52 IOWA STUDIES IN NATURAL HISTORY 



matter in the total plankton of the entire lake may be cal- 

 culated from the volume given in table 2 ; this amounts to 

 98,520 kg. which is equivalent to about 108 tons. Expressed 

 in terms of a unit area this is 64.5 kg. per hectare, or 57.5 

 pounds per acre. In the living state this material would 

 weigh about ten times as much, since various experiments 

 have shown that about 90% of the live weight of these 

 organisms consists of water. 



This represents a small amount of organic matter per 

 unit of area but these figures indicate only the standing 

 crop at this particular time of the year. Both production 

 and destruction of this material are taking place at all 

 seasons of the year and the quantity of organic matter 

 found at a particular time is the resultant of these two 

 processes. Some of the forms reproduce much more rapidly 

 than others and also reproduction is much more vigorous at 

 certain seasons so that there is considerable variation in the 

 quantity of this material at different seasons. The more 

 minute organisms serve as food for the larger ones, such as 

 the Crustacea, and these in turn are eaten by the fish and 

 other aquatic organisms; some forms of the Crustacea, in 

 fact, prey upon others. Thus the whole problem is a very 

 complex one so that it would be extremely difficult to ascer- 

 tain, even roughly, the total annual production of plankton. 



A comparison with results that have been obtained on 

 lake Mendota will serve to show the relative plankton pro- 

 ductiveness of the two lakes. Table 26 gives the amount of 

 net and nannoplankton found in this lake during the last 

 week in July and the first week in August both in 1915 and 

 1916. The figures represent the average amount of organic 

 matter for these two weeks in both years and they indicate 

 the quantity in milligrams per cubic meter of water. It 

 will be noted that the organic matter in the net plankton of 

 lake Mendota in 1915 was substantially the same as that of 

 West Okoboji in 1919, but that it was very much smaller 

 in the former in 1916. On the other hand the nannoplank- 

 ton of lake Mendota was about five times as great in both 

 years as that of West Okoboji lake. The last column of the 

 table shows that the total plankton was more than three 



