a pH below 5. Low pH causes an imbalance in the sodium and chloride ions in aquatic animals' blood. At low pH, hydrogen ions may be taken into cells while expelling sodium ions. These metals were locked up in mineral matter under neutral pH levels, but become mobile when the pH lowers.