Maech 9, 1900.] 



scmjsrcK 



379 



these are an enormous number of diatoms. 

 Chlorophycem are present in larger or smaller 

 numbers and sometimes large numbers of 

 the C'yanophycece. Generally speaking the 

 limnetic flora is not sufficiently abundant 

 to attract attention, but on some lakes they 

 are sometimes multiplied in such quantity 

 as not only to attract attention but even to 

 excite alarm. The surface of the lake has 

 a thick coat of bright green, and as this is 

 cast up on the shore by the waves it forms 

 thick ridges which in their decay become 

 very offensive. This phenomenon has been 

 known in England as ' the breaking of the 

 meres,' in Germany as the ' wasserbliithe,' 

 and in this country as the ' working of the 

 lakes.' The appearance is sometimes as- 

 cribed to the seeds of littoral plants. Only 

 a few species of plants are concerned in this 

 phenomenon, and the species differ some- 

 what in different lakes. Certain diatoms 

 may be present, too, in sufficient numbers 

 to produce an unpleasant ' fishy ' smell in 

 the water. This exaggerated growth of 

 the limnetic flora is most pronounced in 

 shallow or comparatively shallow lakes, 

 and is frequently a source of great annoy- 

 ance to cities which get their water supplies 

 from such bodies. 



The limnetic fauna, as I have said before 

 in this address, has but few kinds of ani- 

 mals, by far the most numerous and char- 

 acteristic being the Crustacea. These are 

 beautiful, transparent and nearly colorless 

 creatures. It is not true, as is sometimes 

 stated, that the limnetic fauna is entirely 

 distinct from the littoral. The general 

 character of the limnetic animals is cer- 

 tainly different, and distinctly different, 

 from that of those found in the littoral 

 region, but many individuals are common 

 to both. It is true, however, that while 

 there is hardlj' a radical distinction be- 

 tween the two faunae, certain species are 

 common in the linmetic regions and only 

 rarelj' found in the littoral, while some that 



are everywhere in littoral collections are 

 rarely found in those taken in the open 

 water. There is, of course, no clear divid- 

 ing 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 cer- 

 tain distinction between the limnetic fauna 

 of the deeper lakes and that of the shal- 

 lower. One species of Diaptomus is found 

 everywhere in the Great Lakes, but in only 

 three of the Wisconsin lakes — Lake Geneva, 

 Green Lake, and Cedar Lake. The shal- 

 lowest 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 faunte into the deep water and the 

 shallow water, suggesting as a possible 

 limit between the two 40 meters. I have 

 since found that Elhart Lake, 117 feet, 

 and Cedar Lake, 95 feet, have many char- 

 acteristics of the deep water fauna, and 

 it is possible 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 

 shallow lakes. This is easily explained. 

 For the number of animals is, of course, 

 dependent on the number of plants. In- 

 asmuch as plants are dependent upon sun- 

 shine, 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 water only to a limited distance, 

 the deeper parts of our lakes are entirely de- 



