Many genera of algae occur freely in both soft, acid water and in limestone spring streams, but others seem to be primarily confined to either hard or soft waters.
The alkalinity of water or some related parameters such as pH or hardness has often been considered to exert a considerable influence on algae. Some researchers have found not only that fewer specimens grew per unit area in acid streams than in alkaline rivers, but that the dominant forms were also different.
The greater abundance of algae in soft water lakes as compared to hard water lakes is dependent upon the bicarbonate content. The algae of soft water regions are largely Desmids and certain species of Chrosophata and Chlorophyta. On the other hand, the Diatoms and Achnanthes and Cocconeis are found only in alkaline water.