90 PLANKTON OF WISCONSIN LAKES 



The maximum number of 60,000 per cubic centimeter noted at 10 

 meters on September 22, 1921, represented twice as much material as 

 the maximum of 1920: that is, wet weight 67.8 milligrams per cubic 

 meter of water, dry weight 10.8 milligrams, and organic matter 9.6 

 milligrams. The 1921 maximum, however, is not the average for all 

 depths, but the number at 10 meters ; the numbers noted at other depths 

 on this date were considerably smaller, but they would still give an 

 average above that of 1920. 



On the basis of their ability to produce living matter in the course 

 of the year, Lohmann 8 estimated that one volume of bacteria is equal 

 to six volumes of protista (protophyta and protozoa) and to three 

 hundred volumes of metazoa. These results obtained on Lake Mendota 

 show a very much larger proportion of protista to bacteria. The 

 nannoplankton alone, exclusive of the protophyta and protozoa of the 

 net plankton, yielded an average of 1,472.0 milligrams of dry organic 

 matter per cubic meter of water during the three summer months of 

 June, July, and August in 1915, and 1,507.0 milligrams during the 

 same months in 1916. For the same periods the net plankton averaged 

 337.0 milligrams of organic matter per cubic meter in 1915 and 181.0 

 milligrams in 1916. Computations based upon numerical data show 

 that the Crustacea and rotifers contribute an average of one-third of 

 the organic matter in the net plankton ; thus, it seems safe to estimate 

 that about half of the net plankton during the summers of 1915 and 

 1916 was derived from the protista. Adding half of the organic matter 

 of the net plankton to that of the nannoplankton gives an average of 

 about 1,640.0 milligrams of dry organic matter in the protista in the 

 summer of 1915 and of 1,589.0 milligrams per cubic meter in 1916. 

 These amounts represent more than three hundred thirty times as much 

 organic matter as the maximum crop of bacteria in 1920 and more than 

 one hundred sixty times as much as the maximum number of bacteria 

 noted at 10 meters on September 22, 1921. They represent more than 

 three thousand times as much organic matter as the late summer crop 

 of bacteria in 1919. 



The value of the bacteria in the plankton economy of Lake Mendota 

 is by no means as small as these figures seem to indicate, because they 

 multiply at a much faster rate than the protista; the bacteria may 

 pass through a number of generations in the course of a day under 

 favorable food and temperature conditions, while the protista may not 

 average more than one or perhaps two divisions per day under similar 

 conditions. In spite of this marked difference in reproductive capacity, 

 it appears from the foregoing results that the bacteria do not play 

 nearly as important a role in the plankton complex of Lake Mendota 

 as Lohmann 's estimate might lead one to expect. 



8 Internationale Eevue, Bd. 4, 1911, pp. 1-38. 



