Marsh — The Planhton of Fresh Water Lakes. 177 



the year, however, are either plant or animal forms wanting, 

 and collections made through the ice will give a considerable 

 variety of kinds, as well as numbers of individuals. 



It will be noticed that the period of maximum plankton de- 

 velopment corresponds to the time of highest temperature of 

 the water, and that tlie period of minimum development corre- 

 sponds to the time of lowest temperature, so that we can be cer- 

 tain that the one important controlling factor in the growth of 

 plankton is the temperature. 



Henscn and his co-laborers who worked out a very elaborate 

 system of measuring the plankton of the sea claimed that the dis- 

 tribution of the plankton over wide stretches of the sea was 

 nearly uniform. Granting this to be true, it evidently is pos- 

 sible by a series of collections and measurements to compare dif- 

 ferent parts of tlie ocean in regard to their productiveness in 

 animal and plant life. This conclusion, that the horizontal 

 plankton of the sea has a practically uniform distribution, has 

 been accepted by the majority of scientists, although vigorously 

 combated bv Hackel. 



Apsteiu has applied the sam.e methods to the examination of 

 lakes, and has concluded from his investigations that the plank- 

 ton of lakes is uniformly distributed. It is easily seen that there 

 is very good reason for assuming this to be the case from the 

 actual conditions under which the plankton exists. The plank- 

 ton consists of organisms that do not move voluntarily from 

 place to place, and therefore do not change their positions. They 

 are dependent upon heat and light for their development, so that 

 their groui:h is mainly within forty feet of the surface ; conse- 

 quently a depth exceeding this would not mean any greater pro- 

 duction of plankton — or in other words, the amount of plankton 

 depends not upon depth, but upon surface. 



This is a most important conclusion, if true, for it gives us 

 an exact method by ^^iiich we can compare one lake with another 

 and determine their relative productiveness, or from a series of 

 collections, determine the absolute annual productiveness of any 

 body of water. Such determinations would have an important 

 commercial value, for by them could be estimated the possibil- 

 ities of fish production in a lake. The method was worked out 

 12 



