Q PLANKTON OF WINNEBAGO AND GREEN LAKES. 



only in the peculiar environment of uniform low temperature 

 and little or no light. Lake Winnebago, on the other hand, can- 

 not he said to have an abyssal fauna, the animals of the bottom 

 in the deeper parts of the lake not differing appreciably from 

 those of the littoral region. 



With such differences in physical conditions we should ex- 

 pect corresponding differences in fauna and flora. The writer 

 has already suggested in a former publication (Marsh '97, 

 181) that lakes may be divided into the two classes of "deep" 

 and "shallow," the dividing line between the two classes being 

 at about forty meters. Later (Marsh, '99, 171) I was led to 

 think that the limit should be placed at a lower figure, — per- 

 haps thirty or thirty-five meters. It would be better, doubt- 

 less, to make the main classification of lakes depend upon the 

 existence or non-existence of a thermocline. In the shallow 

 lakes there is no thermocline. Taking the presence or absence 

 of a thermocline as the basis of classification, the depth limit 

 between deep and shallow lakes would not be definite but would 

 bear a constant relation to the area and character of the shore ; 

 for the thermocline is much more marked in small lakes with 

 high shores. 



Deep lakes are susceptible of a two-fold classification into 

 the large and the small. In the small deep lakes the abysssal 

 water stagnates, and the conditions of life become so hard that 

 an abyssal fauna can hardly be said to exist. In the large deep 

 lakes, on the other hand, the movement of the surface waters 

 under the influence of the winds produces slow return currents 

 along the bottom which serve to relieve the condition of stagna- 

 tion, and permit the existence of a somewhat abundant abyssal 

 fauna. Green lake and Lake Geneva have the characteristics 

 of large deep lakes, while Elkhart and the Waupaca lakes are 

 types of small deep lakes. 



On the whole this seems to me the most satisfactory classi- 

 fication of lakes, and in this paper I shall use the terms "deep," 

 "shallow," "large deep," and "small deep," as indicated above. 

 While, as will be seen later, faunal and floral distinctions do 



