18 PLANKTON OF WISCONSIN LAKES 



Thus the present investigation is a step in the direction of supplying 

 this much needed information ; but it can be regarded only as a begin- 

 ning since it will require similar investigations on a considerable num- 

 ber of different types of lakes. Also, it applies to only a single element 

 of this aquatic life, namely, the plankton. The larger organisms must 

 receive similar treatment. Much more work, therefore, both of a 

 numerical and of a chemical nature, must be done, not only on the 

 plankton but also on the larger aquatic organisms, before we shall 

 have sufficient data for drawing any general conclusions as to the 

 productivity of our fresh-water lakes. 



The problem of aquatic productiveness involves complexities which 

 are not encountered in studies on the productivity of the land. In a 

 lake, for example, the productive soil, so to speak, is coextensive in 

 depth with the depth of the lake and also includes the bottom to a 

 certain depth because many organisms, such as insect larvae and oligo- 

 chaets, inhabit the bottom mud. On land the various crops are easily 

 kept separate and they mature at a definite time so that they may be 

 harvested and their total quantity ascertained. But in a lake the 

 plankton crop alone, for example, comprises a considerable number of 

 forms of which many are present at the same time. Thus the plankton 

 crop, in general, consists of a mixture of forms and only rarely is it 

 possible to obtain a pure catch of any form except the larger Crustacea, 

 which may be sorted out by means of nets having different sizes of 

 mesh. The various forms reach their maximum numbers at different 

 times of the year, some even in winter, so that there is no definite har- 

 vest time at which this material may be collected and the annual pro- 

 duction of it thereby ascertained. The plankton, therefore, must be 

 considered as a " standing crop" since it is present at all seasons of the 

 year and since it does not possess any definite period of maturity; in 

 other words, it constitutes a continuous stream of life which presents 

 different degrees of abundance during the course of its annual cycle. 

 The same is true also of many of the larger aquatic organisms. 



Certain forms of algae frequently appear in a practically pure state 

 in the summer as the socalled "water-bloom" and at such times they 

 can be obtained in sufficient abundance for a chemical analysis. The 

 smaller forms, however, more especially those found in the nannoplank- 

 ton, present a more difficult problem since they do not seem to thrive 

 well in laboratory cultures and they rarely appear in the lake in suffi- 

 cient abundance and purity for one to obtain enough material of the 

 different forms for an analysis. But the problem is not insoluble and 

 the final results will justify the expenditure of much time and energy 

 in its solution. 



