Scientists use pH to describe how acidic or basic a substance is pH is really a measure of the relative amount of free hydrogen (H) in solution. In water at 25°C, the concentration of H* = 1 X 107 moles/liter and the pH is 7. The pH of any
substance is negative logarithm of the hydrogen ion concentration. On the pH scale, a pH of 7 is neutral. The scale ranges
from 0 - 14: a pH less than 7 indicates an acid and a pH greater than 7 indicates a base. Each number represents a 10-fold
change in ion concentration. A substance with a pH of five (H* - 10-5) is ten times more acidic than a substance having a pH
of six (H* = 106).
Most freshwater lakes, streams, and ponds have a natural pH in the range of 6 to 8. Acid deposition has many harmful
ecological effects when the pH of most aquatic systems falls below 6 and especially below 5.
Here are some effects of increased acidity on aquatic systems:
- As the pH approaches 5, non-desirable species of plankton and mosses may begin to invade, and populations of fish such as
smallmouth bass disappear.
- Below a pH of 5, fish populations begin to disappear, the bottom is covered with undecayed material, and mosses may
dominate nearshore areas.
- Below a pH of 4.5, the water is essentially devoid of fish.
- Aluminum ions (Al3+) attached to minerals in nearby soil can be released into lakes, where they can kill many kinds of fish
by stimulating excessive mucus formation. This asphyxiates the fish by clogging their gills. It can also cause chronic stress
that may not kill individual fish, but leads to lower body weight and smaller size and makes fish less able to compete for food
and habitat
When the pH of freshwater becomes highly alkaline (>9), the effects on fish may include death, damage to outer surfaces like
gills, eyes, and skin and an inability to dispose of metabolic wastes.