ENVIRONMENTAL FACTORS 289 



of more than 10,000 sq. km. (Lake Superior has 82,360 sq. km.). Most 

 inland waters are much smaller, and we find all gradations of size 

 down to the ponds and puddles. The depth of inland waters also varies, 

 and compared with the ocean, even the deepest are shallow. The 

 average depth of the ocean is about 3795 m., while only two inland 

 waters (Lake Baikal, 1706 m., and Lake Tanganyika, 1435 m.) have 

 a depth of more than 1000 m., even in their deepest part. Only a few 

 are deeper than 400 m. ; the majority reach a depth of less than 100 m. ; 

 Lake Superior is about 305 m. in greatest depth. Lakes, ponds, and 

 pools decrease in depth until they merge into marsh. The same grada- 

 tion of depth is noticeable in running waters. Consequently the extent 

 of shore line and bottom in relation to amount of water is very much 

 greater in inland waters than it is in the ocean, and as a result, the 

 ratio of extent of suitable habitat for sessile forms and free-swimming 

 forms is very different. 



Salt content. — The variation of the dissolved substances of inland 

 waters is great. In the ocean, in consequence of the free circulation of 

 the masses of water, the differences in the amount of dissolved material 

 are constantly being equalized, so that the variation is always within 

 narrow limits. Nearly all inland waters, on the other hand, have 

 chemical constitutions of their own, depending upon their substrata 

 and their tributary areas. Equalization between different lakes by 

 means of mixing is, of course, out of the question. In streams whose 

 contents change rapidly and whose tributary area, compared with the 

 waters they contain, is more widespread, such differences are equalized, 

 but even here they are present. 



Inland waters are usually fresh waters, i.e., they contain relatively 

 little dissolved material. The content of common salt, especially, is less 

 than in the ocean. The water of the Rhine, for instance, contains only 

 0.14 part of NaCl to 1000 parts of water. There are, nevertheless, a 

 good many inland waters which contain a considerable amount of 

 salt. There are salt springs whose waters, to be sure, soon become fresh 

 when they flow into other streams; but where standing waters are 

 fed the salt content rises, and so, aided by evaporation, salt lakes are 

 formed. The minimum ratio for a salt lake is 0.3 gm. per liter, but the 

 ratio is often much higher as is seen in the Aral Sea where it is 10.8 gm. 

 to 1 liter; or in the salt swamps of Laneuveville in Lorraine, 54 gm. 

 per liter; or in the Dead Sea where it is 237.5 gm. per liter. In the 

 salt seas of the steppes where periodic rains freshen the water and 

 long periods of evaporation during the dry season concentrate it again, 

 the salt content changes with the seasons, and the composition of the 

 fauna changes in consequence. 



