382 



SCIENCE. 



[N. S. Vol. XI. No. 271. 



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 overturning in spring and fall. 

 This overturning may come with consider- 

 able suddenness when the waters have not 

 been much agitated by the winds. We 

 ordinarilj' 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, however, as in the fall, that the 

 upper waters will cool off, and yet i-etain 

 their position, so that the lighter water will 

 actually be below. The lake is in a condi- 

 tion of unstable equilibrium. If, under 

 these circumstances, there comes a heavy 

 wind, the whole body of water will over- 

 turn. 



It is at these two periods of overturning, 

 as shown by Whipple, that the growth of 

 diatoms is especially pronounced, and they 

 are found present in enormous numbers in 

 the limnetic plankton. This great growth of 

 diatoms is explained in the following way : 

 During the periods of stagnation diatoms 

 or their spores, if diatom spores exist, ac- 

 cumulate at the bottom of the lake, inas- 

 much 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 organisms 

 near the surface, and this, under the influ- 

 ence of bacteria, is transformed into mate- 

 rial fitted for food for the diatoms : in this 

 food material the nitrates are perhaps the 

 most important. When the time of over- 

 turning comes, the diatoms or their spores 

 rise to the surface, accompanied by these 

 dissolved organic materials, and, under the 

 influence of sunlight, an exceedingly rapid 

 multiplication takes place. As the food 



materials are used up the numbers of dia- 

 toms decrease again. Other organisms, of 

 course, show the efiect of the overturning 

 of the water, for many are directly or in- 

 directly 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 sur- 

 face. Diaptomus, Epischura, Limnocalaims and 

 two species of Cyclops show quite clearly 

 these two periods of rapid production, al- 

 though in some of these cases the results 

 are complicated by the fact that the tem- 

 perature of the surface water has a direct 

 eflect on the reproduction. 



The matter of the annual distribution of 

 the organisms of the plankton is a very inter- 

 esting question, but is also a very compli- 

 cated one. As has just been stated, certain 

 of the diatoms have a distinct spring and fall 

 maximum, and there are other organisms 

 which, because of their dependence upon 

 the diatoms, have similar periods of maxi- 

 mum growth. But there are other causes 

 at work which control the growth of indi- 

 vidual organisms, so that their optimum 

 periods may come at very diflerent times of 

 the year. 



Generally speaking it is probably true 

 that the largest amount of plankton occurs 

 in midsummer, although Yung says that 

 the maximum development of plankton in 

 Lake Leman is in May or June, and that 

 the minimum is in March and September. 

 Zacharias states that the maximum period 

 for Lake Ploa is about August 1st. Meas- 

 urements of a large number of collections 

 from various lakes in Wisconsin indicate 

 that the maximum of plankton occurs in 

 these lakes in the latter part of July. The 

 exact period of maximum development may 

 vary from year to year, and in different 

 lakes, because of varying local conditions. 

 The reason for this July maximum seems 

 to be not because any considerable number 

 of organisms have their highest develop- 



