CHEMICAL COMPOSITION OF RIVERS AND LAKES 



G9 



EXPLANATION 



Fioube 3.— Seasonal changes in the chemical composition of Imikpuk, a small lake 

 in Alaska near the Arctic Ocean. After Boyd (1959). Reprinted by permission of 

 Ecology. 



Such seasonal changes are of rather restricted 

 extent. In most lakes the major ions, except the 

 components of the carbonate buffer system, remain 

 relatively constant in amount, and large changes in 

 water chemistry are restricted to the scarcer biologi- 

 cally important substances. 



There are also diurnal changes in water chemistry, 

 but these are known to involve only the dissolved 

 gases, oxygen and carbon dioxide. During the day, 

 photosynthetic plants remove carbon dioxide from the 

 water and use it in the manufacture of carbohydrate, 

 giving up oxygen at the same time. During the night 

 the respiration of plants and animals reverses the process. 



In very productive lakes, under the control of carbo- 

 nate buffer systems, uptake of carbon dioxide by 

 photosynthesizing plants occasionally may cause very 

 dramatic changes in pH, as first the free C0 2 , then the 

 HC0 3 , and finally carbonate is used in photosynthesis. 

 The latter step is accomplished by the hydrolysis of 

 calcium carbonate, and leaves calcium hydroxide in 

 the water. An example is given in figure 4. 



Oxygen is easily and accurately measured, and forms 

 part of a great number of chemical analyses of lake 

 waters and of river waters as well. Most of these have 

 been spot analyses taken at a single and unspecified 

 time of day, and yield very little information of value 

 about the oxygen content of the water over a period 

 of 24 hours. Eecently there has been much interest 

 among limnologists in using diurnal oxygen change as 

 a measure of biological productivity (fig. 5), and one 

 may expect a great increase in the amount of informa- 

 tion about the magnitude of changes in this gas. At 

 present it is evident that the change is great in produc- 



643862 — 63 3 



12 p.m. 6 



12 m. 



Figube 4.— Diurnal pH changes in a small freshwater lake near Cape Town. After 

 Schiitle and Elsworth (1954). Reprinted by permission of Blackwell Scientific 

 Publications, Ltd., Oxford, England. 



tive lakes but may not be measurable in unproductive 

 ones, and that the diurnal oxygen change in a single 

 body of water may change with the season. 



In a shallow lake that mixes freely to the bottom, 

 gas changes are only diurnal, for diffusion from the 

 atmosphere makes up any net loss or gain that may take 

 place over 24 hours. Many lakes do not mix freely 

 to the bottom, however, and in the stagnant lower 

 layers of these the gas changes are cumulative and 

 have a profound effect on other aspects of the deep- 

 water chemistry as weU. 



TIME OF DAY, IN HOURS 

 6 12 18 



Light 



Figure 5.— Diumal oxygen change in Silver Springs, Fla. After Odum (1956). 

 Reprinted by permission of American Society of Limnology and Oceanography, 

 Inc. 



