Alkalinity is the water's capacity to resist changes in pH that would make the water more acidic. It is also the protector of your health and piping when it comes to drinking water. This capacity is commonly known as "buffering capacity."
For example, if you add the same weak acid solution to two vials of water - both with a pH of 7, but one with no buffering power (e.g. zero alkalinity) and the other with buffering power (e.g. an alkalinity of 50 mg/l), - the pH of the zero alkalinity water will immediately drop while the pH of the buffered water will change very little or not at all. The pH of the buffered solution would change when the buffering capacity of the solution is overloaded or used up. Pure water (no alkalinity) has a pH of 7 but let that pure water be exposed to the atmosphere and the pH will drop to ~ 5.5 because carbon dioxide in the air will dissolve in the water and create carbonic acid.
This is why I like to call Alkalinity the Protector of the Stream.
Alkalinity is important for fish and aquatic life because it protects or buffers against rapid pH changes. Living organisms, especially aquatic life, function best in a pH range of 6.0 to 9.0. Alkalinity is a measure of how much acid can be added to a liquid without causing a large change in pH. Higher alkalinity levels in surface waters will buffer acid rain and other acid wastes and prevent pH changes that are harmful to aquatic life.
Acid shock may occur in spring when acid snows melt, there are thunderstorms, natural discharges of tannic waters, "acid rain", acidic dryfall, and when other acidic discharges enter the stream. If increasing amounts of acids are added to a body of water, the water's buffering capacity is consumed. If additional buffering material can be obtained from surrounding soils and rocks, the alkalinity level may eventually be restored. However, a temporary loss of buffering capacity can permit pH levels to drop to those harmful to life in the water.
The pH of water does not fall evenly as acid contamination proceeds. The natural buffering materials in water slow the decline of pH to around 6.0. This gradual decline is followed by a rapid pH drop as the bicarbonate buffering capacity is used up. At a pH of 5.5, only very weak buffering materials remain and pH drops rapidly with additional acid. Sensitive species and immature animals are affected first. As food species disappear, even larger, resistant animals are affected.
For the protection of aquatic life, the buffering capacity should be at least 20 mg/L. If alkalinity is naturally low (less than 20 mg/L), there can be no greater than a 25% reduction in alkalinity.