The term “hardness” is applied to the soap neutralizing power of water. Any substance that will form an insoluble curd with soap causes hardness. Since iron, manganese, copper, barium, lead, zinc, and other trace elements are seldom present in appreciable concentrations in natural waters, hardness is attributed principally to calcium and magnesium ions.
Because most of the effect observed with soap results from the presence of calcium and magnesium, hardness is now generally defined in terms of these two constituents alone with some rather indefinite reservations about other ionic interferences. The other ions that might precipitate soap include hydrogen ion and all polyvalent metals, but usually they are present in insignificant amounts in water of the type that is usable domestically and for which hardness data might be obtained.
The hardness of water is conventionally expressed in all water analysis made in the U.S. in terms of an equivalent quantity of calcium carbonate. Some such convention is needed for hardness because this is a property imparted by several different cations which may be present in varying proportions. However, the actual presence of the indicated number of mg/L in the form of calcium carbonate certainly should not be assumed.
The adjectives “hard” and “soft” as applied to water are inexact and some writers have tried to improve on this situation by adding qualifying adverbs.
mg/L calcium carbonate Description
61-120 moderately hard
121-180 hard more than 180 very hard
Hardness in water used for ordinary domestic purposes does not become particularly objectionable until it reaches a level of 100 mg/L or so. Hardness can greatly exceed this level and in many places, especially where waters have contact with limestone or gypsum, 200-300 mg/L or more of hardness will be common. Hardness in water may be caused by the natural accumulation of salts from contact with soil or geologic formations.
The significance of hardness on the aquatic organisms, but conclusive statements of hardness and interaction with the organisms are the exception. Generally, however, it has been noted that hardness can exert a considerable influence on algae. Many genera of algae occur freely both in soft, acid water and in limestone spring streams, but others seem to be primarily confined to either hard or soft waters.
Water hardness also seems to affect some higher aquatic plants, but not others. A few only occur in soft water, whereas other plants seem to require harder water and few are apparently indifferent.
Calcium not only affects the efficiency of osmoregulation, but it has also been shown for groups as far apart as flatworms and fish that both calcium and magnesium affect the rate of respiration. Both ions raise respiratory rates at low temperatures and lower them at high temperatures. It can therefore be expected that water hardness has some important affects on the respiration of aquatic organisms. We can conclude therefore that water hardness is a controlling factor in the ecology of at least some stream invertebrates, but that very often we have little idea as to how it operates.
Most species of fish have a wide tolerance of water hardness, but occasionally it has been suggested that water softness or low pH may account for the absence of certain species from areas where they were expected. It has, however, frequently been observed that brown trout grow faster to a larger size in hard water than in soft water.
Why hardness should affect the growth rate of trout has remained a puzzle.
It is sometimes suggested that the benthic food supply is better in harder waters. Invertebrates with non-seasonal cycles, such as crustaceans and mollusks, are more common and so provide a more consistent food supply than the mostly seasonal insects of soft water.
Another suggestion ties to the relatively greater breeding success in softer waters. This generalization is that soft waters are usually on hard rocks which provide good spawning gravel and that alkaline waters are usually on softer rocks and have much more limited areas for spawning and breeding. The spawning success would then relate to the crowding effect of the juvenile fish reducing the amount of food available for each one.
However, if the slow growth in soft water is caused by lack of food, a reduction of the population should increase the rate of growth of the remaining fish, but this does not apparently happen.
McFadden and Cooper (1962), (from Hynes) compared the populations of fish in three soft water and three hard water streams and found that there were indications that the biomass was larger in the harder streams. To date most reports of a negative correlation between water hardness and growth have been concerned with trout. The situation would seem to be clarified if other types of fish which live in similarly wide ranges of hardness would also be studied.
Another point that may have bearing on fish magnesium raise the respiratory rate of the growth is that calcium and rainbow trout and the chub at low temperatures and decrease it at high ones. This may mean that fish in hard water feed more actively in winter and use up less energy in summer, thus having more energy available for growth. Finally, as one would expect, there are some floral and faunal differences between hard and soft water which affect the available food supply.