Marsh — The Plankton of Fresh Water Lakes. 171 



clear dividing line between the two regions, but one insensibly 

 merges into the other, while, under the influence of the winds 

 and waves, such limits as exist are continually changing. 



In the species of Crustacea there is a certain distinction be- 

 tween the limnetic fauna of the deeper lakes and that of the 

 shallower. One species of Diaptornus is found everywhere in 

 the Great Lakes, but in only three of the Wisconsin lakes — 

 Lake Geneva, Green Lake, and Cedar Lake. The shallowest of 

 these lakes — Cedar Lake — is about a hundred feet in depth. 

 One of the species of Cyclops is very abundant in the Great 

 Lakes, but is seldom found elsewhere except in comparatively 

 deep lakes. 



In a paper published in the Transactions of the Academy, I 

 made the suggestion that lakes might be divided according to 

 their faunse into the deep water and the shallow water, suggest- 

 ing as a possible limit between the two 40 meters. I have since 

 found that Elkhart Lake, 117 feet, and Cedar Lake, 95 feet, 

 have many characteristics of the deep water fauna, and it is pos- 

 sible that the dividing: line should be nearer 30 meters than 40. 

 The German authors make a distinction between "plankton- 

 poor'^ and "plankton-rich" lakes, which very nearly corresponds 

 to my deep-water and shallow-water lakes ; for the total amount 

 of plankton in the deep lakes is very much less than in the shal- 

 low lakes. This is easily explained. For the number of ani- 

 mals is, of course, dependent on the number of plants. Inas- 

 much as plants are dependent upon sunshine, they will grow in 

 water only in those places that are reached by the sun's light. 

 As the light of the sun penetrates in deep w^ater only to a lim- 

 ited distance, the deeper parts of our lakes are entirely devoid 

 of plant life. On the other hand in shallow lakes not only do we 

 find the floating vegetation as in the deep lakes, but as the light 

 reaches the bottom over a larger proportion of its surface, we 

 have in addition a very large flora flourishing on the bottom. 

 In some of the very shallow lakes nearly the whole bottom is 

 covered with a rank vegetation. This is true, for instance, of 

 Lake Vieux Desert. In Green Lake, on the other hand, inas- 

 much as the shores are somewhat precipitous, there is only a 

 comparatively narrow margin on which can be supported a flora 



