Marsh — The Flanklon of Fresh Water Lalces. 175 



hold a great deal of the dead and decaying material falling from 

 the upper layers. It is a curious fact, first pointed out, I think, 

 by Whipple, that, the bottom waters of deep lakes are stagnant 

 during both winter and summer, but have a period of overturn- 

 ing in spring and fall. This overturning may come with con- 

 siderable suddenness when the waters have not been much agi- 

 tated by the winds. We ordinarily think that water is so mobile 

 that the heavier portions will immediately sink, and thus the 

 water of greater density will always be at the bottom. It may 

 happen, how^ever, as in the fall, that the upper waters will cool 

 off, and yet retain their position, so that the lighter water will 

 actually be below. The lake is in a condition of unstable equili- 

 brium. If, under these circumstances, there comes a heavy 

 wind, the whole body of water will overturn. 



It is at these two periods of overturning, as shown by Whip- 

 ple, tliat the growth of diatoms is especially pronounced, and 

 they are found present in enormous numbers in the limnetic 

 plankton. The great growth of diatoms is explained in the fol- 

 lowing way : during the periods of stagnation diatoms or their 

 spores, if diatom spores exist, accumulate at the bottom of the 

 lake, inasmuch as their specific gravity is greater than that of 

 water. They do not grow there, because sunlight is essential to 

 their growth. At the bottom is accumulating during this period, 

 too, a large amount of organic matter from the decay of organ- 

 isms near the surface, and this, under the influence of bacteria, 

 is transformed into material fitted for food for the diatoms: in 

 this food material the nitrates are perhaps the most important 

 When the time of overturning comes, the diatoms or their spores 

 rise to the surface, accompanied by these dissolved organic ma- 

 terials, and, under the influence of sunlight, an exceedingly 

 rapid multiplication takes place. As the food materials are used 

 up the numbers of diatoms decrease again. Other organisms, 

 of course, show the efl^ect of the overturning of the water, for 

 many are directly or indirectly dependent upon the diatoms for 

 food, and, besides, diatoms are not the only organisms which 

 can make use of the food materials which are thus brought to 

 the surface. Diaptormis, Epischura, Limnocalanus and two 

 species of Cyclops show quite clearly these two periods of rapid 



