390 THE FRESH-WATER LOCHS OF SCOTLAND 
The composition of the lake- water is also chemically influenced by 
the organic life. In summer, when the water is only to a very slight 
degree renewed and the evaporation is great owing to the high tem- 
perature, a further concentration takes place from the decomposition of 
the great quantity of organic material. The water, especially in the 
shallow lakes, thus almost assumes the appearance of soup, which is no 
doubt of the greatest importance in determining the maximum develop- 
ment of many Infusoria, Chlorophyceae, Flagellata, and Myxophyceae. 
The phytoplankton may play a principal part in the production of 
oxygen, probably to a less degree in lakes than in ponds (Knauthe, 
1898, p. 785 ; 1899, p. 783). In bright sunshine the Volvocinea; and 
Euglena of the ponds can secrete such large quantities of oxygen under 
the influence of light that the water may contain up to 24 c.c. oxygen 
per litre. Corresponding quantities do not occur, indeed, in the lakes 
(9-12 c.c, Halbfass, 1901, p. 96). The quantity of oxygen does not, 
as a rule, attain in lakes the limit of saturation, or at any rate exceeds 
it but slightly (Halbfass, 1901, p. 96). 
Within recent times the view has come more and more to the 
front that the reduction of the carbonic acid in lake-water, and there- 
with the deposition of lime, is in greater or less degree due to the 
activity of organisms. In fresh water these organisms are the green 
plants and molluscs. If the water at a given tension is saturated 
with calcium bicarbonate, then for every gram of carbonic acid which 
is taken by the plants during the assimilation processes from the water 
and used to build up organic materials, 2 "3 gm. CaCOg are precipitated. 
The molluscs " absorb calcium bicarbonate, retain the monocarbonate, 
but the carbonic-acid-forming bicarbonate is liberated"' (Krogh, 1904, 
p. 382). 
Some authors are inclined to see in the action of the organisms the 
chief source of the reduction of the carbonic acid (Duparc for the 
lake d'Annecy, 1894, p. 199 ; Halbfass, 1901, p. 93, and Passarge, 1901, 
p. 144, for several North German lakes ; and for the lower-lying alpine 
lakes, Bourcart, 1906, p. 118). A co-operating part in the reduction 
of the carbonic acid is ascribed to the organisms by Delebecque 
(1898a, p. 222 ; 1895, p. 790). Dr Krogh takes up a special position. 
He comes to the main result that the organisms " in the long run 
are altogether incapable of either adding to or diminishing the lime 
deposits in a lake (p. 382). Krogh supposes, in fact, that nearly all 
the organic material of plants " is in due course again decomposed, 
whereby the carbonic acid is completely recovered " ; for the molluscs, 
he maintains that the liberated carbonic acid " will increase the 
tension of the water, causing it to dissolve from the lime deposits of 
the bottom, from dead shells, and, indeed, from whatever source, exactly 
the quantity of lime which the living mussels have taken from it." 
