152 PLANKTON OF WISCONSIN LAKES 



One oi! the chief complexities involved in the question of plankton 

 production is the variety of the organisms constituting the crop; that 

 is, the standing crop of plankton is always made up of a number of 

 different kinds of organisms. At certain times of the year perhaps 

 not more than twelve or fifteen genera of organisms may be repre- 

 sented in the total plankton, while at other seasons the number may 

 increase to thirty or forty genera, possibly more ; the number depends 

 partly upon the character of the lake and partly upon the season of the 

 year. The various forms differ widely in character, also, ranging from 

 one-celled plants and animals to organisms as complex in structure as 

 the Crustacea and the insect larvae. 



There is no period of time during the year when one crop of plankton 

 ceases and another begins, so that there is no definite starting point 

 for the estimation of the annual crop of plankton, such as one finds 

 for a land crop, for example. Neither is there any exact date of ma- 

 turity, or harvest season, for the plankton crop as there is for the land 

 crop. The crop of plankton, therefore, represents a continuous stream 

 of life which flourishes at all seasons of the year and which passes on 

 from year to year as long as favorable conditions obtain in a body of 

 water. 



Some of the plankton forms are present in varying degrees of abun- 

 dance at all seasons of the year ; others make their appearance from time 

 to time when conditions are favorable for them, rise to a maximum, and 

 then decline in numbers. The decline of one form may be accompanied 

 by the rise of another, so that the developmental cycles of the two over- 

 lap in time, or two or more forms may rise to their maxima simultane- 

 ously ; therefore, this overlapping may produce a great variety of com- 

 plexities. These complications usually result, however, in a series of 

 waves or pulses of plankton in the course of the year, with the largest 

 crops generally coming in the spring and in the autumn. 



In comparison with land productivity another marked difference is 

 shown by the lake in that plankton production takes place at all depths 

 in a body of water ; the plankton soil of a lake, therefore, is coextensive 

 with the depth of the water. 



The various plankton organisms possess very different rates of re- 

 production. Under favorable conditions the aquatic bacteria may pass 

 through several generations in the course of a single day, while the 

 algae and protozoa may divide only once or perhaps twice in this in- 

 terval of time and the plankton Crustacea may require two weeks or 

 longer to pass from one generation to the next. Temperature is a very 

 important factor in determining the rate of reproduction in the various 

 forms ; that is, the reproductive process is most vigorous at the higher 

 temperatures which prevail during the summer months and it is least 



