SUMMABY AND DISCUSSION 153 



vigorous during the winter period of low temperatures. A decrease of 

 a few degrees in temperature is sufficient to cause a marked decline in 

 the rate of reproduction ; thus even in summer this process will proceed 

 more vigorously in the warm water of the epilimnion than in the cooler 

 water of the hypolimnion. 



Another factor which helps to complicate the problem of determining 

 the plankton productivity of a body of water is the great variation in 

 the length of life of the different forms. In the unicellular plankton 

 organisms, which multiply chiefly by fission, the lifetime of an indi- 

 vidual covers the period from one cell division to the next ; the lifetime 

 of a bacterium, therefore, may be less than an hour under favorable 

 temperature and food conditions, or it may be greatly prolonged, per- 

 haps to several days, by a low temperature of the water. Among the 

 algae and protozoa the span of life may be less than a day at summer 

 temperatures or it may be prolonged to several days at lower tem- 

 peratures. 



In the higher plankton forms reproduction takes place by means of 

 germ cells and the lifetime of the individual is much longer. Some of 

 the Crustacea, for example, may live for several weeks, or even for 

 several months when the temperature of the water is low. The lifetime 

 of the various planktonts, therefore, shows a great variation, ranging 

 from a minimum of less than an hour in some forms to a maximum of 

 several weeks or even months in others. Thus, the determination of the 

 rate of reproduction and of the length of life of individuals belonging 

 to the various forms in their natural environment will be a very im- 

 portant advance toward the solution of the problem in plankton pro- 

 ductivity. In fact, very little progress can be made in answering this 

 question until such data are obtained. 



There is also a great diversity in the size of different planktonts. 

 The coccus forms of bacteria, for example, are only 0.22^ to 0.75/x in 

 diameter, while Leptodora among the plankton Crustacea may reach a 

 length of 18 millimeters. 



The question of plankton productivity is complicated still further by 

 the fact that the losses as well as the gains of material must be taken 

 into account. These losses are sustained in two ways, namely, (1) 

 through the consumption of some of the planktonts as food by various 

 organisms, (2) through the death of some of the material. The losses, 

 like the gains, continue throughout the year so that production and de- 

 struction are simultaneous processes and the quantity of plankton that 

 is present at any given time during the year is the resultant or the 

 algebraic sum of these two processes; this resultant constitutes what 

 has been termed the standing crop of plankton. Whenever production 

 takes place at a faster rate than destruction, there is an increase in the 



