Kutropliic lakes may later develop into ponds and 

 marshes ; dystrophic lakes, into bogs. Biotic succes- 

 sion is scarcely discernible, however, in very large 

 or very deep lakes. The Great Lakes, for instance, 

 will endure until erosion lowers their outlets. 



Carbon dioxide and other gases 



Carbon dioxide is required by plants for photo- 

 synthesis. Its presence in lake waters tends to vary 

 inversely with oxygen. Carbon dioxide is derived 

 from the atmosphere, the respiration of both animals 

 and plants, decaying organic matter, ground water, 

 and bicarbonate salts. It may occur in either the jrcc 

 state (dissolved CO.), Iialj-boiind state (HCO3), 

 or fixed state (CO.i). These three states are asso- 

 ciated respectively with pH values 7, 7 to 10, and 

 above 10. Algae and some rooted aquatic vegetation 

 are able to obtain the half-bound CO2 from the solu- 

 ble bicarbonate salts, thereby converting them into 

 the less soluble carbonates : 



Ca(HC03)2: 



: CaCO:, + CO-, + H2O 



Mollusks, a few insects, and some bacteria are also 

 able to precipitate carbonates. Carbonates precipitate 

 as to make conspicuous marl deposits on the bottom 

 of some lakes. When marl formation becomes consid- 

 erable, there is a decrease in lake fertility and a conse- 

 quent decrease in animal life present, including 

 bottom-inhabiting organisms. 



When there is sufficient free carbon dioxide in 

 the water derived from sources other than carbonates. 

 they are converted back into bicarbonates and marl 

 does not form. The degree of alkalinity of a lake is 

 measured by the amount of carbon dioxide or acid 

 required to convert the excess carbonates into bi- 

 carbonates, yielding neutral water. Soft-water lakes 

 contain not over 5 cc/1 fixed carbon dioxide : 

 medium-class lakes contain 5 to 22 cc/1 ; hard-water 

 lakes may have from 22 to as high as 50 cc/1 ( Birge 

 and Juday 1911). 



Marsh gas (methane) evolves from organic mat- 

 ter decomposing at the bottom. It rises in bubbles to 

 the surface of the water. Methane formation may be 

 extensive during the summer stagnation period. 

 Methane does not appear to be particularly toxic to 

 organisms until it is generated in very large amounts. 



Hydrof/en sulphide results from anaerobic de- 

 composition of sulphurous organic matter. It may be 

 conspicuous in sewage-polluted waters. It is inher- 

 ently very poisonous. 



Nitrogen occurs in water by reason of difTusion 

 from the atmosphere. When present in excessive 

 amount it has been known to form bubbles in the 

 circulatory systems of fish causing death, but this 

 does not commonly occur in natural waters. 



Ammonia may occur naturally in water, a result 

 of decomposition of organic matter. Ammonia may 

 also be dumped into streams and lakes from indus- 

 trial plants, often in concentrations toxic to fish. Fish 

 are apparently unable to detect the ])resenfe of an)- 

 monia in water. 



Dissnlvvd solids 



l-'.-illing r.iin may contain as much as 30 to 40 

 l)pm of solids, and the runoff dissolves more as it 

 drains over the upland into streams and lakes. Water 

 draining off siliceous or sandy soils may contain 50 

 to 80 ppm of dissolved minerals ; off more fertile 

 calcareous soils, 300 to 660 ppm. Lake waters com- 

 monly vary from about 15 to 350 ppm of dissolved 

 minerals, although in some lakes of the Great Basin, 

 the total dissolved salts exceed 100,000 ppm. The 

 ocean contains only 33,000 to 37,370 ppm. 



Inorganic salts especially important for plants in- 

 clude ammonium salts, nitrites, and nitrates as 

 sources of nitrogen ; phosphates to supply phosphorus 

 which, with nitrogen and sulphur, are raw materials 

 for protein synthesis ; silicates, which furnish silicon 

 to diatoms and sponges : and salts of calcium, mag- 

 nesium, manganese, iron, copper, sodium, and po- 

 tassium for proper development of chlorophyll and 

 growth of plants and, indirectly, of animals. Mol- 

 lusks require calcium salts for shells. Crayfish and 

 other arthropods require calcium for the carapace ; 

 vertebrates, for their skeleton. Absence of these 

 necessary salts in lake waters limits the kinds and 

 abundance of animals that can live there. Phosphorus 

 and nitrogen are the most likely to be deficient. 

 Nutrient salts tend to accumulate in the deeper waters 

 and at the lake bottom, but they are brought to the 

 surface at the autumn and spring overturns. Lakes 

 in prairie regions tend to have more salts than those 

 in deciduous or hardwood forests, which, in turn, 

 have more salts than lakes in coniferous forest areas 

 (Moyle 1956). The total dissolved content of a lake 

 is important in determining its general level of pro- 

 ductivity (Northcote and Larkin 1956). 



Little is known about the amount of amino acids, 

 fats, and carbohydrates occurring in natural bodies 

 of water and how much of this nutrient material may 

 be directly absorbed by organisms. Dissolved organic 

 matter is derived chiefly from plankton remains, and 

 other dead plants and animals as well as from bot- 

 tom mud and external sources. In Wisconsin lakes, 

 there is about 15 mg/1, of which crude protein con- 

 stitutes 15 per cent, fats or ether extract 1 per cent, 

 and carbohydrates about 83 per cent. Dissolved or- 

 ganic material becomes higher, of course, in dystro- 

 l)hic lakes and peat bogs (Birge and Juday 1934). 



Lakes 65 



