What is/are the principle ions contributing to the alkalinity of a water sample?

Answer;

The Hydroxide or Hydroxyl ions (OH)

Hydroxide ions are the principle ions contributing to the alkalinity of water sample.

Explanation;

Alkalinity refers to the measure of the buffering capacity of water. In other words this is the water's ability to resist sudden changes in pH.

pH on the other hand which is closely related to alkalinity  is a measure of how acidic or basic water is. It expresses the degree of the concentration of hydrogen ions.

Alkalinity measures the acid neutralizing capacity, as a result of ions such as hydroxide ions, carbonate  and bicarbonate ions.

it is the most commonly accepted one