100 PLANKTON OF WISCONSIN LAKES 



quantity of organic matter in the total plankton in kilograms per hec- 

 tare of surface. The shortest column appears in February, while the 

 next in length is found in August, and the third in March. The longest 

 column is found in December, with April second and November third. 

 Only two columns rise above the 500 kilogram line, namely, December 

 and April, while February-March and August-September fall well be- 

 low the 400 kilogram line, more especially the February average. 



The figures given in table 26 and the columns indicated in diagram 

 36 show only the amount of dry organic matter per unit of area. In 

 the living state, this material weighs about ten times as much as shown 

 here because most of the organisms found in this material contain at 

 least 90.0 per cent of water when alive, some of them in fact as much 

 as 97.0 per cent. On this basis, then, the quantity of living organic 

 matter in the total plankton in this portion of Lake Mendota ranges 

 from a minimum of 2,577 kilograms per hectare (2,300 pounds per 

 acre) in February to a maximum of 5,215 kilograms per hectare (4,652 

 pounds per acre) in December. An additional 10.0 per cent to 15.0 

 per cent of the dry weight would be contributed to this total by the ash 

 of the various organisms. 



Attention may be again called to the fact that these figures show 

 only what has been referred to as the standing or permanent crop, that 

 is, the amount of material that is present constantly during the differ- 

 ent months of the year. Therefore, they do not show how large a 

 quantity of organic matter is produced in the course of a year. Each 

 plankton form that appears during the year plays its particular role 

 in the production of this material; each comes on, passes through its 

 regular cycle when conditions are most favorable for it, and then de- 

 clines to a minimum number or disappears entirely for a certain por- 

 tion of the year. When some forms are on the wane, others are usually 

 ready to take their places to a greater or less extent, so that, while there 

 are marked variations in quantity during the year, there is no period 

 during which all forms are absent. This results in a complex over- 

 lapping of the crops of the various planktonts which continues through- 

 out all seasons of the year. 



The problem of ascertaining the amount of plankton material pro- 

 duced by the lake annually is made extremely difficult by this over- 

 lapping of the various forms and also by the fact that production is 

 a continuous process. There are no definite breaks in the stream of 

 plankton life which might serve to mark off one season 's or one year 's 

 production from another. Concomitant with the process of production, 

 also, is the process of destruction; some of the planktonts are con- 

 stantly being consumed as food by other organisms. Even some mem- 

 bers of the group, such as the plankton Crustacea, feed upon others. 



