The Total Mineral Content of Lake Waters

Abstract

The total dissolved solids or mineral content of lake waters has never been emphasized in limnological studies. In considering the chemical aspects of productivity, we tend to think at once of specific elements such as calcium, nitrogen, and phosphorus. There is reason to believe that in some waters one or more of these substances may be present in limiting quantities, and for this reason fertilization has been used in attempts to increase organic production in small lakes. It is possible that our preoccupation with individual elements has led us to neglect a simple and useful clue to the conditions which lie behind productivity. Recent studies in lakes of northwestern Canada suggest that the total mineral content of waters provides a rough indicator of edaphic conditions which must in some measure affect the productivity of lakes. The range of mineral content in natural waters is exceedingly wide. A few small lakes in Maine, northern Wisconsin and Colorado are known to have between 15 and 20 p.p.m. of total solids (Clarke 1924a). Large lakes occasionally have between 30 and 60 p.p.m. but relatively few have less than 100. Most lakes with free inflow and outflow seem to have between 100 and 200 p.p.m. but as soon as the outflow is seriously restricted the total solids increase to 300 or more. Lakes with more than 300 p.p.m. of total solids are commonly described as saline, although they do not become obviously saline (e.g., to taste) until they contain two or three times this amount. In southern Saskatchewan, we find an array of saline lakes ranging from 500 to 120,000 p.p.m., the latter about four times the salinity of the sea. A general account of these saline lakes has been published (Rawson and Moore 1944). The present discussion will be confined to the range of mineral content in fresh waters. The procedure for determining dissolved solids in lake waters is so simple that it requires little explanation. Samples of 200 to 500 cc. are usually collected. After settling or filtering to remove the suspended material, a sample, usually of 100 cc., is evaporated at 1100 C. The residue, recorded in p.p.m., is usually referred to as total dissolved solids although it is recognized that some bicarbonates and some organic matter may have been decomposed by heating. Evaporation at 1800 C. is sometimes used but this results in a further loss in the water of crystallization of certain salts. Fresh waters are by definition dilute solutions and thus the minerals exist in a highly ionized condition. The total content of electrolytes can be determined with a suitable instrument and will provide a measure of the total mineral content. Rodhe (1949) has demonstrated the validity of this procedure for fresh waters of the commonly occurring carbonate type. In our investigations, it has been convenient to use the evaporation method since it is often desirable to use the residue for determination of one or more specific ions. When the major ions have been determined, it is convenient to plot them in the form of Maucha's (1932) ion field diagrams which make possible a quick visual comparison of different water types.