pH is a measurement of the concentration of hydrogen ions. pH ranges from 0 (very acidic) to 14 (very basic).
	Typical lakes range from pH of 4 to 9.
	Acid rain, organic matter decomposition, and primary production levels all affect pH.  As plants respire, they produce CO2, which has an acidifying effect because CO2 combines with water to form carbonic acid. Increases in respiration rates often result in lower, more acidic pH.