COMMUNITIES IN STANDING WATERS 321 



water plays an important part along with that of the substratum. 

 Biological relations may vary decidedly in the two locations. Thus 

 Weissensee 2 reports that the normal condition of Anodonta in standing 

 water is hermaphroditic while those in flowing waters are bisexual, the 

 transference of the spermatozoa to the eggs being assisted by the 

 current. 



The factor of extent. — Aside from the nature of the substratum 

 and its richness in nutritive salts, there may be noted some important 

 differences, which are due to the ratio of water mass to bottom area. 

 There is relatively less bottom area per cubic meter of water in a lake 

 than in a pond, and less in a pond than in a pool; the relationship is 

 less favorable in deep lakes than in shallow ones. An increase in the 

 relative amount of bottom area, especially of lighted bottom, often 

 means an increase in the abundance of living forms. 3 In running waters, 

 because of their generally rapid current, the smaller streams and brooks 

 contain less life than slowly flowing parts of the large ones; but the 

 smaller bodies of standing waters, if fairly permanent, are more favor- 

 able to life than the larger. 



There is every gradation of size in standing waters between Lake 

 Superior, with 82,360 sq. km. of surface area, and ponds, pools, and 

 puddles. The depths vary similarly. Only in very few do they exceed 

 1000 m., and only rarely are they more than 400 m.; but from this 

 depth there are all gradations to the shallowest puddles, and the ratio 

 of the depth to the surface area is certainly not fixed. As was men- 

 tioned above, the ratio of the amount of water to the extent of sub- 

 stratum is much greater in small, shallow waters, and the amount of 

 salts dissolved from the substratum, other things being equal, is also 

 greater. The shore line, in relationship to the surface area, is longer, 

 and consequently the shore vegetation is richer. A much larger part 

 of the water is penetrated by the sun's rays, a fact which also favors 

 the development of plants. The conditions of oxygen supply are more 

 favorable in shallow waters, for the surface area is larger here in 

 relation to the volume than in deep waters. Thus in Lake Constance 4 

 the upper lake with its 475 sq. km. of surface area and its depth of 

 more than 200 m. is much poorer in fauna than the lower lake, con- 

 nected with the upper, and having a surface area of 63 sq. km. and an 

 average depth much less than the maximum of 45 m. Among ponds in 

 Holstein 5 all those rich in plankton are relatively small and shallow 

 (not more than 7 m. deep, usually much less), and those which are 

 of considerable size, and of a depth reaching 25 m. or more, are invari- 

 ably poor in plankton. Of twenty Swedish lakes, 5 the smallest (with 

 an area of 31 hectares) had the largest amount of bottom fauna and 



